JPS6212289B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a high-strength stainless steel sheet that undergoes almost no material change due to aging. Due to the excellent corrosion resistance of stainless steel, the use of stainless steel in transportation systems such as railway vehicles has recently increased. Under these circumstances, there are active attempts to reduce the increase in vehicle weight due to the increase in vehicle cooling equipment and equipment due to mutual access, etc., by using high strength materials, and reducing the weight of vehicles is also in line with the demands of the times in terms of energy conservation. In the future, there will be an increasing demand for higher strength and higher workability in vehicle materials. Until now, stainless steel manufacturers have used SUS304 or 1/4 hard grade SUS301 as materials, and they have not yet used sufficiently high-strength materials. However, for the reasons mentioned above, there has recently been an active movement to make vehicles entirely stainless steel by using 1/2 hard grade materials while taking advantage of the corrosion resistance of stainless steel. Taking these circumstances into consideration, in order to develop vehicle materials with excellent overall properties, we conducted extensive research and found that C: over 0.04% to 0.10% by weight percentage.
N: 0.05-0.20%, Si: 1.0% or less, Mn: 0.3-2.0
%, Ni: 6.5 to less than 8.0%, Cr: 16.0 to 20.0% and as necessary V: 0.50% or less, Nb: 0.50% or less,
Cold-rolled stainless steel plate containing one or more of Ti: 0.50% or less, Mo: 0.50% or less, and the remainder consisting of Fe and other elements unavoidable in steelmaking, is recrystallized and annealed, then cold tempered. It was confirmed that by rolling, a non-aging, high-strength stainless steel sheet with the best properties as a hard material could be obtained. The first requirement for stainless steel hard materials is high strength, but at the same time, they also have good workability, weldability,
It must also have excellent corrosion resistance. This is because, for vehicle assembly, hard materials of various thicknesses of approximately 6 mm or less are processed into complex cross-sectional shapes and assembled by welding. For workability, especially 0.2 from the spring back characteristics after processing.
% yield strength (σ ₀ . ₂ ) is important, and the material must have good ductility due to the complicated shape. The characteristics of the weld are naturally an important factor in terms of strength, and sensitization of the weld must be avoided as much as possible because it is a remote cause of intergranular corrosion and cracking of the weld. Therefore, although it is a high-strength material, this type of material is required to have a certain level of work hardening properties and an excellent weld sensitization property (yield ratio of 0.80 or less). Another characteristic that is considered problematic is strain aging.
Conventionally, stainless steel is a high-alloy steel containing a large amount of Cr, etc., so the diffusion coefficient of C, N, etc. is small, and there is almost no need to consider aging phenomena. However, since stainless steels that are made to have high strength generally actively utilize deformation-induced transformation, the austenite phase in the component system is unstable, and the dislocation density in the transformed α' martensite becomes extremely high, resulting in so-called strain aging. We found that this phenomenon occurs easily. Considering the environment and storage time after shipping, the amount of increase _{in σ 0.2 is} 10
It may be around Kg/mm ² . As aging progresses, the upper yield point appears from the pre-yield state, and in the over-aged state, the load changes at the lower yield point level, leading to fracture. Therefore, overaging significantly deteriorates workability. The present invention specifically clarifies the basic composition system in order to suppress strain aging and at the same time produce a hard material that has excellent comprehensive properties such as weldability and corrosion resistance without impairing the strength characteristics, ductility, etc. required for hard materials. This is what I did. The progress of strain aging increases σ _0.2 , which increases _the spring back after processing and forming, and further slows work hardening, thereby deteriorating workability. Hard stainless steel materials are generally manufactured by cold temper rolling after recrystallization annealing.
As a result of deformation-induced transformation, strength properties increase and ductility deteriorates. Since the balance of these properties influences the deformability of the hard material, controlling the material temperature, temper rolling rate, rolling speed, etc. during skin pass rolling is important for adjusting the quality of the hard material. Therefore, temper rolling to obtain high strength deteriorates the deformability of the processed material, and it is necessary to prevent strain aging from further degrading the material quality. Furthermore, strain aging not only causes the above-mentioned deterioration in workability, but also affects processing and forming conditions, resulting in a decrease in productivity and also poses a major economic problem. The reason for limiting each component element in the present invention will be described. Si: Necessary in steelmaking as a deoxidizing agent, 1.0%
If it exceeds 1.0%, the ferrite forming ability becomes strong and hot workability deteriorates, so the upper limit was set at 1.0%. Mn: Effective as an austenite-forming element, and if it falls below 0.3%, the austenite stability becomes extremely unstable, so this was set as the lower limit. Also
The upper limit was set at 2.0% because adding more than 2.0% would reduce the effect of suppressing strain aging and cause some problems with the surface properties of the steel sheet due to scaling. Ni: If it exceeds 8.0%, the austenite phase becomes too stable and high strength as a hard material cannot be obtained, so Ni was set at less than 8.0%. The lower limit of 6.5% is the minimum necessary amount from the viewpoint of maintaining the austenite phase and the balance of strength properties. Compared to other elements, Ni is the most convenient element for maintaining austenite balance, and the content range was determined in consideration of austenite balance in addition to the above reasons. Cr: Has a very small effect on austenite stability, but has a large effect on corrosion resistance and ferrite formation.If it is less than 16.0%, corrosion resistance will be a problem, and if it exceeds 20.0%, the amount of ferrite will increase significantly and have a negative impact on hot workability. Therefore, the range was set to 16.0% to 20.0%. C, N: Both elements play the same role in stabilizing austenite. However, it has the opposite effect on strain aging, and while an increase in C accelerates strain aging, N has the effect of suppressing strain aging. Both C and N have effects in the same direction on strength properties, and as the content increases, strength properties after temper rolling increase, and the degree of this is slightly greater for C. However, C is 0.04%
Below this, the amount of increase in σ _{B becomes smaller than the rate of increase in σ 0.2 which} is proportional to the temper rolling rate, resulting in lower work hardening ability _and inferior workability. For this reason, the lower limit of C was set to exceed 0.04%. On the other hand, the upper limit was set at 0.10% in view of the balance between strength characteristics and strain aging. N increases the strength properties and suppresses strain aging, so it is better to have more N, but due to manufacturing problems such as the generation of blowholes during solidification, the upper limit was set at 0.20%.
If it is less than 0.05%, the austenite phase becomes too unstable and strain aging becomes noticeable, so 0.05% was set as the lower limit. V, Nb, Ti, Mo: Strain aging occurs mainly in the α' phase in the two-phase structure (γ phase and α' phase) after temper rolling. Aging of the γ phase also occurs, but it is extremely small compared to the α' phase. Since strain aging is caused by the reaction between high-density dislocations in α' and mainly C atoms, in order to further suppress the phenomenon, we investigated the effects of Nb, V, Ti, and Mo, and found that one of these elements It has also become clear that the overall properties of the hard material can be improved by adding two or more of them. The addition of V, Nb, and Ti has a direct effect on suppressing strain aging, whereas Mo is effective in increasing strength and can reduce the temper reduction rate to obtain constant strength characteristics.
The amount of α′ martensite decreases, indirectly suppressing strain aging. Furthermore, Mo has the effect of improving rust resistance and is a useful element. The effect of each of these elements is sufficiently exhibited at 0.50%, and adding more than this will saturate the effect and is also economically unwise. In the present invention, a cold-rolled stainless steel plate made of such components is subjected to recrystallization annealing and then cold skin pass rolling. Next, as an example, the composition and various properties of a stainless steel plate obtained by the method of the present invention are shown in Tables 1 and 2 in comparison with those obtained by a conventional method. The material in these tables is a 1.1mm thick steel plate made from a 45Kg steel ingot by known cold rolling annealing.
This was measured on a material temper-rolled to a thickness of 0.8 mm at 60°C. The strength properties were measured twice: immediately after temper rolling and after aging treatment (71°C x 50 hours). The yield ratio and ductility (El) are before aging, and _{Δσ 0.2} indicates the difference in σ _0.2 before and after _aging . Therefore, the smaller Δσ _0.2 is _, the more the aging phenomenon is suppressed. From Tables 1 and 2, it can be seen that the examples of the present invention show small aging changes, well-balanced strength properties, and excellent deformability. On the other hand, in the comparative examples, when the aging change is very large or the aging change is small, the yield ratio is large and the workability is poor. As seen in the examples above, the strain aging of the stainless steel sheet obtained by the method of the present invention is small, and there is almost no change in material quality after manufacturing the hard material. For example, once the processing conditions are set, they can be adjusted even if the lot changes. It has great advantages in terms of processing productivity, such as eliminating the need to do so.

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Claims (1)

[Claims] 1. C: more than 0.04% to 0.10% in weight percentage, N:
0.05-0.20%, Si: 1.0% or less, Mn: 0.3-2.0%,
A cold-rolled stainless steel plate containing Ni: 6.5% to less than 8.0%, Cr: 16.0% to 20.0%, and the remainder consisting of Fe and other elements unavoidable in steelmaking is recrystallized and annealed and then cold skin-pass rolled. A manufacturing method for non-aging, high-strength stainless steel sheets. 2 C: more than 0.04% to 0.10% in weight percentage, N:
0.05-0.20%, Si: 1.0% or less, Mn: 0.3-2.0%,
Ni: 6.5 to less than 8.0%, Cr: 16.0 to 20.0%, plus V: 0.50% or less, Nb: 0.50% or less, Ti: 0.50
% or less, Mo: 0.50% or less, and the remainder is Fe and other elements unavoidable in steelmaking. Cold-rolled stainless steel plate is recrystallized and annealed and then cold-pass rolled. A method for producing a non-aging high-strength stainless steel sheet characterized by: