JP3254107B2 - Ultra-high-strength steel sheet excellent in delayed fracture resistance and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high-strength steel sheet having a tensile strength of 1180 MPa or more and excellent in delayed fracture resistance and a method for producing the same. Such ultra-high-strength steel sheets according to the present invention, in particular, thin steel sheets are, for example, as pipe applications, for applications requiring light weight and strength, such as reinforcing members for automobile doors, and for Zn, Cd, Sn, Al, Cr, Ni,
Plating treatment such as Pb, chemical conversion treatment such as chromate treatment and phosphate treatment, and further, anticorrosion surface treatment by organic coating can be performed, and it can be suitably used for various applications in severe corrosive environment.

[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, the need for ultra-high-strength steel sheets having a tensile strength of 1180 MPa or more has been increasing for various reinforcing members of automobiles such as bumpers and door impact beams. But,
Bolts made of ultra-high-strength steel having a strength of 1180 MPa or more are susceptible to cracking due to hydrogen embrittlement, so-called delayed fracture, as described in, for example, JP-A-60-155644.
It is already known as described in Japanese Patent Publication No.
Therefore, even in various members using an ultra-high strength thin steel sheet, hydrogen generated by a corrosion reaction in an atmospheric environment may enter the steel sheet and be suddenly broken during use.

[0003] To prevent delayed fracture of an ultra-high strength thin steel sheet, as described in JP-A-4-268053, Si is added to steel to control the penetration of hydrogen atoms into the steel sheet. Accordingly, a method for preventing the occurrence of hydrogen embrittlement, which is a cause of delayed fracture, has been proposed. However, the cause of the delayed fracture is not necessarily limited to hydrogen intrusion, and the stress concentration due to the 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. Furthermore, Japanese Patent Application Laid-Open No. 5-295481 discloses that by adding Ca to steel and changing MnS extended in the rolling direction to spherical CaS, the bonding force of austenite crystal grain boundaries is strengthened and hydrogen embrittlement resistance is improved. It has been proposed to improve. 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.

[0005]

SUMMARY OF THE INVENTION The present invention is intended 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 sheet, particularly a thin steel sheet, and a method for producing the same.

[0006]

The ultrahigh-strength steel sheet having excellent delayed fracture resistance according to the present invention has a C content of 0.0% by weight.
8 to 0.30%, Si less than 1.0%, Mn 1.5 to 3.0
%, S 0.010% or less, P 0.03 to 0.15%,
Cu 0.10 to 1.00% and Ni 0.10 to 4.00%
, The balance consisting of iron and inevitable impurities, containing at least 40% by volume of at least one of martensite, tempered martensite, and bainite structure, and having a strength of 1
The pressure is 180 MPa or more.

[0007] Such an ultra-high strength steel sheet according to the present invention comprises:
According to the present invention, a steel slab containing the above elements, the balance consisting of iron and unavoidable impurities is heated to a temperature of 1100 ° C. or higher, and hot-rolled at a temperature of 600 ° C. or lower, and then pickled, scaled. , Except that when cold rolling is performed and then continuous annealing is performed, 800 ° C. or more and 100 ° C.
After soaking at a temperature in the range of 0 ° C. or lower, the temperature is gradually cooled to a temperature in the range of 800 to 650 ° C. at a cooling rate of 30 ° C./second or lower, and then cooling from this temperature to 70 ° C./second or more. It can be obtained by cooling at a speed to a temperature of 400 ° C. or less and then reheating, or performing a tempering treatment in which the material is heated at a temperature in a range of 150 to 400 ° C. for 1 to 20 minutes.

First, in the present invention, the range of chemical components of the steel sheet and the reason thereof are as follows. C is
It is an essential element for generating a required structure such as a martensite structure in a steel sheet and increasing the strength of the steel sheet. In particular, in order to obtain a tensile strength of 1180 MPa or more according to the present invention, at least 0.08% Need to be added. However, when the addition amount exceeds 0.30%, the workability may be reduced, or the deterioration of the hydrogen embrittlement resistance may be accelerated due to the deterioration of the corrosion resistance or the like. In particular, in the present invention, from the viewpoint of the strength and corrosion resistance of the steel sheet, the C content is more preferably in the range of 0.12 to 0.20%.

[0009] Si is an element effective for solid solution strengthening of steel without deteriorating ductility. However, 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 Si content is less than 1.0%. Mn is an element that enhances the hardenability of steel. In order to stably generate martensite in continuous annealing equipment, Mn must be added in an amount of 1.5% or more. However, when the content exceeds 3.0%, not only the effect is saturated, but also segregation increases, the structure becomes non-uniform, and the workability deteriorates. Therefore, the upper limit of the addition amount is 3.0%.

Since S forms inclusions with Mn and the like and serves as a starting point of corrosion and degrades bending workability and the like, it is restricted to 0.010% or less. Particularly preferably, 0.0
Not more than 05%. P is effective in densifying the formed rust and significantly reducing the corrosion rate of steel in an atmospheric corrosion environment by adding P together with Cu according to the present invention. P is also effective in increasing the strength and ductility of steel. According to the present invention, in order to obtain these effects effectively, it is necessary to add P in an amount of 0.03% or more.
However, P, on the other hand, tends to segregate at the grain boundaries and may reduce the grain boundary strength.
0.15%. In particular, from the viewpoint of improving corrosion resistance and suppressing embrittlement, the upper limit of the amount of P added is preferably 0.07%.

According to the present invention, Cu is added together with a small amount of P, thereby densifying the formed rust and significantly reducing the corrosion rate of steel in an atmospheric corrosion environment. Also,
Since Cu is electrochemically more noble than iron, it also synergistically improves the corrosion resistance of steel. In order to effectively obtain such an effect, it is necessary to add at least 0.10%. However, on the other hand, Cu
May cause embrittlement during hot rolling,
The upper limit of the amount added is 1.00%. Further, in order to prevent embrittlement at the time of the hot rolling due to the addition of Cu, as described later, it is preferable to simultaneously add an equivalent amount of Ni.
According to the present invention, from a practical viewpoint, the added amount of Cu is:
In particular, the range of 0.20 to 0.60% is preferable.

Ni, by adding 0.1% or more, has an effect of improving the corrosion resistance of steel by densifying the generated rust. However, an excessive addition causes a decrease in tensile strength due to an increase in retained austenite, so the upper limit is made 4.00%. Further, Ni has an effect of suppressing hot rolling embrittlement when Cu is added. Therefore, as described above, it is desirable to add Ni in an amount equivalent to Cu. However, Ni is an expensive metal, and a more preferable addition range is 2.00% or less from the viewpoint of economy.

According to the present invention, the steel sheet contains, in addition to the above elements, Ti 0.01 to 0.50% and Cr 0.10 to 5.0.
It may contain at least one element selected from the group consisting of 0%.

[0014] Ti has the effect of reducing the crystal grain size and the effect of suppressing grain growth, reduces the hydrogen embrittlement susceptibility of the steel material, and also has the effect of densifying the formed rust, thereby improving the corrosion resistance. Let it. In order to obtain these effects effectively, at least 0.
01% addition is required. However, when excessively added, a precipitate with C is formed and a predetermined strength cannot be obtained. Therefore, the upper limit of the added amount is set to 0.50%. Particularly, in the present invention, the addition amount is preferably in the range of 0.03 to 0.20%.

[0015] Cr improves the hardenability of the steel and improves the corrosion resistance of the steel by densifying the formed rust. In order to effectively obtain such effects, it is necessary to add at least 0.10%. However, when added excessively, it may cause a decrease in toughness after quenching and tempering, further promote localization of corrosion form (pit corrosion), and cause hydrogen embrittlement cracking due to tensile stress concentration. Therefore, the upper limit of the addition amount is 5.00%. Particularly, from the viewpoint of corrosion resistance and toughness, in the present invention, the amount added is 1.
A range of 5 to 3.5% is preferred.

Further, according to the present invention, the steel sheet is made of Al.
0.05-2.00%, W 0.05-1.00%, and Co
At least one selected from the group consisting of 0.10-5.00%
Species elements can be included.

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 present invention, the addition amount is preferably 0.15 to 1.00%.

W has an excellent effect of improving the pitting corrosion resistance by the action of adsorbing 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%.

Co is a solid solution strengthening element, has the property of not deteriorating the toughness, and has the effect of increasing the corrosion resistance. To obtain these effects effectively, 0.
Addition of 1% or more is necessary, and addition of 1.0% or more is particularly preferable. However, since Co is an expensive element,
The upper limit of the amount added is 5.00%.

Further, according to the present invention, the steel sheet is La 0.
It may contain at least one element selected from the group consisting of 001 to 0.100%, Ce 0.001 to 0.100%, and misch metal 0.001 to 0.100%.

Any of La, Ce or misch metal dissolves in the aqueous solution when the steel is corroded to form an alkaline hydroxide, and thus hydrolyzes with the elution of iron ions on the corroded surface. Neutralizes acidification by decomposition reaction
It has a suppressing effect, thereby improving the corrosion resistance. Local acidification by corrosion reactions not only promotes corrosion, ie, the hydrogen evolution reaction that causes delayed fracture, but also promotes the formation of pitting, which causes stress concentrations that promote cracking. Therefore, the addition of these elements has the effect of reducing the average corrosion rate and improving pitting corrosion resistance.

In order to effectively exhibit such an effect of improving corrosion resistance, it is necessary to add 0.001% or more of each of the above elements or misch metals. The oxide content increases, the workability is lowered, and the steel wall may be damaged during steelmaking. Therefore, the upper limit is 0.100% for any element or misch metal. And

According to the present invention, Ca can be added to the steel sheet. Ca also has an effect similar to that of La, Ce and misch metal described above, and is an effective element for improving the corrosion resistance of steel. Further, as described above, Ca is obtained by forming MnS expanded in the rolling direction into a spherical shape. Changing to CaS also has the effect of strengthening austenite grain boundaries.
Furthermore, the solid solution Ca also has the effect of increasing the bonding force between iron atoms by emitting electrons. Therefore, a synergistic effect of improving corrosion resistance and suppressing hydrogen embrittlement susceptibility of the material can be realized by the addition of Ca.

In order to obtain such an effect effectively, C
a may be added in an amount of 0.001% or more. However, when excessively added, coarse inclusions are generated and workability is deteriorated. Therefore, the upper limit of the addition amount is set to 0.100%. I do.
Practically, the upper limit is preferably 0.010%.

The delayed fracture of high-strength steel phenomena is that diffusible hydrogen infiltrated into steel concentrates locally at a certain point according to a tensile stress gradient, and at that point, the steel causes hydrogen embrittlement cracking. It seems that it is. 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 countermeasure against hydrogen embrittlement, from the material side, (1) blocking the passage of hydrogen, (2) suppressing 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 absorption of steel in a normal use environment is caused by the fact that hydrogen generated by the cathode reaction penetrates into the steel without corroding when the steel is corroded.
According to the present invention, the countermeasure (1) can be performed by improving the corrosion resistance of steel and preventing hydrogen field occlusion.

As another aspect of the improvement of corrosion resistance,
According to the present invention, by suppressing non-uniform corrosion, stress concentration on the material surface can be avoided,
The above measure (2) can be taken. On the other hand, the decrease in the hydrogen embrittlement susceptibility of the steel itself (3) can be dealt with by reducing the content of grain boundary segregation elements or by making the crystal grains fine.

As described above, according to the present invention, as a result of intensive studies on the additional elements for improving the delayed fracture resistance of ultra-high-strength steel, the use of the above-described predetermined elements makes it possible to obtain a tensile strength of 1180 MPa or more. In spite of this, it succeeded in obtaining an ultra-high-strength steel sheet having excellent delayed fracture resistance.

Next, a method of manufacturing an ultra-high strength steel sheet having excellent delayed fracture resistance 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 hot-rolled in a usual manner at a heating temperature of 1100 ° C. or more and a winding temperature of 600 ° C. or less. In the slab heating, in a high-strength steel as in the present invention, since the rolling load during hot rolling tends to be high, it is preferable that the rolling temperature is not excessively low. Temperature 110
0 ° C or higher. In this case, the heating method is not particularly limited, such as direct rolling of the continuous cast piece as it is, light heating, and a method of reheating after cooling the slab. 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 performed at a temperature of 600 ° C. or less in consideration of the removability of the 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, a part or the whole is transformed into austenite during heating by continuous annealing, and then transformed into martensite by cooling.
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 set to 800 ° C. or more and 1000 ° C. or less. In order to obtain a required low-temperature transformation structure such as martensite, tempered martensite or bainite by cooling treatment after continuous annealing,
It is necessary to precipitate austenite at the time of heating, and therefore, the heating temperature is set to _one or more Ac. But,
Even if the temperature exceeds 1000 ° C., there is no particular advantage and only the energy cost 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./second or less, and then rapidly cooled (secondary cooling) from this temperature. I do. When the slow cooling temperature is higher than 30 ° C./sec, ferrite is not easily 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 ℃ causes transformation such as martensite. When the quenching start temperature is lower than 650 ° C., the transformation of austenite to ferrite proceeds before the start of quenching, and it is difficult to obtain a required low-temperature transformed 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. Although the quenching rate is not particularly limited, the cooling rate by water quenching (1000 to 2000 ° C./second) is usually the upper limit industrially.

The steel sheet according to the present invention has a low-temperature transformation structure of at least one of martensite, tempered martensite and bainite structure in a volume fraction of 40% or more, and all structures are low-temperature transformation products. You may. 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 as to improve its workability and to easily work, for example, a pipe without any trouble. Alternatively, tempering is performed at a temperature in the range of 150 to 400 ° C. as it is from 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.

[0037]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited by these examples.

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

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

With respect to the steel sheet thus obtained, the delayed fracture resistance was examined as follows. That is, a steel sheet is cut out to a width of 20 mm and a length of 100 mm by machining, and this sample is subjected to a U-shaped bending process with a radius of curvature of 10 mm at the center in the longitudinal direction of the plate, and bolted near the end of the plate to bend a certain amount. A test piece to which stress was applied was used. Here, the bolt was covered with a Teflon tube and insulated to avoid galvanic corrosion between the bolt and the test piece. Also,
Bare materials were used for all test pieces.

The test environment was 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 FIG. 1, the steel according to the present invention has a significantly longer time until cracking occurs in any of the tests, indicating that the steel of the present invention has excellent delayed fracture resistance. You.

Example 2 A high-strength steel sheet was manufactured using steel having the chemical components shown in Table 4 under the conditions shown in Tables 4 and 5. Table 5 shows the strength and delayed fracture resistance of the obtained steel sheet. Evaluation of the delayed fracture resistance was performed in the same manner as in Example 1.

[0045]

[Table 4]

[0046]

[Table 5]

[0047]

As described above, the ultra-high-strength steel sheet according to the present invention has a tensile strength of 1180 MPa or more, and at the same time, has excellent resistance to delayed fracture. It can be suitably used for reducing the weight of a vehicle bumper or door reinforcing member.

[Brief description of the drawings]

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

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C22C 38/58 C22C 38/58 (72) Inventor Haruo Tomari 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Steel, Ltd. Kakogawa Works (72) Inventor Gohiro Nakajima 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Steel, Ltd.Kakogawa Works (72) Inventor Fukuta Tanaka 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Kobe Steel Works, Kakogawa Works (56 References JP-A-6-336640 (JP, A) JP-A-55-31123 (JP, A) JP-A-6-306543 (JP, A) JP-A-61-96059 (JP, A) 58-133351 (JP, A) JP-A-7-3383 (JP, A) JP-A-57-107064 (JP, A) JP-A-7-90488 (JP, A) JP-A-1-92317 (JP, A) A) JP-A-4-333524 JP, A) JP flat 5-140652 (JP, A) (58 ) investigated the field (Int.Cl. ^7, DB name) C22C 38/00 - 38/60 C21D 8/02 C21D 9/46

Claims (20)

(57) [Claims] C. 0.08 to 0.30% by weight, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less, and P 0.03 to 0.3% by weight%. 15%, 0.10 to 1.00% Cu, and 0.10 to 4.00% Ni, with the balance being iron and unavoidable impurities, at least one of martensite, tempered martensite, and bainite structure 4 by volume ratio
An ultra-high-strength steel sheet containing not less than 0% and having excellent delayed fracture resistance having a strength of not less than 1180 MPa. 2. The ultra-high strength steel sheet according to claim 1, further comprising: 0.01 to 0.50% of Ti and 0.10 to 5.00% of Cr.
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of: 3. The ultra-high strength steel sheet according to claim 1, further comprising: 0.05 to 2.00% of Al, 0.05 to 1.00% of W, and 0.10 to 5.00% of Co.
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of: 4. The ultra-high strength steel sheet according to claim 1, further comprising: 0.001 to 0.100% of La, 0.001 to 0.100% of Ce, and 0.1% of misch metal.
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of 001 to 0.100%. 5. The ultra-high-strength steel sheet according to claim 1, further comprising: (a) 0.01 to 0.50% of Ti and 0.10% of Cr.
At least one element selected from the group consisting of 5.00%, (b) Al 0.05 to 2.00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of: 6. The ultrahigh-strength steel sheet according to claim 1, further comprising: (a) 0.01 to 0.50% of Ti and 0.10% of Cr.
At least one element selected from the group consisting of 5.00%, (b) 0.001 to 0.100% of La, 0.001 to 0.100% of Ce, and 0.1% of misch metal;
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of 001 to 0.100%. 7. The ultra-high-strength steel sheet according to claim 1, further comprising: (a) 0.05 to 2.00% of Al, 0.05 to 1.00% of W, and 0.10 to 5% of Co. .00%
At least one element selected from the group consisting of:
La 0.001 to 0.100%, Ce 0.001 to 0.100%, and misch metal 0.1.
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of 001 to 0.100%. 8. The ultrahigh-strength steel sheet according to claim 1, further comprising: (a) 0.01 to 0.50% of Ti and 0.10% of Cr.
At least one element selected from the group consisting of 5.00%, (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 0.100%, Ce 0.001 to 0.100%, and misch metal 0.1.
An ultra-high-strength steel sheet containing at least one element selected from the group consisting of 001 to 0.100%. 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. The ultra-high-strength steel sheet according to claim 1, further comprising Ca added in the range of 0.001 to 0.1%. C. 0.08 to 0.30% by weight, Si less than 1.0%, Mn 1.5 to 3.0%, S 0.010% or less, P 0.03 to 0.3% by weight%. 15%, Cu 0.10 to 1.00%, and Ni 0.10 to 4.00%
, A steel slab consisting of iron and unavoidable impurities is heated to a temperature of 1100 ° C or higher, and is rolled at a temperature of 600 ° C or lower. 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.
Slowly 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
A tempering treatment of heating for 1 to 20 minutes at a temperature in the range of 0 to 400 ° C., containing at least 40% by volume of at least one of martensite, tempered martensite and bainite structure; A method for producing an ultra-high strength steel sheet having excellent delayed fracture resistance of 1180 MPa or more. 12. The steel slab further comprises: 0.01 to 0.50% Ti and 0.10 to 5.00% Cr.
The method for producing an ultra-high-strength steel sheet according to claim 11, comprising at least one element selected from the group consisting of: 13. The steel slab further comprises: 0.05 to 2.00% Al, 0.05 to 1.00% W, and 0.10 to 5.00% Co.
The method for producing an ultra-high-strength steel sheet according to claim 11, comprising at least one element selected from the group consisting of: 14. The steel slab further comprises: 0.001 to 0.100% La, 0.001 to 0.100% Ce, and 0.1% of misch metal.
The ultra-high-strength steel sheet according to claim 11, comprising at least one element selected from the group consisting of 001 to 0.100%. 15. The steel slab further comprises: (a) Ti 0.01 to 0.1;
At least one element selected from the group consisting of 50% and Cr 0.10 to 5.00%, and (b) Al 0.05 to 2.
00%, W 0.05 to 1.00%, and Co 0.10 to 5.00%
The method for producing an ultra-high-strength steel sheet according to claim 11, comprising at least one element selected from the group consisting of: 16. The steel slab further comprises: (a) Ti 0.01 to 0.1;
At least one element selected from the group consisting of 50% and Cr 0.10 to 5.00%, and (b) La 0.001 to
0.100%, Ce 0.001 to 0.100%, and misch metal
The method for producing an ultra-high-strength steel sheet according to claim 11, comprising at least one element selected from the group consisting of 001 to 0.100%. 17. The steel slab further comprises: 0.05 to 2.00% Al, 0.05 to 1.00% W, and 0.10 to 5.00% Co.
At least one element selected from the group consisting of: 0.001 to 0.100% of La, 0.001 to 0.100% of Ce, and 0.1% of misch metal.
The method of manufacturing an ultra-high-strength steel according to claim 11, comprising at least one element selected from the group consisting of 001 to 0.100%. 18. The steel slab further comprises: (a) Ti 0.01 to 0.1;
At least one element selected from the group consisting of 50%, Cr 0.10 to 5.00%, Al 0.05 to 2.00%, W 0.05 to 1.00%, and Co 0.005%; 10-5.00%
At least one element selected from the group consisting of: 0.001 to 0.100% of La, 0.001 to 0.100% of Ce, and 0.1% of misch metal.
The method for producing an ultra-high-strength steel sheet according to claim 11, comprising at least one element selected from the group consisting of 001 to 0.100%. 19. A steel slab having a C content of 0.12 to 0.2.
The method for producing an ultra-high strength steel sheet according to any one of claims 11 to 18, wherein the content is 0%. 20. In a steel slab, Ca is 0.001 to 0.00.
The method for producing an ultra-high strength steel sheet according to any one of claims 11 to 19, wherein 1% is added.