JPH0790488A - Ultrahigh strength cold rolled steel sheet excellent in hydrogen brittlement resistance and its production - Google Patents

Abstract

PURPOSE:To produce an ultrahigh strength thin steel sheet for working having ultrahigh strength and excellent in hydrogen embrittlement resistance and to provide the producing method therefor. CONSTITUTION:This ultrahigh strength cold rolled steel sheet excellent in hydrogen embrittlement resistance is the one having a compsn. contg. 0.18 to 0.30% C, 0.2 to 1.5% Si, 1.7 to 3.5% Mn, <=0.010% P, <=0.005% S, 0.025 to 0.120% Al, <=0.0100% N, 3.43XN to 0.150% Ti and <=1.0% Mo, and the balance iron with inevitable impurities and having 1370 to 1960N/mm<2> class tensile strength. The steel slab having the same chemical components is produced, at the time of executing hot rolling, pickling, cold rolling and continuous annealing by the conventional methods, by soaking it at the Ac3 point to 1000 deg.C, thereafter gradually cooling it and executing cooling from the Ar3 point or above to the Ms point or below at >=70 deg.C/sec cooling rate to generate martensitic transformation, and, after reheating or as it is, executing tempering treatment at 150 to 300 deg.C for 1 to 20min.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an ultrahigh strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and a tensile strength of 1370 to 1960 N / mm ² grade, and in particular, processing such as press forming or roll forming. The present invention relates to a method for producing an ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance after being subjected to, specifically, for example, a bumper of a car, a reinforcing member for a door, and the like. It is suitable for manufacturing ultra-high strength steel sheets suitable for various applications.

[0002]

^2. Description of the Related Art As automobiles have become lighter in weight, strength members such as bumpers and door impact beams are press-formed with ultra-high-strength steel sheets of 980 N / mm ² or more, or roll-formed. Due to this, there are many cases where it is adopted in the form of a pipe.

Hydrogen embrittlement occurs in ultra-high-strength steel of 980 N / mm ² or more. For example, the textbook of the Japan Screw Association “Design and Practice of Screw Tightening” training course (1990, 10).
(18th of a month). Therefore, even in an ultra-high-strength thin steel sheet, hydrogen generated by a corrosion reaction in the atmospheric environment may enter the steel sheet and suddenly break brittlely during use. Furthermore, in the case of ultra-high-strength thin steel sheet, since it is used as a molded product by processing such as press forming or roll forming, there is the presence of remarkably high residual stress and the occurrence of voids and cracks in the steel sheet structure due to heavy working. It is considered that hydrogen embrittlement is more likely to occur.

However, the conventional ultra-high-strength thin steel plates for reinforcing members of bumpers and doors are disclosed in, for example, Japanese Patent Laid-Open No. 4-2680.
No. 16 and Japanese Patent Laid-Open No. 4-365814,
It is mainly aimed at improving workability, bake hardenability, and crushing property as a molded product, and it is very rare to take measures against the problem of hydrogen embrittlement that is expected to occur during the use process. It is about the same level as that disclosed in Japanese Patent No. 268053.

The method described in Japanese Patent Laid-Open No. 268053/1992 is a method of preventing hydrogen embrittlement by adding Si to the steel and suppressing the entry of hydrogen atoms into the steel sheet. . However, in actuality, in order to prevent hydrogen embrittlement, it is not enough to suppress the entry of hydrogen atoms into the steel sheet by adding Si, and the steel sheet itself is less likely to be brittle with respect to the invading hydrogen. It is important to keep it as a tissue or ingredient.

Regarding the study of the structure and composition of steel from the viewpoint of preventing hydrogen embrittlement, in the field of bar steel, for example, as described in JP-A-60-155644, a martensite structure of 400 ° C. is used. Above tempering, Fe
A method for sufficiently precipitating a -C compound to prevent it is known.

However, such a steel is inferior in terms of workability, unlike a thin steel sheet which is processed by press forming or roll forming. Further, as described above, no consideration is given to the fact that hydrogen embrittlement is likely to occur due to the presence of residual stress and the occurrence of voids and cracks in the steel sheet structure due to heavy working. In addition, the ultra-high-strength steel sheet, which is usually produced by the continuous annealing method, after soaking of steel having a relatively low C and Mn content, is cooled to a temperature below the martensitic transformation point at a relatively high cooling rate and tempered at 400 ° C or less. However, this manufacturing method is a completely different manufacturing method from the method known for bar steel, and requires measures different from those in the field of bar steel.

The present invention solves the problems of the prior art described above, and has an ultrahigh strength steel sheet having a tensile strength of 1370 to 1960 N / mm ^{2 and} an excellent hydrogen embrittlement resistance, and an ultrahigh strength thin steel sheet for working thereof. It is intended to provide a manufacturing method.

[0009]

[Means for Solving the Problems] As means for solving the above problems, the present invention provides C: 0.18 to 0.30%, S
i: 0.2-1.5%, Mn: 1.7-3.5%, P ≦ 0.0
10%, S ≦ 0.005%, Al: 0.025 to 0.120
%, N ≦ 0.0100%, Ti: 3.43 × N to 0.150
%, Mo ≦ 1.0%, the balance consisting of iron and unavoidable impurities, and a tensile strength of 1370 to 196.
The gist is ultra-high-strength cold-rolled steel sheet with 0N / mm ² grade and excellent hydrogen embrittlement resistance.

Further, in the manufacturing method, a steel slab having the above chemical composition is hot-rolled by a conventional method, pickled, cold-rolled and continuously annealed, and soaked at Ac ₃ point to 1000 ° C. After that, slowly cool it to 70 ℃ / sec from the temperature of Ar ₃ points or more.
It is characterized in that it is cooled to the Ms point or less at the above cooling rate to cause martensitic transformation, and thereafter, reheated or as it is, tempered at 150 to 300 ° C. for 1 to 20 minutes.

[0011]

The present invention will be described in more detail below. First, the reasons for limiting the chemical composition of steel in the present invention will be described.

C: C is an essential element for forming martensite and increasing the strength, and 0.18% or more is necessary to obtain a strength of 1370 N / mm ² or more, but if it increases, it becomes hydrogen embrittlement. Is likely to occur. Particularly in the present invention, the upper limit is set to 0.18% in consideration of workability and spot weldability when obtaining the necessary ultrahigh strength.

Si: Si is an element effective for strengthening steel without deteriorating ductility, and in the present invention, 0.2%.
Add the above. However, if it exceeds 1.5%, not only the effect is saturated, but also there is a problem that the load of the rolling mill in the cold rolling becomes large, and therefore it is specified to be 1.5% or less.

Mn: Mn is an element that enhances the hardenability of steel.
In order to obtain a martensitic structure stably with a continuous annealing equipment, 1.7% or more is required. However, if it exceeds 3.5%, not only the effect is saturated, but also segregation becomes large, the structure becomes nonuniform, and the workability deteriorates.
The upper limit is 5%.

P: P is an element effective for strengthening the steel and increasing the ductility, but it is easily segregated at the grain boundaries and easily causes embrittlement, so the content is made 0.010% or less.

S: S forms inclusions and deteriorates bending workability, so is suppressed to 0.005% or less.

Al: Al is added in an amount of 0.025% or more for deoxidation. From the viewpoint of surface texture, the upper limit is 0.1.
It is specified as 20%.

N: N penetrates from the atmosphere during steelmaking without any particular addition. This N significantly promotes embrittlement when it exists in a solid solution state in steel. Therefore, in the present invention, N is made harmless as a precipitate by adding Ti. However, if N exceeds 0.0100%, the amount of Ti that needs to be added increases, which not only increases the cost but also deteriorates the workability. Therefore, it is suppressed to 0.0100% or less.

Ti: On the other hand, Ti has the purpose of precipitating and fixing N, so it is necessary to add at least the equivalent of N (3.43 × N), but if it is too much, it is necessary for strengthening the steel. Since it forms a precipitate with C and softens, its upper limit is defined as 0.150%.

Mo: Mo is not only an element effective for enhancing the hardenability of steel and stably obtaining martensite in the continuous annealing equipment, but also has the effect of strengthening the grain boundaries and suppressing the occurrence of hydrogen embrittlement. . However, if it exceeds 1.0%, the effect is saturated, so the content is made 1.0% or less.

Next, the manufacturing method of the present invention will be described.

The steel slab having the above chemical components is manufactured by a continuous casting method or an ingot manufacturing method by a conventional method and hot rolling is performed, but the manufacturing conditions are not particularly limited. In hot rolling, it is necessary to heat to a temperature equal to or higher than a predetermined rolling temperature, but after casting, once cooled to around room temperature, reheated or inserted into a heating furnace at a high temperature. Also, there is no particular problem even if it is directly rolled after casting. The hot rolling may be finished at a temperature not lower than the Ar ₃ transformation point, and the cooling conditions and the winding temperature after that are not particularly limited. As in the usual method, for example, cooling is 30 to 100 ° C. /
The winding may be performed at 750 to 400 ° C. within the range of sec.

After hot rolling, pickling and cold rolling are performed, and the cold rolling rate may be, for example, about 25 to 70%. Next, continuous annealing is performed to obtain a steel plate having a predetermined strength, and the continuous annealing is specified under the following conditions.

The soaking in continuous annealing is not less than the Ac ₃ transformation point of 1000.
It is necessary to carry out below ℃. At a temperature below the Ac ₃ transformation point, a ferrite structure is formed during the soaking process, making it difficult to secure strength. On the other hand, even if heated at a temperature of over 1000 ° C, there is no particular problem as long as it has an austenite single-phase structure. However, even if it is unnecessarily heated at a high temperature, the crystal grain size becomes large and the cost is increased. The upper limit.

After soaking, the temperature is gradually cooled to the quenching start temperature, and then quenching is started. The rate of slow cooling is, for example, 1 to 30 ° C / se
c is all right. The quenching start temperature is basically a temperature at which the austenite single-phase structure is formed and martensite is generated to secure a predetermined strength. Therefore, the quenching start temperature is set to the Ar ₃ transformation point or higher. Below this, ferrite is generated and it becomes difficult to secure strength. If the rapid cooling rate is 70 ° C./sec or more, a low temperature transformation product can be obtained, so this is the lower limit. The cooling method may be water quenching, water cooling roll cooling, steam cooling, gas jet cooling, or the like.

The quenching is carried out at the martensitic transformation start temperature (Ms
Point) Do the following, and then 1-2 at 150-300 ° C
By performing a tempering process for 0 min, the strength is adjusted to a predetermined level. At this time, if the quenching end temperature is within the tempering temperature range, the temperature may be kept constant at that temperature, and if it is lower than the tempering temperature, reheating may be performed. If the tempering time is not longer than 1 min, the effect is hardly recognized, while if it is longer than 20 min, the equipment becomes huge, so this is defined as the upper limit. If the tempering temperature is lower than 150 ° C., there is almost no effect, so this is the lower limit. On the other hand, 30
It has been confirmed by the present inventors that, when the temperature exceeds 0 ° C., relatively coarse carbide precipitates, and grain boundary fracture occurs in a short time in a hydrogen embrittlement test in a corrosive environment such as air and salt spray. The reason for this is not clear, but it is considered that voids are generated at the interface with the base material during the molding process, hydrogen atoms are gathered there, stress concentration is increased, and cracks are generated. Therefore, the tempering temperature is limited to 300 ° C.

After annealing, if necessary, temper rolling may be performed to improve the shape, and zinc plating or the like may be performed without any problem.

Next, examples of the present invention will be described.

[0029]

【Example】

After heating the steel having the chemical composition shown in Table 1 to 1200 ° C., it was hot rolled to a plate thickness of 2.8 mm and wound at 560 ° C. After pickling, the sheet was cold-rolled to a plate thickness of 1.8 mm and continuously annealed under the conditions shown in Table 2. After subjecting to 0.3% temper rolling, the mechanical properties and hydrogen embrittlement resistance were investigated. The results are shown in Table 3.

Regarding hydrogen embrittlement resistance, 30 mm × 15
A 0 mm strip test piece was U-bent with a bending radius of 12 R, and was narrowed down to a plate spacing of 2 R (= 24 mm), and a film thickness of 2 on the surface
After applying electrodeposition coating of 0 μm, slit the bending top with a cutter and add 0.5 mol / l sulfuric acid +0.0001 m.
Using a potentiostat in an ol / liter KSCN solution, an electric potential 600 mV lower than the natural potential is applied.
It was evaluated by the time when cracks occurred.

As is clear from Table 3, the invention examples (No.
1, 2, 3, 6, 8) show a tensile strength of 1370 to 1960 N / mm ² and a good workability, and the time until cracking is 700 sec or more and the hydrogen embrittlement resistance is excellent.

On the other hand, Comparative Examples (No. 4, 5, 7)
, The annealing condition is out of the range of the present invention, the ferrite area ratio becomes too high, and the predetermined strength cannot be secured. Further, in Comparative Examples (Nos. 10 and 11), the chemical components are out of the range of the present invention, and the predetermined strength cannot be secured.
On the other hand, the comparative examples (Nos. 9, 12, 13, and 14) are annealing conditions,
Either of the chemical components is out of the range of the present invention, and the time until cracking is as short as 200 to 400 sec, and the difference in hydrogen embrittlement resistance from the examples of the present invention is clear.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

As described in detail above, the cold-rolled steel sheet according to the present invention has a tensile strength and workability of 1370 to 1960 N / mm ² grade, which is optimal as a strength member for automobile bumpers and door impact beams. In addition, it has excellent resistance to hydrogen embrittlement, which is a problem during use, and exerts an excellent effect in reducing the weight of the above-mentioned strength member, reinforcing member, and the like.

Claims (2)

[Claims] 1. C .: 0.18% by weight (hereinafter the same)
0.30%, Si: 0.2-1.5%, Mn: 1.7-3.5
%, P ≦ 0.010%, S ≦ 0.005%, Al: 0.02
5 to 0.120%, N ≦ 0.0100%, Ti: 3.43 ×
N ~ 0.150%, Mo ≤ 1.0%, the balance is composed of iron and unavoidable impurities, the tensile strength is 1
Ultra high-strength cold-rolled steel sheet of 370 to 1960 N / mm ² grade with excellent hydrogen embrittlement resistance. 2. A steel slab having the chemical composition according to claim 1 is hot-rolled by a conventional method, pickled, cold-rolled and continuously annealed, and is soaked at Ac ₃ point to 1000 ° C. After that, it is gradually cooled and cooled from a temperature of Ar ₃ or higher to a temperature of Ms or lower at a cooling rate of 70 ° C./sec or higher to cause martensite transformation, and thereafter, reheated or as it is, 15
A method for producing an ultra-high strength cold-rolled steel sheet having a tensile strength of 1370 to 1960 N / mm ² and excellent hydrogen embrittlement resistance, characterized by performing a tempering treatment at 0 to 300 ° C. for 1 to 20 minutes.