JPH0941079A - Ultra-high strength steel sheet excellent in delayed fracture resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an ultra-high strength steel sheet excellent in delayed fracture resistance by subjecting a slab contg. specified amounts of Cr, Ni, Cu or the like to hot rolling and cold rolling and thereafter executing tempering under specified temp. conditions. SOLUTION: A slab having a compsn. contg., by weight, 0.08 to 0.30% C, <1.0% Si, 1.5 to 3.0% Mn and <0.010% S, furthermore contg. at least one kind among 1.5 to 5.00% Cr, 0.50 to 4.00% Ni and 0.50 to 3.00% Cu or contg. at least one kind among Al, Co and W or at least one kind among La, Cl and misch metal by a small amt. independently or compositely is heated at >=1,100 deg.C, is subjected to hot rolling, is coiled at <=600 deg.C, is subjected to pickling and descaling and is thereafter subjected to cold rolling. Next, it is soaked at 800 to 1,000 deg.C, is gradually cooled to 800 to 650 deg.C at a cooling rate of 30 deg.C/sec, is successively cooled to <=400 deg.C at a cooling rate of >=70 deg.C/sec, is thereafter reheated or, as it is, tempered at 150 to 400 deg.C for 1 to 20min. The thin steel sheet in which the volume rate of either one kind among martensite, tempered martensite and bainite is regulated to >=40% and having >=1,180HPa tensile strength can be produced.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an ultrahigh strength steel sheet having a tensile strength of 1180 MPa or more, which is excellent in delayed fracture resistance, and a manufacturing method thereof. Such an ultra-high strength steel sheet according to the present invention, in particular, a thin steel sheet, is used for pipes, for example, as a reinforcing member for automobile doors, where light weight and strength are required, and Zn, Cd, Sn, Al, Cr,
It can be suitably used for various applications in a severe corrosive environment by performing plating treatment of Ni, Pb, etc., chemical conversion treatment such as chromate treatment, phosphate treatment, etc., as well as anticorrosion surface treatment by organic coating.

[0002]

2. Description of the Related Art Recently, from the viewpoint of environmental protection of the earth, there is a strong demand for improving fuel efficiency of automobiles. Therefore, in order to reduce the weight of the vehicle body, there is an increasing need for ultra-high-strength thin steel sheets having a tensile strength of 1180 MPa or more for various reinforcing member applications such as bumpers and door impact beams. But,
In bolts made of ultra-high strength steel having a strength of 1180 MPa or more, hydrogen embrittlement may cause cracking, so-called delayed fracture, for example, as disclosed in Japanese Patent Laid-Open No. 60-155644.
It is already known as described in Japanese Patent Publication No.
Therefore, even in various members using the ultra-high-strength thin steel plate, hydrogen generated by a corrosion reaction in the atmospheric environment may enter the steel plate and be suddenly broken during use.

Also, when a super-high-strength thin steel sheet having a strength of 1180 MPa or more is used as a pipe for processing a reinforcing member of an automobile door, a high residual stress is generated during the processing, which causes delayed fracture. It is said to be one of the. Regarding the prevention of delayed fracture of an ultra-high strength thin steel sheet, as described in JP-A-4-268053, Si is added to the steel to control the penetration of hydrogen atoms into the steel sheet. Methods have been proposed to prevent the occurrence of hydrogen embrittlement, which is the cause of delayed fracture. But,
The cause of delayed fracture is not necessarily limited to hydrogen invasion, and stress concentration due to formation of corrosion pits is also a major factor. Therefore, it is difficult to sufficiently prevent delayed fracture from occurring only by adding Si.

Japanese Patent Application Laid-Open No. Hei 4-280940 describes improvement of water cracking resistance of a spot welded portion. However, since addition of 3% or more of Ni is required and cost is increased, practical use is not possible. Not a target. Nothing is mentioned about the water cracking resistance of the base material.

Further, in Japanese Unexamined Patent Publication (Kokai) No. 5-295481, by adding Ca to steel and changing MnS extended in the rolling direction into spherical CaS, the binding force of austenite grain boundaries is strengthened and hydrogen embrittlement resistance is obtained. It has been proposed to improve the conversion characteristics. Delayed fracture often shows the form of grain boundary cracking, especially at the crack initiation point, but the whole process of fracture is rarely a grain boundary crack, and thus the strengthening of the grain boundary is difficult. It cannot be a comprehensive measure.

[0006] In addition, from the viewpoint of processing technology, various studies have been made in the past regarding measures for reducing residual stress during processing and making delayed fracture less likely to occur.
Sufficient results have not been obtained.

[0007]

DISCLOSURE OF THE INVENTION The present invention is to solve the above-mentioned problem of delayed fracture in an ultra-high strength thin steel sheet having a tensile strength of 1180 MPa or more and is excellent in delayed fracture resistance. It is an object of the present invention to provide an ultra-high strength steel plate, particularly a thin steel plate, and a method for manufacturing the same.

[0008]

First, according to the present invention,
As shown in (1) to (8) below, (a)
C, Si, Mn and S as essential components, (b) Cr, Ni
And at least one element selected from the group consisting of Cu, or (c) at least one element selected from the group consisting of Al, W and Co, or (d) selected from the group consisting of La, Ce and misch metal. An ultra high strength steel sheet of Group I comprising at least one element is provided.

(1) in% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S
Including 0.010% or less, (b) Cr 1.5 to 5.0
0%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.
Containing at least one element selected from the group consisting of 00%, consisting of balance iron and unavoidable impurities, and containing 40% or more by volume of any one or more of martensite, tempered martensite or bainite structure, Strength is 11
It is characterized by being 80 MPa or more.

Further, according to the present invention, there is provided the following ultrahigh strength steel sheet having excellent delayed fracture resistance. (2) In wt% (a) C 0.08 to 0.30%, Si 1.
Less than 0%, Mn of 1.5 to 3.0%, and S of 0.010%
Including (b) Al 0.10 to 2.00%, W
Martensite, tempered martensite or bainite containing at least one element selected from the group consisting of 0.50 to 1.00% and Co 0.10 to 5.00% and the balance iron and inevitable impurities. Contains at least 40% by volume of any one or more of the tissues and has a strength of 1180 MPa.
An ultra-high strength steel sheet with excellent delayed fracture resistance.

(3) In% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S
Including 0.010% or less, (b) La 0.001-
It contains at least one element selected from the group consisting of 0.000%, Ce 0.001 to 0.100%, and misch metal 0.001 to 0.100%, and the balance is iron and inevitable impurities. Volume ratio of at least one of site, tempered martensite or bainite structure 4
An ultra high strength steel sheet containing 0% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance.

(4) in% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S
Including 0.010% or less, (b) Cr 1.5 to 5.0
0%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.
At least one element selected from the group consisting of 00%, and (c) Al 0.10 to 2.00%, W 0.50 to 1.0
0% and at least one element selected from the group consisting of Co 0.10 to 5.00%, balance iron and unavoidable impurities, and any of martensite, tempered martensite or bainite structure 1 Ultra high strength steel sheet containing 40% or more by volume of 1% or more and having a strength of 1180 MPa or more and excellent delayed fracture resistance.

(5) in% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S
Including 0.010% or less, (b) Cr 1.5 to 5.0
0%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.
At least one element selected from the group consisting of 00% and (c) La 0.001 to 0.100%, Ce 0.001
~ 0.100%, and misch metal 0.001-0.10
Containing at least one element selected from the group consisting of 0%, the balance iron and unavoidable impurities, containing at least 40% by volume of any one or more of martensite, tempered martensite or bainite structure, Strength is 118
Ultra high strength steel plate with excellent delayed fracture resistance of 0 MPa or more.

(6) In% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S
0.010% or less, and (b) Al 0.10-2.
00%, W 0.50 to 1.00%, and Co 0.10
At least one element selected from the group consisting of 5.00% and (c) La 0.001 to 0.100%, Ce 0.001
~ 0.100%, and misch metal 0.001-0.10
Containing at least one element selected from the group consisting of 0%, the balance iron and unavoidable impurities, containing at least 40% by volume of any one or more of martensite, tempered martensite or bainite structure, Strength is 118
Ultra high strength steel plate with excellent delayed fracture resistance of 0 MPa or more.

(7) in% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S
Including 0.010% or less, (b) Cr 1.5 to 5.0
0%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.
At least one element selected from the group consisting of 00%, and (c) Al 0.10 to 2.00%, W 0.50 to 1.0
0% and at least one element selected from the group consisting of Co 0.10 to 5.00%, and (d) La 0.001 to 0.001.
Martensite containing at least one element selected from the group consisting of 100%, Ce 0.001 to 0.100%, and misch metal 0.001 to 0.100%, and the balance iron and inevitable impurities. , Tempered martensite or bainite structure in volume ratio of 4 or more
An ultra high strength steel sheet containing 0% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance.

(8) The ultrahigh strength steel sheet according to any one of (1) to (7) above, further comprising Ti 0.01 to 0.50%.
Ultra high strength steel sheet including.

Further, according to the present invention, the following (9) to (1
As in 6), (a) together with C, Si, Mn and S, Ti,
There is provided an ultra high strength steel sheet of Group II containing Cu and Ni as essential chemical components. (9) C 0.08 to 0.30%, Si 1.0% by weight
Less than, Mn 1.5-3.0%, S 0.010% or less Ti
0.01 to 0.20%, Cu 0.10 to 3.00%, and N
i 0.1 to 3.00%, balance iron and unavoidable impurities, and one or more of martensite, tempered martensite, and bainite structure in a volume ratio of 40
% Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance.

(10) C 0.08 to 0.30% by weight,
Si less than 1.0%, Mn 1.5-3.0%, S 0.01
0% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.0
0%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, balance iron and inevitable impurities, and volume of any one or more of martensite, tempered martensite and bainite structure At a rate of 4
An ultra high strength steel sheet containing 0% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance.

(11)% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.
010% or less Ti 0.01 to 0.20%, Cu 0.10
3.00% and Ni 0.10 to 3.00%, and
(b) Al 0.05 to 2.00%, W 0.05 to 1.00
%, And at least one element selected from the group consisting of Co 0.10 to 5.00%, and the balance iron and unavoidable impurities, and one or more of martensite, tempered martensite, and bainite structure. Volume ratio of 4
An ultra high strength steel sheet containing 0% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance.

(12) in% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.
010% or less Ti 0.01 to 0.20%, Cu 0.10
3.00% and Ni 0.10 to 3.00%, and
(b) La 0.001 to 0.10%, Ce 0.001 to 0.10%.
100%, and misch metal 0.001 to 0.100%
At least one element selected from the group consisting of
The balance consists of iron and unavoidable impurities, martensite,
One of tempered martensite or bainite structure
Contains more than 40% by volume of seeds and has a strength of 1180M
Ultra high strength steel sheet with Pa or more and excellent delayed fracture resistance.

(13) C 0.08 to 0.30% by weight,
Si less than 1.0%, Mn 1.5-3.0%, S 0.01
0% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.0
0%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, and (b) Al 0.0.
5 to 2.00%, W 0.05 to 1.00%, and Co 0.
It contains at least one element selected from the group consisting of 10 to 5.00%, the balance is iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure is contained in a volume ratio of 40 or more. % Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance.

(14) C 0.08 to 0.30% by weight,
Si less than 1.0%, Mn 1.5-3.0%, S 0.01
0% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.0
0%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, and (b) La 0.0.
01 to 0.100%, Ce 0.001 to 0.100%, and misch metal 0.001 to 0.10%, and at least one element selected from the group consisting of iron and inevitable impurities. , A martensite, a tempered martensite, or a bainite structure, containing 40% or more by volume ratio, and having a strength of 1180 MPa or more, an ultrahigh strength steel sheet having excellent delayed fracture resistance.

(15) wt% (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.
010% or less Ti 0.01 to 0.20%, Cu 0.10
3.00% and Ni 0.10 to 3.00%, and
(b) Al 0.05 to 2.00%, W 0.05 to 1.00
%, And at least one element selected from the group consisting of Co 0.10 to 5.00%, and (c) La 0.001 to 0.1
00%, Ce 0.001 to 0.100%, and at least one element selected from the group consisting of misch metal 0.001 to 0.100%, with the balance being iron and inevitable impurities, and martensite. , Tempered martensite or bainite structure of 40% by volume
% Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance.

(16)% by weight (a) C 0.08 to 0.30
%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.
010% or less Ti 0.01 to 0.20%, Cu 0.10
3.00%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, and (b) Al 0.0.
5 to 2.00%, W 0.05 to 1.00%, and Co 0.
At least one element selected from the group consisting of 10 to 5.00%, and (c) La 0.001 to 0.100%, Ce 0.
001-0.100%, and misch metal 0.001-
0.1% and at least one element selected from the group consisting of balance iron and inevitable impurities, and one or more of martensite, tempered martensite, and bainite structure in 40% by volume. Including the above, an ultrahigh strength steel sheet having a strength of 1180 MPa or more and excellent in delayed fracture resistance.

Further, according to the present invention, the above-mentioned group I and group I
A method for manufacturing a group II ultra high strength steel sheet is provided. That is, according to the present invention, each of the ultrahigh-strength steel sheets of Group I and Group II as described above heats a steel slab containing the above elements and the balance iron and unavoidable impurities to a temperature of 1100 ° C or higher. Then, hot rolling is performed at a temperature of 600 ° C. or lower, then pickled, scales are removed, cold rolling is performed, and then continuous annealing is performed at 800 ° C.
Above, after soaking at a temperature in the range of 1000 ° C. or less, 3
It is gradually cooled to a temperature in the range of 800 to 650 ° C. at a cooling rate of 0 ° C./second or less, and then cooled from this temperature to a temperature of 400 ° C. or less at a cooling rate of 70 ° C./second or more. After that, reheat or 150-400 ℃ as it is
It can be obtained by performing a tempering treatment of heating at a temperature in the range of 1 to 20 minutes.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reason for the addition of chemical components and the range of addition in the ultrahigh strength steel sheet of Group I according to the present invention are as follows.

C is an essential element for forming a required low-temperature transformation structure such as martensite in a steel sheet and increasing the strength of the steel sheet. Particularly, according to the present invention, C is 1180 MPa.
In order to obtain the above tensile strength, it is necessary to add at least 0.08%. However, if the addition amount exceeds 0.30%, deterioration of hydrogen embrittlement resistance may be accelerated due to deterioration of workability or deterioration of corrosion resistance. Particularly, in the present invention, the amount of C is more preferably in the range of 0.12 to 0.20% from the viewpoint of the strength and corrosion resistance of the steel sheet.

Si is an element effective for solid-solution strengthening steel and densifying generated rust without deteriorating ductility and suppressing hydrogen intrusion due to corrosion. But,
When the addition amount is 1.0% or more, not only the effect is saturated, but also the paintability decreases. Therefore, in the present invention, the amount of Si is set to less than 1.0%. Mn is an element that enhances the hardenability of steel, and must be added in an amount of 1.5% or more in order to stably generate martensite in continuous annealing equipment. However, when it exceeds 3.0%, not only the effect is saturated, but also segregation becomes large, the structure becomes nonuniform, and the workability deteriorates, so the addition amount is 3.0%.
Is the upper limit.

Since S forms an inclusion with Mn and the like to serve as a starting point of corrosion generation and deteriorate bending workability and the like, it is restricted to 0.010% or less. Particularly preferably, 0.0
Not more than 05%.

[0030] Cr improves the hardenability of the steel and also improves the corrosion resistance of the steel by densifying the generated rust. In order to obtain such effects practically and effectively, it is necessary to add at least 1.5%. However, when added excessively, it causes a decrease in toughness after quenching and tempering, further promotes localization of corrosion form (pitting corrosion), and causes hydrogen embrittlement cracking due to concentration of tensile stress. Therefore, the upper limit of the addition amount is 5.00%. In particular, from the viewpoint of corrosion resistance and toughness, in the present invention, the addition amount is preferably in the range of 1.5 to 3.5%.

When Ni is added in an amount of 0.50% or more, it has the effect of improving the corrosion resistance of the steel by densifying the generated rust. However, if too much is added, it causes a decrease in tensile strength due to an increase in retained austenite, so the upper limit is made 4.00%. Further, Ni has the effect of suppressing hot embrittlement due to the addition of Cu, so it is preferable to add Ni in an amount equal to that of Cu. However, Ni is an expensive metal, and a more preferable addition range is 2.00% or less from the viewpoint of economy.

Since Cu is electrochemically nobler than iron, it has the effect of densifying the generated rust, improving corrosion resistance, particularly weather resistance, and improving pitting corrosion resistance. At least 0.50 for these effects to be effective
% Addition is required. However, even if added in excess of 3.00%, the above effect may be saturated and may cause embrittlement during hot rolling, so the upper limit of the addition amount is set to 3.
00%. In order to suppress embrittlement during hot rolling, it is preferable to add approximately the same amount of Ni together, as described above. From an overall practical standpoint, the addition amount is particularly preferably in the range of 0.50 to 1.00%.

Al has the effect of improving the corrosion resistance of steel. In order to effectively obtain this effect, it is necessary to add 0.10% or more. On the other hand, if too much is added, the surface properties are deteriorated and the workability of the steel is deteriorated. The upper limit is 2.00%. In particular, according to the present invention, the addition amount is particularly preferably from 0.10 to 1.00%.

W has an excellent effect of enhancing pitting corrosion resistance due to the adsorption action of tungstate ions produced by dissolution in an aqueous solution. To achieve this effect effectively, at least
0.50% addition is required. However, even if it is added in excess of 1.00%, the effect is only saturated, so the upper limit is made 1.00%.

Co is a solid solution strengthening element, has the property of not deteriorating the toughness, and also has the effect of enhancing the corrosion resistance. To obtain these effects effectively, 0.
It is necessary to add 10% or more, and particularly preferably 1.0% or more. However, since Co is an expensive element, the upper limit of the amount added is set to 5.00%, preferably 3.00%.
%.

La, Ce and misch metal all dissolve in an aqueous solution when steel corrodes to form an alkaline hydroxide, which causes acidification accompanying elution of iron ions on the corroded surface. It has a neutralizing and suppressing effect, which improves the corrosion resistance. Local acidification due to the corrosion reaction not only promotes corrosion, that is, the hydrogen generation reaction that causes delayed fracture, but also promotes the formation of pitting that causes stress concentration that promotes cracking. Therefore, the addition of these elements and misch metal has an effect of reducing the average corrosion rate and improving pitting corrosion resistance.

In order to effectively exert such an effect of improving the corrosion resistance, it is necessary to add 0.001% or more of each of these elements and misch metal. However, when they are added excessively, In addition, increasing the oxide-based inclusions to lower the workability, and during steel making, there is a risk of melting loss of the furnace wall, therefore, the addition amount, for any element and misch metal, the upper limit is 0. 100%

By forming fine carbides, Ti has the effect of refining the crystal grains and suppressing grain growth, and at the same time acts as a trap site for diffusible hydrogen in the material, and the hydrogen of the steel material is used. It reduces brittleness susceptibility and further has the effect of densifying the generated rust, thereby improving corrosion resistance. To obtain these effects effectively, at least 0.01%
Need to be added. However, when adding too much,
The upper limit of the addition amount is set to 0.50%, because carbides are formed and deposited, and the desired strength cannot be obtained. Particularly, in the present invention, the addition amount of Ti is preferably in the range of 0.03 to 0.20%.

Among the reasons and ranges of addition of the chemical components of the group II ultra high strength steel sheet according to the present invention, C, Si and M are included.
Regarding n, S, Ti, Co, La, Ce, and misch metal, the reason and the range of addition of the chemical components in the ultrahigh-strength steel sheet of Group I are the same. Therefore, next, the reason and the range of addition of the chemical components of the ultrahigh-strength steel sheet of Group II according to the present invention will be explained with respect to the reason and the range of addition of the chemical components in the ultrahigh-strength steel sheet of Group I. .

Cu densifies the formed rust, and significantly reduces the corrosion rate of steel in an atmospheric corrosive environment. Also, C
Since u is electrochemically nobler than iron, it synergistically improves the corrosion resistance of steel together with the above. In order to effectively obtain such effects, it is necessary to add at least 0.10%. However, on the other hand, Cu may cause embrittlement during hot rolling, so the upper limit of the addition amount is set to 3.00%. According to the present invention, in order to suppress such embrittlement during hot rolling, as described above,
An equivalent amount of Ni is also added together. According to the present invention, from the practical point of view, the addition amount is particularly preferably in the range of 0.20 to 2.00%.

When Ni is added in an amount of 0.10% or more, it has the effect of improving the corrosion resistance of the steel by densifying the generated rust. However, if too much is added, it causes a decrease in tensile strength due to an increase in retained austenite, so the upper limit is made 3.00%. Since Ni has the effect of suppressing hot embrittlement due to the addition of Cu, according to the present invention,
It is preferable to add about the same amount as Cu. However, Ni
Is an expensive metal, and a more preferable addition range is in the range of 0.20 to 2.00% from the viewpoint of economy.

[0042] Cr improves the hardenability of the steel and also improves the corrosion resistance of the steel by densifying the generated rust. In order to obtain such effects practically and effectively, it is necessary to add at least 0.10%. But,
If added too much, it may cause deterioration of toughness after quenching and tempering, and may promote localization of corrosion form (pitting corrosion) and cause hydrogen embrittlement cracking due to concentration of tensile stress. Therefore, the upper limit of the amount added is 5.00%. In particular, from the viewpoint of corrosion resistance and toughness, in the present invention, the addition amount is preferably in the range of 1.5 to 3.5%.

Al has the effect of improving the corrosion resistance of steel. In order to effectively obtain this effect, 0.05% or more of addition is necessary. On the other hand, when the addition is excessive, the workability of the steel is reduced, so the upper limit of the addition amount is 2.00%. I do.
In particular, according to the invention, the amount added is in particular between 0.15 and 1.0.
Addition of 0% is preferred.

W has an excellent effect of enhancing pitting corrosion resistance due to the adsorption action of tungstate ions generated by dissolution in an aqueous solution. To achieve this effect effectively, at least
0.05% addition is required. However, even if it is added in excess of 1.00%, the effect is only saturated, so the upper limit is made 1.00%.

Delayed fracture of high-strength steel is a phenomenon in which diffusible hydrogen that has penetrated into the steel locally concentrates at a certain location according to the tensile stress gradient, and the steel causes hydrogen embrittlement cracking at that location. It seems to be that. Although various mechanisms have been proposed for hydrogen embrittlement cracking, such as the theory of surface pressure and the theory of reduced cohesive force between iron atoms, they have not yet been elucidated yet, but they have a tendency to absorb hydrogen and diffuse hydrogen. It is understood that the three factors of easiness and the susceptibility of the steel itself to hydrogen embrittlement are interconnected phenomena.

Therefore, as a measure against hydrogen embrittlement, from the material side, (1) block the passage of hydrogen, and (2) suppress the diffusion of hydrogen in steel and the concentration in the tensile stress portion,
(3) It is considered that three measures for reducing the hydrogen embrittlement susceptibility of the steel itself are effective. Conventionally, there are many measures against hydrogen embrittlement according to (2) and (3), but the present invention focuses on the measure (1) in addition to (2) and (3).

That is, the hydrogen storage of steel in a normal use environment is caused by the fact that when the steel corrodes, the hydrogen generated by the cathode reaction enters the steel without being gasified.
According to the present invention, the measure (1) can be implemented by improving the corrosion resistance of the steel and preventing hydrogen storage. Particularly, in the ultrahigh-strength steel sheet of Group II according to the present invention, the above effects can be more effectively obtained by adding Cu and Ni in combination.

As another aspect of improving the corrosion resistance,
According to the present invention, by suppressing the non-uniform corrosion, it is possible to avoid stress concentration on the material surface,
The measure of (2) above can be taken. On the other hand, the reduction of the hydrogen embrittlement susceptibility of the steel itself of (3) can be dealt with by reducing the content of the grain boundary segregation element or by refining the crystal grains.

In the present invention, as a result of diligent studies on additive elements for improving the delayed fracture resistance of ultra-high strength steel as described above, the tensile strength of 1180 MPa or more was obtained by using the above-mentioned predetermined elements. Nevertheless, it succeeded in obtaining ultrahigh-strength steels of Group I and Group II excellent in delayed fracture resistance.

Next, a method for manufacturing an ultrahigh strength steel sheet having excellent delayed fracture resistance of Group I and Group II according to the present invention will be described. According to the method of the present invention, first, a steel slab having the above-mentioned chemical components is subjected to hot rolling according to a conventional method at a heating temperature of 1100 ° C or more and a winding temperature of 600 ° C or less. In the slab heating, in the high-strength steel as in the present invention, the rolling load during hot rolling tends to be high, so it is preferable that the rolling temperature does not become too low. Temperature 110
0 ° C or higher. In this case, the heating method is not particularly limited, such as direct rolling in which the continuous cast piece is rolled as it is, light heating, or a method in which the slab is cooled and then reheated. However, setting the heating temperature to a temperature exceeding 1300 ° C. requires only heat energy costs, and has no particular advantage. The hot rolling of the steel slab may be performed by a conventional method, and the finish rolling may be performed at a temperature of 800 ° C. or more.

The winding is carried out at a temperature of 600 ° C. or lower in consideration of the removability of scale on the surface. However, when the temperature is too low, the cold rolling property is reduced, so that the lower limit of the winding temperature is preferably 300 ° C. The hot-rolled steel sheet obtained in this manner is subjected to pickling, grinding, shot blasting or other means according to a conventional method to remove the scale of the surface, cold-rolled, and then continuously annealed.

According to the present invention, continuous annealing causes a part or the whole of the material to undergo austenite transformation during heating, and subsequent cooling causes these to undergo martensite transformation.
According to the present invention, a desired strength can be obtained by the amount of the martensite and the amount of the alloying element.

Therefore, in the present invention, in the continuous annealing, the heating temperature is 800 ° C. or higher and 1000 ° C. or lower. In order to obtain the required low temperature transformation structure such as martensite, tempered martensite or bainite by the cooling treatment after continuous annealing, it is necessary to precipitate austenite at the time of heating, and therefore the heating temperature is set to Ac ₁ point or more. . However, even if the temperature exceeds 1000 ° C., there is no particular advantage and the energy cost only increases.

After such continuous annealing, it is gradually cooled (primary cooling) to a temperature in the range of 800 to 650 ° C. at a cooling rate of 30 ° C./sec or less, and then rapidly cooled (secondary cooling) from this temperature. To do. When the slow cooling temperature is higher than 30 ° C./sec, ferrite is hard to be generated and a predetermined strength cannot be stably obtained. The cooling rate during the rapid cooling is 70 ° C. in order to cause low-temperature transformation of martensite and the like.
/ Sec or more is required, and at such a cooling rate 400
Cooling to below ℃, transformation such as martensite occurs. When the quenching start temperature is lower than 650 ° C., transformation of austenite to ferrite progresses by the start of quenching, and it is difficult to obtain a required low temperature transformation structure such as martensite having a volume ratio of 40% or more. On the other hand, when the quenching start temperature is higher than 800 ° C., the shape of the obtained steel sheet deteriorates, which is not preferable. The quenching rate is not particularly limited, but the cooling rate by water quenching (1000 to 2000 ° C./sec) is industrially the upper limit.

The steel sheet according to the present invention has at least 40% by volume of a low temperature transformation structure of at least one of martensite, tempered martensite and bainite structure, and all the structures are low temperature transformation products. May be. When the low-temperature transformation structure is less than 40%, the amount of alloying elements necessary to obtain a desired strength increases, and the production cost increases.

Next, when the quenched structure is martensite, it is reheated after the continuous annealing as described above so that its workability is improved and, for example, pipes and the like can be easily processed without trouble. Alternatively, the tempering treatment is performed at a temperature in the range of 150 to 400 ° C. as it is from the continuous annealing. Performing the tempering treatment at a temperature of 400 ° C. or higher not only increases the production cost due to reheating, but also cannot obtain particularly useful effects.

[0057]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples. I. Examples of group I ultra high strength steel sheets

Example 1 The steels shown in Tables 1 to 3 were heated to 1230 ° C., hot-rolled at a finishing temperature of 800 ° C. to a plate thickness of 3.0 mm, and wound at 480 ° C. This was pickled and cold-rolled to a plate thickness of 1.8 mm. Then, the temperature was maintained at 850 ° C. for 2 minutes, forced air cooling to 750 ° C., water quenching was performed from this temperature, and tempering treatment was performed. The tempering conditions were a temperature of 180 to 400 ° C. and a heating time of 12 minutes to obtain a steel sheet having a tensile strength of 1180 MPa or more. In the table, low temperature transformation products, M
Indicates martensite, Mt indicates tempered martensite, B indicates bainite, and P indicates pearlite.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

The delayed fracture resistance of the steel sheet thus obtained was examined as follows. That is, a steel plate is cut by machining to a width of 20 mm and a length of 100 mm, and this sample is bent in a U shape with a radius of curvature of 10 mm at the central portion in the longitudinal direction of the plate and bolted near the end of the plate to bend a certain amount. The test piece was given a stress. Here, in order to avoid galvanic corrosion between the bolt and the test piece, the bolt was covered with a Teflon tube and insulated. Also,
All the bare materials were used as the test pieces.

The test environment is a salt spray test (JIS Z
2371) is carried out for 12 hours and then left for 12 hours as a cycle test and a 0.1 N hydrochloric acid immersion test (30 ° C.). The delayed fracture resistance of the steel was evaluated by measuring the number and cracking time. As shown in the results in FIG. 1, it is understood that the steel according to the present invention has a significantly long time until crack initiation in any of the tests, and the steel according to the present invention has excellent delayed fracture resistance. It

Example 2 High-strength steel sheets were manufactured under the conditions shown in Tables 4 and 5 using steels having the chemical compositions shown in Table 4. Table 5 shows the strength and delayed fracture resistance of the obtained steel sheet. The delayed fracture resistance was evaluated in the same manner as in Example 1.

[0065]

[Table 4]

[0066]

[Table 5]

II. Examples of group II ultra high strength steel sheets

Example 1 The steels shown in Tables 6 to 8 were heated to 1230 ° C., hot-rolled at a finishing temperature of 800 ° C. to a plate thickness of 3.0 mm, and wound at 480 ° C. This was pickled and cold-rolled to a plate thickness of 1.8 mm. Then, the temperature was maintained at 850 ° C. for 2 minutes, forced air cooling to 750 ° C., water quenching was performed from this temperature, and tempering treatment was performed. The tempering conditions were a temperature of 180 to 400 ° C. and a heating time of 12 minutes to obtain a steel sheet having a tensile strength of 1180 MPa or more. In the table, low temperature transformation products, M
Indicates martensite, Mt indicates tempered martensite, B indicates bainite, and P indicates pearlite.

[0069]

[Table 6]

[0070]

[Table 7]

[0071]

[Table 8]

The delayed fracture resistance of the steel sheet thus obtained was examined as follows. That is, a steel plate is cut by machining to a width of 20 mm and a length of 100 mm, and this sample is bent in a U shape with a radius of curvature of 10 mm at the central portion in the longitudinal direction of the plate and bolted near the end of the plate to bend a certain amount. The test piece was given a stress. Here, in order to avoid galvanic corrosion between the bolt and the test piece, the bolt was covered with a Teflon tube and insulated. Also,
All the bare materials were used as the test pieces.

The test environment is a salt spray test (JIS Z
2371) is carried out for 12 hours and then left for 12 hours as a cycle test and a 0.1 N hydrochloric acid immersion test (30 ° C.). The delayed fracture resistance of the steel was evaluated by measuring the number and cracking time. As shown in the results in FIG. 2, it is understood that the steel according to the present invention has a significantly long time until crack initiation in any of the tests, and the steel according to the present invention has excellent delayed fracture resistance. It

Example 2 Tables 9 and 10 were prepared using steels having the chemical composition shown in Table 9.
A high strength steel plate was manufactured under the conditions shown in. Table 10 shows the strength and delayed fracture resistance of the obtained steel sheet. The evaluation of delayed fracture resistance was carried out in the same manner as in Example 1 for the ultra high strength steel sheet of Group II.

[0075]

[Table 9]

[0076]

[Table 10]

[0077]

INDUSTRIAL APPLICABILITY As described above, the ultrahigh-strength steel sheets according to the present invention are 1180MP for both Group I and Group II.
While having a tensile strength of a or more, at the same time, it has excellent resistance to delayed fracture, and such a steel sheet is preferably used, for example, for weight reduction of automobile bumpers and door reinforcing members. You can

[Brief description of drawings]

FIG. 1 is a graph showing delayed fracture resistance characteristics of a high strength steel sheet of Group I according to the present invention and a steel sheet as a comparative example. In the figure, the subscripts indicate the steel type numbers in the table.

FIG. 2 is a graph showing delayed fracture resistance characteristics of a high-strength steel sheet of Group II according to the present invention and a steel sheet as a comparative example. In the figure, the subscripts indicate the steel type numbers in the table.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoru Nakajima 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kadogawa Steel Works Kakogawa Works (72) Fukuteru Tanaka 1-kanazawa-machi, Kakogawa-shi, Hyogo Kadogawa Works Kakogawa Works (72) Inventor Takayuki Yamamoto 1 Kanazawa-machi, Kakogawa City, Hyogo Prefecture Kamido Works Kakogawa Works (72) Jiro Iwatani 1 Kanazawa-cho, Kakogawa City, Hyogo Prefecture Kadodo Co., Ltd. Inside the Kakogawa Works

Claims (34)

[Claims] 1. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and , (B) Cr 1.5 to 5.00%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of
The balance consists of iron and unavoidable impurities, martensite,
One of tempered martensite or bainite structure
Contains more than 40% by volume of seeds and has a strength of 1180M
Ultra high strength steel sheet with Pa or more and excellent delayed fracture resistance. 2. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less. , (B) Al 0.10 to 2.00%, W 0.50 to 1.00%, and Co 0.10 to 5.00%.
At least one element selected from the group consisting of
The balance consists of iron and unavoidable impurities, martensite,
One of tempered martensite or bainite structure
Contains more than 40% by volume of seeds and has a strength of 1180M
Ultra high strength steel sheet with Pa or more and excellent delayed fracture resistance. 3. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less. , (B) La 0.001 to 0.10%, Ce 0.001 to 0.10%, and misch metal 0.10.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance being iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure at a volume ratio of 40 % Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance. 4. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less. , (B) Cr 1.5 to 5.00%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of:
Al 0.10 to 2.00%, W 0.50 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of, the balance consisting of iron and unavoidable impurities, containing at least 40% by volume of any one or more of martensite, tempered martensite or bainite structure, strength Is 118
Ultra high strength steel plate with excellent delayed fracture resistance of 0 MPa or more. 5. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and , (B) Cr 1.5 to 5.00%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance being iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure at a volume ratio of 40 % Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance. 6. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and further, , (B) Al 0.10 to 2.00%, W 0.50 to 1.00%, and Co 0.10 to 5.00%.
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance being iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure at a volume ratio of 40 % Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance. 7. In weight%, (a) contains C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less. , (B) Cr 1.5 to 5.00%, Ni 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of:
Al 0.10 to 2.00%, W 0.50 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure in volume ratio. An ultra high strength steel sheet containing 40% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance. 8. The ultra high strength steel sheet according to claim 1, further comprising Ti 0.01 to 0.50%. 9. The ultra high strength steel sheet according to claim 1, wherein the C content is in the range of 0.12 to 0.20%. (10) In terms of% by weight, (a) C 0.08 to 0.30.
%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and (b) Cr 1.5 to 5.00%, Ni 0.50 to 4 0.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of
1100 steel slab consisting of iron and unavoidable impurities
After performing hot rolling at a temperature of 600 ° C. or higher and winding at a temperature of 600 ° C. or lower, pickling, removing scale, performing cold rolling, and then performing continuous annealing,
After soaking at a temperature in the range of 0 ° C to 1000 ° C, 800 to 650 ° C at a cooling rate of 30 ° C / sec or less.
Slowly cooled to a temperature in the range of
At a cooling rate of at least 400 g / s to a temperature of 400 ° C. or less, and then reheat, or
A tempering process of heating at a temperature in the range of 00 ° C. for 1 to 20 minutes is performed, and at least one of martensite, tempered martensite, and bainite structure is contained in a volume ratio of 40% or more and a strength of 1180 MPa or more. A method for producing an ultra high strength steel sheet having excellent delayed fracture resistance. 11. A steel slab in weight percent (a) C 0.08.
.About.0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and (b) Al 0.10 to 2.00%, W.sub.0. 0.50 to 1.00% and Co 0.10 to 5.00%
At least one element selected from the group consisting of
The method for producing an ultra-high strength steel sheet according to claim 10, comprising the balance iron and unavoidable impurities. 12. Steel slab in weight percent (a) C 0.08.
.About.0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and (b) La 0.001 to 0.100%, Ce 0. 0.001 to 0.100%, and misch metal 0.1.
The method for producing an ultrahigh-strength steel sheet according to claim 10, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance iron and unavoidable impurities. 13. A steel slab in weight percent (a) C 0.08.
.About.0.30%, Si less than 1.0%, Mn 1.5 to 3.0% and S 0.010% or less, and (b) Cr 1.5 to 5.00%, Ni 0. 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of:
Al 0.10 to 2.00%, W 0.50 to 1.00%, and Co 0.10 to 5.00%
The method for producing an ultrahigh-strength steel sheet according to claim 10, comprising at least one element selected from the group consisting of, and the balance being iron and unavoidable impurities. 14. A steel slab in weight percent (a) C 0.08.
.About.0.30%, Si less than 1.0%, Mn 1.5 to 3.0% and S 0.010% or less, and (b) Cr 1.5 to 5.00%, Ni 0. 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
The method for producing an ultrahigh-strength steel sheet according to claim 10, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance iron and unavoidable impurities. 15. Steel slab in weight percent (a) C 0.08.
.About.0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, and S 0.010% or less, and (b) Al 0.10 to 2.00%, W.sub.0. 0.50 to 1.00% and Co 0.10 to 5.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
The method for producing an ultrahigh-strength steel sheet according to claim 10, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance iron and unavoidable impurities. 16. A steel slab in weight percent (a) C 0.08.
.About.0.30%, Si less than 1.0%, Mn 1.5 to 3.0% and S 0.010% or less, and (b) Cr 1.5 to 5.00%, Ni 0. 0.50 to 4.00%, and Cu 0.50 to 3.00%
At least one element selected from the group consisting of:
Al 0.10 to 2.00%, W 0.50 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
The method for producing an ultrahigh-strength steel sheet according to claim 10, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance iron and unavoidable impurities. 17. The method according to claim 10, wherein the steel slab further contains Ti 0.01 to 0.50%. 18. The method according to claim 10, wherein the C content in the steel slab is 0.12 to 0.20%.
A method for manufacturing an ultra-high strength steel plate within a range of. 19. By weight%, C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20 %, Cu 0.10 to 3.00%, and Ni 0.10 to 3.00%
Containing 40% or more by volume of any one or more of martensite, tempered martensite, and bainite structure, and having a strength of 1
Ultra high strength steel plate with 180 MPa or more excellent in delayed fracture resistance. 20. Weight% C 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20 %, Cu 0.10 to 3.00%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, with the balance being iron and unavoidable impurities, martensite, tempered martensite or bainite. Volume ratio of any one or more of the tissues is 4
An ultra high strength steel sheet containing 0% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance. 21. In terms of% by weight, (a) C 0.08 to 0.30.
%, Less than Si 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, and Ni 0.10. ~ 3.0%
(B) Al 0.05 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%.
At least one element selected from the group consisting of
The balance consists of iron and unavoidable impurities, martensite,
One of tempered martensite or bainite structure
Contains more than 40% by volume of seeds and has a strength of 1180M
Ultra high strength steel sheet with Pa or more and excellent delayed fracture resistance. 22. In terms of% by weight, (a) C 0.08 to 0.30.
%, Less than Si 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, and Ni 0.10. ~ 3.0%
(B) La 0.001 to 0.100%, Ce 0.001 to 0.100%, and misch metal 0.1.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance being iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure at a volume ratio of 40 % Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance. 23. C in weight% 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20 %, Cu 0.10 to 3.00%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, and (b) Al 0.0.
5 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of
The balance consists of iron and unavoidable impurities, martensite,
One of tempered martensite or bainite structure
Contains more than 40% by volume of seeds and has a strength of 1180M
Ultra high strength steel sheet with Pa or more and excellent delayed fracture resistance. 24. C in weight% 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20 %, Cu 0.10 to 3.00%, Ni 0.10 to 3.00% Cr 0.10 to 5.00%, and (b) La 0.0.
01-0.100%, Ce 0.001-0.100%, and misch metal 0.1.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance being iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure at a volume ratio of 40 % Or more, the strength is 1180 MPa or more and the ultra-high strength steel sheet excellent in delayed fracture resistance. 25. Weight% of (a) C 0.08 to 0.30
%, Less than Si 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, and Ni 0.10. ~ 3.0%
(B) Al 0.05 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%.
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
Containing at least one element selected from the group consisting of 001 to 0.100%, the balance iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure in volume ratio. An ultra high strength steel sheet containing 40% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance. 26. A weight percentage of (a) C 0.08 to 0.30
%, Less than Si 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, Ni 0.10. 1.00% Cr 0.10 to 5.00%, and (b) Al 0.0
5 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
And at least one element selected from the group consisting of 001 to 0.100%, the balance iron and unavoidable impurities, and one or more of martensite, tempered martensite or bainite structure in volume ratio. An ultra high strength steel sheet containing 40% or more and having a strength of 1180 MPa or more and excellent in delayed fracture resistance. 27. C in weight% 0.08 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20 %, Cu 0.10 to 3.00%, and Ni 0.10 to 3.00%
A steel slab containing iron and the balance iron and unavoidable impurities is heated to a temperature of 1100 ° C. or higher, and hot rolling is performed at a temperature of 600 ° C. or lower, followed by pickling, removing scale, and cold rolling. Then, when performing continuous annealing, after soaking at a temperature in the range of 800 ° C. or more and 1000 ° C. or less, at a cooling rate of 30 ° C./sec or less, 800 to
Gradually cool to a temperature in the range of 650 ° C., then cool from this temperature to a temperature of 400 ° C. or less at a cooling rate of 70 ° C./sec or more, and then reheat or, as it is, 1
A martensite, which is characterized by performing a tempering treatment by heating at a temperature in the range of 50 to 400 ° C. for 1 to 20 minutes, contains 40% or more by volume of any one or more of martensite, tempered martensite, and bainite structure, and has a strength. 1180 MPa
The above is a method for producing an ultra-high strength steel sheet having excellent delayed fracture resistance. 28. The steel slab, in% by weight, has a C of 0.08 to 0.
30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, Ni 0.10 The method for producing an ultra-high strength steel sheet according to claim 27, wherein the super high-strength steel sheet comprises Cr of 0.10 to 5.00% and balance of iron and inevitable impurities. 29. Steel slab in weight percent (a) C 0.08
~ 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, and Ni 0.10 to 3.00%
(B) Al 0.05 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%.
At least one element selected from the group consisting of
The method for producing an ultra-high strength steel sheet according to claim 27, comprising the balance iron and unavoidable impurities. 30. A steel slab in weight% (a) C 0.08
~ 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, and Ni 0.10 to 3.00%
(B) La 0.001 to 0.100%, Ce 0.001 to 0.100%, and misch metal 0.1.
The method for producing an ultrahigh-strength steel sheet according to claim 27, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance being iron and unavoidable impurities. 31. A steel slab, in% by weight, has a C of 0.08 to 0.
30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, Ni 0.10 .About.3.00% Cr 0.10 to 5.00%, and (b) Al 0.0.
5 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of
The method for producing an ultra-high strength steel sheet according to claim 27, comprising the balance iron and unavoidable impurities. 32. A steel slab, in% by weight, has a C of 0.08 to 0.
30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, Ni 0.10 ˜3.00% Cr 0.10 to 5.00%, and (b) La 0.0
01-0.100%, Ce 0.001-0.100%, and misch metal 0.1.
The method for producing an ultrahigh-strength steel sheet according to claim 27, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance being iron and unavoidable impurities. 33. Steel slab in weight% (a) C 0.08
~ 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, and Ni 0.10 to 3.00%
(B) Al 0.05 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%.
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
The method for producing an ultra-high strength steel plate according to claim 27, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance being iron and unavoidable impurities. 34. A steel slab in weight percent (a) C 0.08.
~ 0.30%, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less Ti 0.01 to 0.20%, Cu 0.10 to 3.00%, Ni 0.1 to 3.00% Cr 0.10 to 5.00%, and (b) Al 0.0
5 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%
At least one element selected from the group consisting of:
La 0.001 to 0.10%, Ce 0.001 to 0.100%, and misch metal 0.10.
The method for producing an ultra-high strength steel plate according to claim 27, comprising at least one element selected from the group consisting of 001 to 0.100%, and the balance being iron and unavoidable impurities.