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. Specifically, the present invention relates to a hot-rolled steel sheet having high strength and excellent ductility, perforation property and toughness, and a method for producing the same.
This application claims priority based on Japanese Patent Application No. 2019-201427 filed in Japan on November 6, 2019, and the contents thereof are incorporated herein by reference.
Background technology
[0002]
In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce carbon dioxide emissions in many fields. Automobile manufacturers are also actively developing technologies for reducing the weight of vehicle bodies with the aim of reducing fuel consumption. However, it is not easy to reduce the weight of the vehicle body because the emphasis is on improving the collision characteristics to ensure the safety of the occupants.
[0003]
In order to achieve both weight reduction of the vehicle body and collision characteristics, it is being considered to thin the members by using high-strength steel plates. Therefore, a steel sheet having both high strength and excellent formability is strongly desired. Among the moldability, a steel sheet having excellent ductility and hole expanding property is particularly desired. Further, the steel plate applied to the automobile body is also required to have excellent toughness in order to sufficiently absorb the impact at the time of a collision.
[0004]
For example, in Patent Document 1, the baynite fraction is 80% or more, and the average particle size r (nm) of the precipitate is expressed by the formula (r ≧ 207 ÷ (27.4 × (V) + 23.5 × (Nb) + 31). .4 × (Ti) ＋ 17.6 × (Mo) ＋ 25.5 × (Zr) ＋ 23.5 × (W)) is satisfied, and the average particle size r and the precipitate fraction f are given by the formula (r / f ≦ 12000). ), A high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretchable flangeability is disclosed.
[0005]
In Patent Document 2, the steel structure at a depth of 1/4 of the thickness from the surface of the steel sheet is, in area%, bainite: 60% or more, polygonal ferrite: 5% or more and less than 30%, retained austenite: less than 3%. The balance excluding bainite, retained austenite and polygonal ferrite: 10% or less, and the polygonal ferrite area ratio at a depth of 100 μm from the steel sheet surface and the polygonal ferrite area ratio at a depth of 1/4 of the plate thickness. Is the formula (Vαs> 1.5Vαq, where Vαs is the area ratio (%) of the polygonal ferrite at a depth of 100 μm from the surface of the steel sheet, and Vαq is the polygonal at a depth of 1/4 of the plate thickness from the surface of the steel sheet. A hot-rolled steel sheet characterized by satisfying (the area ratio of ferrite) is disclosed.
Prior art literature
Patent documents
[0006]
Patent Document 1: Japanese Patent Application Laid-Open No. 2009-84637
Patent Document 2: Japanese Patent Application Laid-Open No. 2016-50335
Outline of the invention
Problems to be solved by the invention
[0007]
However, in Patent Documents 1 and 2, toughness is not considered. The present inventors have found that it is necessary not only to improve ductility and hole-expandability but also to secure toughness in order to achieve both weight reduction of the vehicle body and collision characteristics.
[0008]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent ductility, hole-expanding property and toughness, and a method for producing the same.
[0009]
Further, the steel plate applied to the automobile body may be required to have excellent punching characteristics in addition to the above-mentioned characteristics. Therefore, it is an object of the present invention to preferably provide a hot-rolled steel sheet having excellent punching characteristics in addition to the above-mentioned characteristics and a method for producing the same.
Means to solve problems
[0010]
In view of the above-mentioned problems, the present inventors have obtained the following findings (a) to (e) as a result of intensive studies on the chemical composition of the hot-rolled steel sheet and the relationship between the metallographic structure and the mechanical properties. The invention was completed.
[0011]
(A) In order to obtain excellent ductility and hole-spreading property, it is necessary to make the total area ratio of bainite 80.0% or more.
[0012]
(B) By controlling the grain boundary density having a specific orientation in bainite, ductility, perforation property and toughness can be further improved.
[0013]
(C) In order to keep the grain boundary density having a specific orientation in bainite within a desired range, it is necessary to control the winding temperature, the holding temperature after winding, and the holding time.
[0014]
(D) In ​​order to improve the punching characteristics, it is necessary to control the average particle size and aspect ratio of the old austenite grains.
[0015]
(E) In order to obtain the desired average particle size and aspect ratio of the old austenite grains, it is necessary to control the hot rolling conditions more strictly. Specifically, in the hot rolling process, it is necessary to strictly control the total reduction rate of rough rolling and the reduction rate of the three subsequent stages of finish rolling.
[0016]
The gist of the present invention made based on the above findings is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition of mass%.
C: 0.030 to 0.200%,
Si: 0.05-2.50%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 2.000%,
Ti: 0.030 to 0.200%,
P: 0.020% or less,
S: 0.020% or less,
N: 0.010% or less,
Nb: 0 to 0.200%,
B: 0 to 0.010%,
V: 0 to 1.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0 to 1.00%,
Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0 to 1.00%,
Ca: 0-0.010%,
Mg: 0 to 0.010%,
REM: 0-0.010%, and
Zr: 0 to 0.010%
Containing, the balance consists of iron and impurities,
The metal structure is% of the area
Bainite: 80.0% or more,
Ferrite: 10.0% or less,
Remaining organization: 10.0% or less,
The sum of the densities of the grain boundary length L 7 having a crystal orientation difference of 7 ° and the grain boundary length L 68 having a crystal orientation difference of 68 ° in the bainite about the <110> direction is 0. It is .35 to 0.60 μm / μm 2,
The tensile strength is 780 MPa or more.
(2) The hot-rolled steel sheet according to (1) above has a chemical composition of% by mass.
Nb: 0.005 to 0.200%,
B: 0.001 to 0.010%,
V: 0.005 to 1.00%,
Mo: 0.005 to 1.00%,
Cu: 0.005 to 1.00%,
W: 0.005 to 1.00%,
Cr: 0.005 to 1.00%,
Ni: 0.005 to 1.00%,
Co: 0.005 to 1.00%,
Ca: 0.0005-0.010%,
Mg: 0.0005-0.010%,
REM: 0.0005-0.010%, and
Zr: 0.0005-0.010%
It may contain one or more of the group consisting of.
(3) The hot-rolled steel sheet according to (1) or (2) above has the above-mentioned metal structure.
The average particle size of the old austenite grains is 10 to 30 μm,
The ratio l d / S d of the long axis l d and the short axis S d of the old austenite grains may be 2.0 or less.
(4) The method for producing a hot-rolled steel sheet according to another aspect of the present invention is
A heating step of holding the slab having the chemical composition described in (1) above at a heating temperature of 1200 ° C. or higher for 1.0 hour or longer,
A hot rolling process in which rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher and the total rolling reduction ratio exceeds 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980 ° C. ,
After cooling to a temperature range of 570 to 620 ° C at an average cooling rate of 20 ° C./s or higher and winding, the mixture is held in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours, and then cooled to room temperature. It has a cooling process.
(5) The method for manufacturing a hot-rolled steel sheet according to (4) above is as follows.
In the hot rolling process
The total rolling reduction in the rough rolling was 70% or more.
The finish rolling may be performed so that the rolling reduction of all three steps after the finish rolling is less than 25%.
Effect of the invention
[0017]
According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility, hole-expanding property and toughness, and a method for producing the same. According to the above-mentioned preferred embodiment according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent punching characteristics in addition to the above-mentioned characteristics and a method for producing the same.
Mode for carrying out the invention
[0018]
The chemical composition and metallographic structure of the hot-rolled steel sheet (hereinafter, may be simply referred to as a steel sheet) according to the present embodiment will be specifically described below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.
[0019]
The numerical limit range described below with "~" in between includes the lower limit value and the upper limit value. Numerical values ​​that indicate "less than" or "greater than" do not fall within the numerical range. In the following description,% with respect to chemical composition is mass% unless otherwise specified.
[0020]
Chemical composition
The hot-rolled steel sheet according to the present embodiment has C: 0.030 to 0.200%, Si: 0.05 to 2.50%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 2.000%, Ti: 0.030 to 0.200%, P: 0.020% or less, S: 0.020% or less, N: 0.010% or less, and the balance: Contains Fe and impurities. Each element will be described in detail below.
[0021]
C: 0.030 to 0.200%
C is an element that promotes the formation of bainite by improving the strength of the hot-rolled steel sheet and improving the hardenability. In order to obtain this effect, the C content is set to 0.030% or more. Preferably, the C content is 0.040% or more.
On the other hand, when the C content exceeds 0.200%, it becomes difficult to control the formation of bainite, a large amount of martensite is formed, and both the ductility and the hole-expanding property of the hot-rolled steel sheet, or one of them, become difficult. descend. Therefore, the C content is set to 0.200% or less. The C content is preferably 0.180% or less.
[0022]
Si: 0.05-2.50%
Si is an element that contributes to solid solution strengthening and contributes to improving the strength of hot-rolled steel sheets. Further, Si has an action of making the steel sound by deoxidation (suppressing the occurrence of defects such as blow holes in the steel). If the Si content is less than 0.05%, the effect of the above action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, more preferably 1.00% or more.
On the other hand, Si is an element that promotes the formation of a mixture (MA) of hard martensite (hereinafter, when simply referred to as martensite, it means fresh martensite) and retained austenite. If the Si content exceeds 2.50%, MA is generated and the hole expandability of the hot-rolled steel sheet is lowered. Therefore, the Si content is 2.50% or less. The Si content is preferably 2.30% or less, more preferably 2.00% or less.
[0023]
Mn: 1.00 to 4.00%
Mn dissolves in the steel and contributes to the increase in the strength of the hot-rolled steel sheet, and promotes the formation of bainite by improving the hardenability, improving the hole-expanding property of the hot-rolled steel sheet. In order to obtain such an effect, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.30% or more.
On the other hand, if the Mn content exceeds 4.00%, it becomes difficult to control the formation of bainite, it becomes impossible to obtain a desired amount of bainite, and the ductility and hole expansion property of the hot-rolled steel sheet, or one of them, becomes difficult. descend. Therefore, the Mn content is set to 4.00% or less. Preferably, the Mn content is 3.50% or less.
[0024]
Sol. Al: 0.001 to 2.000%
Al, like Si, has the effect of deoxidizing steel and making it sound. sol. If the Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, sol. The Al content is 0.001% or more. sol. The Al content is preferably 0.010% or more.
On the other hand, sol. If the Al content exceeds 2.000%, it causes an increase in oxide-based inclusions, which causes an increase in oxide-based inclusions. The hole expandability of the hot-rolled steel sheet is reduced. Therefore, sol. The Al content is 2.000% or less. sol. The Al content is preferably 1.500% or less, more preferably 1.300% or less.
In this embodiment, sol. Al means acid-soluble Al, and indicates solid solution Al existing in steel in a solid solution state.
[0025]
Ti: 0.030 to 0.200%
Ti precipitates in steel as carbide or nitride, and has the effect of improving the strength of hot-rolled steel sheet by refining the metal structure by the pinning effect. If the Ti content is less than 0.030%, the effect of the above action cannot be obtained. Therefore, the Ti content is set to 0.030% or more. The Ti content is preferably 0.050% or more, and more preferably 0.080% or more.
On the other hand, when the Ti content exceeds 0.200%, the old austenite grains are less likely to recrystallize, and the rolled texture develops, so that the hole expansion property of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.170% or less, more preferably 0.150% or less.
[0026]
P: 0.020% or less
P is an element that dissolves in steel and contributes to increasing the strength of hot-rolled steel sheets. However, P is also an element that segregates at the grain boundaries, particularly the former austenite grain boundaries, and promotes the grain boundary fracture due to the grain boundary segregation, thereby causing a decrease in the workability of the hot-rolled steel sheet. The P content is preferably as low as possible, but a P content of up to 0.020% is acceptable. Therefore, the P content is set to 0.020% or less. Preferably, the P content is 0.015% or less.
The P content is preferably 0%, but if it is reduced to less than 0.0001%, the manufacturing cost will increase, so the P content may be 0.0001% or more.
[0027]
S: 0.020% or less
S is an element that adversely affects weldability and manufacturability during casting and hot rolling. S combines with Mn to form coarse MnS. This MnS deteriorates the bendability and hole widening property of the hot-rolled steel sheet, and promotes the occurrence of delayed fracture. The S content is preferably as low as possible, but the content of S up to 0.020% is acceptable. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.015% or less.
The S content is preferably 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases and it is economically disadvantageous. Therefore, the S content may be 0.0001% or more.
[0028]
N: 0.010% or less
N is an element that forms a coarse nitride in steel. This nitride deteriorates the bendability and hole expansion property of the hot-rolled steel sheet. Therefore, the N content is set to 0.010% or less. Preferably, the N content is 0.008% or less.
Since reducing the N content to less than 0.0001% causes a significant increase in manufacturing cost, the N content may be 0.0001% or more.
[0029]
The rest of the chemical composition of the hot-rolled steel sheet according to this embodiment consists of Fe and impurities. In the present embodiment, the impurities mean those mixed from ore, scrap, manufacturing environment, etc. as raw materials, and / or those permitted within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
[0030]
The hot-rolled steel sheet according to this embodiment may contain the following elements as optional elements in addition to a part of Fe. The lower limit of the content when the following optional elements are not contained is 0%. Hereinafter, each arbitrary element will be described in detail.
[0031]
Nb: 0 to 0.200%
Nb is an element that forms carbides during hot rolling and contributes to improving the strength of hot-rolled steel sheets by strengthening precipitation. In order to surely obtain this effect, the Nb content is preferably 0.005% or more.
On the other hand, if the Nb content exceeds 0.200%, the recrystallization temperature of the old austenite grains becomes too high, the texture develops, and the hole expansion property of the hot-rolled steel sheet may decrease. Therefore, the Nb content is set to 0.200% or less.
[0032]
B: 0 to 0.010%
B is an element that segregates at the old austenite grain boundaries, suppresses the formation and growth of ferrite, and contributes to the improvement of the strength and hole-spreading property of hot-rolled steel sheets. In order to obtain these effects, the B content is preferably 0.001% or more.
On the other hand, even if B is contained in excess of 0.010%, the above effect is saturated. Therefore, the B content is set to 0.010% or less.
[0033]
V: 0 to 1.00%
V is an element that forms a carbonitride during hot rolling and contributes to the improvement of the strength of the hot-rolled steel sheet by precipitation strengthening. In order to surely obtain this effect, the V content is preferably 0.005% or more.
On the other hand, if the V content exceeds 1.00%, coarse carbides are generated in the slab, which causes cracks in the heating process. Therefore, the V content is set to 1.00% or less.
[0034]
Mo: 0 to 1.00%
Mo is an element that promotes the formation of bainite by improving the hardenability of steel and contributes to the improvement of the strength and hole expansion of hot-rolled steel sheets. In order to surely obtain this effect, the Mo content is preferably 0.005% or more.
On the other hand, if the Mo content exceeds 1.00%, martensite is likely to be generated, and both the elongation and the hole expanding property of the hot-rolled steel sheet, or one of them may decrease. Therefore, the Mo content is set to 1.00% or less.
[0035]
Cu: 0 to 1.00%
Cu is an element that is effective in ensuring the strength of hot-rolled steel sheets in a stable manner. Therefore, Cu may be contained. However, even if it is contained in an amount exceeding 1.00%, the effect of the above action is likely to be saturated and may be economically disadvantageous. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, more preferably 0.50% or less. The Cu content is preferably 0.005% or more in order to obtain the effect of the above action more reliably.
[0036]
W: 0 to 1.00%
W is an element that is effective in improving the strength of hot-rolled steel sheets by solid or precipitation. However, even if it is contained in an amount exceeding 1.00%, the effect of the above action is likely to be saturated and may be economically disadvantageous. Therefore, the W content is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. The W content is preferably 0.005% or more in order to obtain the effect of the above action more reliably.
[0037]
Cr: 0 to 1.00%
Cr is an element that is effective in improving the hardenability and the strength of hot-rolled steel sheets. However, even if it is contained in an amount exceeding 1.00%, the effect of the above action is likely to be saturated and may be economically disadvantageous. Therefore, the Cr content is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. The Cr content is preferably 0.005% or more in order to obtain the effect of the above action more reliably.
[0038]
Ni: 0 to 1.00%
Ni is an element that is effective in improving the hardenability and the strength of hot-rolled steel sheets. However, if it is contained in excess of 1.00%, the hardenability is excessively increased and the martensite structure fraction is increased, which may deteriorate the hole expanding property of the hot-rolled steel sheet. Therefore, the Ni content is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. The Ni content is preferably 0.005% or more in order to obtain the effect of the above action more reliably.
[0039]
Co: 0 to 1.00%
Co is an element that is effective in improving the strength of hot-rolled steel sheets by strengthening solid solution. However, even if it is contained in an amount exceeding 1.00%, the effect of the above action is likely to be saturated and may be economically disadvantageous. Therefore, the Co content is set to 1.00% or less. It is preferably 0.80% or less, more preferably 0.50% or less. The Co content is preferably 0.005% or more in order to obtain the effect of the above action more reliably.
[0040]
Ca: 0 to 0.010%
Mg: 0 to 0.010%
REM: 0-0.010%
Zr: 0 to 0.010%
Ca (calcium), Mg (magnesium), REM (rare earth element), and Zr (zirconium) are all elements that contribute to inclusion control, especially fine dispersion of inclusions, and enhance the toughness of hot-rolled steel sheets. Is. Therefore, these elements may be contained. However, if each of the elements is contained in an amount of more than 0.010%, deterioration of the surface texture may become apparent. Therefore, the content of each of these elements shall be 0.010% or less. The content of each of these elements is preferably 0.005% or less, more preferably 0.003% or less, respectively. In order to obtain the effect of the above action more reliably, it is preferable that each element is 0.0005% or more.
[0041]
In the present embodiment, REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
[0042]
The chemical composition of the hot-rolled steel sheet may be measured by a general analysis method. For example, measurement may be performed using ICP-AES (Inductively Coupled Plasma-Atomic Measurement Spectrometry) or emission spectroscopy (OES). C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-heat conductivity method.
[0043]
Metal structure of hot-rolled steel sheet
Next, the metal structure 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 metal structure having an area% of bainite: 80.0% or more, ferrite: 10.0% or less, and a residual structure of 10.0% or less. With the <110> direction as the axis, the total density of the grain boundary length L 7 having a crystal orientation difference of 7 ° and the grain boundary length L 68 having a crystal orientation difference of 68 ° is 0.35 to 0. It is 60 μm / μm 2.
[0044]
Further, in the hot-rolled steel sheet according to the present embodiment, in the above-mentioned metal structure, the average particle size of the former austenite grains is 10 to 30 μm, and the ratio of the major axis ld and the minor axis S d of the former austenite grains is l d /. S d may be 2.0 or less.
[0045]
In the present embodiment, the metal structure is defined as a cross section parallel to the rolling direction, a depth of 1/4 of the plate thickness from the surface, and a metal structure at the center position in the plate width direction. The reason is that the metallographic structure at this position represents a typical metallographic structure of the steel sheet.
[0046]
Bainite: 80.0% or more
Bainite means a structure having a lath-shaped bainitic ferrite and a structure having Fe-based carbides between and / or inside the bainitic ferrite. Unlike polygonal ferrite, bainitic ferrite has a lath-like shape and has a relatively high dislocation density inside, so it can be easily distinguished from other structures using SEM or TEM.
[0047]
When the area ratio of bainite is less than 80.0%, the toughness and hole expansion property of the hot-rolled steel sheet are significantly reduced. Therefore, the area ratio of bainite is set to 80.0% or more. It is preferably 85.0% or more, and more preferably 90.0% or more. The higher the area ratio of bainite, the more preferable, but since it is difficult to achieve an area ratio of 97.5% or more due to the presence of ferrite, cementite, or MA (mixture of retained austenite and martensite), the practical upper limit is It may be 97.5%.
[0048]
Ferrite: 10.0% or less
Ferrite is polygonal ferrite, and bainitic ferrite is not included in ferrite. If the area ratio of ferrite is more than 10.0%, the desired tensile strength cannot be obtained. Therefore, ferrite The area ratio of is 10.0% or less. It is preferably 5.0% or less. From the viewpoint of ensuring ductility, the area ratio of ferrite may be 1.0% or more.
[0049]
Remaining tissue (cementite, pearlite, martensite, tempered martensite and retained austenite): 10.0% or less in total
Cementite, pearlite, martensite, tempered martensite and retained austenite are all structures that become the starting point of voids during deformation and deteriorate the hole-expandability of hot-rolled steel sheets. If the total area ratio of these residual structures exceeds 10.0%, the desired ductility and perforation property cannot be obtained. Therefore, the area ratio of the residual structure (cementite, pearlite, martensite, tempered martensite and retained austenite) shall be 10.0% or less. It is preferably 5.0% or less.
[0050]
On the other hand, in tissue control, it is practically difficult to control the area ratio of the remaining tissue to less than 1.0%, so the area ratio of the remaining tissue may be 1.0% or more.
In addition, the smaller the total area ratio of martensite and tempered martensite in the remaining structure, the more stable and excellent perforation property can be obtained. Therefore, the total area ratio of martensite and tempered martensite is 5. It is preferably 0.0% or less. More preferably, it is 3.0% or less.
[0051]
The method of measuring the area ratio of each tissue will be described below.
A test piece is collected from a hot-rolled steel sheet so that a metal structure can be observed from the surface at a depth of 1/4 of the plate thickness and at the center position in the plate width direction with a cross section parallel to the rolling direction.
After polishing the cross section of the test piece with silicon carbide paper of # 600 to # 1500, a diamond powder having a particle size of 1 to 6 μm is mirrored using a diluted solution such as alcohol or a liquid dispersed in pure water. Finish. Next, polishing is performed with colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. 50 μm in length, 1/8 depth from surface to plate thickness to 3 Crystal orientation information is obtained by measuring a region with a depth of / 8 by electron backscattering diffraction at a measurement interval of 0.1 μm.
[0052]
For the measurement, an EBSD analyzer composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD analyzer is 9.6 × 10 -5 Pa or less, the acceleration voltage is 15 kv, the irradiation current level is 13, and the electron beam irradiation level is 62. The obtained crystal orientation information is used to calculate the area ratio of retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Those having a crystal structure of fcc are judged to be retained austenite.
[0053]
Next, those having a bcc crystal structure are judged to be bainite, ferrite, and "residual structure other than retained austenite (cementite, pearlite, martensite, and tempered martensite)". For these regions, using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, under the condition that the 15 ° grain boundary is defined as the grain boundary. The region where "Grain Orientation Spread" is 1 ° or less is extracted as ferrite. By calculating the area ratio of the extracted ferrite, the area ratio of ferrite is obtained.
[0054]
Subsequently, in the remaining region (the region where "Grain Orientation Spread" exceeds 1 °), the maximum value of "Grain Average IQ" in the ferrite region is set to Iα under the condition that the 5 ° grain boundary is defined as the grain boundary. Then, the region above Iα / 2 is extracted as bainite, and the region below Iα / 2 is extracted as “residual structure other than retained austenite (cementite, pearlite, martensite and tempered martensite)”. By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained. In addition, the area ratio of the extracted "residual tissue other than retained austenite (cementite, pearlite, martensite and tempered martensite)" is calculated, and the area ratio of the above retained austenite is added to obtain the residual structure (cementite, pearlite). , Martensite, tempered martensite and retained austenite).
[0055]
Regarding the above-extracted "residual tissues other than retained austenite (cementite, pearlite, martensite and tempered martensite)", cementite, pearlite, martensite and tempered martensite can be distinguished by the following method. First, in order to observe the same region as the EBSD measurement region by SEM, a Vickers indentation is imprinted in the vicinity of the observation position. After that, the contamination on the surface layer is removed by polishing, leaving the structure of the observation surface, and nightal etching is performed. Next, the same field of view as the EBSD observation surface is observed by SEM at a magnification of 3000 times.
[0056]
In the EBSD measurement, among the regions determined to be the residual texture, the region having a substructure in the grain and where cementite is precipitated with a plurality of variants is determined to be tempered martensite. The region where cementite is deposited in a lamellar manner is judged to be pearlite. Spherical particles with high brightness and particle size (diameter equivalent to a circle) of 2 μm or less are judged to be cementite. The region where the brightness is high and the substructure is not exposed by etching is judged as "martensite and retained austenite". By calculating the area ratio of each, the area ratio of tempered martensite, pearlite, martensite, and "martensite and retained austenite" is obtained. The area ratio of martensite can be obtained by subtracting the area ratio of retained austenite obtained by the above-mentioned EBSD from the area ratio of the obtained "martensite and retained austenite".
[0057]
For removing contamination on the surface layer of the observation surface, a method such as buffing using alumina particles having a particle size of 0.1 μm or less or Ar ion sputtering may be used.
[0058]
The sum of the densities of the grain boundary length L 7 having a crystal orientation difference of 7 ° and the grain boundary length L 68 having a crystal orientation difference of 68 ° in bainite about the <110> direction: 0. 35-0.60 μm / μm 2
The sum of the densities of the grain boundary length L 7 having a crystal orientation difference of 7 ° and the grain boundary length L 68 having a crystal orientation difference of 68 ° in bainite about the <110> direction is 0. By setting it to 35 to 0.60 μm / μm 2, it is possible to improve the dexterity, hole-expandability and toughness of the hot-rolled steel plate.
[0059]
If the total density of L 7 and L 68 is less than 0.35 μm / μm 2, the toughness of bainite is significantly reduced, and the desired toughness cannot be obtained in the hot-rolled steel sheet. Therefore, the total density of L 7 and L 68 is set to 0.35 μm / μm 2 or more. Preferably, it is 0.40 μm / μm 2 or more. On the other hand, when the total density of L 7 and L 68 exceeds 0.60 μm / μm 2, the ductility of bainite is lowered, and excellent ductility and hole expansion property cannot be obtained in a hot-rolled steel sheet. Therefore, the total density of L 7 and L 68 is set to 0.60 μm / μm 2 or less. Preferably, it is 0.55 μm / μm 2 or less.
[0060]
The grain boundary having a crystal orientation difference of X ° about the <110> direction means that when two adjacent crystal grains A and crystal grains B are specified at a certain grain boundary, one crystal grain B is defined as <110. 110> Refers to a grain boundary having a crystal boundary in which the crystal orientations of the crystal grains A and the crystal grains B are the same when rotated by X ° about the axis. However, considering the measurement accuracy of the crystal orientation, an orientation difference of ± 4 ° is allowed from the matching orientation relation.
[0061]
In the present embodiment, the grain boundary lengths L7 and L68 as described above are measured by using the EBSP-OIM (Electron Backscatter Diffraction Pattern-Orientation Image Microscopic) method. In the EBSP-OIM method, a highly inclined sample is irradiated with an electron beam in a scanning electron microscope (SEM), the Kikuchi pattern formed by backscattering is photographed with a high-sensitivity camera, and the photographed photograph is image-processed by a computer. By doing so, the crystal orientation of the irradiation point can be measured in a short waiting time. The EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer and an OIM Analysis (registered trademark) manufactured by AMETEK.
[0062]
Since the EBSP-OIM method can analyze the fine structure of the sample surface and the crystal orientation, the length of the grain boundary having a specific crystal orientation difference can be quantitatively obtained. The analyzable area of ​​the EBSP-OIM method is an area that can be observed by SEM. Although it depends on the resolution of the SEM, according to the EBSP-OIM method, analysis can be performed with a minimum resolution of 20 nm.
[0063]
When measuring the density of the length of a specific grain boundary in a metal structure at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction with a cross section parallel to the rolling direction, a magnification of 1000 times, 50 μm × 50 μm. L 7 is obtained by performing analysis in at least 5 visual fields in the region and calculating the average value of the lengths of grain boundaries having a crystal orientation difference of 7 ° about the <110> direction in the baynite. Similarly, L 68 is obtained by calculating the average value of the lengths of the grain boundaries having a crystal orientation difference of 68 ° about the <110> direction in bainite. As described above, a directional difference of ± 4 ° is allowed.
[0064]
By dividing the obtained L 7 and L 68 by the measurement area, the length L 7 of the grain boundary having a crystal orientation difference of 7 ° and the crystal orientation difference of 68 in the baynite with the <110> direction as the axis. Obtain the sum of the densities of grain boundary length L 68, which is °. In order to extract only bainite and measure the density of the length of a specific grain boundary, the region having more than Iα / 2 may be extracted as bainite, as in the case of obtaining the area ratio of bainite.
[0065]
Average particle size of old austenite grains: 10 to 30 μm
Ratio l d / S d of the long axis l d of the old austenite grain to the short axis S d: 2.0 or less
In the hot-rolled steel sheet according to the present embodiment, the average particle size of the old austenite grains is 10 to 30 μm, and the ratio l d / S d of the long axis ld and the short axis S d of the old austenite grains is 2.0 or less. It may be. By controlling the average particle size of the old austenite grains and l d / S d within the above range, the punching characteristics of the hot-rolled steel sheet can be improved.
[0066]
The method of measuring the average particle size of the former austenite grains and the ratio l d / S d of the major axis ld and the minor axis S d of the former austenite grains will be described below.
A test piece is collected from a hot-rolled steel sheet so that a metal structure can be observed from the surface at a depth of 1/4 of the plate thickness and at the center position in the plate width direction with a cross section parallel to the rolling direction. Former austenite grain boundaries are revealed by corroding the observation surface with a saturated aqueous solution of picric acid. An enlarged photograph of a cross section parallel to the rolling direction that has been corroded, at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction is taken with a scanning electron microscope (SEM) at a magnification of 1000 times and 5 or more fields. .. The circle-equivalent diameter (diameter) of at least 20 old austenite grains having a circle-equivalent diameter (diameter) of 2 μm or more, which are included in each SEM photograph, is obtained by image processing, and the average value of these is calculated to obtain the old value. Obtain the average particle size of austenite grains. If old austenite grains having a circle-equivalent diameter of less than 2 μm are included, this is excluded and the above measurement is performed.
[0067]
In addition, at least 20 old austenite grains having a circle-equivalent diameter (diameter) of 2 μm or more, which are included in each of the above SEM photographs, are measured on the major axis and the minor axis. Calculate the average value of the long axis and the short axis obtained by measuring each old austenite grain. As a result, the major axis l d and the minor axis S d of the old austenite grains are obtained. By calculating these ratios, the ratio l d / S d of the major axis l d and the minor axis S d of the old austenite grains is obtained.
[0068]
Tensile strength: 780 MPa or more
The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 780 MPa or more. If the tensile strength is less than 780 MPa, the applicable parts are limited, and the contribution of weight reduction of the vehicle body is small. The tensile strength is preferably 980 MPa or more. The upper limit is not particularly limited, but may be 1800 MPa from the viewpoint of suppressing mold wear.
[0069]
Total growth: 14.0% or more
The hot-rolled steel sheet according to the present embodiment may have a total elongation of 14.0% or more. The upper limit of the total elongation is not particularly limited, but may be 30.0% or less or 25.0% or less.
[0070]
Tensile strength and total elongation are measured in accordance with JIS Z 2241: 2011 using JIS Z 2241: 2011 No. 5 test piece. The sampling position of the tensile test piece may be the center position in the plate width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction. The crosshead speed is 3 mm / min.
[0071]
Hole expansion rate: 50% or more
The hot-rolled steel sheet according to this embodiment may have a hole expansion rate of 50% or more. The upper limit of the hole expansion rate is not particularly limited, but may be 90% or less or 85% or less.
The hole expansion rate is obtained by performing a hole expansion test in accordance with JIS Z 2256: 2010.
[0072]
Impact value at -40 ° C: 60 J / cm 2 or more
The hot-rolled steel sheet according to this embodiment may have an impact value of 60 J / cm 2 or more at −40 ° C. The upper limit of the impact value at −40 ° C. is not particularly limited, but may be 180 J / cm 2 or less or 175 J / cm 2 or less.
A sub-sized Charpy impact test piece is taken from an arbitrary position on the hot-rolled steel sheet, and the impact value at -40 ° C is determined according to the test method described in JIS Z 2242: 2005.
[0073]
Plate thickness: 0.6 to 8.0 mm
The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.6 to 8.0 mm. If the thickness of the steel sheet is less than 0.6 mm, it may be difficult to secure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to this embodiment may be 0.6 mm or more. It is preferably 1.2 mm or more, or 1.4 mm or more. On the other hand, when the plate thickness exceeds 8.0 mm, it becomes difficult to miniaturize the metal structure, particularly the old austenite grains, and it may be difficult to secure the above-mentioned metal structure in terms of the structure fraction. Therefore, the plate thickness may be 8.0 mm or less. It is preferably 6.0 mm or less.
[0074]
Plating layer
The hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like to be a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrozinc plating and electroZn—Ni alloy plating. Examples of the hot-dip plating layer include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. To. The amount of plating adhered is not particularly limited and may be the same as the conventional one. Further, it is also possible to further enhance the corrosion resistance by subjecting an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
[0075]
Next, a preferable manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described.
A preferable manufacturing method of the hot-rolled steel sheet according to the present embodiment includes the following steps. The temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
[0076]
A heating process in which a slab having a predetermined chemical composition is held at a heating temperature of 1200 ° C. or higher for 1.0 hour or longer.
A hot rolling process in which rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher and the total rolling reduction ratio exceeds 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980 ° C.
After cooling to a temperature range of 570 to 620 ° C at an average cooling rate of 20 ° C./s or higher and winding, the mixture is held in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours, and then cooled to room temperature. Cooling process.
In the hot rolling step, the finish rolling may be performed so that the total reduction rate in the rough rolling is 70% or more and the reduction rate in the latter three stages of the finish rolling is less than 25%.
Hereinafter, each process will be described in detail.
[0077]
Heating process
In the heating step, the slab having the above-mentioned chemical composition is heated to a heating temperature of 1200 ° C. or higher and held for 1.0 hour. Since the coarse precipitates present at the slab stage cause cracks during rolling and deterioration of material properties, it is preferable to heat the steel material before hot rolling to dissolve the coarse carbides as a solid solution. Therefore, the heating temperature is set to 1200 ° C. or higher, and the holding time is set to 1.0 hour or higher. The preferred heating temperature is 1230 ° C. or higher, and the preferred holding time is 3.0 hours or higher.
[0078]
On the other hand, if the heating temperature becomes too high or the holding time becomes too long, the yield may decrease due to the large amount of scale generated. Therefore, the heating temperature is set to 1400 ° C. or less, and the holding time is 20. It may be 0 hours or less.
The heated slab is preferably produced by continuous casting from the viewpoint of manufacturing cost, but may be produced by another casting method (for example, ingot forming method).
[0079]
Hot rolling process
If rough rolling is performed at a temperature lower than 1000 ° C., the old austenite grains are not sufficiently recrystallized, so that the texture develops and the desired hole-expanding property cannot be obtained. Therefore, rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher. Preferably, it is 1050 ° C. or higher. On the other hand, when rough rolling is performed at a temperature higher than 1300 ° C., the amount of scale generated increases and the yield may decrease. Therefore, the rough rolling completion temperature may be 1300 ° C. or lower.
[0080]
If the total reduction rate in rough rolling is low, the crystal grain size of the old austenite grains becomes non-uniform, which causes a decrease in toughness. Therefore, the total reduction rate in rough rolling is set to more than 65%. The total rolling reduction in rough rolling is preferably 68% or more, more preferably 70% or more, and even more preferably 80% or more. The upper limit of the total rolling reduction in rough rolling is not particularly limited, but may be 90% or less.
The total rolling reduction in rough rolling is expressed as (1-tr / t s) × 100 (%) using the slab thickness: t s and the plate thickness tr at the end of rough rolling. ..
[0081]
By setting the total rolling reduction in rough rolling to 70% or more and strictly controlling the rolling reduction in the latter three stages of finish rolling as described later, the average particle size and aspect ratio of the former austenite grains described above can be realized. Can be done.
[0082]
If the finish rolling completion temperature is less than 860 ° C, the old austenite grains are not sufficiently recrystallized, so that the texture develops and the hole expandability deteriorates. Therefore, the finish rolling completion temperature is set to 860 ° C. or higher. Preferably, the temperature is 900 ° C. or higher. On the other hand, when the finish rolling completion temperature exceeds 980 ° C., the old austenite grains become significantly coarse and the desired toughness cannot be obtained. Therefore, the finish rolling completion temperature is set to 980 ° C. or lower. It is preferably 950 ° C. or lower.
[0083]
In the present embodiment, in order to realize the above-mentioned average particle size and aspect ratio of the former austenite grains and improve the punching characteristics of the hot-rolled steel sheet, the total reduction ratio in the rough rolling and the three steps after the finish rolling are performed. The rolling reduction rate may be strictly controlled. Specifically, as described above, the total rolling reduction in the rough rolling may be 70% or more, and the rolling reduction in the three subsequent stages of the finish rolling may be less than 25%.
[0084]
When the rolling reduction of the third step after the finish rolling, that is, the rolling reduction of at least one of the final pass of the finish rolling, the second pass from the final pass, and the third pass from the final pass is 25% or more, the rolling is performed. , The old austenite grains become flat, and the old austenite grains having a large aspect ratio, which is the starting point of cracking during punching, are formed. Therefore, the reduction rate of the third stage after the finish rolling (the final pass of the finish rolling, the reduction rate of the second pass from the final pass, and the reduction rate of the third pass from the final pass) may be less than 25%. Preferably, both are 20% or less. The rolling reduction ratio can be expressed as (1-h / h 0) × 100 (%) when the plate thickness after rolling in one pass is h and the plate thickness before rolling is h 0.
[0085]
Cooling process
After the hot rolling process, the product is cooled to a temperature range of 570 to 620 ° C at an average cooling rate of 20 ° C / s or higher. In the present embodiment, the average cooling rate is a value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point.
[0086]
If the average cooling rate is less than 20 ° C./s, a large amount of ferrite is precipitated and a desired amount of bainite cannot be obtained. Therefore, the average cooling rate is set to 20 ° C./s or more. It is preferably 30 ° C./s or higher, and more preferably 50 ° C./s or higher. From the viewpoint of suppressing the increase in cooling equipment, the average cooling rate may be 200 ° C./s or less.
[0087]
Further, cooling with an average cooling rate of 20 ° C./s or more is performed up to a temperature range of 570 to 620 ° C. If the cooling shutdown temperature exceeds 620 ° C., a desired amount of bainite cannot be obtained. Therefore, the cooling shutdown temperature is set to 620 ° C. or lower. The cooling shutdown temperature may be any temperature as long as it can be maintained in the temperature range of 620 ° C. or lower and 500 to 580 ° C., but in order to maintain the cooling shutdown temperature in the temperature range of 500 to 580 ° C. for 2.0 hours or more, the cooling shutdown temperature is set. The temperature is preferably 550 ° C. or higher. Further, in order to preferably control the total density of L 7 and L 68 and obtain excellent toughness, the cooling shutdown temperature is preferably 570 ° C. or higher.
[0088]
Since the cooling shutdown temperature is lower than 500 ° C. and the desired amount of bainite cannot be obtained even if the bainite is held in the temperature range of 500 to 580 ° C. after being heated again, it is not desirable to heat the bainite after the cooling is stopped. ..
[0089]
After cooling with an average cooling rate of 20 ° C / s or more, winding is performed. After winding, it is held in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours. If the holding temperature is outside the temperature range of 500 to 580 ° C., or if the holding time is less than 2.0 hours or more than 12.0 hours, the sum of the desired amounts of densities of L 7 and L 68 in bainite. I can't get it. Therefore, the holding temperature is set to a temperature range of 500 to 580 ° C., and the holding time is set to 2.0 to 12.0 hours. The lower limit of the holding temperature is preferably 530 ° C. The upper limit of the holding temperature is preferably 560 ° C. The lower limit of the holding time is preferably 4.0 hours, more preferably 6.0 hours. The upper limit of the holding time is preferably 10.0 hours, more preferably 8.0 hours.
[0090]
For holding in the temperature range of 500 to 580 ° C., the temperature of the steel sheet may be changed or kept constant in the temperature range of 500 to 580 ° C. Further, even if the cooling shutdown temperature of cooling having an average cooling rate of 20 ° C./s or more is less than 580 ° C., it is sufficient that a holding time of 2.0 to 12.0 hours can be secured in a temperature range of 500 to 580 ° C. ..
[0091]
After performing the above-mentioned holding in the temperature range of 500 to 580 ° C., cool to room temperature. Any method may be used for cooling to room temperature, and in addition to air cooling, cooling may be performed by an appropriate method such as mist cooling or rapid cooling using a water cooling tank. The room temperature referred to here is a temperature range of 20 to 30 ° C.
Example
[0092]
Next, the effect of one aspect of the present invention will be described more specifically by way of examples. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one-condition example. The present invention achieves the object of the present invention without departing from the gist of the present invention. As long as it is, various conditions can be adopted.
[0093]
Steels having the chemical compositions shown in the steel types A to AM in Table 1 were melted, and slabs having a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 5 to 7 were obtained under the production conditions shown in Tables 2 to 4. In addition, "F1", "F2" and "F3" in Tables 2 to 4 represent the reduction rate of the final pass of finish rolling, the reduction rate of the second pass from the final pass, and the reduction rate of the third pass from the final pass, respectively. show. In addition, the test material No. in Table 4 63 was heated again after the cooling was stopped, and then held in the temperature range of 500 to 580 ° C.
[0094]
With respect to the obtained hot-rolled steel sheet, the microstructure fraction, the sum of the densities of L 7 and L 68, the average particle size of the former austenite grains, and the major axis ld and the minor axis S d of the former austenite grains were added to the obtained hot-rolled steel sheet by the above method. The ratio l d / S d of was determined. The results obtained are shown in Tables 5-7.
[0095]
Evaluation method of characteristics of hot-rolled steel sheet
Tensile strength (TS) and total elongation (El)
Of the mechanical properties of the obtained hot-rolled steel sheet, tensile strength (TS) and total elongation (El) were measured in accordance with JIS Z 2241: 2011 using JIS Z 2241: 2011 No. 5 test piece. did. The sampling position of the tensile test piece was the central position in the plate width direction, and the direction perpendicular to the rolling direction was the longitudinal direction. The crosshead speed was 3 mm / min.
[0096]
When the tensile strength (TS) was 780 MPa or more, it was judged to be excellent in strength, and when it was less than 780 MPa, it was judged to be inferior in strength and was judged to be unacceptable. Further, when the total elongation (El) was 14.0% or more, it was judged to be excellent in ductility, and when it was less than 14.0%, it was judged to be inferior in ductility, and it was judged to be unacceptable.
[0097]
Hole expansion rate (λ)
The hole expansion rate (λ) was evaluated by performing a hole expansion test in accordance with JIS Z 2256: 2010.
When the hole expansion rate (λ) was 50% or more, it was judged to be acceptable as having excellent hole expansion property, and when it was less than 50%, it was determined to be rejected as being inferior to hole expansion property.
[0098]
Impact value (vE-40)
The toughness was evaluated by performing a Charpy impact test at -40 ° C and determining the impact value. A sub-sized Charpy impact test piece was taken from an arbitrary position on the hot-rolled steel sheet, and the toughness was evaluated by determining the impact value at −40 ° C. according to the test method described in JIS Z 2242: 2005.
When the impact value (vE-40) was 60 J / cm 2 or more, it was judged to be acceptable as having excellent toughness, and when it was less than 60 J / cm 2, it was judged to be unacceptable as being inferior to toughness.
[0099]
Punching characteristics
The punching characteristics were evaluated by conducting a punching test and observing the properties of the punched end face. First, a punched hole was prepared with a hole diameter of 10 mm, a clearance of 12.5%, and a punching speed of 80 mm / s. Next, a cross section perpendicular to the rolling direction of the punched hole was embedded in the resin, and the punched end face was photographed with a scanning electron microscope. When the obtained observation photographs were observed and no end face roughness was observed, "E (Excellent)" was described in Tables 5 to 7 as the punching characteristics were particularly good. Further, when a small elopement of less than 100 μm is observed, “G (Good)” is described in Tables 5 to 7 as having good punching characteristics, and when a large elopement of 100 μm or more is observed, punching is performed. “B (Bad)” is described in Tables 5 to 7 as being inferior in characteristics.
[0100]
Looking at Tables 5 to 7, it can be seen that the examples of the present invention have high strength and excellent ductility, perforation and toughness. Further, the example of the present invention in which the average particle size of the former austenite grains is 10 to 30 μm and the ratio l d / S d of the major axis l d to the minor axis S d of the former austenite grains is 2.0 or less is punched. It can be seen that the characteristics are particularly good.
On the other hand, it can be seen that the comparative example is inferior in any one or more of strength, ductility, perforation property and toughness.
[0101]
[table 1]

[0102]
[Table 2]

[0103]
[Table 3]

[0104]
[Table 4]

[0105]
[Table 5]

[0106]
[Table 6]

[0107]
[Table 7]

Industrial applicability
[0108]
According to the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility, hole-expanding property and toughness, and a method for producing the same. According to the above-mentioned preferred embodiment according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent punching characteristics in addition to the above-mentioned characteristics and a method for producing the same.
The scope of the claims
[Claim 1]
Chemical composition is mass%,
C: 0.030 to 0.200%,
Si: 0.05-2.50%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 2.000%,
Ti: 0.030 to 0.200%,
P: 0.020% or less,
S: 0.020% or less,
N: 0.010% or less,
Nb: 0 to 0.200%,
B: 0 to 0.010%,
V: 0 to 1.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0 to 1.00%,
Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0 to 1.00%,
Ca: 0-0.010%,
Mg: 0 to 0.010%,
REM: 0-0.010%, and
Zr: 0 to 0.010%
Containing, the balance consists of iron and impurities,
The metal structure is% of the area
Bainite: 80.0% or more,
Ferrite: 10.0% or less,
Remaining organization: 10.0% or less,
The sum of the densities of the grain boundary length L 7 having a crystal orientation difference of 7 ° and the grain boundary length L 68 having a crystal orientation difference of 68 ° in the bainite about the <110> direction is 0. It is .35 to 0.60 μm / μm 2,
Tensile strength is 780 MPa or more
A hot-rolled steel sheet characterized by this.
[Claim 2]
The chemical composition is mass%
Nb: 0.005 to 0.200%,
B: 0.001 to 0.010%,
V: 0.005 to 1.00%,
Mo: 0.005 to 1.00%,
Cu: 0.005 to 1.00%,
W: 0.005 to 1.00%,
Cr: 0.005 to 1.00%,
Ni: 0.005 to 1.00%,
Co: 0.005 to 1.00%,
Ca: 0.0005-0.010%,
Mg: 0.0005-0.010%,
REM: 0.0005-0.010%, and
Zr: 0.0005-0.010%
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains one or more of the group consisting of two or more.
[Claim 3]
In the metal structure
The average particle size of the old austenite grains is 10 to 30 μm,
The ratio l d / S d of the long axis l d and the short axis S d of the former austenite grain is 2.0 or less.
The hot-rolled steel sheet according to claim 1 or 2.
[Claim 4]
The method for manufacturing a hot-rolled steel sheet according to claim 1.
A heating step of holding the slab having the chemical composition according to claim 1 at a heating temperature of 1200 ° C. or higher for 1.0 hour or longer,
A hot rolling process in which rough rolling is performed so that the rough rolling completion temperature is 1000 ° C. or higher and the total rolling reduction ratio exceeds 65%, and finish rolling is performed so that the finish rolling completion temperature is 860 to 980 ° C. ,
After cooling to a temperature range of 570 to 620 ° C at an average cooling rate of 20 ° C./s or higher and winding, the mixture is held in a temperature range of 500 to 580 ° C. for 2.0 to 12.0 hours, and then cooled to room temperature. With the cooling process
A method for manufacturing a hot-rolled steel sheet, which comprises.
[Claim 5]
In the hot rolling process
The total rolling reduction in the rough rolling was 70% or more.
The method for manufacturing a hot-rolled steel sheet according to claim 4, wherein the finish-rolling is performed so that the reduction ratios of the three subsequent stages of the finish-rolling are all less than 25%.