JP5662920B2 - High strength steel plate with excellent delayed fracture resistance and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel plate excellent in delayed fracture resistance and a method for producing the same.

  In recent years, steel sheets for automobile structural materials and reinforcing materials have been strengthened and put into practical use in order to achieve both weight reduction and collision safety of automobiles. However, as the strength of steel sheets increases, there is a problem that concerns about the occurrence of delayed fracture due to hydrogen intrusion and the like increase. Although the effects of the structure, material hardness, crystal grain size, various alloy elements, etc. are recognized as steel factors affecting this delayed fracture, its occurrence prevention means are not established, from various viewpoints Attempts have been made to improve delayed fracture resistance. For example, Patent Document 1 proposes to control the form so that oxides, sulfides, nitrides, composite crystallized substances, and composite precipitates of various elements can serve as hydrogen trap sites. However, in order to achieve better delayed fracture resistance, it is considered that further investigation is necessary in consideration of the precipitate other than the hydrogen trap site.

Japanese Patent No. 4167587

  The present invention has been made paying attention to the above-described circumstances, and an object thereof is to realize a high-strength steel sheet that is more excellent in delayed fracture resistance than conventional steel sheets.

The high-strength steel sheet excellent in delayed fracture resistance of the present invention that has solved the above problems has a chemical component composition,
C: 0.10 to 0.40% (meaning mass%. The same applies to the chemical composition).
Si: 0.6-3.0%
Mn: 1.0 to 3.5%
Al: 3% or less (excluding 0%),
P: 0.15% or less (not including 0%), and S: 0.02% or less (not including 0%)
The balance consists of iron and inevitable impurities,
95% or more of martensite in the entire organization, and
It is characterized in that the structure from the position of 10 μm depth to the position of ¼ depth of the sheet thickness in the sheet thickness direction from the steel sheet surface satisfies the following formula (1) and the tensile strength is 1180 MPa or more.
6.7 × 10 ⁻³ × Dγ + 7.4 × 10 ⁻⁹ × ρ ^1/2 −0.073 × Nc + 0.092 × p ² ≦ 0.570 (1)
[In Formula (1),
Dγ: Old γ particle size (μm)
ρ: Dislocation density (m ⁻² )
Nc: concentration of solid solution C in martensite (mass%)
p: ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary (where 0 ≦ p ≦ 1)
Indicates. ]

As the high-strength steel plate, B: 0.0001 to 0.02%, Ti: 0.005 to 0.3%, and N: 0.01% or less (not including 0%) are satisfied, and the following The thing which satisfy | fills Formula (7) is mentioned.
Ti> 3.4 × N (7)
[In formula (7), Ti represents the Ti content (mass%) in the steel, and N represents the N content (mass%) in the steel]

  As the high-strength steel plate, one or more selected from the group consisting of Nb: 0.005 to 0.3%, Cr: 0.003 to 2%, and Mo: 0.01 to 1.0%. The thing containing an element is mentioned.

  As the high-strength steel plate, Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, Co: 0.005 to 0.2%, and V: 0.003 to 1% And those containing one or more elements selected from the group consisting of:

  Examples of the high-strength steel sheet include those containing one or more elements selected from the group consisting of Ca: 0.0005 to 0.005% and Mg: 0.0005 to 0.01%.

  The present invention also includes a method for producing a high-strength steel sheet having excellent delayed fracture resistance, which includes performing cold rolling at a reduction rate of 5% or more, and obtaining the obtained cold-rolled steel sheet. And performing heat treatment so as to satisfy all of the following conditions (A) to (C), and after the coiling step after hot rolling, when the steel plate temperature is 400 ° C. or higher, the oxygen partial pressure of the atmosphere Is characterized in that it is 1200 ppm or less.

(A) Heating before quenching is performed so that the heating temperature T1 (where T1 is 1225K or less) and the heating time t1 satisfy the following formula (4).
5.3 × 10 ⁶ × exp (−1.38 × 10 ⁵ /(8.31×T1))×t1 ^0.2 ≦ 16 (4)
[In Formula (4),
T1: heating temperature (K),
t1: Indicates heating time (seconds)]
(B) After the said heating, it cools to 300 degrees C or less from heating temperature T1. At this time, the heating temperature T1 to 300 ° C. is cooled at 50 ° C./sec or more and quenched.
(C) Tempering is performed so that the tempering temperature T2 (where T2 is 748K or more and 823K or less), the tempering time t2, and the steel plate carbon concentration C satisfy the following formula (6).
T2 × (ln (t2 / 3600) + 21.3−5.8 × C) ≦ 12500 (6)
[In Formula (6),
T2: Tempering temperature (K),
t2: Tempering time (second) (however, t2 is 2 seconds or more and 400 seconds or less),
C: Steel plate carbon concentration (carbon concentration of base material) (% by mass). ]

  The present invention also includes another method for producing the high-strength steel sheet having excellent delayed fracture resistance, and the method performs cold rolling at a rolling reduction of 5% or more, and obtains the obtained cold Using a rolled steel sheet, heat treatment is performed so as to satisfy all of the following conditions (A) to (C), and after the following tempering, the Si deficient layer formed in the steel sheet surface layer part is removed by acid dissolution or It is characterized by being removed by grinding.

(A) Heating before quenching is performed so that the heating temperature T1 (where T1 is 1225K or less) and the heating time t1 satisfy the following formula (4).
5.3 × 10 ⁶ × exp (−1.38 × 10 ⁵ /(8.31×T1))×t1 ^0.2 ≦ 16 (4)
[In Formula (4),
T1: heating temperature (K),
t1: Indicates heating time (seconds)]
(B) After the said heating, it cools to 300 degrees C or less from heating temperature T1. At this time, the heating temperature T1 to 300 ° C. is cooled at 50 ° C./sec or more and quenched.
(C) Tempering is performed so that the tempering temperature T2 (where T2 is 748K or more and 823K or less), the tempering time t2, and the steel plate carbon concentration C satisfy the following formula (6).
T2 × (ln (t2 / 3600) + 21.3−5.8 × C) ≦ 12500 (6)
[In Formula (6),
T2: Tempering temperature (K),
t2: Tempering time (second) (however, t2 is 2 seconds or more and 400 seconds or less),
C: Steel plate carbon concentration (carbon concentration of base material) (% by mass). ]
In addition, said D (gamma), (rho), Nc, and p are calculated | required by the method as described in an Example.

  The steel sheet of the present invention exhibits sufficiently excellent delayed fracture resistance even when the tensile strength is as high as 1180 MPa or higher. Therefore, it can be suitably used as a structural material or a reinforcing material for automobiles.

  In order to solve the above-mentioned problems, the inventors of the present invention have various structural forms for martensite-based steel sheets (area ratio of 95% or more) that can achieve high strength (tensile strength of 1180 MPa or more). We have conducted intensive research on the relationship with delayed fracture resistance. As a result, as structural factors affecting delayed fracture resistance, “old austenite particle size” (hereinafter sometimes referred to as “old γ particle size” or “Dγ”), “dislocation density”, “martensite” It is understood that there are four types, namely, “Solubility C concentration” and “Ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary” (hereinafter, sometimes referred to as “p”). Further research was conducted on the relationship between these factors and delayed fracture resistance.

  As a result, regarding the relationship between the above four factors and delayed fracture resistance, it was understood that the tendency was as shown below. Moreover, although it was difficult to increase the resistance to delayed fracture by independently varying and controlling each factor, if the following formula (1) including these four factors is satisfied, delayed fracture resistance I found that sex improved.

  In the present invention, the control range of the following formula (1) is a range from the surface layer of the steel plate (specifically, a position having a depth of 10 μm in the plate thickness direction from the steel plate surface) to ¼ of the plate thickness. And The reason is as follows. That is, the stress that causes delayed fracture takes a large value particularly near the surface of the processed steel member. In the present invention, even when the steel sheet is processed by making the structure from the surface layer of the steel sheet (before processing) to ¼ depth of the plate thickness excellent in delayed fracture resistance. Excellent delayed fracture resistance can be maintained.

In other words, the structure form of the steel sheet in the range from ¼ depth to ½ depth of the plate thickness (that is, inside the steel plate) is not controlled by the following formula (1).
Below, the tendency is first demonstrated about the relationship between said each factor and delayed fracture resistance.

[Effect of old γ particle size (Dγ)]
As a result of a study by the present inventors, there was a tendency that delayed fracture resistance was improved by reducing the old γ grain size. In a high-strength steel sheet having martensite as a matrix, when delayed fracture occurs, it may break at the old γ grain boundary. When bending is performed under severe conditions, cracks are considered to occur at the site of shear deformation locally (shear band), particularly at the intersection of the shear band and the grain boundary. However, by refining the old γ grains, the locations where shear deformation occurs during bending are dispersed in the steel material, so that local stress concentration parts are not generated (suppressing the occurrence of delayed fracture starting points). It is considered effective for improving delayed fracture resistance.

  Delayed fracture is also likely to occur due to fracture of the embrittled old γ grain boundary. The cause of embrittlement of the old γ grain boundary is a carbide precipitated at the old γ grain boundary. It is considered that the more the carbide occupying the old γ grain boundary, the more easily the grain boundary becomes brittle. On the other hand, when the old γ grain size decreases, the ratio of the old γ grain boundary per unit volume increases. Therefore, when the amount of precipitated carbide is the same, the proportion of carbide occupying the old γ grain boundary decreases as the old γ grain size is small and the old γ grain boundary is large. Conceivable. From this, it is considered that the old γ grain size also affects the later-described p (the ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary).

[Influence of dislocation density (ρ)]
The dislocation density is a major factor in which martensite (particularly tempered martensite) exhibits high strength and high hardness. Generally, when the dislocation density is increased to increase the strength, delayed fracture resistance tends to increase.

[Influence of solid solution C concentration (Nc) in martensite]
The solid solution C concentration in martensite is also a strengthening factor of martensite, but the degree of decrease in delayed fracture resistance is small compared to strengthening by dislocation strengthening. Moreover, when the solid solution C density | concentration in a martensite increases, the precipitation amount of a carbide | carbonized_material will be suppressed relatively. Therefore, it is considered that this factor also affects p (the ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary) described later.
In the present invention, even when a structure other than martensite is present, since the ratio of the structure is small, it is not necessary to consider the solid solution C concentration in the structure other than martensite. .

[Influence of the ratio (p) of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary]
Quenched martensite is too brittle and brittle, so it is tempered to adjust the mechanical properties, but carbides precipitate when tempered. Since the driving force for nucleation / growth of carbide is large at the former γ grain boundary, the carbide tends to precipitate at the old γ grain boundary. However, as described above, it is considered that the more carbides existing in the old γ grain boundary, the more easily the grain boundary becomes brittle and delayed fracture occurs.
Note that p is in the range of 0 ≦ p ≦ 1, and p = 1 means that carbides are present in all (100%) of the old γ grain boundaries.

The relationship between the above four factors and delayed fracture resistance was studied as follows. That is, as shown in the examples described later using various steel plates having the above four factors, a low strain rate tensile test (SSRT test) was performed, and the elongation at break EL ₀ when this test was performed in the atmosphere The elongation at break EL _H when the above test was conducted while charging with hydrogen was determined. Then, a value obtained by dividing the difference between EL ₀ and EL _H by EL ₀ : (EL ₀ −EL _H ) / EL ₀ is obtained as delayed fracture susceptibility S, and the above 4 so that S is 0.50 or less. The following formula (1) was obtained by regression with factors.
6.7 × 10 ⁻³ × Dγ + 7.4 × 10 ⁻⁹ × ρ ^1/2 −0.073 × Nc + 0.092 × p ² ≦ 0.570 (1)
[In Formula (1),
Dγ: Old γ particle size (μm)
ρ: Dislocation density (m ⁻² )
Nc: concentration of solid solution C in martensite (mass%)
p: ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary (where 0 ≦ p ≦ 1)
Indicates. ]

  As described above, it is difficult to improve the delayed fracture resistance by individually varying each of the above four factors, but the present invention can satisfy the above formula (1) including these four factors. It is characterized in that it has been found that the delayed fracture resistance can be reliably improved.

In the steel structure of the steel sheet of the present invention, martensite accounts for 95 area% or more (preferably 98 area% or more, particularly 100 area%) with respect to the entire structure. As other structures, ferrite may be formed along with decarburization that may occur in the vicinity of the steel sheet surface, but this ferrite is preferably less in terms of ensuring high strength, so the area ratio of the entire structure It should be 5% or less.
Further, as a structure other than the martensite and ferrite, residual austenite may be present in an area ratio of about 3% or less.

<Production method>
In order to obtain a steel sheet of the present invention having a tensile strength of 1180 MPa or more and satisfying the above formula (1), a steel material satisfying the chemical composition described below is used, and heating and hot rolling are performed under the generally performed conditions. After that, in particular, it is necessary to perform cold rolling at a rolling reduction of 5% or more and then perform heat treatment under the following conditions.
In the present invention, the cold rolling is performed at a rolling reduction of 5% or more in order to obtain a tough structure. The rolling reduction is preferably 10% or more, and the upper limit of the rolling reduction is not particularly specified.

Next, heat treatment is performed. The steel sheet of the present invention is mainly composed of martensite, but the quenched martensite is too strong and brittle, so it is necessary to temper and adjust the mechanical properties. By performing this quenching and tempering so as to satisfy all of the following conditions (A) to (C), it is possible to obtain a structure satisfying the above formula (1).

[(A) Heating during quenching (keep soaking)]
The heating temperature during quenching (soaking temperature during quenching) T1 (K) and the heating time (holding time at the heating temperature) t1 (seconds) affect the prior γ particle diameter (Dγ). Specifically, the old γ grain size grows according to the following formula (2) with respect to the heating temperature T1 (K) and the heating time t1 (seconds). Within the range studied by the present inventors, the relationship between the prior γ particle diameter and T1 and t1 is as follows: a in the following formula (2) is 5.3 × 10 ⁶ , b is −1.38 × 10 ⁵ , It was found that n is 0.2 and is represented by the following formula (3).

In the present invention, in order to obtain a structure satisfying the formula (1) while ensuring a tensile strength of 1180 MPa or more, the old γ grain size (Dγ) represented by the formula (3) needs to be approximately 16 μm or less. Therefore, the following formula (4) was adopted.
If T1 exceeds 1225K, Dγ tends to increase rapidly, so T1 is set to 1225K or less. T1 is preferably 1208K or less.
Dγ = a × exp (b / (8.31 × T1)) × t1 ⁿ (2)
Dγ = 5.3 × 10 ⁶ × exp (−1.38 × 10 ⁵ /(8.31×T1))×t1 ^0.2 (3)
5.3 × 10 ⁶ × exp (−1.38 × 10 ⁵ /(8.31×T1))×t1 ^0.2 ≦ 16 (4)
[In the formulas (2) to (4),
Dγ: old γ particle size (μm),
T1: heating temperature (K),
t1: Heating time (seconds)
a, b, n: constants. ]

[(B) Cooling conditions during quenching]
After the heating, cooling is performed from the heating temperature T1 to 300 ° C. or less. The cooling rate (C1) from the heating temperature T1 to 300 ° C. is set to 50 ° C./sec or more (for example, water ).

[(C) Tempering conditions]
The tempering temperature T2 (K) and the tempering time t2 (seconds) affect the dislocation density, the solid solution C concentration in martensite, and the grain boundary carbide form. Generally, it is known that the tempering parameter M value represented by the following formula (5) is made constant so that a steel plate having the same level of strength can be obtained even if T2 and t2 are changed.
M = T2 × (ln (t2 / 3600) + 21.3−5.8 × C) (5)
[In Formula (5),
M: Tempering parameter T2: Tempering temperature (K)
t2: Tempering time (seconds)
C: Steel plate carbon concentration (carbon concentration of base material) (% by mass). ]

  However, the present inventors have further studied the tempering conditions. As a result, even when the M value is the same (that is, the same level of strength), the solid solution in martensite is tempered at a higher temperature and in a shorter time. There was a tendency for the C concentration to increase and the amount of grain boundary carbides to decrease, which proved effective in improving delayed fracture resistance.

  In the present invention, in order to set the tensile strength to 1180 MPa or more, the M value is set to 12,500 or less as shown in the following formula (6).

  Moreover, as above-mentioned, in this invention, it tempers in high temperature and a short time. If the tempering temperature T2 is too low, it is necessary to take a long tempering time in order to obtain the same strength. As a result, the dislocation density is high and the solid solution C concentration in the martensite is small. It becomes difficult to satisfy Expression (1). Therefore, T2 was set to 748K or more. Preferably it is 773K or more. On the other hand, if the tempering temperature is too high, the solid solution C concentration in the martensite becomes almost 0 and the tensile strength decreases, so T2 is set to 823K or less. Preferably it is 803K or less.

Furthermore, when the tempering time t2 is too long, the solid solution C concentration in the martensite becomes almost 0 and the tensile strength is lowered. Therefore, t2 is set to 400 seconds or less. Preferably it is 300 seconds or less. From the viewpoint of adjusting the mechanical characteristics, t2 is set to 2 seconds or more.
T2 × (ln (t2 / 3600) + 21.3−5.8 × C) ≦ 12500 (6)
[In Formula (6),
T2: Tempering temperature (K) (however, T2 is 748K or more and 823K or less),
t2: Tempering time (second) (however, t2 is 2 seconds or more and 400 seconds or less),
C: Steel plate carbon concentration (carbon concentration of base material) (% by mass). ]
After tempering so as to satisfy the above conditions, it may be cooled to room temperature by air cooling or the like.

[Suppression or removal of Si-deficient layer]
In the present invention, by containing Si in an amount of 0.6% or more, an effect of suppressing an increase in the solid solution C concentration in the martensite after tempering and the amount of carbides precipitated at the grain boundary during tempering is exhibited. To improve delayed fracture resistance. However, in the process of manufacturing a high Si steel sheet containing the above amount of Si, particularly in a situation where the steel sheet temperature after hot rolling descaling exceeds 400 ° C. (for example, quenching and tempering process), the oxygen potential of the atmosphere Si is preferentially oxidized over iron and moves to the surface of the steel sheet, so that an Si-deficient layer having a reduced Si concentration is easily formed near the surface of the base steel sheet. When the Si concentration in the vicinity of the surface layer is reduced, the carbide suppressing effect at the time of tempering is reduced, so that an appropriate steel sheet structure satisfying the formula (1) cannot be obtained. In addition, the delayed fracture resistance is likely to be lowered at the location where this Si-deficient layer exists.

In the present invention, the following (i) or (ii) is performed in order to prevent the delayed fracture resistance from being deteriorated by the Si-deficient layer.
(I) After the winding step after hot rolling, when the steel sheet temperature is 400 ° C. or higher (for example, quenching and tempering step), the oxygen partial pressure of the atmosphere is 1200 ppm or less (more preferably 1000 ppm or less, further Preferably, it is 800 ppm or less. By suppressing the oxygen partial pressure in the atmosphere, the reaction between Si in the steel and oxygen in the atmosphere is suppressed, and formation of a Si-deficient layer is suppressed.
(Ii) The Si-deficient layer formed on the steel plate surface layer is removed by acid dissolution or ground away.
Next, the reason for defining the chemical component composition in the present invention will be described.

[C: 0.10 to 0.40%]
C is an element necessary for ensuring the strength of the steel sheet, so it is contained by 0.10% or more. Preferably it is 0.12% or more, more preferably 0.15% or more. However, if the C content is excessive, weldability deteriorates, so the content is made 0.40% or less. Preferably it is 0.30% or less, More preferably, it is 0.25% or less.

[Si: 0.6-3.0%]
Si is a substitutional solid solution strengthening element that contributes to hardening of the steel sheet. Si is also an element effective for satisfying the above formula (1) and ensuring delayed fracture resistance by suppressing carbide precipitation during tempering. From such a viewpoint, in the present invention, the Si amount is set to 0.6% or more. Preferably it is 1.0% or more, More preferably, it is 1.4% or more. However, when Si is excessive, SiC is formed, and C is released from SiC during tempering to promote the formation of Fe ₃ C (carbide). Therefore, it is difficult to achieve the formula (1). Ensuring delayed fracture is difficult. Moreover, when Si is excessive, toughness will deteriorate. Therefore, the upper limit of the Si amount is set to 3.0%. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

[Mn: 1.0 to 3.5%]
Mn is an element effective for securing the strength of the steel sheet, and is contained at 1.0% or more. Preferably it is 1.5% or more, More preferably, it is 2.0% or more. On the other hand, when Mn is contained in a large amount, [Fe (Mn)] ₃ C (a carbide in which part of Fe ₃ C is substituted with Mn) is easily formed, and it is difficult to achieve the formula (1). Therefore, it is difficult to ensure delayed fracture resistance. In addition, segregation becomes prominent, workability is reduced, and weldability is easily deteriorated. Therefore, the upper limit of the Mn amount is 3.5%. Preferably it is 3.0% or less, More preferably, it is 2.5% or less.

[Al: 3% or less (excluding 0%)]
Al is an element necessary for deoxidation. Therefore, it is preferable to contain 0.01% or more. However, if added excessively, ductility is lowered and steel becomes brittle, so the upper limit is made 3%. Preferably it is 2.5% or less, More preferably, it is 1.0% or less.

[P: 0.15% or less (excluding 0%)]
Since P is an element that promotes grain boundary fracture due to grain boundary segregation, its upper limit is made 0.15%. Preferably it is 0.10% or less, More preferably, it is 0.05% or less.

[S: 0.02% or less (excluding 0%)]
If S is excessively contained, sulfide inclusions increase and the strength of the steel sheet deteriorates, so the upper limit is made 0.02%. Preferably it is 0.010% or less, More preferably, it is 0.005% or less, More preferably, it is 0.0025% or less.

  The components of the steel of the present invention are as described above, and the balance consists of iron and inevitable impurities. Further, in addition to the above elements, by further containing the following elements in an appropriate amount, it is possible to further increase the strength and improve the corrosion resistance. Hereinafter, these elements will be described in detail.

[B: 0.0001 to 0.02%]
[Ti: 0.005 to 0.3%]
[N: 0.01% or less (excluding 0%)]
[Ti> 3.4 × N (7)
[In formula (7), Ti represents the Ti content (mass%) in the steel, and N represents the N content (mass%) in the steel]

  B is an effective element for enhancing the hardenability of the steel sheet. Further, B has an effect of strengthening the grain boundary and improving delayed fracture resistance. In order to fully exhibit these effects, it is preferable to contain 0.0001% or more of B. More preferably, it is 0.00015% or more, More preferably, it is 0.001% or more. However, since hot workability will deteriorate when it contains excessively, it is preferable to make the upper limit into 0.02%. More preferably, it is 0.005% or less.

By the way, if B is combined with solute N existing in steel to form BN, the above effect is not exhibited. Therefore, in order to exert the effect by B, it is preferable to contain Ti and make the solute N in the steel TiN to make it harmless. Specifically, from the stoichiometric viewpoint, it is preferable to contain Ti so as to satisfy the above formula (7).
Ti is also an element that refines crystal grains, and is an effective element for improving the strength of a steel sheet without impairing toughness.
Furthermore, when the tensile strength exceeds 980 MPa, hydrogen embrittlement is particularly a concern. However, Ti combines with C and N in the steel to form fine carbonitrides, which causes hydrogen embrittlement. It functions effectively as a trap site.

  In order to exert these effects, the Ti content is preferably 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is 0.03% or more. However, even if contained excessively, the effect is not only saturated but also disadvantageous in terms of cost, so the upper limit is preferably made 0.3%. More preferably, it is 0.1% or less.

  N (nitrogen) is an inevitable impurity element that enters during the manufacturing process. Since there is an influence such as reducing the effect of the hardenability improving element B, it is desirable to suppress it to 0.01% or less. More preferably, it is 0.007% or less, More preferably, it is 0.005% or less.

[One or more elements selected from the group consisting of Nb: 0.005 to 0.3%, Cr: 0.003 to 2%, and Mo: 0.01 to 1.0%]
These elements are effective elements for improving the strength of the steel sheet.
Among these elements, Nb is an element that refines crystal grains and is an effective element for improving the strength of a steel sheet without impairing toughness. In order to exhibit such an effect, the Nb content is preferably 0.005% or more, more preferably 0.03% or more. However, even if contained excessively, the effect is not only saturated but also disadvantageous in terms of cost, so the upper limit is preferably made 0.3%. More preferably, it is 0.1% or less.

  Cr is an element that improves hardenability, and is an element that is effective in suppressing the excessive formation of ferrite and ensuring the strength of the steel sheet. Further, when Cr is contained, the corrosion resistance of the steel material itself is improved, so that hydrogen generation due to corrosion that may occur in the use environment can be sufficiently suppressed. This effect works more effectively by making Cr coexist with Cu and Ni described later. In order to exert these effects, the Cr content is preferably 0.003% or more. More preferably, it is 0.1% or more. However, if the Cr amount is excessive, not only the effect is saturated but also the workability deteriorates, so the upper limit of the Cr amount is preferably 2%. More preferably, it is 1% or less.

  Mo is an element effective for increasing the hardenability of the steel sheet. It also has the effect of suppressing hydrogen penetration and improving delayed fracture resistance. Furthermore, it has the effect of strengthening the grain boundaries and suppressing the occurrence of hydrogen embrittlement. In order to exhibit these effects effectively, it is preferable to contain Mo 0.01% or more. More preferably, it is 0.1% or more. However, if it is contained excessively, the effect is not only saturated, but also an expensive alloy element, resulting in an increase in cost. In addition, since the strength of the hot rolled sheet is greatly increased, problems such as difficulty in cold rolling occur. Therefore, the upper limit of the Mo amount is preferably 1.0%. More preferably, it is 0.5% or less.

[Selected from the group consisting of Cu: 0.01-0.3%, Ni: 0.01-0.3%, Co: 0.005-0.2%, and V: 0.003-1%. One or more elements]
Cu is an element that contributes to improving the corrosion resistance of the steel sheet. It is also a solid solution strengthening element and an element that contributes to improving the strength of steel sheets. In order to exhibit these effects, it is preferable to make Cu amount 0.01% or more. More preferably, it is 0.1% or more.
However, if the amount of Cu exceeds 0.3%, flaws and cracks during hot rolling may occur (red hot brittleness). Therefore, the upper limit of the Cu content is preferably 0.3%. More preferably, it is 0.2% or less.

  Ni, like Cu, is an element that contributes to improving the corrosion resistance of the steel sheet itself. Moreover, it is a solid solution strengthening element like Cu, and contributes also to the strength improvement of a steel plate. Furthermore, Ni has the effect of suppressing red heat embrittlement by dissolving in Cu and increasing the melting point. In order to exert these effects, the Ni content is preferably 0.01% or more. More preferably, it is 0.05% or more. However, Ni is an expensive element, and adding a large amount causes an increase in cost, so the upper limit is preferably made 0.3%. More preferably, it is 0.2% or less, More preferably, it is 0.15% or less.

  Co exhibits the same effect as Ni. That is, like Cu, it contributes to improving the corrosion resistance of the steel sheet itself. Co, like Cu, is also a solid solution strengthening element and contributes to improving the strength of the steel sheet. In order to exert these effects, the Co content is preferably set to 0.005% or more. More preferably, it is 0.01% or more. However, if the amount of Co is excessive, the workability is deteriorated and the cost is increased due to the expensive element. Therefore, the upper limit of the amount of Co is preferably 0.2%. More preferably, it is 0.15% or less.

  V contributes to the formation of protective rust, and the formation of protective rust is particularly promoted by the combined addition of Ti and V. V is an element effective for increasing the strength of the steel sheet and making it finer. Further, similarly to Ti, it combines with C and N in steel to form fine carbonitrides, and functions effectively as a hydrogen trap site that causes hydrogen embrittlement as described above. In order to exhibit these effects, it is preferable to contain 0.003% or more of V, and more preferably 0.01% or more. However, when V is excessively contained, precipitates increase and workability is reduced. Therefore, the upper limit of the V amount is preferably 1%, more preferably 0.5% or less.

[One or more elements selected from the group consisting of Ca: 0.0005 to 0.005% and Mg: 0.0005 to 0.01%]
Ca and Mg are elements that control the form of sulfide in steel and are effective for improving workability. Moreover, the rise of the hydrogen ion concentration of the interface atmosphere accompanying corrosion on the steel sheet surface is suppressed. In order to fully exhibit these effects, it is preferable to contain 0.0005% or more of each. More preferably, each is 0.0007% or more. On the other hand, since the workability deteriorates if contained excessively, the upper limit of each content is set to Ca: 0.005% (more preferably 0.001%), Mg: 0.01% (more preferably 0.00). 005%).

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

A steel ingot that satisfies the chemical composition shown in Tables 1 and 2 (the balance is iron and inevitable impurities) is prepared by vacuum melting, and then heated to 1250 ° C. and hot-rolled to a thickness of 2.4 mm. From the end of hot rolling to room temperature, air-cooling was performed in an N ₂ atmosphere to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was pickled with hydrochloric acid and then cold-rolled (rolling rate 67%) to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. Next, this cold-rolled steel sheet was subjected to heat treatment (quenching and tempering) under the conditions described in Tables 3 and 4 using a batch-type atmosphere-controlled heating and cooling furnace. In addition, the cooling rate (C1) described in Tables 3 and 4 is a cooling rate from the heating temperature T1 to 300 ° C.
And as shown below, about the obtained steel plate, while measuring the tensile strength, the measurement of the structure | tissue form and evaluation of delayed fracture resistance were performed. These results are shown in Tables 5 and 6.

<Measurement of tensile strength>
A tensile test was performed using a JIS No. 5 test piece, and the tensile strength (TS) was measured. The strain rate in the tensile test was 10 mm / min. In this invention, the steel plate whose tensile strength measured by the said method is 1180 Mpa or more was evaluated as a high strength steel plate.

<Measurement of tissue morphology>
(Measurement of martensite fraction)
In a plate thickness section parallel to the plate width direction, a sample was taken from a position ¼ of the plate thickness, subjected to nital etching, and observed with an optical microscope (size of one visual field is 160 μm × 200 μm).
And the obtained micrograph was manually binarized into phases other than martensite and martensite, and the martensite fraction (area%) was measured by image analysis. For each steel sheet, the martensite fraction was measured for a total of 3 fields of view, and the average value was determined.

<Measurement of Dγ (old γ grain size) and p (ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary)>
A test piece was taken from the steel plate so that a cross-section of the plate thickness parallel to the plate width direction could be observed, and was subjected to picric acid etching. Then, two places at a position of 10 μm in the thickness direction from the surface of the steel plate and a position at a quarter of the thickness are observed with an optical microscope (the size of one field is 80 μm × 80 μm). The average particle diameter (Dγ) of the grains was determined. Further, the total length (x) of the old γ grain boundary in one field of view and the total length (y) of the part of the old γ grain boundary that is blocked by carbides are measured, and y / x (old γ grain) The ratio of the total length y occupying the old γ grain boundary of carbides precipitated at the old γ grain boundary to the length (full length) x of the boundary, p) was determined.
For each steel plate, the above-mentioned Dγ and p were measured for a total of 5 fields of view, and the average value was obtained.

<Measurement of ρ (dislocation density) and Nc (solid solution C concentration in martensite)>
・ Mechanical polishing and electrolytic polishing are performed so that each of the cross section parallel to the steel plate surface and the cross section parallel to the steel plate surface can be observed at the position of 1/4 of the plate thickness at the position of 10 μm from the steel plate surface in the plate thickness direction. X-ray diffraction was performed on the surface, and the dislocation density and the solid solution C concentration in martensite were measured. More specifically, CAMP-ISIJ vol. 17 (2004) p. According to the method described in 396-399, the dislocation density was measured from the half width of the (200) plane. The X-ray diffraction measurement was performed once per steel plate.
Then, using the measured values of Dγ, p, ρ, and Nc, the left-hand side value of Formula (1) [6.7 × 10 ⁻³ × Dγ + 7.4 × 10 ⁻⁹ × ρ ^1/2 −0.073 × Nc + 0.092 × p ² ] was determined.
In this example, measurement was performed on the position of 10 μm depth from the steel sheet surface in the sheet thickness direction and the position of ¼ depth of the sheet thickness. However, from the position of 10 μm depth, ¼ depth of the sheet thickness was measured. It was also confirmed that a value between these two measured values was obtained between the positions.

<Evaluation of delayed fracture resistance>
The delayed fracture resistance of each steel plate was evaluated as follows. A low strain rate tensile test (SSRT test) was performed under the conditions of a strain rate of 2 × 10 ⁻⁶ sec ⁻¹ using a test piece taken from each steel plate and having a parallel part length of 20 mm and a width of 9 mm. The elongation at break (EL ₀ ) of the test piece in the atmosphere was measured.
Further, using a test piece of the same size, by performing the test with hydrogen charging was determined breaking elongation while hydrogen-charged (EL _H). Specifically, the SSRT test was carried out while maintaining the charge current density after hydrogen charging for 24 hours at a charge current density of 1 μA / mm ² in a state in which an unloaded specimen was immersed in a 5 mass% NaCl + 0.04M-KSCN aqueous solution. The EL _H was determined by performing the strain rate.
Then, the value obtained by dividing the difference between the elongation at break (EL ₀ ) in the atmosphere and the elongation at break (EL _H ) in a hydrogen-charged state by the elongation at break (EL ₀ ): (EL ₀ −EL _H ) / EL ₀ was determined as delayed fracture susceptibility S. And when the tensile strength is 1180 MPa or more and S is 0.50 or less, it is evaluated that the delayed fracture resistance is excellent (determination: ◯), and the tensile strength is 1180 MPa or more, but S is more than 0.50. Was evaluated as inferior in delayed fracture resistance (determination: x). Note that “−” in the delayed fracture susceptibility determination column means that the tensile strength is less than 1180 MPa.

  It can consider as follows from Tables 1-6. That is, no. As shown in 1 to 30, those satisfying the components / structures defined in the present invention exhibit high strength with a tensile strength of 1180 MPa or more and are excellent in delayed fracture resistance.

  On the other hand, those that do not satisfy the requirements defined in the present invention, at least one of the component / structure forms defined in the present invention, even if the tensile strength does not reach 1180 MPa or the tensile strength is 1180 MPa or more. What is not satisfied is inferior in the resistance to breakage after break. Details are as follows.

That is, no. No. 31 was insufficient in C, so the martensite fraction was small and high strength could not be secured.
No. No. 32 has a lack of Si. Since No. 33 was excessive in Si, none of the formulas (1) was satisfied and the delayed fracture resistance was poor.
No. Since Mn was excessive, No. 34 did not satisfy the formula (1) and was inferior in delayed fracture resistance.

No. In No. 36, the heating temperature (T1) at the time of quenching is too high, and the formula (4) is not satisfied. As a result, Dγ rapidly increases and does not satisfy the formula (1), and the delayed fracture resistance is poor. It became.
No. No. 37 is considered to have increased Dγ because no heating was performed so as to satisfy Equation (4) during quenching. As a result, the formula (1) was not satisfied and the delayed fracture resistance was poor.
No. No. 38 had a slow cooling rate (C1) from the heating temperature T1 to 300 ° C., and thus was not sufficiently quenched, the martensite fraction was small, and high strength could not be secured.
No. Since 39 and 40 had low tempering temperature (T2), they did not satisfy | fill Formula (1) and became inferior to delayed fracture resistance.
No. No. 41 had a result that the tempering temperature was too high and the tempering was not performed so as to satisfy the formula (6), so that the tensile strength was insufficient.
No. No. 42 did not satisfy the formula (1) because of its low tempering temperature, and was inferior in delayed fracture resistance.
No. Since No. 43 was not tempered to satisfy the formula (6), high strength could not be secured.
No. Nos. 44 and 45 did not satisfy the formula (1) because the oxygen concentration in the atmosphere during quenching / tempering was high, and the resistance to delayed fracture was poor.
No. 9 and 10; From the comparison with the result of 35, it can be seen that it is preferable to set the recommended range in order to add B and Ti and to fully exhibit the effect of B and Ti.

Claims (6)

The chemical composition is
C: 0.10 to 0.40% (meaning mass%. The same applies to the chemical composition).
Si: 0.6-3.0%
Mn: 1.0 to 3.5%
Al: 3% or less (excluding 0%),
P: 0.15% or less (not including 0%), and S: 0.02% or less (not including 0%)
The balance consists of iron and inevitable impurities,
95% or more of martensite in the entire organization, and
Delay resistance, characterized in that the structure from the position of 10 μm depth to the position of ¼ depth of the sheet thickness in the sheet thickness direction from the surface of the steel sheet satisfies the following formula (1) and the tensile strength is 1180 MPa or more. High strength steel plate with excellent destructibility.
6.7 × 10 ⁻³ × Dγ + 7.4 × 10 ⁻⁹ × ρ ^1/2 −0.073 × Nc + 0.092 × p ² ≦ 0.570 (1)
[In Formula (1),
Dγ: Old γ particle size (μm)
ρ: Dislocation density (m ⁻² )
Nc: concentration of solid solution C in martensite (mass%)
p: ratio of the length of carbide precipitated at the old γ grain boundary to the length of the old γ grain boundary (where 0 ≦ p ≦ 1)
Indicates. ] Furthermore,
B: 0.0001 to 0.02%,
Ti: 0.005 to 0.3%, and N: 0.01% or less (excluding 0%)
The high-strength steel sheet according to claim 1, which satisfies the following equation (7).
Ti> 3.4 × N (7)
[In formula (7), Ti represents the Ti content (mass%) in the steel, and N represents the N content (mass%) in the steel] Furthermore,
Nb: 0.005-0.3%
Cr: 0.003 to 2% and Mo: 0.01 to 1.0%
The high-strength steel sheet according to claim 1 or 2, comprising one or more elements selected from the group consisting of: Furthermore,
Cu: 0.01 to 0.3%,
Ni: 0.01 to 0.3%,
Co: 0.005 to 0.2%, and V: 0.003 to 1%
The high-strength steel sheet according to any one of claims 1 to 3, comprising one or more elements selected from the group consisting of: Furthermore,
Ca: 0.0005 to 0.005%, and Mg: 0.0005 to 0.01%
The high-strength steel plate according to any one of claims 1 to 4, comprising one or more elements selected from the group consisting of: A method for producing a high-strength steel sheet having excellent delayed fracture resistance according to any one of claims 1 to 5, wherein cold rolling is performed at a rolling reduction of 5% or more, and
Using the obtained cold-rolled steel sheet, heat treatment is performed so as to satisfy all of the following conditions (A) to (C), and the temperature of the steel sheet is 400 ° C. or higher after the winding step after hot rolling. A method for producing a high-strength steel sheet having excellent delayed fracture resistance, wherein the oxygen partial pressure in the atmosphere is 1200 ppm or less.
(A) Heating before quenching is performed so that the heating temperature T1 and the heating time t1 satisfy the following formula (4).
5.3 × 10 ⁶ × exp (−1.38 × 10 ⁵ /(8.31×T1))×t1 ^0.2 ≦ 16 (4)
[In Formula (4),
T1: heating temperature (K) (however, T1 is 1225K or less),
t1: Indicates heating time (seconds)]
(B) After the said heating, it cools to 300 degrees C or less from heating temperature T1. At this time, the heating temperature T1 to 300 ° C. is cooled at 50 ° C./sec or more and quenched.
(C) Tempering is performed so that the tempering temperature T2, the tempering time t2, and the steel plate carbon concentration C satisfy the following formula (6).
T2 × (ln (t2 / 3600) + 21.3−5.8 × C) ≦ 12500 (6)
[In Formula (6),
T2: Tempering temperature (K) (however, T2 is 748K or more and 823K or less),
t2: Tempering time (second) (however, t2 is 2 seconds or more and 400 seconds or less),
C: Steel plate carbon concentration (carbon concentration of base material) (% by mass). ]