Title of the invention: Hot-rolled steel sheet and its manufacturing method
Technical field
[0001]
The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2020-013713 filed in Japan on January 30, 2020 and Japanese Patent Application No. 2020-047558 ​​filed in Japan on March 18, 2020. , the contents of which are hereby incorporated by reference.
Background technology
[0002]
In recent years, there has been a desire to reduce the weight of automobiles in order to reduce carbon dioxide emissions and reduce fuel consumption in response to environmental issues. In addition, the demand for improved collision safety is increasing more and more. Increasing the strength of steel is an effective means of reducing the weight of automobiles and improving crash safety. However, when the strength of steel is increased, formability such as ductility and hole expansibility, or toughness deteriorates. Therefore, there is a need for a steel sheet that has both high strength and formability and toughness.
[0003]
In response to such requirements, for example, in Patent Document 1, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 1.5 %, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.1%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05% , Mo: 0.1 to 1.0%, Cr: 0.1 to 1.0%, B: 0.0005 to 0.005%, and the martensite phase or tempered martensite phase is the volume fraction A hot-rolled steel sheet having a main phase of 90% or more, a prior austenite phase having an aspect ratio of 3 to 18, a yield strength YS of 960 MPa or more, and a high toughness of vE-40 of 40 J or more; and A method for its production has been reported.
[0004]
In addition, as a method for reducing the anisotropy of a hot-rolled steel sheet, for example, Patent Document 2 describes, in mass %, C: 0.04 to 0.15%, Si: 0.01 to 0.25%, Mn : 0.1-2.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.005-0.05%, N: 0.01 or less, Ti: 0.01- 0.12%, B: 0.0003 to 0.005%, 90% or more of the structure is martensite, the amount of TiC precipitation is 0.05% or less, and A-type inclusions specified in JIS G0202 A hot-rolled steel sheet having a cleanliness of 0.01% or less and a method for producing the same have been reported.
prior art documents
patent literature
[0005]
Patent Document 1: Japanese Patent No. 5609383
Patent Document 2: Japanese Patent Application Laid-Open No. 2014-47414
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0006]
In the steel sheet of Patent Document 1, the aspect ratio of the prior austenite phase is 3 or more, and there is a problem that the anisotropy of ductility and toughness is large. If there is anisotropy, there are problems in applying it to steel sheets for automobiles because it becomes difficult to maintain the member performance at a high level and the dimensional accuracy due to processing deteriorates.
[0007]
Also, in the steel sheet of Patent Document 2, although the anisotropy of bendability, yield strength and toughness at -20°C is reduced, the anisotropy of ductility is not necessarily reduced. Also, there is no disclosure of absorbed energy or anisotropy at -40°C.
[0008]
As described above, it was difficult to obtain a hot-rolled steel sheet with high strength, excellent ductility, and excellent low-temperature toughness, and with small anisotropy in ductility and toughness with conventional techniques.
[0009]
The present invention aims to solve the above-described problems, and provides a hot-rolled steel sheet having high strength, excellent ductility, and excellent low-temperature toughness, and having small anisotropy of ductility and toughness. An object of the present invention is to provide a manufacturing method thereof. In addition, the present invention provides a hot-rolled steel sheet having high strength, excellent ductility, excellent low-temperature toughness, and excellent hole expansibility, and low anisotropy in ductility and toughness, and a method for producing the same. is a preferred subject.
Means to solve problems
[0010]
The present inventors melted and hot-rolled various steels with varying C content, Si content, and Mn content in the laboratory to obtain the required strength, ductility, toughness, and hole expandability. and various methods for reducing the anisotropy. As a result, while ensuring a high tensile strength of 980 MPa or more, it has excellent ductility and excellent low-temperature toughness, and in order to reduce the anisotropy of ductility and toughness, it is necessary to reduce the anisotropy of the structure. and that it is important to reduce the shape anisotropy of sulfides. Specifically, 1) a structure containing 99% or more martensite (including fresh martensite and tempered martensite), 2) an average aspect ratio of prior austenite grains in a cross section parallel to the rolling direction of 3 3) The proportion of sulfides with an aspect ratio of more than 3.0 among sulfides with an area of ​​1.0 μm 2 or more in a cross section parallel to the rolling direction is 1.0% or less; 4) It has been found that it is important to set the pole density of the {211}<011> orientation to 3.0 or less at the central portion of the plate thickness.
In addition, the present inventors have found that the hole expandability can be further improved by reducing ΔHv, which is the difference between the maximum value and the minimum value of Vickers hardness in a cross section perpendicular to the rolling direction. .
[0011]
The present invention was made based on the above findings. The gist of the present invention is as follows.
[1] The hot-rolled steel sheet according to one aspect of the present invention has, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0 %, P: 0.020% or less, S: 0.001 to 0.010%, Al: 0.005 to 1.000%, N: 0.0010 to 0.0100%, Ti: 0.005 to 0 .30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0% , Ni: 0-5.0%, Cu: 0-3.0%, B: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0500%, REM: 0- 0.050%, and the balance has a chemical composition consisting of Fe and impurities, the microstructure contains 99% or more of martensite in volume fraction, and the balance structure consists of retained austenite and ferrite , In the cross section parallel to the rolling direction, the average aspect ratio of the prior austenite grains is less than 3.0, and among the sulfides with an area of ​​1.0 μm 2 or more, the ratio of sulfides with an aspect ratio of more than 3.0 The content is 1.0% or less, the pole density of the {211}<011> orientation is 3.0 or less at the plate thickness central portion, and the tensile strength TS is 980 MPa or more.
[2] The hot-rolled steel sheet described in [1] above may have a tensile strength TS of 1180 MPa or more.
[3] In the hot-rolled steel sheet described in [2] above, the volume fraction of tempered martensite may be less than 5%.
[4] The hot-rolled steel sheet described in [1] above may have ΔHv, which is the difference between the maximum value and the minimum value of Vickers hardness, of 50 or less in a cross section perpendicular to the rolling direction.
[5] In the hot-rolled steel sheet described in [4] above, the volume fraction of fresh martensite may be less than 3%.
[6] The hot-rolled steel sheet according to any one of [1] to [5] above may have a galvanized layer on the surface.
[7] In the hot-rolled steel sheet described in [6] above, the galvanized layer may be an alloyed galvanized layer.
[8] The hot-rolled steel sheet according to any one of [1] to [7] above, wherein the chemical composition is, in mass%, Nb: 0.005 to 0.30%, V: 0.01 to 0.00%. 50%, Cr: 0.05-3.0%, Mo: 0.05-3.0%, Ni: 0.05-5.0%, Cu: 0.10-3.0%, B: 0 0003-0.0100%, Mg: 0.0005-0.0100%, Zr: 0.0010-0.0500%, REM: 0.0010-0.050%, one selected from the group Or you may contain 2 or more types.
[9] A method for producing a hot-rolled steel sheet according to another aspect of the present invention is a method for producing a hot-rolled steel sheet according to any one of [1] to [3] above, wherein C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 to 0.010%, Al: 0.005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0. 30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3.0%, B: Casting having a chemical composition containing 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance being Fe and impurities A heating step of heating the slab directly or after cooling once to 1350° C. or higher and 1400° C. or lower, and a hot rolling step of hot rolling the cast slab after the heating step to obtain a hot rolled steel sheet. and a winding step of winding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 100 ° C. or less, and in the hot-rolling step, the cast slab is finished at a finish rolling temperature. Rolling is performed so that the temperature is 1000 ° C. or higher, cooling is started within 0.10 seconds after the end of the rolling, and the temperature is lowered by 50 ° C. or higher at an average cooling rate of 100 ° C./second or higher. After cooling, after the first cooling, light reduction rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a reduction rate of 5% or more and 20% or less, and the average cooling rate from the completion of the light reduction rolling to 200 ° C. or less is 50. The second cooling is performed so that the temperature becomes ℃/second or more.
[10] A method for producing a hot-rolled steel sheet according to another aspect of the present invention is a method for producing a hot-rolled steel sheet according to [4] or [5] above, wherein C: 0.08 in mass% ~0.25%, Si: 0.01-1.00%, Mn: 0.8-2.0%, P: 0.020% or less, S: 0.001-0.010%, Al: 0 .005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3.0%, B: 0-0 .0100%, Mg: 0-0.0100%, Zr: 0-0.0500%, REM: 0-0.050%, and a cast slab having a chemical composition with the balance consisting of Fe and impurities, A heating step of heating to 1350 ° C. or higher and 1400 ° C. or lower directly or after cooling once, a hot rolling step of hot rolling the cast slab after the heating step to a hot rolled steel sheet, and the hot rolling step A winding step of winding the hot-rolled steel sheet after the inter-rolling step in a temperature range of 100 ° C. or less; A temper rolling step to be performed, a tempering step of performing a tempering treatment of heating to 430 to 560 ° C. after the temper rolling,
In the hot rolling step, the cast slab is rolled so that the finish rolling temperature is 1000 ° C. or higher, and cooling is started within 0.10 seconds after the end of the rolling, First cooling is performed at an average cooling rate of 100° C./sec or more so that the temperature is lowered by 50° C. or more, and after the first cooling, light reduction with a reduction rate of 5% or more and 20% or less at a temperature of the Ar3 transformation point or more. Rolling is performed, and second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200° C. or lower is 50° C./second or more.
[11] A method for producing a hot-rolled steel sheet according to another aspect of the present invention is a method for producing a hot-rolled steel sheet according to [6] above, wherein C: 0.08 to 0.25 in mass% %, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 to 0.010%, Al: 0.005 to 1 .000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0- 0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3.0%, B: 0-0.0100%, A cast slab having a chemical composition containing Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance being Fe and impurities, is cooled directly or once. After that, a heating step of heating to 1350 ° C. or higher and 1400 ° C. or lower, and a hot rolling step of hot rolling the cast slab after the heating step to obtain a hot rolled steel sheet. , a winding step of winding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 100 ° C. or less; and a galvanizing step of performing Ni pre-plating on the hot rolled steel sheet, heating it to 430 to 480 ° C. at a temperature rising rate of 20 ° C./sec or more, and then galvanizing it. In the hot rolling step, the cast slab is rolled so that the finish rolling temperature is 1000 ° C. or higher, and cooling is started within 0.10 seconds after the end of the rolling, and the temperature is 100 ° C. First cooling is performed at an average cooling rate of / second or more so that the temperature is lowered by 50 ° C. or more, and after the first cooling, light reduction rolling is performed at a temperature of the Ar3 transformation point or more and a reduction rate of 5% or more and 20% or less. Then, the second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200° C. or lower is 50° C./second or more.
[12] A method for producing a hot-rolled steel sheet according to another aspect of the present invention is a method for producing a hot-rolled steel sheet according to [7] above, wherein C: 0.08 to 0.25 in mass% %, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 to 0.010%, Al: 0.005 to 1 .000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0- 0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3.0%, B: 0-0.0100%, A cast slab having a chemical composition containing Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance being Fe and impurities, is cooled directly or once. After that, a heating step of heating to 1350 ° C. or higher and 1400 ° C. or lower, a hot rolling step of hot rolling the cast slab after the heating step to make a hot rolled steel sheet, and after the hot rolling step A winding step of winding the hot-rolled steel sheet in a temperature range of 100 ° C. or less, and a temper rolling of performing temper rolling with an elongation of 0.7% or more on the hot-rolled steel sheet after the winding step. a zinc plating step of performing Ni pre-plating on the hot-rolled steel sheet, heating to 430 to 480 ° C. at a temperature increase rate of 20 ° C./sec or more, and then galvanizing; an alloying step of performing an alloying treatment at 560° C. for 10 to 40 seconds, and in the hot rolling step, the cast slab is rolled so that the finish rolling temperature is 1000° C. or higher; After the end of the rolling, cooling is started within 0.10 seconds, and the first cooling is performed so that the temperature is lowered by 50 ° C. or more at an average cooling rate of 100 ° C./sec or more. Light reduction rolling is performed at a temperature equal to or higher than the transformation point at a reduction rate of 5% or more and 20% or less, and the second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200 ° C. or less is 50 ° C./sec or more. I do.
Effect of the invention
[0012]
According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility (elongation), excellent low-temperature toughness, and low anisotropy of ductility and toughness, and a method for producing the same. can be done. Further, according to a preferred embodiment of the present invention, a hot-rolled steel sheet having high strength, excellent ductility (elongation), excellent low-temperature toughness, and excellent hole expansibility, and having small anisotropy of ductility and toughness and its manufacturing method. This hot-rolled steel sheet can be suitably applied to automobile parts and the like, and can contribute to the weight reduction of automobiles by application, so that the industrial contribution is extremely remarkable.
MODE FOR CARRYING OUT THE INVENTION
[0013]
A hot-rolled steel sheet according to one embodiment of the present invention (hot-rolled steel sheet according to this embodiment) and a method for manufacturing the same will be described below.
The hot-rolled steel sheet according to the present embodiment is mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0 .020% or less, S: 0.001 to 0.010%, Al: 0.005 to 1.000%, N: 0.0010 to 0.0100%, Ti: 0.005 to 0.30%, Ca : 0.0005 to 0.0100%, and if necessary, Nb: 0.30% or less, V: 0.50% or less, Cr: 3.0% or less, Mo: 3.0% or less , Ni: 5.0% or less, Cu: 3.0% or less, B: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.0500% or less, REM: 0.050% or less and the balance has a chemical composition consisting of Fe and impurities,
　The microstructure contains 99% or more of martensite in volume fraction, and the remaining structure consists of retained austenite and ferrite,
In the cross section parallel to the rolling direction, the average aspect ratio of the prior austenite grains is less than 3.0, and among the sulfides with an area of ​​1.0 μm 2 or more, the ratio of sulfides with an aspect ratio of more than 3.0 is 1 .0% or less, and the pole density of the {211} <011> orientation at the center of the plate thickness is 3.0 or less,
　The tensile strength (TS) is 980 MPa or more.
The hot-rolled steel sheet according to this embodiment will be described in detail below.
[0014]
First, the reason for limiting the range of each element contained in the chemical composition of the hot-rolled steel sheet according to this embodiment will be described. Hereinafter, % in the content of each element is mass %.
[0015]
C: 0.08-0.25%
　C is an element that increases the strength of steel. If the C content is less than 0.08%, it is difficult to ensure a tensile strength of 980 MPa or more. Therefore, the C content is made 0.08% or more. Preferably, it is 0.10% or more.
On the other hand, if the C content exceeds 0.25%, the ductility, weldability, toughness, etc. are significantly degraded. Therefore, the C content should be 0.25% or less. The C content is preferably 0.20% or less.
[0016]
　Si: 0.01 to 1.00%
　Si is a useful element for increasing the strength of steel through solid-solution strengthening. Moreover, Si is an element useful for suppressing the formation of cementite. If the Si content is less than 0.01%, these effects cannot be sufficiently obtained. Therefore, the Si content is set to 0.01% or more.
On the other hand, if the Si content exceeds 1.00%, the detachability of scale generated by hot rolling and the chemical conversion treatability are significantly deteriorated. Moreover, the desired tissue may not be obtained. Therefore, the Si content is set to 1.00% or less.
[0017]
　Mn: 0.8 to 2.0%
　Mn is an effective element for enhancing the hardenability of steel. If the Mn content is less than 0.8%, the effect of improving hardenability cannot be sufficiently obtained. Therefore, the Mn content is set to 0.8% or more.
On the other hand, when the Mn content exceeds 2.0%, the toughness deteriorates. Therefore, the Mn content is set to 2.0% or less.
[0018]
P: 0.020% or less
P is an impurity element that segregates at grain boundaries to reduce grain boundary strength and deteriorate toughness. Therefore, it is desirable to reduce it. The P content is set to 0.020% or less in consideration of the current refining technology and manufacturing cost. Although the lower limit of the P content is not limited, it may be 0.001% in view of the steelmaking cost.
[0019]
S: 0.001-0.010%
S is an impurity element that deteriorates hot workability and toughness, and it is desirable to reduce it. The S content is set to 0.010% or less in consideration of the current refining technology and manufacturing cost. The lower limit of the S content is set to 0.001% in view of the steelmaking cost. The lower limit of the S content is preferably 0.003%.
[0020]
Al: 0.005-1.000%
Al is an effective element as a deoxidizing agent. In addition, Al is an element that forms AlN and contributes to suppression of grain coarsening. If the Al content is less than 0.005%, these effects cannot be sufficiently obtained. Therefore, the Al content is set to 0.005% or more.
On the other hand, when the Al content exceeds 1.000%, the toughness deteriorates. Therefore, the Al content is set to 1.000% or less.
[0021]
N: 0.0010-0.0100%
N is an element that forms nitrides and contributes to the suppression of grain coarsening. If the N content is less than 0.0010%, the effect cannot be obtained. Therefore, the N content is made 0.0010% or more.
On the other hand, if the N content exceeds 0.0100%, the toughness deteriorates. Therefore, the N content is made 0.0100% or less.
[0022]
Ti: 0.005-0.30%
Ti is an element that forms TiN and is an element that is effective in suppressing coarsening of crystal grains. If the Ti content is less than 0.005%, this effect cannot be sufficiently obtained. Therefore, the Ti content is set to 0.005% or more. The Ti content is preferably 0.01% or more.
On the other hand, if the Ti content exceeds 0.30%, TiN may coarsen and the toughness may deteriorate. Therefore, the Ti content is set to 0.30% or less.
[0023]
Ca: 0.0005-0.0100%
Ca is an element that is effective in suppressing deterioration of hot workability and toughness due to S through control of the morphology of sulfides. If the Ca content is less than 0.0005%, the effect cannot be sufficiently obtained. Therefore, Ca content is made 0.0005% or more.
On the other hand, even if Ca is contained excessively, not only the effect is saturated, but also the cost increases. Therefore, Ca content shall be 0.0100% or less.
[0024]
The above are the basic components of the hot-rolled steel sheet according to the present embodiment, and usually consist of Fe and impurities other than the above. , V, B, Mg, Zr, and REM may be further contained within the following range. Since the hot-rolled steel sheet according to the present embodiment can obtain the effect without containing the above optional element, the lower limit of the content of the above optional element is 0%. In the present embodiment, the term "impurities" refers to ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment, etc., and are permissible within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. do. The optional elements will be described in detail below.
[0025]
Nb: 0-0.30%
Nb is an element that forms fine carbonitrides and is an element that is effective in suppressing coarsening of crystal grains. Therefore, it may be contained. When the toughness is enhanced by suppressing grain coarsening, the Nb content is preferably 0.005% or more.
On the other hand, if the Nb content is excessive, the precipitates become coarse and the toughness may deteriorate. Therefore, when Nb is contained, the Nb content is preferably 0.30% or less.
[0026]
　V: 0 to 0.50%
V is an element that forms fine carbonitrides like Nb. Therefore, it may be contained. In order to suppress coarsening of crystal grains and improve toughness, the V content is preferably 0.01% or more.
On the other hand, if the V content exceeds 0.50%, toughness may deteriorate. Therefore, when it is contained, the V content is preferably 0.50% or less.
[0027]
Cr: 0-3.0%
Mo: 0-3.0%
Ni: 0-5.0%
Cu: 0-3.0%
Cr, Mo, Ni, and Cu are effective elements for improving ductility and toughness. Therefore, it may be contained. In order to improve ductility and toughness, it is preferable that the Cr content is 0.05% or more, the Mo content is 0.05% or more, the Ni content is 0.05% or more, and the Cu content is 0.1% or more. . More preferably, the Cr content is 0.1% or more, the Mo content is 0.1% or more, the Ni content is 0.1% or more, and the Cu content is 0.2% or more.
On the other hand, when the contents of Cr, Mo, and Cu exceed 3.0%, respectively, and the content of Ni exceeds 5.0%, toughness may decrease due to an increase in strength. Therefore, when they are contained, it is preferable that the Cr content is 3.0% or less, the Mo content is 3.0% or less, the Ni content is 5.0% or less, and the Cu content is 3.0% or less.
[0028]
B: 0-0.0100%
B is an element that segregates at grain boundaries and suppresses the grain boundary segregation of P and S. It is also an effective element for enhancing the hardenability of steel. Therefore, it may be contained. The B content is preferably 0.0003% or more in order to improve ductility, toughness and hot workability, and to improve hardenability by strengthening grain boundaries.
On the other hand, when the B content exceeds 0.0100%, grainsCoarse precipitates may form in the boundary, deteriorating hot workability and toughness. Therefore, when B is contained, the B content is preferably 0.0100% or less.
[0029]
　Mg: 0 to 0.0100%
Zr: 0-0.0500%
REM: 0-0.050%
Mg, Zr, and REM are elements that are effective in suppressing deterioration of hot workability and toughness due to S by controlling the morphology of sulfides. Therefore, it may be contained. When improving the toughness, it is preferable to set the Mg content to 0.0005% or more, the Zr content to 0.0010% or more, and the REM content to 0.001% or more.
On the other hand, even if Mg, Zr and/or REM are contained excessively, the effect is saturated. Therefore, when they are contained, it is preferable that the Mg content is 0.0100% or less, the Zr content is 0.0500% or less, and the REM content is 0.050% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanides, and the content of REM above refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metals.
[0030]
The content of each element in the hot-rolled steel sheet according to this embodiment can be obtained by a known method such as ICP emission spectroscopic analysis.
[0031]
Next, the microstructure of the hot-rolled steel sheet according to this embodiment will be described.

The hot-rolled steel sheet according to the present embodiment has a microstructure of 99% by volume fraction of martensite (including fresh martensite and tempered martensite) in order to improve the uniformity of the structure and reduce the anisotropy. The structure contains the above and the residual structure consists of retained austenite and ferrite.
　Retained austenite and ferrite have different distributions in the rolling direction and in the direction perpendicular to it, so the anisotropy increases as the volume fraction of these increases. Therefore, it is necessary to make the total volume fraction of these elements 1% or less and to make the homogeneous martensite structure 99% or more.
　Fresh martensite is generated during cooling after hot rolling. Further, tempered martensite is generated by tempering fresh martensite by subsequent heat treatment (heating in tempering process or plating process).
[0032]
If you want to increase the strength, it is preferable to reduce the volume fraction of tempered martensite in the martensite and use fresh martensite as the main structure. For example, when the tensile strength is 1180 MPa or more, the area fraction of tempered martensite is preferably less than 5%.
Also, in order to improve the homogeneity of the structure and improve the hole expansibility, it is preferable to reduce the volume fraction of fresh martensite and use tempered martensite as the main structure. For example, the area fraction of fresh martensite is preferably less than 3%.
[0033]
　The volume fraction of each structure in the microstructure is obtained by the following method.
First, a sample is taken from the central portion of the hot-rolled steel sheet in the width direction so that the cross section parallel to the rolling direction serves as the observation surface.
The area fraction of martensite (fresh martensite and tempered martensite) and ferrite is the position (of the plated steel sheet) at a depth of 1/4 of the plate thickness in the plate thickness direction from the surface of the observation surface (cross section in the rolling direction). In the case, the structure of 1/4 depth of the plate thickness in the thickness direction of the steel plate which is the base material from the interface between the plating layer and the base material) is exposed by repeller etching or nital etching, and optical Observe with a microscope, SEM or TEM, determine each phase from the structure morphology, carbide precipitation state, dislocation density, etc., and measure the area fraction of each phase using an image analyzer or the like. The obtained area fraction of each phase is regarded as the volume fraction.
Fresh martensite and tempered martensite do not necessarily need to be distinguished from each other in this embodiment, but when they are distinguished, they are distinguished by Vickers hardness (Hv) and C concentration (% by mass). The Vickers hardness (HvM) of martensite is determined by measuring the Vickers hardness at three points within the martensite grain with a test force of 5 gf in accordance with JIS Z 2244:2009, and calculating the average value of the Vickers hardness. Get with Next, the C concentration (CM: mass %) of the martensite is measured.
In the present embodiment, when cementite is present in martensite grains, the C concentration of martensite is the concentration including the C concentration of cementite. The C concentration (CM) of martensite is measured using an electron probe microanalyzer (EPMA) attached to the FE-SEM at a pitch of 0.5 μm or less, and the average value of the obtained C concentrations is calculated. Earn by doing Tempered martensite and fresh martensite are distinguished from the obtained Vickers hardness (HvM) and C concentration (CM) of martensite. Specifically, when the obtained HvM and CM satisfy the following formula 1, it is determined as tempered martensite, and in other cases, it is determined as fresh martensite.
HvM / (-982.1 × CM 2 + 1676 × CM + 189) ≤ 0.60 Equation 1
The value (−982.1×CM 2 +1676×CM+189) obtained by substituting the C concentration (CM) of martensite into the denominator on the left side of Equation 1 above represents the original hardness of martensite at that C concentration. The tempered martensite contained in the metal structure of the hot-rolled steel sheet according to the present embodiment is a structure generated by tempering the martensite generated during cooling after hot rolling by subsequent heat treatment. The hardness is lower than that of the original martensite due to cementite precipitation in martensite grains due to On the other hand, the fresh martensite contained in the hot-rolled steel sheet according to the present embodiment is a structure formed by transforming the austenite remaining until after cooling after hot rolling into martensite in the cooling process of the subsequent heat treatment. It has not been returned and has a hardness close to that of the original martensite. Therefore, in the present embodiment, tempered martensite and fresh martensite are distinguished from each other by determining the ratio between the original hardness of martensite and the hardness of martensite actually obtained by measurement.
[0034]
In addition, the volume fraction of retained austenite is measured by the following method.
A sample is taken from the center of the steel plate in the width direction so that the cross section parallel to the plate surface becomes the observation surface. After grinding the surface of the sample to a position of 1/4 depth (in the case of a plated steel sheet, a position of 1/4 depth of the base steel plate from the interface between the plating layer and the base material), chemically polished, Mo By X-ray diffraction using a tube, based on the following formula, the diffraction intensity of ferrite (200) Iα (200), the diffraction intensity of ferrite (211) Iα (211), and the diffraction intensity of austenite (200) The volume fraction of retained austenite is obtained from the intensity ratio of the diffraction intensity Iγ(311) of Iγ(220) and (311). Vγ in the following formula indicates the volume fraction of retained austenite.
Vγ=0.25×{Iγ(220)/(1.35×Iα(200)+Iγ(220))+Iγ(220)/(0.69×Iα(211)+Iγ(220))+Iγ(311)/ (1.5×Iα(200)+Iγ(311))+Iγ(311)/(0.69×Iα(211)+Iγ(311))}
[0035]

In the hot-rolled steel sheet according to this embodiment, the average aspect ratio of the prior austenite grains in the cross section parallel to the rolling direction is less than 3.0. When the average aspect ratio of prior austenite grains is 3.0 or more, the anisotropy of ductility and toughness increases.
[0036]

In the hot-rolled steel sheet according to the present embodiment, the grain size of prior austenite grains (prior γ grain size) in a cross section parallel to the rolling direction is preferably 12 μm or more and 100 μm or less.
If the prior austenite grain size is less than 12 μm, unrecrystallized grains tend to remain, and there is concern that the uniformity of the structure will decrease. On the other hand, if the prior austenite grain size is more than 100 μm, the low temperature toughness is lowered.
[0037]
The average aspect ratio and grain size of prior austenite grains are obtained by the following method.
First, a sample is taken from the central portion of the hot-rolled steel sheet in the width direction so that the cross section parallel to the rolling direction serves as the observation surface.
The structure at a depth of 1/4 of the plate thickness from the surface of the steel plate in the observation plane (cross section in the rolling direction) was measured with a corrosive liquid (ethanol, 2% picric acid, 1% iron chloride (II )), observed with an optical microscope or SEM, observed 100 or more prior austenite grains using an image analyzer or the like, and measured the grain size and aspect ratio of each prior austenite grain. Let the value which averaged these be a prior austenite grain size and an average aspect-ratio. Here, the aspect ratio of the prior austenite grains is (aspect ratio)=(major axis in rolling direction)/(minor axis in plate thickness direction).
[0038]

In a cross section parallel to the rolling direction, among sulfides with an area of ​​1.0 μm 2 or more, if the number of sulfides with an aspect ratio of more than 3.0 exceeds 1.0%, these sulfides are the starting points As a result, voids are generated, and the anisotropy of ductility and toughness increases. Moreover, when sulfides having a large aspect ratio are formed, the difference in Vickers hardness in the cross section perpendicular to the rolling direction tends to increase. Therefore, in the hot-rolled steel sheet according to the present embodiment, the ratio of the number of sulfides having an aspect ratio of more than 3.0 among the sulfides having an area of ​​1.0 μm 2 or more in the cross section parallel to the rolling direction is 1.0 % or less.
The reason why the target is sulfides with an area of ​​1.0 μm 2 or more is that sulfides with an area of ​​less than 1.0 μm 2 are unlikely to form voids.
In the hot-rolled steel sheet according to this embodiment, sulfides are, for example, MnS, TiS, CaS, and the like.
[0039]
The ratio of sulfides with an aspect ratio exceeding 3.0 is obtained by the following method.
In the present embodiment, sulfides are defined as inclusions with a mass fraction of S of 5% or more. Therefore, when determining the ratio of sulfides with an aspect ratio of more than 3.0, first, a sample is taken from the center of the hot-rolled steel sheet in the width direction so that the cross section parallel to the rolling direction becomes the observation surface. . Observe the as-polished structure at a depth of 1/4 of the plate thickness from the surface of the steel plate on the observation surface (cross section in the rolling direction) with an SEM, and measure the composition of each inclusion using the EDX attached to the SEM. The sulfide is discriminated by using an image analyzer or the like to measure the area of ​​the sulfide, and the aspect ratio of the sulfide having an area of ​​1.0 μm 2 or more is measured. The aspect ratio of 1000 or more sulfides having an area of ​​1.0 μm 2 or more is measured by the above method, and the number ratio of sulfides having an aspect ratio of more than 3.0 is obtained. Here, the aspect ratio of sulfide is (aspect ratio)=(major axis in rolling direction)/(minor axis in plate thickness direction).
[0040]
orientation at the plate thickness center of the cross section parallel to the rolling direction: 3.0 or less>
The hot-rolled steel sheet according to the present embodiment has a pole density of 3.0 or less in the {211}<011> orientation at the thickness center of the cross section parallel to the rolling direction. When the hot-rolled steel sheet has a texture with a {211}<011> orientation pole density of more than 3.0, the anisotropy of the structure increases, and the anisotropy of ductility and toughness increases. The pole density is preferably 2.5 or less, more preferably 2.0 or less.
[0041]
　Polar density can be obtained from crystal orientation information by EBSD analysis, and is synonymous with the X-ray random intensity ratio. Specifically, the pole density of the {211}<011> orientation is obtained by the following method.
Using a device that combines a scanning electron microscope and an EBSD analysis device and AMETEK's OIM Analysis (registered trademark), the center of the plate thickness (from the plate thickness center position to the front and back directions of the steel plate, respectively) In the range of about 1/10 of the plate thickness), fcc and bcc are distinguished, and 1000 or more bcc Orientation information of crystal grains is measured and obtained by ODF analysis using a harmonic series expansion method.
[0042]
<ΔHv, which is the difference between the maximum value and the minimum value of Vickers hardness: 70 or less>
In the hot-rolled steel sheet according to the present embodiment, in a cross section perpendicular to the rolling direction, ΔHv (Hvmax−Hvmin), which is the difference between the maximum value (Hvmax) and the minimum value (Hvmin) of Vickers hardness, is 70 or less. is preferred. When ΔHv increases, stress concentrates on the boundary between the soft portion with low Vickers hardness and the hard portion with high Vickers hardness when an external force is applied, promoting the initiation and propagation of cracks and increasing the hole expansibility of the hot-rolled steel sheet. It may deteriorate. ΔHv is more preferably 50 or less to obtain particularly excellent hole expansibility.
[0043]
ΔHv, which is the difference between the maximum and minimum values ​​of Vickers hardness, is measured by the following method.
A test piece is taken from the center of the hot-rolled steel sheet in the width direction so that the cross section perpendicular to the rolling direction is the measurement surface. The obtained test piece is subjected to a Vickers hardness test with a test force of 5 gf in accordance with JIS Z 2244:2009. The Vickers hardness is measured at a pitch of 0.05 mm from the surface of the steel sheet to a depth of 1/2 of the thickness of the cross section perpendicular to the rolling direction. Vickers hardness tests are performed on at least three specimens in this manner. Hvmax is obtained by calculating the average value of the maximum values ​​of Vickers hardness of each test piece. Also, Hvmin is obtained by calculating the average value of the minimum values ​​of Vickers hardness of each test piece. By subtracting Hvmin from the obtained Hvmax, ΔHv (Hvmax−Hvmin) is obtained.
[0044]

Considering the contribution to weight reduction of automobiles, it is assumed that the hot-rolled steel sheet according to the present embodiment is a high-strength steel sheet having a tensile strength of 980 MPa or more. The tensile strength is preferably 990 MPa or higher, more preferably 1080 MPa or higher, and still more preferably 1180 MPa or higher.
Although it is not necessary to specify the upper limit of the tensile strength, there is a concern that the elongation will decrease as the tensile strength increases, so the tensile strength may be set to 1470 MPa or less. Alternatively, it may be 1270 MPa or less.
In addition, in the hot-rolled steel sheet according to the present embodiment, TS×λ, which is the product of tensile strength (TS) and hole expansion ratio (λ), is targeted to be 38000 MPa·% or more. TS×λ is more preferably 40000 MPa·% or more, and even more preferably 50000 MPa·% or more.
[0045]
Tensile strength (TS) was obtained by performing a tensile test in accordance with JIS Z 2241: 2011 on JIS No. 5 test pieces cut so that the longitudinal direction was parallel or perpendicular to the rolling direction of the hot-rolled steel sheet. It is obtained from the stress-strain curve. Moreover, the hole expansion rate is measured by conducting a hole expansion test in accordance with JIS Z 2256:2010.
[0046]

The hot-rolled steel sheet according to this embodiment may have a galvanized layer on its surface.
The galvanized layer included in the hot-rolled steel sheet according to the present embodiment may be a galvanized layer (hot-dip galvanized layer) formed by hot-dip galvanizing, and is formed by performing an alloying treatment on the galvanized layer. It may be an alloyed galvanized layer.
The galvanized layer of the hot-rolled steel sheet according to the present embodiment preferably contains less than 7.0% by mass of Fe and 0.5 to 2.0 g/m 2 of Ni. When the galvanized layer is an alloyed galvanized layer, it preferably contains 7.0 to 15.0% by mass of Fe and 0.5 to 2.0 g/m 2 of Ni. In this embodiment, the preferred range of the Fe content in the galvanized layer differs between the case where the alloying treatment is not performed and the case where the alloying treatment is performed.
[0047]
　Fe content: less than 7.0% by mass or 7.0 to 15.0% by mass
First, the case of performing alloying treatment will be described. By applying an alloying treatment to a galvanized steel sheet having a galvanized layer on its surface, the galvanized layer is alloyed, and spot weldability and paintability are further improved. Specifically, by immersing the steel sheet in a hot-dip galvanizing bath and then performing an alloying treatment, Fe is incorporated into the galvanized layer, and the Fe concentration in the galvanized layer becomes 7.0% by mass or more. A galvannealed steel sheet having excellent spot weldability and paintability can be obtained. On the other hand, if the Fe content exceeds 15.0% by mass, the adhesion of the galvanized layer deteriorates, and the galvanized layer breaks or falls off during processing and adheres to the mold, resulting in scratches on the galvanized steel sheet. Occur. Therefore, the range of the Fe content in the alloyed zinc plating layer obtained by the alloying treatment is preferably 7.0 to 15.0% by mass. More preferably, it is 8.0% by mass or more, or 14.0% by mass or less.
When alloying treatment is not performed, the Fe content in the galvanized layer is preferably less than 7.0% by mass. Even if the Fe content in the galvanized layer is less than 7.0% by mass, the galvanized steel sheet is excellent in corrosion resistance, formability and hole expansibility. Although the lower limit of the Fe content in the galvanized layer when alloying treatment is not performed is not particularly limited, the lower limit may be 1.0% by mass in terms of actual operation. By omitting the alloying treatment, it is excellent in economic efficiency and manufacturability.
[0048]
Ni content: 0.5-2.0 g/m 2
The galvanized layer (including the alloyed galvanized layer) of the hot-rolled steel sheet according to this embodiment preferably contains 0.5 to 2.0 g/m 2 of Ni. If the Ni content in the galvanized layer is less than 0.5 g/m 2 or more than 2.0 g/m 2 , good adhesion and alloying promotion effects may not be sufficiently obtained.
The Ni content in the plating layer can be adjusted by Ni pre-plating or the like.
[0049]
Al content: 0.1 to 1.0% by mass
　Al is added to the galvanizing bath to control the alloying reaction in the galvanizing bath. Therefore, the galvanized layer contains a small amount of Al. If the Al content in the galvanized layer is less than 0.1% by mass or more than 1.0% by mass, the alloying reaction in the galvanizing bath cannot be controlled, and the galvanized layer is properly alloyed. may not be possible. Therefore, the Al content in the galvanized layer is preferably 0.1 to 1.0% by mass.
[0050]
The content of Fe and Al in the above-mentioned zinc plating layer was determined by dissolving and removing only the zinc plating layer with an inhibitor-added 5% HCl aqueous solution, and using ICP to determine the content of Fe and Al in the solution (% by mass). obtained by measuring Regarding the Ni content (g/m2) in the galvanized layer, the Ni content (mass%) in the galvanized layer was measured in the same manner as described above, and the amount of zinc plating deposited (g/m2) was measured. 2) is obtained by measuring
[0051]
Although the coating weight of the galvanized layer according to the present embodiment is not particularly limited, it is preferable that the coating weight on one side is 5 g/m 2 or more from the viewpoint of corrosion resistance.
For the purpose of further improving paintability and weldability on the galvanized steel sheet according to the present embodiment, upper layer plating is applied, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement It does not deviate from the scope of the present invention even if it is subjected to a treatment or the like.
[0052]
Next, I will explain the reasons for limiting the manufacturing conditions.
The hot-rolled steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps.
(I) a heating step of directly or once cooling a cast slab having a predetermined chemical composition and then heating it to 1350° C. or more and 1400° C. or less;
(II) A hot-rolling step of hot-rolling the cast slab after the heating step to obtain a hot-rolled steel sheet,
(III) A winding step of winding the hot rolled steel sheet after the hot rolling step in a temperature range of 100° C. or less.
Further, when making ΔHv smaller in a cross section perpendicular to the rolling direction, it is preferable to further include the following steps.
(IV) a skin-pass rolling step of subjecting the hot-rolled steel sheet after the winding step to a skin-pass rolling with an elongation of 0.7% or more;
(V) A tempering step of heating to 430 to 560° C. after the temper rolling.
However, when the hot-rolled steel sheet is to be a galvanized steel sheet having a galvanized layer on the surface, it is preferable to perform the following step (V') instead of the above step (V).
(V') A hot-dip galvanizing step in which the hot-rolled steel sheet is subjected to Ni pre-plating, heated to 430 to 480°C at a temperature rising rate of 20°C/sec or more, and then hot-dip galvanized.
In addition, when the galvanized layer on the surface of the hot-rolled steel sheet is to be an alloyed galvanized layer, it is preferable to further perform the following step (VI) after the above step (V').
(VI) An alloying step of subjecting the hot-rolled steel sheet having the galvanized layer to alloying treatment at 470 to 560° C. for 10 to 40 seconds.
[0053]
Preferred conditions for each step are described below.
In manufacturing the hot-rolled steel sheet according to this embodiment, the manufacturing process preceding the heating process is not particularly limited. That is, following smelting by a blast furnace, an electric furnace, etc., various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting, casting by the ingot method, or thin slab casting. In the case of continuous casting, the cast slab may be cooled to a low temperature once, then heated again and then hot rolled, or the cast slab may be hot rolled directly after casting without cooling to a low temperature. . Scrap may be used as the raw material.
[0054]

In the heating process, the cast slab is directly or once cooled and then heated to 1350°C or higher and 1400°C or lower.
If the heating temperature is less than 1350°C, the dissolution of sulfide becomes insufficient, and undissolved sulfide remains. This sulfide extends in the rolling direction during hot rolling, causing an increase in anisotropy. Therefore, the heating temperature is set to 1350° C. or higher. Preferably, the heating temperature is above 1350°C.
On the other hand, if the heating temperature exceeds 1,400°C, scale formation is severe, the surface properties are deteriorated, and crystal grains are coarsened, resulting in a decrease in strength and low-temperature toughness of the hot-rolled steel sheet. Therefore, the heating temperature is set to 1400° C. or lower.
[0055]

In the hot rolling process, the cast slab is rolled so that the finish rolling temperature is 1000°C or higher, and cooling (first cooling) is started within 0.10 seconds after rolling. In the first cooling, the temperature is lowered by 50° C. or more at an average cooling rate of 100° C./sec or more.
After the first cooling, light reduction rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a reduction rate of 5% or more and 20% or less, and then the average cooling rate from the completion of light reduction rolling to the cooling stop temperature of 200 ° C. or less. Second cooling is performed so that the temperature is 50° C./sec or more. This makes the slab a hot-rolled steel sheet.
[0056]
When the finish rolling temperature is less than 1000°C, the texture develops and the anisotropy of the texture increases. Therefore, finish rolling temperature shall be 1000 degreeC or more.
On the other hand, when the finish rolling temperature exceeds 1100°C, the crystal grains become coarse. Therefore, the finishing temperature is preferably 1100° C. or lower.
[0057]
After finish rolling, the time until the start of cooling (the time from the completion of finish rolling to the start of cooling) exceeds 0.10 seconds, or the average cooling rate of the first cooling is less than 100 ° C./second, or the temperature drop due to cooling is If the temperature is lower than 50°C, the desired sulfide cannot be obtained and the toughness is lowered. Therefore, in the first cooling, cooling is started within 0.10 seconds after finish rolling, and the steel is cooled by 50°C or more at an average cooling rate of 100°C/second or more (the temperature drop is 50°C or more). In the first cooling, the subsequent light reduction is performed at a temperature equal to or higher than the Ar3 transformation point, so the cooling stop temperature is preferably equal to or higher than the Ar3 transformation point. Although it is not necessary to limit the upper limit of the average cooling rate of the first cooling, it may be set to 1000° C./sec or less in consideration of facilities and the like.
When cooling within 0.10 seconds after finish rolling, for example, a method of cooling using a cooling device between stands of a tandem rolling mill is exemplified.
In the present embodiment, sulfides are finely precipitated under light pressure, which will be described later. If sulfides are precipitated before the soft reduction process, the sulfides are elongated by the reduction and the aspect ratio increases.Control so that sulfide does not precipitate.
[0058]
In the method for manufacturing a hot-rolled steel sheet according to the present embodiment, since sulfides are finely precipitated after the completion of the first cooling described above, rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a rolling reduction of 5% or more and 20% or less. (Light reduction rolling) is performed.
When the light reduction rolling temperature is lower than the Ar3 transformation point, ferrite is generated. Therefore, the light reduction rolling temperature is set to the Ar3 transformation point or higher in order to suppress the formation of ferrite. Also, if the rolling reduction in light reduction rolling is less than 5%, the effect of finely precipitating sulfides cannot be obtained sufficiently, and if the rolling reduction exceeds 20%, the anisotropy becomes large. Therefore, the reduction ratio of light reduction rolling is set to 5% or more and 20% or less.
Here, the Ar3 transformation point is obtained by using a fully automatic transformation recording measuring device manufactured by Fuji Denpa Koki Co., Ltd., and heating a test piece of a predetermined shape at 950 ° C. for 30 minutes, then heating it at a rate of 30 ° C./sec. It can be measured by cooling with and measuring the expansion curve.
[0059]
After light reduction rolling, the steel is cooled to the coiling temperature so that the average cooling rate from the light reduction rolling completion temperature to 200°C or less is 50°C/sec or more, and coiled in a temperature range of 100°C or less. . If the cooling rate from the rolling completion temperature to a temperature of 200 ° C. or less is less than 50 ° C./sec or the coiling temperature (cooling stop temperature) is more than 100 ° C., a large amount of retained austenite, ferrite, and bainite are generated, and marten The site volume fraction cannot be greater than 99%.
[0060]

After winding, temper rolling may be performed for the purpose of correcting the shape of the steel sheet, preventing yield point elongation, and homogenizing the hardness distribution in the thickness direction. From the viewpoint of shape correction and prevention of yield point elongation, the elongation rate is preferably 0.2% or more. From the viewpoint of homogenizing the hardness distribution in the plate thickness direction, the elongation rate is preferably 0.7% or more. If the elongation percentage is less than 0.7%, the above effect cannot be sufficiently obtained. On the other hand, if the elongation exceeds 3.0%, the yield ratio increases significantly and the elongation deteriorates.
The elongation rate during temper rolling can be obtained, for example, from the difference between the number of revolutions of the payoff reel on the entry side and the number of rotations of the tension reel on the delivery side.
[0061]

If necessary, pickling may be performed after hot rolling or after temper rolling in order to remove scale generated during hot rolling. When pickling is carried out, the pickling conditions may be known conditions.
[0062]

In the case of controlling ΔHv to 50 or less and not forming a galvanized layer, the hot-rolled steel sheet according to the present embodiment is subjected to temper rolling or pickling after temper rolling. After that, it is preferable to perform a tempering treatment by heating to a temperature range of 430 to 560°C.
If the heating temperature is less than 430°C, the desired structure cannot be obtained due to insufficient tempering. On the other hand, when the heating temperature exceeds 560°C, the retained austenite is decomposed to form ferrite and cementite, and the metal structure of the finally obtained steel sheet becomes heterogeneous, resulting in an uneven hardness distribution in the thickness direction. become homogenous.
[0063]

In the case of controlling ΔHv to 50 or less and forming a galvanized layer on the surface of the hot-rolled steel sheet according to the present embodiment, pickling is performed after temper rolling or after temper rolling. After that, a galvanizing step is performed instead of the tempering step described above. In this galvanizing step, first, Ni pre-plating is performed, and after Ni pre-plating is performed, after heating to a temperature range of 430 to 480 ° C. at an average temperature increase rate of 20 ° C./sec or more, for example, in a hot dip galvanizing bath to obtain a galvanized steel sheet. The temperature here is the surface temperature of the steel plate.
If the average heating rate before hot-dip galvanizing is less than 20°C/sec, the strain introduced by temper rolling is relaxed and the effect of promoting alloying cannot be obtained. If the heating temperature before hot-dip galvanizing is less than 430° C., non-plating tends to occur during hot-dip galvanizing. If the heating temperature before hot-dip galvanizing exceeds 480° C., the strain introduced by temper rolling is relaxed and the effect of promoting alloying cannot be obtained. Also, the tensile strength may decrease. When alloying is not performed, press formability, weldability, and paint corrosion resistance are inferior to those when alloying is performed.
The Ni pre-plating method may be electroplating, immersion plating, or spray plating, and the plating weight is preferably about 1.0 to 4.0 g/m 2 . If Ni pre-plating is not performed, the effect of promoting alloying cannot be obtained, and the alloying temperature must be increased, so the effect of improving the hole expansibility cannot be obtained in the galvanized steel sheet.
[0064]

The galvanized hot-rolled steel sheet may, if necessary, be subjected to an alloying treatment by holding it in a temperature range of 470-560°C for 10-40 seconds. Accordingly, by increasing the Fe concentration in the galvanized layer to 7.0% by mass or more, the spot weldability and paintability of the galvanized steel sheet can be further improved. If the temperature during the alloying treatment is less than 470°C, the alloying will be insufficient. If the temperature during the alloying treatment exceeds 560° C., the retained austenite is decomposed to form cementite, so that the desired microstructure cannot be obtained and the ductility and strength are lowered. Moreover, sufficient hole expansibility may not be obtained. The time for the alloying treatment is determined by the balance with the alloying temperature, but is preferably in the range of 10 to 40 seconds. If the alloying treatment time is less than 10 seconds, the alloying is difficult to proceed, and if it exceeds 40 seconds, the retained austenite is decomposed to form cementite, so that a predetermined microstructure cannot be obtained, and a sufficient hole expandability improvement effect is obtained. may not be obtained.
[0065]
After the tempering process, galvanizing process, or alloying process, the finally obtained hot-rolled steel sheet is subjected to temper rolling at an elongation rate of 0.2 to 1.0% for the purpose of shape correction and prevention of yield point elongation. You can go further. If the elongation percentage is less than 0.2%, the above effect cannot be sufficiently obtained, and if the elongation percentage exceeds 1.0%, the yield ratio increases significantly and the elongation deteriorates.
Example
[0066]
Hereinafter, the effects of the present invention will be described more specifically with reference to examples. These examples are examples for confirming the effect of the present invention, and do not limit the present invention.
[0067]
Cast steel with chemical compositions shown in Tables 1-1 and 1-2, and shown in Tables 2-1, 2-2, 4-1, 4-2, and 6-1 to 6-4. Heating, rolling, first cooling, light reduction rolling, second cooling, and coiling were performed under the conditions. The heating temperature in Tables 6-1 to 6-4 indicates the heating temperature of the slab, and the rolling completion temperature indicates the finish temperature of hot rolling before the first cooling.
After that, No. in Tables 2-1 and 2-2. 1 to 24 were subjected to temper rolling, Ni pre-plating, hot-dip galvanizing, and alloying treatment under the conditions shown in Table 2-2, resulting in galvanized hot-rolled steel shown in Tables 3-1 and 3-2. A steel sheet (alloyed hot-dip galvanized hot-rolled steel sheet) was obtained.
Also, No. in Tables 4-1 and 4-2. For 25 to 46, under the conditions shown in Tables 4-1 and 4-2, temper rolling, Ni pre-plating and hot-dip galvanizing (45 g/m 2 on one side on both sides) were performed, , to obtain a galvanized hot-rolled steel sheet (hot-dip galvanized hot-rolled steel sheet) shown in Table 5-2.
Also, No. in Tables 6-1 to 6-4. For 47 to 88, some of the steel sheets were subjected to skin pass rolling and tempering under the conditions shown in Tables 6-1 to 6-4 to obtain the heat shown in Tables 7-1 to 7-4. A rolled steel sheet (hot-rolled steel sheet without galvanization) was obtained.
Both the galvanized hot-rolled steel sheet and the hot-rolled steel sheet finally obtained had a thickness of 5.0 mm. In addition, the prior austenite grain size of the finally obtained galvanized hot-rolled steel sheet and hot-rolled steel sheet was as follows. 13, No. 37, No. 59, No. All except for No. 81 were within the range of 12 μm or more and 100 μm or less. No. 13, No. 37, No. 59, No. The prior austenite grain size of 81 was greater than 100 μm.
[0068]
Fractions of martensite (fresh martensite and tempered martensite), retained austenite, ferrite and other structures of the obtained hot-dip galvanized hot-rolled steel sheet or hot-rolled steel sheet, average aspect ratio of prior austenite grains, and prior austenite grains Ratio of sulfides with an aspect ratio of more than 3.0 among sulfides with a diameter and area of ​​1.0 μm 2 or more, extreme density of {211} <011> orientation, difference between maximum and minimum values ​​of Vickers hardness ΔHv, and the Fe content, Ni content and Al content of the galvanized layer were evaluated by the methods described above.
[0069]
In addition, as mechanical properties, JIS No. 5 tensile test pieces were taken from the L direction (rolling direction) and C direction (perpendicular to the rolling direction) in accordance with JIS Z 2241:2011, and tensile tests were performed. Tensile strength (TS) and total elongation (EL) were obtained from the stress-strain curve of the tensile test.
The toughness was evaluated by taking sub-size V-notch Charpy test specimens of 5 mm width (x 10 mm x 55 mm length) from the L direction and C direction and conducting the Charpy test in accordance with JIS Z 2242:2018.
Tensile strength (L direction and C direction) is 980 MPa or more, total elongation is 10.0% or more, and Charpy absorbed energy (vE-40°C) at -40°C (L direction and C direction) is 50 J/cm 2 or more. If there is, it is determined that it has high strength, excellent ductility, and excellent toughness.
In addition, if the product of the tensile strength (TS) in the C direction and the hole expansion rate (λ) is TS (MPa) × λ (%) ≥ 38000 MPa ·%, it has good hole expandability, and TS (MPa )×λ(%)≧40000 MPa·%, it was judged to have excellent hole expansibility.
Also, if the ratio of the value in the L direction to the value in the C direction of each characteristic value (value in the L direction/value in the C direction) was 0.90 or more and 1.10 or less, it was judged that the anisotropy was small.
[0070]
The presence or absence of non-plating was determined by visual observation of the plating appearance. When non-plating was not visually observed, the appearance of the plating was judged to be excellent and the sample was judged to be acceptable. If there was non-plating, it was judged to be unacceptable because it was inferior in practicality as a plated steel sheet.
[0071]
The adhesion of the galvanized layer was evaluated by performing a cylindrical deep drawing test (punch diameter: 40 mm, BHF (Blank Holder Force): 1 ton, drawing ratio: 2.0). and measured the degree of blackness of the tape. The degree of blackness was determined by measuring the lightness (L value), and the difference from the L value of the blank tape was defined as the degree of blackness. When the degree of blackness was less than 30%, it was judged as acceptable, and "OK" was written in the column of adhesion in the table. When the degree of blackness was 30% or more, it was judged as unacceptable, and "NG" was written in the column of adhesion in the table.
[0072]
The respective results are shown in Tables 3-1, 3-2, 5-1, 5-2, and 7-1 to 7-4.
[0073]
The Fe content shown in Tables 3-2 and 5-2 indicates the Fe content in the galvanized layer. In the alloyed hot-dip galvanized steel sheets (examples of the present invention) in Tables 3-1 and 3-2 that were alloyed, the Fe content was 7.0 to 15.0% by mass, and the alloying was It shows that you are well advanced. The hot-dip galvanized steel sheets (invention examples) in Tables 5-1 and 5-2 that were not alloyed had an Fe content of less than 7.0% by mass.
[0074]
[Table 1-1]

[0075]
[Table 1-2]

[0076]
[Table 2-1]

[0077]
[Table 2-2]

[0078]
[Table 3-1]

[0079]
[Table 3-2]

[0080]
[Table 4-1]

[0081]
[Table 4-2]

[0082]
[Table 5-1]

[0083]
[Table 5-2]

[0084]
[Table 6-1]

[0085]
[Table 6-2]

[0086]
[Table 6-3]

[0087]
[Table 6-4]

[0088]
[Table 7-1]

[0089]
[Table 7-2]

[0090]
[Table 7-3]

[0091]
[Table 7-4]

[0092]
From Tables 1-1 to 7-4, it can be seen that the steel sheets of the invention examples all have the target properties. On the other hand, none of the comparative examples whose chemical composition or manufacturing method was outside the scope of the present invention It can be seen that one or more properties are inferior.
The scope of the claims
[Claim 1]
in % by mass,
C: 0.08-0.25%,
Si: 0.01 to 1.00%,
Mn: 0.8-2.0%,
P: 0.020% or less,
S: 0.001 to 0.010%,
Al: 0.005 to 1.000%,
N: 0.0010 to 0.0100%,
Ti: 0.005 to 0.30%,
Ca: 0.0005 to 0.0100%,
Nb: 0 to 0.30%,
　V: 0 to 0.50%,
Cr: 0 to 3.0%,
Mo: 0-3.0%,
Ni: 0 to 5.0%,
Cu: 0 to 3.0%,
B: 0 to 0.0100%,
　Mg: 0 to 0.0100%,
Zr: 0 to 0.0500%,
REM: 0-0.050%,
and has a chemical composition with the balance consisting of Fe and impurities,
　The microstructure contains 99% or more of martensite in volume fraction, and the remaining structure consists of retained austenite and ferrite,
In the cross section parallel to the rolling direction,
　　The average aspect ratio of the prior austenite grains is less than 3.0,
Among sulfides with an area of ​​1.0 μm 2 or more, the proportion of sulfides with an aspect ratio of more than 3.0 is 1.0% or less,
{211}<011>orientation pole density is 3.0 or less at the center of the plate thickness,
　Tensile strength TS is 980 MPa or more
A hot-rolled steel sheet characterized by:
[Claim 2]
The tensile strength TS is 1180 MPa or more,
The hot-rolled steel sheet according to claim 1, characterized in that:
[Claim 3]
The volume fraction of tempered martensite is less than 5%,
The hot-rolled steel sheet according to claim 2, characterized in that:
[Claim 4]
　In a cross section perpendicular to the rolling direction, ΔHv, which is the difference between the maximum value and the minimum value of Vickers hardness, is 50 or less,
The hot-rolled steel sheet according to claim 1.
[Claim 5]
The volume fraction of fresh martensite is less than 3%,
The hot-rolled steel sheet according to claim 4, characterized in that:
[Claim 6]
The hot-rolled steel sheet according to any one of claims 1 to 5, characterized by having a galvanized layer on the surface.
[Claim 7]
The hot-rolled steel sheet according to claim 6, wherein the galvanized layer is an alloyed galvanized layer.
[Claim 8]
The chemical composition, in % by mass,
Nb: 0.005 to 0.30%,
　V: 0.01 to 0.50%,
Cr: 0.05 to 3.0%,
Mo: 0.05 to 3.0%,
Ni: 0.05 to 5.0%,
Cu: 0.10 to 3.0%,
B: 0.0003 to 0.0100%,
　Mg: 0.0005 to 0.0100%,
Zr: 0.0010 to 0.0500%,
REM: 0.0010-0.050%,
containing one or more selected from the group consisting of
The hot-rolled steel sheet according to any one of claims 1 to 7, characterized in that:
[Claim 9]
A method for manufacturing the hot-rolled steel sheet according to any one of claims 1 to 3,
% by mass, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 ~0.010%, Al: 0.005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3 .0%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance is Fe and impurities a heating step of heating a cast slab having a chemical composition of 1350° C. or higher and 1400° C. or lower directly or after cooling once;
A hot-rolling step of hot-rolling the cast slab after the heating step into a hot-rolled steel sheet,
A winding step of winding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 100°C or less;
has
　In the hot rolling process,
　　The cast slab is rolled so that the finish rolling temperature is 1000°C or higher,
After the end of the rolling, the cooling is started within 0.10 seconds, and the first cooling is performed so that the temperature is lowered by 50°C or more at an average cooling rate of 100°C/second or more,
After the first cooling, light reduction rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a reduction rate of 5% or more and 20% or less,
　　The second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200 ° C or below is 50 ° C / sec or more,
A method for manufacturing a hot-rolled steel sheet, characterized by:
[Claim 10]
A method for manufacturing the hot-rolled steel sheet according to claim 4 or 5,
% by mass, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 ~0.010%, Al: 0.005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3 .0%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance is Fe and impurities a heating step of heating a cast slab having a chemical composition of 1350° C. or higher and 1400° C. or lower directly or after cooling once;
A hot-rolling step of hot-rolling the cast slab after the heating step into a hot-rolled steel sheet,
A winding step of winding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 100 ° C. or less; A temper rolling process for rolling,
A tempering process of heating to 430 to 560°C after the temper rolling,
has
　In the hot rolling process,
　　The cast slab is rolled so that the finish rolling temperature is 1000°C or higher,
After the end of the rolling, the cooling is started within 0.10 seconds, and the first cooling is performed so that the temperature is lowered by 50°C or more at an average cooling rate of 100°C/second or more,
After the first cooling, light reduction rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a reduction rate of 5% or more and 20% or less,
　　The second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200 ° C or below is 50 ° C / sec or more,
A method for manufacturing a hot-rolled steel sheet, characterized by:
[Claim 11]
A method for manufacturing the hot-rolled steel sheet according to claim 6,
% by mass, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 ~0.010%, Al: 0.005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3 .0%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance is Fe and impurities a heating step of heating a cast slab having a chemical composition of 1350° C. or higher and 1400° C. or lower directly or after cooling once;
A hot-rolling step of hot-rolling the cast slab after the heating step into a hot-rolled steel sheet,
A winding step of winding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 100°C or less;
A temper rolling step of performing temper rolling with an elongation of 0.7% or more on the hot rolled steel sheet after the winding step;
a galvanization step of performing Ni pre-plating on the hot-rolled steel sheet, heating it to 430 to 480°C at a temperature rising rate of 20°C/sec or more, and then galvanizing;
has
　In the hot rolling process,
　　The cast slab is rolled so that the finish rolling temperature is 1000°C or higher,
After the end of the rolling, the cooling is started within 0.10 seconds, and the first cooling is performed so that the temperature is lowered by 50°C or more at an average cooling rate of 100°C/second or more,
After the first cooling, light reduction rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a reduction rate of 5% or more and 20% or less,
　　The second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200 ° C or below is 50 ° C / sec or more,
A method for manufacturing a hot-rolled steel sheet, characterized by:
[Claim 12]
A method for manufacturing the hot-rolled steel sheet according to claim 7,
% by mass, C: 0.08 to 0.25%, Si: 0.01 to 1.00%, Mn: 0.8 to 2.0%, P: 0.020% or less, S: 0.001 ~0.010%, Al: 0.005-1.000%, N: 0.0010-0.0100%, Ti: 0.005-0.30%, Ca: 0.0005-0.0100%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3 .0%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0500%, REM: 0 to 0.050%, and the balance is Fe and impurities a heating step of heating a cast slab having a chemical composition of 1350° C. or higher and 1400° C. or lower directly or after cooling once;
A hot-rolling step of hot-rolling the cast slab after the heating step into a hot-rolled steel sheet,
A winding step of winding the hot-rolled steel sheet after the hot-rolling step in a temperature range of 100°C or less;
A temper rolling step of performing temper rolling with an elongation of 0.7% or more on the hot rolled steel sheet after the winding step;
a galvanization step of performing Ni pre-plating on the hot-rolled steel sheet, heating it to 430 to 480°C at a temperature rising rate of 20°C/sec or more, and then galvanizing;
an alloying step of performing an alloying treatment at 470-560°C for 10-40 seconds after the zinc plating step;
has
　In the hot rolling process,
　　The cast slab is rolled so that the finish rolling temperature is 1000°C or higher,
After the end of the rolling, the cooling is started within 0.10 seconds, and the first cooling is performed so that the temperature is lowered by 50°C or more at an average cooling rate of 100°C/second or more,
After the first cooling, light reduction rolling is performed at a temperature equal to or higher than the Ar3 transformation point at a reduction rate of 5% or more and 20% or less,
　　The second cooling is performed so that the average cooling rate from the completion of the light reduction rolling to 200 ° C or below is 50 ° C / sec or more,
A method for manufacturing a hot-rolled steel sheet, characterized by: