Title of invention: Hot-rolled steel plate
Technical field
[0001]
The present invention relates to a hot-rolled steel plate. Specifically, the present invention relates to a high-strength hot-rolled steel plate having excellent formability.
This application claims priority based on Japanese Patent Application No. 2019-222162 filed in Japan on December 9, 2019, and the contents thereof are incorporated herein by reference.
Background technology
[0002]
The strength of steel plates is increasing in order to ensure the collision safety of automobiles and reduce the environmental load. As the strength of the steel sheet increases, the formability decreases. Therefore, improvement of the formability of the 980 MPa class steel sheet is required. Generally, ductility, hole expandability and bendability are used as indicators of formability, but these characteristics are in a trade-off relationship, and a steel plate having excellent ductility, hole expandability and bendability is required. ..
[0003]
Further, when press-molding a complicated part shape such as an undercarriage part, it is particularly necessary to have excellent ductility and hole widening property.
[0004]
In Patent Document 1, a baynite phase having an area ratio of 85% or more is used as a main phase, a martensite phase or a martensite-austenite mixed phase having an area ratio of 15% or less is used as the second phase, and the balance is composed of a ferrite phase. The average particle size of the second phase is 3.0 μm or less, the average aspect ratio of the old austenite grains is 1.3 or more and 5.0 or less, and the area of ​​the recrystallized old austenite grains with respect to the unrecrystallized old austenite grains. It has a structure with a ratio of 15% or less, and the precipitate having a diameter of less than 20 nm deposited in the hot-rolled steel plate is 0.10% or less in mass%, and the tensile strength TS is 980 MPa or more. A strong hot-rolled steel plate is disclosed.
[0005]
Patent Document 2 includes a baynite phase having an area ratio of more than 90% as a main phase, or further having one or more of a ferrite phase, a martensite phase and a retained austenite phase as a second phase. The average particle size of the baynite phase is 2.5 μm or less, and the interval of Fe-based carbides precipitated in the bainitic ferrite grains in the baynite phase is 600 nm or less, and the tension is increased. A high-strength hot-rolled steel plate having a strength TS of 980 MPa or more is disclosed.
Prior art literature
Patent documents
[0006]
Patent Document 1: International Publication No. 2017/017933
Patent Document 2: International Publication No. 2015/12919
Outline of the invention
Problems to be solved by the invention
[0007]
Patent Document 1 does not consider bendability. The present inventors have found that in the high-strength hot-rolled steel plate disclosed in Patent Document 1, it may not be possible to obtain excellent bendability, and it is necessary to further improve the hole-expandability.
[0008]
Patent Document 2 does not consider hole expandability and bendability. The present inventors have found that in the high-strength hot-rolled steel plate disclosed in Patent Document 2, it may not be possible to obtain excellent hole-spreading property and bendability.
[0009]
In view of the above circumstances, an object of the present invention is to provide a hot-rolled steel plate having excellent strength, ductility, bendability and hole-spreading property.
Means to solve the problem
[0010]
As a result of studies by the present inventors in order to solve the above problems, the present inventors obtained the following findings (a) to (g).
[0011]
(A) By making the metal structure a single phase, the difference in hardness between the structures can be reduced and the generation of voids at the structure interface can be suppressed, so that the hole expandability of the hot-rolled steel plate can be improved.
[0012]
(B) When the metal structure is a bainite single phase, a strength of 980 MPa or more cannot be obtained. Therefore, by including a desired amount of a hard phase (martensite phase), the hole expandability of the hot-rolled steel plate can be improved. The desired strength can be obtained while ensuring.
[0013]
(C) By performing tempering, the hard phase remaining after hot rolling is tempered and detoxified (the difference in hardness between structures is reduced and the generation of voids is suppressed), and hot spreading is performed. The hole expandability of the steel plate is improved.
[0014]
(D) By setting the polar density in the (110) <112> orientation to 3.0 or less, the anisotropy can be reduced and the hole expandability of the hot-rolled steel plate can be further improved.
[0015]
(E) By using baynite as the main phase (95.00% or more), high ductility (preferably, total elongation is 13.0% or more) can be achieved, and desired ductility can be obtained.
[0016]
(F) The bendability of the hot-rolled steel plate can be improved by controlling the texture in the surface layer (position of 1/16 of the plate thickness in the plate thickness direction from the surface to the surface).
[0017]
(G) In order to obtain the above-mentioned metal structure, it is particularly effective to control the cooling conditions after hot rolling, the cooling conditions after winding into a coil, and the tempering conditions in a complex and indivisible manner. be.
[0018]
The gist of the present invention made based on the above findings is as follows.
(1) The hot-rolled steel plate according to one aspect of the present invention has a chemical composition of% by mass.
C: 0.040 to 0.150%,
Si: 0.50 to 1.50%,
Mn: 1.00 to 2.50%,
P: 0.100% or less,
S: 0.010% or less,
Al: 0.010 to 0.100%,
N: 0.0100% or less,
Ti: 0.005 to 0.150%,
B: 0.0005 to 0.0050%,
Cr: 0.10 to 1.00%,
Nb: 0 to 0.06%,
V: 0 to 0.50%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Sb: 0 to 0.020%,
Ca: 0-0.010%,
REM: 0-0.010%, and
Mg: 0 to 0.010%
Containing, the balance is iron and impurities,
In the metal structure at the position of 1/4 of the plate thickness in the plate thickness direction from the surface
In terms of area ratio, the main phase is the baynite phase of 95.00 to 98.00%, and the second phase is the tempered martensite phase of 2.00 to 5.00%.
The average particle size of the second phase is 1.5 μm or less,
(110) <112> The extreme density of the orientation is 3.0 or less,
The average particle size of iron-based carbide is 0.100 μm or less,
In the metal structure at the position 1/16 of the plate thickness in the plate thickness direction from the surface to the surface, the extreme density in the (110) <1-11> orientation is 3.0 or less.
The tensile strength TS is 980 MPa or more.
(2) The hot-rolled steel plate according to (1) above has a chemical composition of% by mass.
Nb: 0.005 to 0.06%,
V: 0.05 to 0.50%,
Mo: 0.05-0.50%,
Cu: 0.01-0.50%,
Ni: 0.01-0.50%,
Sb: 0.0002 to 0.020%,
Ca: 0.0002 to 0.010%,
REM: 0.0002-0.010%, and
Mg: 0.0002 to 0.010%
It may contain one or more selected from the group consisting of.
Effect of the invention
[0019]
According to the above aspect according to the present invention, it is possible to provide a hot-rolled steel plate having excellent strength, ductility, bendability and hole-spreading property.
Embodiment for carrying out the invention
[0020]
The chemical composition and metallographic structure of the hot-rolled steel plate (hereinafter, may be simply referred to as a steel plate) 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 gist of the present invention.
[0021]
The lower limit value and the upper limit value are included in the numerical limitation range described below with "~" in between. Numerical values ​​indicated as "less than" and "greater than" do not include the value in the numerical range. All% for chemical composition indicate mass%.
[0022]
The hot-rolled steel plate according to the present embodiment has a chemical composition of% by mass, C: 0.040 to 0.150%, Si: 0.50 to 1.50%, Mn: 1.00 to 2.50%. , P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Ti: 0.005 to 0.150%, B: 0 It contains 0005 to 0.0050%, Cr: 0.10 to 1.00%, and the balance: iron and impurities. Hereinafter, each element will be described.
[0023]
C: 0.040 to 0.150%
C is an element that promotes the formation of baynite by improving the strength of the hot-rolled steel plate and improving the hardenability. In order to obtain this effect, the C content is 0.040% or more. Preferably, the C content is 0.050% or more, 0.060% or more, and 0.070% or more.
On the other hand, when the C content exceeds 0.150%, it becomes difficult to control the formation of baynite, a large amount of martensite phase is formed, and the hot-rolled steel plate has both ductility and / or perforation property. Decreases. Therefore, the C content is set to 0.150% or less. The C content is preferably 0.140% or less, 0.120% or less, and 0.100% or less.
[0024]
Si: 0.50 to 1.50%
Si is an element that contributes to solid solution strengthening and contributes to improving the strength of hot-rolled steel sheets. Further, Si is an element that suppresses the formation of carbides in steel. By suppressing the formation of charcoal during the baynite transformation, a fine martensite phase is formed at the lath interface of the baynite phase. Since the martensite phase present in the baynite phase is fine, it does not deteriorate the hole expandability of the hot-rolled steel plate. In order to obtain the above effect due to the Si content, the Si content is 0.50% or more. Preferably, the Si content is 0.55% or more, 0.60% or more, and 0.65% or more.
On the other hand, Si is an element that promotes the formation of ferrite, and when the Si content exceeds 1.50%, ferrite is formed, and the hole-spreading property and strength of the hot-rolled steel plate are lowered. Therefore, the Si content is 1.50% or less. Preferably, the Si content is 1.30% or less, 1.20% or less, and 1.00% or less.
[0025]
Mn: 1.00 to 2.50%
Mn dissolves in the steel and contributes to the increase in the strength of the hot-rolled steel plate, and at the same time, it promotes the formation of bainite by improving the hardenability and improves the hole-expanding property of the hot-rolled steel plate. 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, 1.50% or more, and 1.70% or more.
On the other hand, if the Mn content exceeds 2.50%, it becomes difficult to control the formation of baynite, the martensite phase increases, and both the ductility and the hole-expanding property of the hot-rolled steel plate decrease. Therefore, the Mn content is set to 2.50% or less. Preferably, the Mn content is 2.00% or less and 1.95% or less.
[0026]
P: 0.100% 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 destruction due to the grain boundary segregation, thereby causing a decrease in the ductility, bendability, and hole expanding property of the hot-rolled steel plate. The P content is preferably as low as possible, but a P content of up to 0.100% is acceptable. Therefore, the P content is set to 0.100% or less. Preferably, the P content is 0.090% or less and 0.080%.
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. Preferably, the P content is 0.001% or more and 0.010% or more.
[0027]
S: 0.010% or less
S is an element that adversely affects the weldability and the 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 plate, and promotes the occurrence of delayed fracture. The S content is preferably as low as possible, but the content of S up to 0.010% is acceptable. Therefore, the S content is 0.010% or less. Preferably, the S content is 0.008% 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. Preferably, the S content is 0.001% or more.
[0028]
Al: 0.010 to 0.100%
Al is a deoxidizer It is an effective element to work and improve the cleanliness of steel. In order to obtain this effect, the Al content is 0.010% or more. Preferably, the Al content is 0.015% or more and 0.020% or more.
On the other hand, if Al is excessively contained, an increase in oxide-based inclusions is caused, and the hole expanding property of the hot-rolled steel plate is lowered. Therefore, the Al content is set to 0.100% or less. Preferably, the Al content is 0.050% or less, 0.040% or less, and 0.030% or less.
[0029]
N: 0.0100% or less
N is an element that forms coarse nitrides in steel. This nitride deteriorates the bendability and hole widening property of the hot-rolled steel plate and also deteriorates the delayed fracture resistance. Therefore, the N content is 0.0100% or less. Preferably, the N content is 0.0080% or less, 0.0060% or less, 0.0050% 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. Preferably, the N content is 0.0005% or more and 0.0010% or more.
[0030]
Ti: 0.005 to 0.150%
Ti is an element that forms a nitride in the austenite phase high temperature region (high temperature region in the austenite phase region and higher temperature region than the austenite phase region (casting stage)). By containing Ti, precipitation of BN is suppressed, and B is in a solid-dissolved state, so that the hardenability required for the formation of baynite can be obtained. As a result, the strength and hole expandability of the hot-rolled steel plate can be improved. In addition, Ti forms carbides in the steel during hot rolling to suppress the recrystallization of old austenite grains. In order to obtain these effects, the Ti content is 0.005% or more. Preferably, the Ti content is 0.030% or more and 0.050% or more, 0.070% or more, and 0.090% or more.
On the other hand, when the Ti content exceeds 0.150%, 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 plate deteriorates. Therefore, the Ti content is set to 0.150% or less. Preferably, the Ti content is 0.130% or less and 0.120% or less.
[0031]
B: 0.0005 to 0.0050%
B is an element that segregates into the former austenite grain boundary, suppresses the formation and growth of ferrite, and contributes to the improvement of the strength and hole expandability of the hot-rolled steel plate. In order to obtain these effects, the B content is 0.0005% or more. Preferably, the B content is 0.0007% or more and 0.0010% or more.
On the other hand, even if B is contained in excess of 0.0050%, the above effect is saturated. Therefore, the B content is set to 0.0050% or less. Preferably, the B content is 0.0030% or less and 0.0025% or less.
[0032]
Cr: 0.10 to 1.00%
Cr is an element that forms carbides in the steel and contributes to increasing the strength of the hot-rolled steel plate, promotes the formation of bainite by improving the hardenability, and promotes the precipitation of Fe-based carbides in the bainite grains. .. In order to obtain these effects, the Cr content is set to 0.10% or more. Preferably, the Cr content is 0.30% or more, 0.40% or more, 0.50% or more.
On the other hand, when the Cr content exceeds 1.00%, the martensite phase is likely to be formed, and the ductility and / or bendability of the hot-rolled steel plate are lowered. Therefore, the Cr content is set to 1.00% or less. Preferably, the Cr content is 0.90% or less, 0.80% or less, 0.70% or less.
[0033]
The balance of the chemical composition of the hot-rolled steel plate according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., or those permitted within a range that does not adversely affect the characteristics of the hot-rolled steel plate according to the present embodiment. do.
[0034]
The hot-rolled steel plate according to the present embodiment may contain the following elements as optional elements instead of a part of Fe. When the following optional elements are not contained, the lower limit of the content is 0%. Hereinafter, each arbitrary element will be described in detail.
[0035]
Nb: 0 to 0.06%
Nb is an element that has the effect of forming carbides during hot rolling and suppressing the recrystallization of austenite, and contributes to improving the strength of hot-rolled steel sheets. In order to surely obtain this effect, the Nb content is preferably 0.005% or more. The Nb content is more preferably 0.02% or more.
On the other hand, if the Nb content exceeds 0.06%, the recrystallization temperature of the old austenite grains becomes too high, the texture develops, and the hole expansion property of the hot-rolled steel plate may deteriorate. Therefore, the Nb content is 0.06% or less. Preferably, the Nb content is 0.04% or less.
[0036]
V: 0 to 0.50%
V is an element that has the effect of forming carbon nitride during hot rolling and suppressing the recrystallization of austenite, and contributes to improving the strength of the hot-rolled steel plate. In order to surely obtain this effect, the V content is preferably 0.05% or more. The V content is more preferably 0.10% or more.
On the other hand, when the V content exceeds 0.50%, the recrystallizing temperature of the old austenite grains becomes high, and the recrystallizing temperature of the austenite grains after the finishing rolling is completed becomes high, so that the texture develops and the heat spreads. The hole expandability of the steel plate may decrease. Therefore, the V content is set to 0.50% or less. Preferably, the V content is 0.25% or less.
[0037]
Mo: 0 to 0.50%
Mo is an element that promotes the formation of the baynite phase by improving the hardenability and contributes to the improvement of the strength and hole expansion of the hot-rolled steel plate. In order to surely obtain this effect, the Mo content is preferably 0.05% or more. The Mo content is more preferably 0.10% or more.
On the other hand, if the Mo content exceeds 0.50%, the martensite phase is likely to be formed, and both the ductility and the hole-expanding property of the hot-rolled steel plate may decrease, or one of them may decrease. Therefore, the Mo content is set to 0.50% or less. Preferably, the Mo content is 0.30% or less.
[0038]
Cu: 0 to 0.50%
Cu is an element that dissolves in steel and contributes to increasing the strength of hot-rolled steel sheets. Further, Cu is an element that promotes the formation of a baynite phase by improving the hardenability and contributes to the improvement of the strength and the hole expanding property of the hot-rolled steel plate. In order to surely obtain these effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more.
On the other hand, if the Cu content exceeds 0.50%, the surface texture of the hot-rolled steel plate may deteriorate. Therefore, the Cu content is set to 0.50% or less. Preferably, the Cu content is 0.20% or less.
[0039]
Ni: 0 to 0.50%
Ni is an element that dissolves in steel and contributes to increasing the strength of hot-rolled steel sheets. In addition, Ni is an element that promotes the formation of a baynite phase by improving the hardenability and contributes to the improvement of the strength and the hole expanding property of the hot-rolled steel plate. In order to surely obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.02% or more.
On the other hand, if the Ni content exceeds 0.50%, the martensite phase is likely to be formed, and both the bendability and the hole expanding property of the hot-rolled steel plate, or either one may be deteriorated. Therefore, the Ni content is set to 0.50% or less. Preferably, the Ni content is 0.20% or less.
[0040]
Sb: 0 to 0.020%
Sb has the effect of suppressing nitriding of the slab surface at the slab heating stage. By containing Sb, precipitation of BN on the surface layer of the slab is suppressed. In order to surely obtain this effect, the Sb content is preferably 0.0002% or more. The Sb content is more preferably 0.001% or more.
On the other hand, even if Sb is contained in excess of 0.020%, the above effect is saturated, so the Sb content should be 0.020% or less.
[0041]
Ca: 0 to 0.010%
Ca is an element that controls the shape of sulfide-based inclusions and improves the hole expandability of hot-rolled steel sheets. In order to surely obtain this effect, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.001% or more.
On the other hand, if the Ca content exceeds 0.010%, surface defects of the hot-rolled steel plate may occur and the productivity may decrease. Therefore, the Ca content is 0.010% or less. Preferably, the Ca content is 0.008% or less.
[0042]
REM: 0 to 0.010%
Like Ca, REM is an element that controls the shape of sulfide-based inclusions and improves the hole-spreading property of hot-rolled steel sheets. In order to surely obtain this effect, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.001% or more.
On the other hand, if the REM content exceeds 0.010%, the cleanliness of the steel deteriorates, and both the hole-spreading property and the bendability of the hot-rolled steel plate, or one of them, deteriorates. Therefore, the REM content is 0.010% or less. Preferably, the REM content is 0.008% or less.
Here, 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 lantanoids, it is industrially added in the form of misch metal.
[0043]
Mg: 0 to 0.010%
Mg is an element whose morphology of sulfide can be controlled by containing it in a small amount. In order to surely obtain this effect, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0005% or more.
On the other hand, if the Mg content exceeds 0.010%, the cold formability is deteriorated due to the formation of coarse inclusions. Therefore, the Mg content is 0.010% or less. Preferably, the Mg content is 0.008% or less.
[0044]
The chemical composition of the hot-rolled steel plate may be measured by a general analysis method. For example, it may be measured using ICP-AES (Industrial Coupled Plusma-Atomic Emission Spectrometri). In addition, 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.
[0045]
Next, the metal structure of the hot-rolled steel plate according to the present embodiment will be described.
The hot-rolled steel plate according to the present embodiment is a baynite phase having a main phase of 95.00 to 98.00% in terms of area ratio in a metal structure at a position of 1/4 of the plate thickness in the plate thickness direction from the surface. The two phases are 2.00 to 5.00% tempered martensite phases, the average particle size of the second phase is 1.5 μm or less, and the extreme density in the (110) <112> orientation is 3.0. The (110) <1-11> orientation in the metal structure having the following, the average particle size of the iron-based carbide is 0.100 μm or less, and the metal structure is located at 1/16 of the plate thickness in the plate thickness direction from the surface to the surface. The extreme density of is 3.0 or less, and the tensile strength TS is 980 MPa or more.
[0046]
In this embodiment, the types of the main phase and the second phase at the position 1/4 of the plate thickness in the plate thickness direction from the surface, the average particle size of the second phase, and the extreme density in the (110) <112> orientation are determined. It is specified because the metal structure at this position indicates the typical metal structure of the steel plate. Further, the position defining the metal structure is preferably the center position in the plate width direction.
Below, each regulation will be explained.
[0047]
Baynite phase (main phase): 95.00-98.00%
The hot-rolled steel plate according to this embodiment has a baynite phase as the main phase. The area ratio of the baynite phase, which is the main phase, is 95.00% or more. In the present embodiment, the main phase means that the area ratio is 95.00% or more.
[0048]
The baynite phase means a structure having Fe-based carbides between and / or inside the lath-shaped bainitic ferrite. Unlike polygonal ferrite, vanitic ferrite has a lath-like shape and is internal.Since the part has a relatively high dislocation density, it can be easily distinguished from other structures by using SEM or TEM.
[0049]
It is necessary to use the baynite phase as the main phase in order to realize a tensile strength of 980 MPa or more and improve the hole expanding property. If the area ratio of the baynite phase is less than 95.00%, the hole-spreading property due to the difference in hardness from the second phase may decrease or the ductility may decrease. Therefore, the area ratio of the baynite phase is set to 95.00% or more. It is preferably 96.00% or more.
On the other hand, if the area ratio of the baynite phase exceeds 98.00%, the tensile strength may not be 980 MPa or more, so the area ratio of the baynite phase should be 98.00% or less. Preferably, it is 97.50% or less and 97.00% or less.
[0050]
Reheated martensite phase (Phase 2): 2.00-5.00%
The hot-rolled steel plate according to this embodiment has the tempered martensite phase as the second phase. The tempered martensite phase is a collection of lath-shaped crystal grains, and means a structure in which the extension direction of iron carbide is two or more inside the crystal grains.
[0051]
The higher the area ratio of the second phase, the better the tensile strength of the hot-rolled steel plate. If the area ratio of the second phase is less than 2.00%, the desired tensile strength cannot be obtained. Therefore, the area ratio of the second phase is set to 2.00% or more. It is preferably 3.00% or more.
On the other hand, if the area ratio of the second phase exceeds 5%, the desired hole-expanding property cannot be obtained. Therefore, the area ratio of the second phase is set to 5.00% or less. It is preferably 4.00% or less.
[0052]
The hot-rolled steel plate according to the present embodiment may contain 3% or less ferrite in addition to the baynite phase and the second phase. However, since it is not always necessary to include ferrite, the area ratio of ferrite may be 0%.
[0053]
The method of measuring the area ratio of the metal structure will be described below.
First, from the hot-rolled steel plate, it is a plate thickness cross section perpendicular to the rolling direction, and is a 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3 in the plate thickness direction from the surface). The test piece is collected so that the region at the / 8 position, that is, the region starting from the 1/8 position in the plate thickness direction from the surface and ending at the 3/8 position in the plate thickness direction from the surface) can be observed. The cross section of the test piece is mirror-polished, corroded with a repera corrosive solution, and then the structure is observed using an optical microscope.
[0054]
The second phase appears as a white part by the repera corrosive liquid, and the other structure (baynite phase) is stained, so that it can be easily distinguished. The area ratio of the white part is calculated by binarizing the white part (bright part) and the other areas. For example, by binarizing the white portion and the other region using image analysis software such as Image-J, the area ratio of the white portion and the area ratio of the other region can be obtained. The observation fields shall be three or more, and the area of ​​each field shall be 300 μm × 400 μm or more.
[0055]
The area ratio of the second phase is obtained by calculating the average value of the area ratio of the white part measured in a plurality of fields. The area ratio of the baynite phase is obtained by calculating the average value of the area ratios of the regions other than the white part measured in a plurality of fields. When the ferrite phase is present in the metal structure, the ferrite phase is dyed white in the same manner as the baynite phase. However, the baynite phase and the ferrite phase can be easily distinguished by observing their morphology. When a ferrite phase is present, the area ratio of the baynite phase is obtained by subtracting the area ratio of the white portion determined to be the ferrite phase from the area ratio of the region other than the white portion. The baynite phase is observed as lath-shaped crystal grains, and the ferrite phase is observed as mass-shaped crystal grains containing no lath inside.
[0056]
Average particle size of the second phase: 1.5 μm or less
When the average particle size of the second phase becomes large, voids are likely to occur, and the hole expanding property of the hot-rolled steel plate decreases. In order to suppress the generation of voids and improve the hole widening property, it is preferable that the average particle size of the second phase is small. If the average particle size of the second phase is more than 1.5 μm, the desired hole-expanding property cannot be obtained. Therefore, the average particle size of the second phase is 1.5 μm or less. It is preferably 1.4 μm or less and 1.3 μm or less.
Since it is technically difficult to make the average particle size of the second phase less than 0.1 μm, the average particle size of the second phase may be 0.1 μm or more.
[0057]
The method for measuring the average particle size of the second phase will be described below.
First, from the hot-rolled steel plate, it is a plate thickness cross section perpendicular to the rolling direction, and is a 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3 in the plate thickness direction from the surface). The test piece is collected so that the region at the / 8 position, that is, the region starting from the 1/8 position in the plate thickness direction from the surface and ending at the 3/8 position in the plate thickness direction from the surface) can be observed. The cross section of the test piece is mirror-polished, corroded with a repera corrosive solution, and then the structure is observed using an optical microscope. Using image analysis software (Image-J), a binarized image of the white part and the other areas is created. After that, particle analysis is performed based on the binarized image, and the area of ​​each particle is calculated. The average particle size of the second phase is obtained by calculating the average value of the average particle size obtained in each visual field with three or more observation fields.
[0058]
The second phase, which has an area of ​​less than 0.5 μm 2, is excluded from the above-mentioned measurement (measurement of the average particle size of the second phase) because it does not affect the hole expandability of the hot-rolled steel plate. do.
[0059]
(110) <112> Extreme density of orientation: 3.0 or less
The extreme density of the (110) <112> orientation in the metal structure at the position of 1/4 of the plate thickness in the plate thickness direction from the surface is an index for evaluating the development condition of the rolled texture. The more the extreme density of the (110) <112> orientation develops, that is, the larger the extreme density of the (110) <112> orientation, the greater the anisotropy of the structure and the lower the hole expandability of the hot-rolled steel plate. If the polar density of the (110) <112> orientation exceeds 3.0, the hole expanding property is lowered, so the polar density of the (110) <112> orientation is set to 3.0 or less. It is preferably 2.8 or less, 2.5 or less, and 2.3 or less.
[0060]
The smaller the extreme density in the (110) <112> orientation, the more random the structure is and the better the hole expanding property of the hot-rolled steel plate. Therefore, the smaller the extreme density in the (110) <112> orientation is preferable. (110) The extreme density of the <112> orientation is 1.0 when it does not have an texture, so the lower limit may be 1.0.
[0061]
The method for measuring the extreme density in the (110) <112> orientation will be described below.
(110) The extreme density of the <112> orientation was measured by the EBSD (Electron Back Scattering Diffraction) method using a device combining a scanning electron microscope and an EBSD analyzer and an OIM Analysis (registered trademark) manufactured by AMETEK. The orientation data can be obtained from the crystal orientation distribution function (ODF: Orientation Distribution Function) that displays the three-dimensional aggregate structure calculated by using the spherical harmonization function. The measurement range is 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3/8 position in the plate thickness direction from the surface, that is, 1 in the plate thickness direction from the surface. A region starting from the / 8 position and ending at the 3/8 position in the plate thickness direction from the surface), and a region of 400 μm in the rolling direction. It is preferable to set the measurement pitch so that the measurement pitch is 0.5 μm / step or less.
[0062]
Average particle size of iron-based charcoal: 0.100 μm or less
In the present embodiment, the iron-based charcoal means cementite (Fe 3C). When the average particle size of the iron-based carbide becomes coarse, it becomes a void generation starting point at the time of hole expansion, and the hole expansion property of the hot-rolled steel plate deteriorates. Therefore, the average particle size of the iron-based carbide is set to 0.100 μm or less. It is preferably 0.080 μm or less, 0.070 μm or less, 0.060 μm or less, and 0.050 μm or less.
The smaller the average particle size of the iron-based carbide is, the more preferable it is for improving the hole-spreading property, so the lower limit may be 0 μm.
[0063]
The method for measuring the average particle size of iron-based carbides will be described below.
It is a plate thickness cross section perpendicular to the rolling direction from the hot-rolled steel plate, and is 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3/8 in the plate thickness direction from the surface). The test piece is collected so that the region of the position, that is, the region starting from the 1/8 position in the plate thickness direction from the surface and ending at the 3/8 position in the plate thickness direction from the surface) can be observed. After the cross section of the test piece is Nital corroded, 10 fields are photographed with SEM at a magnification of 5000 times. The interface of vanitic ferrite in the shooting field and the granular or needle-like substances dispersed in it are judged to be iron-based carbides, and the image analysis of the iron-based carbides is performed to calculate the diameter equivalent to a circle and one field of view. The average value of iron-based charcoal in. By calculating the average value of the iron-based charcoal obtained for 10 fields, the average particle size of the iron-based charcoal is obtained.
[0064]
Extreme density of (110) <1-11> orientation in the metal structure at 1/16 of the plate thickness in the plate thickness direction from the surface to the surface: 3.0 or less
(110) <1 in the metal structure at the 1/16 position of the plate thickness in the plate thickness direction from the surface to the surface (the region starting from the surface and ending at the position of 1/16 of the plate thickness in the plate thickness direction from the surface) -11> Extreme density of orientation is an index for evaluating the development of the shear texture in the surface layer region of the hot-rolled steel plate. When the polar density of the (110) <1-11> orientation at this position develops, that is, when the polar density of the (110) <1-11> orientation increases, the anisotropy of the structure increases and the bending of the hot-rolled steel plate becomes large. The sex is reduced. If the polar density in the (110) <1-11> orientation exceeds 3.0, the bendability of the hot-rolled steel plate decreases, so the polar density in the (110) <1-11> orientation shall be 3.0 or less. .. Preferably, it is 2.8 or less, 2.6 or less, 2.4 or less, 2.2 or less.
[0065]
The smaller the extreme density in the (110) <1-11> orientation, the more random the structure is and the better the bendability of the hot-rolled steel plate. Therefore, the smaller the extreme density in the (110) <1-11> orientation is preferable. (110) The extreme density of the <1-11> orientation is 1.0 when it does not have an texture, so the lower limit may be 1.0.
[0066]
The method for measuring the extreme density in the (110) <1-11> orientation will be described below.
(110) The extreme density of the <1-11> orientation is determined by the EBSD (Electron Back Scattering Diffraction) method using a device combining a scanning electron microscope and an EBSD analysis device and OIM Analysis (registered trademark) manufactured by AMETEK. The measured orientation data can be obtained from a crystal orientation distribution function (ODF: Orientation Distribution Function) that displays a three-dimensional aggregate structure calculated by using a spherical harmonization function. The measurement range is the region from the surface to the plate thickness direction from the surface to the plate thickness 1/16 position (the region starting from the surface and the end point from the surface to the plate thickness direction 1/16 of the plate thickness) and rolling. In the direction, a region of 400 μm or more is evaluated. It is preferable to set the measurement pitch so that the measurement pitch is 0.5 μm / step or less.
[0067]
Tensile strength TS: 980 MPa or more
Tensile strength is an index showing the strength of steel, and by using a material with high tensile strength, it is possible to make automobile parts with the same characteristics at a lighter weight. The tensile strength of the hot-rolled steel plate according to this embodiment is 980 MPa or more. If the tensile strength is less than 980 MPa, the effect of reducing the weight of the vehicle body is not sufficient. Preferably, the tensile strength is 1000 MPa or more and 1030 MPa or more. The higher the tensile strength, the more preferable, but the upper limit may be 1600 MPa or less.
[0068]
The tensile strength TS is measured by performing a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241: 2011. The crosshead speed is 10 mm / min.
[0069]
Next, a preferable manufacturing method of the hot-rolled steel plate according to the present embodiment will be described.
A preferred method for manufacturing a hot-rolled steel plate according to the present embodiment includes the following steps.
A slab with a predetermined chemical composition at 1100 ° C Above, the heating step of heating to less than 1350 ° C.
A hot rolling process in which hot rolling is performed so that the hot rolling start temperature is 1050 to 1200 ° C and the finish rolling completion temperature is more than 950 ° C and 1050 ° C or lower.
A cooling step in which cooling is started within 1.0 second after the completion of the hot rolling and is cooled to a cooling stop temperature of 500 to 600 ° C at an average cooling rate of 30 to 150 ° C / s.
A winding process in which the product is cooled to the cooling stop temperature and then wound in a temperature range of 500 to 600 ° C.
After the winding, a coil cooling step of cooling at an average cooling rate of more than 25 ° C / h and 100 ° C / h or less,
A rewinding process in which rebashing is performed at 350 to 550 ° C for 30 seconds to 12 hours so that the rebirth parameter LMP is 12500 to 15500.
Below, each process will be explained in detail.
[0070]
Heating process
In the heating step, the slab having the above-mentioned chemical composition is heated to 1100 ° C or higher and lower than 1350 ° C. Since the coarse precipitates present in 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. Therefore, the heating temperature is preferably 1100 ° C. or higher. More preferably, it is 1150 ° C. or higher. On the other hand, even if the heating temperature becomes too high, the amount of scale generated increases and the yield decreases. Therefore, the heating temperature is preferably 1350 ° C. or lower. More preferably, it is 1300 ° C. or lower.
[0071]
The slab to be heated is preferably produced by continuous casting from the viewpoint of manufacturing cost, but may be produced by another casting method (for example, ingot formation method).
[0072]
Hot rolling process
The temperature of the steel plate in hot rolling affects the precipitation of carbides and nitrides of Ti and Nb in austenite. If the hot rolling start temperature is less than 1050 ° C., precipitation starts before the start of hot rolling and the precipitate becomes coarse, so that the precipitate cannot be controlled to a desired form and a uniform slab can be obtained. May not be possible. Therefore, the hot rolling start temperature is preferably 1050 ° C. or higher. More preferably, it is 1070 ° C. or higher.
On the other hand, when the hot rolling start temperature exceeds 1200 ° C., it becomes difficult to start the precipitation of the precipitate during the hot rolling, and the precipitate may not be controlled to a desired form. Therefore, the hot rolling start temperature is preferably 1200 ° C. or lower. More preferably, it is 1170 ° C. or lower.
[0073]
Finish rolling completion temperature is a factor that affects the texture of old austenite grains. When the finish rolling completion temperature is 950 ° C. or lower, the texture of the old austenite grains may develop and the anisotropy of the steel material characteristics may increase. Therefore, the finish rolling completion temperature is preferably over 950 ° C. More preferably, it is 960 ° C. or higher.
On the other hand, if the finish rolling completion temperature is too high, the coarsening of the old austenite grains becomes remarkable, and the coarsening of the second phase may make it impossible to obtain the desired hole-expanding property. Therefore, the finish rolling completion temperature is preferably 1050 ° C. or lower. More preferably, it is 1020 ° C. or lower.
[0074]
Before hot rolling, the slab may be roughly rolled to form a rough bar and then hot rolled.
[0075]
Also, before finish rolling, the scale formed on the surface of the steel plate is usually removed (descaling). In the present embodiment, descaling may be performed by a conventional method, for example, the impact pressure of the jetted water may be less than 3.0 MPa. If high-pressure descaling is performed in which the collision pressure of the injected water is 3.0 MPa or more, the texture of the surface layer may not be preferably controlled.
[0076]
Cooling process
In the present embodiment, in order to obtain a desired metal structure, the cooling condition after hot rolling in the cooling step, the cooling condition after winding into a coil in the coil cooling step, and the tempering condition in the tempering step are combined. It is effective to control it in an inseparable manner.
[0077]
In the above-mentioned hot rolling, since the rolling is performed at a relatively high temperature, the coarsening of the old austenite grains tends to proceed. Therefore, it is necessary to start cooling in a short time after the completion of finish rolling to suppress the coarsening of the old austenite grains. If the time from the completion of finish rolling to the start of cooling is long, the old austenite grains may become coarse and the desired average particle size of the second phase may not be obtained. The earlier the cooling start time is, the better, and in the present embodiment, it is preferable to start cooling within 1.0 second after the completion of hot rolling. It is more preferably within 0.5 seconds, and more preferably 0 seconds.
[0078]
The cooling start time here means the elapsed time from the completion of finish rolling to the start of cooling (cooling with an average cooling rate of 30 to 150 ° C./s), which will be described later.
[0079]
It is preferable that the cooling after hot rolling is performed at an average cooling rate of 30 to 150 ° C./s to a cooling stop temperature of 500 to 600 ° C. If the average cooling rate is too slow, ferrite may precipitate, making it impossible to obtain the desired amount of baynite phase, and it may not be possible to obtain the desired tensile strength and / or hole expandability. Further, when the average cooling rate is slow, carbon dioxide-forming elements Ti, V, Nb and the like may be bonded to carbon to form a large amount of precipitates, and the bendability of the hot-rolled steel plate may be deteriorated. Therefore, the average cooling rate of cooling after the completion of hot rolling is preferably 30 ° C./s or more. The average cooling rate in cooling after hot rolling is more preferably 60 ° C./s or higher.
[0080] [0080]
On the other hand, if the average cooling rate after the completion of hot rolling is too fast, the surface temperature becomes too low and martensite is likely to be generated on the surface of the steel plate, and the desired ductility may not be obtained. Therefore, the average cooling rate of cooling after the completion of hot rolling is preferably 150 ° C./s or less. More preferably, it is 120 ° C./s or less, and more preferably 100 ° C./s or less.
[0081]
The average cooling rate in the present embodiment is a value obtained by dividing the temperature difference between the start point and the end point in the set range by the elapsed time from the start point to the end point.
[0082]
If the cooling stop temperature is outside the temperature range of 500 to 600 ° C., the winding step described later cannot be performed in the desired temperature range. Further, in order to obtain a desired metal structure, it is desirable not to perform air cooling in the cooling after hot rolling.
[0083]
Winding process
The take-up temperature is preferably 500 to 600 ° C. in order to suppress the ferrite transformation and promote the baynite transformation, and to control the distribution, morphology, and fraction of the second phase.
[0084]
Baynite transformed at high temperature has excellent ductility. If the winding temperature is less than 500 ° C., the precipitation strengthening does not work at the time of winding, so that the strength after rewinding may be insufficient. Therefore, the winding temperature is preferably 500 ° C. or higher.
On the other hand, if the winding temperature exceeds 600 ° C, ferrite may precipitate and the strength may decrease. Therefore, the winding temperature is preferably 600 ° C. or lower.
[0085]
Coil cooling process
The cooling rate after winding into a coil affects the microstructural fraction of the second phase. In the coil cooling step, carbon enrichment to untransformed austenite is performed. Untransformed austenite is a tissue before it is transformed into "Phase 2 (Martensite phase)". When the product is wound into a coil and then cooled at an average cooling rate of 25 ° C./h or less, untransformed austenite may be decomposed and a desired amount of the second phase may not be obtained. In addition, carbon concentration to untransformed austenite progresses excessively, the hardness of the second phase becomes excessive, and the difference in hardness between the structures of the main phase and the second phase becomes large, so that the holes in the hot-rolled steel plate are formed. Spreadability may decrease. Therefore, the average cooling rate is preferably more than 25 ° C./h. More preferably, it is 30 ° C./or.
[0086]
On the other hand, if the average cooling rate is too fast, there may be a difference in the cooling rate between the inside and outside of the coil, and uniform cooling may not be possible. Therefore, the average cooling rate is preferably 100 ° C./h or less. It is more preferably 80 ° C./h or less, and even more preferably 60 ° C./h or less.
[0087]
Rewinding process
In the tempering step, it is preferable to perform tempering at 350 to 600 ° C. for 30 seconds to 12 hours so that the tempering parameter LMP is 12500 to 15500.
[0088]
When the tempering parameter LMP is within the above range, a desired amount of tempered maltensite and an iron-based carbide having a desired average particle size can be obtained. If the tempering parameter LMP is less than 12500, the martensite phase remains, so that the desired metal structure cannot be obtained, and sufficient ductility and perforation may not be obtained. Therefore, the tempering parameter LMP is preferably 12500 or more. The burnback parameter LMP is more preferably 13500 or more and 14000 or more.
[0089]
On the other hand, if the tempering parameter LMP exceeds 15,500, iron-based carbides may become coarse. The coarsened iron-based charcoal causes stress concentration on the end face at the time of punching, and tends to become a defect, and this defect reduces the hole expandability of the hot-rolled steel plate. Further, ferrite may precipitate to obtain a desired metal structure, and the strength of the hot-rolled steel plate may decrease. Therefore, the tempering parameter LMP is preferably 15500 or less. The burnback parameter LMP is more preferably 15,000 or less.
[0090]
The tempering parameter LMP is calculated by LMP = (273 + T) × (20 + logt) when the holding temperature T (° C.) at the time of tempering and the holding time t (h). The log is a common log with a base of 10.
[0091]
The tempering parameter LMP can be obtained by LMP = (T + 273) × (20 + log (t)) when the heat treatment temperature is constant. In the formula, T is the heat treatment temperature (° C.), and t is the heat treatment time (h). However, when the heat treatment temperature is not constant, that is, when the temperature changes continuously as in continuous quenching, the literature (interpretation of the physical meaning of the tempering parameter and its application to the continuous heating / cooling heat treatment process, heat treatment volume 42. It can be calculated as an integrated burnback parameter by a method considering the heat treatment process as described in No. 3, pp. 163 to 168, June 2002). In the present embodiment, the integrated tempering parameter calculated based on the method described in the above document is referred to as the tempering parameter LMP.
[0092]
The burnback parameter LMP is specifically obtained by the following method.
The time from the start of heating to the end of heating is divided by a minute time Δt of the total number N. Here, the average temperature in the (n-1) th section is Tn-1 (° C.), and the average temperature in the nth section is Tn (° C.). The burnback parameter P (1) corresponding to the first minute time (interval in the case of n = 1) can be obtained by the following equation. Note that log indicates a common logarithm with a base of 10.
P (1) = (T1 + 273) x (20 + log (Δt))
[0093]
P (1) can be expressed as a value equivalent to P calculated based on the temperature T2 and the heating time t2 by the following formula.
(T1 + 273) x (20 + log (Δt)) = (T2 + 273) x (20 + log (t2))
[0094]
Time t2 is the time required (equivalent time) to obtain P equivalent to the integrated value of P calculated based on heating in the section before the second section (that is, the first section) at the temperature T2. Is. The heating time in the second section (temperature T2) is the time obtained by adding the actual heating time Δt to the time t2. Therefore, the integrated value P (2) of P at the time when the heating of the second section is completed can be obtained by the following formula.
P (2) = (T2 + 273) x (20 + log (t2 + Δt))
[0095]
The generalization of this formula is the following formula (4).
P (n) = (Tn + 273) x (20 + log (tn + Δt)) (4)
[0096]
Time nt is the product of P at the time when heating in the (n-1) th section is completed.It is an equivalent time for obtaining P equivalent to the calculated value at the temperature Tn. The time nt can be obtained by the equation (5).
Log (tn) = ((Tn-1 + 273) / (Tn + 273)) x (20 + log (tn-1)) -20 (5)
[0097]
The Nth tempering parameter P (n) obtained by the above method is the integrated value of P at the time when the heating of the Nth section is completed, and this is the tempering parameter LMP.
Example
[0098]
Next, an embodiment of the present invention will be described, but 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 may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0099]
Steel Nos. in Tables 1 and 2. Steels having the chemical compositions shown in 1 to 36 were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. Using the obtained slab, a hot-rolled steel plate was obtained under the production conditions shown in Tables 3 and 4. The "average cooling rate between FT and CT" in Tables 3 and 4 indicates the average cooling rate from the start of cooling after hot rolling to the winding (stopping of cooling). Further, the product was tempered at 350 to 600 ° C. for 30 seconds to 12 hours so as to have the value of the "burnback parameter LMP" shown in Tables 3 and 4. Further, before finish rolling, descaling was performed by a conventional method (collision pressure of injected water is less than 3.0 MPa). No. Only for 42, descaling was performed so that the collision pressure of the injected water was 3.5 MPa.
[0100]
[table 1]

[0101]
[Table 2]

[0102]
[Table 3]

[0103]
[Table 4]

[0104]
With respect to the obtained hot-rolled steel plate, by the above-mentioned method, the structure fraction at the position of 1/4 of the plate thickness in the plate thickness direction from the surface, the average particle size of the second phase, and the extreme density in the (110) <112> orientation. The average particle size of the iron-based carbide and the extreme density in the (110) <1-11> orientation in the metal structure at 1/16 of the plate thickness in the plate thickness direction from the surface to the surface were determined.
The obtained results are shown in Tables 5 and 6. In the case where the total area ratio of baynite and the second phase did not reach 100%, the balance of the metal structure was ferrite.
[0105]
[Table 5]

[0106]
[Table 6]

[0107]
For the obtained hot-rolled steel plate, the tensile strength TS, the total elongation El, the hole expansion ratio λ, and the limit bending radius R were obtained by the method described later.
[0108]
Tensile strength TS and total elongation El
A tensile strength TS and total elongation El were obtained by performing a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241: 2011. The crosshead speed was set to 10 mm / min. When the tensile strength TS was 980 MPa or more, it was determined to be acceptable as having excellent strength, and when it was less than 980 MPa, it was determined to be rejected as being inferior in strength. When the total elongation El was 13.0% or more, it was judged to be acceptable as having excellent ductility, and when it was less than 13.0%, it was judged to be rejected as being inferior in ductility.
[0109]
Hole expansion rate λ
The hole widening property is obtained by punching a circular hole with a diameter of 10 mm under the condition that the clearance is 12.5% ​​using a 60 ° conical punch and performing a hole widening test so that the burr is on the die side. It was evaluated by the spread ratio λ. For each test number, five hole expansion tests were performed, and the average value thereof was calculated to obtain a hole expansion rate λ. When the hole expanding rate was 60% or more, it was judged to be acceptable as having excellent hole expanding property, and when it was less than 60%, it was determined to be rejected as being poor in hole expanding property.
[0110]
Limit bending radius R
The bendability was evaluated by the limit bending radius R obtained by performing the V bending test. The limit bending radius R is V-bent using the No. 1 test piece in accordance with JIS Z2248: 2014 so that the direction perpendicular to the rolling direction is the longitudinal direction (the bending ridge line coincides with the rolling direction). Obtained by conducting a test. The angle between the die and the punch was set to 60 °, and the V-bending test was performed by changing the tip radius of the punch in 0.1 mm increments to obtain the maximum value of the tip radius of the punch that could be bent without cracking. I asked. The maximum value of the tip radius of the punch that could be bent without cracking was defined as the limit bending radius R. When the value (R / t) obtained by dividing the limit bending radius R by the plate thickness t of the test piece is 1.0 or less, it is judged to be acceptable as having excellent bendability, and is described as "Good" in Tables 7 and 8. did. On the other hand, if the value (R / t) obtained by dividing the limit bending radius R by the plate thickness t of the test piece is more than 1.0, it is judged as inferior in bendability and rejected, and "Bad" is shown in Tables 7 and 8. ".
The above test results are shown in Tables 7 and 8.
[0111]
[Table 7]

[0112]
[Table 8]

[0113]
Looking at Tables 5 to 8, it can be seen that the examples of the present invention have excellent strength, ductility, bendability and hole widening property. On the other hand, it can be seen that the comparative example is inferior in one or more of strength, ductility, bendability and hole widening property.
Industrial availability
[0114]
According to the present invention, it is possible to provide a hot-rolled steel plate having excellent strength, ductility, bendability and hole-spreading property, and a method for manufacturing the same.
The scope of the claims
[Claim 1]
The chemical composition is mass%,
C: 0.040 to 0.150%,
Si: 0.50 to 1.50%,
Mn: 1.00 to 2.50%,
P: 0.100% or less,
S: 0.010% or less,
Al: 0.010 to 0.100%,
N: 0.0100% or less,
Ti: 0.005 to 0.150%,
B: 0.0005 to 0.0050%,
Cr: 0.10 to 1.00%,
Nb: 0 to 0.06%,
V: 0 to 0.50%,
Mo: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Sb: 0 to 0.020%,
Ca: 0-0.010%,
REM: 0-0.010%, and
Mg: 0 to 0.010%
Containing, the balance is iron and impurities,
In the metal structure at the position of 1/4 of the plate thickness in the plate thickness direction from the surface
In terms of area ratio, the main phase is the baynite phase of 95.00 to 98.00%, and the second phase is the tempered martensite phase of 2.00 to 5.00%.
The average particle size of the second phase is 1.5 μm or less,
(110) <112> The extreme density of the orientation is 3.0 or less,
The average particle size of iron-based charcoal is 0.100 μm or less,
In the metal structure at the position 1/16 of the plate thickness in the plate thickness direction from the surface to the surface, the extreme density in the (110) <1-11> orientation is 3.0 or less.
Tensile strength TS is 980 MPa or more
A hot-rolled steel plate characterized by this.
[Claim 2]
The chemical composition is by mass%
Nb: 0.005 to 0.06%,
V: 0.05 to 0.50%,
Mo: 0.05-0.50%,
Cu: 0.01-0.50%,
Ni: 0.01-0.50%,
Sb: 0.0002 to 0.020%,
Ca: 0.0002 to 0.010%,
REM: 0.0002-0.010%, and
Mg: 0.0002 to 0.010%
The hot-rolled steel plate according to claim 1, wherein the hot-rolled steel plate contains one kind or two or more kinds selected from the group consisting of.