JPH04214842A - High strength stainless steel excellent in workability - Google Patents

Abstract

PURPOSE:To obtain stainless steel excellent in workability as annealed, low in the degree of work hardening and provided with high strength by subzero treatment at about -73 deg.C. CONSTITUTION:This is stainless steel in which the content of each element is regulated so as to contain, by weight, 0.01 to 0.30% C, 0.2 to 3.0% Si, <=4.0% Mn, <=0.030% P, <=0.008% S, 3.0 to 7.0% Ni, 12.0 to 18.0% Cr, 0.01 to 0.25% N and 0.0003 to 0.30% B and so as to regulate A value defined by A value = Ni+0.61XMn+22.60XC+13.77XN+0.43XCr+0.33XSi+0.20XB to the range of 15.7 to 18.0 and the balance Fe with inevitable impurities.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention relates to a high-strength stainless steel that has excellent workability in an annealed state and develops high strength through sub-zero treatment after working. It is something. [Prior art and problems] Conventionally, work-hardening type SUS301, SUS304 or age-hardening type SU have been used as high-strength stainless steels.
S630, SUS631, etc. are frequently used and are applied to various spring materials. Work-hardening stainless steel, typified by SUS301, has high strength through work-hardening of the austenite phase through cold working and the formation of highly hard work-induced martensite. Additionally, after cold working, aging treatment may be used to further increase the strength. However, these steels need to be work-hardened by cold working, and as a result, they have the disadvantage of poor formability. On the other hand, although age-hardening stainless steel can obtain high strength through aging treatment, it generally has poor workability and requires aging treatment at around 400°C, which results in temper coloring and deformation during aging treatment. There are problems such as the risk of Additionally, there are stainless steels that can be made stronger by undergoing sub-zero treatment at an industrially easily obtained temperature of around -73°C instead of such aging treatment, but this type of material is subject to severe work hardening. Therefore, it has the disadvantage that it requires a large load during processing and is inferior in workability. Furthermore, this type of material has the disadvantage that if the alloy composition changes slightly, a large amount of martensite phase will be generated in the annealed state, resulting in poor workability, or the material will not harden even after sub-zero treatment. ing. [Object of the Invention] Therefore, the object of the present invention is to provide a stainless steel that has excellent workability in an annealed state, has a low degree of work hardening, and can achieve high strength through sub-zero treatment at about -73°C. be. [Structure of the Invention] The gist of the present invention, which aims to achieve the above-mentioned object, is that, in weight %, C: 0.01 to 0.30%, Si: 0
．． 2 to 3.0%, Mn: 4.0% or less, P: 0.
030% or less, S: 0.008% or less, Ni: 3.0
~7.0%, Cr:12.0~18.0%, N:0
．． 01~0.25%, B:0.0003~0.30%
and A value=Ni+0.61×Mn+22
．． 60×C+13.77×N+0.43×Cr+0.3
Whether the content of each element is adjusted so that the A value defined by 3 x Si + 0.20 x B is in the range of 15.7 to 18.0, and 3.
0% or less Mo, 3.0% or less Cu, 3.0% or less Co, 0.5% or less Nb, 0.5% or less V
, contains one or more of 0.5% or less W or 0.5% or less Ti, and has an A value = Ni + 0
．． 61×Mn+22.60×C+13.77×N+0.
43×Cr+0.33×Si+0.2
0×B+0.25×Mo+0.18×Cu+0.29×
Co+0.36×Nb+0.33×V
A value defined by +0.27×W+0.35×Ti is 1
The content of each element is adjusted to be in the range of 5.7 to 18.0, with the remainder consisting of Fe and inevitable impurities.
It is a high-strength stainless steel with excellent workability that has excellent workability in the annealed state and develops high strength through sub-zero treatment. [Detailed Description of the Invention] Generally, austenitic stainless steel contains Cr,
The Ms point (temperature at which the quenched martensitic phase begins to form) varies greatly depending on the content of main components such as Ni. According to the research conducted by the present inventors, if the composition is designed so that the main structure in the annealed state is an austenite phase and the Ms point is near room temperature, good workability will be exhibited.
When an appropriate amount of Si is added to this, the workability is significantly improved, and when an appropriate amount of B is added, sufficient hardenability can be obtained without increasing work hardening, thus reducing the load during processing. It was found that workability could be improved. After further processing such as molding or bending, -73℃
Although martensitic transformation is induced in a temperature range of
At this time, it was revealed that B allows a large amount of martensitic phase to be generated stably against fluctuations in composition, and that Si strengthens the martensitic phase and is extremely effective in increasing strength. In other words, in a Cr-Ni metastable stainless steel whose composition has been adjusted so that the Ms point is near room temperature, the combined addition of Si and B shows excellent workability in the annealed state, and the sub-zero treatment provides high strength. We discovered that it is possible to transform The present invention has been made based on these findings. First, the various components of the steel of the present invention and the reason for limiting their content will be explained. C is a strong austenite-forming element and is an essential element for adjusting the A value. Further C
is an essential element for increasing the strength of the martensitic phase produced after sub-zero treatment, and must be contained in an amount of 0.01% or more. However, if it is contained in a large amount, the excellent workability that characterizes the steel of the present invention cannot be obtained, and the corrosion resistance also deteriorates, so the upper limit is set at 0.30%. [0008]Si is an essential element for imparting excellent workability in the annealed state, which is a characteristic of the steel of the present invention. It is also an element that significantly and effectively contributes to increasing the strength of the martensitic phase produced after sub-zero treatment. In order to exhibit these effects, it is necessary to contain 0.2% or more of Si. However, Si is a ferrite-forming element, and if it is contained in a large amount, a large amount of δ-ferrite phase will be generated after annealing, making it impossible to obtain high strength even if sub-zero treatment is performed. For this reason, the upper limit of Si is set to 3.0%. [0009] Like Ni, Mn is an austenite-forming element and is an essential element for adjusting the A value. However, if a large amount of Mn is contained, manufacturing becomes difficult because a large amount of Mn fume is generated during melting and the furnace wall is severely damaged, so the upper limit is set at 4.0%. [0010] When P is contained in a large amount, processability deteriorates, so the upper limit is set at 0.030%. [0011] Most of S exists as inclusions and deteriorates workability, so the upper limit of S is set at 0.008%. Ni, like Mn, is an austenite-forming element, and is an element necessary to adjust the A value within the range specified in the present invention and obtain an austenite phase in the annealed state. % or more is necessary. However, if a large amount of Ni is contained, the A value will increase too much, and even if sub-zero treatment is performed, the martensite phase will not be formed much and high strength will not be obtained, so the upper limit is set at 7.0%. [0013] Cr is an essential element for stainless steel, and must be contained in an amount of 12.0% or more in order to obtain good corrosion resistance. It is also an essential element for adjusting the A value within the range specified by the present invention. However, Cr is a strong ferrite-forming element, and if it is contained in too large a quantity, a δ-ferrite phase will be generated in the annealing state, making it impossible to achieve high strength after sub-zero treatment. Therefore, the upper limit is set to 18.0%. [0014] Like C, N is a strong austenite-forming element, and is an essential element for adjusting the A value. Also N
Like C, C is a solid solution strengthening element for the martensitic phase produced after sub-zero treatment, and is also an extremely effective element in obtaining high strength through sub-zero treatment. For this purpose 0
．． It is necessary for the content to be 0.1% or more, but if the content is too large, blowholes will occur in the steel ingot, making it impossible to obtain a sound steel ingot, so the upper limit is set at 0.25%. B is a characteristic element of the steel of the present invention in that it provides the effect of improving workability without increasing work hardening in the annealed state. Moreover, B effectively contributes to generating a large amount of martensite phase by subzero treatment. Furthermore, B is an effective element for stably generating a large amount of martensitic phase despite variations in alloy composition. In order to exhibit these effects, it is necessary to contain B at 0.0003% or more. However, if it is contained in a large amount, a large amount of boride will be generated and hot workability will deteriorate, so the upper limit is set at 0.30%. Mo is an element that contributes to improving corrosion resistance. It is also an effective element that contributes to high strength after sub-zero treatment. However, like Cr, Mo is a ferrite-forming element, and if it is contained in a large amount, a δ-ferrite phase will be generated, making it impossible to obtain high strength even if sub-zero treatment is performed. Therefore, the upper limit is set at 3.0%. [0017] Cu is an effective element for improving workability in an annealed state. However, if too large a content is contained, the manufacturability will deteriorate, so the upper limit is set at 3.0%. [0018] Co is an element that contributes to improving corrosion resistance. In addition, Co is an austenite-forming element that is effective in adjusting the A value and obtaining an austenite phase in the annealed state, and is also an element that increases the amount of martensite formed after sub-zero treatment and contributes to high strength. be. However, if it is contained in a large amount, the A value will increase, and even if sub-zero treatment is performed, martensitic phase may not be generated, making it impossible to obtain high strength, so the upper limit is set at 3.0%. Nb, V, W and Ti are effective elements for improving intergranular corrosion resistance. However, if the amount added increases in either case, δ ferrite is generated and hot workability deteriorates, so the upper limit for each is set at 0.5%. [0020] The A value was obtained in a laboratory during the development of the steel of the present invention as an index for achieving excellent workability in the annealed state, which is a characteristic of the steel, and obtaining high strength through sub-zero treatment. It is defined by the following formula, and even if the content of each component is within the above range, the amount of each component can be adjusted so that the A value is in the range of 15.7 to 18.0. This is necessary to achieve the objectives of the present invention. A value=Ni+0.61×Mn+22.60×C+1
3.77×N+0.43×Cr+0.33×Si
+0.20×B+0.25×Mo+0.18
×Cu+0.29×Co+0.36×Nb
+0.33×V+0.27×W+0.35×Ti [
[0021] Figure 1 shows the measurement of the amount of martensite in annealed steel with different A values by varying the amount of each component within the above-mentioned ranges and in annealed steel under the same conditions and sub-zero treated steel under the same conditions. However, we looked at the relationship with the A value. As seen in the figure, when the A value is less than 15.7, a large amount of martensite phase is generated in the annealing state, resulting in already high strength and poor workability. Furthermore, even when sub-zero treatment is applied, the increase in the amount of martensite reaches a plateau, and the rate of increase in strength is also small. On the other hand, the A value is 18.
If it exceeds 0, the austenite phase becomes a single phase in the annealed state and the workability is excellent, but the austenite phase becomes too stable and even if sub-zero treatment is performed, the martensite phase is not generated much and high strength cannot be obtained. On the other hand,
When the A value is in the range of 15.7 to 18.0, almost no martensite is produced in the annealed state, or even if it is produced, it is extremely small, so workability is good, and sufficient martensite is produced when sub-zero treatment is performed. This results in high strength. Therefore, in order to achieve the object of the present invention, it is necessary to adjust the amount of each component within the above-mentioned range so that the A value is in the range of 15.7 to 18.0. Note that the broken line in Figure 1 indicates the same as above without adding B.
The relationship between the amount of martensite and the A value is shown for annealed control steels and sub-zero treated steels whose values have been changed. When compared with the B-added steel indicated by the solid line, there is no significant difference in the amount of martensite between the B-added steel according to the present invention and the control B-free steel in the annealed material, but in the sub-zero treated material, the B-added steel of the present invention The amount of martensite is larger in the case of , and the effect of B addition is recognized. EXAMPLES The effects of the present invention will be specifically illustrated by examples below. [Example] Table 1 shows the chemical composition and A value of the test steel. A1 to A3 steels are conventional steels. B1 to B20 steels are steels of the present invention. Further, C1 to C3 steels are comparative steels and have component contents outside the range specified by the present invention. 12 kg of these steels were melted and cast using the vacuum melting method, and the resulting steel ingot was forged and then hot rolled into a 3 mm thick plate. After solution treatment, cold rolling and annealing were repeated for 1 hour. .0m
An annealed plate having a thickness of m was obtained. This was used as a test material and placed in methyl alcohol (-73℃) cooled with dry ice.
Sub-zero processing to maintain time was applied. Table 2 shows the amount of martensite (M amount) in the annealed material and sub-zero treated material of each steel.
, hardness and tensile properties. [Table 1] [Table 2] [Table 2] As seen from the results in Table 2, conventional steel A
1, A2, and A3 steels all have A values exceeding 18.0, and both the annealed material and the sub-zero treated material have a martensite content of 0.5% or less, and the austenite phase is stable. Therefore, the hardness and tensile properties hardly change even after subzero treatment. In C1 steel, which is a comparison steel, the content of each element is within the range of the present invention, but since the A value is as low as 14.65, a large amount of martensitic phase already exists in the annealed material, and the hardness is 51.
4, elongation is low at 4.1%, and workability is poor. In C2 steel, the content of each element is within the range of the present invention, but A
Since the value is high at 19.38, there is no martensite phase in the annealed material, and it has good workability with a hardness of 238 and an elongation of 37.8%. However, the amount of martensite produced in the sub-zero treated material is There is little change in hardness and tensile strength. In addition, C3 steel has an A value of 17.54, which is within the range of the present invention, but because the C content is low, the hardness after sub-zero treatment is 27.
It is low at 8 and cannot be called high strength. On the other hand, all of the steels of the present invention have A values in the range of 15.7 to 18.0, and the hardness of annealed materials is 3.
It has a low elongation of less than 0.00 and has an elongation of more than 30%, and has excellent workability. It can be seen that after sub-zero treatment, the hardness increased to over 400 and the strength was increased. As described above, the steel of the present invention has excellent workability in the solution treatment state by adding Si and B in combination to the steel whose composition has been adjusted so that the Ms point is near room temperature, and -73 High strength can be obtained by performing sub-zero treatment in a temperature range of approximately ℃, and there is no need for post-treatment such as descaling after sub-zero treatment. Therefore, the present invention provides a high-strength stainless steel material that has extremely high industrial value.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the A value and the amount of martensite in an annealed material and a sub-zero treated material according to the present invention.

Claims (2)

[Claims] [Claim 1] C: 0.01 to 0.30% by weight
, Si: 0.2 to 3.0%, Mn: 4.0% or less,
P: 0.030% or less, S: 0.008% or less, N
i: 3.0-7.0%, Cr: 12.0-18.0%
, N: 0.01~0.25%, B: 0.0003~
Contains 0.30% and A value=Ni+0.61×
Mn+22.60×C+13.77×N+0.43×C
Stainless steel in which the content of each element is adjusted so that the A value defined by r + 0.33 x Si + 0.20 x B is in the range of 15.7 to 18.0, with the remainder consisting of Fe and unavoidable impurities. A high-strength stainless steel with excellent workability that has excellent workability in the annealed state and develops high strength through sub-zero treatment. [Claim 2] C: 0.01 to 0.30 in weight%
%, Si: 0.2 to 3.0%, Mn: 4.0% or less, P: 0.030% or less, S: 0.008% or less,
Ni: 3.0-7.0%, Cr: 12.0-18.
0%, N: 0.01-0.25%, B: 0.000
In addition to containing 3 to 0.30%, Mo: 3.0
% or less, Cu: 3.0% or less, Co: 3.0% or less, Nb: 0.5% or less, V: 0.5% or less, W:
0.5% or less, contains one or more of Ti: 0.5% or less, and A value = Ni + 0.61 × Mn
+22.60×C+13.77×N+0.43×Cr+
0.33×Si +0.20×B+0.
25×Mo+0.18×Cu+0.29×Co+0.3
6×Nb +0.33×V+0.27×
A value defined by W + 0.35 x Ti is 15.7 to 18
．． This is a stainless steel in which the content of each element is adjusted to be in the 0 range, with the remainder consisting of Fe and unavoidable impurities, and has excellent workability in the annealed state and develops high strength through sub-zero treatment. High strength stainless steel with excellent durability.