JP7359331B1 - High strength steel plate and its manufacturing method - Google Patents

Abstract

Provided is a high-strength steel plate having a tensile strength of 1320 MPa or more and excellent ductility, hole expandability, and delayed fracture resistance. The above-mentioned high-strength steel sheet has a specific composition containing Ti, etc., the amount of diffusible hydrogen in the steel is 0.50 mass ppm or less, tempered martensite and bainite are 70.0 to 95.0%, fresh marten Precipitation that is a carbide, nitride, or carbonitride containing 15.0% or less of sites, 5.0 to 15.0% of retained austenite, and at least one element selected from the group consisting of Ti, Nb, and V. Precipitates A with an average particle diameter of 0.001 to 0.050 μm and a major axis of 0.050 μm or less, a number density N _S of 10 pieces/μm ² or more, a precipitate with a number density N _S and a major axis of more than 0.050 μm The ratio of A to the number density _NL is 10.0 or more.

Description

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

In recent years, from the perspective of global environmental conservation, there has been an increasing need to reduce the weight of automobile bodies in order to improve automobile fuel efficiency.
At this time, the weight of the vehicle body is reduced while maintaining its strength. For example, it is desired to use high-strength steel plates with a tensile strength (TS) of 1320 MPa or more for frame parts around the cabin of a vehicle body.

By the way, as the strength of steel plates increases, the ductility of the steel plates tends to decrease. In this case, the formability of the steel plate becomes insufficient, making it difficult to press the steel plate into a complicated shape.
Therefore, for example, Patent Documents 1 and 2 disclose techniques for achieving both strength and ductility of steel plates.

JP 2019-2078 Publication Japanese Patent Application Publication No. 2011-184756

Conventionally, hot pressing has been applied when processing high-strength steel plates into parts, etc., but recently, in consideration of productivity, application of cold pressing is being considered.
However, components obtained by cold pressing high-strength steel plates with a tensile strength of 1320 MPa or more may suffer delayed fracture.
Delayed fracture is when a stressed component is placed in a hydrogen intrusion environment, hydrogen intrudes into the interior of the component, reducing interatomic bonding strength and causing local deformation. This is a phenomenon in which micro-cracks are generated and the components are destroyed as the micro-cracks propagate.
For this reason, high-strength steel sheets are required to have sufficient formability (ductility and hole expandability) as well as good delayed fracture resistance.

The present invention has been made in view of the above points, and aims to provide a high-strength steel plate having a tensile strength of 1320 MPa or more and excellent formability (ductility and hole expandability) and delayed fracture resistance. purpose.

As a result of extensive studies, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

That is, the present invention provides the following [1] to [8].
[1] In mass%, C: 0.130 to 0.350%, Si: 0.50 to 2.50%, Mn: 2.00 to 4.00%, P: 0.100% or less, S: 0.0500% or less, Al: 0.010 to 2.000%, N: 0.0100% or less, and Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100% and V : Contains at least one element selected from the group consisting of 0.001 to 0.500%, with the balance consisting of Fe and unavoidable impurities, and has a microstructure and diffusible hydrogen in steel. amount is 0.50 mass ppm or less, and in the above microstructure, the total area ratio of tempered martensite and bainite is 70.0 to 95.0%, and the area ratio of fresh martensite is 15%. .0% or less, the area ratio of retained austenite is 5.0 to 15.0%, and a carbide, nitride or carbon containing at least one element selected from the group consisting of Ti, Nb and V. The average particle diameter of the precipitate A, which is a nitride, is 0.001 to 0.050 μm, and the number density N _S of the precipitate _A , which is the above precipitate A, whose major axis is 0.050 μm or less is 10 pieces/μm. ² or more, _and the ratio N _S /N _{L of the number density N S of} the precipitates A S to the number density N _L of the precipitates A _L , which is the precipitate A having a major axis of more than 0.050 μm, is 10 .0 or more, a high strength steel plate.
[2] The above component composition further includes, in mass %, W: 0.500% or less, B: 0.0100% or less, Ni: 2.000% or less, Co: 2.000% or less, Cr: 1. 000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.500% or less, Sb: 0.500% or less, Ta: 0.100% or less, Zr: 0.200% Containing at least one element selected from the group consisting of: Hf: 0.020% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less, The high-strength steel plate according to [1] above.
[3] The high-strength steel plate according to [1] or [2] above, which has a plating layer on its surface.
[4] The high-strength steel plate according to [3] above, wherein the plating layer is an alloyed plating layer.
[5] A method for producing a high-strength steel plate according to [1] or [2] above, which comprises heating a steel slab having the composition described in [1] or [2] above to 1100°C or higher. , a hot-rolled steel plate is obtained by hot rolling at a finishing rolling temperature of 850 to 950°C, and the hot-rolled steel plate is coiled and retained at a winding temperature T of 400 to 700°C, and then cooled. A cold-rolled steel plate is obtained by inter-rolling, the cold-rolled steel plate is heat-treated, and during the residence, the temperature of the coiled hot-rolled steel plate is equal to or higher than the coiling temperature T - 50 ° C. When the sum of t is expressed in units of s, the following formula 1 is satisfied, and in the heat treatment, the cold-rolled steel sheet is held in the temperature range T1 of 800 to 950°C for 30 seconds or more, and then cooled to 150 to 250°C. A method for manufacturing a high-strength steel plate, which comprises cooling to a stop temperature T2, and then holding at a temperature range T3 of 250 to 400°C for 30 seconds or more.
Formula 1: 0.001<[1.17×10 ⁻⁶ ×{t/(T+273.15)}] ^1/3 <0.050
[6] Before the hot rolling, the steel slab is cast and cooled, and in the cooling of the steel slab, the average cooling rate v1 at 700 to 600°C is 5.0°C/h or more, The method for producing a high-strength steel plate according to [5] above, wherein the average cooling rate v2 at 600 to 500°C is 2.5°C/h or more.
[7] The method for producing a high-strength steel sheet according to [5] or [6] above, wherein after the heat treatment, the cold-rolled steel sheet is subjected to a plating treatment to form a plating layer.
[8] The method for producing a high-strength steel sheet according to [7] above, wherein the plating treatment includes an alloying plating treatment for alloying the plating layer.

According to the present invention, it is possible to provide a high-strength steel plate having a tensile strength of 1320 MPa or more and excellent formability (ductility and hole expandability) and delayed fracture resistance.

[High strength steel plate]
The high-strength steel sheet of the present invention has the composition and microstructure described below, and satisfies the amount of diffusible hydrogen in steel described below.
Hereinafter, "high-strength steel plate" is also simply referred to as "steel plate."
The thickness of the steel plate is not particularly limited, and is, for example, 0.3 to 3.0 mm, preferably 0.5 to 2.8 mm.
High strength means that the tensile strength (TS) is 1320 MPa or more.

The high-strength steel plate of the present invention has a tensile strength of 1320 MPa or more, and is also excellent in formability (ductility and hole expandability) and delayed fracture resistance. Therefore, the high-strength steel sheet of the present invention has great utility in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of frame parts of automobile bodies.

<Component composition>
The composition of the high-strength steel sheet of the present invention (hereinafter also referred to as "the composition of the present invention" for convenience) will be explained.
"%" in the component composition of the present invention means "% by mass" unless otherwise specified.

《C: 0.130-0.350%》
C increases the strength of tempered martensite, bainite and fresh martensite.
Further, C improves the stability of retained austenite and improves the ductility of the steel sheet.
Furthermore, C precipitates fine precipitates (precipitates A _S to be described later) that serve as hydrogen trap sites inside tempered martensite and bainite, thereby improving delayed fracture resistance.
In order to sufficiently obtain these effects, the amount of C is 0.130% or more, preferably 0.150% or more, more preferably 0.160% or more, and even more preferably 0.170% or more.

On the other hand, if the amount of C is too large, C distribution to austenite will progress during reheating in the heat treatment described below, suppressing martensitic transformation and bainite transformation when cooling after heat treatment, and reducing the area ratio of retained austenite. become excessive. Furthermore, if the amount of C is too large, the strength of the steel sheet becomes excessively high, and the steel becomes susceptible to hydrogen embrittlement, making it impossible to obtain sufficient delayed fracture resistance. Furthermore, weldability, which is important when joining automobile parts, deteriorates.
Therefore, the amount of C is 0.350% or less, preferably 0.330% or less, and more preferably 0.310% or less.

《Si: 0.50-2.50%》
By suppressing the formation of carbides, Si suppresses the decrease in hole expandability caused by the difference in hardness between the carbides and each structure. Furthermore, Si provides stable retained austenite and ensures good ductility.
From the viewpoint of obtaining these effects, the amount of Si is 0.50% or more, preferably 0.55% or more, and more preferably 0.60% or more.

On the other hand, excessive content of Si causes the steel plate to become brittle, resulting in poor hole expandability, making it difficult to obtain desired formability.
Therefore, the amount of Si is 2.50% or less, preferably 2.30% or less, and more preferably 2.00% or less.

《Mn: 2.00-4.00%》
Mn forms a microstructure mainly composed of tempered martensite and bainite, thereby suppressing the difference in hardness between each structure and improving hole expandability.
Furthermore, Mn is an element that contributes to stabilizing retained austenite and is effective in ensuring good ductility.
From the viewpoint of obtaining these effects, the Mn amount is 2.00% or more, preferably 2.20% or more, and more preferably 2.50% or more.

On the other hand, if the amount of Mn is too large, the steel plate becomes brittle, the hole expandability is poor, and it becomes difficult to obtain the desired formability.
Therefore, the Mn amount is 4.00% or less, preferably 3.70% or less, and more preferably 3.50% or less.

《P: 0.100% or less》
P embrittles the steel plate due to grain boundary segregation and has an adverse effect on delayed fracture resistance and weldability. Therefore, the amount of P is 0.100% or less, preferably 0.070% or less, more preferably 0.050% or less, even more preferably 0.030% or less, and particularly preferably 0.010% or less.

《S: 0.0500% or less》
S segregates at grain boundaries and embrittles the steel sheet during hot working. Furthermore, S adversely affects delayed fracture resistance by forming sulfides. Therefore, the amount of S is 0.0500% or less, preferably 0.0100% or less, and more preferably 0.0050% or less.

《Al: 0.010-2.000%》
Al reduces inclusions in the steel sheet by acting as a deoxidizing agent. Therefore, the amount of Al is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more.
On the other hand, if there is too much Al, there is an increased risk that cracks will occur in the steel slab when casting the steel slab, reducing manufacturability. Therefore, the amount of Al is 2.000% or less, preferably 1.500% or less, more preferably 1.000% or less, even more preferably 0.500% or less, and particularly preferably 0.100% or less.

《N: 0.0100% or less》
If coarse nitrides are present in the steel plate, voids are formed when the steel plate is sheared, and delayed fracture starting from these voids is likely to occur, deteriorating the delayed fracture resistance of the steel plate. Therefore, it is preferable that the amount of N be as small as possible. Specifically, the amount of N is 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.

<At least one element selected from the group consisting of Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100% and V: 0.001 to 0.500%>
Ti, Nb, and V contribute to precipitation strengthening, so they are effective in increasing the strength of steel sheets. Furthermore, Ti, Nb, and V refine the grain size of prior austenite, thereby refine tempered martensite and bainite, and create fine precipitates (precipitates A _S described later) that become trap sites for hydrogen. This improves delayed fracture resistance.
From the viewpoint of obtaining these effects, the amount of Ti, the amount of Nb, and the amount of V are each 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.

On the other hand, if Ti, Nb, and V are too large, when the steel slab is heated during hot rolling, Ti, Nb, and V will remain undissolved and coarse precipitates (precipitates A _L described later) will be formed. This may cause the delayed fracture characteristics to deteriorate.
Therefore, the amount of Ti and the amount of Nb are each 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.
The amount of V is 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

《Other elements》
The component composition of the present invention may further contain, in mass %, at least one element selected from the group consisting of the elements described below.

(W: 0.500% or less)
W improves the hardenability of the steel plate. Furthermore, W generates fine carbides containing W, which serve as hydrogen trap sites, and refines tempered martensite and bainite, thereby improving delayed fracture resistance.
However, if there is too much W, coarse precipitates such as WN and WS that remain undissolved increase when the steel slab is heated during hot rolling, and the delayed fracture resistance deteriorates. Therefore, the amount of W is preferably 0.500% or less, more preferably 0.300% or less, and even more preferably 0.150% or less.
The lower limit of the amount of W is not particularly limited, but from the viewpoint of obtaining the effect of adding W, it is, for example, 0.010%, and preferably 0.050%.

(B: 0.0100% or less)
B is effective in improving hardenability. Furthermore, B forms a microstructure mainly composed of tempered martensite and bainite, thereby preventing a decrease in hole expandability.
However, if there is too much B, moldability may deteriorate. Therefore, the amount of B is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less.
The lower limit of the amount of B is not particularly limited, but from the viewpoint of obtaining the effect of adding B, it is, for example, 0.0005%, preferably 0.0010%.

(Ni: 2.000% or less)
Ni is an element that stabilizes retained austenite and is effective in ensuring good ductility. Furthermore, Ni increases the strength of steel through solid solution strengthening.
However, if the amount of Ni is too large, the area ratio of fresh martensite becomes excessive, and the hole expandability decreases. Therefore, the Ni amount is preferably 2.000% or less, more preferably 1.000% or less, and even more preferably 0.500% or less.
The lower limit of the amount of Ni is not particularly limited, but from the viewpoint of obtaining the effect of adding Ni, it is, for example, 0.010%, and preferably 0.050%.

(Co: 2.000% or less)
Co is an element that is effective in improving hardenability and is effective in strengthening steel sheets.
However, too much Co causes deterioration in formability. Therefore, the Co amount is preferably 2.000% or less, more preferably 1.000% or less, and even more preferably 0.500% or less.
The lower limit of the amount of Co is not particularly limited, but from the viewpoint of obtaining the effect of adding Co, it is, for example, 0.010%, and preferably 0.050%.

(Cr: 1.000% or less)
Cr improves the balance between strength and ductility.
However, if there is too much Cr, the cementite solid solution rate will be delayed during reheating in the heat treatment described below, and relatively coarse carbides containing Fe as a main component such as cementite will remain undissolved, resulting in Delayed fracture characteristics deteriorate. Therefore, the Cr content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less.
The lower limit of the amount of Cr is not particularly limited, but from the viewpoint of obtaining the effect of adding Cr, it is, for example, 0.030%, and preferably 0.050%.

(Mo: 1.000% or less)
Mo improves the balance between strength and ductility. Furthermore, Mo generates fine carbides containing Mo, which serve as hydrogen trap sites, and refines tempered martensite and bainite, thereby improving delayed fracture resistance.
However, if there is too much Mo, chemical conversion treatment properties will be significantly deteriorated. Therefore, the amount of Mo is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less.
The lower limit of the amount of Mo is not particularly limited, but from the viewpoint of obtaining the effect of adding Mo, it is, for example, 0.010%, and preferably 0.050%.

(Cu: 1.000% or less)
Cu is an element effective in strengthening steel. Furthermore, since Cu suppresses hydrogen from entering the steel sheet, it has better delayed fracture resistance.
However, if there is too much Cu, the area ratio of fresh martensite becomes excessive and the hole expandability deteriorates. Therefore, the amount of Cu is preferably 1.000% or less, more preferably 0.500% or less, and even more preferably 0.200% or less.
The lower limit of the amount of Cu is not particularly limited, but from the viewpoint of obtaining the effect of adding Cu, it is, for example, 0.010%, and preferably 0.050%.

(Sn: 0.500% or less, Sb: 0.500% or less)
Sn and Sb suppress decarburization of the surface layer region of the steel sheet (an area several tens of μm deep from the surface of the steel sheet) caused by nitriding or oxidation of the surface of the steel sheet, and increase the area ratio of tempered martensite on the surface of the steel sheet. prevent a decrease in
However, too much of these elements leads to a decrease in toughness. Therefore, the amount of Sn and the amount of Sb are each preferably at most 0.500%, more preferably at most 0.100%, and even more preferably at most 0.050%.
The lower limits of the amount of Sn and the amount of Sb are not particularly limited, but from the viewpoint of obtaining the effect of adding Sn and Sb, they are each, for example, 0.001%, preferably 0.003%.

(Ta: 0.100% or less)
Ta generates alloy carbide or alloy carbonitride and contributes to high strength. Furthermore, Ta is partially dissolved in Nb carbide or Nb carbonitride and forms composite precipitates such as (Nb, Ta) (C, N), thereby significantly suppressing coarsening of the precipitates. Stabilizes the contribution to strength from precipitation strengthening.
However, even if Ta is added in excess, these effects will be saturated and the cost will also increase. Therefore, the Ta amount is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.070% or less.
The lower limit of the amount of Ta is not particularly limited, but from the viewpoint of obtaining the effect of adding Ta, it is, for example, 0.005%, preferably 0.010%.

(Zr: 0.200% or less)
Zr improves the hardenability of the steel plate. Furthermore, Zr generates fine carbides containing Zr that serve as hydrogen trap sites, and refines tempered martensite and bainite, thereby improving delayed fracture resistance.
However, too much Zr causes an increase in inclusions, etc., which causes defects on the surface and inside of the steel sheet, and deteriorates delayed fracture resistance. Therefore, the Zr amount is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.
The lower limit of the amount of Zr is not particularly limited, but from the viewpoint of obtaining the effect of adding Zr, it is, for example, 0.005%, preferably 0.010%.

(Hf: 0.020% or less)
Hf affects the distribution state of oxides and improves delayed fracture resistance.
However, too much Hf deteriorates the formability of the steel sheet. Therefore, the Hf amount is preferably 0.020% or less, more preferably 0.015% or less, and even more preferably 0.010% or less.
The lower limit of the amount of Hf is not particularly limited, but from the viewpoint of obtaining the effect of adding Hf, it is, for example, 0.001%, preferably 0.003%.

(Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less)
Ca, Mg and REM (rare earth metals) spheroidize the shape of sulfide and improve the negative effect of sulfide on hole expandability.
However, if there are too many of these elements, inclusions and the like will increase, causing defects on the surface and inside of the steel sheet, and deteriorating the delayed fracture resistance. Therefore, the amount of Ca, the amount of Mg, and the amount of REM are each preferably at most 0.0090%, more preferably at most 0.0080%, and even more preferably at most 0.0070%.
The lower limits of the amount of Ca, the amount of Mg, and the amount of REM are not particularly limited, but from the viewpoint of obtaining the effect of adding Ca, Mg, and REM, they are each, for example, 0.0005%, and preferably 0.0010%.

《Remainder: Fe and inevitable impurities》
The remainder in the component composition of the present invention consists of Fe and unavoidable impurities.

<Microstructure>
Next, the microstructure of the high-strength steel plate of the present invention (hereinafter also referred to as "microstructure of the present invention" for convenience) will be explained.
In order to obtain the effects of the present invention, it is not sufficient to satisfy the component composition of the present invention described above, but it is necessary to satisfy the microstructure of the present invention described below.
Hereinafter, the area ratio is the area ratio with respect to the entire microstructure. The area ratio of each tissue is determined by the method described in [Examples] below.

《Total area ratio of tempered martensite and bainite: 70.0 to 95.0%》
Tempered martensite and bainite contribute to tensile strength.
Moreover, using tempered martensite and bainite as main components is effective in increasing hole expandability while maintaining high strength.
In order to sufficiently obtain these effects, the total area ratio of bainite and tempered martensite is 70.0% or more, preferably 72.0% or more, and more preferably 74.0% or more.

On the other hand, too much tempered martensite and bainite results in too little retained austenite.
Therefore, the total area ratio of bainite and tempered martensite is 95.0% or less, preferably 93.0% or less, and more preferably 90.0% or less.

《Fresh martensite area ratio: 15.0% or less》
Fresh martensite produces a large hardness difference between tempered martensite and bainite, and therefore, during punching, hole expandability is reduced due to the hardness difference. For this reason, it is necessary to avoid excessive presence of fresh martensite in the steel sheet.
Specifically, from the viewpoint of obtaining good hole expandability, the area ratio of fresh martensite is 15.0% or less, preferably 14.0% or less, and more preferably 13.0% or less.
On the other hand, the lower limit is not particularly limited, but from the viewpoint of tensile strength, the area ratio of fresh martensite is preferably 1.0% or more, more preferably 3.0% or more, and even more preferably 5.0% or more.

《Area ratio of retained austenite: 5.0 to 15.0%》
During processing, retained austenite undergoes martensitic transformation due to the TRIP (Transformation Induced Plasticity) effect, increasing strength and at the same time improving ductility by increasing strain dispersion ability.
Therefore, the area ratio of retained austenite is 5.0% or more, preferably 6.0% or more, and more preferably 7.0% or more.

On the other hand, if there is too much retained austenite, voids are likely to occur at the interface between the retained austenite and tempered martensite during processing, and hydrogen embrittlement occurs starting from the voids, which deteriorates the delayed fracture resistance of the steel sheet. .
Furthermore, since residual austenite undergoes stress-induced martensitic transformation during press molding, hole expandability is reduced.
Therefore, the area ratio of retained austenite is 15.0% or less, preferably 14.0% or less, more preferably 13.0% or less, and even more preferably 12.0% or less.

《Remnant organization》
Examples of the structure excluding tempered martensite, bainite, fresh martensite, and retained austenite (residual structure) include ferrite, pearlite, and the like. The area ratio of the remaining structure in the microstructure of the present invention is preferably 5.0% or less because it does not impede the effects of the present invention.

《Precipitate A》
Next, the precipitate A will be explained.
Precipitate A is a carbide, nitride, or carbonitride containing at least one element selected from the group consisting of Ti, Nb, and V.

(Average particle size of precipitate A: 0.001 to 0.050 μm)
If the precipitate A is too small, the effect of precipitation strengthening will not be obtained and the strength will be insufficient.
Therefore, the average particle size of the precipitate A is 0.001 μm or more, preferably 0.005 μm or more, and more preferably 0.010 μm or more.

On the other hand, if the precipitate A is too large, it will adversely affect the delayed fracture resistance of the sheared end face.
Therefore, the average particle size of the precipitate A is 0.050 μm or less, preferably 0.040 μm or less, more preferably less than 0.040 μm, even more preferably 0.035 μm or less, particularly preferably 0.030 μm or less, Most preferably, it is 0.020 μm or less.
The average particle size of the precipitate A is determined by the method described in [Examples] below.

( _NS : 10 pieces/μm ² or more)
Precipitate A _S is precipitate A with a major axis of 0.050 μm or less.
The number density (number per unit area) of the precipitates A _S is 10 pieces/μm ² _or more.
This increases the strength of the steel plate due to precipitation strengthening. Furthermore, the delayed fracture resistance is improved by the fine _precipitates acting as hydrogen trap sites.
For the reason that delayed fracture resistance is more excellent, N _S is preferably more than 125 pieces/μm ² , more preferably 200 pieces/μm ² or more, and even more preferably 310 pieces/μm ² or more.

The upper limit of N _S is not particularly limited. However, if the absolute amount of fine precipitates _AS increases, the rolling load will increase, which may make it difficult to manufacture the steel sheet. Therefore, N _S is preferably 1,000 pieces/μm ² or less, more preferably 800 pieces/μm ² or less.

( _NS / _NL : 10.0 or more)
Precipitate A _L is precipitate A having a major axis of more than 0.050 μm.
The ratio (N _S /N _L ) between the number density N _S (unit: pieces/μm ² ) of precipitates A _S and the number density N _L (unit: pieces/μm ² ) of precipitates A _L is 10. It is 0 or more. This provides good delayed fracture resistance. The reason is assumed to be as follows.

Since the fine precipitates A _S have a small particle size, it is thought that it is difficult to accumulate strain and stress. Furthermore, since the fine precipitates A _S have a circular shape, their surfaces are considered to be curved surfaces, and it is thought that strain and stress tend to escape along the curved surfaces.
On the other hand, it is thought that strain and stress tend to accumulate in the coarse precipitates A _L because the strain and stress travel distance is larger than that in the fine precipitates A _S.
In particular, the coarse precipitates _AL include the rectangular precipitates A, whose surfaces are considered to be flat, and are thought to more easily accumulate strain and stress. In this case, it is presumed that the local strain and residual stress inside the sheared end face become high. When the local strain and residual stress inside the sheared end face increases, initial cracks are more likely to occur on the sheared end face, and the delayed fracture resistance of the sheared end face deteriorates.
Therefore, by reducing the abundance ratio of coarse precipitates _AL , the delayed fracture resistance of the steel sheet can be improved.

For the reason that delayed fracture resistance is more excellent, N _S /N _L is preferably 11.0 or more, more preferably 12.0 or more, even more preferably more than 12.1, particularly preferably 12.2 or more, and 13 .0 or more is most preferable.
The upper limit of N _S /N _L is not particularly limited, but is preferably 100.0 or less, more preferably 80.0 or less, even more preferably 50.0 or less, and particularly preferably 30.0 or less.

The upper limit of N _L is not particularly limited. However, it is considered that as the absolute amount of coarse precipitates _A increases, local strain and residual stress within the sheared end face become high, making it easier for initial cracks to occur on the sheared end face. Therefore, N _L is preferably 50 pieces/μm ² or less, more preferably 35 pieces/μm ² or less.

N _S and N _L are determined by the method described in [Examples] below.

<Amount of diffusible hydrogen in steel: 0.50 mass ppm or less>
From the viewpoint of ensuring good delayed fracture resistance, the amount of diffusible hydrogen in steel is 0.50 mass ppm or less, preferably 0.40 mass ppm or less, more preferably 0.30 mass ppm or less, and 0.50 mass ppm or less. More preferably, it is 25 mass ppm or less.
Although the lower limit of the amount of diffusible hydrogen in steel is not particularly limited, it is, for example, 0.01 mass ppm due to constraints on production technology.
The amount of diffusible hydrogen in steel is determined by the method described in Examples below.

<Plating layer>
The high-strength steel plate of the present invention may have a plating layer on its surface. The plating layer is formed by a plating process described below.
Examples of the plating layer include a zinc plating layer (Zn plating layer), an Al plating layer, and the like, and among them, a zinc plating layer is preferable. The galvanized layer may contain elements such as Al and Mg. The plating layer may be an alloyed plating layer (alloyed plating layer).

The amount of adhesion of the plating layer (amount of adhesion per one side) is preferably 20 g/m2 or ^more , more preferably 25 g/m2 or ^more , and 30 g/m2 from the viewpoint of controlling the amount of plating layer adhesion and from the viewpoint of corrosion resistance. More preferably ² or more.
On the other hand, from the viewpoint of adhesion, the amount of the plating layer deposited is preferably 120 g/m ² or less, more preferably 100 g/m ² or less, and even more preferably 70 g/m ² or less.

[Manufacturing method of high-strength steel plate]
Next, the method for manufacturing a high-strength steel plate of the present invention (hereinafter also referred to as "the manufacturing method of the present invention" for convenience) will be explained.

<Steel slab>
In the manufacturing method of the present invention, first, a steel slab (steel material) having the above-mentioned composition of the present invention is prepared.
Steel slabs are cast from molten steel by known methods, such as continuous casting.
The method for producing molten steel is not particularly limited, and any known method using a converter, electric furnace, etc. can be employed.

《Average cooling rate v1: 5.0°C/h or more and average cooling rate v2: 2.5°C/h or more》
After the steel slab is cast, it may be cooled, for example, by being allowed to rest, before being subjected to hot rolling, which will be described later.
In this cooling, the average cooling rate v1 at 700 to 600°C is preferably 5.0°C/h or more, more preferably 10.0°C/h or more, and even more preferably 15.0°C/h or more.
The average cooling rate v2 at 600 to 500°C is preferably 2.5°C/h or more, more preferably 5.0°C/h or more, and even more preferably 10.0°C/h or more.
Coarse precipitates _AL may be deposited on the steel slab during casting. When the average cooling rate v1 and the average cooling rate v2 satisfy the above range, the distribution state of precipitates in the steel slab becomes uniform, and the coarse precipitates A _L precipitated during casting are removed from the steel during hot rolling, which will be described later. It is easier to remelt when heating the slab. That is, the value of N _S /N _L tends to become large.

The upper limit of the average cooling rate v1 is not particularly limited, and is, for example, 150.0°C/h, preferably 100.0°C/h.
The upper limit of the average cooling rate v2 is not particularly limited, and is, for example, 200.0°C/h or less, preferably 150.0°C/h.

<Hot rolling>
In the manufacturing method of the present invention, a hot rolled steel plate is obtained by hot rolling a prepared steel slab under the conditions described below (heating temperature and finish rolling end temperature).

《Heating temperature: 1100℃ or more》
During hot rolling, the steel slab is heated.
If the heating temperature of the steel slab is too low, it will be difficult to sufficiently dissolve at least one element selected from the group consisting of Ti, Nb, and V. Moreover, excessive growth of the precipitates A occurs, so that the number density N _S of the fine precipitates A _S becomes too small, and the number density N _L of the coarse precipitates A _L becomes too large. That is, the value of N _S /N _L tends to be too small. For this reason, the heating temperature of the steel slab is 1100°C or higher, preferably 1150°C or higher.

The heating temperature of the steel slab should be within the above range from the viewpoint of reducing the rolling load and scaling off the surface defects (bubbles, segregation, etc.) of the steel slab and smoothing the surface of the obtained steel plate. It is preferable that there be.

The upper limit of the heating temperature of the steel slab is not particularly limited, but if it is too high, scale loss will increase as the amount of oxidation increases. For this reason, the heating temperature of the steel slab is preferably 1400°C or lower, more preferably 1350°C or lower.

《Final rolling temperature: 850-950℃》
The steel slab heated to the above heating temperature is subjected to hot rolling including finish rolling to become a hot rolled steel plate.

If the finish rolling end temperature is too low, the rolling load will increase and the rolling load will increase. Furthermore, the average particle size of the precipitates A becomes too large, or the value of N _S /N _L becomes too small. Moreover, each structure of the obtained hot rolled steel sheet becomes coarse, and each structure during the subsequent heat treatment may also become coarse. In this case, for example, when cooling is stopped, it becomes difficult to stably obtain fine retained austenite, which is mechanically stable and is difficult to transform into martensitic material. do.
Therefore, the finish rolling end temperature is 850°C or higher, preferably 855°C or higher, and more preferably 860°C or higher.

On the other hand, if the finish rolling end temperature is too high, the austenite in the hot-rolled steel sheet becomes coarse, and as a result, the austenite grains in the cold-rolled steel sheet become coarse. In that case, the diffusion distance of C increases, so that sufficient C concentration to obtain stable austenite is not caused in the heat treatment described below. As a result, sufficient retained austenite is not obtained in the final microstructure, resulting in reduced ductility.
Moreover, the amount of oxide (scale) produced increases rapidly, and the surface quality tends to deteriorate after pickling and cold rolling, which will be described later.
Moreover, if scale cannot be sufficiently removed by pickling, ductility and hole expandability will be adversely affected.
Furthermore, the crystal grain size becomes excessively coarse, which may cause surface roughness during press working.
Therefore, the finish rolling end temperature is 950°C or lower, preferably 940°C or lower, and more preferably 930°C or lower.

<Take-up>
The hot rolled steel sheet obtained by hot rolling is wound up under the conditions (winding temperature T) described below.

《Winding temperature T: 400-700℃》
If the winding temperature T is too low, the precipitates A will not be sufficiently formed, and the number density N _S of fine precipitates A _S will become too small, or the value of N _S /N _L will become too small.
Moreover, the strength of the hot-rolled steel sheet increases, and the rolling load during cold rolling increases, and the cold-rolled steel sheet obtained by cold rolling becomes defective in shape, resulting in a decrease in productivity.
Therefore, the winding temperature T is 400°C or higher, preferably 420°C or higher, and more preferably 430°C or higher.

On the other hand, if the winding temperature T is too high, the growth of the precipitates A progresses, and the average particle size of the precipitates A becomes too large or the value of N _S /N _L becomes too small.
Therefore, the winding temperature T is 700°C or lower, preferably 680°C or lower, and more preferably 670°C or lower.

The winding temperature T is the end surface temperature of the hot rolled steel sheet (that is, the coil) that has been wound up.

<Retention>
The hot-rolled steel plate (coil) that has been wound up is retained until cold rolling, which will be described later, is carried out.
In this residence, when the total time (unit: s) during which the temperature of the hot-rolled steel sheet is equal to or higher than the coiling temperature T - 50°C is t (also referred to as "residence time t"), the following formula 1 satisfy.
Formula 1: 0.001<[1.17×10 ⁻⁶ ×{t/(T+273.15)}] ^1/3 <0.050

Hereinafter, for convenience, "[1.17×10 ⁻⁶ ×{t/(T+273.15)}] ^1/3 " in the above formula 1 will be expressed as "X".

If X in the above formula 1 is below the lower limit, the coil will stop staying in a state where sufficient nucleation has not occurred or nucleation is insufficient, and the average particle size of precipitates A will be too small. or the value of N _S /N _L becomes too small.

On the other hand, if X in formula 1 above exceeds the upper limit, the precipitates A will undergo excessive Ostwald growth due to the diffusion of Ti, Nb or V, and C or N, and the average grain size of the precipitates A will become excessively large. or the value of N _S /N _L becomes too small.

The method of controlling the thermal history of the hot-rolled steel sheet (coil) that has been wound up is not particularly limited, and examples thereof include a method of covering the coil, a method of applying hot air and/or cold air to the coil, and the like.
The temperature of the coiled hot rolled steel sheet (coil) is the temperature of the coil surface measured using a radiation thermometer if there is no cover, and the temperature of the coil surface measured using a thermocouple if there is a cover. Let it be the internal temperature.

The coil retained under the conditions satisfying the above formula 1 may be pickled, if necessary, before cold rolling, which will be described later. A conventional pickling method may be used. Skin pass rolling may be performed to correct the shape and improve pickling properties.

<Cold rolling>
The hot-rolled steel sheet that has been wound up is retained under conditions that satisfy the above formula 1, pickled if necessary, and then cold-rolled to become a cold-rolled steel sheet.
The reduction ratio in cold rolling is preferably 25% or more, more preferably 30% or more.
On the other hand, excessive rolling causes an excessive rolling load, leading to an increase in the load on the mill used for cold rolling. Therefore, the rolling reduction ratio is preferably 75% or less, more preferably 70% or less.

<Heat treatment>
The cold rolled steel sheet obtained by cold rolling is heat treated under the conditions described below.
Roughly speaking, a cold rolled steel plate is held (heated) in a temperature range T1, then cooled to a cooling stop temperature T2, and then held (reheated) in a temperature range T3.

《Temperature range T1: 800-950℃》
If the temperature in the temperature range T1 is too low, it will be maintained in a two-phase range, and the total area ratio of tempered martensite and bainite will be too small in the finally obtained microstructure.
Therefore, the temperature in the temperature range T1 is 800°C or higher, preferably 830°C or higher, and more preferably 850°C or higher.

On the other hand, if the temperature in the temperature range T1 is too high, the precipitates A formed during hot rolling become coarse, the average grain size of the precipitates A becomes too large, or the value of N _S /N _L becomes too small. It becomes.
Therefore, the temperature in the temperature range T1 is 950°C or lower, preferably 940°C or lower, and more preferably 930°C or lower.

《Holding time in temperature range T1: 30 seconds or more》
If the holding time in the temperature range T1 is too short, sufficient recrystallization will not be performed. In addition, the formation of austenite becomes insufficient, and the area ratio of retained austenite becomes too small.
Therefore, the holding time in the temperature range T1 is 30 seconds or more, preferably 65 seconds or more, and more preferably 100 seconds or more.
The upper limit of the holding time in the temperature range T1 is not particularly limited, and is, for example, 800 seconds, preferably 500 seconds, and more preferably 200 seconds.

《Cooling stop temperature T2: 150-250℃》
If the cooling stop temperature T2 is too low, only a small amount of untransformed austenite remains when cooling is stopped, and ultimately the area ratio of retained austenite becomes too small.
Therefore, the cooling stop temperature T2 is 150°C or higher, preferably 160°C or higher, and more preferably 170°C or higher.

On the other hand, if the cooling stop temperature T2 is too high, a large amount of austenite remains when cooling is stopped, and eventually the area ratio of the retained austenite becomes excessive.
Therefore, the cooling stop temperature T2 is 250°C or lower, preferably 240°C or lower, and more preferably 230°C or lower.

《Temperature range T3: 250-400℃》
If the temperature in the temperature range T3 is too low, C will not be sufficiently concentrated in untransformed austenite, and the area ratio of retained austenite will become too small.
Therefore, the temperature in the temperature range T3 is 250°C or higher, preferably 260°C or higher, and more preferably 270°C or higher.

On the other hand, if the temperature in the temperature range T3 is too high, the decomposition of untransformed austenite will proceed excessively, and the area ratio of retained austenite will become too small, resulting in poor ductility.
Therefore, the temperature in the temperature range T3 is 400°C or lower, preferably 380°C or lower, and more preferably 360°C or lower.

《Holding time in temperature range T3: 30 seconds or more》
If the holding time in the temperature range T3 is too short, the area ratio of fresh martensite may become excessive in the final microstructure, or the area ratio of retained austenite may become insufficient due to insufficient C concentration in retained austenite. It may become too small.
Therefore, the holding time in the temperature range T3 is 30 seconds or more, preferably 100 seconds or more, and more preferably 180 seconds or more.
The upper limit of the holding time in the temperature range T3 is not particularly limited, and is, for example, 800 seconds, preferably 500 seconds, and more preferably 300 seconds.

<Plating treatment>
The cold-rolled steel sheet that has been subjected to the heat treatment described above may be subjected to a plating treatment to form a plating layer. Examples of the plating treatment include hot-dip galvanizing treatment. In this case, a galvanized layer is formed as the plating layer.
When hot-dip galvanizing is carried out, for example, a cold-rolled steel sheet that has been subjected to the heat treatment described above is immersed in a hot-dip galvanizing bath at 440 to 500°C. After dipping, adjust the amount of plating layer adhered by gas wiping or the like.
The hot-dip galvanizing bath may contain elements such as Al, Mg, and Si, and may further contain elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn. Elements may be mixed. The amount of Al in the hot dip galvanizing bath is preferably 0.08 to 0.30%.

The plating process may include an alloying process to alloy the formed plating layer.
When alloying is performed after hot-dip galvanizing, the galvanized layer is alloyed at a temperature of 450 to 600° C. (alloying temperature). If the alloying temperature is too high, untransformed austenite transforms into pearlite, and the area ratio of retained austenite becomes too small.
The Fe concentration of the alloyed galvanized layer is preferably 8 to 17% by mass.

When plating is applied, a cold-rolled steel sheet that has been subjected to heat treatment and plating corresponds to the high-strength steel sheet of the present invention.
On the other hand, when no plating treatment is applied, a heat-treated cold-rolled steel sheet corresponds to the high-strength steel sheet of the present invention.

The present invention will be specifically described below with reference to Examples. However, the present invention is not limited to the embodiments described below.

<Manufacture of steel plates>
Molten steel having the composition shown in Table 1 below, with the remainder consisting of Fe and unavoidable impurities, was produced in a converter, and a steel slab was obtained by a continuous casting method.
The obtained steel slab was cooled under the conditions shown in Table 2 below.
Next, the cooled steel slab was subjected to hot rolling, coiling, retention, cold rolling, and heat treatment under the conditions shown in Table 2 below to obtain a cold rolled steel plate (CR) having a thickness of 1.4 mm. I got it. The reduction ratio of cold rolling was 50%.

Some cold-rolled steel sheets were subjected to hot-dip galvanizing treatment to form galvanized layers on both sides to obtain hot-dip galvanized steel sheets (GI). The amount of adhesion of the galvanized layer (the amount of adhesion per one side) was 45 g/m ² .
Further, some hot-dip galvanized steel sheets (GI) were subjected to alloying treatment to alloy the formed galvanized layers to obtain alloyed hot-dip galvanized steel sheets (GA). In the alloying treatment, the Fe concentration of the alloyed galvanized layer was adjusted to be within the range of 9 to 12% by mass.
For the hot-dip galvanized steel sheet (GI), a hot-dip galvanizing bath containing 0.19% by mass of Al was used. For the alloyed hot-dip galvanized steel sheet (GA), a hot-dip galvanizing bath containing 0.14% by mass of Al was used. The bath temperature was 465°C in all cases.

Hereinafter, for convenience, cold-rolled steel sheets (CR), hot-dip galvanized steel sheets (GI), and alloyed hot-dip galvanized steel sheets (GA) are all simply referred to as "steel sheets."
In the "Type" column of Table 2 below, one of "CR", "GI", and "GA" was written depending on the obtained steel plate.

<Observation of microstructure>
The obtained steel plate was placed so that the cross section (L cross section) at the 1/4 plate thickness position parallel to the rolling direction (the position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) was the observation surface. , and polished to prepare an observation sample.
Using the prepared observation sample, the microstructure was observed in the following manner, and the area ratio of each structure was determined. The results are shown in Table 3 below.
In Table 3 below, tempered martensite is expressed as "TM", bainite as "B", fresh martensite as "FM", and retained austenite as "γR".

《Area ratio of tempered martensite, bainite and fresh martensite》
The observation surface of the observation sample was corroded using nital, and then 10 fields of view were observed using a scanning electron microscope (SEM) at a magnification of 2000 times to obtain a SEM image.
Regarding the obtained SEM images, the area ratio (unit: %) of each tissue was determined. The average area ratio of 10 visual fields was taken as the area ratio of each tissue.
In the SEM image, light gray areas were determined to be fresh martensite, and dark gray areas where carbides were precipitated were determined to be tempered martensite and bainite.
Fresh martensite and retained austenite cannot be clearly distinguished in the SEM image, so the area rate of fresh martensite is determined by calculating the area rate of retained austenite from the area rate of the light gray area using the method described below. The value was calculated by subtracting the value.

《Area ratio of retained austenite》
The area ratio (unit: %) of retained austenite was determined by X-ray diffraction method.
Specifically, first, the observation surface of the observation sample was polished by 0.1 mm in the plate thickness direction, and further polished by 0.1 mm by chemical polishing to obtain a polished surface.
Regarding this polished surface, diffraction of the {200}, {220} and {311} planes of FCC iron and the {200}, {211} and {220} planes of BCC iron was performed using CoKα radiation. The integrated intensity of the peak was measured.
Then, the ratio of the integrated intensity of each plane of {200}, {220}, and {311} of FCC iron to the integrated intensity of each plane of {200}, {211}, and {220} of BCC iron (integrated intensity) I asked for
The average value of the nine integrated intensity ratios obtained was taken as the volume fraction of retained austenite, and this volume fraction was regarded as the area fraction (unit: %) of retained austenite.

<<Average particle size of precipitate A, N _S and N _L >>
A replica sample was collected from the observation surface of the observation sample by the replica method.
The collected replica sample was observed using a transmission electron microscope (TEM) at an accelerating voltage of 200 kV and a magnification of 20,000 times over 10 fields of view to obtain a TEM image. The size of one field of view was 0.5 μm×0.5 μm.
The presence of precipitates was confirmed by viewing the obtained TEM image.
Furthermore, energy dispersive X-ray spectroscopy (EDS) was performed in the same field of view as the TEM image to confirm the elements contained in the precipitates.

Among the precipitates confirmed in the TEM image, a precipitate containing at least one selected from the group consisting of Ti, Nb, and V was identified as precipitate A.
The equivalent circle diameter of each precipitate identified as Precipitate A was determined, and the average value for 10 fields of view was taken as the average particle diameter (unit: μm) of Precipitate A.

Furthermore, the major axis of Precipitate A was measured.
Specifically, for each precipitate particle identified as precipitate A, the longest length passing through the particle was measured, and this was defined as the major axis of precipitate A.
Then, by measuring the number of precipitates A with a major axis of 0.050 μm or less (that is, precipitates A _S ) and dividing the measured number by the area of 10 fields, the number density N of precipitates A _S is determined. _S (unit: pieces/μm ² ) was determined.
Similarly, the number density of precipitates A L can be determined by measuring the number of precipitates A ₍ that is, precipitates A _L ) with a major axis of more than 0.050 μm and dividing the measured number by the area of 10 fields of view. N _L (unit: pieces/μm ² ) was determined.
Furthermore, the ratio between N _S and N _L (N _S /N _L ) was determined.

<Measurement of the amount of diffusible hydrogen in steel>
A test piece with a size of 5 mm x 30 mm was cut out from the obtained steel plate. If a plating layer (a galvanized layer or an alloyed galvanized layer) was formed, the plating layer was removed using a router (precision grinder).
The test piece was placed in a quartz tube, and the inside of the quartz tube was replaced with argon gas (Ar). Thereafter, the temperature inside the quartz tube was raised to 400° C. at a rate of 200° C./hr, and the amount of hydrogen generated from inside the quartz tube during the temperature rise was measured by a heating analysis method using a gas chromatograph.
The cumulative value of the amount of hydrogen detected in the temperature range from room temperature (25° C.) to less than 250° C. was determined as the amount of diffusible hydrogen in steel (unit: mass %). The results are shown in Table 3 below.

<evaluation>
The obtained steel plate was evaluated by the following tests. The results are shown in Table 3 below.

《Tensile test》
A JIS No. 5 test piece whose tensile direction was perpendicular to the rolling direction was taken from the obtained steel plate. Using the sampled test pieces, a tensile test was conducted in accordance with JIS Z 2241 (2011) to measure tensile strength (TS) and total elongation (EL).
If the TS was 1320 MPa or more, it was evaluated as having high strength.
If EL was 10.0% or more, it was evaluated as having excellent ductility.

《Hole Expansion Test》
A hole expansion test was conducted on the obtained steel plate in accordance with JIS Z 2256 (2010).
Specifically, the obtained steel plate was cut to take a test piece with a size of 100 mm x 100 mm. A hole with a diameter of 10 mm was punched into the sample specimen with a clearance of 12±1%. Then, using a die with an inner diameter of 75 mm, a conical punch with an apex angle of 60° was pushed into the hole with a wrinkle pressing force of 9 tons, and the hole diameter D _f (unit: mm) at the crack generation limit was measured. Setting the initial hole diameter as D ₀ (unit: mm), the hole expansion rate λ (unit: %) was determined from the following formula. If λ was 25% or more, it was evaluated that the hole expandability was excellent.
λ={(D _f −D ₀ )/D ₀ }×100

《Delayed fracture resistance evaluation test》
A test piece was taken from the obtained steel plate. If a plating layer was formed, it was dissolved and removed using diluted hydrochloric acid, stored at room temperature for one day (dehydrogenation treatment), and then a test piece was collected.
The size of the test piece was such that the length of the long side (length in the direction perpendicular to rolling) was 100 mm, and the length of the short side (length in the rolling direction) was 30 mm.
In the test piece, the end face on the long side was used as the evaluation end face, and the end face on the short side was used as the non-evaluation end face.
The end face for evaluation was cut out by shearing. The clearance for shearing was 10%, and the rake angle was 0.5 degrees. The end face for evaluation was left as sheared. In other words, no machining was performed to remove burrs. On the other hand, the non-evaluation end face was machined to remove burrs.

Bending processing was performed on such a test piece. The bending process was carried out under the conditions that the ratio (R/t) between the bending radius R and the plate thickness t of the test piece was 4.0, and the bending angle was 90 degrees (V-shaped bending).
For example, when the plate thickness t was 2.0 mm, a punch with a tip radius of 8.0 mm was used. More specifically, a punch having the above-mentioned tip radius and having a U-shape (the tip portion is semicircular and the body thickness is 2R) was used.
Furthermore, a die with a corner bending radius of 30 mm was used for the bending process.

In the bending process, a bent part having a bending angle of 90 degrees was formed on the test piece by adjusting the depth into which the punch pushed the test piece.
The test piece with the bent portion formed thereon was clamped and tightened using a hydraulic jack, and the bolts were tightened with the following residual stresses S1, S2, or S3 being applied to the outermost layer of the bent portion.
・Residual stress S1: Residual stress of 1300 MPa or more and 1500 MPa or less ・Residual stress S2: Residual stress of more than 1500 MPa and 1700 MPa or less ・Residual stress S3: Residual stress of more than 1700 MPa and 1900 MPa or less The number was set to two.

The required tightening amount was calculated by CAE (Computer Aided Engineering) analysis.
Bolt tightening was performed by passing a bolt through an elliptical hole (short axis: 10 mm, long axis: 15 mm) that was previously provided 10 mm inside from the non-evaluation end face of the test piece.
The test piece after bolting was immersed in hydrochloric acid (hydrogen chloride aqueous solution) having a pH of 4, and the pH was controlled to be constant at 25°C. The amount of hydrochloric acid was 1 L or more per test piece.
After 48 hours had passed after immersion, the presence or absence of visible microcracks (having a length of about 1 mm) was confirmed for the test pieces in hydrochloric acid. These microcracks represent the initial state of delayed fracture.
The results according to the presence or absence of microcracks ("×", "△", "○", or "◎" shown below) are shown in Table 3 below.

×: One or more microcracks were observed in the test piece to which residual stress S1 was applied.
Δ: No microcracks were observed in the test piece to which the residual stress S1 was applied, but one or more microcracks were observed in the test piece to which the residual stress S2 was applied.
○: No microcracks were observed in the test piece to which residual stress S1 and S2 were applied, but one or more microcracks were observed in the test piece to which residual stress S3 was applied.
◎: No microcracks were observed in any of the test pieces.

If it was "△", "○" or "◎", it was evaluated that the delayed fracture resistance was excellent.
"○" or "◎" is preferable because the delayed fracture resistance is more excellent, and "◎" is more preferable because the delayed fracture resistance is even more excellent.

The underline in Tables 1 to 3 below means outside the scope of the present invention.

<Summary of evaluation results>
As shown in Table 3 above, No. Steel plates Nos. 1, 3 to 5, 8 to 9, 11 to 13, 15 to 17, 21 to 22, 26, 30 to 31 and 39 have at least any of tensile strength, ductility, hole expandability and delayed fracture resistance. was insufficient.

On the other hand, No. Steel plates Nos. 2, 6 to 7, 10, 14, 18 to 20, 23 to 25, 27 to 29, 32 to 38, and 40 to 45 all have a tensile strength of 1320 MPa or more, and have ductility and holes. It was found to have excellent spreadability and delayed fracture resistance.

Among these steel plates, the steel plates satisfying all of the following (i) to (v) had an evaluation result of delayed fracture resistance of "◎".
(i) Area ratio of retained austenite: 12.0% or less (ii) Average particle size of precipitate A: 0.020 μm or less (iii) N _S : 310 particles/μm ² or more (iv) N _S /N _L : 13.0 or more (v) Amount of diffusible hydrogen in steel: 0.25 mass ppm or less

Steel plates that did not satisfy at least any of the above (i) to (v) had an evaluation result of delayed fracture resistance of "○".
In addition, No. Steel plate No. 44 (steel code: T) satisfies all of the above (i) to (v), but because the amount of C is somewhat small, it is presumed that the evaluation result of delayed fracture resistance was "○".

Steel plates in which N _S /N _L was 12.1 or less or the average grain size of precipitates A was 0.040 μm or more had an evaluation result of delayed fracture resistance of “△”.

Claims (10)

In mass%,
C: 0.130-0.350%,
Si: 0.50 to 2.50%,
Mn: 2.00-4.00%,
P: 0.100% or less,
S: 0.0500% or less,
Al: 0.010-2.000%,
N: 0.0100% or less, and
Contains at least one element selected from the group consisting of Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, and V: 0.001 to 0.500%, the balance being Fe. and a component composition consisting of unavoidable impurities, and a microstructure,
The amount of diffusible hydrogen in the steel is 0.50 mass ppm or less,
In the microstructure,
The total area ratio of tempered martensite and bainite is 70.0 to 95.0%,
The area ratio of fresh martensite is 15.0% or less,
The area ratio of retained austenite is 5.0 to 15.0%,
The average particle size of the precipitates A, which are carbides, nitrides or carbonitrides containing at least one element selected from the group consisting of Ti, Nb and V, is 0.001 to 0.050 μm,
The number density N _S of the precipitates A _S having a major axis of 0.050 μm or less is 10 pieces/μm ² or more,
The ratio N _S /N _L between the number density N _S of the precipitates A S and the number density _{N L} of the precipitates A _L having a major axis of more than 0.050 μm is 10.0 or more. , high strength steel plate. The component composition further includes, in mass%,
W: 0.500% or less,
B: 0.0100% or less,
Ni: 2.000% or less,
Co : 2.000% or less,
Cr: 1.000% or less,
Mo: 1.000% or less,
Cu: 1.000% or less,
Sn: 0.500% or less,
Sb: 0.500% or less,
Ta: 0.100% or less,
Zr: 0.200% or less,
Hf: 0.020% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less, and
The high-strength steel plate according to claim 1, containing at least one element selected from the group consisting of REM: 0.0100% or less. The high-strength steel plate according to claim 1 or 2, which has a plating layer on the surface. The high-strength steel plate according to claim 3, wherein the plating layer is an alloyed plating layer. A method for manufacturing the high-strength steel plate according to claim 1 or 2, comprising:
Obtaining a hot rolled steel plate by heating a steel slab having the composition according to claim 1 or 2 to 1100 ° C. or higher and hot rolling at a finishing rolling temperature of 850 to 950 ° C.,
The hot-rolled steel sheet is wound up at a winding temperature T of 400 to 700° C., retained, and then cold-rolled to obtain a cold-rolled steel sheet,
Heat-treating the cold-rolled steel plate,
In the residence, when the total time during which the temperature of the coiled hot rolled steel sheet is equal to or higher than the coiling temperature T - 50 ° C. is expressed in units of s and t, the following formula 1 is satisfied,
In the heat treatment, the cold rolled steel sheet is held in a temperature range T1 of 800 to 950°C for 30 seconds or more, then cooled to a cooling stop temperature T2 of 150 to 250°C, and then heated in a temperature range T3 of 250 to 400°C. A method for producing a high-strength steel plate that is held for 30 seconds or more.
Formula 1: 0.001<[1.17×10 ⁻⁶ ×{t/(T+273.15)}] ^1/3 <0.050 Before the hot rolling, the steel slab is cast and then cooled;
In the cooling of the steel slab, an average cooling rate v1 at 700 to 600°C is 5.0°C/h or more, and an average cooling rate v2 at 600 to 500°C is 2.5°C/h or more. 5. The method for producing a high-strength steel plate according to 5. The method for manufacturing a high-strength steel plate according to claim 5, wherein after the heat treatment, the cold-rolled steel plate is subjected to a plating treatment to form a plating layer. The method for manufacturing a high-strength steel plate according to claim 6, wherein after the heat treatment, the cold-rolled steel plate is subjected to a plating treatment to form a plating layer. The method for manufacturing a high-strength steel plate according to claim 7, wherein the plating treatment includes an alloying plating treatment for alloying the plating layer. The method for manufacturing a high-strength steel plate according to claim 8, wherein the plating treatment includes an alloying plating treatment for alloying the plating layer.