Steel sheet and manufacturing method thereof

Information

  • Patent Grant
  • 11859260
  • Patent Number
    11,859,260
  • Date Filed
    Tuesday, January 7, 2020
    4 years ago
  • Date Issued
    Tuesday, January 2, 2024
    11 months ago
Abstract
A steel sheet has a predetermined chemical composition, in which a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%, the metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%, the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, and in the surface layer region, the average grain size of the secondary phase is 0.5 μm to 4.0 μm.
Description
TECHNICAL FIELD, OF THE INVENTION

The present invention relates to a steel sheet and a method for manufacturing the same.


Priority is claimed on Japanese Patent Application No. 201′9-000671, filed on Jan. 7, 2019, the content of which is incorporated herein by reference.


RELATED ART

Recently, in order to protect the global environment, it is desired to improve the fuel consumption of a vehicle. Regarding the improvement of the fuel consumption of a vehicle, high-strengthening is further required for a steel sheet for a vehicle in order to reduce the weight of a vehicle body while securing safety. This high-strengthening is required not only for a structural member such as a member or a pillar but also for an exterior component (for example, a roof, a hood, a fender, or a door) of a vehicle. For this requirement, a material has been developed in order to simultaneously achieve strength and elongation (formability).


On the other hand, the forming of an exterior component of a vehicle tends to become more complicated. When the thickness of a steel sheet is reduced through high-strengthening, a surface of the steel sheet is likely to be uneven during forming into a complicated shape. When the surface is uneven, the external appearance after forming deteriorates. Regarding an exterior panel component, not only characteristics such as strength but also design and surface quality are important. Therefore, the external appearance after forming is required to be excellent. The unevenness occurring after forming described herein refers to unevenness occurring on a surface of a formed component even when the steel sheet surface after manufacturing is not uneven. Even when the formability of the steel sheet is improved, the occurrence is not necessarily suppressed. Therefore, when a high strength steel sheet is applied to an exterior panel of a vehicle, there is a large problem.


Regarding a relationship between the external appearance after forming and mater al characteristics in a steel sheet to be applied to an exterior panel, for example, Patent Document 1 discloses a ferritic steel sheet in which, in order to improve surface properties after stretching, an area fraction of crystal having a crystal orientation of ±15° from {001} plane parallel to a steel sheet surface is 0.25 or less and, the average grain size of the crystal is 25 μm or less.


However. Patent Document 1 relates to a ferritic steel sheet in which a C content is 0.0060% or less. For high-strengthening of a steel sheet, it is effective to increase the C content such that a dual phase structure including ferrite and a hard phase is obtained. However, as a result of an investigation by the present inventors, it was found that, when the C content is increased to obtain a dual phase structure, the area fraction of crystal having a crystal orientation of ±15° from {001} plane parallel to a steel sheet surface cannot be reduced unlike Patent Document 1. That is, with the method disclosed in Patent Document 1, the high-strengthening and the improvement of surface properties after working (suppression of the occurrence of unevenness) cannot be achieved simultaneously.


For example, Patent Document 2 discloses a dual phase structure steel including ferrite and a secondary phase, and describes that it is effective to decrease a yield point as a countermeasure against surface strain during forming. However, Patent Document 2 does not disclose a relationship between the external appearance after forming and a structure from the viewpoint of a countermeasure against surface roughness or pattern.


That is, in the related art, a high-strength dual phase structure steel in which surface roughness or pattern defects after forming is improved is not disclosed.


PRIOR ART DOCUMENT
Patent Document



  • [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2016-156079

  • [Patent Document 2] PCT International Publication No. WO2013/046476



DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention

The present invention has been made in consideration of the above-described problems. An object of the present invention is to provide: a high strength steel sheet in which the occurrence of surface unevenness during forming is suppressed; and a method for manufacturing the same.


Means for Solving the Problem

The present inventors conducted an investigation on a method for achieving the object. In particular, the present inventors conducted a thorough investigation on a relationship between surface unevenness in a manufactured steel sheet or surface unevenness after forming and a microstructure or, a texture of the steel sheet and found that: i) unevenness after forming occurs even when unevenness does not occur on the steel sheet surface after manufacturing; ii) surface unevenness after forming occurs due to inhomogeneity of deformation in a range from the steel sheet surface to a position of 20 μm in the sheet thickness direction; and iii) the reason for the uneven deformation is non-uniform dispersion of a hard structure or development of a specific texture.


In addition, as a result of further investigation, the present inventors found that DP (dual phase) steel including ferrite and a secondary phase is preferable in order to simultaneously achieve strength and formability, and by adjusting the fraction of the secondary phase, the average grain size of the secondary phase, and the texture of ferrite in the metallographic structure in a surface layer region ranging from the surface to a range of 0 to 20 μm in the sheet thickness direction (range from the surface to a position of 20 μm from the surface in the sheet thickness direction) to be different from those in an internal region of the steel sheet, a steel sheet in which the occurrence of surface unevenness after forming is suppressed and the external appearance (surface appearance quality) after forming is excellent can be obtained, while securing strength.


In addition, as a result of investigation, the present inventors found that, in order to control the metallographic structure in the surface layer region, it is effective to apply strain after hot rolling instead of after cold rolling and to set a cold-rolling reduction and heat treatment conditions after the strain application depending on the working amount.


The present invention has been made based on the above findings, and the scope thereof is as follows.


[1] According to one aspect of the present invention, there is provided a steel sheet including, as a chemical composition, by mass %: C: 0,050% to 0.145%; Mn: 0.70% to 2.25%; Al and Si in total: 0.60% or less; P: 0.030% or less; S: 0.020% or less; N: 0.0050% or less; Mo: 0% to 0.80%; B: 0% to 0.0050%; Ti: 0% to 0.200%; N 0% to 0.100%; Cr: 0% to 0.700%; Ni: 0% to 0.200%; and a remainder of iron and impurities, in which a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%, a metallographic structure in, an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%, the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, and in the surface layer region, the average grain size of the secondary phase is 0.5 μm to 4.0 μm, and a texture in which an XODF{001}/{111} as the ratio of an intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.70 to 2.50 is included.


[2] In the steel sheet according to [1], the average grain size of the secondary phase in the internal region may be 1.0 μm to 5.0 μm and may be more than the average grain size of the secondary phase in the surface layer region.


[3] In the steel, sheet according to [1] or [2], the chemical composition may include, by mass %, one or more selected from the group consisting of: Mo: 0.001% to 0.80%; B: 0.0001% to 0.0050%; Ti: 0.001% to 0.200%; Nb: 0.001% to 0.100%©; Cr: 0.001% to 0.700%; and Ni: 0.001% to 0.200%.


[4] In the steel sheet according to any one of [1] to [3], the chemical composition may satisfy the following Expression (1), the tensile strength may be 550 MPa or higher, a sheet thickness may be 0.10 mm to 0.55 mm, and the sheet, width may be 1300 mm to 2000 mm.

7.00≥[C]×20+[Si]×3.0[Mn]×2.0+[Al]×2.0+[Cr]+[Ti]×20+[Nb]×40+[Mo]×2.0+[Ni]×2.0+[B]×200  (1)


In the expression, a symbol of each of the elements in Expression (1) represents the amount of the element by mass %, and when the element is not included, 0 is substituted into the symbol.


[5] In the steel sheet according to any one of [1] to [4] the secondary phase in the surface layer region may include one or more selected from the group consisting of martensite, bainite, and tempered martensite.


[6] In the steel sheet according to any one of [1] to [5], a plating layer may be provided on the surface.


[7] According to another aspect of the present invention, there is provided a method for manufacturing a steel sheet including: a heating process of heating a slab having the chemical composition according to [1] at 1000° C. or higher; a hot-rolling process of hot-rolling the slab such that the rolling finishing temperature is 950° C. or lower to obtain a hot-rolled steel sheet; a stress application process of applying a stress to the hot-rolled steel sheet after the hot-rolling process such that the absolute value of a residual stress σS on a surface is 165 MPa to 400 MPa; a cold-rolling process of cold-rolling the hot-rolled steel sheet after the stress application process such that a cumulative rolling reduction RCR is 70% to 90% to obtain a cold-rolled steel sheet; an annealing process of heating the cold-rolled steel sheet such that the average heating rate in a range from 300° C. to a soaking temperature T1° C. that satisfies the following Expression (2) is 1.5° C./sec to 10.0° C./sec and holding the heated steel sheet at the soaking temperature T1° C. for 30 seconds to 150 seconds for annealing; and a cooling process of cooling the cold-rolled steel sheet after the annealing process to a temperature range of 550° C. to 650° C. such that the average cooling rate in a range from the soaking temperature T1° C. to 650° C. is 1.0° C./sec to 10.0° C./sec and further cooling the cooled steel sheet to a temperature range of 200° C. to 490° C. such that the average cooling rate is 5.0° C./sec to 500.0° C./sec.

1275−25×ln(σS)−4.5×RCR≤T1≤1275−25×ln(σS)−4×RCR  (2).


[8] In the method for manufacturing a steel sheet according to [7], the stress application process may be performed at 40° C. to 500° C.


[9] In the method for manufacturing a steel sheet according to [7] or [8], in the hot-rolling process, a finish rolling start temperature may be 850° C. or lower.


[10] The method for manufacturing a steel sheet according to any one of [7] to [9] may further include a holding process of holding the cold-rolled steel sheet after the cooling process in a temperature range of 200° C. to 490° C. for 30 seconds to 600 seconds.


Effects of the Invention

In the steel sheet according to the aspect of the present invention, the occurrence of surface unevenness is suppressed even after various deformation during press forming as compared to a material in the related art. Therefore, the steel sheet according to the aspect of the present invention has excellent appearance quality of the surface after forming and can contribute to improvement of the vividness and design of coating. In addition, the steel sheet according to the present invention has high strength and can contribute to further reduction in the weight of, a vehicle. In the present invention, the high strength represents a tensile strength of 550 MPa or higher.


In addition, with the method for manufacturing a steel sheet according to the aspect of the present invention, a high strength steel sheet in which the occurrence of surface unevenness is suppressed even after various deformation during press forming can be manufactured.





BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a diagram showing a relationship between surface properties after forming and a texture parameter.





EMBODIMENTS OF THE INVENTION

A steel sheet according to an embodiment of the present invention (the steel sheet according to the embodiment) includes, as a chemical composition, by mass %: C: 0.050% to 0.145%; Mn: 0.70% to 2.25%; Al and Si in total: 0.60% or less; P: 0.030% or less; S: 0.020% or less; N: 0.0050% or less; Mo: 0% to 0.80%; B: 0% to 0.0050%; Ti: 0% to 0.200%; Nb: 0% to 0.100%; Cr: 0% to 0.700%; Ni: 0% to 0.200%; and a remainder of iron and impurities.


In addition, in the steel sheet according to the embodiment, a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%, a metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%, and the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region.


In addition, in the steel sheet according to the embodiment, in the surface layer region, the average grain size of the secondary phase is 0.5 μm to 4.0 μm, and a texture in which an XODF{001}/{111} as the ratio of an intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.70 to 2.50 is included.


In the steel sheet according to the embodiment, it is preferable that the average grain size of the secondary phase in the internal region is 1.0 μm to 5.0 μm and is more than the average grain size of the secondary phase in the surface layer region.


Hereinafter, the steel sheet according to the embodiment will be described in detail. The present, invention is not limited only to the configuration disclosed in the embodiment and can be modified within a range not departing from the scope of the present invention. A limited numerical range described below includes a lower limit and an upper limit. A numerical value shown together with “more than” or “less than” is not included in a numerical range. All the “%” in the chemical composition represents “mass %”. First, the reason for limiting the chemical composition of the steel sheet according to the embodiment will be described.


<Regarding Chemical Composition>


[C: 0.050% to 0.145%]


C (carbon) is an element that increases the strength of the steel sheet and is essential for securing the volume fraction of the secondary phase. In order to secure a desired volume fraction of the secondary phase, the C content is set to be 0.050% or more. The C content is preferably 0.070% or more.


On the other hand, when the C content is more than 0.145%, the number of hard phase (secondary phase) grains increases, and the hard phase is likely to be linked. A portion other than the linked hard phase is promoted to be deformed during forming. In a case where hard phase grains are, non-uniformly dispersed, pattern defects are likely to be formed on the surface after forming. In addition, when the C content is more than 0.145%, a cold rolling force during cold rolling at a high rolling reduction increases, the productivity decreases, and the formability or weldability of the steel sheet deteriorates. Therefore, the C content is set to be 0.145% or less. The C content is preferably 0.130% or less and more preferably 0.110% or less.


[Mn: 0.70% to 2.25%]


Mn (manganese) is an element that is effective for increasing the mechanical strength of the steel sheet. In addition, Mn is an element that immobilizes S (sulfur) in the steel as MnS or the like to prevent cracking during hot rolling. In order to obtain the effects, the Mn content is set to be 0.70% or more. The Mn content is preferably 0.90% or more.


On the other hand, when the Mn content is more than 2.25%, a cold rolling force during cold rolling at a high rolling reduction increases, and the productivity decreases. In addition, segregation of Mn is likely to occur. Therefore, the hard phase aggregates after annealing such that pattern defects are likely to be formed on the surface after forming. Therefore, the Mn content is set to be 2.25% or less. The Mn content is preferably 2.00% or less and more preferably 1.75% or less.


[Al and Si in Total: 0.60% or less]


Al (aluminum) is a deoxidizing element of steel that is effective for increasing the mechanical strength of the steel sheet. In addition, Si (silicon) is a deoxidizing element of steel that is effective for increasing the mechanical strength of the steel sheet. However, when the total amount of Al and Si is more than 0.60%, scale peelability during production deteriorates, and surface defects are likely to be formed on the product. In addition, a cold rolling force during cold rolling at a high rolling reduction increases, and the productivity decreases. Further, the weldability or the deformability of the steel sheet deteriorates. Therefore, the total amount of Al and Si is set to be 0.60% or less. The total amount of Al and Si is preferably 0.50% or less.


In addition by setting the Si content to be 0.10% or less, the scale peelability during production can be improved, and the occurrence of surface defects in the product can be suppressed. Therefore, the Si content is preferably 0.10% or less and more preferably 0.05% or less.


[P: 0.030% or less]


P (phosphorus) is an, impurity. When an excess amount of P is included in the steel, cracking is promoted during hot rolling or cold rolling, and the weldability or ductility of the steel sheet deteriorates. Therefore, the P content is limited to 0.030% or less. It is preferable that the P content is limited to 0.020% or less. The P content is preferably small and may be 0%. In consideration of existing general refining (including secondary refining), the P content may be 0.0005% or more.


[S: 0.020% or less]


S (sulfur) is an impurity. When an excess amount of S is included in the steel, MnS stretched by hot rolling is formed, and the deformability of the steel sheet deteriorates. Therefore, the S content is limited to 0.020% or less. The S content is preferably small and may be 0%. In consideration of existing general refining (including secondary refining), the S content may be 0.0005% or more.


[N: 0.0050% or less]


N (nitrogen) is an impurity and is an element that deteriorates the deformability of the steel sheet. Accordingly, the N content is limited to 0.0050% or less. The N content is preferably small and may be 0%. However, in consideration of existing general refining (including secondary refining), the N content may be 0.0005% or more.


That is, the steel sheet according to the embodiment may include the above-described elements and a remainder consisting of Fe and impurities. However, in order to improve various characteristics, the following elements (optional elements) may be included instead of a part of Fe. From the viewpoint of reducing the alloy cost, it is, not necessary to add the optional elements to the steel on purpose. Therefore, the lower limit of the amount of each of the optional elements is 0%. The impurities refer to components that are unintentionally included from raw materials or other manufacturing processes in the process of manufacturing the steel sheet.


[Mo: 0% to 0.80%]


Mo (molybdenum) is an element that contributes to the improvement of the mechanical strength of the steel sheet. In addition, in a case where the Mo content is less than the Mn content, Mo is an element that is less likely to segregate than Mn and contributes to uniform dispersion of the hard phase. Therefore, Mo may be optionally included. In order to obtain the effect, the Mo content is preferably 0.001% or more.


On the other hand, when the Mo content is excessively large, the deformability of the steel sheet may deteriorate. Therefore, even in a case where Mo is included, the Mo content is 0.80% or less. In addition, Mo is an expensive element, and an increase in Mo content increases an increase in alloy cost. From this viewpoint, the Mo content is preferably 0.15% or less.


[B: 0% to 0.0050%]


B (boron) is an element that immobilizes carbon and nitrogen in the steel to form a fine carbonitride. The fine carbonitride contributes to precipitation hardening, microstructure control, grain refinement strengthening, and the like of the steel. Therefore, B may be optionally included. In order to obtain the effect, the B content is preferably 0.0001% or more.


On the other hand, when the B content is more than 0.0050%, the effect is saturated, and the workability (deformability) of the steel sheet may deteriorate. In addition, the strength of the steel sheet provided for cold rolling increases by including B. Therefore, a cold rolling force during cold rolling at a high rolling reduction increases. Therefore, in a case where B is included, the B content is set to be 0.0050% or less.


In addition, by setting the Al content to be 0.10% or more and setting the B content to be 0.0010% to 0.0030%, the strength of the steel sheet can be more efficiently improved while reducing a cold rolling force. Therefore, the Al content, is preferably 0.10% or more, and the B content is preferably 0.0010% to 0.0030%. In this case the upper limit of the Al content may be set to be 0.50% in consideration of the total amount of Al and Si.


[Ti: 0% to 0.200%]


Ti (titanium) is an element that immobilizes carbon and nitrogen in the steel to form a fine carbonitride. The fine carbonitride contributes to precipitation, hardening, microstructure control, grain refinement strengthening, and the like of the steel. Therefore, Ti may be optionally included. In order to obtain this effect, the Ti content is preferably 0.001% or higher.


On the other hand, when the Ti content is more than 0.200%, the effect is saturated, the strength, of the steel sheet provided for cold rolling increases, and a cold rolling force during cold rolling at a high rolling reduction increases. Therefore, even in a case where Ti is included, the Ti content is 0.200% or less.


[Nb: 0% to 0.100%]


Nb (niobium) is an element that immobilizes carbon and nitrogen in the steel to form a fine carbonitride. The fine Nb carbonitride contributes to precipitation hardening, microstructure control, grain refinement strengthening, and the like of the steel. Therefore, Nb may be optionally included. In order to obtain the effect, the Nb content is preferably 0.001% or more.


On the other hand, when the Nb content is more than 0.100%, the effect is saturated, the strength of the steel sheet provided for cold rolling increases, and a cold rolling force during cold rolling at a high rolling reduction increases. Therefore, even in a case where Nb is included, the Nb content is 0.100% or less.


[Cr: 0% to 0.700%]


Cr (chromium) is an element that contributes to the improvement of the mechanical strength of the steel sheet. Therefore, Cr may be optionally included. In order to obtain the effect, the Cr content is preferably 0.001% or more.


On the other hand, when the Cr content is excessively large, the strength of the steel sheet provided for cold rolling increases, and a cold rolling force during cold rolling at a high rolling reduction increases. In addition, excessive inclusion of Cr causes an increase in alloy cost. Therefore, even in a case where Cr is included, the Cr content is 0.700% or less.


[Ni: 0% to 0.200%]


Ni (nickel) is an element that contributes to the improvement of the mechanical strength of the steel sheet. Therefore, Ni may be optionally included. In order to obtain the effect, the Ni content is preferably 0.001% or more.


On the other hand, when the Ni content is excessively large, the strength of the steel sheet provided for cold rolling increases, and a cold rolling force during cold rolling at a high rolling reduction increases. In addition, excessive inclusion of Ni causes an increase in alloy cost. Therefore, even in a case where Ni is included, the Ni content is 0.200% or less.


The above-described chemical composition of the steel sheet may be measured using a general analysis method. For example, the chemical composition may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using an infrared absorption method after combustion, and N may be measured using an inert gas fusion-thermal conductivity method. In a case where the steel sheet includes a plating layer on the surface, the chemical composition may be analyzed after removing the plating layer from the surface by mechanical grinding.


<Metallographic Structure of Surface Layer Region>


In the steel sheet according to the embodiment, when the sheet thickness is represented by t, a depth range from the surface to t/4 in a sheet thickness direction is divided into two regions, a depth range from the surface as a starting point to a depth position of 20 μm in a depth direction as an end point is represented by a surface layer region, and a range over the surface layer region to a center side of the steel sheet is represented by an internal region.


As a result of a thorough investigation by the present inventors, it was found that the surface unevenness during forming occurs due to inhomogeneous deformation occurs during forming caused by inhomogeneity in strength in a microscopic region. In particular, it was found that the occurrence of the unevenness of the surface is largely affected by the metallographic structure in the surface layer region ranging from the surface to a range of 0 to 20 μm in the sheet thickness direction (range from the surface to a position of 20 μm in the sheet thickness direction). Therefore, in the steel sheet according to the embodiment, the metallographic structure in the surface layer region is controlled as follows.


[Consisting of Ferrite and Secondary phase having Volume Fraction of 1.0% to 15.0% and Volume fraction of Secondary Phase being less than Volume fraction of Secondary Phase in Internal region]


When the volume fraction of the secondary phase in the surface layer region is less than 1.0%, the strength of the steel sheet is not sufficiently improved. Therefore, the volume fraction of the secondary phase is set to be 1.0% or more. On the other hand, when the volume fraction of the secondary phase is more than 15.0%, the hard phase is likely to be non-uniformly dispersed. Therefore, surface unevenness occurs during forming, and the external appearance after forming deteriorates.


In addition, the volume fraction of the secondary phase in the metallographic structure of the surface layer region is set to be less than the volume fraction of the secondary phase in the internal region. By setting the volume fraction of the secondary phase in the surface layer to be less than the volume fraction, of the secondary phase in the internal region and further increasing the volume fraction in the internal region as described below, the suppression of the occurrence of the surface unevenness and the material strength can be achieved simultaneously.


In the steel sheet according to the embodiment, the secondary phase in the surface layer region is a hard structure other than ferrite and is, for example, one or more among pearlite, martensite, residual austenite, bainite, and tempered martensite. From the viewpoint of improving the strength, it is preferable that the secondary phase in the surface layer region includes one or more among martensite, bainite, and tempered martensite, and it is more preferable that secondary phase in the surface layer region includes martensite.


The volume fraction of the secondary phase in the surface layer region can be obtained using the following method.


A sample (the size is substantially 20 mm in the rolling direction×20 mm in the width direction×the thickness of the steel sheet) for metallographic structure (microstructure) observation is collected from a W/4 position or a 3W/4 position of a sheet width W of the obtained steel sheet (that is, an end portion of the steel sheet in the width direction to the W/4 position in the width direction), and a metallographic structure (microstructure) in a range from the surface to the ¼ thickness position is observed using an optical microscope to calculate the area fraction of the secondary phase in a range from the surface of the steel sheet (in a case where a plating layer is present, the surface excluding the plating layer) to 20 μm. In order to prepare the sample, a sheet thickness cross section in an orthogonal-to-rolling direction is polished as an observation section and is etched with, the LePera reagent.


“Microstructures” are classified based on an optical microscope image at a magnification by 500-times. When the optical microscope observation is performed after the LePera corrosion, the respective structures are observed with different colors, for example, bainite is observed to be black, martensite (including tempered martensite) is observed to be white, and ferrite is observed to be gray. Therefore, ferrite and other hard structures can be easily distinguished from each other. In the optical microscope image, a region other than gray representing ferrite is the secondary phase.


A region ranging from the surface to a ¼ thickness position in the sheet thickness direction from the surface in the steel sheet etched with the LePera reagent is observed in 10 viewing fields at a magnification by 500-times, a region from the surface to a position of 20 μm of the steel sheet in the obtained optical microscope image is designated, and the image analysis is performed using image analysis software “Photoshop CS5” (manufactured by Adobe Inc.) to obtain the area fraction of the secondary phase. In an image analysis method, for example, a maximum luminosity value Lmax and a minimum luminosity value Lmin of the image are acquired from the image, a portion that has pixels having a luminosity of Lmax−0.3×(Lmax−Lmin) to Lmax is defined as a white region, a portion that has pixels having a luminosity of Lmin to Lmin+0.3×(Lmax−Lmin) is defined as a black region, a portion other than the white and black regions is defined as a gray region, and the area fraction of the secondary phase that is the region other than gray is calculated. By performing the image analysis as described above in 10 observed viewing fields in total, the area fraction of the secondary phase is measured. Further, the area fraction values are averaged to calculate the average value. Assuming that the area fraction, is the same as the volume fraction, the average value is calculated as the volume fraction of the secondary phase in the surface layer region.


[Average Grain Size of Secondary Phase being 0.5 to 4.0 μm]


When the average grain size of the secondary phase is more than 4.0 μm, the external appearance after forming deteriorates. Therefore, the average grain size of the secondary phase in the surface layer region is set to be 4.0 μm or less.


On the other hand, when the average grain size of the secondary phase is less than 0.5 μm, grains of the secondary phase are likely to aggregate. Even in a case where individual grains of the secondary phase are fine, when the grains aggregate, the external appearance after forming deteriorates. Therefore, the average grain size of the secondary phase in the surface layer region is set to be 0.5 μm or more. The average grain size is preferably 1.0 μm or more.


The average grain size of the secondary phase in the surface layer region can be obtained using the following method.


Using the same method as described above, a region ranging from the surface to a ¼ thickness position in the sheet thickness direction from the surface in the steel sheet etched with the LePera reagent is observed in 10 viewing fields at a magnification by 500-times, a region from (the surface to a position of 20 μm)×200 μm of the steel sheet in the optical microscope image is selected, and the image analysis is performed using image analysis software “Photoshop CS5” (manufactured by. Adobe Inc.) to calculate the area fraction of the secondary phase and the number of grains of the secondary phase, respectively. By adding up the values and dividing the area fraction of the secondary phase by the number grains of the secondary phase, the average area fraction per grain of the secondary phase is calculated. The circle equivalent diameter is calculated based on the average area fraction and the number of grains, and the obtained circle equivalent diameter is set as the average grain size of the secondary phase.


[Texture in which XODF{001}/{111} as Ratio of Intensity of {001} Orientation to Intensity of {111} Orientation in Ferrite is 0.70 to 2.50 being included in Surface Layer Region]


When a texture in which an XODF{001}/{111} as a ratio of an intensity of {001} orientation to an intensity of {111} orientation in the ferrite (ratio between maximum values of X-ray random intensity ratios) is 0.70 to 2.50 is included in the surface layer region, the external appearance after forming is improved. The reason for this is not clear but is presumed to be that the inhomogeneous deformation on the surface is suppressed due to an interaction between the existence form of the secondary phase and the crystal orientation distribution of ferrite.


When XODF{001}/{111} is less than 0.70, inhomogeneous deformation caused by an orientation distribution and a difference in intensity of each crystal of the material (steel sheet) is likely to occur, and deformation concentration on the orientation in the vicinity of {001} in ferrite is significant. On the other hand, it is presumed that, when XODF{001}/{111} is more than 2.50, inhomogeneous deformation caused by an orientation distribution and a difference in intensity of each the crystal of the material (steel sheet) is likely to occur, inhomogeneous deformation is likely to occur in a boundary between ferrite and the secondary phase and a boundary between crystal grains in an orientation in the vicinity of {111} and crystal grains in another orientation in ferrite, and surface unevenness is likely to occur.


In addition, it is more preferable that a difference between XODF{001}/{111} of ferrite in the surface layer region and XODF{001}/{111} of ferrite in the internal region is −0.30 to 0.40 because inhomogeneous deformation in the ferrite in the sheet thickness direction is suppressed, and contributes to the improvement of strain hardening property of the material.


Whether or not the texture in which the intensity ratio XODF{001}/{111} is 0.70 to 2.50 is included in ferrite of the surface layer region can be determined in the following manner using EBSD (Electron Backscattering Diffraction) method.


Regarding a sample provided for EBSD method, the steel sheet is polished by mechanical grinding, strain is removed by chemical polishing or electrolytic polishing, the sample is prepared such that the cross section in the sheet thickness direction including the range from the surface to the ¼ thickness position is a measurement surface, and the texture is measured. Regarding a sample collection position in the sheet width direction, the sample is collected in the vicinity of a sheet width position of W/4 or 3W/4 (position at a distance of ¼ from an end surface of the steel sheet in the sheet width direction).


In the region of the sample ranging from the surface of the steel sheet to 20 μm from the surface in the sheet thickness direction, a crystal orientation distribution is measured by EBSD method at a pitch of 0.5 μm or less. Ferrite is extracted using an IQ (Image Quality) map that is analyzable by EBSP-OIM (registered trade name, Electron Backscatter Diffraction Pattern-Orientation Image Microscopy). Ferrite has a characteristic in that the IQ value is high, and thus can be simply classified from other metallographic structures using this method. A threshold of the IQ value is set such that the area fraction of ferrite that is calculated by the observation of the microstructure obtained by the LePera corrosion matches the area fraction of ferrite calculated based on the IQ value.


In a cross section of ϕ2=45° in a three-dimensional texture (ODF: Orientation Distribution Functions) calculated using crystal orientations of the extracted ferrite, a ratio of a maximum value of X-ray random intensity ratios of a {001} orientation group to a maximum value of X-ray random intensity ratios of a {111} orientation group (γ-fiber) (the maximum value of X-ray random intensity ratios of {001} orientation group/the maximum value of X-ray random intensity ratios of {111} orientation group (γ-fiber)) is obtained as XODF{001}/{111}. The X-ray random intensity ratio is a numerical value obtained by measuring a diffraction intensity of a standard sample having no pile-up in a specific orientation and a diffraction intensity of a sample material by X-ray diffraction under the same conditions and dividing the obtained diffraction intensity of the sample material by the diffraction intensity of the standard sample. For example, in a case where the steel sheet is rolled at a high rolling reduction of 70% or higher and annealed, the texture is developed, and the X-ray random intensity of the {111} orientation group (γ-fiber) increases.


Here, {hkl} represents that, when a sample is collected using the above-described method, the normal direction of a sheet surface is parallel to <hkl>. Regarding the crystal orientation, typically, an orientation orthogonal to a sheet surface is represented by (hkl) or {hkl}. {hkl} is a generic term for equivalent planes, and (hkl) represents each, of crystal planes. That is, in the embodiment, a body-centered cubic structure (bee structure) is targeted. Therefore, for example, the respective planes (111), (−111), (1−11), (11−1), (−1−11), (−11−1), (1−1−1), and (−1−1−1) are equivalent and cannot be distinguished from each other. In this case, these orientations are collectively referred to as “{111} orientation group”. The ODF representation is used for representing other orientations of a crystal structure having low symmetry. Therefore, in the ODF representation, each of orientations is generally represented by (hkl)[uvw]. However, in the embodiment, attention is paid to the normal direction orientation {hkl} from which the finding that the normal direction orientation of a sheet surface has a large effect on the development of unevenness was obtained. {hkl} and (hkl) have the same definition.


In a case where the product is a steel sheet including, a plating layer, the surface of the steel sheet excluding the plating layer is defined as an origin of the surface layer region.


<Regarding Metallographic Structure in Internal Region>


In the steel sheet according to the embodiment, it is necessary that, in a state where the metallographic structure in the surface layer region is controlled as described above, a metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position (in a case where the sheet thickness is represented by t: t/4) from the surface in the sheet thickness direction is also controlled.


[Consisting of Ferrite and Secondary phase having Volume Fraction of 5.0% to 25.0%]


When the volume fraction of the secondary phase in the internal region is less than 5.0%, the strength of the steel sheet cannot be sufficiently improved. Therefore, the volume fraction of the secondary phase is set to be 5.0% or more.


On the other hand, when the volume fraction of the secondary phase is more than 25.0%, the volume fraction of ferrite decreases excessively, and the workability such as elongation or hole expansibility of the steel sheet deteriorates. Therefore, the volume fraction of the secondary phase is set to be 25.0% or less.


[Average Grain Size of Secondary Phase being 1.0 μm to 5.0 μm and being more than Average Grain Size of Secondary Phase in Surface Layer Structure]


When the average grain size of the secondary phase in the internal region is 1.0 μm to 5.0 μm and is more than the average grain size of the secondary phase in the surface layer structure, the average grain size of the secondary phase in the surface layer region is less than that in the internal region and inhomogeneous deformation in the surface layer structure is suppressed, which is preferable.


Therefore, the average grain size in the internal region may be controlled to be in the above-described range.


The volume fraction and the average grain size of the secondary phase in the internal, region can be obtained by using a steel sheet etched with the LePera reagent, selecting a range from a position of more than 20 μm from the surface of the sample in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction, and analyzing the range with the same method as that of the surface layer region.


In addition, a texture of ferrite in the internal region can be obtained by selecting a range from a position of more than 20 μm from the surface of the sample in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction using the above-described EBSD method and analyzing the range with the same method as that of the surface layer region.


When the sheet thickness of the product is more than 0.40 mm, it is preferable that the internal, region is a range from a position of more than 20 μm from the surface in the sheet thickness direction to a position of 100 μm from the surface in the sheet thickness direction.


<Regarding Expression (1), Tensile Strength, Sheet Thickness, and Sheet Width>


In the steel sheet according to the embodiment, it is preferable that the chemical composition satisfies the following Expression (1), the tensile strength is 550 MPa or higher, the sheet thickness is 0.10 mm to 0.55 mm, and the sheet width is 1300 mm to 2000 mm. By satisfying all the conditions, a steel sheet having excellent surface appearance quality over the entire width in the sheet width direction can be obtained.

7.00≥[C]×20+[Si]×3.0+[Mn]×2.0+[Al]×2.0+[Cr]+[Ti]×20+[Nb]×40+[Mo]×2.0+[Ni]×2.0+[B]×200  (1)


In the expression, a symbol of each of the elements in Expression (1) represents the amount of the element by mass %, and when the element is not included, 0 is substituted into the symbol.


The tensile strength can be obtained using a method described in “JIS Z 2241:2011, Metallic materials—Tensile testing”. As a test piece, a JIS No. 5 test piece cut from the direction orthogonal to the rolling direction is used.


In addition, the sheet thickness of the steel sheet can be obtained by sampling a sheet from an end portion of a steel sheet coil in a longitudinal direction, collecting a sample for sheet thickness measurement from a position of 300 mm from the end portion in the sheet width direction, and measuring the thickness of the sample using a micrometer. Regarding the sheet width, positions of both end portions of the steel sheet in the width direction are detected by image analysis using two cameras on an outlet side of a cold rolling line, and the sheet width is calculated from a relationship between the positions of the two ends. The sheet width can be obtained by performing the measurement on the line as needed and averaging sheet width data corresponding to one coil. In a case where it is difficult, to obtain the sheet width using this method, the sheet width may be measured using a caliper after sampling a sheet from an end portion of the steel sheet coil in the longitudinal direction.


<Regarding Plating Layer>


The steel sheet according to the embodiment may include a plating layer on the surface (on the surface of the steel sheet). By including the plating layer on the surface, corrosion resistance is improved, which is preferable.


A plating to be applied is not particularly limited, and examples thereof include hot-dip galvanizing, hot-dip galvannealing, electrogalvanizing, Zn—Ni plating (electrogalvanizing), Sn plating, Al—Si plating, electrogalvannealing, hot-dip zinc-aluminum alloy plating, hot-dip zinc-aluminum-magnesium alloy plating, hot-dip zinc-aluminum-magnesium alloy-Si plating, and zinc-Al alloy deposition.


<Regarding Manufacturing Method>


Next, a preferable method for manufacturing the steel sheet according to the embodiment will be described. The effects can be obtained as long as the steel sheet according to the embodiment, has the above-described characteristics irrespective of the manufacturing method. However, with the following method, the steel sheet can be stably manufactured, which is preferable.


Specifically, the steel sheet according to the embodiment can be manufactured with a manufacturing method including the following processes (i) to (vi).


(i) A heating process of heating a slab having the above-described chemical composition at 1000° C. or higher.


(ii) A hot-rolling process of hot-rolling the slab such that a rolling finishing temperature is 950° C. or lower to obtain a hot-rolled steel sheet.


(iii) A stress application process of applying a stress to the hot-rolled steel sheet after the hot-rolling process such that an absolute value of a residual stress σS on a surface is 165 MPa to 400 MPa.


(iv) A cold-rolling process of cold-rolling the hot-rolled steel sheet after the stress application process such that a cumulative rolling reduction RCR is 70% to 90% to obtain a cold-rolled steel sheet.


(v) An annealing process of heating the cold-rolled steel sheet such that the average heating rate in a range from 300° C. to a soaking temperature T1° C. that satisfies the following Expression (2) is 1.5° C./sec to 10.0° C./sec and holding the heated steel sheet at the soaking temperature T1° C. for 30 seconds to 150 seconds for annealing.

1275−27×ln(σS)−4.5×RCR≤T1≤1275−25×ln(σS)−4×RCR  (2)


(vi) a cooling process of cooling the cold-rolled steel sheet after the annealing process to a temperature range of 550° C. to 650° C. such that the average cooling rate in a range from the soaking temperature T1° C. to 650° C. is 1.0° C./sec to 10.0° C./sec: and further cooling the cooled steel sheet to a temperature range of 200° C. to 490° C. such that the average cooling rate is 5.0° C./sec to 500.0° C./sec.


In addition, in a case where ductility is improved by tempering martensite such that a cold-rolled steel sheet or a plated steel sheet having higher formability is obtained, the manufacturing method may further include the following process.


(vii) A holding process of holding the cold-rolled steel sheet after the cooling process in a temperature range of 200° C. to 490° C. for 30 seconds to 600 seconds.


Hereinafter, the each process will be described.


[Heating Process]


In the heating process, a slab having the predetermined chemical composition is heated to 1000° C. or higher before rolling. When the heating temperature is lower than 1000° C., a rolling reaction force during hot rolling increases, sufficient hot rolling cannot be performed, and there may be a case where the desired thickness of the product cannot be obtained. Alternatively, there may a case where the steel sheet cannot be coiled due to deterioration in the sheet shape.


It is not necessary to limit the upper limit of the heating temperature, and it is not preferable that the heating temperature is excessively high from the viewpoint of economy. Due to this reason, it is preferable that the slab heating temperature is lower than 1300° C. In addition, the slab provided for the heating process is not limited. For example, a slab that is manufactured using a continuous casting method after preparing molten steel having the above-descried chemical composition using an converter or an electric furnace can be used. For example, an ingot-making method or a thin slab casting method may be adopted instead of the continuous casting method.


[Hot-Rolling Process]


In the hot-rolling process, the slab heated to 1000° C. or higher in the heating process is hot-rolled and coiled to obtain a hot-rolled steel sheet.


When the rolling finishing temperature is higher than 950° C., the average grain size of the hot-rolled steel sheet excessively increases. In this case, the average grain size of the final product sheet increases, and an increase in average grain size causes a decrease in yield strength and deterioration in the surface appearance quality after forming, which is not preferable. Therefore, the rolling finishing temperature is preferably 950° C. or lower.


In addition, the finish rolling start temperature is preferably 850° C. or lower.


When a temperature change (finish rolling finishing temperature—finish rolling start temperature) in the hot-rolling process is +5° C. or higher, recrystallization is promoted by deformation heating in the hot-rolling process, and crystal grains are refined, which is preferable.


In addition, in order to refine crystal grains, the coiling temperature in the coiling process is preferably 750° C. or lower and more preferably 650° C. or lower. In addition, from the viewpoint of reducing the strength of the steel sheet provided for cold rolling, the coiling temperature is preferably 450° C. or higher and more preferably 500° C. or higher.


[Stress Application Process]


In the stress application process, a stress is applied to the hot-rolled steel sheet after the hot-rolling process such that an absolute value of a residual stress σS on a surface is 165 MPa to 400 MPa. For example, a stress can be applied by grinding the hot-rolled steel sheet using a surface grinding brush after hot rolling or pickling. At that time, while changing a contact pressure of the grinding brush on the steel sheet surface, a surface layer residual stress is measured on-line using a portable X-ray residual stress analyzer and may be controlled to be in the above-described range. By performing cold rolling, annealing, and cooling in a state where the residual stress is applied to the surface to be in the above-described range, a steel sheet including ferrite having a desired texture and having a desired hard phase distribution can be obtained.


When the residual stress σs is lower than 165 MPa or higher than 400 MPa, the desired texture of ferrite cannot be obtained after cold rolling, annealing, and cooling to be performed after the stress application. In addition, in a case where the residual stress is applied after cold rolling instead of after hot rolling, the residual stress is widely distributed in the sheet thickness direction. Therefore, the desired hard phase distribution and the texture cannot be obtained only on the surface layer of the material.


A method for applying the residual stress to the surface of the hot-rolled steel sheet is not limited to the above-described grinding brush. For example, a method for performing shot blasting may also be used. In the case of shot blasting, fine unevenness may occur on the surface due, to collision with shot media, or shot media may be trapped and cause defects during the next cold rolling or the like. Therefore, the method for applying the stress by grinding using a brush is preferable.


In addition, during rolling using a roll such as a skin pass, a stress is applied to the entire steel sheet in the thickness direction and the desired hard phase distribution and the texture cannot be obtained only on the surface layer of the material.


It is preferable that the stress application process is performed at a steel sheet temperature of 40° C. to 500° C. By performing the stress application process in this temperature range, the residual stress can be efficiently applied to the range corresponding to the surface layer region, and the cracking caused by the residual stress of the hot-rolled steel sheet can be suppressed.


[Cold-Rolling Process]


In the cold-rolling process, the hot-rolled steel sheet is cold-rolled, such that a cumulative rolling reduction RCR is 70% to 90% to obtain a cold-rolled steel sheet. By cold-rolling the hot-rolled steel sheet to which the predetermined residual stress is applied at the above-described cumulative rolling reduction, ferrite having the desired texture can be obtained after annealing and cooling.


When the cumulative rolling reduction RCR is less than 70%, the texture of the cold-rolled steel sheet is not sufficiently developed. Therefore, the desired texture cannot be obtained after annealing. In addition, when the cumulative rolling reduction RCR is more than 90%, the texture of the cold-rolled steel sheet is excessively developed. Therefore, the desired texture cannot be obtained after annealing. In addition, the rolling force increases, and the homogeneity of the material in the sheet width direction deteriorates. Further, the production stability also deteriorates. Therefore, the cumulative rolling reduction RCR during cold rolling is set to be 70% to 90%.


[Annealing Process]


In the annealing process, the cold-rolled steel sheet is heated to the soaking temperature at the average heating rate corresponding to the residual stress applied in the stress application process and the cumulative rolling reduction RCR in the cold-rolling process, and is held at the soaking temperature corresponding to the residual stress applied in the stress application process and the cumulative rolling reduction RCR in the cold-rolling process.


Specifically, in the annealing process, the cold-rolled steel sheet is heated such that the average heating rate in a range from 300° C. to a soaking temperature T1° C. that satisfies the following Expression (2) is 1.5° C./sec to 10.0° C./sec and holding the heated steel sheet at the soaking temperature T1° C. for 30 seconds to 150 seconds for annealing.

1275−25×ln(σS)−4.5×RCR≤T1≤1275−25×ln(σS)−4×RCR  (2).


When the average heating rate is slower than 1.5° C./sec, a long period of time is required for heating, and the productivity deteriorates, which is not preferable. In addition, when the average heating rate is faster than 10.0° C./sec, the uniformity of the temperature in the sheet width direction deteriorates, which is not preferable.


In addition, when the soaking temperature T1 is lower than 1275−25×ln(σS)−4.5×RCR, although recrystallization of ferrite and reversible transformation from ferrite to austenite do not sufficiently progress, and the desired texture cannot be obtained. In addition, inhomogeneous deformation during forming, is promoted due to a difference in strength between non-recrystallized crystal grains and recrystallized crystal grains, which is not preferable. In addition, when the soaking temperature T1 is higher than 1275−25×ln(σS)−4×RCR, although recrystallization of ferrite and reversible transformation from ferrite to austenite sufficiently progresses, crystal grains are coarsened, and the desired texture cannot be obtained, which is not preferable.


The average heating rate can be obtained from (Heating End Temperature−Heating Start Temperature)/(Heating Time).


[Cooling Process]


In the cooling process, the cold-rolled steel sheet after soaking in the annealing process is cooled. During cooling, the cold-rolled steel sheet is cooled to a temperature range of 550° C. to 650° C. such that the average cooling rate in a range from the soaking temperature T1° C. to 650° C. is 1.0° C./sec to 10.0° C./sec and is cooled to a temperature range of 200° C. to 490° C. such that the average cooling rate is 5.0° C./sec to 500.0° C./sec.


When the average cooling rate in a range from T1° C. to 650° C. is slower than 1.0° C./sec, ferritic transformation is excessively promoted, and the desired volume fraction of the secondary phase cannot be obtained. On the other hand, when the average cooling rate in a range from T1° C. to 650° C. is faster than 10.0° C./sec, ferritic transformation do not sufficiently progress, and concentration of carbon on austenite does not sufficiently progress. Therefore, the desired volume fraction of the secondary phase cannot be obtained.


In addition, when the average cooling rate from this temperature range to a temperature range of 200° C. to 490° C. after cooling is performed in a temperature range of 550° C. to 650° C. is slower than 5.0° C./sec, ferritic transformation is excessively promoted. Therefore, the desired volume fraction of the secondary phase cannot be obtained. On the other hand, it is difficult to set the average cooling rate to be faster than 500.0° C./sec due to the facility restriction. Therefore, the upper limit is set to be 500.0° C./sec.


The average cooling rate can be obtained from (Cooling Start Temperature−Cooling End Temperature)/(Cooling Time).


[Holding Process]


The cold-rolled steel sheet that is cooled to 200° C. to 490° C. may be held in the temperature range of 200° C. to 490° C. for 30 to 600 seconds.


By holding the cold-rolled steel sheet in the temperature range for the predetermined time, the effect of improving ductility through tempering martensite can be obtained, which is preferable.


The cold-rolled steel sheet that is cooled to 200° C. to 490° C. or the cold-rolled steel sheet after the holding process may be cooled to room temperature at 10° C./sec or faster.


A plating process of forming a plating layer on the surface may be further performed on the cold-rolled steel sheet obtained using the above-described method. Examples of the plating process include the following process.


[Electroplating Process]


[Galvannealing Process]


The cold-rolled steel sheet after the cooling process or the holding process may be electroplated to form an electroplating layer on the surface. An electroplating method is not particularly limited. The hot-dip galvanizing method may be determined depending on required characteristics (for example, corrosion resistance or adhesion).


In addition, after electroplating, the cold-rolled steel sheet may be heated to alloy plating metal.


[Hot-Dip Galvanizing Process]


[Galvannealing Process]


The cold-rolled steel sheet after the cooling process or the holding process may be hot-dip galvanized to form a hot-dip galvanized layer on the surface. A hot-dip galvanizing method is not particularly limited. The hot-dip galvanizing method may be determined depending on required characteristics (for example, corrosion resistance or adhesion).


In addition, the cold-rolled steel sheet after hot-dip galvanizing may be heat-treated to alloy a plating layer. In a case where alloying is performed, it is preferable that the cold-rolled steel sheet is heat-treated in a temperature range of 400° C. to 600° C. for 3 to 60 seconds.


With the above-described manufacturing method, the steel sheet according to the embodiment can be obtained.


EXAMPLES

Next, examples of the present invention will be described. However, conditions of the examples are merely exemplary to confirm the operability and the effects of the present invention, and the present invention is not limited to these condition examples. The present invention can adopt various conditions within a range not departing from the scope of the present invention as long as the object of the present invention can be achieved under the conditions.


Steels having chemical compositions shown in “Steel A to AB” of Table 1 were melted, and slabs having a thickness of 240 to 300 mm were manufactured by continuous casting. The obtained slabs were heated at a temperature shown in Tables 2A and 2B. The heated slabs were hot-rolled under conditions shown in Tables 2A and 2B and were coiled.


Next, the coil was uncoiled and a stress was applied to the hot-rolled steel sheet. At this time, while measuring the surface layer residual stress on-line using a portable X-ray residual stress analyzer at a working temperature (steel sheet temperature) shown in Tables 2A and 2B, a contact pressure of a grinding brush on the steel sheet surface was changed such that the residual stress was as shown in Tables 2A and 2B. Next, by performing cold rolling at a cumulative rolling reduction RCR shown in Tables 2A and 2B, steel sheets A1 to AB1 were obtained.


“Temperature Change in Hot-Rolling Process” in Tables 2A and 2B shows a temperature change (finish rolling finishing temperature—finish rolling start temperature) in the hot-rolling process.


Next, by performing annealing and cooling under conditions shown in Tables 3A to 3C, some steel sheets were held at 200° C. to 490° C. for 30 to 600 seconds. After cooling or holding, the steel sheets were air-cooled to room temperature. Next, some steel sheets were plated in various ways to form a plating layer on the surface. In Tables 3A to 3C, CR represents that no plating was performed, GI represents that hot-dip galvanizing was performed, GA represents that hot-dip galvannealing was performed, EG represents that electroplating was performed, EGA represents that electrogalvannealing was performed, and Zn—Al Mg, Al—Si, or the like represents that plating including these elements was performed. In addition, in Tables 3A to 3C, phosphate coating EG represents that phosphate coating electrogalvanizing was performed, and lubricant GA represents lubricant hot-dip galvannealing.


Regarding each of the product sheets No. A1a to AB1a, the observation of the metallographic structures in the surface layer region and the internal region and the measurement of XODF{001}/{111}, the sheet thickness, the sheet width, and the tensile strength were performed using the above-described method. The results are shown in Tables 4A to 4C.


“ΔXODF{001}/{111} Surface Layer Region-Internal Region” in Tables 4A to 4C represents a difference between XODF{001}/{111} of ferrite in the surface layer region and XODF{001}/{111} of ferrite in the internal region.


[Evaluation of Surface Properties of Steel Sheet]


In addition, regarding each of the manufactured product sheets, the surface properties of the steel sheet were evaluated.


Specifically, the surface of the manufactured steel sheet was observed by visual inspection to evaluate the surface properties. The evaluation criteria of the surface properties of the steel sheet were as follows. In a case where the surface properties of the steel sheet were evaluated as C or D, the steel sheet was not able to be used as an exterior material, or a component and was determined to be unacceptable.


A: no pattern was formed (more desirably, can be used as an exterior material)


B: an acceptable small pattern was formed (can be used as an exterior material)


C: an unacceptable pattern was formed (can be used as a component but cannot be used as an exterior material)


D: a significant pattern defect was formed (cannot be used as a component)


The results are shown in Tables 4A to 4C.


[Forming Test of Steel Sheet]


A forming test was not performed on the material (Product Sheets No. S2a, No. X1a to No. Z1a) for which the surface properties of the steel sheet were evaluated as C or D, and the forming test was performed only on the material for which the surface properties of the steel sheet was evaluated as A or


Regarding forming, plastic strain of 10% in the rolling width direction was applied to the steel sheet of which the surface properties was measured in a cylinder drawing forming test with the Marciniak method using a deep drawing tester, a cylindrical punch of ϕ50 mm, and a cylindrical die of ϕ54 mm.


A test piece of 100 mm in the rolling width direction×50 mm in the rolling direction was prepared from a portion deformed by forming, and an arithmetic mean height Pa of a profile curve defined by JIS B0601 (2001) was measured in the direction orthogonal to the rolling direction according to JIS B0633 (2001). The evaluation was performed in the portion deformed by forming, and the evaluation length was 30 mm.


A test piece of 100 mm in the rolling width direction×50 mm in the rolling direction was prepared from a flat portion of the formed article, and an arithmetic mean height Pa of a profile curve defined by JIS B0601 (2001) was measured in the direction orthogonal to the rolling direction according to JIS B0633 (2001). The evaluation length was 30 mm.


The amount ΔPa of increase in roughness (ΔPa=Pa of Formed Article−Pa of Steel Sheet) was calculated using Pa of the formed article and Pa of the steel sheet obtained in the above-described measurement test.


The surface properties of the steel sheet after forming were evaluated based on the ΔPa. The evaluation criteria were as follows. In a case where the surface of the steel sheet after forming was evaluated as C or D, the steel sheet was not able to be used as an exterior material or a component and was determined to be unacceptable.


A: ΔPa≤0.25 μm (more desirably, can be used as an exterior material)


B: 0.25 μm<ΔPa≤0.35 μm (can be used as an exterior material)


C: 0.35 μm<ΔPa≤0.55 μm (can be used as a component but cannot be used as an exterior material)


D: 0.55 μm<ΔPa (cannot be used as a component)


As shown in Tables 1 to 4C, in the examples (Examples) where the chemical composition, the metallographic structure in the surface layer region, the metallographic structure in the internal region, and XODF{001}/{111} were in the preferable ranges, the evaluation of the surface properties and the evaluation of the surface properties after forming were A or B, and the formation of the surface unevenness was suppressed in the stage of the steel sheet and after working. On the other hand, in the examples (comparative examples) where one or more of the chemical composition, the metallographic structure in the surface layer region, the metallographic structure in the internal region, and XODF{001}/{111} were outside of the ranges according to the present invention, a pattern was formed or unevenness occurred in the stage of the steel sheet or after forming such that the material was not able to be used as an exterior material or a component.


The FIGURE is a diagram showing a relationship between surface properties after forming and a texture parameter obtained in Examples. Referring to the FIGURE, it can be seen that the surface properties after forming were excellent in the examples where the texture parameter was in the range of the present invention (the ratio XODF{001}/{111} of the intensity of {001} orientation to the intensity of {111} orientation in ferrite was 0.70 to 2.50). In the FIGURE, there are points where XODF{001}/{111} is 0.70 to 2.50 but ΔPa is more than 0.35 μm, and the points correspond to Comparative Examples where the fraction of the secondary phase in the surface layer is outside of the range of the present invention










TABLE 1






Chemical Composition mass % (Remainder: Fe + Impurities)























SlabNo.
C
Mn
Al
Si
Al + Si
P
S
N





A
0.070
1.85
0.03
0.45
0.48
0.015
0.005
0.0050


B
0.085
1.80
0.03
0.54
0.57
0.015
0.005
0.0030


C
0.070
2.05
0.04
0.45
0.49
0.010
0.006
0.0030


D
0.078
1.50
0.01
0.50
0.51
0.025
0.007
0.0030


E
0.089
1.95
0.05
0.40
0.45
0.010
0.004
0.0050


F
0.075
1.90
0.01
0.45
0.46
0.020
0.010
0.0040


G
0.079
2.00
0.02
0.15
0.17
0.010
0.002
0.0030


H
0.073
1.73
0.40
0.01
0.41
0.020
0.004
0.0030


I
0.080
1.49
0.30
0.01
0.31
0.010
0.003
0.0030


J
0.095
1.50
0.15
0.01
0.16
0.015
0.005
0.0030


K
0.050
2.10
0.14
0.01
0.15
0.010
0.006
0.0035


L
0.110
1.73
0.15
0.02
0.17
0.020
0.008
0.0020


M
0.050
1.69
0.15
0.05
0.20
0.030
0.006
0.0030


N
0.142
1.15
0.20
0.01
0.21
0.030
0.007
0.0040


O
0.075
2.25
0.03
0.28
0.31
0.020
0.001
0.0040


P
0.095
1.72
0.30
0.01
0.31
0.020
0.003
0.0033


Q
0.090
0.70
0.30
0.20
0.50
0.020
0.004
0.0030


R
0.060
1.20
0.03
0.01
0.04
0.010
0.003
0.0030



S


0.150

1.50
0.40
0.02
0.42
0.020
0.004
0.0030



T


0.007


0.10

0.05
0.01
0.06
0.015
0.005
0.0035



U


0.016

1.80
0.03
0.02
0.05
0.030
0.005
0.0015



V

0.050
2.00

0.70

0.10

0.80

0.008
0.005
0.0030



W

0.095

0.50

0.03
0.45
0.48
0.010
0.006
0.0030



X

0.055
1.68
0.06
0.10
0.16
0.030
0.008
0.0026



Y

0.090
1.70
0.05
0.58

0.63

0.020
0.006
0.0020



Z

0.070

2.34

0.40
0.10
0.50
0.020
0.006
0.0025



AA

0.082
1.51
0.35
0.08
0.43
0.030
0.004
0.0033


AB
0.075
2.25
0.03
0.32
0.31
0.020
0.001
0.0040













Expression



Slab No.
Mo
B
Ti
Nb
Cr
Ni
(1)






A
0.00
0.0000
0.000
0.000
0.001
0.000
6.51



B
0.00
0.0000
0.000
0.000
0.001
0.000
6.98



C
0.00
0.0000
0.000
0.000
0.001
0.000
6.93



D
0.00
0.0015
0.020
0.000
0.000
0.000
6.78



E
0.00
0.0000
0.000
0.000
0.005
0.001
6.99



F
0.00
0.0000
0.000
0.000
0.300
0.000
6.97



G
0.00
0.0000
0.015
0.015
0.000
0.001
6.96



H
0.00
0.0015
0.000
0.000
0.000
0.000
6.05



I
0.00
0.0016
0.010
0.000
0.000
0.000
5.73



J
0.00
0.0011
0.000
0.000
0.000
0.000
5.46



K
0.00
0.0002
0.000
0.000
0.350
0.010
5.92



L
0.00
0.0012
0.000
0.000
0.500
0.000
6.76



M
0.30
0.0010
0.000
0.000
0.500
0.005
6.14



N
0.00
0.0011
0.000
0.025
0.150
0.000
6.94



O
0.00
0.0000
0.000
0.000
0.010
0.006
6.92



P
0.00
0.0015
0.012
0.000
0.000
0.007
6.52



Q
0.80
0.0015
0.010
0.000
0.000
0.000
6.50



R
0.20
0.0016
0.020
0.000
0.700
0.000
5.51




S

0.00
0.0015
0.000
0.000
0.000
0.000
7.16




T

0.00
0.0008
0.050
0.005
0.000
0.000
1.82




U

0.01
0.0027
0.000
0.000
0.050
0.000
4.65




V

0.00
0.0000
0.010
0.000
0.500
0.000
7.40




W

0.50
0.0000
0.020
0.000
0.100
0.000
5.81




X

0.01

0.0051

0.000
0.000
0.100
0.000
6.02




Y

0.00
0.0000
0.000
0.000
0.000
0.000
7.04




Z

0.00
0.0000
0.010
0.000
0.000
0.000
7.38




AA

0.10
0.0000
0.000
0.000

1.050

0.000
6.85



AB
0.00
0.0000
0.000
0.000
0.000
0.000
7.02





The underline represents that the value is outside of the range of the present invention.
















TABLE 2A










Finish Rolling














Heating


Temperature




Process
Rolling
Rolling
Change in



Steel
Heating
Start
Finishing
Hot-Rolling


Steel
Sheet
Temperature
Temperature
Temperature
Process


No.
No.
° C.
° C.
° C.
° C.





A
A1
1200
950
890
−60


A
A2
1300
1080
950
−130


A

A3

1200
950
905
−45


A

A4

1250
1040
930
−110


B
B1
1200
920
890
−30


B
B2
1200
930
880
−50


B
B3
1200
930
880
−50


B

B4

1100
835
870
35


B
B5
1100
835
870
35


B
B6
1230
970
890
−80


C
C1
1050
800
850
50


C
C2
1280
1050
930
−120


C

C3

1050
800
850
50


C
C4
1100
850
885
35


C

C5

1200
850
890
40


D
D1
1200
850
890
40


D
D2
1200
950
900
−50


D

D3

1200
950
900
−50


E
E1
1200
925
895
−30


E
E2
1250
960
910
−50


E
E3
1200
925
870
−55


F
F1
1200
950
890
−60


F

F2

1200
950
890
−60


G
G1
1100
850
885
35


G

G2

1100
850
870
20


G

G3

1010
790
860
70


H
H1
1050
800
850
50


H

H2

1200
910
880
−30


H

H3

1200
1000
930
−70


I
I1
1250
950
895
−55


I

I2

1200
920
880
−40


I

I3

1280
1080

970

−110


















Cold-Rolling






Process














Coiling


Cumulative













Process
Stress Application Process
Rolling













Steel
Coiling
Residual
Steel Sheet
Reduction


Steel
Sheet
Temperature
Stress σs
Temperature
RCR


No.
No.
° C.
MPa
° C.
%





A
A1
700
174
112
78


A
A2
600
220
 40
85


A

A3

650
182
112

92



A

A4

650
35
 30
85


B
B1
550
165
 30
78


B
B2
500
198
142
75


B
B3
500
212
 43
75


B

B4

600
88
 30
78


B
B5
600
166
 40
90


B
B6
620
23
140
80


C
C1
650
186
 30
85


C
C2
550
278
103
80


C

C3

600

411

 20
90


C
C4
700
168
 40
75


C

C5

680
20
 *1
78


D
D1
680
166
 30
88


D
D2
680
168
 30
85


D

D3

680
302
 30

92



E
E1
520
177
120
85


E
E2
760
190
 50
85


E
E3
480
198
 40
87


F
F1
750
170
 27
80


F

F2

750

414

 25
87


G
G1
480
185
 30
85


G

G2

650
56
510
87


G

G3

550
58
510

68



H
H1
650
174
 45
71


H

H2

700
170
 45

65



H

H3

650
25
 40
80


I
I1
550
172
 30
82


I

I2

650
38
 *1
80


I

I3

550
166
 45
82





The underline represents that the value is outside of the range of the present invention.


*1 represents that the stress application process was not performed.


*2 represents that slab cracking occurred.
















TABLE 2B









Heating
Finish Rolling














Process
Rolling
Rolling
Temperature



Steel
Heating
Start
Finishing
Change in Hot-


Steel
Sheet
Temperature
Temperature
Temperature
Rolling Process


No.
No.
° C.
° C.
° C.
° C.





J
J1
1200
930
880
−50


J
J2
1200
910
890
−20


J
J3
1200
930
880
−50


J
J4
1100
850
880
30


K
K1
1200
810
850
40


K
K2
1200
845
870
35


K

K3

1100
850
885
35


K
K4
1100
850
885
35


L

L1

1200
930
870
−60


L
L2
1200
820
845
25


M
M1
1100
850
885
35


M

M2

1100
850
885
35


N

N1

1200
930
890
−40


N
N2
1200
930
890
−40


O
O1
1200
930
890
−40


P
P1
1200
820
845
25


P

P2

1200
820
845
25


Q
Q1
1250
850
880
30


R
R1
1200
910
890
−20


R

R2

1200
910
890
−20



S


S1

1100
860
887
27



S


S2

1250
850
880
30



T


T1

1200
910
890
−20



U


U1

1200
920
890
−30



U


U2

1200
925
895
−30



V


V1

1050
800
850
50



W


W1

1050
800
850
50



X


X1

1100
860
865
5



Y


Y1

1100
850
885
35



Z


Z1

1100
850
870
20



AA


AA1

1010
790
860
70


AB
AB1
1200
880
890
10


















Cold-Rolling




Coiling
Stress Application Process
Process














Process

Steel
Cumulative



Steel
Coiling
Residual
Sheet
Rolling


Steel
Sheet
Temperature
Stress σs
Temperature
Reduction RCR


No.
No.
° C.
MPa
° C.
%





J
J1
600
169
100
83


J
J2
600
192
 30
77


J
J3
600
213
 25
83


J
J4
550
178
 50
83


K
K1
680
241
 20
88


K
K2
580
220
 35
88


K

K3

530

409

 20
85


K
K4
480
167
100
82


L

L1

650
178
 50

66



L
L2
700
250
 25
81


M
M1
700
184
110
87


M

M2

700
192
110

92



N

N1

560

141

300
82


N
N2
560
323
 30
87


O
O1
560
263
 45
72


P
P1
700
180
 40
80


P
P2
700
12
 *1
78


Q
Q1
580
174
 40
75


R
R1
600
211
 30
75


R

R2

700

441

 10

*2




S


S1

640
172
 55
80



S


S2

580
177
300

94




T


T1

760
169
 50

91




U


U1

600
168
 30
77



U


U2

520
72
208
71



V


V1

600
177
 50
70



W


W1

600
192
 30
72



X


X1

670
352
 40
88



Y


Y1

480
170
 30
77



Z


Z1

650
198
 25
80



AA


AA1

550
240
 70
85


AB
AB1
560
213
 41
72





The underline represents that the value is outside of the range of the present invention.


*1 represents that the stress application process was not performed.


*2 represents that slab cracking occurred.
















TABLE 3A










Cooling





Process





















Average









Cooling












Annealing Process
Rate in

















Left
Right


Temperature




Average
Side
Side
Soaking

Range of


Steel
Product
Heating
in
in
Temperature
Holding
T1° C. to


Sheet
Sheet
Rate
Expression
Expression
T1
Time
650° C.


No.
No
° C./s
(2)
(2)
° C.
sec
º C./s





A1
A1a
8.3
795
834
800
 40
9.2


A2
A2a
3.8
758
800
780
 70
6.7



A3


A3a

5.4
731
777
770
 50
9.5



A4


A4a

4.2
804
846
820
 70
3.3


B1
B1a
2.5
796
835
800
110
4.3


B2
B2a
2.1
805
843
810
150
3.3


B3
B3a
3.4
804
841
810
 80
6.0



B4


B4a

6.6
812
851

810

 50
7.6


B5
B5a
6.6
742
787
770
 50
7.6


B5
B5b
6.6
742
787
770
 50
7.6


B6
B6a
2.3
837
877
850
 90
4.6


C1
C1a
2.2
762
804
780
120
3.9


C2
C2a
2.2
774
814
780
120
3.9



C3


C3a

2.2
720
765
760
120
3.9


C4
C4a
4.4
809
847
820
 60
7.7



C5


C5a

2.5
849
888
860
110
4.3


D1
D1a
4.7
751
795
780
 60
7.7


D2
D2a
3.4
764
807
780
 80
6.0



D3


D3a

4.4
718
764
755
 60
7.7


E1
E1a
3.7
763
806
790
 90
4.3


E2
E2a
3.7
761
804
780
 90
4.3


E3
E3a
2.7
751
795
780
120
3.1


F1
F1a
5.8
787
827
800
 60
6.2



F2


F2a

2.7
733
776
770
120
3.1

















Cooling Process



















Average









Cooling









Rate in









Temperature








Cooling
Range of
Cooling

Surface



Steel
Product
Stop
200° C. to
Stop

Treatment



Sheet
Sheet
Temperature
490° C.
Temperature
Holding
Type of



No.
No
° C.
° C./s
° C.
Process
Plating






A1
A1a
570
92.0
350
Not
CR








Provided




A2
A2a
570
67.0
250
Not
EG








Provided





A3


A3a

570
95.0
250
Not
EG








Provided





A4


A4a

570
6.0 
460
Not
GA








Provided




B1
B1a
570
8.6 
300
Provided
CR



B2
B2a
570
6.4 
300
Not
CR








Provided




B3
B3a
570
12.0
250
Not
CR








Provided





B4


B4a

570
15.2
250
Not
CR








Provided




B5
B5a
570
15.2
350
Provided
CR



B5
B5b
570
15.2
350
Provided
Sn



B6
B6a
580
10.9
460
Not
GA








Provided




C1
C1a
580
39.0
250
Not
EG








Provided




C2
C2a
570
39.0
250
Provided
EG




C3


C3a

580
39.0
250
Provided
CR



C4
C4a
570
77.0
250
Not
CR








Provided





C5


C5a

570
43.0
250
Not
CR








Provided




D1
D1a
560
77.0
250
Provided
CR



D2
D2a
560
60.0
250
Not
EG








Provided





D3


D3a

610
77.0
300
Not
CR








Provided




E1
E1a
570
8.6
440
Not
GA








Provided




E2
E2a
570
8.6
440
Not
GA








Provided




E3
E3a
600
5.1
450
Not
GI








Provided




F1
F1a
570
62.0
490
Not
GA








Provided





F2


F2a

570
31.0
490
Not
GA








Provided





The underline represents that the value is outside of the range of the present invention.
















TABLE 3B










Cooling





Process





















Average









Cooling












Annealing Process
Rate in

















Left
Right


Temperature




Average
Side
Side
Soaking

Range of


Steel
Product
Heating
in
in
Temperature
Holding
T1° C. to


Sheet
Sheet
Rate
Expression
Expression
T1
Time
650° C.


No.
No
° C./s
(2)
(2)
° C.
sec
º C./s





G1
G1a
2.7
762
804
790
120
3.1



G2


G2a

5.8
783
826
820
 60
6.2



G3


G3a

5.8
868
902

820

 60
6.2


H1
H1a
2.2
826
862
830
120
3.9



H2


H2a

2.9
854
887

830

 90
5.2



H3


H3a

4.5
835
875
850
 65
3.2


I1
I1a
6.6
777
818
800
 50
7.6



I2


I2a

6.6
824
864
850
 50
7.6



I2


I2b

6.6
857
897

850

 50
7.6



I3


I3a

6.6
778
819
800
 50
7.6


J1
J1a
2.9
773
815
800
 90
5.2


J2
J2a
3.7
797
836
810
 90
4.4


J3
J3a
2.9
767
809
800
 90
5.2


J4
J4a
3.7
772
813
800
 90
4.4


K1
K1a
9.5
742
786
780
 30
9.9


K2
K2a
3.7
744
788
780
 90
4.3



K3


K3a

2.0
742
785
780
140
2.9


K4
K4a
2.0
778
819
780
140
2.9



L1


L1a

2.0
849
882

770

140
2.9


L2
L2a
4.2
772
813
790
 80
4.8


L2
L2b
2.0
767
807
770
150
1.1


L2
L2c
9.8
767
807
800
 35
9.9









Average









Cooling








Cooling
Rate in








Process
Temperature








Cooling
Range of
Cooling

Surface



Steel
Product
Stop
200° C. to
Stop

Treatment



Sheet
Sheet
Temperature
490° C.
Temperature
Holding
Type of



No.
No
° C.
° C./s
° C.
Process
Plating






G1
G1a
570
 31.0
420
Not
Zn-Al-Mg








Provided





G2


G2a

570
 62.0
450
Not
GA








Provided





G3


G3a

580
 62.0
300
Provided
CR



H1
H1a
570
 39.0
400
Not
CR








Provided





H2


H2a

590
 52.0
400
Not
CR








Provided





H3


H3a

570
  7.0
450
Not
GA








Provided




I1
I1a
570
 76.0
400
Not
CR








Provided





I2


I2a

550
 76.0
200
Not
EG








Provided





I2


I2b

570
 76.0
250
Not
CR








Provided





I3


I3a

570
 76.0
250
Not
CR








Provided




J1
J1a
570
 52.0
250
Provided
Phosphate









coating EG



J2
J2a
570
 44.0
250
Not
CR








Provided




J3
J3a
600
 52.0
250
Provided
CR



J4
J4a
570
 44.0
250
Not
CR








Provided




K1
K1a
550
495.0
450
Not
GA








Provided




K2
K2a
580
 43.0
450
Provided
Lubricant GA




K3


K3a

570
 29.0
450
Not
GI








Provided




K4
K4a
570
 29.0
450
Provided
Al-Si




L1


L1a

600
 29.0
420
Not
GA








Provided




L2
L2a
570
 48.0
440
Provided
CR



L2
L2b
570
  5.5
440
Not
Zn-Al








Provided




L2
L2c
590
200.0
440
Provided
GA





The underline represents that the value is outside of the range of the present invention.
















TABLE 3C










Cooling





Process





















Average









Cooling









Rate in












Annealing Process
Temperature
















Average:
Left Side
Right Side
Soaking

Range of


Steel
Product
Heating
in
in
Temperature

T1° C. to


Sheet
Sheet
Rate
Expression
Expression
T1
Holding
650° C.


No.
No
° C./s
(2)
(2)
° C
Times
º C./s





M1
M1a
3.2
753
797
780
110
3.6



M2


M2a

2.5
730
776
775
130
2.9



N1


N1a

2.7
776
817
815
120
3.1


N2
N2a
3.3
739
783
770
100
3.8


O1
O1a
2.7
812
848
820
120
3.1


P1
P1a
3.7
785
825
800
 90
4.3



P2


P2a

2.7
862
901

780

120
3.1


Q1
Q1a
2.7
808
846
820
120
3.1


R1
R1a
3.7
804
841
810
 90
4.3



S1


S1a

3.7
786
826

782

 90
4.3



S2


S2a

3.7
723
770
765
 90
4.3



T1


T1a

4.2
737
783
775
 80
4.8



U1


U1a

5.4
800
839

792

 60
6.2



U2


U2a

2.7
849
884
850
120
3.1


V1
V1a
2.7
831
866
850
120
3.1



W1


W1a

3.7
820
856

815

 90
4.3



X1


X1a

3.8
732
776
770
100
5.4


Y1
Y1a
5.4
800
839
796
 60
6.2



Z1


Z1a

2.9
783
823

780

 90
5.2



AA1


AA1a

3.7
755
798
790
 90
4.4


AB1
AB1a
3.7
817
853
820
120
3.1

















Cooling Process



















Average









Cooling









Rate in









Temperature









Range of
Cooling

Surface



Steel
Product
CoolingStop
200° C. to
Stop

Treatment



Sheet
Sheet
Temperature
490° C.
Temperature
Holding
Type of



No.
No
° C.
° C./s
° C.
Process
Plating






M1
M1a
620
36.0
440
Provided
GA




M2


M2a

570
14.5
460
Provided
GA




N1


N1a

560
 7.1
480
Provided
Zn-Al-









Mg-Si



N2
N2a
570
19.0
460
Provided
GA



O1
O1a
570
15.5
250
Provided
GI



P1
P1a
570
21.5
250
Provided
GI




P2


P2a

570
15.5
450
Provided
GA



Q1
Q1a
570
15.5
450
Provided
GA



R1
R1a
570
21.5
250
Provided
EGA




S1


S1a

570
21.5
350
Provided
GA




S2


S2a

570
21.5
350
Provided
GA




T1


T1a

570
48.0
300
Provided
CR




U1


U1a

570
62.0
440
Provided
GA




U2


U2a

590
 6.2
350
Not
GI








Provided




V1
V1a
560
 6.2
350
Provided
GI




W1


W1a

560
 8.6
450
Not
GA








Provided





X1


X1a

570
10.8
450
Not
GA








Provided




Y1
Y1a
580
12.4
250
Provided
GA




Z1


Z1a

570
10.4
200
Provided
CR




AA1


AA1a

600
 8.8
250
Provided
EG



AB1
AB1a
570
15.5
250
Provided
GA





The underline represents that the value is outside of the range of the present invention.


















TABLE 4A












Internal Region


















Surface Layer Region
























Average


Average







Fraction
Grain Size

Fraction
Grain Size







of
of

of
of



Steel
Product
Sheet
Sheet
Secondary
Secondary
XODF of
Secondary
Secondary
XODF of


Sheet
Sheet
Thickness
Width
Phase
Phase
Ferrite
Phase
Phase
Ferrite


No.
No.
mm
mm
%
μm
(001)/(111)
%
μm
(001)/(111)





A1
A1a
0.50
1300
 8.8
2.1
1.19
13.4
3.6
0.84


A2
A2a
0.35
1300
10.2
2.4
1.82
11.7
3.7
1.08



A3


A3a

0.20
1300
 9.0
3.0

3.02

12.9
4.3
0.80



A4


A4a

0.40
1300

15.8

2.5

2.59

5.4
2.6
1.61


B1
B1a
0.35
1300
 9.4
2.6
1.11
10.1
4.8
0.62


B2
B2a
0.40
1300
 6.1
3.3
0.78
9.4
2.5
0.55


B3
B3a
0.40
1300
 7.3
3.1
0.86
11.7
3.9
0.45



B4


B4a

0.35
1300
10.4
2.8

0.58

12.1
3.1
0.43


B5
B5a
0.16
1300
 9.1
3.5
1.93
11.3
4.4
0.79


B5
B5b
0.16
1300
 9.2
3.5
1.70
11.3
4.6
0.70


B6
B6a
0.40
1300

16.8

3.3
1.89
14.9
5.2
1.71


C1
C1a
0.35
1300
12.7
3.2
2.47
14.2
1.9
1.87


C2
C2a
0.50
1300
 9.7
2.7
1.17
13.4
3.4
0.78



C3


C3a

0.20
1300
10.7
3.4

2.60

14.5
4.7
2.88


C4
C4a
0.55
1300
 8.2
1.7
1.51
12.9
1.6
1.19



C5


C5a

0.50
1300
 7.6
3.3

0.37

10.8
3.8
0.57


D1
D1a
0.40
1300
 9.2
2.4
2.05
13.6
2.9
1.57


D2
D2a
0.45
1300
11.2
3.4
1.47
12.6
3.4
0.97



D3


D3a

0.25
1300
12.6
2.7

3.28

14.2
4.4
0.88


E1
E1a
0.45
1450
13.6
1.8
1.55
14.2
3.7
0.51


E2
E2a
0.45
1450
11.6
2.1
1.17
13.5
4.9
1.04


E3
E3a
0.40
1400
 4.7
1.2
2.20
9.2
4.0
0.88


F1
F1a
0.50
1300
 7.9
3.0
1.38
11.8
3.8
0.68



F2


F2a

0.30
1300
 6.7
4.0

2.97

12.4
3.8
2.41









ΔXODE
























(001)/(111)


Forming Test





















Surface

Evaluation
Amount
Evaluation







Layer

of Surface
ΔPa of
of Surface





Steel
Product
Region
Tensile
Properties
Increase in
Properties





Sheet
Sheet
Internal
Strength
(Steel
Roughness
after





No.
No.
Region
MPa
Sheet)
μm
Forming
Note







A1
A1a
0.35
613
B
0.21
A
Example




A2
A2a
0.74
595
B
0.22
A
Example





A3


A3a

2.22
608
B
0.47
C
Comparative











Example





A4


A4a

−0.02 
432
B
0.36
C
Comparative











Example




B1
B1a
0.49
574
B
0.21
A
Example




B2
B2a
0.23
564
B
0.29
B
Example




B3
B3a
0.41
591
B
0.30
B
Example





B4


B4a

0.15
600
B
0.56
D
Comparative











Example




B5
B5a
1.14
588
B
0.27
B
Example




B5
B5b
1.10
589
B
0.24
A
Example




B6
B6a
0.18
642
B
0.41
C
Comparative











Example




C1
C1a
0.60
628
B
0.34
B
Example




C2
C2a
0.39
615
B
0.22
A
Example





C3


C3a

−0.28 
629
B
0.48
C
Comparative











Example




C4
C4a
0.32
607
B
0.19
A
Example





C5


C5a

−0.20 
580
B
0.54
C
Comparative











Example




D1
D1a
0.48
616
B
0.25
A
Example




D2
D2a
0.50
607
B
0.22
A
Example





D3


D3a

2.40
628
B
0.58
D
Comparative











Example




E1
E1a
1.04
629
B
0.23
A
Example




E2
E2a
0.13
618
B
0.23
A
Example




E3
E3a
1.32
553
B
0.31
B
Example




F1
F1a
0.70
593
B
0.22
A
Example





F2


F2a

0.56
599
B
0.49
C
Comparative











Example





The underline represents that the value is outside of the range of the present invention.


















TABLE 4B












Internal Region


















Surface Layer Region

Average





















Fraction
Average

Fraction
Grain Size







of
Grain Size

of
of



Steel
Product
Sheet
Sheet
Secondary
of
XODF of
Secondary
Secondary
XODF of


Sheet
Sheet
Thickness
Width
Phase
Secondary
Ferrite
Phase
Phase
Ferrite


No.
No.
mm
mm
%
Phase
(001)/(111)
%
μm
(001)/(111)





G1
G1a
0.35
1300
 7.6
2.4
2.10
13.5
3.5
0.97



G2


G2a

0.30
1300
14.8
3.9

2.86

23.5
4.1
2.25



G3


G3a

0.75
1300
12.9
3.7

0.59

18.9
3.4
0.39


H1
H1a
0.55
1950
 9.1
3.0
0.85
13.5
3.2
0.62



H2


H2a

0.70
1950
 7.5
3.5

0.38

12.0
3.4
0.48



H3


H3a

0.40
1300

16.2

2.7

2.67

 6.1
2.6
1.65


I1
I1a
0.40
1450
 8.7
4.0
1.50
11.8
3.4
1.13



I2


I2a

0.45
1450
 9.4
1.6

0.65

10.4
3.1
0.25



I2


I2b

0.45
1450
 7.5

0.4


0.56

11.4
2.8
0.39



I3


I3a

0.45
1450
10.1
3.8

0.69

10.8
3.5
0.60


J1
J1a
0.40
1600
 9.5
3.3
0.98
11.7
3.5
0.75


J2
J2a
0.55
1600
 7.9
2.5
0.77
10.9
2.7
0.50


J3
J3a
0.40
1600
 9.3
2.5
1.04
10.7
3.2
0.67


J4
J4a
0.40
1600
 9.3
3.1
0.82
10.9
2.0
0.69


K1
K1a
0.40
1500
 8.6
3.4
2.38
12.1
3.5
0.86


K2
K2a
0.40
1500
 8.8
3.8
2.28
12.6
3.6
0.90



K3


K3a

0.50
1500
 8.1
3.1

2.96

11.9
3.4
1.23


K4
K4a
0.55
1500
10.0
2.0
1.48
12.3
2.8
0.84



L1


L1a

0.55
1550
12.1
1.7

0.19

13.4
1.9
0.48


L2
L2a
0.12
1550
12.9
1.4
0.90
13.9
2.0
0.47


L2
L2b
0.30
1550
11.1
1.9
0.71
11.8
3.4
0.98


L2
L2c
0.30
1550
10.6
2.1
0.97
15.2
1.6
0.67









ΔXODE
























(001)/(111)


Forming Test





















Surface

Evaluation
Amount
Evaluation







Layer

of Surface
ΔPa of
of Surface





Steel
Product
Region
Tensile
Properties
Increase in
Properties





Sheet
Sheet
Internal
Strength
(Steel
Roughness
after





No.
No.
Region
MPa
Sheet)
μm
Forming
Note







G1
G1a
1.13
551
A
0.26
B
Example





G2


G2a

0.61
743
A
0.56
D
Comparative











Example





G3


G3a

0.20
685
A
0.60
D
Comparative











Example




H1
H1a
0.23
615
A
0.31
B
Example





H2


H2a

−0.10 
595
A
0.54
C
Comparative











Example





H3


H3a

0.02
439
A
0.39
C
Comparative











Example




I1
I1a
0.37
594
A
0.23
A
Example





I2


I2a

0.40
578
A
0.52
C
Comparative











Example





I2


I2b

0.17
588
A
0.46
C
Comparative











Example





I3


I3a

0.09
584
B
0.47
C
Comparative











Example




J1
J1a
0.23
594
A
0.24
A
Example




J2
J2a
0.28
582
A
0.35
B
Example




J3
J3a
0.37
582
A
0.26
B
Example




J4
J4a
0.13
584
A
0.35
B
Example




K1
K1a
1.52
597
A
0.34
B
Example




K2
K2a
1.38
604
A
0.32
B
Example





K3


K3a

1.73
594
A
0.44
C
Comparative











Example




K4
K4a
0.64
602
A
0.24
A
Example





L1


L1a

−0.29 
618
A
0.38
C
Comparative











Example




L2
L2a
0.43
625
A
0.29
B
Example




L2
L2b
−0.27 
597
A
0.35
B
Example




L2
L2c
0.30
638
A
0.24
A
Example





The underline represents that the value is outside of the range of the present invention.


















TABLE 4C












Internal Region


















Surface Layer Region

Average





















Fraction
Average

Fraction
Grain Size







of
Grain Size

of
of



Steel
Product
Sheet
Sheet
Secondary
of
XODF of
Secondary
Secondary
XODF of


Sheet
Sheet
Thickness
Width
Phase
Secondary
Ferrite
Phase
Phase
Ferrite


No.
No.
mm
mm
%
Phase
(001)/(111)
%
μm
(001)/(111)





M1
M1a
0.20
1450
12.0
2.5
2.02
12.1
2.8
1.69



M2


M2a

0.45
1450
10.9
2.3

3.03

11.8
4.9
2.41



N1


N1a

0.35
1400
 1.5
3.0

3.04

 5.4
2.2
1.02


N2
N2a
0.30
1400
 6.4
0.5
2.20
11.2
1.2
0.87


O1
O1a
0.45
1300
 9.1
3.5
1.55
12.3
3.0
1.25


P1
P1a
0.30
1300
 8.9
2.8
1.66
11.3
2.1
1.27



P2


P2a

0.35
1300

17.2

1.6

0.21

14.0
4.3
0.31


Q1
Q1a
0.55
1300
11.8
3.2
1.63
14.3
3.6
0.66


R1
R1a
0.55
2000
 7.6
3.2
1.40
 8.1
3.8
0.49



S1


S1a

0.32
1100

25.9


4.5


0.32


25.1

5.5
0.55



S2


S2a

0.10
1100

28.9


5.1


2.94


28.2

6.1
2.59



T1


T1a

0.60
1950
0.0

-


0.61

0.0

0.55



U1


U1a

0.55
1600
 5.8
3.3

0.50

4.9
3.7
0.64



U2


U2a

0.50
1600
 3.8

4.4


0.61

4.2
4.5
0.94



V1


V1a

0.45
1100

15.3


4.9

2.06
23.1
5.1
1.79



W1


W1a

0.85
1650
 3.4

0.5


0.13

 5.3
1.0
0.17



X1


X1a

0.25
1350
 1.9
1.1
1.65
2.3
2.0
1.72



Y1


Y1a

0.55
1300

15.2

3.2
1.52
18.3
3.3
0.66



Z1


Z1a

0.50
1100

26.0


4.2

0.90

26.4

6.5
0.60



AA1


AA1a

0.35
1250

17.6


6.4

0.92
21.5
6.7
0.80


AB1
AB1a
0.45
1200
11.3
2.7
0.95
15.9
3.0
0.75









ΔXODE
























(001)/(111)


Forming Test





















Surface

Evaluation
Amount
Evaluation







Layer

of Surface
ΔPa of
of Surface





Steel
Product
Region-
Tensile
Properties
Increase in
Properties





Sheet
Sheet
Internal
Strength
(Steel
Roughness
after





No.
No.
Region
MPa
Sheet)
μm
Forming
Note







M1
M1a
0.33
622
A
0.25
A
Example





M2


M2a

0.62
617
A
0.57
D
Comparative











Example





N1


N1a

2.02
542
B
0.17
A
Comparative











Example




N2
N2a
1.33
619
B
0.31
B
Example




O1
O1a
0.30
621
B
0.23
A
Example




P1
P1a
0.39
608
A
0.20
A
Example





P2


P2a

−0.10 
649
A
0.43
C
Comparative











Example




Q1
Q1a
0.97
648
B
0.21
A
Example




R1
R1a
0.91
568
A
0.19
A
Example





S1


S1a

−0.23 
777
B
0.55
C
Comparative











Example





S2


S2a

0.35
819
C


Comparative











Example





T1


T1a

0.06
441
A
0.40
C
Comparative











Example





U1


U1a

−0.14 
507
A
0.48
C
Comparative











Example





U2


U2a

−0.33 
496
A
0.45
C
Comparative











Example





V1


V1a

0.27
739
B
0.48
C
Comparative











Example





W1


W1a

−0.04 
508
B
0.36
C
Comparative











Example





X1


X1a

−0.07 
470
D


Comparative











Example





Y1


Y1a

0.86
681
C


Comparative











Example





Z1


Z1a

0.30
793
C


Comparative











Example





AA1


AA1a

0.12
723
B
0.60
D
Comparative











Example




AB1
AB1a
0.20
652
B
0.26
B
Example





The underline represents that the value is outside of the range of the present invention.






INDUSTRIAL APPLICABILITY

In the steel sheet according to the aspect of the present invention, a high strength steel sheet in which the occurrence of surface unevenness is suppressed even after various deformation during press forming can be manufactured. Therefore, the industrial applicability is high.

Claims
  • 1. A steel sheet comprising, as a chemical composition, by mass %: C: 0.050% to 0.145%;Mn: 0.70% to 2.25%;Al and Si in total: 0.60% or less;P: 0 to 0.030%;S: 0 to 0.020%;N: 0 to 0.0050%;Mo: 0% to 0.80%;B: 0% to 0.0050%;Ti: 0% to 0.200%;Nb: 0% to 0.100%;Cr: 0% to 0.700%;Ni: 0% to 0.200%; anda remainder of iron and impurities,wherein a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%,a metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%,the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, andin the surface layer region,an average grain size of the secondary phase is 0.5 μm to 4.0 μm, anda texture in which an XODF{001}/{111} as a ratio of an intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.70 to 2.50 is included,wherein an average grain size of the secondary phase in the internal region is 1.0 μm to 5.0 μm and is more than the average grain size of the secondary phase in the surface layer region, andwherein the chemical composition satisfies following Expression (1),a tensile strength is 550 MPa or higher,a sheet thickness is 0.10 mm to 0.55 mm, anda sheet width is 1300 mm to 2000 mm, 7.00≥[C]×20+[Si]×3.0+[Mn]×2.0+[Al]×2.0+[Cr]+[Ti]×20+[Nb]×40+[Mo]×2.0+[Ni]×2.0+[B]×200  (1),where a symbol of each of the elements in Expression (1) represents an amount of the element by mass %, and when the element is not included, 0 is substituted into the symbol.
  • 2. The steel sheet according to claim 1, wherein a plating layer is provided on the surface.
  • 3. A method for manufacturing the steel sheet according to claim 1, the method comprising: a heating process of heating a slab having said chemical composition at 1000° C. or higher;a hot-rolling process of hot-rolling the slab such that a rolling finishing temperature is 950° C. or lower to obtain a hot-rolled steel sheet;a stress application process of applying a stress to the hot-rolled steel sheet after the hot-rolling process such that an absolute value of a residual stress σs on the surface is 165 MPa to 400 MPa;a cold-rolling process of cold-rolling the hot-rolled steel sheet after the stress application process such that a cumulative rolling reduction RCR is 70% to 90% to obtain a cold-rolled steel sheet;an annealing process of heating the cold-rolled steel sheet such that an average heating rate in a range from 300° C. to a soaking temperature T1° C. that satisfies following Expression (2) is 1.5° C./sec to 10.0° C./sec and holding the heated steel sheet at the soaking temperature T1° C. for 30 seconds to 150 seconds for annealing; anda cooling process of cooling the heated steel sheet after the annealing process to a temperature range of 550° C. to 650° C. such that an average cooling rate in a range from the soaking temperature T1° C. to 650° C. is 1.0° C./sec to 10.0° C./sec and further cooling the cooled steel sheet to a temperature range of 200° C. to 490° C. such that an average cooling rate is 5.0° C./sec to 500.0° C./sec, 1275−25×ln(σs)−4.5×RCR≤T1≤1275−25×ln(σs)−4×RCR  (2).
  • 4. The method for manufacturing the steel sheet according to claim 3, wherein the stress application process is performed at 40° C. to 500° C.
  • 5. The method for manufacturing the steel sheet according to claim 3, wherein in the hot-rolling process, a finish rolling start temperature is 850° C. or lower.
  • 6. The method for manufacturing the steel sheet according to claim 3, further comprising a holding process of holding the cooled steel sheet after the cooling process in a temperature range of 200° C. to 490° C. for 30 seconds to 600 seconds.
  • 7. A steel sheet comprising, as a chemical composition, by mass %: C: 0.050% to 0.145%;Mn: 0.70% to 2.25%;Al and Si in total: 0.60% or less;P: 0% to 0.030%;S: 0% to 0.020%;N: 0% to 0.0050%;Mo: 0% to 0.80%;B: 0% to 0.0050%;Ti: 0% to 0.200%;Nb: 0% to 0.100%;Cr: 0% to 0.700%;Ni: 0% to 0.200%; anda remainder of iron and impurities,wherein a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction comprises ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%,a metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction comprises ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%,the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, andin the surface layer region,an average grain size of the secondary phase is 0.5 μm to 4.0 μm, anda texture in which an XODF{001}/{111} as a ratio of an intensity of {001} orientation to an intensity of {111} orientation in the ferrite is 0.70 to 2.50 is included,wherein an average grain size of the secondary phase in the internal region is 1.0 μm to 5.0 μm and is more than the average grain size of the secondary phase in the surface layer region, andwherein the chemical composition satisfies following Expression (1),a tensile strength is 550 MPa or higher,a sheet thickness is 0.10 mm to 0.55 mm, anda sheet width is 1300 mm to 2000 mm, 7.00≥[C]×20+[Si]×3.0+[Mn]×2.0+[Al]×2.0+[Cr]+[Ti]×20+[Nb]×40+[Mo]×2.0+[Ni]×2.0+[B]×200  (1),where a symbol of each of the elements in Expression (1) represents an amount of the element by mass %, and when the element is not included, 0 is substituted into the symbol.
  • 8. The steel sheet according to claim 1, wherein the chemical composition includes, by mass %, one or more of:Mo: 0.001% to 0.80%;B: 0.0001% to 0.0050%;Ti: 0.001% to 0.200%;Nb: 0.001% to 0.100%;Cr: 0.001% to 0.700%; andNi: 0.001% to 0.200%.
  • 9. The steel sheet according to claim 1, wherein the secondary phase in the surface layer region comprises at least one of martensite, bainite, and tempered martensite.
Priority Claims (1)
Number Date Country Kind
2019-000671 Jan 2019 JP national
PCT Information
Filing Document Filing Date Country Kind
PCT/JP2020/000139 1/7/2020 WO
Publishing Document Publishing Date Country Kind
WO2020/145259 7/16/2020 WO A
US Referenced Citations (2)
Number Name Date Kind
20120282487 Ogawa Nov 2012 A1
20150047751 Ono et al. Feb 2015 A1
Foreign Referenced Citations (5)
Number Date Country
2 500 445 Sep 2012 EP
2007-291514 Nov 2007 JP
2009-132981 Jun 2009 JP
2016-156079 Sep 2016 JP
WO2013046476 Apr 2013 WO
Non-Patent Literature Citations (1)
Entry
Milenin et al., “Model of residual stresses in hot-rolled sheets with taking into account relaxation process and phase transformation,” Procedia Engineering, vol. 81, 2014, pp. 108-113, 6 pages total.
Related Publications (1)
Number Date Country
20220074015 A1 Mar 2022 US