JP7364942B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate and a method for manufacturing the same.

From the viewpoint of reducing vehicle body weight and ensuring collision safety, high-strength steel plates are being used as steel plates for automobiles. Automobile members include reinforcing members such as bumpers and door guard bars, as well as frame members such as pillars, sills, and members. High-strength steel plates applied to these members are required to have collision resistance properties that can ensure the safety of passengers in the event of a collision (for example, Patent Documents 1 to 3). Here, the collision resistance property is a property that has a high reaction force property and is capable of absorbing energy during collision deformation without causing brittle destruction even if the member is significantly deformed during collision deformation.

Examples of steel sheets with excellent energy absorption performance include DP steel sheets that have a two-phase structure of ferrite and martensite (for example, Patent Document 4), or TRIP steel sheets that have a structure of residual γ in addition to ferrite and bainite (transformation-induced steel sheets). Plastic steel plate) is used (for example, Patent Document 5). Further, steel plates and members having high yield stress and having a structure mainly composed of martensite have been disclosed (for example, Patent Documents 6 to 8).

Japanese Patent Application Publication No. 2009-185355 Japanese Patent Application Publication No. 2011-111672 Japanese Patent Application Publication No. 2012-251239 Japanese Patent Application Publication No. 11-080878 Japanese Patent Application Publication No. 11-080879 JP2010-174280A JP2013-117068A JP2015-175050A

“Steel Bainite Photo Collection-1” Japan Iron and Steel Institute (1992) p. 4 “Structural control of steel”, Masashi Maki, Rokaku Uchida (2015) Five people including Liu Xiao, Lattice-parameter variation with carbon content of martensite. I. X-ray-diffraction experimental study, Physical Review B, Vol. 52 (1995), pp. 9970-9978

However, in the DP steel plate or TRIP steel plate described in Patent Documents 4 or 5, the yield stress obtained is low and the reaction force characteristics are insufficient, and in addition, cracks occur from the end face of the shear punching during impact deformation. Therefore, there were cases where it was not possible to obtain a predetermined amount of energy absorption.

Furthermore, although high yield stress can be obtained with steel plates having a martensite-based structure described in Patent Documents 6 to 8, stress on the punched end face or bent portions of the plate during impact deformation after forming the member In some cases, brittle cracks occurred in concentrated areas, and the impact energy could not be absorbed sufficiently.

The present invention solves the above problems, exhibits good reaction force characteristics when an impact load is applied after being molded into a part shape, and is less likely to cause cracks from the end face of the part or the part bent during impact. , an object of the present invention is to provide a steel plate having a yield stress of 1000 MPa or more and a method for manufacturing the same.

The present inventors have diligently studied methods for solving the above problems, and as a result, have come to the following findings.

(a) By optimizing the crystal structure of martensite and further reducing the block grain size to a certain value or less, it is possible to suppress the occurrence and propagation of cracks in stress concentration areas during high-speed large deformation.

(b) By optimizing the ingredients and optimizing the martensitic transformation start temperature Ms, it is possible to suppress the occurrence and propagation of cracks in stress concentration areas during high-speed deformation.

(c) By suppressing the cracking described above and having a high yield stress, high reaction force characteristics and impact energy absorption ability can be obtained.

The present invention has been made based on the above findings, and its gist is the following steel plate and method for manufacturing the same.

(1) The chemical composition is in mass%,
C: 0.14-0.60%,
Si: more than 0% and less than 3.00%,
Al: more than 0% and less than 3.00%,
Mn: 5.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
N: 0.015% or less,
B: 0 to 0.0050%,
Ni: 0-5.00%,
Cu: 0 to 5.00%,
Cr: 0-5.00%,
Mo: 0-1.00%,
W: 0-1.00%,
Ti: 0 to 0.20%,
Zr: 0 to 0.20%,
Hf: 0-0.20%,
V: 0-0.20%,
Nb: 0 to 0.20%,
Ta: 0 to 0.20%,
Sc: 0-0.20%,
Y: 0-0.20%,
Sn: 0 to 0.020%,
As: 0 to 0.020%,
Sb: 0 to 0.020%,
Bi: 0-0.020%,
Mg: 0 to 0.005%,
Ca: 0-0.005%,
REM: 0-0.005%,
The remainder: Fe and impurities, and
The following formulas (i) to (v) are satisfied,
The value of Ms expressed by the following formula (vi) is 200 or more,
The metal structure is in volume %,
Martensite: 85% or more,
Retained austenite: 15% or less,
The remainder: Bainite,
Average block grain size of martensite and bainite: 3.0 μm or less,
Average axial ratio of martensite and bainite: 1.0004 to 1.0100,
The yield stress is 1000 MPa or more,
steel plate.
Si+Al≦3.00...(i)
C×Mn≦0.80...(ii)
Mn+Ni+Cu+1.3Cr+4(Mo+W)≧0.80...(iii)
0.003≦Ti+Zr+Hf+V+Nb+Ta+Sc+Y≦0.20...(iv)
Sn+As+Sb+Bi≦0.020...(v)
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
However, the element symbol in the above formula represents the content (mass %) of each element in the steel sheet, and if it is not contained, 0 is substituted.

(2) the average grain size of iron carbides contained in the metal structure is 0.005 to 0.20 μm;
The steel plate according to (1) above.

(3) having a plating layer on the surface;
The steel plate according to (1) or (2) above.

(4) A method for manufacturing a steel plate according to any one of (1) to (3) above,
A slab having the chemical composition described in (1) above is sequentially subjected to a hot rolling process, a cold rolling process, an annealing process, and a heat treatment process,
In the hot rolling process, the average cooling rate from the rolling end temperature to 650 ° C. is 8 ° C. / s or more, and cooling to room temperature,
In the annealing step, the temperature is maintained in the temperature range of ₃ points Ac to ( ₃ points Ac + 100) ° C. for 3 to 90 seconds, and
The average cooling rate from 700 °C to (Ms point -50) °C is 10 °C / s or more,
In the heat treatment step,
If the Ms point is 250℃ or higher,
(Ms point +50) ~ 250 ° C. Residence time in the temperature range of 100 ~ 10000 s,
If the Ms point is less than 250°C,
(Ms point +80) ~ 100 ° C. Residence time in the temperature range of 100 ~ 50000 s,
Method of manufacturing steel plates.
However, the above Ms point (℃) and Ac ₃ point (℃) are expressed by the following formula, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and if it is not contained. shall be assigned 0.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ac ₃ =910-203×C ^0.5 +44.7(Si+Al)-30×Mn+700×P-15.2×Ni-26×Cu-11×Cr+31.5×Mo...(vii)

(5) A method for manufacturing a steel plate according to any one of (1) to (3) above,
The slab having the chemical composition described in (1) above is subjected to a hot rolling process, an annealing process, and a heat treatment process in order,
In the hot rolling process, the average cooling rate from the rolling end temperature to 650 ° C. is 8 ° C. / s or more, and cooling to room temperature,
In the annealing step, a temperature range of Ac ₃ to (Ac ₃ +100)° C. is maintained for 3 to 90 seconds, and
The average cooling rate from 700°C to (Ms-50)°C is 10°C/s or more,
In the heat treatment step,
If the Ms point is 250℃ or higher,
The residence time in the temperature range of (Ms+50) to 250°C is 100 to 10,000 s,
If the Ms point is less than 250°C,
(Ms + 80) ~ 100 ° C. Residence time in the temperature range of 100 ~ 50000 s,
Method of manufacturing steel plates.
However, the above Ms point (℃) and Ac ₃ point (℃) are expressed by the following formula, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and if it is not contained. shall be assigned 0.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ac ₃ =910-203×C ^0.5 +44.7(Si+Al)-30×Mn+700×P-15.2×Ni-26×Cu-11×Cr+31.5×Mo...(vii)

(6) A method for manufacturing a steel plate according to any one of (1) to (3) above,
A slab having the chemical composition described in (1) above is subjected to a hot rolling process and a heat treatment process in order,
In the hot rolling step, the rolling end temperature is set to Ar ₃ points or higher, and
The average cooling rate from the rolling end temperature to (Ms-50) °C is 10 °C / s or more,
In the heat treatment step,
If the Ms point is 250℃ or higher,
The residence time in the temperature range of (Ms+50) to 250°C is 100 to 10,000 s,
If the Ms point is less than 250°C,
(Ms + 80) ~ 100 ° C. Residence time in the temperature range of 100 ~ 50000 s,
Method of manufacturing steel plates.
However, the above Ms point (°C) and Ar ₃ point (°C) are expressed by the following formula, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and if it is not contained. shall be assigned 0.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ar ₃ =910-310×C+33×Si-80×Mn-55×Ni-20×Cu-15×Cr-80×Mo...(viii)

According to the present invention, when an impact load is applied after forming the part into a shape, it exhibits good reaction force characteristics, is less prone to cracking from the end face of the part or the part bent at the time of impact, and has a yield of 1000 MPa or more. It becomes possible to obtain a high-strength steel plate with stress.

It is a figure for explaining the shape of the test object used for a collision test.

Hereinafter, each requirement of the present invention will be explained in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding content means "mass %".

C: 0.14-0.60%
C is an element that is effective in improving strength and making the block grain size finer. In order to maintain a yield stress of 1000 MPa, the C content is set to 0.14% or more. On the other hand, when the C content exceeds 0.60%, the Ms point decreases and the average axial ratio described below tends to increase. As a result, brittle fracture occurs at the stress concentration portion during impact deformation, reducing the impact energy absorption ability. Therefore, the C content is set to 0.14 to 0.60%. The C content is preferably 0.15% or more, more preferably 0.18% or more, and preferably 0.50% or less.

Si: more than 0% and less than 3.00% and Al: more than 0% and less than 3.00%, and
Si+Al≦3.00...(i)
Si and Al are effective elements for deoxidizing steel, but in the present invention, they have the effect of increasing the average axial ratio of martensite, the effect of suppressing the formation of iron carbides, and the effect of increasing the block grain size of martensite. This has the effect of reducing the size of the material, thereby suppressing cracking during collision deformation of the member and improving energy absorption ability. In order to obtain a deoxidizing effect, Si and Al are each contained in an amount exceeding 0%. It is preferable to contain 0.01% or more of each of Si and Al.

However, if the total content exceeds 3.00%, there is a strong tendency for brittle fracture to occur during impact deformation, and impact energy absorption ability decreases. Therefore, the total content of Si and Al is 3.00% or less. The total content is preferably 2.50% or less. Although the lower limit of the total content is not particularly limited, it is preferably 0.10% or more in order to reliably obtain the effect of reducing the block particle size.

Mn: 5.00% or less Mn has the effect of suppressing the formation of ferrite and improving yield stress, and is also an element useful for controlling the average axial ratio. However, when the Mn content exceeds 5.00%, the Ms point decreases and the average axial ratio described below tends to increase. As a result, brittle fracture occurs at the stress concentration portion during impact deformation, reducing the impact energy absorption ability. Therefore, the Mn content is set to 5.00% or less. The Mn content is preferably 4.00% or less, 3.00% or less, or 2.00% or less. In order to reliably obtain the above effects, the content is preferably 0.01% or more.

C×Mn≦0.80...(ii)
The product of the C and Mn contents is a parameter that correlates with brittle fracture at stress concentration areas during impact deformation. If the value of C×Mn exceeds 0.80, the brittle fracture tendency becomes strong, so it is set to 0.80 or less. This value is preferably 0.60 or less, more preferably 0.40 or less.

P: 0.030% or less P is an element that contributes to improving strength. However, when the P content exceeds 0.030%, the grain boundary fracture tendency increases during impact deformation, and the impact energy absorption ability decreases. Therefore, the P content is set to 0.030% or less. From the viewpoint of resistance weldability, the P content is preferably 0.020% or less. Although the lower limit is not particularly limited, reducing the content to less than 0.001% will increase manufacturing costs, so 0.001% is the lower limit in practice.

S: 0.0050% or less S is an impurity element, and if its content exceeds 0.0050%, breakage occurs from the punched portion or bent portion upon collision. Therefore, the S content is set to 0.0050% or less. The S content is preferably 0.0040% or less, or 0.0030% or less. The lower limit is not particularly limited, but reducing the content to less than 0.0002% will increase manufacturing costs, so 0.0002% is practically the lower limit.

N: 0.015% or less N is an element that can be used to control the average axial ratio. However, when the N content exceeds 0.015%, the toughness of the steel plate decreases, and cracks tend to occur from stress concentration areas during a collision. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.010% or less, or 0.005% or less. Although the lower limit is not particularly limited, reducing the content to less than 0.001% will increase manufacturing costs, so 0.001% is the lower limit in practice.

B: 0-0.0050%
Since B is an element that has the effect of improving the hardenability of the steel sheet, it may be included as necessary. However, if the B content exceeds 0.0050%, cracks may occur during impact deformation. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0040% or less, or 0.0030% or less. The lower limit is not particularly limited and may be 0%, but if the above effects are desired, the B content is preferably 0.0003% or more.

Ni: 0 to 5.00%, Cu: 0 to 5.00%, Cr: 0 to 5.00%, Mo: 0 to 1.00%, and W: 0 to 1.00%, and
Mn+Ni+Cu+1.3Cr+4(Mo+W)≧0.80...(iii)
Like Mn, Ni, Cu, Cr, Mo, and W have the effect of suppressing the formation of ferrite and improving yield stress, and are also useful elements for controlling the average axial ratio. Therefore, one or more selected from these elements may be contained. In order to obtain this effect, the content of these elements needs to satisfy formula (iii).

From the viewpoint of stably suppressing the formation of ferrite and bainite, the left-hand side value of the above formula (iii) is preferably 1.00 or more. The upper limit is not particularly limited, but if it exceeds 4.00, the Ms point decreases and the average axial ratio described below tends to increase. As a result, there is a risk that brittle fracture will occur at the stress concentration part during collision deformation, and the impact energy absorption ability will decrease. Therefore, it is preferable that the left-hand side value of the above equation (iii) is 4.00 or less.

Further, the content of Ni and Cu is preferably 4.00% or less, more preferably 3.00% or less, and even more preferably 1.00% or less. The Cr content is preferably 3.00% or less, more preferably 1.00% or less. The contents of Mo and W are each preferably at most 0.80%, more preferably at most 0.60%.

Ti: 0-0.20%, Zr: 0-0.20%, Hf: 0-0.20%, V: 0-0.20%, Nb: 0-0.20%, Ta: 0-0 .20%, Sc: 0 to 0.20%, and Y: 0 to 0.20%, and
0.003≦Ti+Zr+Hf+V+Nb+Ta+Sc+Y≦0.20...(iv)
These elements have the effect of reducing the block grain size of martensite and the effect of suppressing the formation of iron carbides, thereby suppressing the generation and propagation of cracks from stress concentration areas during impact deformation. Therefore, at least one of these elements is contained, and the total content is set to 0.003% or more. On the other hand, if the total content exceeds 0.20%, a large amount of alloy precipitates will precipitate and cracks will easily occur during impact deformation, so the content should be 0.20% or less. The total content is preferably 0.010% or more.

Sn: 0 to 0.020%, As: 0 to 0.020%, Sb: 0 to 0.020%, and Bi: 0 to 0.020%, and
Sn+As+Sb+Bi≦0.020...(v)
Since Sn, As, Sb, and Bi are elements used to obtain a predetermined metal structure, one or more selected from these may be included as necessary. However, if the total content exceeds 0.020%, the tendency for grain boundary fracture to occur during impact deformation increases, so the upper limit is set to 0.020%. Although the lower limit is not particularly limited, reducing the content to less than 0.00005% causes an increase in manufacturing costs, so 0.00005% is practically the lower limit.

Mg: 0-0.005%, Ca: 0-0.005%, and REM: 0-0.005%
Since Mg, Ca, and REM are elements that have the effect of controlling the morphology of oxides and sulfides, one or more selected from these may be included as necessary. However, if the content of any element exceeds 0.005%, the effect of addition will be saturated and the energy absorption ability during impact deformation will decrease, so it is set to 0.005% or less. It is preferable that the contents of Mg, Ca and REM are all 0.003% or less. In order to obtain the above effects, it is preferable to contain one or more selected from Mg: 0.001% or more, Ca: 0.001% or more, and REM: 0.001% or more.

Here, in the present invention, REM refers to 15 elements of lanthanoids, and the content of REM means the total content of lanthanides. Note that lanthanoids are industrially added in the form of mischmetal.

Value of Ms: 200 or more Ms means the martensitic transformation start temperature (° C.). If the Ms point of the steel plate is less than 200°C, the axial ratio will increase, and with the configuration of the present invention, it will be difficult to suppress brittle fracture during collision deformation. Therefore, the value of Ms is set to 200 or more. The value of Ms is preferably 220 or more.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)

In the chemical composition of the steel sheet of the present invention, the remainder is Fe and impurities. Here, "impurities" are components that are mixed in during the industrial production of steel sheets due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are allowed within the range that does not adversely affect the present invention. means something that

(B) Metal structure The metal structure of the steel plate according to one embodiment of the present invention will be explained. In the following description, "%" means "volume %".

Martensite: 85% or more Having martensite as the main structure is essential to ensuring a yield stress of 1000 MPa or more. When the volume fraction of martensite is less than 85%, it becomes difficult to ensure a yield stress of 1000 MPa or more. Therefore, the volume fraction of martensite is set to 85% or more. In order to ensure stable yield stress, the volume fraction of martensite is preferably 90% or more. Note that martensite includes tempered martensite, that is, martensite in which carbides are formed inside. Further, the form of martensite may be lath, butterfly, twin, thin plate, etc.

Retained austenite: 15% or less Retained austenite is a metal structure that is effective in improving formability and impact energy absorption characteristics. However, when the volume fraction exceeds 15%, the yield stress decreases and brittle cracks tend to occur during impact deformation. Therefore, the volume fraction of retained austenite is set to 15% or less. The volume fraction of retained austenite is preferably 12% or less. Although the lower limit is not particularly limited, it is preferably 0.1% or more.

The remaining structure other than the above structure is bainite. Here, bainite includes lower bainite and upper bainite, and bainitic ferrite (α°B) described in Non-Patent Document 1 is classified as bainite. Note that, even according to Reference 1, it may be difficult to separate the structure of martensite from bainite after tempering. In cases where tissue separation is difficult as described above, the tissue fraction is calculated assuming that the material is martensite. Although it is not necessary to set an upper limit to the area ratio of the remaining bainite, it is substantially 15% or less, preferably 10% or less.

The volume fraction of the metal structure is determined by the following procedure. First, a 1/4 thickness portion of the surface parallel to the rolling direction and thickness direction of the steel plate is mirror polished and then subjected to nital corrosion. Then, the structure of the surface is observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and the area ratio of martensite and bainite is determined using a point counting method or image analysis using the photographed structure photograph. , this is taken as the volume ratio. Further, the volume fraction of retained austenite is determined by an X-ray diffraction method. The area to be observed is 1000 μm ² or more when using SEM, and 100 μm ² or more when using TEM.

Further, in the present invention, the average block grain size and average axial ratio of martensite and bainite are also defined as follows.

Average block grain size of martensite and bainite: 3.0 μm or less The block grain size of martensite affects the occurrence and propagation of brittle fracture during impact deformation, and the smaller the value, the better the impact properties can be obtained. If the average block grain size exceeds 3.0 μm, plate breakage may occur at the bent portion during collision deformation, so it is set to 3.0 μm or less. Preferably, the average block particle size is 2.7 μm or less, 2.5 μm or less, or 2.4 μm or less.

Here, the block particle size will be explained. Non-Patent Document 2, p. As shown in Table 223, martensite and bainite can be classified as being composed of 24 different crystal units (variants) as their substructures. One method of grouping these 24 variants is as described in Non-Patent Document 2, p. There is a Bain group described in No. 223, which allows martensite and bainite to be classified into three types of crystal units. The block grain size in the present invention refers to the average size of group grains when classified by this Bain group.

Moreover, the average block particle size is measured by the following procedure. First, each steel plate is cut so that the plane parallel to the rolling direction and the thickness direction serves as the observation plane, and the area between the 1/4 position and the 1/2 position of the plate thickness of this cross section is 5000 μm ² or more. Measure by EBSD method. The step size of the measurement is 0.2 μm. Then, based on the crystal orientation information obtained by EBSD measurement, the orientation is classified into three Bain groups, displayed as an image, and the size of this crystal unit is determined using the intersecting line segment method described in Annex 2 of JIS G 0552. Seek the truth.

Average axial ratio of martensite and bainite: 1.0004 to 1.0100
The crystal structure of the metallographic parts other than retained austenite, that is, martensite and bainite, influences the cracking behavior at stress concentration areas and bending areas during impact deformation. In particular, it is necessary to appropriately adjust the average axial ratio of martensite and bainite having a tetragonal structure. Here, the axial ratio is a value expressed as c/a, where a and c are the lattice constants of the a-axis and c-axis in the tetragonal structure, respectively. Although it is not clear why the magnitude of the axial ratio c/a is related to the cracking behavior during high-speed large deformation during a collision test, it is possible that crystal lattice strain has some influence.

If the average axial ratio is less than 1.0004, cracks may occur during collision deformation, or impact energy tends to be difficult to absorb. On the other hand, if it exceeds 1.0100, there is a tendency for the member to break brittle from the end face or bent portion during collision deformation. Therefore, the average axial ratio is set to 1.0004 to 1.0100. From the viewpoint of stably securing yield stress, the average axial ratio is preferably 1.0006 or more. Further, in order to more reliably suppress cracking during collision deformation, the average axial ratio is preferably 1.0007 or more. On the other hand, from the viewpoint of better absorbing impact energy, the average axial ratio is preferably 1.0080 or less.

Here, the average axial ratio of martensite and bainite is measured by the following procedure using an X-ray diffraction method. At this time, the average axial ratio c/a shall be determined by one of the following two methods depending on whether or not the diffraction lines of tetragonal iron or cubic iron are split. Here, the area of the X-ray irradiation area on the sample is 0.2 mm ² or more.

(a) When the 200 diffraction line and the 002 diffraction line are clearly split into two The peak separation of the diffraction line from the {200} plane is performed using the pseudo Voigt function, and the lattice constant calculated from the 200 diffraction angle is a , 002 The lattice constant calculated from the diffraction angle is c, and the ratio thereof is the average axial ratio c/a.

(b) When the diffraction line is not clearly split into two, the lattice constant calculated from the diffraction angle from the {200} plane is a, and the lattice constant calculated from the diffraction angle from the {110} plane is c. ', and the ratio c'/a is approximated to the average axial ratio c/a (see Non-Patent Document 3).

Average particle size of iron carbide: 0.005-0.20μm
The metal structure of the steel sheet according to another embodiment of the present invention may contain iron carbide. When the average grain size of iron carbide exceeds 0.20 μm, fracture from the bending part tends to be promoted during impact deformation. On the other hand, when the average grain size of iron carbide is less than 0.005 μm, iron impact deformation tends to accelerate. There is a tendency for brittle fracture to occur at bent portions. Therefore, the average particle size of the iron carbide is preferably 0.005 to 0.20 μm. Note that the iron carbide may contain alloying elements such as Mn or Cr in addition to Fe.

The average grain size of iron carbides in martensite and bainite is measured by observing the structure of an area of 10 μm ² or more using SEM and TEM. Fine iron carbides that cannot be determined by TEM are measured using the atom probe method. In this case, five or more iron carbides shall be measured.

(C) Plating layer The steel plate according to another embodiment of the present invention may have a plating layer on its surface. The composition of the plating is not particularly limited, and any of hot-dip plating, alloyed hot-dip plating, and electroplating may be used.

(D) Mechanical properties Yield stress: 1000 MPa or more If the yield stress is less than 1000 MPa, the advantage of reducing the weight of the member by making the member thinner cannot be obtained, so the yield stress is set to be 1000 MPa or more. Here, the yield stress is the flow stress (0.2% proof stress) at a strain of 0.002 when a tensile test is conducted in accordance with JIS Z 2241 2011.

Further, there is no particular restriction on the tensile strength, but from the viewpoint of improving impact energy absorption characteristics, it is preferably 1400 MPa or more.

(E) Manufacturing method Although there are no particular restrictions on the manufacturing conditions for the steel plate according to the present invention, a slab having the above-mentioned chemical composition may be subjected to a treatment including the steps shown in (a) to (c) below. It can be manufactured by Each method will be explained in detail.

Incidentally, the slab can be obtained by a conventional method from molten steel having the above chemical composition. The slab to be subjected to hot rolling is not particularly limited. That is, it may be manufactured by continuous casting slabs or thin slab casters. It is also compatible with continuous casting-direct rolling processes in which hot rolling is performed immediately after casting.

In addition, in the following explanation, Ms point (°C), Ac ₃ point (°C), and Ar ₃ point (°C) are expressed by the following formula, and the element symbol in the formula is the content of each element in the steel plate. (% by mass), and if it is not contained, 0 is substituted.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ac ₃ =910-203×C ^0.5 +44.7(Si+Al)-30×Mn+700×P-15.2×Ni-26×Cu-11×Cr+31.5×Mo...(vii)
Ar ₃ =910-310×C+33×Si-80×Mn-55×Ni-20×Cu-15×Cr-80×Mo...(viii)

(a) Method including a hot rolling process, a cold rolling process, an annealing process, and a heat treatment process The above slab is sequentially subjected to a hot rolling process, a cold rolling process, an annealing process, and a heat treatment process. In this case, the obtained steel plate will be a cold rolled steel plate. Each process will be explained in detail.

In the hot rolling process, the slab is first heated. Although the heating temperature is not particularly limited, it is preferably 1200° C. or higher in order to remelt alloy carbonitrides precipitated during casting or rough rolling.

Hot rolling is performed after heating. At this time, the average cooling rate from the rolling end temperature to 650°C is set to 8°C/s or more. If the above average cooling rate is less than 8° C./s, the martensite block grain size of the final product becomes large and the impact properties deteriorate. After that, the steel plate is rolled up. The winding temperature is not particularly limited, but is preferably 630°C or lower. After winding, the film is further cooled to room temperature.

Subsequently, after performing treatments such as pickling, cold rolling is performed. Regarding the cold rolling conditions, there is no need to particularly specify the number of rolling passes and the rolling reduction ratio, and conventional methods may be followed.

In the annealing step, the steel plate after cold rolling is maintained at a temperature range of ₃ points Ac to ( ₃ points Ac + 100)° C. for 3 to 90 seconds. If the annealing temperature is less than ₃ points of Ac, a predetermined amount of martensite cannot be obtained, and if the annealing temperature exceeds ₃ points of Ac + 100°C, the block grain size becomes large. Further, if the holding time within this temperature range is less than 3 seconds, a predetermined amount of martensite cannot be obtained, and a yield stress of 1000 MPa or more cannot be obtained. On the other hand, when the holding time exceeds 90 seconds, the block particle size increases. From the viewpoint of reducing the block grain size, the annealing temperature is preferably lower, and is preferably at most (Ac ₃ points + 80)°C. Further, the holding time is preferably 10 seconds or more, and preferably 60 seconds or less.

After maintaining the above temperature range for a predetermined time, cooling is performed under conditions such that the average cooling rate from 700°C to (Ms point -50)°C is 10°C/s or more. If this average cooling rate is less than 10°C/s, it will not be possible to obtain a predetermined amount of martensite, and the yield stress will decrease, and the block grain size will further increase, making it more likely that cracks will occur during impact deformation. . When the above-mentioned average axial ratio is set to 1.0007 or more and cracking during collision deformation is desired to be suppressed more reliably, the average cooling rate is preferably 20° C./s or more. Note that the temperature at which this cooling is stopped may be (Ms-50)° C. or lower, and is not particularly limited, but is preferably 100° C. or higher from the viewpoint of anti-destruction properties.

In the heat treatment process, the heat treatment is performed so that the following heat history is obtained according to Ms calculated from the chemical composition of the steel plate. In addition, after stopping the cooling described above, the following heat treatment may be performed successively, or heating may be performed to an extent that does not exceed the upper limit of the temperature range of the heat treatment step described below.

When the Ms point calculated from the chemical composition of the steel plate is 250°C or higher, the residence time in the temperature range of (Ms point + 50) to 250°C is set to 100 to 10,000 s. If the residence time is less than 100 seconds, the average axial ratio may exceed a predetermined value and brittle fracture may occur during a collision test, or a predetermined yield stress may not be obtained. On the other hand, if it exceeds 10,000 seconds, the average axial ratio becomes less than a predetermined value, and the iron carbide becomes coarser, making it easier to crack during a collision. The residence time is preferably 400 seconds or more, and preferably 5000 seconds or less. In particular, when the above-mentioned average axial ratio is set to 1.0007 or more and it is desired to more reliably suppress cracking during collision deformation, the residence time is more preferably 1500 seconds or less.

Further, when the Ms point calculated from the chemical composition of the steel plate is less than 250°C, the residence time in the temperature range of (Ms point + 80) to 100°C is set to 100 to 50,000 s. If the residence time is less than 100 seconds, the average axial ratio will exceed a predetermined value, and there is a risk of brittle fracture during a collision test. On the other hand, if it exceeds 50,000 seconds, the average axial ratio becomes less than a predetermined value, and the iron carbides become coarser, making it easier to crack during a collision. The residence time is preferably 400 seconds or more, preferably 30,000 seconds or less, and more preferably 10,000 seconds or less.

(b) Method including a hot rolling process, an annealing process, and a heat treatment process The above slab is sequentially subjected to a hot rolling process, an annealing process, and a heat treatment process. In this case, the obtained steel plate will be a hot rolled steel plate. Each process will be explained in detail.

In contrast to the step shown in (a) above, this step does not include a cold rolling step. In the annealing process, while heating the cold rolled steel sheet from room temperature to the annealing temperature, ferrite, which is the parent phase, recrystallizes and a texture develops. Under the influence of this crystal orientation, texture develops even in austenite that exists when maintained in the temperature range of Ac ₃ point to (Ac ₃ point + 100)°C. When austenite, whose orientation is biased due to the development of texture, transforms into martensite, martensite crystals are generated and grown in a specific direction.

Furthermore, since steel expands due to the formation and growth of martensite crystals, from a macroscopic point of view, the steel sheet expands in a particular direction. However, freely expanding or deforming the steel strip during the annealing process will result in a decrease in threadability, so tension is usually applied to correct the shape of the steel sheet and maintain threading stability.

Note that if martensitic transformation occurs under such excessive tension, residual stress will be imparted to the steel sheet, making it difficult to obtain the effect of suppressing cracking. Furthermore, when the residual stress within the steel plate increases, cracks that occur when the steel plate is deformed become more likely to generate and propagate. Therefore, from the viewpoint of more reliably suppressing cracking during impact deformation, it is preferable to omit the cold rolling process.In other words, in the annealing process, the development of texture due to recrystallization of the matrix ferrite is prevented, and the crystal orientation is For the purpose of randomization, the steel plate according to one embodiment of the present invention is preferably a hot rolled steel plate.

In the hot rolling process, the slab is first heated. Although the heating temperature is not particularly limited, it is preferably 1200° C. or higher in order to remelt alloy carbonitrides precipitated during casting or rough rolling.

Hot rolling is performed after heating. At this time, the average cooling rate from the rolling end temperature to 650°C is set to 8°C/s or more. If the above average cooling rate is less than 8° C./s, the martensite block grain size of the final product becomes large and the impact properties deteriorate. Thereafter, the steel plate may be rolled up or may be cooled to room temperature without being rolled up. Further, after cooling, a treatment such as pickling or the like may be performed, or shape correction may be performed.

In the annealing step, the hot-rolled steel plate is maintained at a temperature range of ₃ points Ac to ( ₃ points Ac + 100)° C. for 3 to 90 seconds. If the annealing temperature is less than ₃ points of Ac, a predetermined amount of martensite cannot be obtained, and if the annealing temperature exceeds ₃ points of Ac + 100°C, the block grain size becomes large. Further, if the holding time within this temperature range is less than 3 seconds, a predetermined amount of martensite cannot be obtained, and a yield stress of 1000 MPa or more cannot be obtained. On the other hand, when the holding time exceeds 90 seconds, the block particle size increases. From the viewpoint of reducing the block grain size, the annealing temperature is preferably lower, and is preferably at most (Ac ₃ points + 80)°C. Further, the holding time is preferably 10 seconds or more, and preferably 60 seconds or less.

After maintaining the above temperature range for a predetermined time, cooling is performed under conditions such that the average cooling rate from 700°C to (Ms point -50)°C is 10°C/s or more. If this average cooling rate is less than 10°C/s, it will not be possible to obtain a predetermined amount of martensite, and the yield stress will decrease, and the block grain size will further increase, making it more likely that cracks will occur during impact deformation. . When the above-mentioned average axial ratio is set to 1.0007 or more and cracking during collision deformation is desired to be suppressed more reliably, the average cooling rate is preferably 20° C./s or more. Note that the temperature at which this cooling is stopped may be (Ms-50)°C or lower, and is not particularly limited, but preferably 100°C or higher from the viewpoint of anti-destruction properties.

In the heat treatment step, the treatment is performed so that the following thermal history is obtained according to Ms calculated from the chemical composition of the steel plate. Note that after stopping the cooling in the annealing step described above, the following heat treatment may be performed successively, or heating may be performed to an extent that does not exceed the upper limit of the temperature range of the heat treatment described below.

When the Ms point calculated from the chemical composition of the steel plate is 250°C or higher, the residence time in the temperature range of (Ms point + 50) to 250°C is set to 100 to 10,000 s. If the residence time is less than 100 seconds, the average axial ratio may exceed a predetermined value and brittle fracture may occur during a collision test, or a predetermined yield stress may not be obtained. On the other hand, if it exceeds 10,000 seconds, the average axial ratio becomes less than a predetermined value, and the iron carbide becomes coarser, making it easier to crack during a collision. The residence time is preferably 400 seconds or more, and preferably 5000 seconds or less. In particular, when the above-mentioned average axial ratio is set to 1.0007 or more and it is desired to more reliably suppress cracking during collision deformation, the residence time is more preferably 1500 seconds or less.

Further, when the Ms point calculated from the chemical composition of the steel plate is less than 250°C, the residence time in the temperature range of (Ms point + 80) to 100°C is set to 100 to 50,000 s. If the residence time is less than 100 seconds, the average axial ratio will exceed a predetermined value, and there is a risk of brittle fracture during a collision test. On the other hand, if it exceeds 50,000 seconds, the average axial ratio becomes less than a predetermined value, and the iron carbides become coarser, making it easier to crack during a collision. The residence time is preferably 400 seconds or more, preferably 30,000 seconds or less, and more preferably 10,000 seconds or less.

(c) Method including a hot rolling step and a heat treatment step The above slab is sequentially subjected to a hot rolling step and a heat treatment step. In this case, the obtained steel plate will be a hot rolled steel plate. Each process will be explained in detail.

In contrast to the step shown in (b) above, this step does not include an annealing step. When annealing is performed, interfacial movement of the martensitic structure occurs during heating from room temperature to the annealing temperature in the annealing process. Furthermore, in this interface movement, crystal interfaces in specific orientations with high mobility move preferentially, so randomization of crystal orientation is impaired, and a slight residual stress remains in the steel plate after the annealing process. Therefore, from the viewpoint of reducing residual stress as much as possible, it is preferable to omit the annealing step.

In the hot rolling process, the slab is first heated. Although the heating temperature is not particularly limited, it is preferably 1200° C. or higher in order to remelt alloy carbonitrides precipitated during casting or rough rolling.

Hot rolling is performed after heating. At this time, rolling is performed so that the end temperature of the rolling becomes ₃ points or higher. If the rolling end temperature is less than ₃ points of Ar, ferrite is formed and it becomes difficult to obtain a predetermined yield stress.

After hot rolling, cooling is performed under conditions such that the average cooling rate from the rolling end temperature to (Ms point -50)°C is 10°C/s or more. If this average cooling rate is less than 10°C/s, the volume fraction of ferrite or bainite increases, making it impossible to obtain a predetermined amount of martensite, lowering the yield stress, and further increasing the block grain size. Therefore, cracks are more likely to occur during impact deformation. Note that the temperature at which this cooling is stopped may be (Ms-50)° C. or lower, and is not particularly limited, but is preferably 100° C. or higher from the viewpoint of anti-destruction properties.

In the heat treatment step, the treatment is performed so that the following thermal history is obtained according to Ms calculated from the chemical composition of the steel plate. Note that after stopping the cooling in the hot rolling process described above, the following heat treatment may be performed successively, or heating may be performed to an extent that does not exceed the upper limit of the temperature range of the heat treatment described below.

When the Ms point calculated from the chemical composition of the steel plate is 250°C or higher, the residence time in the temperature range of (Ms point + 50) to 250°C is set to 100 to 10,000 s. If the residence time is less than 100 seconds, the average axial ratio may exceed a predetermined value and brittle fracture may occur during a collision test, or a predetermined yield stress may not be obtained. On the other hand, if it exceeds 10,000 seconds, the average axial ratio becomes less than a predetermined value, and the iron carbide becomes coarser, making it easier to crack during a collision. The residence time is preferably 400 seconds or more, and preferably 5000 seconds or less. In particular, when the above-mentioned average axial ratio is set to 1.0007 or more and it is desired to more reliably suppress cracking during collision deformation, the residence time is more preferably 1500 seconds or less.

Further, when the Ms point calculated from the chemical composition of the steel plate is less than 250°C, the residence time in the temperature range of (Ms point + 80) to 100°C is set to 100 to 50,000 s. If the residence time is less than 100 seconds, the average axial ratio will exceed a predetermined value, and there is a risk of brittle fracture during a collision test. On the other hand, if it exceeds 50,000 seconds, the average axial ratio becomes less than a predetermined value, and the iron carbides become coarser, making it easier to crack during a collision. The residence time is preferably 1,000 seconds or more, preferably 30,000 seconds or less, and more preferably 10,000 seconds or less.

After completing any of the steps (a) to (c) above, temper rolling may be performed to correct the shape. The elongation rate is not particularly limited. Furthermore, plating treatment may be performed during or after the heat treatment within a range that satisfies the heat history. Further, the plating method may be manufactured using a continuous annealing/plating line, or may use a dedicated plating equipment separate from the annealing line. The composition of the plating is not particularly limited, and any of hot-dip plating, alloyed hot-dip plating, and electroplating may be used.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

A slab was manufactured by melting steel having the composition shown in Table 1, and this slab was heated at 1220 to 1260° C. and hot rolled. Subsequently, finish rolling was performed, and after cooling, winding treatment was performed at 500 to 620° C., and cooling was performed to room temperature. As shown in Tables 2 and 3, the average cooling rate (CR1) from the rolling end temperature (FT) to 650°C was varied.

After cooling to room temperature, pickling treatment was performed to remove scale, cold rolling was performed at a cold rolling rate of 30 to 70% to a thickness of 1.2 mm, and then annealing was performed.

In annealing, the annealing temperature (ST), the annealing holding time (t1), and the average cooling rate (CR2) between 700°C and (Ms point -50)°C are changed, and in the heat treatment process, the steel with Ms of 250°C or higher is The residence time (t2) between (Ms+50)°C and 250°C was changed, and the residence time (t3) between (Ms+80)°C and 100°C was changed for steel with Ms less than 250°C. After the heat treatment step, temper rolling was performed to correct the shape.

Next, the metal structure of the obtained steel plate was observed, and the volume fraction of each structure was measured. Specifically, after mirror-polishing a 1/4 thickness portion of a surface parallel to the rolling direction and thickness direction of the steel plate, the nital-corroded surface was observed by SEM. Using the tissue photograph, the area ratio of each tissue was determined by measurement using the point counting method, and the value was taken as the volume ratio of each tissue. At this time, the observation area was 2500 μm ² or more. Moreover, the volume fraction of retained austenite was measured by an X-ray diffraction method.

In addition, in the remaining structure column in the table, F indicates ferrite, B indicates bainite, P indicates pearlite, and fM and fA indicate the volume fraction of martensite and retained austenite, respectively, with respect to the total structure.

In addition, the average block grain size of martensite and bainite was measured using the following procedure. First, each steel plate is cut so that the plane parallel to the rolling direction and the thickness direction serves as the observation plane, and the area between the 1/4 position and the 1/2 position of the plate thickness of this cross section is 5000 μm ² or more. Measured by EBSD method. The step size of the measurement was 0.2 μm.

Next, based on the crystal orientation information obtained by the EBSD measurement, the following procedure is performed as described in p. The directions were classified into three Bain groups shown in the table of No. 223. Next, the boundary between these groups was defined as a block grain boundary, and the area surrounded by this boundary was defined as a block grain, and the size (db) of the block grain was determined by the intersecting line segment method described in Appendix 2 of JIS G0552. .

The average axial ratios of martensite and bainite were measured by X-ray diffraction according to the following procedure. At this time, the axial ratio c/a was measured by the following two methods depending on the presence or absence of splitting of the diffraction lines of tetragonal iron or cubic iron, and the average axial ratio was determined.

(a) When the 200 diffraction line and the 002 diffraction line are clearly split into two The peak separation of the diffraction line from the {200} plane is performed using the pseudo Voigt function, and the lattice constant calculated from the 200 diffraction angle is a , 002 The lattice constant calculated from the diffraction angle was defined as c, and the ratio thereof was defined as the average axial ratio c/a.

(b) When the diffraction line is not clearly split into two, the lattice constant calculated from the diffraction angle from the {200} plane is a, and the lattice constant calculated from the diffraction angle from the {110} plane is c. ', and the ratio c'/a was taken as the average axial ratio c/a.

Further, the structure was observed by SEM and TEM, and the average particle size of iron carbides present in an area of 10 μm ² or more was measured and calculated as equivalent circle diameter (dcar). Fine iron carbides that could not be determined by TEM were measured by the atom probe method.

Subsequently, a tensile test piece as described in JIS Z 2241 (2011) was taken from the obtained steel plate with the longitudinal direction in the direction perpendicular to rolling (plate width direction). Then, using the tensile test piece, a tensile test was conducted in accordance with JIS Z 2241 (2011), and mechanical properties (yield stress YS, tensile strength TS) were measured.

Furthermore, in order to investigate the collision resistance characteristics of the steel plate, a collision test was conducted according to the following procedure, and the presence or absence of fracture at that time was evaluated.

First, a hat-shaped part A is formed by cold bending or roll forming a steel plate, and then the hat-shaped part A and the lid B are joined by spot welding to form a test specimen having the shape shown in Fig. 1. Created. Next, the test specimen was placed on table D with A facing upward, and a cylindrical weight C weighing 500 kg was made to collide with the center of the test specimen from a height of 3 m. Then, the portion bent due to the collision and the end face of the test piece were visually observed to evaluate cracks. When evaluating, a score is given according to the maximum length of the crack: E if the maximum length is 10 mm or more, D if it is 7 mm or more and less than 10 mm, C if it is 4 mm or more and less than 7 mm, B if it is 2 mm or more and less than 4 mm, and A if the maximum length is less than 2 mm. Ta.

The above measurement results and evaluation results are summarized in Tables 2 and 3. As is clear from the results shown in Tables 2 and 3, in the case of the example of the present invention that satisfies all the regulations, the yield stress is 1000 MPa or more, and it can be seen that no cracking occurs after the impact test of the member. From this, it is clear that the steel plate according to the present invention has excellent collision characteristics.

A slab is manufactured by melting steel having the composition shown in Table 1, and this slab is heated at 1,220 to 1,260°C, subjected to hot rough rolling, then finish rolled, and cooled to room temperature. Ta. As shown in Table 4, the average cooling rate (CR1) from the final rolling temperature (FT) to 650°C was varied, and below 650°C, cooling was performed in the range of 10°C/s to 20°C/h. Ta.

After hot rolling, the shape was corrected and annealed. In annealing, the annealing temperature (ST), the annealing holding time (t1), and the average cooling rate (CR2) between 700°C and (Ms point -50)°C are changed, and in the heat treatment process, the steel with Ms of 250°C or higher is The residence time (t2) between (Ms+50)°C and 250°C was changed, and the residence time (t3) between (Ms+80)°C and 100°C was changed for steel with Ms less than 250°C. After the heat treatment step, temper rolling was performed to correct the shape.

The obtained steel plate was subjected to measurement of metallographic structure and mechanical properties and evaluation of collision resistance properties in the same manner as in Example 1. The measurement results and evaluation results are shown in Table 4.

As is clear from the results shown in Table 4, in the case of the example of the present invention that satisfies all the regulations, the yield stress is 1000 MPa or more, and it can be seen that no cracking occurs after the impact test of the member. From this, it is clear that the steel plate according to the present invention has excellent collision characteristics.

A slab is manufactured by melting steel having the composition shown in Table 1, and this slab is heated at 1220 to 1260°C, hot rough rolled, and then finish rolled. Changed. As shown in Table 5, the final rolling temperature (FT) and the average cooling rate (CR3) between the final rolling temperature and (Ms-50)°C were varied. Furthermore, in the heat treatment process, the residence time (t2) between (Ms+50)℃ and 250℃ for steel with Ms of 250℃ or higher, and the residence time (t3) between (Ms+80)℃ and 100℃ for steel with Ms of less than 250℃. ) changed. After the heat treatment step, temper rolling was performed to correct the shape.

The obtained steel plate was subjected to measurement of metallographic structure and mechanical properties and evaluation of collision resistance properties in the same manner as in Example 1. Table 5 shows the measurement results and evaluation results.

As is clear from the results shown in Table 5, in the case of the example of the present invention that satisfies all the regulations, the yield stress is 1000 MPa or more, and it can be seen that no cracking occurs after the impact test of the member. From this, it is clear that the steel plate according to the present invention has excellent collision characteristics.

According to the present invention, when an impact load is applied after forming the part into a shape, it exhibits good reaction force characteristics, is less prone to cracking from the end face of the part or the part bent at the time of impact, and has a yield of 1000 MPa or more. It becomes possible to obtain a high-strength steel plate with stress. Therefore, the steel sheet according to the present invention is suitable for use in automobile frame parts, reinforcing parts, and parts of construction industry machinery.

Claims (6)

The chemical composition is in mass%,
C: 0.14-0.60%,
Si: more than 0% and less than 3.00%,
Al: more than 0% and less than 3.00%,
Mn: 5.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
N: 0.015% or less,
B: 0 to 0.0050%,
Ni: 0 to 5.00%,
Cu: 0 to 5.00%,
Cr: 0-5.00%,
Mo: 0-1.00%,
W: 0-1.00%,
Ti: 0 to 0.20%,
Zr: 0 to 0.20%,
Hf: 0-0.20%,
V: 0-0.20%,
Nb: 0 to 0.20%,
Ta: 0 to 0.20%,
Sc: 0-0.20%,
Y: 0-0.20%,
Sn: 0 to 0.020%,
As: 0 to 0.020%,
Sb: 0 to 0.020%,
Bi: 0-0.020%,
Mg: 0 to 0.005%,
Ca: 0-0.005%,
REM: 0-0.005%,
The remainder: Fe and impurities, and
The following formulas (i) to (v) are satisfied,
The value of Ms expressed by the following formula (vi) is 200 or more,
The metal structure is in volume %,
Martensite: 85% or more,
Retained austenite: 15% or less,
The remainder: Bainite,
Average block grain size of martensite and bainite: 2.4 μm or less,
Average axial ratio of martensite and bainite: 1.0004 to 1.0100,
The yield stress is 1000 MPa or more,
steel plate.
0.10≦ Si+Al≦3.00...(i)
C×Mn≦0.80...(ii)
Mn+Ni+Cu+1.3Cr+4(Mo+W)≧0.80...(iii)
0.003≦Ti+Zr+Hf+V+Nb+Ta+Sc+Y≦0.20...(iv)
Sn+As+Sb+Bi≦0.020...(v)
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
However, the element symbol in the above formula represents the content (mass %) of each element in the steel sheet, and if it is not contained, 0 is substituted. The average particle size of iron carbides contained in the metal structure is 0.005 to 0.20 μm,
The steel plate according to claim 1. Has a plating layer on the surface,
The steel plate according to claim 1 or claim 2. A method for manufacturing a steel plate according to any one of claims 1 to 3,
A slab having the chemical composition according to claim 1 is sequentially subjected to a hot rolling process, a cold rolling process, an annealing process, and a heat treatment process,
In the hot rolling process, the average cooling rate from the rolling end temperature to 650 ° C. is 8 ° C. / s or more, and cooling to room temperature,
In the annealing step, the temperature is maintained at a temperature range of ₃ points Ac to ( ₃ points Ac + 80 °C) for 3 to 90 seconds, and
The average cooling rate from 700 °C to (Ms point -50) °C is 10 °C / s or more,
In the heat treatment step,
If the Ms point is 250℃ or higher,
(Ms point +50) ~ 250 ° C. Residence time in the temperature range of 100 ~ 10000 s,
If the Ms point is less than 250°C,
(Ms point +80) ~ 100 ° C. Residence time in the temperature range of 100 ~ 50000 s,
Method of manufacturing steel plates.
However, the above Ms point (℃) and Ac ₃ point (℃) are expressed by the following formula, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and if it is not contained. shall be assigned 0.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ac ₃ =910-203×C ^0.5 +44.7(Si+Al)-30×Mn+700×P-15.2×Ni-26×Cu-11×Cr+31.5×Mo...(vii) A method for manufacturing a steel plate according to any one of claims 1 to 3,
A slab having the chemical composition according to claim 1 is sequentially subjected to a hot rolling process, an annealing process, and a heat treatment process,
In the hot rolling process, the average cooling rate from the rolling end temperature to 650 ° C. is 8 ° C. / s or more, and cooling to room temperature,
In the annealing step, a temperature range of Ac ₃ to (Ac ₃ + 80 )° C. is maintained for 3 to 90 seconds, and
The average cooling rate from 700°C to (Ms-50)°C is 10°C/s or more,
In the heat treatment step,
If the Ms point is 250℃ or higher,
The residence time in the temperature range of (Ms+50) to 250°C is 100 to 10,000 s,
If the Ms point is less than 250°C,
(Ms + 80) ~ 100 ° C. Residence time in the temperature range of 100 ~ 50000 s,
Method of manufacturing steel plates.
However, the above Ms point (℃) and Ac ₃ point (℃) are expressed by the following formula, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and if it is not contained. shall be assigned 0.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ac ₃ =910-203×C ^0.5 +44.7(Si+Al)-30×Mn+700×P-15.2×Ni-26×Cu-11×Cr+31.5×Mo...(vii) A method for manufacturing a steel plate according to any one of claims 1 to 3,
A slab having the chemical composition according to claim 1 is sequentially subjected to a hot rolling process and a heat treatment process,
In the hot rolling step, the rolling end temperature is set to Ar ₃ points or higher, and
The average cooling rate from the rolling end temperature to (Ms-50) °C is 10 °C / s or more,
In the heat treatment step,
If the Ms point is 250℃ or higher,
The residence time in the temperature range of (Ms+50) to 250°C is 100 to 10,000 s,
If the Ms point is less than 250°C,
(Ms + 80) ~ 100 ° C. Residence time in the temperature range of 100 ~ 50000 s,
Method of manufacturing steel plates.
However, the above Ms point (°C) and Ar ₃ point (°C) are expressed by the following formula, and the element symbol in the formula represents the content (mass%) of each element in the steel sheet, and if it is not contained. shall be assigned 0.
Ms=546×exp(-1.362×C)-11×Si-30×Mn-18×Ni-20×Cu-12×Cr-8(Mo+W)...(vi)
Ar ₃ =910-310×C+33×Si-80×Mn-55×Ni-20×Cu-15×Cr-80×Mo...(viii)

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