JP2000063947A - Manufacture of high strength stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To satisfactorily increase strength even in an as-cold-rolled state by subjecting a steel, having a composition which contains C, Si, Mn, Cr, and Ni and in which an austenite stability index is regulated to a value in a specific range, to hot rolling and cold rolling, applying final annealing to the resultant steel sheet, and further applying temper rolling at specific draft to regulate hardness to a value in a specific range. SOLUTION: The steel has a composition which consists of, by weight, <=0.15% C, <=2.0% Si, <=2.0% Mn, 16.0-20.0% Cr, 6.0-10.5% Ni, and the balance Fe with inevitable impurities and in which the austenite stability index Md30, represented by equation Md30=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+ Cu)-18.5Mo-60Nb, is regulated to 0 to 39 deg.C. This steel is hot rolled and cold rolled, and the resultant steel sheet is subjected to final annealing and then to temper rolling at 20 to 70% draft to regulate hardness to Hv 300 to 600.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength stainless steel used as a spring material or a gasket material for an engine of an automobile or the like. Conventionally, metastable austenitic stainless steel such as SUS 304 or SUS 301 has been used as a spring material or a metal gasket material requiring high strength. These steels are cold-rolled after the final annealing to generate a martensite phase in the austenite phase, and the strength is increased by the hard martensite phase. The feature is that it is made of stainless steel with excellent properties. [0003] However, for example, stainless steel for a spring having a metastable austenitic structure represented by SUS 301 described above is generally tempered at 200 to 550 ° C after cold working. Usually, a predetermined strength is secured by performing aging treatment at 400 to 600 ° C. It is well known that stainless steel for springs treated in this way has good spring properties, but in recent years,
The demand for improving the properties of spring materials is becoming more and more severe. In particular, stainless steel for springs having higher strength (higher hardness) than conventional materials has been desired. [0004] As described above, recent stainless steels for springs maintain a necessary strength level mainly by heat treatment after cold rolling. The reason for this is that, in general, when these steels are used in a salt water environment, stress corrosion cracking can occur from pitting as a starting point. This is due to stress corrosion cracking due to exhaust gas condensate. Since the tendency becomes remarkable as the strength is increased by increasing the cold rolling rate, when used in the above environment, the cold rolling rate is reduced and the strength is suppressed. . That is, the cold rolling reduction was reduced, and thereafter, the high strength was realized by heat treatment. Therefore, a main object of the present invention is to develop and propose a method for obtaining a sufficiently high-strength stainless steel without cold-rolling without performing heat treatment such as aging treatment. Another object of the present invention is to provide low cold rolling,
That is, it is an object of the present invention to propose a method capable of advantageously producing stainless steel used in fields such as springs having a sufficiently high strength even with a temper rolling having a small rolling reduction. [0006] As a result of earnest research aimed at establishing a manufacturing technique that meets the above-mentioned objects, the inventors have found that
The present inventors have found that the above-mentioned problems can be overcome by suitable control of the austenite stability index Md ₃₀ (° C.), and have come to think of the present invention. That is, the present invention provides a method for producing C ≦ 0.15 wt%, Si ≦ 2.
0 wt%, Mn ≦ 2.0 wt%, Cr: 16.0-20.0 wt%, Ni: 6.0
A steel having a composition of about 10.5 wt% and the balance consisting of Fe and inevitable impurities and having an austenite stability index Md ₃₀ represented by the following formula adjusted to be 0 ° C. to 39 ° C .: And then subjected to final annealing and then temper rolling at a reduction rate of 20 to 70% to increase the hardness to Hv: 300 to
A method for producing high-strength stainless steel, characterized by adjusting to 600. Md ₃₀ = 551-462 (C + N) -9.2Si -8.1Mn -13.7Cr
-29 (Ni + Cu) -18.5Mo-60Nb The steel material used in the present invention is as follows:
C ≦ 0.15 wt%, Si ≦ 2.0 wt%, Mn ≦ 2.0 wt%, Cr: 16 ~
20.0 wt%, Ni: 6.0 to 10.5 wt%, contains 4.0 wt% or less of Mo, 4.0 wt% or less of Cu, 1.0 wt% or less of Nb if necessary, and contains P and S unavoidably mixed , N, and the balance is mainly made of Fe. The reasons for each limitation will be described below. C: 0.15% or less; C is an austenite-forming element, and as C increases, harder martensite is formed. But,
If the content is too large, coarse carbides are formed and the fatigue properties important as a spring material are deteriorated, and the formation of martensite itself becomes difficult because austenite is too stabilized, so the upper limit was 0.15%. Si: 2.0 wt% or less; at least about 0.3 wt% of deoxidizer is required. In order to develop hardness by tempering, the higher the content, the better. However, since solidification cracking during solidification and welding is promoted, the upper limit is 2.0 wt%. Mn: 2.0 wt% or less; at least 0.5% is required as a deoxidizing agent. Also, in order to harden the material by rolling, it is better that the content is large, but if it is too large, it interferes with the formation of martensite and deteriorates the fatigue resistance.
2.0%. Cr: 16.0 to 20.0% or less; an element characterizing the alloy in stainless steel, and at least 16.0% is necessary to ensure corrosion resistance. However, if too much is added, δ ferrite precipitates in the mother phase and hot workability is hindered, so 16.0-
20.0%. Ni: 6.0 to 10.5%; Ni is an essential component of austenitic stainless steel, and is required to be 6.0% or more to prevent precipitation of δ ferrite and complete martensitic transformation in a solution-treated state, but not much. If it is too large, it not only hinders the formation of martensite due to processing, but also increases the production cost, so it was limited to 6.0 to 10.5%. Mo: Mo is effective in improving corrosion resistance, increasing the strength of reverse-transformed austenite, and lowering the Ms point. However, it is an expensive material, and if too much, increases the price of steel, so the upper limit is 4.0%. I do. Cu: Cu is an element originally effective for improving corrosion resistance, but is effective for lowering the Ms point in the present invention. If it exceeds about 4.0%, the hot workability is significantly impaired, so the upper limit is about 4.0%. N
b: Nb is an effective component for suppressing the formation of a Cr deficiency layer due to precipitation of Cr carbide during welding and for suppressing the growth of crystal grains of a reverse transformed austenite phase. Not only does it accelerate, but it also impairs the ductility of the material, so the upper limit is 1.0%. Next, what is important in the manufacturing method according to the present invention is to make the stainless steel sufficiently high in strength even when cold-rolled, that is, to set the hardness Hv during final annealing to 20%.
Temper rolling (cold rolling) performed after controlling to 0 to 300
Sometimes Hv: 300 to 600 can be secured. For this purpose, in the present invention, in addition to adjusting the component composition of the steel to be smelted as described above, the austenite stability index indicated as Md ₃₀ is controlled as follows. [0010] In the present invention, the austenite stability index Md ₃₀ is given by the following equation. Md ₃₀ = 551-462 (C% + N%)-9.2Si% -8.1Mn%
-13.7Cr% -29 (Ni% + Cu %) - 18.5Mo% -60Nb% This Md _30, when given a strain of 30% to the material, the tissue
The working temperature (° C.) at which 50% is transformed into martensite indicates that the higher the value of Md ₃₀ , the more easily martensite is generated during working, indicating that the material is unstable. That is, in the present invention, the components are adjusted so that the value of Md ₃₀ falls within the range of 0 ° C. to 39 ° C. Although the desirable composition range of the steel material component according to the present invention is as described above, it does not mean that any component design may be made within this range, and in the present invention, Md ₃₀
Must be adjusted to be in the range of 0 ° C to 39 ° C.
If Md ₃₀ is 0 ° C. or lower, austenite is too stabilized to form martensite by processing, and the required strength is not obtained due to low work hardening. On the other hand, if the temperature is + 39 ° C or higher, austenite becomes too unstable and excessive martensite is generated in the early stage of processing.
In any case, this is an obstacle to stably obtaining the optimum strength range. Next, in the present invention, the smelting is performed according to a conventional method, hot and cold rolling is performed, and a final annealing is performed to obtain Hv: 20.
Use stainless steel of about 0 to 300, and then continue
By performing temper rolling at a reduction rate of 20 to 70%, Hv: 30
High strength stainless steel of 0-600. As described above, the feature of the present invention is that, in cold rolling before final annealing, corrosion resistance is ensured at the expense of strength by suppressing a reduction in working ratio, and thereafter, under high pressure in order to secure necessary strength. To perform temper rolling at a low rate. The purpose of this processing is to stabilize the martensite structure and to adjust the hardness of the final product (Hv: 300 to 600). That is, by adjusting Md ₃₀ in the range of 0 ° C. to 39 ° C., the amount of generated work-induced martensite is controlled to an appropriate amount. In this sense, the reason why the rolling reduction of the temper rolling performed in this stage is limited to 20% or more is that the rolling reduction of the allowable reduction with the current rolling thickness control accuracy is compared to the normally used minimum thickness of 0.2 mm. There is variation. On the other hand, the reason for setting the upper limit of the rolling reduction to 70% or less is that if rolling is performed at a rolling reduction higher than this, workability hardens and the rollability deteriorates, and a stable shape cannot be obtained. EXAMPLE In this example, the test materials shown in Table 1 were melted by a method according to a conventional method, and after passing through known hot rolling, heat treatment, cold rolling, and annealing steps, Md ₃₀ , hardness after annealing, and hardness after temper rolling were examined for those subjected to temper rolling at a required reduction ratio. This is shown together with a comparative example that deviates from the constituent features of the present invention. FIG. 1 shows that in this example, the hardening coefficient (hardness after temper rolling / hardening rolling) was calculated for the values of Md ₃₀ for (No. 2) with a rolling reduction of 28% and (No. 3) of 65%. (The previous hardness). [Table 1] As is clear from the above table and the results shown in the drawings, in the case of the invention example, it is understood that stable and high hardness stainless steel can be obtained by the combined use of the control of Md ₃₀ and the suitable temper rolling. As described above, according to the present invention, high hardness stainless steel can be stably manufactured by suitably adjusting the austenite stability index Md ₃₀ and the temper rolling. .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the Md ₃₀ and the hardening coefficient.

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Claims (1)

[Claim 1] C ≦ 0.15 wt%, Si ≦ 2.0 wt%, Mn ≦ 2.0
wt%, Cr: 16.0 to 20.0 wt%, Ni: 6.0 to 10.5 wt%, the balance consisting of Fe and inevitable impurities, and an austenite stability index Md ₃₀ represented by the following formula of 0 ° C.
After the steel of the component composition adjusted to be 39 ° C. is hot and cold rolled, and subjected to final annealing, a further reduction of 20 to 70
A method for producing high-strength stainless steel, wherein the hardness is adjusted to Hv: 300 to 600 by subjecting the steel to a temper rolling of 0.1%. Md ₃₀ = 551-462 (C + N) -9.2Si -8.1Mn -13.7Cr
−29 (Ni + Cu) −18.5Mo−60Nb