Patent Application
20180100213
The present invention relates to a hot-rolled steel sheet and a method for producing the hot-rolled steel sheet.
In order to compatibly achieve both collision safety and a reduction in weight of automobiles, the application of high strength steel sheets to automobiles is progressing. A high strength steel sheet contains a large amount of alloying elements to increase strength. In particular, a high strength steel sheet having a tensile strength of 980 MPa or more contains a large amount of Si and Mn.
A high strength steel sheet is usually produced by the following method. First, a slab is hot rolled to produce a hot-rolled steel sheet, which is then coiled in a coil shape. Next, the hot-rolled steel sheet is pickled, cold-rolled, and annealed.
The temperature of the hot-rolled steel sheet when being coiled into a coil shape (hereunder, referred to as “coiling temperature”) may be raised to enhance the cold workability of the hot-rolled steel sheet. If the coiling temperature is high, an internal oxidized layer is formed in the vicinity of an outer layer of the hot-rolled steel sheet. The internal oxidized layer is formed with a thickness of several tens of μm from the surface of the base metal of the hot-rolled steel sheet toward the center of the plate thickness. The internal oxidized layer reduces the surface properties, formability and weldability of the steel sheet after cold rolling (cold-rolled steel sheet). Therefore, the internal oxidized layer is removed before cold rolling, by subjecting the hot-rolled steel sheet to a pickling treatment.
Further, during production of the hot-rolled steel sheet, an oxide film (scale) is formed on the surface of the hot-rolled steel sheet. The scale reduces the surface properties, formability and weldability of the steel sheet. Therefore, similarly to the internal oxidized layer, the scale is also removed by subjecting the hot-rolled steel sheet to a pickling treatment.
However, if the internal oxidized layer or the scale is thick, an excessive workload is applied in the pickling treatment for the hot-rolled steel sheet. In addition, if the internal oxidized layer or the scale remains, as described above, the surface properties, formability and weldability of the cold-rolled steel sheet are reduced. Furthermore, the internal oxidized layer or the scale peel off during forming of the cold-rolled steel sheet, and cause surface defects such as indentation defects.
The internal oxidized layer is formed as a result of an alloying element in the base metal being selectively oxidizing. Si and Mn are easily oxidized. Therefore, an internal oxidized layer is liable to arise in a hot-rolled steel sheet in which the content of Si and Mn is high. Scale is similarly liable to become thick on a hot-rolled steel sheet having a high Si and Mn content.
In addition, as a time period in which the steel sheet has a high temperature continues, the thicknesses of the internal oxidized layer and scale increase. If the coiling temperature is raised in order to enhance the cold workability of the hot-rolled steel sheet as described above, an internal oxidized layer is more liable to arise, and is liable to be thick. This situation similarly applies with respect to scale.
Technology for suppressing the formation of the above described internal oxidized layer and scale is proposed in JP62-13520A (Patent Literature 1), JP2010-535946A (Patent Literature 2), JP2013-253301A (Patent Literature 3), JP2011-184741A (Patent Literature 4), JP2011-231391A (Patent Literature 5), JP2012-036483A (Patent Literature 6), JP2013-216961A (Patent Literature 7), JP2013-103235A (Patent Literature 8), JP2010-503769A (Patent Literature 9), JP2011-523441A (Patent Literature 10), JP2015-113505A (Patent Literature 11), JP2004-332099A (Patent Literature 12), JP2013-060657A (Patent Literature 13) and JP2011-523443A (Patent Literature 14).
According to Patent Literature 1, an antioxidant agent is coated onto a steel sheet surface. It is described in Patent Literature 1 that by this means the formation of an internal oxidized layer and scale is suppressed.
According to Patent Literature 2, a hot-rolled steel sheet is coiled at a comparatively low temperature of 530 to 580° C. It is described in Patent Literature 2 that by this means the formation of an oxidized layer is suppressed.
According to Patent Literature 3, a hot-rolled steel sheet after rolling is coiled at a temperature between 750° C. and 600° C. to form a coil. After coiling, the coil is maintained for 10 to 30 minutes, and thereafter the hot-rolled steel sheet is cooled while dispensing the coil. Subsequently, when the temperature of the hot-rolled steel sheet reaches 550° C. or less, the hot-rolled steel sheet is coiled again to form a coil. It is described in Patent Literature 3 that in this case the oxidized layer can be thin.
According to Patent Literatures 4 to 6, a heat treatment or a cooling treatment is performed on a steel sheet after hot rolling or after coiling in an atmosphere in which the oxygen concentration is reduced. It is described in the aforementioned Patent Literatures 4 to 6 that the heat treatment or cooling treatment in the atmosphere in which the oxygen concentration is reduced is effective to reduce scale and an internal oxidized layer.
According to Patent Literature 7, descaling is performed prior to coiling on a hot-rolled steel sheet after hot rolling, to thereby remove oxide scale from the surface thereof. By removing the oxide scale, an oxygen supply source that is utilized for formation of an internal oxidized layer during coil cooling decreases. It is described in Patent Literature 7 that, consequently, not only the scale but also the internal oxidized layer decreases.
In Patent Literature 8, a cooling method is proposed for uniformizing an internal oxidation amount of a hot-rolled steel sheet within a preferable range across the width direction, in the longitudinal direction thereof.
On the other hand, in Patent Literatures 9 to 14, technology that is different from the technology disclosed in the above described Patent Literatures is proposed. According to Patent Literature 9, internal oxidation is suppressed by appropriately controlling the alloy elements of a steel and the conditions for heat treatment of a hot-rolled steel sheet. Specifically, according to Patent Literature 9, Sb of a content of 0.001 to 0.1% is contained in the steel, the steel sheet is reheated to 1100 to 1250° C. and subjected to hot rolling, and coiled at a temperature of 450 to 750° C. Thereafter, the hot-rolled steel sheet is subjected to pickling and cold rolling, and is annealed at 700 to 850° C. By this means, formation of an internal oxidized layer is suppressed.
In Patent Literature 10, technology is proposed to appropriately control alloy elements to suppress formation of an oxide, and thereby improve a plating property. According to Patent Literature 10, a steel slab is used that contains 0.005 to 0.1% of Sb, and in which the relation between the contents of Ni, Mn, Al and Ti is adjusted. The steel slab is subjected to hot working, and hot rolled and coiled at 500 to 700° C. In addition, the steel slab is subjected to pickling, cold rolling and annealing. By this means, internal oxidation is suppressed.
According to Patent Literature 11, a slab containing 0.02 to 0.10% of Sb is subjected to hot rolling, pickling, cold rolling, annealing and cooling. In this case, the finish rolling temperature for the hot rolling is 800 to 1000° C., and the draft during the cold rolling is set to 20% or more. In addition, annealing is performed under conditions of being held for 60 seconds or more in a temperature range of 750 to 900° C. in an atmosphere in which a dew-point temperature is −35° C. or less. After annealing, cooling is performed to 300° C. or less at an average cooling rate of 30° C./sec or more, followed by tempering. By this means internal oxidation is suppressed.
Patent Literatures 12 to 14 describe suppressing scale by appropriately adjusting a content of Si, a heating temperature of a slab, a temperature of finish rolling and a coiling temperature and the like.
However, even when the respective technologies described in Patent Literatures 1 to 14 are implemented, in some cases a deep internal oxidized layer is formed or thick scale is formed.
An objective of the present invention is to provide a hot-rolled steel sheet in which formation of an internal oxidized layer and scale is suppressed.
A hot-rolled steel sheet according to the present embodiment has a chemical composition consisting of, in mass %, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to 0.50% and rare earth metal: 0 to 0.50%, with a balance being Fe and impurities, and satisfying Formula (1):
Si+Mn≥3.20 (1)
where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1).
In the hot-rolled steel sheet of the present embodiment, formation of an internal oxidized layer and scale is suppressed.
The present inventors conducted investigations and studies regarding an internal oxidized layer and scale with respect to a steel with a high Si and high Mn content, and obtained the following findings.
In an outer layer of a hot-rolled steel sheet after coiling, an internal oxidized layer is formed within the base metal (ground metal), and scale is formed adjoining the surface.
It is considered that the internal oxidized layer and scale are formed by the following mechanism. Oxygen ions penetrate into the hot-rolled steel sheet through grain boundaries of a near-surface portion of the hot-rolled steel sheet and the surface of the hot-rolled steel sheet. An internal oxidized layer is formed as a result of the oxygen ions that penetrated into the inside of the hot-rolled steel sheet oxidizing iron of the base metal. On the other hand, iron ions in the base metal move to the surface of the hot-rolled steel sheet through the grain boundaries. Scale is formed as a result of the Fe that moved to the surface being oxidized.
It is effective to block the movement path (grain boundaries and surface) of the oxygen ions and iron ions in order to suppress formation of an internal oxidized layer and scale. If elements that easily segregate at the grain boundaries and surface (hereunder, referred to as “segregation elements”) are contained in a hot-rolled steel sheet, the segregation elements segregate at the surface and grain boundaries of the hot-rolled steel sheet and suppress movement of oxygen ions and iron ions. Therefore, penetration of oxygen ions into the inside of the hot-rolled steel sheet can be suppressed. In addition, movement of iron ions to the hot-rolled steel sheet surface can be suppressed. As a result, formation of an internal oxidized layer and scale can be suppressed.
The segregation elements are, for example, P, B, and Sb. However, although P and B segregate at grain boundaries and block the movement path of oxygen ions and iron ions, they also reduce the mechanical properties of the hot-rolled steel sheet.
On the other hand, Sb segregates at the surface of the hot-rolled steel sheet. Therefore, the present inventors produced a hot-rolled steel sheet from steel with a high Si and high Mn content, which also contained Sb, and examined the thickness of scale and an internal oxidized layer.
Referring now to the SEM images in
In addition, Sb mapping was performed using EPMA. As a result, in the steel containing Sb, a layer 30 containing Sb (white region in the drawing; hereunder referred to as “Sb concentrated layer”) was observed at the interface between the scale 10 and the base metal.
As described above, in a case where Sb is contained in steel with a high Si and high Mn content, an Sb concentrated layer is formed. It is considered that, because of the formation of the Sb concentrated layer, the situation is as follows. In a case where steel with a high Si and high Mn content contains a suitable amount of Sb, in a hot rolling process, an Sb concentrated layer is formed at the interface (surface of the hot-rolled steel sheet) between scale and the base metal. The Sb concentrated layer blocks the penetration of oxygen ions into the base metal. Consequently, iron in the base metal is not oxidized, and it is difficult for an internal oxidized layer to form. The Sb concentrated layer also suppresses movement of iron ions contained in the base metal to the scale. Consequently, growth of the scale is suppressed, and the thickness of the scale is thin.
Thus, the Sb concentrated layer functions as a so-called “barrier layer” that blocks the movement of oxygen ions and iron ions. Therefore, by formation of the Sb concentrated layer, the penetration of oxygen ions into the base metal from the scale after coiling of the hot-rolled steel sheet can be suppressed. In addition, movement of iron ions to the scale from the base metal can be suppressed. Therefore, the formation of an internal oxidized layer and scale is suppressed.
A barrier layer like the Sb concentrated layer is not formed even if P and B that are elements segregable at grain boundaries are contained in steel with a high Si and high Mn content. Accordingly, Sb is suitable for suppressing scale and an internal oxidized layer.
In the present embodiment, the Sb concentrated layer also suppresses movement of carbon contained in the base metal, and not just movement of oxygen ions and iron ions. Consequently, it is easy to maintain a uniform micro-structure in the plate thickness direction, and the obtainment of strength in a cold-rolled steel sheet after cold rolling and annealing is facilitated.
A hot-rolled steel sheet according to the present embodiment that was completed based on the above described findings has a chemical composition consisting of, in mass %, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to 0.50% and rare earth metal: 0 to 0.50%, with a balance being Fe and impurities, and satisfying Formula (1):
Si+Mn≥3.20 (1)
where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1).
The above described chemical composition may also contain one or more types of element selected from the group consisting of Ti: 0.005 to 0.15%, V: 0.001 to 0.30% and Nb: 0.005 to 0.15%.
The above described chemical composition may also contain one or more types of element selected from the group consisting of Cr: 0.10 to 1.0%, Ni: 0.10 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to 1.0% and B: 0.0001 to 0.010%.
The above described chemical composition may also contain Cu: 0.10 to 0.50%.
The above described chemical composition may also contain one or more types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As in a total amount of 0.0001 to 0.30%.
The above described chemical composition may also contain one or more types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal in a total amount of 0.0001 to 0.50%.
A hot-rolled steel sheet according to the present embodiment includes an Sb concentrated layer having a thickness of 0.5 μm or more between the surface and scale.
In the micro-structure of the above described hot-rolled steel sheet, a total area fraction of ferrite and pearlite may be 75% or more, and the tensile strength of the hot-rolled steel sheet may be 800 MPa or less.
In the micro-structure of the above described hot-rolled steel sheet, a total area fraction of bainite and martensite may be 75% or more, and the tensile strength of the hot-rolled steel sheet may be 900 MPa or more.
In the micro-structure of the above described hot-rolled steel sheet, a total area fraction of bainite and martensite may be 75% or more, and the tensile strength of the hot-rolled steel sheet may be 800 MPa or less.
Preferably, the thickness of an internal oxidized layer of the hot-rolled steel sheet is 5 μm or less.
Preferably, in the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of ferrite and pearlite is 75% or more and which has a tensile strength of 800 MPa or less, a scale thickness is 10 μm or less.
Preferably, in the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of ferrite and pearlite is 75% or more and which has a tensile strength of 800 MPa or less, a decarburization layer thickness in an outer layer of the hot-rolled steel sheet is 20 μm or less.
Preferably, in the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of bainite and martensite is 75% or more and which has a tensile strength of 900 MPa or more, a scale thickness is 7 μm or less.
Preferably, in the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of bainite and martensite is 75% or more and which has a tensile strength of 800 MPa or less, a scale thickness is 7 μm or less.
A production method for producing the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of ferrite and pearlite is 75% or more and which has a tensile strength of 800 MPa or less includes: a process of preparing a steel material having the above described chemical composition; a process of heating the steel material to 1100 to 1350° C. and thereafter performing hot rolling so as to form the steel material into a steel sheet; and a process of coiling the steel sheet at a temperature of 600 to 750° C., preferably 650 to 750° C., and more preferably 700 to 750° C.
A production method for producing the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of bainite and martensite is 75% or more and which has a tensile strength of 900 MPa or more includes: a preparation process of preparing a steel material having the above described chemical composition; a hot rolling process of heating the steel material to 1100 to 1350° C., thereafter performing hot rolling so as to form the steel material into a steel sheet, and cooling the steel sheet to a coiling temperature; and a process of coiling the steel sheet after cooling, at a temperature of 150 to 600° C., preferably 350 to 500° C., and more preferably 400 to 500° C.
A production method for producing the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of bainite and martensite is 75% or more and which has a tensile strength of 800 MPa or less includes: a preparation process of preparing a steel material having the above described chemical composition; a hot rolling process of heating the steel material to 1100 to 1350° C., thereafter performing hot rolling so as to form the steel material into a steel sheet, and cooling the steel sheet to a coiling temperature; a process of coiling the steel sheet after cooling, at a temperature of 150 to 600° C., preferably 350 to 500° C., and more preferably 400 to 500° C.; and a process of tempering the steel sheet after coiling at a temperature of 550° C. or more.
Hereunder, hot-rolled steel sheets according to the present embodiments are described in detail.
The chemical composition of the hot-rolled steel sheet according to the present embodiment contains the following elements. The symbol “%” with respect to the chemical composition means “percent by mass” unless specified otherwise.
C: 0.07 to 0.30%
Carbon (C) forms retained austenite in the hot-rolled steel sheet and enhances the strength and formability of the steel. If the C content is too low, the aforementioned effect will not be obtained. On the other hand, if the C content is too high, the strength of the hot-rolled steel sheet will be too high and a cold rolling property will be reduced. If the C content is too high, the weldability of the steel will also decrease. Therefore, the C content is from 0.07 to 0.30%. A preferable lower limit of the C content is 0.10%, more preferably is 0.12%, and further preferably is 0.15%. A preferable upper limit of the C content is 0.25%, and more preferably is 0.22%.
Si: More than 1.0 to 2.8%
Silicon (Si) suppresses the formation of iron-based carbides and facilitates formation of retained austenite. The strength and formability of the steel is improved by formation of retained austenite. If the Si content is too low, the aforementioned effect will not be obtained. On the other hand, if the Si content is too high, an internal oxidized layer will grow noticeably and the surface properties of the hot-rolled steel sheet will decrease. If the Si content is too high, the hot-rolled steel sheet will also become brittle and the ductility will decrease. Therefore, the Si content is from more than 1.0 to 2.8%. A preferable lower limit of the Si content is 1.3%, and more preferably is 1.5%. A preferable upper limit of the Si content is 2.5%, and more preferably is 2.0%.
Mn: 2.0 to 3.5%
Manganese (Mn) increases the strength of the steel sheet. If the Mn content is too low, a large amount of soft micro-structure is formed during cooling after annealing, and the strength is lowered. On the other hand, if the Mn content is too high, coarse Mn concentrated parts form at a central portion of the plate thickness and the steel becomes brittle. Consequently, a slab that is cast is liable to crack. If the Mn content is too high, the weldability of the steel also decreases. If the Mn content is too high, the hot-rolled steel sheet will also harden and a cold rolling property will decrease. Therefore, the Mn content is from 2.0 to 3.5%. A preferable lower limit of the Mn content is 2.2%, more preferably is 2.3%, and further preferably is 2.5%. A preferable upper limit of the Mn content is 3.2%, and more preferably is 3.0%.
P: 0.030% or Less
Phosphorus (P) segregates at a central portion of the plate thickness of the steel sheet and embrittles a weld zone. Therefore, the P content is 0.030% or less. A low P content is preferable. However, making the P content low increases the production costs. Therefore, when taking the production cost into consideration, the lower limit of the P content is, for example, 0.0010%.
S: 0.010% or Less
Sulfur (S) reduces the weldability of the steel. S also reduces the producibility during casting and heat rolling. S also combines with Mn to form MnS, and reduces the ductility and stretch flangeability of the steel. Therefore, the content of S is 0.010% or less. A preferable upper limit of the S content is 0.005%, and more preferably is 0.0025%. A lower limit of the S content is not particularly limited. However, when taking the production cost into consideration, the lower limit of the S content is, for example, 0.0001%.
Al: 0.01 to Less than 1.0%
Aluminum (Al) suppresses formation of iron-based carbides and facilitates formation of retained austenite. The strength and formability of the steel is enhanced by the formation of retained austenite. Al also deoxidizes the steel. If the Al content is too low, the aforementioned effects are not obtained. On the other hand, if the Al content is too high, the weldability of the steel decreases. Therefore, the Al content is from 0.01 to less than 1.0%. A preferable lower limit of the Al content is 0.02%. A preferable upper limit of the Al content is 0.8%, and more preferably is 0.5%. In the present description, the “Al” content means the content of “sol. Al” (acid-soluble Al).
N: 0.01% or Less
Nitrogen (N) forms coarse nitrides and reduces the ductility and stretch flangeability of the steel. N is also a cause of the occurrence of blowholes during welding. Therefore, a low N content is preferable. The N content is 0.01% or less. A preferable upper limit of the N content is 0.005%. The lower limit of the N content is not particularly limited. However, when taking the production cost into consideration, the lower limit of the N content is, for example, 0.0001%.
O: 0.01% or Less
Oxygen (O) forms oxides and reduces the toughness and stretch flangeability of the steel. Therefore, a low O content is preferable. The O content is 0.01% or less. A preferable upper limit of the O content is 0.008%, and more preferably is 0.006%. The lower limit of the O content is not particularly limited. However, when taking the production cost into consideration, a preferable lower limit of the O content is, for example, 0.0001%.
Sb: 0.03 to 0.30%
Antimony (Sb) is, as described above, an element that easily segregates at the surface of the steel. Sb forms an Sb concentrated layer in the surface (interface between scale and base metal) of the hot-rolled steel sheet during hot rolling. The Sb concentrated layer suppresses penetration of oxygen ions into the inside of the hot-rolled steel sheet from grain boundaries that are exposed on the surface of the hot-rolled steel sheet. The Sb concentrated layer also suppresses movement of iron ions contained in the base metal to scale. Therefore, formation of an internal oxidized layer in the hot-rolled steel sheet and the growth of scale are suppressed. Sb also restricts movement of C to suppress formation of a decarburization layer.
If the Sb content is too low, it is difficult to form an Sb concentrated layer and the above described effects are not obtained. On the other hand, if the Sb content is too high, the workability of the steel sheet decreases. Further, if the Sb content is too high, the mechanical properties of the hot-rolled steel sheet also decrease. Therefore, the Sb content is from 0.03 to 0.30%. A preferable lower limit of the Sb content is 0.05%, more preferably is 0.07%, further preferably is 0.10%, and further preferably is 0.11%. A preferable upper limit of the Sb content is 0.25%, and more preferably is 0.20%.
The chemical composition of the above described hot-rolled steel sheet further satisfies Formula (1):
Si+Mn≥3.20 (1)
where, a content (mass %) of a corresponding element is substituted for each symbol of an element in Formula (1).
If the total content of Si and Mn is less than 3.20%, retained austenite will not be stable during annealing performed after cold rolling. In this case, there is a possibility that the strength or ductility of the steel sheet after annealing will be low. Therefore, the lower limit of the total content of Si and Mn is 120%. In this case, the strength and ductility of the steel sheet will be high even after cold rolling and annealing. The lower limit of the total content of Si and Mn is preferably 3.50%. On the other hand, if the total content of Si and Mn is 5.0% or less, a phase transformation delay during annealing can be suppressed. Therefore, carbon (C) sufficiently concentrates in untransformed austenite, and the retained austenite is more stable. Accordingly, an upper limit of the total content of Si and Mn is preferably 5.0%, and more preferably is 4.5%.
The balance of the chemical composition of the hot-rolled steel sheet of the present embodiment is Fe and impurities. Here, the term “impurities” refers to elements that, during industrial production of the hot-rolled steel sheet, are mixed in from ore or scrap used as a raw material, or from the production environment or the like, and that are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
The chemical composition of the hot-rolled steel sheet that is described above may contain optional elements that are described hereunder in addition to the above described essential elements. The chemical composition need not contain optional elements.
The above described chemical composition may contain one or more types of element selected from the group consisting of Ti, V and Nb as a substitute for a part of Fe. Each of Ti, V and Nb is an optional element, and each element increases the strength of the steel.
Ti: 0 to 0.15%
Titanium (Ti) is an optional element, and need not be contained in the steel. In a case where Ti is contained, Ti forms carbo-nitrides and increases the strength of the steel. Ti also contributes to fine-grain strengthening of the steel by suppressing growth of ferrite grains. Ti also contributes to dislocation strengthening of the steel through suppression of recrystallization. However, if the Ti content is too high, carbo-nitrides are excessively formed and the formability of the steel decreases. Therefore, the Ti content is 0 to 0.15%. A preferable upper limit of the Ti content is 0.10%, and more preferably is 0.07%. A preferable lower limit of the Ti content is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.
V: 0 to 0.30%
Vanadium (V) is an optional element, and need not be contained in the steel. In a case where V is contained, similarly to Ti, V increases the strength of the steel by precipitation strengthening, fine-grain strengthening and dislocation strengthening. However, if the V content is too high, carbo-nitrides precipitate excessively and the formability of the steel decreases. Therefore, the V content is from 0 to 0.30%. A preferable upper limit of the V content is 0.20%, and more preferably is 0.15%. A preferable lower limit of the V content is 0.001%, and more preferably is 0.005%.
Nb: 0 to 0.15%
Niobium (Nb) is an optional element, and need not be contained in the steel. In a case where Nb is contained, similarly to Ti and V, Nb increases the strength of the steel by precipitation strengthening, fine-grain strengthening and dislocation strengthening. However, if the Nb content is too high, carbo-nitrides precipitate excessively and the formability of the steel decreases. Therefore, the Nb content is from 0 to 0.15%. A preferable upper limit of the Nb content is 0.10%, and more preferably is 0.06%. A preferable lower limit of the Nb content is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.
The above described chemical composition may contain one or more types of element selected from the group consisting of Cr, Ni, Mo, W and B as a substitute for a part of Fe. Each of Cr, Ni, Mo, W and B is an optional element, and each element increases the strength of the steel.
Cr: 0 to 1.0%
Chromium (Cr) is an optional element, and need not be contained in the steel. In a case where Cr is contained, Cr suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the Cr content is too high, the workability of the steel decreases and productivity is reduced. Therefore, the Cr content is from 0 to 1.0%. A preferable lower limit of the Cr content is 0.10%.
Ni: 0 to 1.0%
Nickel (Ni) is an optional element, and need not be contained in the steel. In a case where Ni is contained, Ni suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the Ni content is too high, the weldability of the steel decreases. Therefore, the Ni content is from 0 to 1.0%. A preferable lower limit of the Ni content is 0.10%.
Mo: 0 to 1.0%
Molybdenum (Mo) is an optional element, and need not be contained in the steel. In a case where Mo is contained, Mo suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the Mo content is too high, the hot workability of the steel decreases and productivity is reduced. Therefore, the Mo content is from 0 to 1.0%. A preferable lower limit of the Mo content is 0.01%.
W: 0 to 1.0%
Tungsten (W) is an optional element, and need not be contained in the steel. In a case where W is contained, W suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the W content is too high, the hot workability of the steel decreases and productivity is reduced. Therefore, the W content is from 0 to 1.0%. A preferable lower limit of the W content is 0.01%.
B: 0 to 0.010%
Boron (B) is an optional element, and need not be contained in the steel. In a case where B is contained, B suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the B content is too high, the hot workability of the steel decreases and productivity is reduced. Therefore, the B content is from 0 to 0.010%. A preferable upper limit of the B content is 0.005%, and more preferably is 0.003%. A preferable lower limit of the B content is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
The above described chemical composition may contain Cu as a substitute for a part of Fe.
Cu: 0 to 0.50%
Copper (Cu) is an optional element, and need not be contained in the steel. In a case where Cu is contained, Cu precipitates in the steel as fine particles and increases the strength of the steel. However, the weldability of the steel will decrease if the Cu content is too high. Therefore, the Cu content is from 0 to 0.50%. A preferable lower limit of the Cu content is 0.10%.
The above described chemical composition may contain one or more types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As as a substitute for a part of Fe. Each of Sn, Bi, Se, Te, Ge and As is an optional element, and each element suppresses formation of an internal oxidized layer.
Sn: 0 to 0.30%
Bi: 0 to 0.30%
Se: 0 to 0.30%
Te: 0 to 0.30%
Ge: 0 to 0.30%
As: 0 to 0.30%
Tin (Sn), bismuth (Bi), selenium (Se), tellurium (Te), germanium (Ge) and arsenic (As) are optional elements, and need not be contained in the steel. If contained, these elements suppress formation of an internal oxidized layer by suppressing segregation of Mn and Si. However, if the content of these elements is too high, the formability of the steel decreases. Therefore, the Sn content is from 0 to 0.30%, the Bi content is from 0 to 0.30%, the Se content is from 0 to 0.30%, the Te content is from 0 to 0.30%, the Ge content is from 0 to 0.30% and the As content is from 0 to 0.30%. A preferable upper limit of the Sn content is 0.25%, and more preferably is 0.20%. A preferable upper limit of the Bi content is 0.25%, and more preferably is 0.20%. A preferable upper limit of the Se content is 0.25%, and more preferably is 0.20%. A preferable upper limit of the Te content is 0.25%, and more preferably is 0.20%. A preferable upper limit of the Ge content is 0.25%, and more preferably is 0.20%. A preferable upper limit of the As content is 0.25%, and more preferably is 0.20%. A preferable lower limit of the Sn content is 0.0001%. A preferable lower limit of the Bi content is 0.0001%. A preferable lower limit of the Se content is 0.0001%. A preferable lower limit of the Te content is 0.0001%. A preferable lower limit of the Ge content is 0.0001%. A preferable lower limit of the As content is 0.0001%. Note that, when two or more types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As are to be contained in the steel, it is preferable to make the total content thereof from 0.0001 to 0.30%.
The above described chemical composition may contain one or more types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal (REM) as a substitute for a part of Fe. Each of these elements is an optional element, and each element increases the formability of the steel.
Ca: 0 to 0.50%
Mg: 0 to 0.50%
Zr: 0 to 0.50%
Hf: 0 to 0.50%
Rare earth metal (REM): 0 to 0.50%
Calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf) and rare earth metal (REM) are each an optional element and need not be contained in the steel. If contained, these elements enhance the formability of the steel. However, if the content of these elements is too high, the ductility of the steel decreases. Therefore, the Ca content is from 0 to 0.50%, the Mg content is from 0 to 0.50%, the Zr content is from 0 to 0.50%, the Hf content is from 0 to 0.50%, and the rare earth metal (REM) content is from 0 to 0.50%. A preferable lower limit of the Ca content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. A preferable lower limit of the Mg content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. A preferable lower limit of the Zr content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. A preferable lower limit of the Hf content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. A preferable lower limit of the rare earth metal (REM) content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. Note that, when two or more types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal (REM) are to be contained in the steel, it is preferable to make the total content thereof from 0.0001 to 0.50%.
The term “REM” as used in the present description refers to one or more types of element selected from Sc, Y, and lanthanoids (elements with atomic numbers 57 through 71 from La to Lu). The term “REM content” refers to the total content of these elements.
The micro-structure of the hot-rolled steel sheet of the present embodiment is not particularly limited. The micro-structure of the hot-rolled steel sheet of the present embodiment, for example, mainly consists of ferrite and pearlite. Specifically, in the micro-structure, the combined area fraction of ferrite and pearlite is 75% or more. In the micro-structure, a region (balance) other than ferrite and pearlite is one or more types of micro-structure selected from the group consisting of bainite (including tempered bainite), martensite (including tempered martensite) and retained austenite.
If the total area fraction of ferrite and pearlite in the micro-structure is 75% or more, the strength of the hot-rolled steel sheet can be suppressed. In this case, the cold workability is enhanced.
The area fraction of the respective phases can be determined by the following methods.
The hot-rolled steel sheet is cut along a plane perpendicular to the rolling direction. The cut surface is mirror polished. Of the entire area of the cut surface that was mirror polished, a width-wise central portion of the hot-rolled steel sheet (range of ±10 mm in the width direction from the center in the width direction) that is a range of ±5 mm from a position that is equal to ¼ of the plate thickness from the surface is defined as an observation region. The observation region is corroded with a nital etching reagent. After corrosion, an arbitrary range of 200 μm×150 μm of the observation region is photographed using a scanning electron microscope (SEM). Ferrite and pearlite are identified using the image of the region that was photographed (hereunder, referred to as “photographing region”). The total of the areas of ferrite and pearlite that are identified is determined, and the determined total area is divided by the sum total of the areas of the entire photographing region to obtain the total area fraction (%) of ferrite and pearlite. The areas of ferrite and pearlite are measured using a mesh method or image processing software (product name: ImagePro).
A method for measuring the area fraction of bainite and martensite is as follows. A photographing region (200 μm×150 μm) that is the same as in the above described method for measuring an area fraction of ferrite and pearlite is photographed using an electron backscattering diffraction method (EBSD method) to generate a photographic image.
A portion excluding pearlite and retained austenite is extracted from the photographic image by image processing. With respect to a low temperature transformation phase of the remaining region, 15 degrees is defined as a threshold value of an orientation difference with adjacent grains, and grains are identified. With respect to each identified grain, the average visibility of the Kikuchi Diffraction pattern in the grains (Grain Average Image Quality: GAIQ) is converted into numerical values. A histogram of area fractions is created with respect to the GAIQ values that were converted into numerical values. In a case where the created histogram has two peaks, a distribution on a side on which the GAIQ is high is taken as originating from bainite, and a distribution on a side on which the GAIQ is low is taken as originating from martensite. The total area fraction of grains having a GAIQ identified as originating from bainite is defined as a bainite area fraction. In a case where two peaks are overlapping in the histogram, grains that are identified by the GAIQ up to a boundary at which the distributions overlap are defined as bainite, and the area fraction of the bainite is determined.
A value (%) obtained by deducting the sum total (%) of the aforementioned ferrite area fraction, pearlite area fraction and bainite area fraction as well as a retained austenite area fraction that is described later from 100(%) is defined as the area fraction of martensite.
The area fraction of retained austenite is determined by X-ray diffractometry. Specifically, in a photographing region (200 μm×150 pin) that is the same as in the above described method for measuring an area fraction of ferrite and pearlite, the proportion of retained austenite is determined experimentally by X-ray diffractometry using the property that the reflection surface intensity differs between austenite and ferrite. A retained austenite area fraction Vγ is determined using the following formula based on an image obtained by X-ray diffractometry using the Kα ray of Mo:
Vγ=(⅔){100/(0.7×α(211)/γ(220)+1)}+(⅓){100/(0.78×α(211)/γ(311)+1)}
where, α(211) represents the reflection surface intensity at a (211) surface of ferrite, γ(220) represents the reflection surface intensity at a (220) surface of austenite, and γ(311) represents the reflection surface intensity at a (311) surface of austenite.
A preferable tensile strength of the hot-rolled steel sheet of the present embodiment is 800 MPa or less, and more preferably is 700 MPa or less. Because the tensile strength is low, the cold workability is enhanced. Although a lower limit of the tensile strength is not particularly limited, for example, the lower limit is 400 MPa. The tensile strength can be determined by a tensile testing method for metal materials in accordance with JIS Z 2241 (2011).
As described above, an Sb concentrated layer is formed at an interface between a base metal surface and scale of a hot-rolled steel sheet. The existence or non-existence of the Sb concentrated layer can be observed by electron probe microanalysis (EPMA). Specifically, the hot-rolled steel sheet is cut along a plane perpendicular to the rolling direction, and of the entire cut surface, an arbitrary region of 50 μm in the width direction×45 μm in the depth direction of the hot-rolled steel sheet among a width-wise central portion (range of ±10 mm in the width direction from the center in the width direction) that includes the surface is defined as an observation region. A sample including the observation region is extracted. Mapping analysis using EPMA is conducted with respect to the observation region. A portion at which an Sb concentration is 1.5 times or more greater than the region average is defined as an Sb concentrated layer. In a case where an Sb concentrated layer is confirmed in 90% or more of the width (50 μm) of the observation region, it is determined that an Sb concentrated layer is formed.
The thickness of the identified Sb concentrated layer is measured at a pitch of 5 μm in the width direction of the observation region, and an average value thereof is defined as the thickness of the Sb concentrated layer. A preferable thickness of the Sb concentrated layer is 0.5 μm or more, more preferably is 1.0 μm or more, and further preferably is 1.5 μm or more.
At a high temperature, Sb segregates at grain boundaries and the surface of the steel, and in particular there is a strong tendency for Sb to segregate and concentrate at the surface. Even in a case where scale covers the base metal, Sb segregates strongly toward the base metal surface. To sufficiently induce segregation of Sb and promote the formation of an Sb concentrated layer, it is preferable to retain the hot-rolled steel sheet in a high temperature region for a long time period. The Sb concentrated layer is also formed during hot rolling, and is extended by rolling. Therefore, as described above, the finish rolling temperature is preferably a high temperature.
In the hot-rolled steel sheet of the present embodiment, because an Sb concentrated layer is formed, the thickness of an internal oxidized layer is suppressed. A preferable thickness of the internal oxidized layer is 5 μm or less.
The internal oxidized layer is measured by the following method. A small piece that includes a part of the surface of the hot-rolled steel sheet is cut out from an arbitrary position within a width-wise central portion (range of ±10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet. Of the entire surface of the small piece, a cross-section that is perpendicular to the rolling direction (hereunder, referred to as “observation face”) is mirror polished. The observation face is subjected to C vapor deposition. After the C vapor deposition, a portion in the vicinity of the surface of the observation face is photographed for arbitrary visual fields at a magnification of ×1000 using a field-emission scanning electron microscope (FE-SEM) to obtain images (each visual field is 200 μm×180 μm). The thickness (μm) of the internal oxidized layer is determined based on the obtained images. Oxides of Si and Mn arise in the base metal in the internal oxidized layer. Therefore, the scale, the internal oxidized layer and the base metal can be easily distinguished by means of a backscattered electron image obtained by a backscattered electron detector that is normally mounted in a common SEM.
In the obtained image, a distance from the interface of the scale and base metal to the lowest edge of the internal oxidized layer is determined at each interval of 10 μm in the rolling direction. This measurement is performed for an arbitrary three visual fields, and the average value of the obtained distances is defined as the internal oxidized layer thickness (μm).
In the hot-rolled steel sheet of the present embodiment, because the Sb concentrated layer is formed, formation of scale is also suppressed. A preferable thickness of the scale is 10 μm or less.
The scale thickness is measured by the following method. Images are obtained using an FE-SEM, similarly to when measuring the thickness of the internal oxidized layer. In the obtained images (it is sufficient to use the same images as when measuring the internal oxidized layer), the scale is identified, and a distance between an uppermost edge of the scale and the interface is determined at each interval of 10 μm in the rolling direction. This measurement is performed for an arbitrary three visual fields, and the average value of the obtained distances is defined as the scale thickness (μm).
In the hot-rolled steel sheet of the present embodiment, because the Sb concentrated layer is formed, the thickness of a decarburization phase is also suppressed. A preferable thickness of the decarburization layer is 20 μm or less.
The decarburization layer is measured by the following method. A small piece that includes a part of the surface of the hot-rolled steel sheet is cut out from an arbitrary position within a width-wise central portion (range of ±10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet. Line analysis of the C-Kα line is performed by means of EPMA with respect to the surface of the small piece, and C strengths (line analysis results) in the depth direction from the surface of the steel sheet are obtained. Among the obtained line analysis results, a distance from a position at which the C strength is smallest in the steel sheet to a depth position at which a difference between the average C strength (C strength of the base metal) of the steel sheet and the smallest C strength in the steel sheet is 98% is defined as the thickness (μm) of the decarburization layer.
As described above, in the hot-rolled steel sheet of the present embodiment, an Sb concentrated layer suppresses formation of an internal oxidized layer. The Sb concentrated layer also suppresses formation of scale. Furthermore, the Sb concentrated layer suppresses formation of a decarburization layer. In addition, in the hot-rolled steel sheet of the present embodiment, a total area fraction of ferrite and pearlite in the micro-structure is 75% or more. Therefore, the tensile strength is suppressed to 800 MPa or less, preferably 700 MPa or less, and the hot-rolled steel sheet is thus excellent in cold workability.
The hot-rolled steel sheet of the present embodiment may also be subjected to descaling that is described later. In this case, an area fraction of the surface of island-shaped scale that is caused by fayalite and formed on the surface is lowered. Consequently, a pickling property is further enhanced.
An example of a method for producing the above described hot-rolled steel sheet will now be described. The production method includes a preparation process, a hot rolling process and a coiling process.
In the preparation process, a steel material having the above described chemical composition is prepared. Specifically, molten steel having the above described chemical composition is produced. A slab as the steel material is produced using the molten steel. The slab may be produced by a continuous casting process. Alternatively, an ingot may be produced using the molten steel, and the ingot may be subjected to blooming to produce the slab.
The prepared steel material (slab) is heated. The heating temperature is from 1100 to 1350° C. Preferably, the heating time period is set to 30 minutes or more. The heated slab is subjected to hot rolling using a roughing mill and a finish rolling mill and made into a steel sheet. The roughing mill includes a plurality of roll stands that are arranged in a single row, with each roll stand having a pair of rolls. The roughing mill may be a reverse-type roughing mill. The finish rolling mill includes a plurality of roll stands that are arranged in a single row, with each roll stand having a pair of rolls.
During the hot rolling, descaling may also be performed on the steel sheet that is being subjected to rolling, by means of one or a plurality of high-pressure water descaling devices that are installed between the plurality of roll stands (roughing mill or finish rolling mill). The descaling is preferably performed on the steel sheet having a temperature of 1050° C. or more. In this case, Fe2SiO4 (fayalite) arising on the surface of a steel with a high Si and high Mn content, as in a steel sheet having the chemical composition of the present embodiment, can be effectively removed. If fayalite remains, island-shaped scale will be formed on the surface of the hot-rolled steel sheet. If island-shaped scale remains on the surface of the hot-rolled steel sheet, it is difficult to remove the scale during pickling. Further, if fayalite remains, indentation defects arise during cold rolling, and such defects may mar the external appearance of the cold-rolled steel sheet. Fayalite can be removed by performing descaling.
In a case where one or a plurality of high-pressure water descaling devices are installed between finishing roll stands during finish rolling, it is preferable that a steel sheet (rough bar) in a state after rough rolling and before finish rolling is heated to 1050° C. or more by a heating apparatus arranged in the vicinity of the entrance side of the initial roll stand of the finish rolling mill. The method of heating the rough bar is not particularly limited. For example, the rough bar is heated by an induction heating apparatus or a reflow furnace.
In hot rolling, the surface temperature of the steel sheet of the exit side of the final stand of the finish rolling mill is defined as a finish rolling temperature FT (° C.). A preferable finish rolling temperature FT (° C.) is the Ar3 transformation temperature +50° C. or more. If the finish rolling temperature FT is less than the Ar3 transformation temperature +50° C., rolling resistance of the steel sheet increases and productivity decreases. In addition, the steel sheet is rolled in a two-phase region of ferrite and austenite. In this case, the micro-structure of the steel sheet forms a layered micro-structure and the mechanical properties decrease. Therefore, the finish rolling temperature FT is the Ar3 transformation temperature +50° C. or more. A preferable finish rolling temperature FT is more than 920° C., and more preferably is 950° C. or more.
After finish rolling is completed, the steel sheet is cooled to the coiling temperature. The cooling method is not particularly limited. The cooling methods are, for example, water cooling, forced air-cooling and allowing cooling.
The hot-rolled steel sheet produced in the hot rolling process is coiled to form a coil. A surface temperature (hereunder, referred to as “coiling temperature”) CT of the hot-rolled steel sheet when starting coiling of a coil is preferably from 600° C. to 750° C.
If the coiling temperature CT is too high, formation of an internal oxidized layer in the hot-rolled steel sheet is promoted. On the other hand, if the coiling temperature CT is too low, in steel that contains a large amount of Si, as in the case of the hot-rolled steel sheet of the present embodiment, the strength of the hot-rolled steel sheet will be too high and the cold rolling property will decrease.
If the coiling temperature CT is made a temperature in the range of 600° C. to 750° C., an increase in the strength of the hot-rolled steel sheet can be suppressed, and formation of an internal oxidized layer in the steel composition defined in the present embodiment is suppressed. The coiling temperature CT is preferably from 650° C. to 750° C., and more preferably is from 700° C. to 750° C.
The hot-rolled steel sheet of the present embodiment can be produced by the above described processes. Note that, the above described production method is one example of a method for producing a hot-rolled steel sheet in which a total area fraction of ferrite and pearlite is 75% or more, and a method for producing the hot-rolled steel sheet of the present embodiment is not limited thereto.
The micro-structure of the hot-rolled steel sheet may be a micro-structure that consists mainly of bainite and martensite. Specifically, a combined area fraction of bainite and martensite may be 75% or more.
The chemical composition of a hot-rolled steel sheet according to the present embodiment (second embodiment) is the same as the chemical composition of the hot-rolled steel sheet of the first embodiment, and satisfies Formula (1). If the chemical composition does not satisfy Formula (1), the ductility of the cold-rolled steel sheet may decrease. If Formula (1) is satisfied, excellent ductility is obtained in a cold-rolled steel sheet after annealing also.
On the other hand, the micro-structure of the hot-rolled steel sheet of the present embodiment is different from the first embodiment. In the micro-structure of the hot-rolled steel sheet of the present embodiment, a combined area fraction of bainite and martensite is 75% or more.
A region (balance) other than bainite and martensite is one or more types of micro-structure selected from the group consisting of ferrite, pearlite and retained austenite. In the present embodiment, preferably tempering is performed on the hot-rolled steel sheet after coiling. By this means, the strength of the steel sheet can be reduced to a certain extent, and the cold workability can be enhanced while maintaining a certain degree of strength. If tempering is performed, the bainite is mainly tempered bainite, and the martensite is mainly tempered martensite. Methods for measuring the area fractions of the respective phases in the micro-structure are the same as in the first embodiment.
The hot-rolled steel sheet of the present embodiment has the above described chemical composition and micro-structure. In a case where tempering is not performed after coiling, the tensile strength of the hot-rolled steel sheet of the present embodiment is 900 MPa or more.
On the other hand, if tempering is performed after coiling, the tensile strength of the hot-rolled steel sheet is 800 MPa or less. In this case the cold workability can be enhanced, and the load placed on the equipment system during cold rolling can be reduced. Although the lower limit of the tensile strength is not particularly limited, for example, the lower limit is 400 MPa. The tensile strength is determined by a method that is in accordance with JIS Z 2241 (2011).
An example of a method for producing the hot-rolled steel sheet according to the present embodiment will now be described. The production method includes a preparation process, a hot rolling process and a coiling process. In comparison to the production method of the first embodiment, in the present embodiment the coiling temperature CT in the coiling process differs from the first embodiment. Preferably, tempering is also performed after the coiling process. The other processes are the same as in the first embodiment.
The steel sheet produced in the hot rolling process is coiled to form a coil. If the surface temperature (coiling temperature) of the steel sheet when starting coiling of a coil is too low, the strength of the steel sheet increases and the load placed on a coiling apparatus becomes large. Therefore, the surface temperature (coiling temperature) CT of the steel sheet when starting coiling is from 150 to 600° C., preferably from 350 to 500° C., and more preferably from 400° C. to 500° C.
Because the coiling temperature CT of the hot-rolled steel sheet of the present embodiment is 600° C. or less, preferably 500° C. or less, the hardness of the hot-rolled steel sheet is high. Therefore, tempering may be performed to lower the strength and enhance the cold rolling property. In the tempering, the steel sheet after coiling is tempered at a temperature of 550° C. or more (Ac1 transformation temperature or less). If the tempering time period is too short, it is difficult to obtain the above described effect. On the other hand, if the tempering time period is too long, the effect saturates. Therefore, a preferable tempering time period is 0.5 to 8 hours in a temperature range of 550° C. or more.
The hot-rolled steel sheet of the second embodiment can be produced by the above described processes.
Note that tempering need not be performed. In a case where tempering is not performed also, in the micro-structure of the hot-rolled steel sheet, a combined area fraction of bainite and martensite is 75% or more, and the balance is one or more types of micro-structure selected from the group consisting of ferrite, pearlite and retained austenite. However, the bainite micro-structure and martensite micro-structure in a case where tempering is not performed are not micro-structures that are mainly composed of tempered bainite and tempered martensite, but rather are micro-structures mainly composed of bainite and martensite that contain some tempered bainite and tempered martensite formed during the coiling process.
In a case where tempering is not performed, the tensile strength of the hot-rolled steel sheet is 900 MPa or more. A hot-rolled steel sheet that has not been subjected to tempering is useful in particular in a case where a high tensile strength is required as a hot-rolled steel sheet and the like.
Note that the above described production method is one example of a method for producing a hot-rolled steel sheet in which the total area fraction of bainite and martensite is 75% or more, and a method for producing the hot-rolled steel sheet of the present embodiment is not limited to the above described method.
In the above described first and second embodiments, the micro-structure is defined. However, the micro-structure of a hot-rolled steel sheet of the present embodiments is not particularly limited. As long as the above described chemical composition and Formula (1) are satisfied, an Sb concentrated layer can be formed and formation of an internal oxidized layer and/or scale can be suppressed while maintaining the necessary workability and strength.
The above described production method is merely one example. Accordingly, in some cases the hot-rolled steel sheets of the first and second embodiments can also be produced by other production methods.
Note that, in the finish rolling of the hot rolling process, it is preferable to make the average cooling rate (hereunder, referred to as “ADFT”) from the temperature of the steel sheet at the time when the final descaling is performed until the temperature of the steel sheet reaches 800° C. by cooling after the finish rolling, a rate of 10° C./sec or more. In this case, formation of scale on the hot-rolled steel sheet surface can be further suppressed.
Examples of the hot-rolled steel sheet of the present invention will now be described. The conditions adopted in the Examples are merely one example of conditions adopted to confirm the operability and advantageous effects of the present invention. Therefore, the present invention is not limited to this one example of the conditions. The present invention can adopt various conditions as long as the objective of the present invention is achieved without departing from the gist of the present invention.
Molten steels having the chemical compositions shown in Table 1 were produced.
0.01
0.429
2.88
0.70
0.40
5.61
0.02
Referring to Table 1, steel types A to O are within the range of the chemical composition of the steel material of the present embodiments. On the other hand, the chemical compositions of steel types P to U are outside the range of the chemical composition of the steel material of the present embodiments.
Steel materials (ingots) were produced by an ingot-making process using the above described molten steel. Hot-rolled steel sheets were produced by hot rolling the steel materials under the hot rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 2 using a hot rolling mill for testing that was composed of a plurality of hot rolling stands. In addition, a heat history equivalent to a coil that was coiled at a coiling temperature CT (° C.) shown in Table 2 was imparted to the respective hot-rolled steel sheets after hot rolling by an N2-purged annealing furnace.
Note that, a reheating furnace simulating a rough bar heater was installed on the entrance side of the finish rolling mill, and reheating of the hot-rolled steel sheets was conducted under the conditions shown in Table 2. Further, a high-pressure water descaling device was disposed between roll stands of the finish rolling mill, and descaling was performed with respect to steel sheets undergoing finish rolling. The surface temperatures (descaling temperatures) of the respective steel sheets immediately before performing descaling were as shown in Table 2.
A small piece was cut out from a width-wise central portion (range of ±10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet of each test number. Of the entire surface of the small piece, a cross-section (hereunder, referred to as “observation face”) perpendicular to the rolling direction was mirror polished. The observation face was subjected to C (carbon) vapor deposition. After the C vapor deposition, a portion in the vicinity of the surface of the observation face was photographed using a field-emission scanning electron microscope (FE-SEM) and images were obtained. The thickness (μm) of the internal oxidized layer and the scale thickness (μm) were determined by the above described method using the obtained images.
A small piece was cut out from a width-wise central portion (range of ±10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet of each test number. Line analysis of C-Kα line was performed by means of EPMA with respect to the surface of the small piece. Among the obtained line analysis results, a distance from a position at which the C strength was smallest in the steel sheet to a depth position at which a difference between the average C strength (C strength of the base metal) of the steel sheet and the smallest C strength in the steel sheet was 98% was defined as the thickness (μm) of the decarburization layer.
The presence or absence of an Sb concentrated layer and the thickness (μm) of an Sb concentrated layer were measured by the measurement method described in the first embodiment.
No. 5 tensile test coupons specified in the JIS Standards were taken at a width-wise central portion of the respective steel sheets that from a position that was ¼ of the thickness from the surface. Parallel portions were made parallel in the rolling direction. A tension test in accordance with JIS Z 2241 (2011) was performed using the respective tensile test specimens at normal temperature (25° C.) in the atmosphere, and a tensile strength (MPa) was obtained.
A test specimen including part of the steel sheet surface was taken from a width-wise central portion of the steel sheet of each test number. In each test specimen, a region including part of the steel sheet surface was 50 mm in width×70 mm in length. A pickling test was performed on each test specimen. In the pickling test, the test specimen was immersed in an 8% hydrochloric acid aqueous solution heated to 85° C., and scale on the surface of the test specimen was removed. A time period (pickling completion time period) taken to remove scale from the entire surface of the test specimen was measured. The pickling property was determined as being excellent if the pickling completion time period was within 60 seconds.
The test results are shown in Table 3.
In Table 3, the item “micro-structure” shows the micro-structure at a depth position at ¼ of the plate thickness at a width-wise central portion (range of ±10 mm in the width direction from the center in the width direction) of the steel sheet. In each of the micro-structures shown in Table 3, an area fraction (%) of ferrite in the respective micro-structures is described in an “F” column, and an area fraction of pearlite is described in a “P” column. The area fractions of phases other than ferrite and pearlite are described in an “Other” column. The character “B” denotes the area fraction (%) of bainite (including tempered bainite), and the character “M” denotes the area fraction (%) of martensite (including tempered martensite). “Rg” denotes the area fraction (%) of retained austenite. The area fraction of each phase was measured by the above described measurement method.
The item “tensile strength TS” shows the tensile strength TS (MPa) at a width-wise central portion of the steel sheet. The cold rolling property was determined as excellent if the tensile strength was 800 MPa or less.
An “O” mark in the “pickling property” column in Table 3 indicates that the pickling completion time period was within 60 seconds. An “x” mark indicates that the pickling completion time period was more than 60 seconds.
Referring to Table 3, the chemical compositions of test numbers 1 to 19 were appropriate, and also satisfied Formula (1). Therefore, an Sb concentrated layer was confirmed. The thickness of each of the Sb concentrated layers was 0.5 μm or more. In addition, the scale thickness was 10 μm or less, and the thickness of an internal oxidized layer was 5 μm or less. Thus, the internal oxidized layer and scale could be suppressed. Consequently, the pickling property of the respective chemical compositions of test numbers 1 to 19 was excellent. Further, the thickness of the decarburization layer was 20 μm or less.
In test numbers 1 to 5, test number 7 and test numbers 9 to 19, the production conditions were suitable for formation of ferrite and pearlite. Therefore, in the micro-structure of the hot-rolled steel sheet of each of these test numbers, the total area fraction of ferrite and pearlite was 75% or more. Therefore, the tensile strength was 800 MPa or less.
In test number 6 and test number 8, because the coiling temperature CT was from 150 to 600° C., the total area fraction of bainite and martensite in the micro-structure was 75% or more, and the tensile strength was 900 MPa or more.
On the other hand, in steel type P used in test number 20, the Sb content was too low. Therefore, the thickness of the Sb concentrated layer was less than 0.5 μm, and the thickness of the internal oxidized layer was more than 5 μm. Consequently, the pickling property was poor.
Steel type Q used in test number 21 did not contain Sb. Therefore an Sb concentrated layer was not confirmed. Consequently, the thickness of an internal oxidized layer was more than 5 μm, and the scale thickness was more than 10 μm. Therefore, the pickling property was poor. In addition, the decarburization layer thickness was more than 20 μm.
In steel type R used in test number 22, the C content was too high. In addition, Sb was not contained therein. The coiling temperature CT was also too low. Consequently, the micro-structure mainly consisted of bainite and martensite, and did not contain ferrite and pearlite. Therefore, the tensile strength was more than 800 MPa. In addition, because there was no Sb concentrated layer, the thickness of the internal oxidized layer was more than 5 μm, and the scale thickness was more than 10 μm. Therefore, the pickling property was poor.
In steel type S used in test number 23, the Si content was too high, and Sb was not contained therein. Consequently, an Sb concentrated layer was not formed, and the thickness of the internal oxidized layer was more than 5 μm and the scale thickness was more than 10 μm. Therefore, the pickling property was poor. In addition, the decarburization layer thickness was more than 20 μm.
Note that, in test number 23, heating with a rough bar heater was not performed during hot rolling. Consequently, the descaling temperature was less than 1050° C. As a result, the ratio of island-shaped scale was more than 6%.
In steel type T used in test number 24, the Mn content was too low, the Al content was too low, and the Sb content was too high. Consequently, embrittlement of the steel material was severe and the evaluation test was abandoned. In test number 25, the Mn content was high. Consequently, embrittlement of the steel material was severe and the evaluation test was abandoned.
Molten steels having the chemical compositions shown in Table 4 were produced.
0.41
0.93
1.55
2.96
3.99
0.02
3.07
3.04
Using the above described molten steel, steel materials (ingots) were produced by an ingot-making process. Steel sheets were produced by hot rolling the steel materials under the hot rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 5 using a hot rolling mill for testing. In addition, a heat treatment that simulated coiling at a coiling temperature CT (° C.) shown in Table 5 was performed on the respective steel sheets after hot rolling. Specifically, the steel sheets were stacked and charged into a furnace that was set to the coiling temperature CT (° C.). The inside of the furnace was a nitrogen atmosphere, and the steel sheet surface was in a state in which the surface was blocked-off from the atmosphere. That is, the surface state of the steel sheet was equal to the surface state of a coil obtained by actual production. After holding the steel sheet inside the furnace for 30 minutes at the coiling temperature CT (° C.), the steel sheet was gradually cooled to room temperature at 20° C./hour.
By the same method as in Example 1, the total of the area fractions of ferrite and pearlite in the steel sheet (hot-rolled steel sheet) after hot rolling was measured. The results are shown in Table 5. In the “steel micro-structure” column in Table 5, “F+P” indicates that the total area fraction of ferrite and pearlite in the micro-structure of the hot-rolled steel sheet was 75% or more. In the “steel micro-structure” column in Table 5, “B+M” indicates that the total area fraction of bainite and martensite in the micro-structure of the hot-rolled steel sheet was 75% or more.
The thickness of an internal oxidized layer in the steel sheet after hot rolling (hot-rolled steel sheet) of each test number was measured by the same method as in Example 1. Specifically, a small piece including a part of the surface of the hot-rolled steel sheet was cut out from a width-wise central portion of the hot-rolled steel sheet. Of the entire surface of the small piece, a cross-section (hereunder, referred to as “observation face”) perpendicular to the rolling direction was mirror polished. The observation face was subjected to C (carbon) vapor deposition. After the C vapor deposition, a portion in the vicinity of the surface of the observation face was photographed at an observation magnification of ×1000 using a field-emission scanning electron microscope (FE-SEM) and images were obtained. The thickness of the internal oxidized layer was measured by the above described method based on the obtained images. The results are shown in Table 5.
Note that scale is a layer that is formed when iron ions at the exterior of the hot-rolled steel sheet are oxidized. On the other hand, an internal oxidized layer is a layer that contains oxides of Si and Mn and is formed inside the hot-rolled steel sheet. Therefore, scale, an internal oxidized layer and the base metal can be easily distinguished using a common SEM.
The tensile strength TS of the hot-rolled steel sheet of each test number was measured by a method in accordance with JIS Z 2241 (2011). The results are shown in Table 5. In Table 5, the symbol “-” in the “TS (MPa)” column indicates that cracking occurred at an edge of the hot-rolled steel sheet and measurement was not possible.
The hot-rolled steel sheet of each test number was cold-rolled at a draft of 50%. After cold rolling, each steel sheet was subjected to annealing. The annealing was performed under the following conditions. The steel sheet was heated to an HC temperature (Ae3 temperature +10° C.) at an average heating rate of 5° C./second, and the steel sheet was subjected to annealing for 90 seconds at this HC temperature. Thereafter, the steel sheet was gradually cooled at a cooling rate of 2° C./sec to an AC temperature (HC temperature −120° C.). In addition, the steel sheet was rapidly cooled at 80° C./sec from the AC temperature to 420° C. After being held at 420° C. for 300 seconds, the steel sheet was allowed to cool to room temperature. A tension test was performed by a method in accordance with JIS Z 2241 (2011) on the steel sheet after annealing. During the tension test, a length that the test specimen stretched until necking occurred in the test specimen (a section in which the test specimen exhibited uniform elongation) was measured. The obtained length was divided by the length of the test specimen and the result was adopted as a uniform elongation EL. The results are shown in Table 5. In Table 5, the symbol “-” in the “EL (%)” column indicates that cracking occurred at an edge of the steel sheet and measurement was not possible.
Referring to Table 4 and Table 5, the chemical compositions of test numbers 1 to 10 were appropriate. In addition, the production conditions of test numbers 1 to 10 were appropriate. Therefore, in the micro-structure of the hot-rolled steel sheets of test numbers 1 to 10, the total area fraction of ferrite and pearlite was 75% or more. Further, in the hot-rolled steel sheets of test numbers 1 to 10, an Sb concentrated layer with a thickness of 0.5 μm or more was formed. Furthermore, the thickness of an internal oxidized layer was 5 μm or less, and thus formation of an internal oxidized layer was suppressed.
In addition, the tensile strength of the hot-rolled steel sheets of test numbers 1 to 10 was 800 MPa or less, and the workability during cold rolling was excellent. The uniform elongation of the cold-rolled steel sheets of test numbers 1 to 10 was 10.0% or more, indicating excellent workability after cold rolling also.
Steel type K used for test number 11 did not contain Sb. Consequently, in the hot-rolled steel sheet of test number 11, an Sb concentrated layer was not formed and an internal oxidized layer had a thick thickness of 47 μm.
In steel type L used for test number 12, the Sb content was too high. Consequently, cracking occurred at an edge of the hot-rolled steel sheet of test number 12, and a tension test could not be performed. Therefore, the workability during cold rolling was poor.
In steel type M used for test number 13, the Sb content was too low. Consequently, an internal oxidized layer in the hot-rolled steel sheet of test number 13 had a thick thickness of 34 μm.
In steel type N used in test number 14, a total content of Si and Mn was 3.07%, and thus Formula (1) was not satisfied. Consequently, in the cold-rolled steel sheet of test number 14, in comparison to test numbers 1 to 10 in which the total area fraction of ferrite and pearlite was 75% or more similarly to test number 14, the uniform elongation EL was a low value of less than 10%.
In steel type O used in test number 15, the Si content was a low value of 0.93%. In addition, in steel type O, the total content of Si and Mn was 3.04%, and thus Formula (1) was not satisfied. Therefore, the uniform elongation of the cold-rolled steel sheet of test number 15 was 8.7%, which was low in comparison to test numbers 1 to 10 in which, similarly to test number 15, the total area fraction of ferrite and pearlite was 75% or more.
In steel type P used in test number 16, the Mn content was a low value of 1.55%. Consequently, the uniform elongation of the cold-rolled steel sheet of test number 16 was 6.7%, which was low in comparison to test numbers 1 to 10 in which, similarly to test number 16, the total area fraction of ferrite and pearlite was 75% or more.
In steel type Q used in test number 17, the Si content was a high value of 2.96%. Consequently, the uniform elongation of the cold-rolled steel sheet of test number 17 was 7.8%, and the workability was poor in comparison to test numbers 1 to 10 in which, similarly to test number 17, the total area fraction of ferrite and pearlite was 75% or more.
In steel type R used in test number 18, the Mn content was a high value of 3.99%. Consequently, the uniform elongation of the cold-rolled steel sheet of test number 18 was 3.2%, and the workability was poor.
In steel type S used in test number 19, the Sb content was a low value of 0.02%. Consequently, in the hot-rolled steel sheet of test number 19, the thickness of the Sb concentrated layer was less than 0.5 μm, and the internal oxidized layer had a thick thickness of 25 μm.
Molten steels having the chemical compositions shown in Table 4 were produced.
Using the above described molten steel, steel materials (ingots) were produced by an ingot-making process. Steel sheets were produced by hot rolling the steel materials under the hot rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 6 using a hot rolling mill for testing. In addition, a heat treatment that simulated coiling at a coiling temperature CT (° C.) shown in Table 6 was performed on the respective steel sheets after hot rolling. Specifically, the steel sheets were stacked and charged into a furnace that was set to the coiling temperature CT (° C.). The inside of the furnace was a nitrogen atmosphere, and the steel sheet surface was in a state in which the surface was blocked-off from the atmosphere. That is, the surface state of the steel sheet was equal to the surface state of a coil obtained by actual production. After holding the steel sheet inside the furnace for 30 minutes at the coiling temperature CT (° C.), the steel sheet was gradually cooled to room temperature at 20° C./hour. In addition, with respect to the steel sheets of test numbers other than test numbers 2, 5, 7, 13 and 15, tempering was performed at a tempering temperature (° C.) and for a tempering time period (hr) as shown in Table 6. In Table 6, the column “tempering time period (hr)” shows the time period for which the relevant steel sheet was kept at the tempering temperature shown in Table 6.
The area fraction of bainite and martensite in the hot-rolled steel sheet was measured by the above described method. The results are shown in Table 6. In the “micro-structure” columns in Table 6, “F” shows the area fraction of ferrite, “P” shows the area fraction of pearlite, “B” shows the area fraction of bainite, “M” shows the area fraction of martensite, and “γ” shows the area fraction of austenite.
The internal oxidized layer thickness and scale thickness of the hot-rolled steel sheet of each test number were measured by the same method as in Example 1. The results are shown in Table 6.
The presence or absence of an Sb concentrated layer and the thickness (μm) of an Sb concentrated layer were measured for the hot-rolled steel sheet of each test number by the same method as in Example 1. The results are shown in Table 6.
The tensile strength TS (MPa) of each test number was measured by the same method as in Example 1. The results are shown in Table 6. In Table 6, the symbol “-” in the “tensile strength” column indicates that cracking occurred at an edge of the hot-rolled steel sheet and measurement was not possible.
The uniform elongation EL of each test number was measured by the same method as in Example 2. The results are shown in Table 6.
Referring to Table 4 and Table 6, the chemical compositions of test numbers 1 to 15 were appropriate. In addition, the production conditions of test numbers 1 to 15 were appropriate. Consequently, in the micro-structure of the hot-rolled steel sheets of test numbers 1 to 15, the total area fraction of bainite and martensite was 75% or more. In the hot-rolled steel sheets of test numbers 1 to 15, an Sb concentrated layer having a thickness of 0.5 μm or more was also confirmed. As a result, the thickness of the internal oxidized layer was 5 μm or less, and formation of an internal oxidized layer was suppressed. In addition, the scale thickness of the hot-rolled steel sheets of test numbers 1 to 15 was 7 μm or less, and scale was suppressed.
In test numbers 1, 3, 4, 6, 8 to 12 and 14, tempering was performed. Consequently, the tensile strength TS was 800 MPa or less and the uniform elongation EL was 10% or more, and excellent workability was obtained after cold rolling. On the other hand, in test numbers 2, 5, 7, 13 and 15, tempering was not performed. Consequently, the tensile strength was 900 MPa or more and excellent strength was obtained.
In contrast, steel type K used in test number 16 did not contain Sb. Consequently, an Sb concentrated layer was not formed. As a result, the thickness of an internal oxidized layer was more than 5 μm, and the scale thickness was more than 7 μm.
In steel type L used in test number 17, the Sb content was 0.41%, which was too high. Consequently, in test number 17, cracking occurred at an edge of the hot-rolled steel sheet, and the workability was poor. Therefore a tension test could not be performed.
In steel type M used in test number 18, the Sb content was 0.004%, which was too low. Therefore, an Sb concentrated layer was not formed in the hot-rolled steel sheet of test number 18. Consequently, the thickness of the internal oxidized layer was more than 5 μm, and the scale thickness was more than 7 μm.
In steel type N used in test number 19, the total content of Si and Mn was 3.07%, and thus Formula (1) was not satisfied. Therefore, even though tempering was performed, the uniform elongation EL was less than 10%.
In steel type O used in test number 20, the Si content was a low value of 0.93%. In addition, in steel type O, the total content of Si and Mn was 3.04%, and thus Formula (1) was not satisfied. Consequently, even though tempering was performed, the uniform elongation EL was less than 10%.
In steel type P used in test number 21, the Mn content was a low value of 1.55%. Consequently, in the micro-structure, the area fraction of ferrite was 30%, and the combined area fraction of martensite and bainite was less than 75%. As a result, even though tempering was performed, the uniform elongation EL was less than 10%.
In steel type Q used in test number 22, the Si content was a high value of 2.96%. Consequently, even though tempering was performed, the uniform elongation EL was less than 10%.
In steel type R used in test number 23, the Mn content was a high value of 3.99%. Consequently, even though tempering was performed, the uniform elongation EL was less than 10%.
In steel type S used in test number 24, the Sb content was a low value of 0.02%. Therefore, the thickness of an Sb concentrated layer was less than 0.5 μm. Consequently, the thickness of an internal oxidized layer was more than 10 μm, and the scale thickness was more than 7 μm.
Embodiments of the present invention have been described above. However, the above described embodiments are merely examples for implementing the present invention. Accordingly, the present invention is not limited to the above described embodiments, and the above described embodiments can be appropriately modified within a range that does not deviate from the technical scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-083603 | Apr 2015 | JP | national |
| 2015-083604 | Apr 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/061991 | 4/14/2016 | WO | 00 |
