Patent Application
20240384381
The present invention relates to a hot-rolled steel sheet, and specifically, to a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability.
Priority is claimed on Japanese Patent Application No. 2021-122173, filed Jul. 27, 2021, the content of which is incorporated herein by reference.
In recent years, in order to improve durability of automobiles and improve collision safety, application of high-strength steel sheets to automobile members has been actively examined. In particular, for high-strength steel sheets applied to automobile members, it is important to secure fatigue durability of parts.
When a steel sheet is processed into a part, a part shape is punched out from the steel sheet to produce a blank material. In this case, if cracks occur in the punched sheared surface, fatigue durability of the part may not always be improved even if a high-strength steel sheet is used.
For example, Patent Document 1 proposes a hot-rolled steel sheet having excellent fatigue properties at sheared edges by increasing the volume proportion of martensite and decreasing the volume proportion of pearlite.
Patent Document 2 proposes a steel sheet that is mainly composed of ferrite and bainite structure, and has excellent fatigue properties at a punched shear section by reducing the maximum height of the steel sheet surface.
However, in the techniques described in Patent Documents 1 and 2, since it is not possible to sufficiently minimize the occurrence of cracks of the punched sheared surface, fatigue properties are not sufficient when parts are shaped by processing.
Patent Document 3 proposes a steel sheet with a fatigue crack occurrence lifespan prolonged by securing <011> and <111> of ferrite and martensite and minimizing <001>.
Patent Document 4 proposes a method of controlling crystal orientation of a ferrite or bainite main phase structure by controlling the shape ratio up to the final pass in finish rolling.
However, in order to further improve shear processability in addition to fatigue properties, there is room for improvement in the techniques described in Patent Documents 3 and 4.
In recent years, with the increasing demands for further weight reduction of automobiles and against the background of complication of part shapes, there has been a demand for high-strength hot-rolled steel sheets having better fatigue properties and shear processability.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability.
It is known that a dual-phase structure in which hard fresh martensite is dispersed in ferrite which inhibits dislocation movement is effective as a structure with excellent fatigue properties. Steel sheets having this dual-phase structure are widely used, for example, for wheel disk parts of automobiles.
On the other hand, since hard fresh martensite is a structure that inhibits plastic deformation, during strong processing such as punching, voids are formed around fresh martensite and cracks easily occur in the punched sheared surface. Therefore, in a hot-rolled steel sheet having a dual-phase structure utilizing ferrite and fresh martensite, shear processability generally deteriorates.
In order to break down these mutual relationships, the inventors analyzed each deformation mechanism in detail. As a result, the inventors found that, when the crystal orientation of ferrite and bainite in a desired region is strictly controlled, it is possible to improve shear processability while securing fatigue properties of the hot-rolled steel sheet. That is, it was found that, when area proportions of hard fresh martensite and tempered martensite, which is the main phase, are controlled, fatigue properties are secured, and when the crystal orientations of ferrite and bainite undergoing large crystal rotation due to punching are appropriately formed in a desired region, it is possible to achieve both fatigue properties and shear processability of the hot-rolled steel sheet at a high level.
The present invention has been made based on the above findings and the gist of the present invention is as follows.
According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability. According to the hot-rolled steel sheet of the present invention, it is possible to integrally mold parts for reducing the weight of a vehicle body of an automobile or the like, shorten the processing process, improve fuel efficiency, and reduce production costs.
A hot-rolled steel sheet according to one embodiment of the present invention (sometimes referred to as a hot-rolled steel sheet according to the present embodiment) will be described. However, the present invention is not limited only to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
Hereinafter, respective constituent elements of the present invention will be described in detail. First, the reason for limiting the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described.
Hereinafter, numerical values limiting a range indicated by “to” include both the lower limit value and the upper limit value. Numerical values indicated by “less than” or “more than” are not included in this numerical value range. Unless otherwise specified, % related to the chemical composition is mass %.
The hot-rolled steel sheet according to the present embodiment has a chemical composition containing, in mass %, C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.10 to 1.00%, N: 0.0100% or less, and Ti: 0.06 to 0.20%, with the remainder: Fe and impurities.
C is an element important for improving the strength of the hot-rolled steel sheet. If the C content is less than 0.02%, it is not possible to obtain a desired strength. Therefore, the C content is 0.02% or more, and preferably 0.04% or more, 0.06% or more, or 0.10% or more.
On the other hand, if the C content is more than 0.30%, the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the C content is 0.30% or less, and preferably 0.25% or less, or 0.20% or less.
Si is an element that has an effect of inhibiting formation of carbides during ferrite transformation and improving the fatigue properties of the hot-rolled steel sheet. If the Si content is less than 0.10%, it is not possible to obtain this effect. Therefore, the Si content is 0.10% or more, and preferably 0.20% or more, 0.30% or more, or 0.50% or more.
On the other hand, if the Si content is more than 2.00%, the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the Si content is 2.00% or less, and preferably 1.80% or less, 1.60% or less, or 1.50% or less.
Mn is an element effective in improving the strength of the hot-rolled steel sheet according to improvement in hardenability and solid-solution strengthening. If the Mn content is less than 0.5%, it is not possible to obtain this effect. Therefore, the Mn content is 0.5% or more, and preferably 0.7% or more, or 1.0% or more.
On the other hand, if the Mn content is more than 3.0%, the fatigue properties of the hot-rolled steel sheet deteriorate due to formation of MnS. Therefore, the Mn content is 3.0% or less and preferably 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.
P is an impurity, and a lower P content is desirable, and the P content is preferably 0%. If the P content is more than 0.100%, the processability and weldability of the hot-rolled steel sheet significantly deteriorate and the fatigue properties also deteriorate. Therefore, the P content is 0.100% or less, and preferably 0.070% or less, 0.050% or less, or 0.030% or less.
The P content may be 0.001% or more in consideration of refining costs.
S is an impurity, and a lower S content is desirable, and the S content is preferably 0%. If the S content is more than 0.010%, a large amount of inclusions such as MnS are formed, and the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the S content is 0.010% or less and preferably 0.008% or less, or 0.007% or less. If better shear processability is required, the S content is preferably 0.006% or less.
The S content may be 0.001% or more in consideration of refining costs.
Al is an element important to control ferrite transformation. If the Al content is less than 0.10%, it is not possible to preferably control the area proportion of ferrite. Therefore, the Al content is 0.10% or more, and preferably 0.20% or more, 0.30% or more, or 0.40% or more.
On the other hand, if the Al content is more than 1.00%, alumina precipitated in clusters is formed and the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the Al content is 1.00% or less, and preferably 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.
N is an impurity, and a lower N content is desirable and the N content is preferably 0%. If the N content is more than 0.0100%, coarse Ti nitrides are formed at a high temperature, and the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the N content is 0.0100% or less, and preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less.
The N content may be 0.0001% or more in consideration of refining costs.
Ti is an element that strengthens precipitation of ferrite, and is an element important for controlling ferrite transformation to obtain a desired amount of ferrite. If the Ti content is less than 0.06%, it is not possible to obtain an effect of precipitation strengthening and ferrite transformation control. Therefore, the Ti content is 0.06% or more, and preferably 0.08% or more, or 0.10% or more.
On the other hand, if the Ti content is more than 0.20%, inclusions caused by TiN are formed, and the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is 0.20% or less, and preferably 0.18% or less, or 0.16% or less.
The hot-rolled steel sheet according to the present embodiment may have the above chemical composition, with the remainder being made up of Fe and impurities. Here, impurities are elements that are mixed in from raw materials such as ores and scrap, and other factors when steel materials are industrially produced, and/or are allowable as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment.
In order to reduce production variations and further improve the strength, the chemical composition of the hot-rolled steel sheet according to the present embodiment may contain the following optional elements that are not essential for satisfying required properties. However, since none of the following optional elements is essential to satisfy required properties, the lower limits of the contents thereof are 0%.
Nb is an element that has an effect of increasing the strength of the hot-rolled steel sheet according to refining the crystal grain size and strengthening precipitation of NbC. In order to obtain this effect, the Nb content is preferably 0.01% or more.
On the other hand, if the Nb content is more than 0.10%, the above effect is maximized. Therefore, even if Nb is contained, the Nb content is 0.10% or less, and preferably 0.06% or less.
Ca is an element that fixes S in steel as spherical CaS, inhibits formation of elongated inclusions such as MnS, and improves hole expandability of the hot-rolled steel sheet. In order to obtain these effects, the Ca content is preferably 0.0005% or more.
On the other hand, if the Ca content is more than 0.0060%, the above effect is maximized. Therefore, even if Ca is contained, the Ca content is 0.0060% or less, and preferably 0.0040% or less.
Mo is an element effective in improving the strength of the hot-rolled steel sheet according to strengthening precipitation of ferrite. In order to obtain this effect, the Mo content is preferably 0.02% or more and more preferably 0.10% or more.
On the other hand, if the Mo content is more than 1.00%, cracking sensitivity of the slab increases and it is difficult to handle the slab. Therefore, even if Mo is contained, the Mo content is 1.00% or less, and preferably 0.60% or less, 0.50% or less, or 0.30% or less.
Cr is an element effective in improving the strength of the hot-rolled steel sheet. In order to obtain this effect, the Cr content is preferably 0.02% or more and more preferably 0.10% or more.
On the other hand, if the Cr content is more than 1.00%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, even if Cr is contained, the Cr content is 1.00% or less, and preferably 0.80% or less.
V is an element that improves the strength of the hot-rolled steel sheet by dislocation strengthening according to precipitation strengthening and recrystallization inhibition. In order to obtain these effects, the V content is preferably 0.01% or more. On the other hand, if the V content is more than 0.40%, a large amount of carbonitrides are precipitated and the moldability of the hot-rolled steel sheet decreases. Therefore, the V content is 0.40% or less and preferably 0.20% or less.
Ni is an element that inhibits phase transformation at a high temperature and improves the strength of the hot-rolled steel sheet. In order to obtain this effect, the Ni content is preferably 0.01% or more.
On the other hand, if the Ni content is more than 0.40%, the weldability of the hot-rolled steel sheet decreases. Therefore, the Ni content is 0.40% or less and preferably 0.20% or less.
B is an element that inhibits phase transformation at a high temperature and improves the strength of the hot-rolled steel sheet. In order to obtain this effect, the B content is preferably 0.0001% or more.
On the other hand, if the B content is more than 0.0020%, B precipitates are formed and the strength of the hot-rolled steel sheet decreases. Therefore, the B content is 0.0020% or less, and preferably 0.0005% or less.
Cu is element that is present in steel in the form of fine particles and improves the strength of the hot-rolled steel sheet. In order to obtain this effect, the Cu content is preferably 0.02% or more.
On the other hand, if the Cu content is more than 1.00%, the weldability of the hot-rolled steel sheet deteriorates. Therefore, the Cu content is 1.00% or less, and preferably 0.80% or less.
Sn is an element that inhibits coarsening of crystal grains and improves the strength of the hot-rolled steel sheet. In order to obtain this effect, the Sn content is preferably 0.01% or more.
On the other hand, if the Sn content is more than 0.50%, steel becomes brittle and easily broken during rolling. Therefore, the Sn content is 0.50% or less, and preferably 0.30% or less.
Zr is an element that contributes to improving the moldability of the hot-rolled steel sheet. In order to obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, if the Zr content is more than 0.050%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Zr content is 0.050% or less, and preferably 0.030% or less.
The chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method. For example, inductively coupled plasma-atomic emission spectrometry (ICP-AES) may be used for measurement. Here, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.
Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
The microstructure of the hot-rolled steel sheet according to the present embodiment contains, in area proportion, a total amount of ferrite and bainite: 30 to 47%, tempered martensite: 50 to 70%, and fresh martensite: 3 to 10%, and within regions obtained by dividing a sheet thickness cross section parallel to a rolling direction into three parts in a sheet thickness direction, when a pole density of the {001} plane of ferrite and bainite in a center region is Pi and a pole density of the {001} plane of ferrite and the bainite in a surface layer region is Ps, Pi/Ps is 1.2 to 2.0.
The hot-rolled steel sheet according to the present embodiment preferably has a microstructure composed of only ferrite, bainite, tempered martensite and fresh martensite. That is, the hot-rolled steel sheet according to the present embodiment preferably has a microstructure composed of only, in area proportion, a total amount of ferrite and bainite: 30 to 47%, tempered martensite: 50 to 70%, and fresh martensite: 3 to 10%.
Here, in the present embodiment, in the region of a depth of ⅛ from the surface to a depth of ⅜ from the surface, area proportions of ferrite, bainite, tempered martensite, and fresh martensite are specified. The reason for this is that the microstructure in this region is a typical microstructure of the hot-rolled steel sheet.
Ferrite and bainite improve the shear processability of the hot-rolled steel sheet. If the total area proportion of ferrite and bainite is less than 30%, the shear processability of the hot-rolled steel sheet may deteriorate or the fatigue strength of the hot-rolled steel sheet may deteriorate. Therefore, the total area proportion of ferrite and bainite is 30% or more, and preferably 33% or more, 35% or more, or 37% or more.
On the other hand, if the total area proportion of ferrite and bainite is more than 47%, the strength and the fatigue properties of the hot-rolled steel sheet may deteriorate or the shear processability of the hot-rolled steel sheet may deteriorate. Therefore, the total area proportion of ferrite and bainite is 47% or less, and preferably 45% or less, or 43% or less.
Here, in the present embodiment, it is not necessary to contain both ferrite and bainite, and only one of ferrite and bainite may be contained, and the area proportion thereof may be within the above range.
It is effective to incorporate fresh martensite in order to improve fatigue properties of the hot-rolled steel sheet, but in order to achieve both shear processability and fatigue properties, it is effective to incorporate tempered martensite, which has a high martensite formation temperature and is formed by tempering during cooling.
If the area proportion of tempered martensite is less than 50%, the strength and the fatigue properties of the hot-rolled steel sheet deteriorate. Therefore, the area proportion of tempered martensite is 50% or more, and preferably 53% or more, or 55% or more.
On the other hand, if the area proportion of tempered martensite is more than 70%, the shear processability of the hot-rolled steel sheet may deteriorate or the strength of the hot-rolled steel sheet may deteriorate. Therefore, the area proportion of tempered martensite is 70% or less, and preferably 65% or less, or 60% or less.
Fresh martensite improves the fatigue strength of the hot-rolled steel sheet. If the area proportion of fresh martensite is less than 3%, the fatigue strength of the hot-rolled steel sheet may deteriorate and/or the strength of the hot-rolled steel sheet may deteriorate. Therefore, the area proportion of fresh martensite is 3% or more, and preferably 4% or more, or 5% or more.
On the other hand, if the area proportion of fresh martensite is more than 10%, the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the area proportion of fresh martensite is 10% or less, and preferably 9% or less, or 8% or less.
The area proportion of each structure can be obtained by the following method.
First, a test piece is collected from the hot-rolled steel sheet so that the microstructure can be observed in the sheet thickness cross section parallel to the rolling direction at a depth of ¼ of the sheet thickness from the surface (the region of a depth of ⅛ from the surface to a depth of ⅜ from the surface) and a center position in the sheet width direction.
The cross section of the test piece is polished using silicon carbide paper of #600 to #1500 and then mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 μm in a diluted solution such as an alcohol or pure water. Next, polishing is performed using colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At an arbitrary position on the cross section of the sample in the longitudinal direction, in order to observe the position of a depth of ¼ of the sheet thickness from the surface, a region with a length of 50 μm and to a depth of ⅛ of the sheet thickness from the surface to a depth of ⅜ of the sheet thickness from the surface is measured by a back scattering electron beam diffraction method at 0.1 μm measurement intervals to obtain crystal orientation information.
For measurement, an EBSD analysis device composed of a thermal field emission scanning electron microscope (JSM-7001F commercially available from JEOL) and an EBSD detector (DVC5 type detector commercially available from TSL) is used. In this case, the degree of vacuum in the EBSD analysis device is 9.6×10−5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. From the obtained crystal orientation information, using the “Grain Orientation Spread” function installed in the software “OIM Analysis (registered trademark)” bundled in the EBSD analysis device, under the condition in which the 15° grain boundary is the crystal grain boundary, a region in which the “Grain Orientation Spread” is 1° or less is extracted as ferrite. If the area proportion of the extracted ferrite is calculated, the area proportion of ferrite is obtained.
Subsequently, within the remaining region (a region in which the “Grain Orientation Spread” is more than 1°), under the condition in which the 5° grain boundary is the crystal grain boundary, when the maximum value of the “Grain Average IQ” in the ferrite region is Iα, a region of more than Iα/2 is extracted as bainite, and a region of Iα/2 or less is extracted as “fresh martensite and tempered martensite.” If the area proportion of the extracted bainite is calculated, the area proportion of bainite is obtained.
Regarding the extracted “fresh martensite and tempered martensite,” fresh martensite and tempered martensite are distinguished by the following method.
In order to observe the same region as the EBSD measurement region with an SEM, a Vickers indentation is stamped in the vicinity of the observation position. Then, leaving the structure of the observation surface, contaminants on the surface layer are removed by polishing and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed with an SEM at a magnification of 3,000.
In the EBSD measurement, within the region determined to as “fresh martensite and tempered martensite,” a region having a substructure within grains and in which cementites with a plurality of variants are precipitated is determined as tempered martensite. A region in which the luminance is large and the substructure is not exposed by etching is determined as fresh martensite. When respective area proportions are calculated, the area proportion of tempered martensite and the area proportion of fresh martensite are obtained.
Here, in order to remove contaminants on the surface layer of the observation surface, a method such as buff polishing using alumina particles having a particle size of 0.1 μm or less or Ar ion sputtering may be used.
Pi/Ps: 1.2 to 2.0
When the rolled plane and the {001} plane are parallel, there are few slip systems of dislocation, crystal rotation does not occur during shear processing, and cracks are likely to occur in the punched sheared surface, and thus the shear processability of the hot-rolled steel sheet deteriorates. The inventors have found that cracks during shear processing tends to occur in the center region within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction. In the present embodiment, when the pole density of the {001} plane of ferrite and bainite in the center region and the surface layer region is preferably controlled, the shear processability of the hot-rolled steel sheet is improved.
Within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction, when the pole density of the {001} plane of ferrite and bainite in the center region is Pi and the pole density of the {001} plane of ferrite and the bainite in the surface layer region is Ps, Pi/Ps is less than 1.2, which indicates that the {001} planes are uniformly distributed from the surface of the hot-rolled steel sheet. In this case, crystal rotation occurs from the sheared surface during punching, sagging during shearing increases, and cracks are likely to occur in the punched sheared surface, and as a result, the shear processability of the hot-rolled steel sheet deteriorates. Therefore, Pi/Ps is 1.2 or more, and preferably 1.3 or more, 1.4 or more, or 1.5 or more.
On the other hand, if Pi/Ps is more than 2.0, it indicates that the {001} planes are excessively concentrated in the center region. In this case, there are many {001} planes, which are brittle fracture surfaces, on the fracture surface, and cracks are likely to occur in the punched sheared surface, and as a result, the shear processability of the hot-rolled steel sheet deteriorates. Therefore, Pi/Ps is 2.0 or less and preferably 1.9 or less, 1.8 or less, or 1.7 or less.
Here, the center region is a region of a depth of ⅓ of the sheet thickness from the surface to a depth of ⅔ of the sheet thickness from the surface within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction. In addition, the surface layer region is a region from the surface to a depth of ⅓ of the sheet thickness from the surface, or a region from the surface to a depth of ⅔ of the sheet thickness to the back surface (another surface different from the surface) within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction, and in the present embodiment, it is not particularly limited to any region.
In addition, {hkl} indicates a crystal plane parallel to the rolled plane. That is, {hkl} indicates that the rolling direction and the {hkl} plane are parallel.
The pole density of the {001} plane of ferrite and bainite is measured using a device in which a scanning electron microscope and an EBSD analysis device are combined and OIM Analysis (registered trademark, commercially available from TSL). The pole density can be obtained from the crystal orientation distribution function (ODF) that represents a 3D texture calculated using orientation data measured by an electron back scattering diffraction (EBSD) method and spherical surface harmonics. Here, the measurement pitch is 5 μm/step.
The measurement ranges are the center region (a region of a depth of ⅓ of the sheet thickness from the surface to a depth of ⅔ of the sheet thickness from the surface within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction) and the surface layer region (a region from the surface to a depth of ⅓ of the sheet thickness from the surface, or a region of a depth of ⅔ of the sheet thickness from the surface to the back surface (another surface different from the surface) within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction). In addition, the pole density of the regions identified as ferrite and bainite is measured by the same method as in the above EBSD measurement.
The tensile strength of the hot-rolled steel sheet according to the present embodiment is 950 MPa or more, and preferably 1,000 MPa or more. If the tensile strength is less than 950 MPa, application parts are limited, and contribution to vehicle body weight reduction is small. The upper limit is not necessarily particularly limited and may be 1,500 MPa or less, or 1,300 MPa or less in order to reduce mold wear.
In addition, the fatigue limit ratio (fatigue strength/tensile strength) of the hot-rolled steel sheet according to the present embodiment may be 0.35 or more.
The tensile strength is evaluated by performing a tensile test according to JIS Z 2241:2011. The test piece is the No. 5 test piece according to JIS Z 2241:2011. A position at which the tensile test piece is collected may be a quarter from the edge in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
The fatigue strength is measured by collecting No. 1 test piece from the hot-rolled steel sheet according to JIS Z 2275:1978 using a Schenck plane bending fatigue testing machine. For the stress load during measurement, the test speed is set to 30 Hz in both swings and the fatigue strength is measured over 107 cycles. Then, the fatigue strength over 107 cycles is divided by the tensile strength measured by the above tensile test to calculate the fatigue limit ratio (fatigue strength/tensile strength).
The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be 1.2 to 8.0 mm. If the sheet thickness of the hot-rolled steel sheet is less than 1.2 mm, it may become difficult to secure the rolling completion temperature, the rolling load may become excessive, and hot rolling may become difficult. On the other hand, if the sheet thickness is more than 8.0 mm, it may become difficult to obtain the above microstructure after hot rolling.
The hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment may be a surface-treated steel sheet that has a plating layer on the surface in order to improve the corrosion resistance. The plating layer may be an electroplating layer or a melting plating layer. Examples of electroplating layers include zinc electroplating and electro Zn—Ni alloy plating. Examples of melting plating layers include melting zinc plating, alloying melting zinc plating, melting aluminum plating, melting Zn—Al alloy plating, melting Zn—Al—Mg alloy plating, and melting Zn—Al—Mg—Si alloy plating. The amount of plating adhered is not particularly limited, and may be the same as in the related art. In addition, it is possible to further improve the corrosion resistance by applying appropriate chemical conversion (for example, applying a silicate-based chromium-free chemical conversion solution and drying) after plating.
When the hot-rolled steel sheet according to the present embodiment has the above chemical composition and microstructure, the effect can be obtained regardless of the production method. However, according to the following production method, this is preferable because the hot-rolled steel sheet according to the present embodiment can be stably obtained.
In a preferable method of producing the hot-rolled steel sheet according to the present embodiment, hot rolling conditions and subsequent cooling conditions are strictly controlled. Hereinafter, details will be described.
The heating temperature of the slab has a great effect on solutionization and elimination of element segregation. If the heating temperature of the slab is lower than 1,100° C., solutionization and elimination of element segregation become insufficient, it is not possible to obtain sufficient precipitation strengthening of the finally obtained product, and the tensile strength deteriorates. In addition, if the heating temperature of the slab is higher than 1,350° C., not only is the effect of solutionization and elimination of element segregation maximized, but also the average grain size of austenite becomes coarse, which causes non-uniformity in crystal rotation during rolling, and it is difficult to obtain a desired texture. Therefore, the heating temperature of the slab is preferably 1,100 to 1,350° C. and more preferably 1,150 to 1,300° C.
Here, the temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
In finish rolling, rolling is performed by continuously passing the slab through rolling stands for finish rolling a plurality of times. In this case, in a plurality of continuous rolling stands, it is preferable that rolling conditions at the rolling stands of the final three stands (the final rolling stand, the rolling stand one stand before the final stand, and the rolling stand two stands before the final stand) satisfy the following Formula (1) and Formula (2). In the present embodiment, it is preferable that the average value of the final three stands satisfy the following Formula (1) and Formula (2).
2.0≤2×{R(H1−H2)}0.5/(H1+H2)≤10.0 Formula (1)
However, respective symbols in Formula (1) are as follows.
5≤ΔT≤35 Formula (2)
However, in Formula (2), ΔT is the difference between the steel sheet inlet side temperature and the steel sheet outlet side temperature at each rolling stand.
The middle side of Formula (1) is a formula for obtaining a rolling shape ratio. When the rolling shape ratio is controlled, it is possible to control crystal rotation by rolling, and it is possible to obtain a desired crystal orientation in the desired region. If the average value of the rolling shape ratio of the final three stands is less than 2.0, the compressive strain inside the steel sheet increases due to rolling, and formation of a rolling recrystallized texture causes the pole density of the {001} plane in the center region to decrease. As a result, Pi/Ps becomes less than 1.2.
In addition, if the average value of the rolling shape ratio of the final three stands is more than 10.0, the steel sheet surface is subjected to strong shear deformation, and the pole density of the {001} plane in the surface layer region excessively increases. As a result, Pi/Ps may be less than 1.2.
Therefore, the average value of the rolling shape ratio of the final three stands is preferably 2.0 to 10.0. That is, the average value of the rolling shape ratio at the final rolling stand, the rolling shape ratio at the rolling stand one stand before the final stand and the rolling shape ratio at the rolling stand two stands before the final stand is preferably 2.0 to 10.0.
In Formula (2), when ΔT, which is the difference between the steel sheet inlet side temperature and the steel sheet outlet side temperature at each rolling stand, is controlled, this is effective for controlling the temperature inside the steel sheet. During hot rolling, removal of heat due to contact with the rolling roll and heat generation from the inside of the steel sheet due to processing energy and frictional heat with the rolls are performed at the same time. In the latter stage of finish rolling, particularly, the sheet thickness becomes thin, and the rolling speed increases, and thus the amount of heat removed becomes small and an effect of heat generated during processing becomes large. Therefore, it is important to perform production at an appropriate sheet passing speed according to the diameter and surface condition of the rolling roll and the sheet thickness of the sheet to be produced.
If the average value of ΔT in the final three stands is less than 5, the temperature difference in the sheet thickness direction with the inside of the steel sheet decreases. As a result, the difference in the pole density of the {001} plane between the surface layer region and the center region becomes small, and Pi/Ps becomes less than 1.2.
In addition, if the average value of ΔT of the final three stands is more than 35, since the amount of heat removed from the steel sheet surface increases, the shear deformation of the steel sheet surface increases. As a result, the pole density of the {001} plane in the surface layer region excessively decreases, and Pi/Ps may be more than 2.0.
Therefore, the average value of ΔT in the final three stands is preferably 5 to 35. That is, the average value of ΔT at the final rolling stand, ΔT at the rolling stand one stand before the final stand and ΔT at the rolling stand two stands before the final stand is preferably 5 to 35.
After finish rolling is completed, it is preferable to start cooling within 1.6 sec. If the time until cooling starts exceeds 1.6 sec, since the strain due to rolling is recovered, it may not be possible to preferably control the pole density of the {001} plane in the surface layer region. As a result, Pi/Ps may be less than 1.2. The time until cooling starts is more preferably within 0.6 sec.
After finish rolling, as primary cooling, it is preferable to cool to a temperature range of 600 to 750° C. at an average cooling rate of 50° C./sec or faster. Then, it is preferable to perform air cooling for 2.0 to 6.0 sec in the temperature range. If the temperature range in which air cooling is performed is lower than 600° C. and higher than 750° C., since ferrite transformation does not proceed sufficiently, a desired amount of ferrite may not be obtained. As a result, the total area proportion of ferrite and bainite may not be a desired amount. The average cooling rate for primary cooling may be 250° C./s or slower in order to reduce extension of cooling facilities.
In addition, if the air cooling time in the temperature range of 600 to 750° C. is longer than 6.0 sec, a large amount of ferrite is formed, and the total area proportion of ferrite and bainite may not reach a desired amount. If the air cooling time in the temperature range is shorter than 2.0 sec, the area proportion of tempered martensite may increase, and the area proportion of fresh martensite may not reach a desired amount.
After the air cooling, as secondary cooling, it is preferable to cool to a temperature range of 200° C. or lower at an average cooling rate of 40° C./sec or faster. If the average cooling rate for secondary cooling is slower than 40° C./sec, since the cooling rate is less than the critical cooling rate required for martensite transformation, it may not be possible to obtain a desired amount of fresh martensite and/or tempered martensite. The average cooling rate for secondary cooling may be 250° C./s or slower in order to reduce extension of cooling facilities.
Here, in the present embodiment, the average cooling rate is a value obtained by dividing the temperature drop range of the steel sheet from start of cooling to end of cooling by the time required from start of cooling to end of cooling. The start of cooling is the time when injection of a cooling medium to a steel sheet starts in the cooling facility, and the end of cooling is the time when the steel sheet is taken out of the cooling facility.
In addition, cooling facilities include a facility having no air cooling section midway and a facility having one or more air cooling sections midway. In the present embodiment, any cooling facility may be used.
After cooling to a temperature range of 200° C. or lower by secondary cooling, the steel sheet is wound into a coil shape. Since the steel sheet is wound immediately after secondary cooling, the coiling temperature is almost equal to the cooling stop temperature during secondary cooling. If the coiling temperature is higher than 200° C., a large amount of ferrite or bainite is formed, and a desired microstructure may not be obtained. Therefore, the coiling temperature, which is the cooling stop temperature, is preferably 200° C. or lower.
Here, after coiling, the hot-rolled steel sheet may be temper-rolled according to a general method or may be pickled to remove scale formed on the surface. Alternatively, a plating treatment such as melting plating and electroplating or a chemical conversion may be performed.
According to the above production method, it is possible to stably produce the hot-rolled steel sheet according to the present embodiment.
Next, effects of one aspect of the present invention will be described in more detail with reference to examples, but conditions in the examples are one condition example used for confirming the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. In the present invention, various conditions can be used without departing from the gist of the present invention and as long as the object of the present invention can be achieved.
Steels having chemical compositions shown in Table 1A and Table 1B were melted, and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Table 3 were obtained under production conditions shown in Table 2A and Table 2B. Here, since finish rolling was performed using a finish rolling machine having seven rolling stands, the rolling shape ratio and AT in F5 (rolling stand two stands before the final stand), F6 (rolling stand one stand before the final stand) and F7 (final rolling stand) are shown in the table.
The area proportion of the microstructure, Pi/Ps, the tensile strength and the fatigue limit ratio of the obtained hot-rolled steel sheets were obtained by the above methods. The obtained measurement results are shown in Table 3.
If the tensile strength TS was 950 MPa or more, it was determined as satisfactory because the hot-rolled steel sheet had high strength. On the other hand, if the tensile strength TS was less than 950 MPa, it was determined as unsatisfactory because the hot-rolled steel sheet did not have high strength.
If the fatigue limit ratio was 0.35 or more, it was determined as satisfactory because the hot-rolled steel sheet had excellent fatigue strength. On the other hand, if the fatigue limit ratio was less than 0.35, it was determined as unsatisfactory because the hot-rolled steel sheet did not have excellent fatigue strength.
In addition, the shear processability of the hot-rolled steel sheet was evaluated by the following method.
According to JIS Z 2256:2020, three punched holes were formed by punching at a clearance of 15% and a punching speed of 3 m/s using a φ10 mm punch. For the three punched holes, the maximum length of cracks on the punched sheared surface (cross section perpendicular to the sheet surface) was measured. If the maximum length of cracks was 300 μm or more, it was determined as unsatisfactory because the hot-rolled steel sheets did not have excellent shear processability. On the other hand, if the maximum length of cracks was less than 300 μm, it was determined as satisfactory because the hot-rolled steel sheet had excellent shear processability.
0.35
10.5
40
I
1.8
1.9
38
8.0
2.3
580
20
780
1.4
300
20
23
570
30
17
1.0
610
17
0.9
350
65
23
12
340
44
0.9
380
2.2
340
11
0.8
340
13
28
325
16
24
72
330
18
25
74
1
0.31
19
375
21
75
18
0.34
24
0
0.28
27
0.7
315
31
95
0
0.8
320
32
98
0
2.3
415
Based on Table 3, it can be understood that the hot-rolled steel sheets of examples of the present invention had high strength and excellent fatigue properties and shear processability. On the other hand, it can be understood that one or more of the above properties of the hot-rolled steel sheets according to comparative examples were inferior.
According to the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability. According to the hot-rolled steel sheet of the present invention, it is possible to reduce the weight of a vehicle body of an automobile or the like, integrally mold parts, and shorten the processing process, thereby improving fuel efficiency and reducing production costs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-122173 | Jul 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/017623 | 4/12/2022 | WO |
