JP3330207B2 - Ultra-high strength cold rolled steel sheet with excellent hydrogen embrittlement resistance and method for producing 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 a method for producing an ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and a tensile strength of 980 to 1670 N / mm ² , and particularly to a method such as press forming or roll forming. The present invention relates to a method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance characteristics after receiving the same. Specifically, for example, bumpers for automobiles, reinforcing members for doors, and the like are required to be lightweight and resistant. Suitable for the production of ultra-high strength steel sheet suitable for use.

[0002]

^2. Description of the Related Art As automobiles have become lighter in weight, ultrahigh-strength steel sheets of 980 N / mm ² or more have been press-formed or roll-formed on strength members such as bumpers and door impact beams. In many cases, it has been adopted in a pipe shape.

[0003] Hydrogen embrittlement occurs in ultra-high-strength steels of 980 N / mm ² or more, for example, in the textbook of the Japan Screw Association “Design and Practice of Screw Fastening” training course (October 1990).
18). Therefore, even in an ultra-high strength thin steel sheet, it is conceivable that hydrogen generated by a corrosion reaction in an atmospheric environment enters the steel sheet and is suddenly brittlely broken during use. Furthermore, in the case of ultra-high strength thin steel sheets, since they are used as molded products by processing such as press forming or roll forming, there is the existence of extremely high residual stress, and the occurrence of voids and cracks in the steel sheet structure due to strong processing. It is considered that hydrogen embrittlement is more likely to occur.

[0004] However, conventional ultra-high strength thin steel sheets for bumper and door reinforcing members are disclosed in, for example, Japanese Patent Application Laid-Open No. H4-2680.
No. 16, JP-A-4-365814,
It mainly aims at improving workability, baking hardenability, and crushing properties 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 a degree that is found in -268053 and the like.

[0005] The method described in JP-A-4-268053 is a method in which Si is added to steel to suppress the intrusion of hydrogen atoms into the steel sheet to thereby prevent the occurrence of hydrogen embrittlement. . However, in actuality, it is not enough to prevent hydrogen embrittlement by merely suppressing the intrusion of hydrogen atoms into the steel sheet by adding Si, and the steel sheet itself is unlikely to be brittle against the invading hydrogen. It is important to keep the structure and components.

In order to examine the structure and composition of steel from the viewpoint of preventing hydrogen embrittlement, in the field of strip steel, for example, as described in JP-A-60-155644, a martensitic structure is reduced to 400 ° C. Tempering, Fe
There is known a method of sufficiently precipitating and preventing a -C-based compound.

[0007] However, such a steel is inferior in workability, unlike a thin steel sheet subjected to processing such as press forming or roll forming. Further, as described above, no consideration is given to the point that hydrogen embrittlement is likely to occur due to the presence of residual stress or the occurrence of voids and cracks in the steel sheet structure due to heavy working. In addition, ultra-high-strength steel sheets usually manufactured by the continuous annealing method, after soaking a steel having a relatively low C and Mn content, are cooled at a relatively high cooling rate to below the martensitic transformation point and tempered at 400 ° C or less. However, this manufacturing method is completely different from the method known for bar steel, and requires different measures from the field of bar steel.

The present invention solves the above-mentioned problems of the prior art, and provides an ultra-high-strength steel sheet for processing having an ultra-high strength of 980 to 1670 N / mm ² and excellent hydrogen embrittlement resistance and its production. It is intended to provide a way.

[0009]

Means for Solving the Problems As means for solving the above problems, the present invention provides C: 0.05 to 0.17%,
Si: 0.2 to 1.5%, Mn: 1.7 to 3.5%, P
≦ 0.030%, S ≦ 0.010%, Al: 0.025
０．１0.120%, N ≦ 0.0100%, Ti: 3.43
× N-0.150%, and if necessary, Mo ≦
A tensile strength of 980 to 1670 N / mm ² , characterized in that the balance comprises iron and unavoidable impurities, including 1.0%.
The gist is an ultra-high strength cold rolled steel sheet having excellent hydrogen embrittlement resistance.

[0010] In addition, the method of producing the steel sheet is such that when a steel slab having the above-mentioned chemical composition is hot-rolled by a conventional method, pickled, cold-rolled and continuously annealed, the steel is soaked at an Ac temperature of _{3 to} 1000 ° C. After that, the temperature was gradually cooled, and the temperature was changed from Ar ₃ points −30 ° C. or more to 7 °
It is characterized by cooling to a Ms point or less at a cooling rate of 0 ° C./sec or more to cause martensitic transformation, and thereafter reheating or performing a tempering treatment at 150 to 300 ° C. for 1 to 15 minutes. .

[0011]

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

C: C is an element which forms martensite and is indispensable for increasing the strength. In order to obtain a strength of 980 N / mm ² or more, C is required.
5% or more is necessary, but when it is increased, hydrogen embrittlement is likely to occur. In the present invention, in particular, the upper limit is set to 0.17% in consideration of workability and spot weldability when obtaining the required ultra-high strength. However, in order to obtain a tensile strength of 980 to 1670 N / mm ² , the amount of C Is enough.

Si: Si is an element effective for strengthening steel without deteriorating ductility, and is 0.2% in the present invention.
Add the above. However, when the content 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, so that the content is specified to 1.5% or less.

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

P: P is an effective element for strengthening the steel and increasing the ductility. However, P is set to 0.030% or less because it is liable to segregate at the grain boundaries and easily become brittle.

S: Since S forms inclusions and deteriorates bending workability and the like, S is suppressed to 0.010% or less.

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

N: N enters from the atmosphere at the time of steel making without any particular addition. When N is present in a solid solution state in steel, it significantly promotes embrittlement. Therefore, in the present invention, N is rendered 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.

Ti: On the other hand, since Ti aims to precipitate and fix N, it is necessary to add at least the equivalent of N (3.43 × N) or more. If it is too much, it is necessary to strengthen the steel. Since C forms a precipitate and softens, the upper limit is defined as 0.150%.

Mo: Mo is not only an element effective for improving the hardenability of steel and stably obtaining martensite in continuous annealing equipment, but also has the effect of strengthening grain boundaries and suppressing the occurrence of hydrogen embrittlement. , If necessary. When adding
If the content exceeds 1.0%, the effect is saturated, so the content is set to 1.0% or less.

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

The steel slab having the above-mentioned chemical components is manufactured by a conventional method by continuous casting or an ingot-making method and is subjected to hot rolling, 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 normal temperature, re-heated, or inserted into a heating furnace at a high temperature. Also, there is no particular problem even if rolling is performed directly after casting. The hot rolling may be finished at a temperature equal to or higher than the Ar ₃ transformation point, and the cooling conditions and the winding temperature thereafter are not particularly limited. For example, cooling is performed at an average of 30 to 100 ° C. as usual.
The winding may be performed at 750 to 400 ° C. within the range of sec.

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

The soaking in continuous annealing is 1000 or more at the transformation point of Ac ₃
It is necessary to carry out at a temperature of not more than ° C. At a temperature lower than the Ac ₃ transformation point, a ferrite structure is formed during the soaking process, and it is difficult to secure strength. Although the existence of a ferrite structure is advantageous from the viewpoint of hydrogen embrittlement, it is actually difficult to control the formation of a predetermined amount of ferrite in accordance with the chemical composition in the soaking process. It is easier to do it. On the other hand, even if it is heated at a temperature exceeding 1000 ° C., there is no particular problem as long as it is an austenitic single phase structure, but if it is unnecessarily heated to a high temperature, the crystal grain size becomes large and the cost increases. Upper limit.

After soaking, the temperature is gradually cooled to a rapid cooling start temperature, and then rapid cooling is started. The slow cooling rate is, for example, 1 to 30 ° C./se.
c is fine. The quenching start temperature is basically a temperature at which austenite single phase structure is formed and martensite is formed to secure a predetermined strength. Therefore, the quenching start temperature is desirably equal to or higher than the Ar ₃ transformation point.
From the viewpoint of hydrogen embrittlement, it is more advantageous to have a ferrite structure, so that there is no problem even if a small amount of ferrite structure is formed as long as the strength can be ensured. Therefore, the quenching start temperature is limited to the Ar ₃ transformation point of −30 ° C. Note that the small amount of ferrite structure here is 20% in area ratio.
% Or less. If the rapid cooling rate is 70 ° C./sec or more, a low-temperature transformation product can be obtained, and therefore the lower limit is set. The cooling method is not limited, such as water quenching, water-cooled roll cooling, and gas-water cooled gas jet cooling.

The quenching is carried out at the martensitic transformation start temperature (Ms).
Point) to the following, and then at 150 to 300 ° C for 1 to 1
The strength is adjusted to a predetermined value by performing a tempering process for 5 minutes. At this time, if the quenching end temperature is within the tempering treatment temperature range, the temperature may be kept constant at that temperature, and if lower than the tempering treatment temperature, reheating may be performed. If the tempering treatment time is not more than 1 min, the effect is hardly recognized. On the other hand, if the tempering treatment time is longer than 15 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
The present inventors have confirmed that if the temperature exceeds 0 ° C., relatively coarse carbides are precipitated, and a grain boundary fracture occurs in a short time in a hydrogen embrittlement test under a corrosive environment such as air and salt water spray. Although the reason is not clear, it is considered that voids are formed at the interface between the carbide and the base material during the forming process, and hydrogen atoms gather there to increase stress concentration and lead to crack generation. Therefore, the upper limit of the tempering temperature is 30.
0 ° C.

After annealing, if necessary, temper rolling may be performed to improve the shape, and plating with zinc or the like does not cause any problem.

Next, an embodiment of the present invention will be described.

[0029]

【Example】

After heating steel having the chemical composition shown in Table 1 to 1200 ° C., it was hot-rolled to a thickness of 3.0 mm and wound at 480 ° C. After pickling, the sheet was cold-rolled to a sheet thickness of 1.8 mm and continuously annealed under the conditions shown in Table 2. After subjecting to a 0.3% temper rolling, the mechanical properties and the hydrogen embrittlement resistance were investigated. Table 3 shows the results.

Regarding the hydrogen embrittlement resistance, 30 mm × 15
A 0 mm strip test piece is U-bend-formed at a bending radius of 9R, and squeezed until the distance between the plates becomes 2R (= 18 mm).
After applying an electrodeposition coating of μm, a slit was made at the top of the bend with a cutter, and 0.5 mol / l sulfuric acid + 0.0001 mol
Using a potentiostat in a 1 / liter KSCN solution, a potential 600 mV lower than the natural potential was applied, and the evaluation was made based on the time when cracks occurred.

As is clear from Table 3, the present invention example (N
o. 1, 2, 3, 6, 8) are 980 to 1670 N / mm
^It shows a tensile strength of ² and good workability, and the time to crack generation is as long as 1000 sec or more, and the hydrogen embrittlement resistance is excellent. On the other hand, the comparative examples (Nos. 4, 5,
7) is that the annealing conditions are out of the range of the present invention, and
In Comparative Examples (Nos. 10 and 12), the chemical components are out of the range of the present invention, and the area ratios of the ferrites are too high, and the predetermined strength cannot be secured. On the other hand, the comparative example (N
o. In Nos. 9 and 11), the chemical components are out of the range of the present invention, the time until crack generation is as short as 300 to 500 sec, and the difference in hydrogen embrittlement resistance from the present invention example is apparent.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

As described above in detail, the cold-rolled steel sheet according to the present invention has a tensile strength and workability of 980 to 1670 N / mm ^{2 which} are optimal as strength members such as bumpers and door impact beams for automobiles. In addition, it has excellent resistance to hydrogen embrittlement that becomes a problem during use, and the above strength member,
Excellent effect for weight reduction of reinforcing members.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-131327 (JP, A) JP-A-63-38520 (JP, A) JP-A-6-220576 (JP, A) JP-A-7- 90488 (JP, A) JP-A-51-88420 (JP, A) JP-A-4-99227 (JP, A) Nagataki et al. Strength and steel composition affecting delayed fracture resistance of ultra-high strength cold rolled steel sheet , Materials and Processes, Japan, March 2, 1993, Vol. 6, No. 3, p. 668 (58) Fields surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02-8/04 C21D 9/46-9/48

Claims (3)

(57) [Claims] C. 0.05 in% by weight (hereinafter the same).
0.17%, Si: 0.2-1.5%, Mn: 1.7
３3.5%, P ≦ 0.030%, S ≦ 0.010%, A
l: 0.025 to 0.120%, N ≦ 0.0100%,
Ti: contains 3.43 × N to 0.150%, with the balance being iron and unavoidable impurities, and having a tensile strength of 980 to 1670 N / mm ² and an ultra-high strength excellent in hydrogen embrittlement resistance. Cold rolled steel sheet. 2. The cold-rolled steel sheet according to claim 1, further comprising Mo ≦ 1.0%. 3. A hot rolling by a conventional method a steel slab having a chemical composition according to claim 1 or 2, pickled, when continuous annealing and cold rolling, soaking in Ac ₃ point to 1000 ° C. After heating, it is gradually cooled, and is cooled from a temperature of Ar ₃ point −30 ° C. or more to 7 ° C.
Cooling to below the Ms point at a cooling rate of 0 ° C / sec or more,
A martensitic transformation is caused, and thereafter, reheating or tempering at 150 to 300 ° C. for 1 to 15 minutes is performed, and the tensile strength is 980 to 167.
A method for producing an ultra-high strength cold rolled steel sheet excellent in hydrogen embrittlement resistance at 0 N / mm ² .