JPH01275737A - High strength stainless steel having excellent cold workability - Google Patents

Abstract

PURPOSE:To provide the title steel with high strength and to improve its corrosion resistance and cold workability by specifying the content of C, Cr, Mo, Ca, etc. in a stainless steel. CONSTITUTION:The compsn. of a high strength stainless steel is constituted of, by weight, 0.3-0.6% C, <=0.3% Si, <=0.3% Mn, <=0.03% P, <=0.005% S, <=0.6% Ni, 13.0-19.0% Cr, 0.3-3.0% Mo, 0.002-0.02% Ca and <=0.05% Al, furthermore of 0.002-0.02% B and one or more kinds among 0.005-0.05% rare earths and balance consisting of Fe with inevitable impurities. If required, one or more kinds among 0.03-0.30% V, 0.03-0.30% Nb and 0.03-0.30% Ta, and, moreover, 0.3-2.0% Cu are incorporated thereto. The stainless steel has excellent cold workability as cast, furthermore has high hardness after quenching and tempering and has excellent corrosion resistance as well.

Description

[Detailed description of the invention]

[Object of the Invention] (Industrial Application Field) The present invention is applicable to OA type equipment, automobiles, and various industrial machines. Prevents rust from forming on various agricultural machinery, chemical equipment, etc.
High-strength Cr stainless steel is used as a material for parts that require high strength including wear resistance, and has excellent cold workability and is particularly suitable as a material for parts formed by cold working. It concerns high-strength stainless steel. (Conventional technology) High-strength parts that are more resistant to rust, such as various shafts and self-tapping screws. 5US is used for materials such as electronic fuel injection nozzles and food bearings.
Martensitic Cr stainless steel such as 410, 5US420J2, 5US440C is used. (Problem to be solved by the invention) However, although these are stainless steel,
Rust may occur during use, leading to deterioration of product performance and reliability. Furthermore, since high C and high Cr materials such as 5US440C for high hardness contain coarse carbides, their workability in hot and cold conditions is not very good and manufacturing may be difficult. It is also difficult to perform deformation rolling or cold forging. Therefore, 5U
When processing a material such as S440C into a part shape, most of the part is cut in an annealed state, quenched and tempered, and then ground for final finishing. In addition, precipitation hardening martensitic stainless steels such as 5US630, which are more expensive than Cr stainless steels, are also used when strength and corrosion resistance are required, and solution treatment - machining -
It is often processed into parts through the age hardening process,
This is also difficult to process such as cold forging. As described above, conventional martensitic Cr stainless steels generally have poor corrosion resistance, and in the case of high-hardness materials, there are problems in that the material has poor manufacturability and workability. In addition, expensive precipitation hardening martensitic stainless steel (SUS63
0) also has the problem of poor cold workability. Currently, there are no relatively inexpensive materials to replace them, so they are used out of necessity. For this reason, processing costs have increased, and parts prices have become higher than material costs. Therefore, in fields such as automobile parts, which are produced in large quantities and where there is a strong demand for cost reduction, there is a problem in that this high price limits the range of application of the material and the spread of parts using new technology. (Objective of the Invention) The present invention solves the above-mentioned conventional problems by solving both the problems of corrosion resistance and cold workability, and providing a relatively inexpensive high-corrosion-resistant, high-strength stainless steel for cold working. The purpose is to

[Structure of the invention]

(Means for solving the problem) In order to improve the corrosion resistance of steel materials, Cr, Mo, Cu, etc. are expected to stabilize the protective film, improve the film repair ability, and improve the passivation film forming ability. The addition of alloying elements is effective. Furthermore, in order to improve cold workability, it is effective to remove coarse carbides that become starting points for cracks and voids. This carbide is M
, C3 (M is mainly Cr), so consideration must be given to the steel composition to prevent the formation of Cr. Specifically, this is achieved by reducing the amount of C while taking into account the relationship between the amounts of Cr and Mo and the required strength. Furthermore, in order to improve the yield and provide the material at low cost, it is essential that the material has excellent hot workability. For this purpose, it is effective to reduce Si and S, as well as add a small amount of Ca. For further improvement, addition of B and REM is also effective. The inventors carried out research and development in consideration of strength, corrosion resistance, and cold and hot workability as described above, and discovered a Cr-based high-strength martensitic stainless steel that is excellent in these properties. That is, the high strength stainless steel according to the present invention contains 1% by weight.
So, C: 0.3 to 0.6%, Si 0.3% or less, M
n + 0, 3% or less, P: 0.03% or less, S: 0
．． 005% or less, Ni: 0.6% or less, Cr: 13.
0-19.0%, Mo: 0.3-3.0%, Ca: 0
．． 002 to 0.02%, Al: 0.05% or less, further B: 0.002 to 0.02%, and rare earth element 20.0
One or more types selected from 0.05 to 0.05%, and further V: 0.03 to 0.30% as necessary.
, Nb: 0.03~0.30% and Ta: 0.03~
Contains one or more selected from 0.30%, Cu: 0.3 to 2.0% if necessary, and the remainder is Fe and impurities, and has excellent cooling properties in the annealed state. It is characterized by having machinability, and by having such a composition of high Cr stainless steel, it is a means to solve the above-mentioned conventional problems. Next, the reasons for limiting the composition (wt%) of the high-strength stainless steel with excellent cold workability according to the present invention will be explained. C: 0.3-0.6% C is an essential element to obtain the strength of steel. That is,
During quenching, most of the matrix becomes a martensite structure, which is solidly dissolved in it to significantly increase its strength. In addition, when the subsequent restoring is performed, carbides are formed and contribute to strengthening. In order to obtain sufficient strength, it is necessary to add 0.3% or more. However, when a large amount of C is added, coarse M and C3 (M is mainly Cr) not dissolved in the matrix are formed in the matrix when softening treatment is performed by spheroidizing annealing (SA) for cold working. It remains and impairs cold workability. Furthermore, C lowers the martensitic transformation point and causes an austenite phase to remain during quenching, lowering the strength and making it more likely to cause cracking. Therefore, the upper limit of C was set to 0.6%. Si: 0.3% or less Si is usually added as a deoxidizing agent. Since this Si impairs cold and hot workability, it is desirable to suppress its addition as much as possible, but considering the manufacturability during melting, the upper limit is set at 0.3
%. Mn: 0.3% or less Mn is also added as a deoxidizing agent like Si, but since it impairs cold workability and corrosion resistance, it is set to 0.3% or less. P: 0.03% or less P is mixed into steel as an impurity from raw materials, etc., but if it is too large, it impairs the toughness of the steel, so it is set to 0.03% or less. S: 0.005% or less S is also mixed into steel as an impurity and deteriorates hot workability, so the upper limit was set at 0.005%. Ni: 0.6% or less Ni is an element that improves the toughness of the martensitic phase, but if it is included in a large amount, it becomes difficult to anneale and at the same time lowers the transformation point and makes it easier to form retained austenite.
The upper limit was set at 0.8%. Cr: 13.0-19.0% Cr is the most basic element that imparts corrosion resistance to steel, and at the same time forms carbides in the present invention. In order to obtain sufficient corrosion resistance, the Cr content must be 13.0% or more.
is required to be dissolved in the matrix. Further, although it depends on the amount of C contained, if the Cr content is less than 13.0%, coarse carbide Cr7C3 will be generated and impair cold workability, so the Cr content is set to 13.0% or more. Further, if a large amount of Cr is contained, a δ-ferrite phase is generated and the elasticity is lowered, so the upper limit is set to 19.0%. Mo: 0.3-3.0% MO is an effective element for improving the corrosion resistance of steel,
To obtain such an effect, it is necessary to contain 0.3% or more. In addition, if it exceeds 3.0%, δ-
Ferrite is easily generated and hot workability is also deteriorated. Ca: 0.002 to 0.02% Ca improves hot workability and machinability by containing a small amount. However, if it is less than 0.002%, the effect cannot be expected, and if it is contained in a large amount, the hot workability will deteriorate, so the upper limit was set at 0.02%. AIL: 0.05% or less AIL is used as a deoxidizing agent, but there is a risk that it may remain in the steel as an oxide that impairs cold workability, so unnecessary addition must be avoided. The upper limit was set at 0.05%. B: 0.002-0.02% Rare earth element (REM): 0.005-0.05% B and REM (one or more selected from rare earth elements) both improve hot workability. At least one or two or more of B and REM are added because they have the same effect as that of B and REM. The effect of this B and REM is that B is 0.002% or more,
A significant effect cannot be obtained unless REM is contained in an amount of 0.005% or more, and if added in large amounts, either element will actually reduce hot workability. When a large amount of B is present, it forms borides at grain boundaries or becomes concentrated, lowering the melting point of those portions and making hot cracking more likely to occur. Also, RE
When M is present in a large amount, a large number of inclusions are formed, which act as starting points for single cracks and reduce hot workability. Therefore, for these reasons, the upper limit of B was set to 0.02%, and the upper limit of REM was set to 0.05%. V: 0.03-0.30% Nb: 0.03-0.30% Ta: 0.03-0.30% V, Nb, and Ta form fine carbides and contribute to improving strength. , add as necessary. However, 0
．． If the content is less than 0.3%, the effect will be small, and if the content exceeds 0.30%, the carbide will become coarse and the hot and cold workability will deteriorate. Therefore, when added, the contents of V, Nb, and Ta are each 0.03 to 0.30%. Cu: 0.3 to 2.0% Cu is an effective element for improving corrosion resistance, and is added as necessary. And to get a clear effect, 0.
It is necessary to contain 3% or more, but if added in a large amount, it not only impairs hot workability but also lowers the transformation point like Ni, so if it is added, the upper limit of the content should be 2.
It was set to 0%. (Example) Example steel No. of the present invention having the chemical composition shown in Table 1. 1 to 8 and Comparative Example Steel N001 to 7 were melted in a vacuum and in an argon atmosphere in an amount of 50 kg and forged to produce a material with a diameter of 16 mm.Next, from these materials, materials for evaluating hot workability were obtained. After collecting it, it was subjected to spheroidizing annealing to soften it. The hot workability was evaluated based on the degree of cracking during hot rolling at 1000°C. The results are also shown in Table 1. In the hot workability column of Table 1, A indicates that there were no cracks, B indicates that there were some cracks, and C indicates that there were many cracks. As shown in Table 1, invention example steel Nos. 1 to 8
Comparative example steel No. 5 (SUS410) and No.
, 6 (It was confirmed that it has excellent hot workability comparable to SUS42 (lJ2).Next, in order to evaluate the cold workability, a room temperature compression test was performed using the spheroidized annealed material. The relationship between compressive true stress and compressive strain during this implementation was determined for Invention Example Steel No. 2.
, Comparative example steels N004 and N006 are shown in Fig. 1, and the maximum compression range n (h
o/h) (ho: Height of specimen before compression, h
: height of the test piece after the test) are also shown in Table 1. As shown in FIG. 1, Example Steel No. 002 of the present invention has significantly better cold workability than Comparative Example Steel No. 5US440C, which is also Comparative Example Steel No. It is equivalent to or more than 5US420J2 of 6,
It can be fully used for cold processing. Further, the critical compressive strain at which cracking does not occur for other steels of the present invention was 1.80 or more as shown in Table 1, and it was recognized that they had good cold workability. Furthermore, we investigated the presence of coarse carbides that impair cold workability.
Similarly, as shown in the microstructure column of Table 1 (0 mark indicates no coarse carbide, X mark indicates presence of coarse carbide), there was a good correspondence with cold workability. Furthermore, when the hardness of each sample material after quenching and low-temperature tempering was investigated, as shown in the hardness column of Table 1, all of the steels of the present invention had a hardness of HRC50 or higher. ,
It was confirmed that the strength was also excellent. Furthermore, the corrosion resistance of each sample material after quenching and low-temperature tempering was investigated by a wet test. This humidity test was conducted at a temperature of 49°C and a humidity of 95%.
The presence or absence of rust was investigated after being kept in the above humid environment for 72 hours, and the results are also shown in the corrosion resistance column of Table 1. In Table 1, A means that there was no rust, B means that there was a little rust, C means that there was a lot of rust, and D means that there was rust all over the surface. It shows. As shown in Table 1, the example steels of the present invention are comparative example steel No. 4 (SUS440C) and N006 (SUS4
It was recognized that the wet corrosion resistance was one rank higher than that of 20J2).

【Effect of the invention】

As described above, the stainless steel according to the present invention has C: 0.3 to 0.6%, Si: 0.3% or less, M n
: 0, 3% or less, P: 0.03% or less, S: 0.
005% or less, Ni: 0.6% or less, Cr: 13.0
~19.0%, M o + 0, 3~3.0%, Ca
: 0.002 to 0.02%, Al: 0.05% or less, B: 0.002 to 0.02%, and rare earth elements:
One or more types selected from 0.005 to 0.05%, and if necessary, V: 0.03 to 0.05%.
30%, Nb: 0.03-0.30% and Ta: 0.
The material contains one or more selected from 0.03 to 0.30%, and if necessary Cu: 0.3 to 2.0%, with the balance consisting of Fe and impurities. Because it has good hot workability during annealing, it has excellent manufacturability, so the material can be provided at a low price, and it also has good cold workability in the annealed state compared to conventional steel. In addition, it has a high hardness and relatively high strength after quenching and tempering, and it also has significantly superior corrosion resistance compared to conventional steel.
A significant effect is brought about in that it is possible to solve the conventional problems of both corrosion resistance and cold workability, and to provide a relatively inexpensive high corrosion resistance and high strength stainless steel for cold working.

[Brief explanation of the drawing]

Figure 1 shows Example Steel No. 002 of the present invention and Comparative Example Steel No.
4 is a graph illustrating the results of examining the relationship between compressive true stress and compressive strain when a room temperature compression test was performed on Samples No. 4, No. 6, and No. 6.

Claims (4)

[Claims] (1) In weight%, C: 0.3-0.6%, Si: 0.3
% or less, Mn: 0.3% or less, P: 0.03% or less, S
: 0.005% or less, Ni: 0.6% or less, Cr: 13
．． 0-19.0%, Mo: 0.3-3.0%, Ca: 0
．． 002 to 0.02%, Al: 0.05% or less, further B: 0.002 to 0.02%, and rare earth elements: 0.0
0.05 to 0.05%, the balance being Fe and impurities, and having excellent cold workability in an annealed state. High strength stainless steel. (2) In weight%, C: 0.3 to 0.6%, Si: 0.3
% or less, Mn: 0.3% or less, P: 0.03% or less, S
: 0.005% or less, Ni: 0.6% or less, Cr: 13
．． 0-19.0%, Mo: 0.3-3.0%, Ca: 0
．． 002 to 0.02%, Al: 0.05% or less, further B: 0.002 to 0.02%, and rare earth elements: 0.0
05 to 0.05%, one or more selected from 0.05 to 0.05%, furthermore V: 0.03 to 0.30%, Nb: 0.
03 to 0.30% and Ta: 0.03 to 0.30%, the balance being Fe and impurities, and having excellent cold workability in an annealed state. High strength stainless steel with excellent cold workability. (3) In weight%, C: 0.3 to 0.6%, Si: 0.3
% or less, Mn: 0.3% or less, P: 0.03% or less, S
: 0.005% or less, Ni: 0.6% or less, Cr: 13
．． 0-19.0%, Mo: 0.3-3.0%, Ca: 0
．． 002 to 0.02%, Al: 0.05% or less, further B: 0.002 to 0.02%, and rare earth elements: 0.0
Cu: 0.3-2.0%, balance Fe and impurities, and has excellent cold workability in the annealed state. A high-strength stainless steel with excellent cold workability. (4) In weight%, C: 0.3 to 0.6%, Si: 0.3
% or less, Mn: 0.3% or less, P: 0.03% or less, S
: 0.005% or less, Ni: 0.6% or less, Cr: 13
．． 0-19.0%, Mo: 0.3-3.0%, Ca: 0
．． 002 to 0.02%, Al: 0.05% or less, further B: 0.002 to 0.02%, and rare earth elements: 0.0
05 to 0.05%, one or more selected from 0.05 to 0.05%, furthermore V: 0.03 to 0.30%, Nb: 0.
03 to 0.30% and one or more selected from Ta: 0.03 to 0.30%, furthermore Cu
: 0.3 to 2.0%, the balance consisting of Fe and impurities,
A high-strength stainless steel with excellent cold workability, which is characterized by excellent cold workability in an annealed state.