JPH07197183A - Ultra-high strength thin steel sheet without developing hydrogen brittleness and its production - Google Patents

Abstract

PURPOSE:To obtain an ultra-high strength thin steel by making number of Fe-Cr series precipitation having a specific size or larger to a specific value or lower, in the ultra-high strength thin steel sheet having specific composition and martensite containing ratio. CONSTITUTION:The steel containing, by wt.%, 0.05-0.25C, 1.0-3.0 Mn, 0.025-0.100 Al, <=0.01S, <=0.008N, and as necessary, further containing one or more kinds of <=3.0Si, <=0.1 each of P, Cr, W and/or one or more kinds of <=0.2Ti and <=0.1 each of Nb, V, Zr, is used. This steel is hot-rolled according to the ordinary method, and when continuously annealing under pickling condition or after pickling and cold-rolling, after soaking at Ac3 point or higher, slow cooling from 850 deg.C to 650 deg.C is executed and cooling is executed from 650 deg.C to 300 deg.C at >=5 deg.C/sec cooling rate and, thereafter, reheating to <=300 deg.C or tempering treatment by keeping at <=300 deg.C for 1-20min is executed. Then, the objective thin steel sheet having >=980N/mm<2> tensile strength and containing >=70% martensite by vol. ratio and <=3X10<5>/mm<2> Fe-Cr series precipitates having >=0.1mum size of precipitates is obtd.

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 use in automobiles such as bumpers for automobiles and reinforcing members for doors, which is lightweight and requires strength resistance, and a method for producing the same.

[0002]

[Problems to be Solved by the Prior Art and Invention] The weight reduction of automobiles is promoted by the proposal of strengthening the regulations of CAFE (Corporate Average Fuel Economy) in the United States, and the ultra high strength of 980 N / mm ² or more is applied to the reinforcing members of the bumpers and doors. High strength steel sheets have been adopted.

Hydrogen embrittlement occurs in ultra-high strength steel of 980 N / mm ² or more, for example, issued by Japan Screw Association.
It is known for the training text "Design and practice of screw fastening" (October 18, 1990). 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 during use.

Regarding the hydrogen embrittlement of ultra-high strength thin steel sheet, it is proposed to add Si to the steel to suppress the penetration of hydrogen atoms into the steel sheet, as described in JP-A-4-268053. Has been done. However, the generation of rust changes variously depending on the corrosive environment, and it is difficult to prevent the hydrogen embrittlement by sufficiently suppressing the intrusion of hydrogen atoms into the steel sheet by adding Si.

Further, in the field of bar steel, which has been reported to prevent hydrogen embrittlement of steel, for example, JP-A-60-155644.
40 martensite texture as described in
A method is known in which tempering is performed at 0 ° C. or higher to sufficiently prevent the Fe—C compound from precipitating. However, such steel is inferior in workability as compared with the ultra-high-strength thin steel sheet that is processed by press forming or the like. Further, it is considered that the ultra-high strength thin steel sheet is likely to cause hydrogen embrittlement due to the increase in strength due to cold working, but this characteristic is not taken into consideration.

Usually, the ultra-high-strength thin steel sheet produced by the continuous annealing method has a relatively low amount of C and Mn in order to secure cold workability such as press forming, and is tempered at 400 ° C. or higher. In this case, the strength is lowered and the desired characteristics cannot be obtained.
Therefore, it is manufactured by cooling after soaking at a predetermined cooling rate to 400 ° C. or lower, or by once cooling to normal temperature and then tempering at 400 ° C. or lower, which is completely different from the method known for bar steel.

The present invention solves the problems of the prior art in the ultrahigh strength thin steel sheet having a tensile strength of 980 N / mm ² or more, and provides an ultrahigh strength thin steel sheet for working in which hydrogen embrittlement does not occur, and It is intended to provide a manufacturing method thereof.

[0008]

Means for Solving the Problems As a means for solving the above problems, the present invention provides 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,
One or more of 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
An ultra high strength thin steel sheet containing at least 70% by weight of martensite, having a composition of unavoidable impurities, the balance of which is unavoidable impurities, and a tensile strength of 980 N / mm ² or more.
Fe-C based precipitates of 1 μm or more are 3 × 10 ⁵ per 1 mm ^2.
The gist is an ultrahigh-strength thin steel sheet that is free from hydrogen embrittlement and is 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 pickled or subsequently cold-rolled and continuously annealed, after soaking to an Ac ₃ point or more, 85
By gradually 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 directly performing tempering treatment at 300 ° C or less for 1 to 20 minutes , Tensile strength of 980 N / mm ² or more, martensite volume ratio of 70% or more, and
Fe-C based precipitates of 1 μm or more are 3 × 10 ⁵ per 1 mm ^2.
The gist is a method for producing an ultrahigh-strength thin steel sheet that does not cause hydrogen embrittlement, which is characterized by obtaining the following.

[0010]

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

The inventors of the present invention have earnestly studied hydrogen embrittlement in a corrosive environment such as air and salt spray in an ultra-high strength thin steel sheet that has been cold worked such as press forming and bending.

As a result, the fracture due to hydrogen embrittlement occurs from the cold-worked portion, and the fracture occurs in a short time in the steel sheet, which is the fracture surface mainly due to the intergranular fracture and is of the Fe-C system of 0.1 μm or more. More than 3 × 10 ⁵ precipitates were deposited per 1 mm ² . On the other hand, a steel sheet with a long fracture time is a fracture surface mainly due to intragranular fracture, and similarly to a steel sheet that does not cause hydrogen embrittlement, Fe—C-based precipitates of 0.1 μm or more are 3 × 10 ⁵ per 1 mm ^2. I found the following:

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

As described above, when the Fe-C-based precipitates having a size of 0.1 μm or more are deposited in an amount of more than 3 × 10 ⁵ per 1 mm ² , the intergranular fracture becomes the main fracture surface and hydrogen embrittlement is caused. Although the cause of shortening the breakdown time is not clear, it is presumed 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,
If Fe-C based precipitates of 0.1 μm or more are deposited,
High stress or voids are generated at the interface between the carbide and the base metal structure due to cold working during pressing, and atomic hydrogen generated by the corrosion reaction is collected at that location, further increasing stress concentration or cracking. To do. Since precipitates are preferentially deposited at grain boundaries with many lattice defects, Fe of 0.1 μm or more is used.
When the amount of —C-based precipitates exceeds 3 × 10 ⁵ per 1 mm ² , the stress concentration at grain boundaries or the frequency of cracks increases,
Hydrogen embrittlement occurs mainly due to intergranular fracture. On the other hand, 0.1 μm
Fe-C-based precipitates smaller than the above-mentioned size are less likely to generate high stress or voids at the interface between the carbide and the base metal structure due to cold working. Therefore, the occurrence frequency of cracks at the grain boundaries also decreases, It is considered that hydrogen embrittlement, which is mainly due to fracture, is unlikely to occur.

Next, the reasons for limiting the chemical composition of steel in the present invention will be explained.

C: C is an element essential for increasing the strength by forming martensite, and 0.05% or more is necessary to obtain a strength of 980 N / mm ² or more. But 0.2
If it exceeds 5%, workability such as bending deteriorates, so this is made the upper limit.

Mn: Mn is an element that enhances the hardenability of steel, and is required to be 1.0% or more in order to stably obtain martensite in continuous annealing equipment. However, if it exceeds 3.0%, not only the effect is saturated, but also segregation becomes large, the structure becomes nonuniform, and the workability deteriorates, so this is made the upper limit.

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

N: N is a solid solution in steel and deteriorates press workability, so it is specified to be 0.008% or less.

Although the above elements are essential components, as shown below, if necessary, at least one member selected from the group consisting of Si, P, Cr, Mo and W, or Ti, Nb, V and Zr. One or more of the groups can be included in suitable amounts.

Si: Si is an element effective for strengthening the steel and enhancing the ductility, but if it exceeds 3.0%, not only the effect is saturated, but also the load in cold rolling becomes high. Therefore, it is specified below.

P: P is an element effective for strengthening the steel and increasing the ductility, but if it exceeds 0.1%, embrittlement is likely to occur, so the content is made less than this.

Cr, Mo: Cr and Mo are effective elements for enhancing the hardenability of the steel and stably obtaining martensite in the continuous annealing equipment. However, if each exceeds 1.0%, the effect is saturated. Therefore, the upper limit of each is 1.0%.

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

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

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

The steel slab having the above chemical composition is manufactured by continuous casting or ingot casting, and hot rolling is performed by a conventional method.
It is pickled or cold-rolled thereafter.

In hot rolling, it is necessary to heat above a predetermined rolling temperature, but after casting, once cooled to near room temperature and reheated, or when it is inserted into a heating furnace at a high temperature,
Further, there is no particular problem even if it is rolled as it is after casting. The rolling may be finished at a temperature not lower than the Ar ₃ transformation point, and the cooling conditions and the coiling temperature thereafter are not particularly specified, and the usual method may be used. For example, the cooling may be performed in the range of 30 to 100 ° C./sec on average, and the winding temperature may be 750 to 400 ° C.

Next, continuous annealing is performed. In continuous annealing, even after pickling after hot rolling, 25% to 80% afterwards
You may use the steel plate which cold-rolled. The soaking temperature of continuous annealing is performed at a temperature of Ac ₃ or higher. 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 deteriorate. In addition, it becomes difficult to secure strength, which is not desirable. After soaking, 1 to 3 until 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 with a martensite structure volume ratio of 70% or more, but if it exceeds 850 ° C., the shape of the steel sheet deteriorates during quenching, so the upper limit is 850 ° C. To do. If the quenching cooling rate is 5 ° C./sec or more, a martensite structure is obtained, so this is the lower limit. The cooling method may be water quenching, water-cooled roll cooling, steam cooling, gas jet cooling, or the like. The quenching stop temperature is set to 300 ° C. or lower because the martensite structure is the main constituent.

After that, tempering treatment is performed at 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 1 min
If it is not above, the effect is hardly recognized, but 20mi
If it 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 exceeds the predetermined amount and the fracture characteristics are deteriorated, which is not desirable.

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

This ultrahigh-strength steel sheet is excellent in hydrogen embrittlement because the delayed fracture generation time becomes longer or does not fracture in hydrogen embrittlement tests under corrosive environments such as salt spray, hydrochloric acid immersion and cathode charge tests. Have tolerance.

Regarding the structure, the martensite containing the above-mentioned carbide may be 70% or more in volume ratio in order to secure a predetermined strength. Even if the martensite content is 100% or if the content of ferrite, bainite, and retained austenite alone or in combination is 30% or less, the effect of the present invention is not changed.

After the continuous annealing, there is no problem in temper rolling or plating treatment with zinc or the like.

Next, examples of the present invention will be shown.

【Example】

Steels having the chemical composition shown in Table 1 were heated to 1200 ° C., then hot rolled to a thickness of 3.2 mm and wound at 560 ° C.
After pickling, cold rolling was performed to a plate thickness of 0.8 mm, and continuous annealing was performed under the conditions shown in Table 2. After temper rolling of 0.3%,
The mechanical properties such as strength and bending workability, and hydrogen embrittlement resistance were investigated. The results are shown in Table 1.

Regarding hydrogen embrittlement resistance, 30 mmw × 150
Bending a strip test piece of mml with a bending radius of 12 mm, tightening it until the width between the plates became 24 mm, and applying electrodeposition coating of 20 μm on the surface, then 0.5 mol / liter 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 at which cracking 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 showed tensile strength of 980 N / mm ² or more and good workability, and as shown in FIG. 1, there were few carbides of 0.1 μm or more, The time until cracking is as long as 980 seconds or more and hydrogen embrittlement resistance is excellent.

On the other hand, steel Nos. 1, 7, and 8 of the comparative examples
Has a chemical component outside the scope of the present invention, and cannot secure a predetermined strength, or has poor workability. Steel No. 3, 6,
Nos. 16, 22, 23, 24, and 25 have annealing conditions outside the scope of the present invention, and the volume ratio of martensite is insufficient and a predetermined strength cannot be ensured, or as shown in FIG.
The above carbides are abundant, the time until cracking is short, and the hydrogen embrittlement resistance is poor.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

As described above, according to the present invention,
It has an optimal tensile strength of 980 N / mm ² or more as a reinforcing member for automobile bumpers and door impact beams, good workability, and excellent resistance to hydrogen embrittlement, which is a problem during use. Since the high-strength thin steel sheet can be provided, the effect of contributing to weight reduction of the above-described strength member, reinforcing member, etc. is remarkable.

[Brief description of drawings]

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

Claims (4)

[Claims] 1. In weight% (hereinafter the same), C: 0.05-
0.25%, Mn: 1.0 to 3.0%, Al: 0.025
0.10%, S: 0.01% or less, N: 0.008% or less, with the balance being inevitable impurities.
0.1 μm in an ultra-high strength thin steel sheet having a tensile strength of 980 N / mm ² or more and containing martensite in a volume ratio of 70% or more.
Ultra high strength steel sheet without hydrogen embrittlement, characterized in that the Fe-C-based precipitates are 3 × 10 ⁵ or less per 1 mm ² . 2. Further, Si: 3.0% or less, P: 0.1%
Below, Cr: 1.0% or less, Mo: 1.0% or less, W: 1.
The ultrahigh-strength thin steel sheet according to claim 1, wherein the ultrahigh-strength thin steel sheet contains 0% or less of one or more kinds. 3. Further, Ti: 0.2% or less, Nb: 0.1%
The ultrahigh-strength thin steel sheet without hydrogen embrittlement according to claim 1 or 2, further comprising one or more of V: 0.1% or less and Zr: 0.1% or less. 4. A steel having the composition according to claim 1, 2 or 3 is hot-rolled by a conventional method, and as it is pickled or cold-rolled and continuously annealed, it is evened to an Ac ₃ point or more. After heating, gradually cool to 850 to 650 ℃, and from that temperature 5 ℃ / sec
After cooling to below 300 ° C, reheat to below 300 ° C or 1300 min below 300 ° C
Tensile strength of 980 N / mm
² or more, martensite contains 70% or more by volume ratio,
Fe-C based precipitates of 0.1 μm or more are 3 × 1 per 1 mm ^2.
A method for producing an ultra-high-strength thin steel sheet in which hydrogen embrittlement does not occur, which is characterized by obtaining 0 ⁵ or less.