JP7444018B2 - Steel plates, their manufacturing methods, and members - Google Patents

Description

The present invention relates to a steel plate used for automobile parts, a method for manufacturing the same, and a member made using the steel plate.

In recent years, from the perspective of preserving the global environment, there has been a demand for further improvement in the fuel efficiency of automobiles with the aim of suppressing CO ₂ emissions. An effective way to improve automobile fuel efficiency is to reduce the weight of automobiles by making parts thinner. Therefore, in recent years, the amount of high-strength steel sheets used for automobile parts has been increasing.

However, in parts using high-strength steel plates, there is generally a concern that delayed fracture resistance may deteriorate. Here, delayed fracture is a phenomenon that occurs when a component is placed in a hydrogen intrusion environment under stress, and hydrogen enters the steel plate that makes up the component, reducing the interatomic bonding force and causing local damage. This is a phenomenon in which micro-cracks occur due to severe deformation, and the micro-cracks propagate leading to destruction.

Particularly, parts for automobiles are usually manufactured by shearing a steel plate to cut it into a predetermined size and then pressing it into a predetermined shape. At the end face of a steel plate cut by shearing (hereinafter also referred to as a sheared end face), residual stress due to strain introduced during shearing is large, and this residual stress is one of the causes of delayed fracture.

As a technique for suppressing delayed fracture of a steel plate having such a sheared edge surface, for example, Patent Document 1 describes
``A method for shearing a metal plate made of high-strength steel plate,
Shearing is performed twice on at least one end of the metal plate,
A method for shearing a metal plate, characterized in that a cutting allowance in the second shearing process of the two-time shearing process is set to 1.2 times or more and less than 20 times the thickness of the metal plate. ”
is disclosed.

International Publication 2020/145063

However, although the technique of Patent Document 1 has the effect of suppressing delayed fracture, the effect is not necessarily sufficient, and there is currently a need for further improvement in this respect.

The present invention was developed in view of the above-mentioned current situation, and an object of the present invention is to provide a steel plate having a sheared end face, high strength, and excellent delayed fracture resistance, and a method for manufacturing the same. shall be.
Another object of the present invention is to provide a member using the above-mentioned steel plate.

Now, the inventors have made extensive studies to achieve the above object, and have obtained the following knowledge.
(a) The occurrence of delayed fracture in a steel plate having a sheared edge surface is correlated with the size (width) of the region where strain is introduced into the steel plate by shearing (hereinafter also referred to as the processed region) and the amount of strain introduced.
(b) In particular, the size of the machining area must be below a certain level in relation to the thickness of the steel plate, specifically, the hardness of the machining area must be such that it satisfies equation (1) described below, and is proportional to the amount of strain introduced. By suppressing the hardness to 1.20 times or less of the hardness of the base material region where no strain is introduced, in other words, by setting the ratio of the hardness of the processed region to the hardness of the base material region to 120% or less. , the delayed fracture resistance is significantly improved.
(c) In addition, in order to obtain a steel plate like the one in (b) above, the (same) end face of the raw steel plate that will be the material to be sheared (hereinafter also referred to as the workpiece) must be sheared multiple times (n times, n is an integer of 2 or more), and the cutting allowance in the last (nth) shearing process is appropriately controlled according to the clearance in the previous (n-1st) shearing process and the thickness of the material steel plate. Specifically, it is effective to satisfy formulas (I) to (III) described below, which greatly reduces the size of the machining area and the amount of strain introduced, and greatly improves delayed fracture resistance. improve.
The present invention was completed based on the above findings and further studies.

That is, the gist of the present invention is as follows.
1. A steel plate having a sheared end surface,
The steel plate has a processing area including the sheared end surface and a base material area,
The following formula (1) is satisfied,
The ratio of the hardness of the processed region to the hardness of the base material region is 120% or less,
A steel plate having a tensile strength of 1180 MPa or more.
0<S/t ² ≦0.012 (1)
here,
S: Area of processing area in a plane perpendicular to the sheared end face and parallel to the plate thickness direction (mm ² )
t: Thickness of steel plate (mm)
It is.

2. Having a structure in which martensite: 40% or more and 100% or less, ferrite: 0% or more and 60% or less, and other metal phases: 5% or less, in terms of area ratio to the entire structure,
The number density of inclusion particles having a circular equivalent diameter of 4.0 μm or more in a depth region from the surface of the steel plate to a position of 1/4 of the plate thickness is 5 pieces/mm ² or more and 27 pieces/mm ² or less. 1. The steel plate according to 1.

3. In mass%,
C: 0.05% or more and 0.60% or less,
Si: 0.01% or more and 2.00% or less,
Mn: 0.10% or more and 3.20% or less,
P: 0.050% or less,
S: 0.0050% or less,
3. The steel sheet according to 1 or 2 above, which has a composition containing 0.10% or less of Al and 0.010% or less of N, with the remainder being Fe and inevitable impurities.

4. The component composition further includes, in mass%,
Cr: 0.50% or less,
Mo: 0.15% or less,
V: 0.05% or less,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
Ni: 0.10% or less,
B: 0.0020% or less,
3. The steel sheet according to 3 above, containing at least one selected from Sb: 0.10% or less and Sn: 0.10% or less.

5. A preparation process of preparing a steel plate as a workpiece,
a shearing process in which shearing is performed n times on at least one end surface of the raw steel plate, where n is an integer of 2 or more;
The cutting allowance T (mm) of the n-th shearing process in the shearing process is expressed by the following formula in relation to the plate thickness t (mm) of the raw steel plate and the clearance a (%) of the n-1th shearing process. A method for manufacturing a steel plate that satisfies any of (I) to (III).
(I) When a>30 0.002×a×t≦T≦0.07×a×t
(II) When 30≧a>5 0.06×t≦T≦2.1×t
(III) When 5≧a 0.012×a×t≦T≦0.42×a×t

6. The material steel plate has a structure in which martensite: 40% or more and 100% or less, ferrite: 0% or more and 60% or less, and other metal phases: 5% or less, in area ratio to the entire structure,
In the depth region from the surface of the material plate to the position of 1/4 of the plate thickness, the number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more is 5 pieces/mm ² or more and 27 pieces/mm ² or less, 5. The method for manufacturing a steel plate as described in 5 above.

7. The material steel plate is in mass%,
C: 0.05% or more and 0.60% or less,
Si: 0.01% or more and 2.00% or less,
Mn: 0.10% or more and 3.20% or less,
P: 0.050% or less,
S: 0.0050% or less,
7. The method for producing a steel plate according to 5 or 6 above, which has a composition containing 0.10% or less of Al and 0.010% or less of N, with the remainder being Fe and inevitable impurities.

8. The component composition further includes, in mass%,
Cr: 0.50% or less,
Mo: 0.15% or less,
V: 0.05% or less,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
Ni: 0.10% or less,
B: 0.0020% or less,
7. The method for producing a steel plate as described in 7 above, containing at least one selected from Sb: 0.10% or less and Sn: 0.10% or less.

9. The preparation step is
a slab heating step of heating and holding the slab;
a hot rolling step of hot rolling the slab into a hot rolled steel plate;
A cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;
an annealing step of annealing the cold rolled steel sheet;
has
In the slab heating step,
The surface temperature of the slab has an average heating rate of 0.10°C/s or more in the temperature range from 300°C to 1220°C,
In the temperature range, the average temperature ratio Tc/Ts of the center temperature Tc of the slab to the surface temperature Ts of the slab is 0.60 or more and 0.85 or less,
The holding time at the surface temperature of the slab is 1220°C or more for 30 minutes or more,
In the annealing process,
9. The method for producing a steel plate according to any one of 5 to 8 above, wherein the annealing temperature is at least A _C1 point and the annealing time is 30 seconds or more.

10. A member made using the steel plate according to any one of 1 to 4 above.

According to the present invention, a steel plate having a sheared end face, high strength, and excellent delayed fracture resistance can be obtained. In particular, by applying the steel plate of the present invention and members made using the steel plate to automobile parts, it is possible to improve the performance of the automobile body by reducing the weight of the automobile body.

This is an example of a SEM image of the vicinity of the sheared end surface on the cut surface of the test piece. This is an example of a SEM image in which the processing area has been determined. This is an example of a SEM image in which the determined processing area is binarized and displayed.

The present invention will be explained based on the following embodiments.
A steel plate according to an embodiment of the present invention is a steel plate having a sheared end face, and the steel plate has a processed region including the sheared end face and a base material region.
Here, the processed area is an area into which strain is introduced by shearing, and a part thereof includes a sheared end face. The base material region is a region other than the processed region (a region in which no strain is introduced by shearing). Note that the processing area and the base material area are defined by a method described later. Moreover, in the steel plate according to one embodiment of the present invention, if one of the end faces is a sheared end face, the other end face may or may not be a sheared end face. Here, the end surface is a surface other than the flat portion (front surface and back surface) of the steel plate. Moreover, the sheared end surface is a surface (end surface) cut by shearing.

In the steel plate according to an embodiment of the present invention, it is extremely important that the following formula (1) is satisfied and that the ratio of the hardness of the processed region to the hardness of the base material region is 120% or less.
0<S/t ² ≦0.012 (1)
here,
S: Area of processing area in a plane perpendicular to the sheared end face and parallel to the plate thickness direction (mm ² )
t: Thickness of steel plate (mm)
It is.

0<S/t ² ≦0.012
As mentioned above, the occurrence of delayed fracture in a steel plate having a sheared edge surface is correlated with the size of the processed area. In particular, in order to improve the delayed fracture resistance of a steel plate having a sheared edge, the area S of the processed area in a plane perpendicular to the sheared edge and parallel to the plate thickness direction (hereinafter also referred to as the area S of the processed area) It is necessary to keep S/t ² (hereinafter also referred to as S/t ² ), which is the value obtained by dividing t by the square of the thickness t of the steel plate, to 0.012 or less. S/t ² is preferably 0.011 or less, more preferably 0.010 or less, even more preferably 0.009 or less. The lower limit of S/t ² is not particularly limited, but since S does not become 0 as long as it has a sheared end surface, S/t ² exceeds 0.

Here, the area S of the processing area is measured as follows.
That is, the steel plate to be measured is cut so that a surface perpendicular to the sheared end face and parallel to the plate thickness direction is exposed, and a test piece is taken. Then, the cut surface of the test piece is polished to a mirror finish, and the flow of the structure (metal flow) is revealed using picric acid. Then, a region near the sheared end face is observed using a scanning electron microscope (hereinafter also referred to as SEM) at a magnification of 200 times. Then, as shown in FIG. 1, reference points are set at 200 μm from the sheared end surface of the cut surface at intervals of 50 μm in the thickness direction. Then, identify the tissue flow closest to each reference point at a position of 200 μm from the sheared end surface, and follow the tissue flow, that is, pass through the tissue flow at a position of 200 μm from the sheared end surface, and , draw a straight line (hereinafter also referred to as the reference line) parallel to the direction perpendicular to the sheared end surface. Then, points (hereinafter also referred to as boundary reference points) where the flow of each tissue is shifted by 10 μm from the reference line in the thickness direction are connected to adjacent boundary reference points in the thickness direction. Then, the processing area (the area from the boundary line to the sheared end face) and the base material area are determined by a line connecting these boundary reference points (hereinafter also referred to as a boundary line). Next, the area of the defined processing area is determined by binarizing the SEM photograph of the defined processing area, and the obtained area is set as the area S of the processing area.
For reference, Fig. 1 shows an example of an SEM image near the sheared end surface of the cut surface of the test piece, Fig. 2 shows an example of an image with the processed area determined, and an SEM image in which the determined processed area is binarized and displayed. Examples of the images are shown in FIG. 3.

Here, the cutting position of the steel plate that is the object to be measured is the center position of the projection line of the sheared end face (hereinafter also referred to as the projection line of the sheared end face) when the steel plate is projected in the plate thickness direction (equally distanced from both ends). position).
In addition, if the projection line of the sheared end face is a continuous tangent, it is counted as one sheared end face.
In addition, if the projection line of the sheared end surface is not a straight line, in the projection view, the direction perpendicular to the straight line connecting both ends of the projection line of the sheared end surface is defined as the direction perpendicular to the sheared end surface. In addition, when the projection line of the sheared end surface is a straight line, in the projection view, the direction perpendicular to the projection line of the sheared end surface is defined as the direction perpendicular to the sheared end surface. In addition, when the projection line of the sheared end surface is approximately circular and extends over the entire circumference of the end surface of the steel plate that is the object to be measured, the cutting position of the steel plate that is the object to be measured is taken as the reference position. In the projection view, the direction perpendicular to the tangential direction at the reference position on the projection line of the sheared end surface is defined as the direction perpendicular to the sheared end surface.
Note that the direction perpendicular to the sheared end surface is parallel to the surface of the steel plate, that is, a direction perpendicular to the plate thickness direction. Further, the plane perpendicular to the sheared end face and parallel to the plate thickness direction is a plane including the direction perpendicular to the sheared end face and the plate thickness direction.

Ratio of hardness of processed area to hardness of base material area: 120% or less As described above, the occurrence of delayed fracture at the sheared end face is correlated with the amount of strain introduced into the processed area. In particular, in order to improve the delayed fracture resistance of a steel plate with a sheared edge, the hardness of the processed region, which is proportional to the amount of strain introduced, should be 1.20 times or less than the hardness of the base material region where no strain is introduced. In other words, it is necessary to make the ratio of the hardness of the processed region to the hardness of the base material region 120% or less. The ratio of the hardness of the processed region to the hardness of the base material region is preferably 117% or less, more preferably 115% or less, even more preferably 112% or less. Note that when shearing is performed, the hardness of the region where strain is introduced increases, so the ratio of the hardness of the processed region to the hardness of the base material region becomes 100% or more.

Here, the hardness of the processed region and the hardness of the base material region are each measured as follows.
That is, the steel plate to be measured is cut so that a surface perpendicular to the sheared end face and parallel to the plate thickness direction is exposed, and a test piece is taken. Then, the Vickers hardness measured at 20 μm and 1 mm from the sheared end surface at the thickness center position of the cut surface of the test piece (plate thickness 1/2 position) was calculated as the hardness of the processed area and the hardness of the base material area, respectively. Satoru. Further, the Vickers hardness shall be measured in accordance with JIS Z 2244 (2009), and the measurement load shall be 10 gf in all cases.

Note that the cutting position of the steel plate, which is the object to be measured, is the same as in the case of measuring the area S of the processing region.

In addition, for steel plates having two or more sheared end faces, the above formula (1) must be satisfied for each sheared end face, and the ratio of the hardness of the processed area to the hardness of the base metal area must be 120% or less. It is preferable to

Tensile strength (TS): 1180 MPa or more The tensile strength of the steel plate according to one embodiment of the present invention is 1180 MPa or more. The tensile strength of the steel plate according to one embodiment of the present invention is preferably 1250 MPa or more, more preferably 1300 MPa or more, and still more preferably 1400 MPa or more. Note that the upper limit of the tensile strength of the steel plate according to an embodiment of the present invention is not particularly limited, but from the viewpoint of easy balance with other properties and from the viewpoint of preventing damage to the blade during shearing, it is preferably 2500 MPa or less. preferable.

Moreover, "excellent delayed fracture resistance" means that the critical load stress determined by the method described below is greater than or equal to the yield strength (hereinafter also simply referred to as YS). The critical load stress is preferably (YS+100) MPa or more, more preferably (YS+200) MPa or more.

Here, tensile strength (TS) and yield strength (YS) are measured as follows.
That is, a JIS No. 5 test piece with a gauge distance of 50 mm and a gauge width of 25 mm is taken from the center of the plate width in the base material region of the steel plate so that the rolling direction is the longitudinal direction. Next, using the sampled JIS No. 5 test piece, a tensile test is conducted in accordance with the provisions of JIS Z 2241 (2011), and the tensile strength (TS) and yield strength (YS) are measured. Note that the tensile speed is 10 mm/min.

Moreover, the plate thickness t of the steel plate according to one embodiment of the present invention is preferably 0.2 mm or more and 3.2 mm or less.
Note that the thickness of a material steel plate that will be a workpiece to be described later is the same as the thickness of a steel plate according to an embodiment of the present invention. Therefore, these plate thicknesses are all expressed as t.

Next, a preferred structure of a steel plate according to an embodiment of the present invention will be described.

Area ratio of martensite to the entire structure: 40% or more and 100% or less In order to obtain high strength of TS≧1180MPa, the area ratio of martensite to the entire structure (hereinafter also simply referred to as area ratio) should be 40% or more. preferable. If the area ratio of martensite is less than 40%, the area ratio of ferrite, retained austenite, pearlite, bainite, etc. increases, which may lead to a decrease in strength. The area ratio of martensite is more preferably 50% or more, still more preferably 60% or more. Moreover, the area ratio of martensite may be 100%.
Note that martensite here refers to a hard structure generated from austenite below the martensite transformation point (also simply referred to as the Ms point), and refers to so-called fresh martensite as quenched, and fresh martensite that is reheated. It shall include both so-called tempered martensite.

Area ratio of ferrite: 0% or more and 60% or less From the viewpoint of ensuring the strength of the steel plate, the area ratio of ferrite is preferably 60% or less. The area ratio of ferrite is more preferably 50% or less, still more preferably 40% or less. Further, the area ratio of ferrite may be 0%.
Note that ferrite here is a structure consisting of crystal grains in a BCC lattice, and is generated by transformation from austenite at a relatively high temperature.

Area ratio of other metal phases: 5% or less The structure of the steel sheet according to an embodiment of the present invention may include metal phases other than martensite and ferrite. Here, the area ratio of other metal phases is allowed as long as it is 5% or less.
Other metal phases include, for example, retained austenite, pearlite, and bainite.
Note that the retained austenite here refers to austenite that remains without undergoing martensitic transformation. Pearlite is a structure consisting of ferrite and acicular cementite. Bainite is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite, and is generated from austenite at a relatively low temperature (above the martensitic transformation point).

Here, the area ratio of each phase is measured as follows.
That is, a test piece is taken from the base material region of the steel plate so that the L cross section parallel to the rolling direction serves as the test surface. Next, the test surface of the test piece is mirror polished and the structure is exposed using nital solution. The test surface of the test piece on which the structure has appeared is observed using a SEM at a magnification of 1,500 times, and the area ratio of martensite and the area ratio of ferrite at a position of 1/4 of the plate thickness is measured using a point counting method. Further, the area ratio of other metal phases is calculated by subtracting the area ratio of martensite and the area ratio of ferrite from 100%.

Note that in the SEM image, martensite exhibits a white structure. Further, among martensite, tempered martensite has fine carbides precipitated inside. Ferrite exhibits a black structure. From these points, each phase is identified in the SEM image. However, depending on the plane orientation of the block grains and the degree of etching, internal carbides may be difficult to reveal, so in that case, sufficient etching should be performed to confirm.

Number density of inclusion particles with an equivalent circle diameter of 4.0 μm or more in the depth region from the surface of the steel plate to 1/4 of the plate thickness: 5 particles/mm ² or more and 27 particles/mm ² or less Steel plate surface to plate In the depth region up to the 1/4th thickness position, large strain occurs during shearing. Therefore, when the number of inclusion particles having a circular equivalent diameter of 4.0 μm or more in the area increases (hereinafter also simply referred to as coarse inclusion particles), the processing area increases and the delayed fracture resistance tends to deteriorate. Therefore, the number density of coarse inclusion particles is preferably 27 particles/mm ² or less. The number density of coarse inclusion particles is more preferably 25 pieces/mm ² or less, and even more preferably 20 pieces/mm ² or less.
On the other hand, coarse inclusion particles play a role in promoting crack growth during shearing. That is, when the number of coarse inclusion particles is excessively reduced, the growth of cracks during shearing is suppressed. Therefore, the processing area increases and the delayed fracture resistance tends to deteriorate. Therefore, the number density of coarse inclusion particles is preferably 5 pieces/mm ² or more. The number density of coarse inclusion particles is more preferably 10 pieces/mm ² or more, and even more preferably 12 pieces/mm ² or more.

Here, the number density of coarse inclusion particles is measured as follows.
That is, a test piece is taken from the base metal region of the steel plate so that the L cross section parallel to the rolling direction serves as the test surface. Next, the test surface of the test piece is polished to a mirror surface, and the structure is revealed using a nital solution. The test surface of the test piece on which the structure has been exposed is observed at a magnification of 5,000 times using a scanning electron microscope (SEM), and images are continuously taken from the surface to the 1/4 position of the plate thickness. Then, the obtained SEM image is binarized, the area of each white inclusion particle is measured, and the circle equivalent diameter of each inclusion particle is calculated using the following formula.
[Equivalent circular diameter of inclusion particles (μm)]
= ([Area of inclusion particles (μm ² )] ÷ π) ^0.5 ×2
Then, the number of inclusion particles with a circle equivalent diameter of 4.0 μm or more observed in the SEM image is counted, and the number of inclusion particles with a circle equivalent diameter of 4.0 μm or more is calculated based on the area of the observation range in the SEM image. By dividing by , the number density of coarse inclusion particles is calculated.

Next, a preferred composition of a steel plate according to an embodiment of the present invention will be described. Note that the units in the component compositions are all "% by mass", but hereinafter, unless otherwise specified, they will be simply expressed as "%".
A preferred composition of the steel plate according to one embodiment of the present invention is:
C: 0.05% or more and 0.60% or less,
Si: 0.01% or more and 2.00% or less,
Mn: 0.10% or more and 3.20% or less,
P: 0.050% or less,
S: 0.0050% or less,
Contains Al: 0.10% or less, and N: 0.010% or less,
Furthermore, optionally
Cr: 0.50% or less,
Mo: 0.15% or less,
V: 0.05% or less,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
Ni: 0.10% or less,
B: 0.0020% or less,
It contains at least one selected from Sb: 0.10% or less and Sn: 0.10% or less, and the remainder is Fe and inevitable impurities.

C: 0.05% or more and 0.60% or less C is an element that improves the hardenability of steel, and is an effective element for ensuring a predetermined amount of martensite. Further, C is an effective element from the viewpoint of increasing the strength of martensite and ensuring a predetermined strength. Therefore, from the viewpoint of obtaining a predetermined strength while obtaining excellent delayed fracture resistance, the C content is preferably 0.05% or more. In addition, from the viewpoint of obtaining TS≧1250 MPa, the C content is more preferably 0.11% or more. Moreover, from the viewpoint of obtaining TS≧1300 MPa, it is more preferable that the C content is 0.125% or more. On the other hand, when the C content exceeds 0.60%, the strength increases excessively. Therefore, the C content is preferably 0.60% or less. The C content is more preferably 0.50% or less, still more preferably 0.40% or less.

Si: 0.01% or more and 2.00% or less Si is an element that increases the strength of the steel plate through solid solution strengthening. In order to sufficiently obtain such effects, the Si content is preferably 0.01% or more. The Si content is more preferably 0.02% or more, still more preferably 0.03% or more. On the other hand, if the Si content becomes too large, coarse MnS tends to be generated in the thickness direction. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Si content is preferably 2.00% or less. The Si content is more preferably 1.70% or less, still more preferably 1.50% or less.

Mn: 0.10% or more and 3.20% or less Mn is an element that improves the hardenability of steel, and is an effective element for ensuring a predetermined amount of martensite. Here, if the Mn content is less than 0.10%, ferrite is generated in the surface layer of the steel sheet, and the strength tends to decrease. Therefore, the Mn content is preferably 0.10% or more. The Mn content is more preferably 0.20% or more, still more preferably 0.30% or more. On the other hand, Mn is an element that particularly promotes the formation and coarsening of MnS. In particular, when the Mn content exceeds 3.20%, the amount of coarse MnS produced increases. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Mn content is preferably 3.20% or less. The Mn content is more preferably 3.00% or less, still more preferably 2.80% or less.

P: 0.050% or less P is an element that strengthens steel. However, when the P content increases, P segregates at grain boundaries. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the P content is preferably 0.050% or less. The P content is more preferably 0.030% or less, still more preferably 0.010% or less. Note that the lower limit of the P content is not particularly limited, but is preferably about 0.003%.

S: 0.0050% or less S is an element that forms inclusions such as MnS, TiS, Ti (C, S), etc., and deteriorates delayed fracture resistance. From the viewpoint of suppressing deterioration of such delayed fracture resistance, the S content is preferably 0.0050% or less. The S content is more preferably 0.0020% or less, still more preferably 0.0010% or less, even more preferably 0.0005% or less. Note that the lower limit of the S content is not particularly limited, but is preferably about 0.0002%.

Al: 0.10% or less Al is added to perform sufficient deoxidation and reduce coarse inclusions in the steel. From the viewpoint of obtaining sufficient effects, the Al content is preferably 0.005% or more. The Al content is more preferably 0.010% or more. On the other hand, when the Al content exceeds 0.10%, carbides mainly composed of Fe such as cementite generated during coiling after hot rolling become difficult to dissolve in the annealing process, resulting in coarse inclusions and carbides. tends to be generated. As a result, the strength of the steel plate may decrease. In addition, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Al content is preferably 0.10% or less. The Al content is more preferably 0.08% or less, still more preferably 0.06% or less.

N: 0.010% or less N is an element that forms coarse inclusions of nitrides and carbonitrides such as TiN, (Nb, Ti) (C, N), and AlN in steel. That is, if N is contained excessively, the delayed fracture resistance may deteriorate due to the formation of coarse inclusions. From the viewpoint of preventing such deterioration of delayed fracture resistance, the N content is preferably 0.010% or less. The N content is more preferably 0.007% or less, still more preferably 0.005% or less. Note that the lower limit of the N content is not particularly limited, but is preferably about 0.0003%.

The basic composition of the steel plate according to one embodiment of the present invention has been explained above, but furthermore,
Cr: 0.50% or less,
Mo: 0.15% or less,
V: 0.05% or less,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
Ni: 0.10% or less,
B: 0.0020% or less,
At least one optionally added element selected from Sb: 0.10% or less and Sn: 0.10% or less, for example,
At least one selected from Cr: 0.50% or less, Mo: 0.15% or less, and V: 0.05% or less,
At least one selected from Nb: 0.020% or less and Ti: 0.020% or less,
At least one selected from Cu: 0.20% or less and Ni: 0.10% or less,
B: 0.0020% or less, and/or at least one selected from Sb: 0.10% or less and Sn: 0.10% or less,
can be contained.
In addition, when an optionally added element is included in an amount less than a preferable lower limit value, the element is included as an unavoidable impurity.

Cr: 0.50% or less Cr can be contained for the purpose of improving the hardenability of steel. In order to obtain such effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more, still more preferably 0.03% or more. On the other hand, when the Cr content increases too much, carbides become coarse. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Cr content is preferably 0.50% or less. The Cr content is more preferably 0.10% or less.

Mo: 0.15% or less Like Cr, Mo can be included for the purpose of improving the hardenability of steel. In order to obtain such effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.02% or more, still more preferably 0.03% or more. On the other hand, when the Mo content increases too much, carbides become coarse. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Mo content is preferably 0.15% or less. Mo content is more preferably 0.10% or less.

V: 0.05% or less Similar to Cr and Mo, V can be included for the purpose of improving the hardenability of steel. In order to obtain such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.002% or more, still more preferably 0.003% or more. On the other hand, when the V content becomes too large, carbides become coarse. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the V content is preferably 0.05% or less. The V content is more preferably 0.04% or less, still more preferably 0.03% or less.

Nb: 0.020% or less Nb is an element that contributes to high strength by refining prior γ grains. In order to obtain such an effect, it is preferable that the Nb content is 0.001% or more. The Nb content is more preferably 0.002% or more, still more preferably 0.003% or more. On the other hand, when the Nb content becomes excessive, coarse Nb-based substances such as NbN, Nb(C,N), (Nb,Ti)(C,N), etc. that remain undissolved during slab heating in the hot rolling process Precipitates increase. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Nb content is preferably 0.020% or less. The Nb content is more preferably 0.015% or less, still more preferably 0.010% or less.

Ti: 0.020% or less Ti, like Nb, is an element that contributes to higher strength by refining prior γ grains. In order to obtain such an effect, it is preferable that the Ti content is 0.001% or more. The Ti content is more preferably 0.002% or more, still more preferably 0.003% or more. On the other hand, when the Ti content becomes excessive, coarse Ti-based precipitates such as TiN, Ti(C,N), Ti(C,S), TiS, etc. that remain undissolved during slab heating in the hot rolling process occur. increases. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Ti content is preferably 0.020% or less. The Ti content is more preferably 0.015% or less, still more preferably 0.010% or less.

Cu: 0.20% or less Cu has the effect of improving corrosion resistance in the environment of automobile use and suppressing hydrogen intrusion into the steel plate by coating the surface of the steel plate with corrosion products. In order to obtain this effect, the Cu content is preferably 0.001% or more. The Cu content is more preferably 0.002% or more. On the other hand, when the Cu content becomes excessive, surface defects are caused and the plating properties and chemical conversion treatment properties are deteriorated. Therefore, the Cu content is preferably 0.20% or less. The Cu content is more preferably 0.15% or less, still more preferably 0.10% or less.

Ni: 0.10% or less Ni, like Cu, has the effect of improving corrosion resistance in the environment of automobile use and suppressing hydrogen intrusion into the steel plate by coating the surface of the steel plate with corrosion products. In order to obtain such an effect, it is preferable that the Ni content is 0.001% or more. The Ni content is more preferably 0.002% or more. On the other hand, when the Ni content becomes excessive, surface defects are caused and the plating properties and chemical conversion treatment properties are deteriorated. Therefore, the Ni content is preferably 0.10% or less. The Ni content is more preferably 0.08% or less, still more preferably 0.06% or less.

B: 0.0020% or less B is an element that improves the hardenability of steel, and is an effective element for ensuring a predetermined amount of martensite. In order to obtain such an effect, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0002% or more, still more preferably 0.0003% or more. On the other hand, if the B content exceeds 0.0020%, the solid solution rate of cementite during annealing will be reduced, and carbides mainly composed of Fe such as undissolved cementite will remain. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the B content is preferably 0.0020% or less. The B content is more preferably 0.0015% or less, still more preferably 0.0010% or less.

Sb: 0.10% or less Sb suppresses oxidation and nitridation in the surface layer of the steel sheet, and further suppresses the reduction of C and B in the steel due to oxidation and nitridation in the surface layer of the steel sheet. Furthermore, Sb suppresses the formation of ferrite in the surface layer of the steel sheet and contributes to increasing the strength. In order to obtain such an effect, it is preferable that the Sb content is 0.002% or more. The Sb content is more preferably 0.003% or more, still more preferably 0.004% or more. On the other hand, when the Sb content exceeds 0.10%, Sb segregates at prior γ grain boundaries. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Sb content is preferably 0.10% or less. The Sb content is more preferably 0.08% or less, still more preferably 0.06% or less.

Sn: 0.10% or less Like Sb, Sn suppresses oxidation and nitridation in the surface layer of the steel sheet, and further suppresses the reduction of C and B in the steel due to oxidation and nitridation in the surface layer of the steel sheet. Moreover, Sn suppresses the formation of ferrite in the surface layer of the steel sheet and contributes to increasing the strength. In order to obtain such an effect, it is preferable that the Sn content is 0.002% or more. The Sn content is more preferably 0.003% or more, still more preferably 0.004% or more. On the other hand, when the Sn content exceeds 0.10%, Sn segregates at prior γ grain boundaries. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Therefore, the Sn content is preferably 0.10% or less. The Sn content is more preferably 0.08% or less, still more preferably 0.06% or less.

The remainder other than the above elements is Fe and unavoidable impurities.

In addition, the steel plate according to one embodiment of the present invention may have a plating layer. Examples of the plating layer include Zn-based plating and Al-based plating.

Next, a method for manufacturing a steel plate according to an embodiment of the present invention will be described.
A method for manufacturing a steel plate according to an embodiment of the present invention includes:
A preparation process of preparing a steel plate as a workpiece,
A shearing process is provided, in which at least one end face of the raw steel plate is subjected to shearing process n times, where n is an integer of 2 or more.

・Preparation process For example, the material steel plate to be processed is:
Having a structure in which martensite: 40% or more and 100% or less, ferrite: 0% or more and 60% or less, and other metal phases: 5% or less, in terms of area ratio to the entire structure,
In the depth region from the surface of the material plate to the position of 1/4 of the plate thickness, the number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more is 5 pieces/mm ² or more and 27 pieces/mm ² or less, Steel plate is preferred. By using such a steel plate, the processing area formed in the shearing process described below can be more advantageously reduced.
Further, as the material steel plate to be processed, a steel plate having the above-mentioned composition is suitable.

Such material steel plates are, for example,
a slab heating step of heating and holding the slab;
a hot rolling step of hot rolling the slab into a hot rolled steel plate;
A cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;
an annealing step of annealing the cold rolled steel sheet;
has
In the slab heating step,
The surface temperature of the slab has an average heating rate of 0.10°C/s or more in the temperature range from 300°C to 1220°C,
In the temperature range, the average temperature ratio Tc/Ts of the thickness center temperature Tc of the slab to the surface temperature Ts of the slab is 0.60 or more and 0.85 or less,
The holding time at the surface temperature of the slab is 1220°C or more for 30 minutes or more,
In the annealing process,
It can be prepared by a method for producing a raw steel plate in which the annealing temperature is at least A _C1 point and the annealing time is at least 30 seconds.
Hereinafter, preferred conditions for the slab heating step and annealing step in the method for manufacturing the above-mentioned raw steel plate will be explained.

Average heating rate in the slab surface temperature range from 300°C to 1220°C: 0.10°C/s or more 4.0 μm in equivalent circle diameter in the depth region from the surface of the steel plate to 1/4 of the plate thickness In order to control the number density of the above inclusion particles within a predetermined range, it is preferable that the average heating rate (hereinafter also referred to as average heating rate) in the temperature range of 300°C to 1220°C at the surface temperature of the slab is faster. In particular, it is preferably 0.10° C./s or more. The average heating rate is more preferably 0.11°C/s or more, and still more preferably 0.12°C/s or more. The upper limit of the average heating rate is not particularly limited, but is preferably 0.17° C./s or less.

Average temperature ratio Tc/Ts of slab thickness center temperature Tc to slab surface temperature Ts in the temperature range from 300°C to 1220°C in slab surface temperature: 0.60 or more and 0.85 or less During heating of the slab, Particularly in the slab surface temperature range of 300°C to 1220°C, by slowing down the heating rate at the center of the thickness of the slab relative to the slab surface, coarse inclusion particles can be reduced at the center of the thickness of the slab. The production can be promoted. Furthermore, on the other hand, it is possible to suppress the formation of coarse inclusion particles at a position from the slab surface to 1/4 of the plate thickness. That is, the smaller the average temperature ratio Tc/Ts (hereinafter also referred to as Tc/Ts) of the thickness center temperature Tc of the slab to the slab surface temperature Ts, the larger the coarse inclusions at the slab surface to 1/4 thickness position. It becomes possible to suppress the generation of particles. Therefore, Tc/Ts is preferably 0.85 or less. Tc/Ts is more preferably 0.83 or less, still more preferably 0.80 or less. On the other hand, if Tc/Ts becomes too small, the generation of coarse inclusion particles at a position from the slab surface to 1/4 of the plate thickness is excessively suppressed, and the number density of coarse inclusion particles becomes less than 5 pieces/mm ² . As a result, the delayed fracture resistance is rather deteriorated. Therefore, it is preferable that Tc/Ts be 0.60 or more. Tc/Ts is more preferably 0.65 or more, still more preferably 0.70 or more.

Here, Tc/Ts is calculated as follows.
By drilling a small hole in the center of the slab in the longitudinal and width directions so as to reach the center of the plate thickness, and embedding a temperature measurement sensor at the center of the plate thickness, the temperature at the center of the plate thickness (Tc) during heating of the slab can be measured. Measure. Furthermore, the surface temperature (Ts) during slab heating is measured using a thermocouple or a non-contact radiation thermometer. Then, calculate the time integral values of Tc and Ts until the surface temperature (Ts) of the slab during slab heating increases from 300°C to 1220°C, and calculate the value obtained by dividing the time integral value of Tc by the time integral value of Ts. , Tc/Ts.

Holding time at a slab surface temperature of 1220°C or higher: 30 minutes or more The longer the holding time at a slab surface temperature of 1220°C or higher (hereinafter also referred to as slab holding time), the more coarse particles remained in undissolved form. Things become easier to dissolve. Therefore, the slab holding time is preferably 30 minutes or more. The slab holding time is more preferably 60 minutes or more, still more preferably 90 minutes or more. Further, the upper limit of the slab holding time is preferably 180 minutes or less from the viewpoint of leaving only a few coarse inclusions, promoting crack propagation during shearing, and reducing the processing area.

Annealing temperature: A _C1 point or more If the annealing temperature is less than A _C1 point, austenite will not be generated, and it will be difficult to generate a predetermined amount of martensite and, by extension, to obtain the desired strength. Therefore, it is preferable that the annealing temperature be set to the A _C1 point or higher. The annealing temperature is more preferably ( _AC1 point +10°C) or higher. Although the upper limit of the annealing temperature is not particularly limited, when the austenite grain size increases as the annealing temperature increases, a metal phase other than martensite is generated. Therefore, it is preferable that the annealing temperature is 900°C or less.
Note that A _C1 point is calculated using the following formula.
A _C1 point (℃) = 723 + 22 (%Si) - 18 (%Mn) + 17 (%Cr) + 4.5 (%Mo) + 16 (%V)
In the formula, (% element symbol) is the content (mass%) of each element.

Annealing time: 30 seconds or more If the holding time at the annealing temperature (hereinafter also referred to as annealing time) is less than 30 seconds, the dissolution of carbides will not proceed sufficiently, so they will remain when heat treatment is performed in subsequent steps. The carbides that are present become coarser. As a result, strain is likely to be introduced during shearing, the processing area increases, and delayed fracture resistance may deteriorate. Furthermore, since the austenite transformation does not proceed sufficiently, it becomes difficult to generate a predetermined amount of martensite and, by extension, to obtain a desired strength. Therefore, the annealing time is preferably 30 seconds or more. The annealing time is more preferably 35 seconds or more. The upper limit of the annealing time is not particularly limited, but from the viewpoint of suppressing coarsening of the austenite grain size, the annealing time is preferably 900 seconds or less.
Note that the annealing temperature may be constant during holding, and does not need to be constant during holding as long as it is within the above temperature range. The same goes for holding the slab.

Further, it is preferable that the component composition of the slab be the above-mentioned component composition.

Note that the conditions for each step other than those described above are not particularly limited, and may be according to conventional methods.
For example, in the hot rolling process, it is preferable that the finish rolling temperature is 840°C or higher and the winding temperature is 630°C or lower.
If the finish rolling temperature is less than 840° C., it takes time to lower the temperature, and inclusions and coarse carbides may be generated, which may deteriorate the delayed fracture resistance. Furthermore, the quality of the inside of the steel plate may also deteriorate. Therefore, the finish rolling temperature is preferably 840°C or higher. The finish rolling temperature is more preferably 860°C or higher. Note that the upper limit of the finish rolling temperature is not particularly limited, but the finish rolling temperature is preferably 950° C. or lower since cooling to the winding temperature in the subsequent step becomes difficult. The finish rolling temperature is more preferably 920°C or lower.
Furthermore, if the winding temperature exceeds 630° C., there is a possibility that the surface of the steel base will decarburize, and a difference in structure will occur between the inside and the surface of the steel sheet, which may cause uneven alloy concentration. In addition, ferrite is generated in the surface layer due to decarburization, which may reduce the strength. Therefore, the winding temperature is preferably 630°C or lower. The winding temperature is more preferably 600°C or lower. Although the lower limit of the winding temperature is not particularly limited, the winding temperature is preferably 500° C. or higher in order to prevent deterioration of cold rollability.
Furthermore, the hot-rolled steel sheet after winding may be pickled. Pickling conditions are not particularly limited, and any conventional method may be used. In addition, the hot-rolled steel sheet after winding may be subjected to heat treatment for softening the structure.

Moreover, in the cold rolling process, it is preferable that the rolling reduction is 20% or more. This is because if the rolling reduction ratio is less than 20%, the surface flatness may be poor and the structure may become non-uniform.

In addition, a tempering treatment may be performed after the annealing step. Further, the steel plate may be subjected to a plating treatment such as Zn-based plating or Al-based plating. Furthermore, after the annealing process or after the plating process, the steel plate may be subjected to temper rolling for shape adjustment.

- Shearing process Next, at least one end face of the raw steel plate prepared as described above is subjected to shearing process n times.
Here, n is an integer of 2 or more. There is no particular limit to the upper limit of n, but as described later, if the final shearing conditions are appropriately controlled according to the immediately preceding shearing conditions, the previous shearing conditions will be resistant to delayed fracture. Does not affect properties. Therefore, it is preferable that n be smaller, and n is preferably 3 or less.

In the method for manufacturing a steel plate according to an embodiment of the present invention, the cutting allowance T in the last (n-th) shearing process is calculated by the clearance a in the immediately preceding (n-1st) shearing process and the plate thickness t of the material steel plate. Specifically, it is important to satisfy the following equations (I) to (III).
(I) When a>30 0.002×a×t≦T≦0.07×a×t
(II) When 30≧a>5 0.06×t≦T≦2.1×t
(III) When 5≧a 0.012×a×t≦T≦0.42×a×t

That is, when shearing is performed under general conditions, a processed region is formed near the sheared end face. Therefore, it is important to adjust the cutting allowance during the last (nth) shearing process and remove (cut off) the processed area created by the n-1st shearing process according to the thickness t of the material steel plate. It is. However, if the cutting allowance during the last (nth) shearing process becomes too large, a processed area is formed by the shearing process, and the hardness of the processed area increases.
Further, when the clearance in the n-1th shearing process is 30≧a>5, the increase or decrease in the processed area due to the influence of the clearance is very small. Therefore, in this case, the cutting allowance T in the n-th shearing process is controlled only according to the plate thickness t of the material steel plate.
On the other hand, if the clearance in the n-1th shearing process is relatively large, specifically if a>30, the machining area introduced in the n-1st shearing process is (30≧a>5) ) becomes relatively large. Further, the machining area increases according to the clearance. Therefore, in addition to the plate thickness t of the material steel plate, it is necessary to control (increase) the cutting allowance T in the n-th shearing process according to the clearance.
In addition, if the clearance in the n-1th shearing process is relatively small, specifically if 5≧a, the machining area introduced in the n-1st shearing process is (30≧a>5) (compared to ) will be relatively small. Further, the machining area is reduced depending on the clearance. Therefore, in addition to the thickness t of the material steel plate, it is necessary to control (reduce) the cutting allowance T in the n-th shearing process according to the clearance.
From the above, the cutting allowance T in the n-th shearing process can be calculated according to the above formulas (I) to (III) according to the clearance a and the thickness t of the material steel plate in the n-1th shearing process immediately before. It is important to control this to satisfy the following.

Note that a machining area is also formed during the last (nth) shearing process, but the cutting distance is within the upper limit of the above formulas (I) to (III), in other words, the nth shearing process By setting the cutting position for machining near the boundary between the machining area formed in the n-1th shearing process and the base material area, the final (nth) shearing process is easier than normal shearing process, which has a large cutting allowance. The range of the processing area formed during this process is significantly reduced.
That is, in normal shearing with a large cutting allowance, cracks that occur from the upper blade of the punch progress in the thickness direction toward the lower blade (along the straight line connecting the upper and lower blades of the punch). On the other hand, as in the n-th shearing process, when the cutting position is near the boundary between the processing area formed in the n-1st shearing process and the base metal area, the n-1st shearing process occurs directly under the punch upper blade. Compressive stress increases due to the influence of strain introduced by processing. Therefore, in order to avoid the high compressive stress field, the cracks generated by the nth shearing process dig into the base metal side from the straight line connecting the upper and lower blades of the punch, and then progress toward the lower blade. As a result, the processed area formed in the n-1th shearing process and a part of the processed area formed in the nth sheared process are removed at the same time. Therefore, the range of the processing area formed during the last (nth) shearing process is the range of the processing area formed during normal shearing process with a large cutting allowance (shearing process before the n-1th time). is significantly reduced compared to .

Here, the lower limit of the above formula (I) is preferably 0.0023 x a x t, more preferably 0.003 x a x t, and still more preferably 0.0033 x a x t.
The upper limit of the above formula (I) is preferably 0.053×axt, more preferably 0.04×axt, and even more preferably 0.027×axt.

The lower limit of the above formula (II) is preferably 0.07×t, more preferably 0.09×t, and even more preferably 0.10×t.
The upper limit of the above formula (II) is preferably 1.6×t, more preferably 1.2×t, and still more preferably 0.8×t.

The lower limit of the above formula (III) is preferably 0.014×axt, more preferably 0.018×axt, and still more preferably 0.020×axt.
The upper limit of the above formula (III) is preferably 0.32 x a x t, more preferably 0.24 x a x t, and still more preferably 0.16 x a x t.

Here, the cutting allowance T in the n-th shearing process is the center position of the projection line of the sheared end face in the n-1st shearing process when the steel plate is projected in the plate thickness direction (a position equidistant from both ends) and the distance between the center position (position equidistant from both ends) of the projection line of the sheared end surface obtained by the n-th shearing process. The same applies to the cutting allowances in other shearing operations.

Moreover, the clearance a (%) is calculated by the following formula.
a=L/t×100
here,
L: Gap between upper blade (movable blade) and lower blade (fixed blade) (mm)
It is.
More specifically, the gap L between the upper and lower blades (of the cutting device used for shearing) is measured using a measuring jig such as calipers, and the measured L is calculated as the plate thickness t (mm) of the material steel plate. Calculate by dividing by .

Note that conditions other than those described above are not particularly limited, and conventional methods may be followed.
For example, the clearance in the n-th shearing process is preferably 5% or more and 30% or less, and the shear angle is preferably 0° or more and 2° or less. In particular, if the clearance is less than 5%, the machining area tends to become large, and if the clearance exceeds 30% and the shear angle exceeds 2°, delayed fracture resistance tends to deteriorate due to excessive burr formation.
In addition, the clearance in the n-1th shearing process is not particularly limited as long as the above formulas (I) to (III) are satisfied, but it is preferably 1% or more and 40% or less, and 5% or more and 30% or less. % or less is more preferable. Further, the shear angle in the (n-1)th shearing process is preferably 0° or more and 2° or less. Note that the cutting allowance in the (n-1)th shearing process is not particularly limited, and may be set as appropriate depending on the shape of the member to be the final product.
In addition, the shearing conditions before the (n-1)th time are not particularly limited, and may be set as appropriate depending on the shape of the member to be the final product.
Furthermore, cutting in the shearing process is performed by shear cutting (mechanical cutting). Here, shear cutting (mechanical cutting) is a method of mechanically cutting a workpiece using shear force, and includes, for example, cutting using a shear cutter or a punching machine.

Next, a member according to an embodiment of the present invention will be described.
The member according to one embodiment of the present invention is made using the steel plate according to one embodiment of the present invention described above, and includes, for example, a press-formed product obtained by press-forming the steel plate according to one embodiment of the present invention. can be mentioned.
A member according to an embodiment of the present invention is particularly suitable for use in automobile parts and the like.

[Example 1]
A raw steel plate having the structure and tensile properties shown in Table 1 was prepared, and the raw steel plate was sheared using a shear cutter under the conditions shown in Table 2 to obtain a steel plate having a sheared end surface. Note that the shearing process using the shear cutter was performed on the same end face.
Here, the identification of the structure of the raw steel sheet, the measurement of the number density of coarse inclusion particles, and the measurement of the mechanical properties were each performed as described above (note that even in the steel sheet having the sheared end surface described above, The same structure, number density of coarse inclusion particles, and tensile properties as the material steel sheet were obtained.)
Further, for tissue identification (point counting method), a 16×15 grid was placed at equal intervals on the SEM observation area (82 μm×57 μm area). Then, the number of points of each phase in the lattice points was counted, and the ratio of the number of lattice points occupied by each phase to the total number of lattice points was defined as the area ratio of each phase. Moreover, the area ratio of each phase was taken as the average value of the area ratio of each phase obtained from three separate SEM images.
Furthermore, in the measurement of the number density of coarse inclusion particles, the observation area by SEM was set to 25 μm×17 μm.

Using the steel plate having the sheared end surface obtained as described above, determine the machining area and the base material area in a plane perpendicular to the shear end surface and parallel to the plate thickness direction in the manner described above, and S (mm ² ) was determined. Then, the value of S/t ² was calculated from the obtained area S of the processing area. The results are also listed in Table 2.

In addition, using the steel plate having the sheared end surface obtained as described above, the hardness of the processed area and the hardness of the base metal area were measured in the manner described above, and the hardness of the processed area relative to the hardness of the base metal area was measured. The ratio of The results are also listed in Table 2.

Furthermore, delayed fracture resistance was evaluated in the following manner.
That is, a 110 mm x 30 mm test piece was taken from the steel plate having the sheared end surface obtained as described above so that the longitudinal direction was the sheared end surface, and the test piece was subjected to a V-shaped bending process in the longitudinal direction. provided. Then, using bolts, nuts, and taper washers, the V-shaped bent test piece was tightened with bolts from both sides of the plate surface, and various applied stresses at 10 MPa intervals from 700 MPa to 2400 MPa were applied to the V-shaped bent part. Various molded member test pieces were prepared as described above. Here, the load stress was adjusted using the correlation between the load stress and the amount of bolt tightening. Note that the correlation between the applied stress and the amount of bolt tightening was determined by CAE analysis using the YU model. Further, in the CAE analysis, a stress-strain curve obtained by a tensile test was used.
Then, the various molded member test pieces prepared were immersed in a hydrochloric acid aqueous solution of pH=3 (25°C) for 96 hours, and the maximum value of the applied stress of the molded member test pieces that did not show delayed fracture (cracking) after immersion was determined. , was taken as the critical load stress. The determined critical load stress is also listed in Table 2.
Delayed fracture was determined visually and using images magnified to 20x using a stereomicroscope. If no cracks with a length of 200 μm or more were observed, it was determined that there was no delayed fracture (cracking). , Length: If even one crack with a length of 200 μm or more was confirmed, it was determined that there was “delayed fracture (cracking)”.
Then, the delayed fracture resistance was evaluated using the determined critical load stress based on the following criteria.
Pass (excellent): Critical load stress is greater than or equal to yield stress YS Fail: Critical load stress is less than yield stress YS

As shown in Table 2, all of the steel plates having sheared end surfaces of the invention examples had high strength and excellent delayed fracture resistance.
On the other hand, in the comparative example, sufficient delayed fracture resistance was not obtained.

[Example 2]
A slab having the component composition shown in Table 3 (the remainder being Fe and unavoidable impurities) was heated under the conditions shown in Table 4, and then hot rolled to obtain a hot rolled steel plate. The obtained hot-rolled steel sheets were ground and then cold-rolled to the thicknesses shown in Table 5 to obtain cold-rolled steel sheets. Then, the obtained cold-rolled steel sheets were annealed under the conditions shown in Table 4 to prepare raw steel sheets. This raw steel plate was subjected to shearing using a shear cutter under the conditions shown in Table 6 to obtain a steel plate having a sheared end surface. Note that the shearing process using the shear cutter was performed on the same end face.
In addition, a blank column for each element in Table 3 indicates that the element is not intentionally added, and not only when the element is not contained (0% by mass), but also when the element is unavoidably contained. Including cases.
Then, in the same manner as in Example 1, the structure was identified, the number density of coarse inclusion particles was measured, the mechanical properties were measured, the value of S/t ² was calculated, and the hardness of the processed area was determined relative to the hardness of the base material area. The ratio was calculated and the delayed fracture resistance was evaluated. The results are also listed in Tables 5 and 6.

As shown in Table 6, all of the steel plates having sheared end surfaces of the invention examples had high strength and excellent delayed fracture resistance.
On the other hand, in the comparative example, sufficient delayed fracture resistance was not obtained.

Claims (7)

A steel plate having a sheared end surface,
The steel plate has a processing area including the sheared end surface and a base material area,
The following formula (1) is satisfied,
The ratio of the hardness of the processed region to the hardness of the base material region is 120% or less,
The tensile strength is 1180 MPa or more,
In mass%,
C: 0.05% or more and 0.60% or less,
Si: 0.01% or more and 2.00% or less,
Mn: 0.10% or more and 3.20% or less,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.10% or less, and
N: 0.010% or less
, with the remainder being Fe and unavoidable impurities,
Having a structure in which martensite: 40% or more and 100% or less, ferrite: 0% or more and 60% or less, and other metal phases: 5% or less, in terms of area ratio to the entire structure,
A steel plate in which the number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more in a depth region from the surface of the steel plate to a position of 1/4 of the plate thickness is 5 pieces/mm ² or more and 27 pieces/mm ² or less. .
0<S/t ² ≦0.012 (1)
here,
S: Area of processing area in a plane perpendicular to the sheared end face and parallel to the plate thickness direction (mm ² )
t: Thickness of steel plate (mm)
It is. The component composition further includes, in mass%,
Cr: 0.50% or less,
Mo: 0.15% or less,
V: 0.05% or less,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
Ni: 0.10% or less,
B: 0.0020% or less,
The steel plate according to claim 1 , containing at least one selected from Sb: 0.10% or less and Sn: 0.10% or less. A method of manufacturing a steel plate having a sheared end surface , the method comprising:
The steel plate has a processing area including the sheared end surface and a base material area,
The following formula (1) is satisfied,
The ratio of the hardness of the processed region to the hardness of the base material region is 120% or less,
The tensile strength is 1180 MPa or more,
Having a structure in which martensite: 40% or more and 100% or less, ferrite: 0% or more and 60% or less, and other metal phases: 5% or less, in terms of area ratio to the entire structure,
In the depth region from the surface of the steel plate to 1/4 of the plate thickness, the number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more is 5 pieces/mm ² or more and 27 pieces/mm ² or less,
The method includes:
A preparation process of preparing a steel plate as a workpiece,
a shearing process in which at least one end face of the raw steel plate is subjected to shearing n times, where n is an integer of 2 or more,
The cutting allowance T (mm) of the n-th shearing process in the shearing process is expressed by the following formula in relation to the plate thickness t (mm) of the raw steel plate and the clearance a (%) of the n-1th shearing process. Satisfies any of (I) to (III),
The material steel plate is in mass%,
C: 0.05% or more and 0.60% or less,
Si: 0.01% or more and 2.00% or less,
Mn: 0.10% or more and 3.20% or less,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.10% or less, and
N: 0.010% or less
A method for manufacturing a steel sheet having a chemical composition in which the remainder is Fe and unavoidable impurities .
0<S/t ² ≦0.012 (1)
here,
S: Area of processing area in a plane perpendicular to the sheared end face and parallel to the plate thickness direction (mm ² )
t: Thickness of steel plate (mm)
It is.
(I) When a>30 0.002×a×t≦T≦0.07×a×t
(II) When 30≧a>5 0.06×t≦T≦2.1×t
(III) When 5≧a 0.012×a×t≦T≦0.42×a×t The material steel plate has a structure in which martensite: 40% or more and 100% or less, ferrite: 0% or more and 60% or less, and other metal phases: 5% or less, in area ratio to the entire structure,
In the depth region from the surface of the material plate to the position of 1/4 of the plate thickness, the number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more is 5 pieces/mm ² or more and 27 pieces/mm ² or less, The method for manufacturing a steel plate according to claim 3 . The component composition further includes, in mass%,
Cr: 0.50% or less,
Mo: 0.15% or less,
V: 0.05% or less,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
Ni: 0.10% or less,
B: 0.0020% or less,
The method for producing a steel plate according to claim 3 or 4 , containing at least one selected from Sb: 0.10% or less and Sn: 0.10% or less. The preparation step is
a slab heating step of heating and holding the slab;
a hot rolling step of hot rolling the slab into a hot rolled steel plate;
A cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet;
an annealing step of annealing the cold rolled steel sheet;
has
In the slab heating step,
The surface temperature of the slab has an average heating rate of 0.10°C/s or more in the temperature range from 300°C to 1220°C,
In the temperature range, the average temperature ratio Tc/Ts of the center temperature Tc of the slab to the surface temperature Ts of the slab is 0.60 or more and 0.85 or less,
The holding time at the surface temperature of the slab is 1220°C or more for 30 minutes or more,
In the annealing process,
The method for producing a steel plate according to any one of claims 3 to 5 , wherein the annealing temperature is at least A _C1 point and the annealing time is at least 30 seconds. A member made of the steel plate according to claim 1 or 2 .