JP2826058B2 - Ultra-high strength thin steel sheet without hydrogen embrittlement and manufacturing method - 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 thin steel sheet suitable for applications requiring light weight and high strength, such as bumpers for automobiles and reinforcing members for doors, and a method for producing the same.

[0002]

^2. Description of the Related Art The weight of automobiles has been reduced due to the proposal of CAFE (Corporate Average Fuel Economy) in the United States to strengthen regulations, and bumpers and door reinforcing members have a height of 980 N / mm ² or more. High strength steel sheets have been adopted.

[0003] Hydrogen embrittlement occurs in ultra-high-strength steel of 980 N / mm ² or more, for example, according to the Japan Screw Association.
(October 18, 1990) Training text "Design and practice of screw fastening". 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 destroyed during use.

With respect to hydrogen embrittlement of ultra-high strength thin steel sheets, as described in Japanese Patent Application Laid-Open No. Hei 4-268053, it has been proposed to add Si to steel to suppress intrusion of hydrogen atoms into the steel sheet. Have been. However, the state of rust changes variously depending on the corrosive environment, and it is difficult to sufficiently suppress the intrusion of hydrogen atoms into the steel sheet by Si addition and prevent hydrogen embrittlement.

[0005] In the field of steel bars in which there is a report on prevention of hydrogen embrittlement of steel, for example, Japanese Patent Application Laid-Open No. 60-155644.
As described in the issue, the martensitic structure is 40
There is known a method of tempering at 0 ° C. or higher to sufficiently precipitate and prevent the Fe—C compound. However, such a steel is inferior in workability as compared with an ultra-high strength thin steel sheet that is processed by press forming or the like. Further, it is considered that hydrogen embrittlement is likely to occur in an ultra-high strength thin steel sheet due to an increase in strength due to cold working, but this property is not considered.

[0006] Normally, ultra-high strength thin steel sheets produced by the continuous annealing method have relatively low C and Mn contents in order to ensure cold workability such as press forming, and are tempered at 400 ° C or more. In such a case, the strength is reduced and the desired characteristics cannot be obtained.
For this reason, it is manufactured by cooling to a temperature of 400 ° C. or lower at a predetermined cooling rate after soaking or by tempering at a temperature of 400 ° C. or lower after once cooling to room temperature, which is completely different from the method known for strip steel.

The present invention solves the above-mentioned problems of the prior art in an ultra-high-strength thin steel sheet having a tensile strength of 980 N / mm ² or more, and provides an ultra-high-strength thin steel sheet for processing which does not cause hydrogen embrittlement; It is an object of the present invention to provide a method of manufacturing the same.

[0008]

Means for Solving the Problems As means for solving the above problems, the present invention relates to a method for producing C: 0.05 to 0.25%, M:
n: 1.0 to 3.0%, Al: 0.025 to 0.100%,
S: 0.01% or less, N: 0.008% or less, and if necessary, Si: 3.0% or less, P: 0.1% or less,
Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less, and / or Ti: 0.2% or less, Nb:
0.1% or less, V: 0.1% or less, Zr: 0.1% or less 1
The ultrahigh-strength thin steel sheet containing at least 980 N / mm ² and having a tensile strength of 980 N / mm ² or more and containing 70% or more by volume of martensite has a composition of at least 0.1%.
3 × 10 ⁵ Fe-C based precipitates of 1 μm or more per mm ²
The gist of the present invention is an ultra-high-strength steel sheet free from hydrogen embrittlement, characterized by the following.

In another aspect of the present invention, when a steel having the above composition is hot-rolled by a conventional method, and then continuously rolled while being pickled or cold-rolled, and after soaking at a temperature of ₃ or more Ac, the steel is soaked.
Slowly cooling to 0 to 650 ° C, cooling from that temperature to 300 ° C or less at 5 ° C / sec or more, and then reheating to 300 ° C or less or tempering at 300 ° C or less for 1 to 20 minutes With a tensile strength of 980 N / mm ² or more, a martensite content of 70% or more by volume,
3 × 10 ⁵ Fe-C based precipitates of 1 μm or more per mm ²
The gist of the present invention is a method for producing an ultra-high-strength steel sheet free from hydrogen embrittlement, characterized by obtaining the following.

[0010]

The present invention will be described below in more detail. First, the reasons for limiting the carbide in the present invention will be described.

The present inventors have conducted intensive studies on hydrogen embrittlement of corrosive environments such as air and salt water in ultrahigh-strength thin steel sheets which have been subjected to cold working such as press forming and bending.

As a result, the fracture due to hydrogen embrittlement occurs from the cold-worked portion, and the steel sheet in which the fracture occurs in a short time is a fracture surface mainly due to grain boundary fracture, and is a Fe-C based material of 0.1 μm or more. More than 3 × 10 ⁵ precipitates were deposited per mm ² . On the other hand, the long steel plate breaking time is fracture transgranular fracture is mainly similar to the steel sheet hydrogen embrittlement does not occur, Fe-C based precipitates more than 0.1μm is 1 mm ² per 3 × 10 ⁵ We found that:

A large number of Fe—C-based or other carbides smaller than 0.1 μm are present in any steel sheet.
No relationship was observed with the length of time of occurrence of fracture due to hydrogen embrittlement.

As described above, when more than 3 × 10 ⁵ Fe—C based precipitates are precipitated per 1 mm ² , grain boundary fracture mainly occurs, and the fracture time due to hydrogen embrittlement is increased. It is not clear why this is shortened,
It is estimated as follows.

That is, a product of an ultra-high-strength thin steel sheet that has been subjected to processing such as press forming in which a high tensile external force exists.
When Fe-C based precipitates of 0.1 μm or more are precipitated,
High stress or voids are generated at the interface between carbide and base metal structure by cold working at the time of pressing, atomic hydrogen generated by corrosion reaction gathers at that location, further increasing stress concentration or cracking I do. Precipitates originally preferentially precipitate at grain boundaries with many lattice defects, so that Fe of 0.1 μm or more
When the amount of -C based precipitates is more than 3 × 10 ⁵ per 1 mm ² , the frequency of stress concentration at the grain boundaries or the occurrence of cracks increases,
Hydrogen embrittlement mainly due to grain boundary fracture occurs. On the other hand, 0.1 μm
Fe-C-based precipitates, which are smaller in size, are less likely to generate high stresses or voids at the interface between the carbide and the base metal structure by cold working, and thus the frequency of cracks at the grain boundaries is reduced. It is considered that hydrogen embrittlement mainly due to fracture hardly occurs.

Next, 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, 0.05% or more is required. However, 0.2
If it exceeds 5%, the workability such as bending deteriorates, so this is made the upper limit.

Mn: Mn is an element that enhances the hardenability of steel, and must be at least 1.0% in order to obtain martensite stably with continuous annealing equipment. However, if the content exceeds 3.0%, not only the effect is saturated, but also segregation increases, the structure becomes nonuniform, and the workability deteriorates. Therefore, the upper limit is set.

S: Since S forms inclusions and deteriorates bending workability and the like, S is suppressed to 0.01% or less.

N: N is specified as 0.0008% or less because N forms a solid solution in steel and deteriorates press workability and the like.

The above elements are essential components. As shown below, if necessary, one or more of the group consisting of Si, P, Cr, Mo, W, or Ti, Nb, V, Zr. One or more of the groups can be included in appropriate amounts.

Si: Si is an element effective for strengthening steel and increasing ductility, but if it exceeds 3.0%, not only its effect is saturated, but also the load in cold rolling increases. Because of the problem described above, it is stipulated below.

P: P is an element effective for strengthening steel and increasing ductility. However, if it exceeds 0.1%, embrittlement is liable to occur.

Cr, Mo: Cr and Mo are effective elements for increasing the hardenability of steel and stably obtaining martensite in a continuous annealing facility, but the effect is saturated when each exceeds 1.0%. Therefore, the upper limit is 1.0%.

W: W is an element effective for increasing the strength of steel, but if it exceeds 1.0%, the workability deteriorates.

Ti, Nb, V and Zr: Ti, Nb, V and Zr
Is an element that forms carbides, is effective for grain refinement, and is effective for strengthening steel. However, when Ti exceeds 0.2% and other elements each exceed 0.1%, cold rolling of Due to problems such as an increased load, the upper limit of Ti is 0.2% or less, and the upper limit of each of the other elements is 0.1%.

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

A steel slab having the above chemical composition is manufactured by continuous casting or ingot casting, and hot-rolled by a conventional method.
Pickling or subsequent cold rolling is performed.

In hot rolling, it is necessary to heat the material to a predetermined rolling temperature or higher.
Also, there is no particular problem if rolling is performed as it is after casting. The rolling may be performed 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, and may be in accordance with a conventional method. For example, the cooling may be performed at an average of 30 to 100 ° C./sec, and the winding temperature may be performed at 750 to 400 ° C.

Next, continuous annealing is performed. In continuous annealing, even after pickling after hot rolling, 25 to 80%
May be used. The soaking temperature in the continuous annealing is at a temperature of _three or more Ac. If the Ac is less than ₃ points, the structure becomes non-uniform due to the growth of ferrite during the soaking process, and the bending workability and the like decrease. Further, it is not desirable because it is difficult to secure the strength. After soaking, 1-3 to the quenching start temperature
Slowly cool at 0 ° C / sec. The quenching start temperature has a lower limit of 650 ° C. in order to secure a predetermined strength by setting the martensite structure volume ratio to 70% or more. However, if the temperature exceeds 850 ° C., the shape of the steel sheet deteriorates during quenching. I do. If the quenching cooling rate is 5 ° C./sec or more, a martensite structure can be obtained. The cooling method does not matter, such as water quenching, water-cooled roll cooling, steam-water cooling, and gas jet cooling. The quenching stop temperature is set to 300 ° C. or less in order to mainly have a martensite structure.

Thereafter, tempering treatment is performed at a temperature of 300 ° C. or lower for 1 to 20 minutes to adjust the strength to a predetermined value. At this time, if the quenching start 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. Tempering time is 1min
Otherwise the effect is hardly noticeable, but 20mi
If the length is longer than n, not only the effect is saturated, but also the equipment becomes huge, which is not desirable. The tempering temperature is 3
If the temperature is higher than 00 ° C., the amount of carbides becomes larger than a predetermined amount, and the fracture characteristics are deteriorated.

According to the above manufacturing method, the tensile strength is 980 N
/ Mm ² or more, contains 70% or more by volume of martensite, and contains Fe-C based precipitates of 0.1 μm or more per 3 mm ^2.
An ultra-high strength steel sheet having a size of × 10 ⁵ or less is obtained.

This ultra-high-strength steel sheet has a delayed fracture occurrence time in a hydrogen embrittlement test in a corrosive environment such as salt spray, hydrochloric acid immersion and a cathode charge test, or does not break, and is excellent in hydrogen embrittlement. Resistant.

Further, as for the structure, in order to secure a predetermined strength, it is sufficient that martensite containing the above-mentioned carbide is at least 70% by volume. Even if the martensite content is 100%, or the content of ferrite, bainite and retained austenite alone or in combination containing 30% or less, the effect of the present invention does not change at all.

It should be noted that there is no problem if the temper rolling is performed after the continuous annealing, or if plating treatment with zinc or the like is performed.

Next, examples of the present invention will be described.

【Example】

A steel having the chemical composition shown in Table 1 was heated to 1200 ° C., hot-rolled to a thickness of 3.2 mm, and wound at 560 ° C.
After pickling, the sheet was cold-rolled to a sheet thickness of 0.8 mm and continuously annealed under the conditions shown in Table 2. After subjecting to 0.3% temper rolling,
Mechanical properties such as strength and bending workability, and hydrogen embrittlement resistance were investigated. Table 1 shows the results.

Regarding the hydrogen embrittlement resistance, 30 mmw × 150
A mml strip test piece was bent at a bending radius of 12 mm, tightened until the width between the plates became 24 mm, applied with a 20 μm electrodeposition coating on the surface, and then added with 0.5 mol / l sulfuric acid + 0.0.
Using a potentiostat in a 001 mol / liter KSCN solution, a potential 550 mV lower than the natural potential was applied to evaluate the time when cracks occurred. The bending workability was evaluated by a 45 degree V bending test.

As is clear from Table 1, all of the examples of the present invention show a tensile strength of 980 N / mm ² or more and good workability, and as shown in FIG. 1, there are few carbides of 0.1 μm or more. The time until crack generation is as long as 980 sec or more, and the hydrogen embrittlement resistance is excellent.

On the other hand, steel Nos. 1, 7, and 8 of Comparative Examples
Has a chemical component deviating from the range of the present invention, cannot secure a predetermined strength, or has poor workability. In addition, steel No. 3, 6,
16, 22, 23, 24, and 25, the annealing conditions were out of the range of the present invention, the volume ratio of martensite was insufficient, and a predetermined strength could not be secured, or 0.1 μm as shown in FIG.
Many of the above carbides are present, and the time until crack generation is short and hydrogen embrittlement resistance is inferior.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

As described above, according to the present invention,
Has a car bumper and optimal 980 N / mm ² or more tensile strength and good processability as a reinforcing member such as a door impact beam, yet has excellent resistance for hydrogen embrittlement becomes a problem when using super Since a high-strength thin steel plate can be provided, the effect of reducing the weight of the above-described strength member, reinforcement member, and the like is remarkable.

[Brief description of the drawings]

FIG. 1 is a photograph showing a metal structure of a steel sheet obtained in an example, showing a distribution state of carbides by an extracted replica, (a) showing a case of the present invention, and (b) showing a case of a comparative example.

(56) References JP-A-6-122936 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 301 C21D 8/02 C21D 9/46

Claims (4)

(57) [Claims] (1) C: 0.05 to 5% by weight (the same applies hereinafter)
0.25%, Mn: 1.0-3.0%, Al: 0.025-
0.10% or less, S: 0.01% or less, N: 0.008% or less, with the balance being composed of unavoidable impurities,
An ultra-high-strength steel sheet having a tensile strength of 980 N / mm ² or more and containing 70% or more of martensite by volume is 0.1 μm
An ultra-high-strength steel sheet free from hydrogen embrittlement, characterized in that the Fe-C-based precipitates are 3 × 10 ⁵ or less per 1 mm ² . 2. Si: 3.0% or less, P: 0.1%
Hereinafter, Cr: 1.0% or less, Mo: 1.0% or less, W: 1.0% or less.
The ultra-high-strength steel sheet free from hydrogen embrittlement according to claim 1, comprising one or more of 0% or less. 3. Ti: 0.2% or less, Nb: 0.1%
The ultrahigh-strength steel sheet free from hydrogen embrittlement according to claim 1 or 2, wherein the steel sheet contains at least one of V: 0.1% or less and Zr: 0.1% or less. 4. A hot rolled in a conventional manner a steel having a composition according to claim 1, 2 or 3, pickling remain or thereafter cold rolling, when continuous annealing, soaking the Ac ₃ point or more After heating, slowly cool down to 850-650 ° C, then 5 ° C / sec from that temperature
The above is cooled to 300 ° C. or lower, and thereafter, reheated to 300 ° C. or lower or as it is at 300 ° C. or lower for 1 to 20 minutes
Tempering treatment, the tensile strength is 980N / mm
² or more, contains 70% or more by volume of martensite,
Fe × C precipitates of 0.1 μm or more are 3 × 1 per mm ²
0 ⁵ method for manufacturing ultra-high strength thin steel sheet without occurrence of hydrogen embrittlement, characterized in that to obtain the following.