JPH03138335A - Martensitic stainless steel for cold forging and its manufacture - Google Patents

Abstract

PURPOSE:To provide the stainless steel with excellent cold forgeability as annealed and to improve its corrosion resistance and hardening capacity by specifying the content of Si, Mn, S, etc., and the heat treating conditions in a steel and regulating its grain size into a specified range. CONSTITUTION:The compsn. of a martensitic stainless steel for cold forging is formed from, by weight, 0.30 to 0.40% C, <=0.30% Si, <=0.40% Mn, <=0.005% S, 12.00 to 14.00% Cr and the balance Fe with inevitable impurities. The steel is subjected to heat treatment for one or two times of which heating is executed at 800 to 950 deg.C for 2 to 16hr. By this heat treatment, its grain size is regulated to 7 to 10 and carbides therein are spheroidized. In this way, its cold forgeability and mechanical strength are improved. The steel is useful as a steel for structural purposes such as house appliances, pins for stopping shingles, screws and shafts.

Description

[Detailed Description of the Invention] [Industrial Application Field] Does the present invention apply to houses? 412 products, high-strength martensitic stainless steel that has excellent cold forgeability in the annealed state and has outstanding corrosion resistance and quench hardenability, used for roof plate fixing bolts, screws, shafts, etc. and its manufacturing method.

[Conventional technology] Low carbon 13% C "martensitic stainless steel (SU
S410, etc.) not only have excellent corrosion resistance and heat resistance, such as oxidation resistance up to high temperatures without decreasing strength up to 500℃, but also have high strength and toughness, so they are used for home appliances and bins for roofing. , widely used as structural steel for screws and shafts, etc.

In addition, medium carbon 13% Cr martensitic stainless steel (SUS420J or 5US420Jz) has a high C content, which increases strength and wear resistance. It is used for the required structure.

[Problems to be Solved by the Invention] For any of the above-mentioned steel types, quenching and tempering treatment is desirable in order to obtain not only toughness but also corrosion resistance, and the steel is used after being tempered. By the way, the market demand for #IN is that it has higher mechanical strength, with a quenching hardness of 600 Hv or more, and a tensile strength in the annealed state to facilitate cold forging. Strength is 55kgf/am
``It is desired that the corrosion resistance is the same as that of

However, among the above-mentioned steel types, 5US410 has a tensile strength of about 55 kgf/am" in the annealed state and has good cold forgeability, but has a quenched hardness of Hv430 and has poor mechanical strength. not something to be satisfied with,
In addition, for 5US420J, the quenching hardness is Hv
519, the mechanical strength is somewhat insufficient, the tensile strength in the annealed state is 62 kgf/am", and the cold forgeability is poor. Regarding 5US420J2, the quenched hardness is Hv. 608, the mechanical strength is satisfactory, but the tensile strength in the annealed state is 65 kgf/
As'', cold forging is nearly impossible, and even if the crystal grain size is coarsened by heat treatment, cold forging is difficult.

Generally, when trying to improve cold forgeability, strength decreases; cold forgeability and strength are contradictory properties, and it has been difficult to satisfy both.

In order to deal with such problems, the present invention particularly aims to solve the problem of Si, Mrl
This was made after studying the S component content and grain size, and it shows excellent cold forgeability in the annealed state without reducing the strength in the tempered state, and has excellent corrosion resistance and hardening properties. The purpose is to provide a high-strength martensitic stainless steel with excellent hardenability.

[Means for solving the problems] The martensitic stainless steel for cold forging of the present invention has the following features:
As the first invention, C: 0.30 to 0.40% by weight
, Si: 0.30% or less, Mn: 0.40% or less, S:
The gist is that it contains 0.005% or less, Cr; 12.00 to 14.00%, and the remainder consists of Fe and impurity elements.

The present invention is applicable to 5US410, SUS420J+
In view of the above-mentioned drawbacks of 5US420J2,
Effects of C1Si, Mn, and Cr on tensile strength in annealed state for 13%Cr martensitic stainless steel
``As a result of intensive research on the influence of alloying elements on i,
First, Si and M improve the strength of the substrate through solid solution action.
The second step is to reduce the S content as much as possible, which generates MnS and becomes the starting point for cracks during cold forging, and thirdly, by coarsening the grain size through a unique heat treatment. By making the carbide spheroidal, cold forgeability is improved and tensile strength is increased to 55 kgf/l11m.
It has been found that the temperature can be reduced to below. That is, in the present invention, by adding C and Cr equivalent to SUS 420 J 2, the quenching hardness can be increased to Hv6.
00 or higher, and succeeded in lowering the tensile strength in the annealed state by approximately 10 kgr/am" compared to conventional steel.

Further, the second invention of the present invention further provides M in the first invention steel.
o: 1.5% or less, Ti: 0.20% or less, V: 0.1
5% or less, Nb; Q, 10% or less of 1 or 21
The gist is that 11 or more are contained.

This further improves the tensile strength of the first invention steel in the quenched and tempered state, and also improves the strength and corrosion resistance when the material is used.

Furthermore, the method for producing a martensitic stainless steel for cold forging according to the third invention of the present invention includes a method for manufacturing a martensitic stainless steel for cold forging according to the third invention.
The gist is that a heat treatment of heating at a temperature of 50° C. for 2 to 16 hours is performed once or twice or more to obtain a crystal grain size of 7 to 10 and to make the carbide spheroidal, thereby improving cold forgeability. Further, the method for manufacturing a cold-forgeable martensitic stainless steel according to the fourth invention is such that the steel of the second invention is subjected to a heat treatment of heating the steel at a temperature of 800 to 950°C for 2 to 16 hours once or twice or more, thereby crystallizing the steel. The gist is that the cold rrf1 forgeability is improved by setting the grain size to 7 to 10 and making the carbide spheroidal.

Next, the reason for limiting the composition of the martensitic stainless steel for cold forging of the present invention will be explained.

C: 0.30-0.40% C is an element necessary to ensure a hardness of Hv600 or more, and in order to obtain this strength, the lower limit was set to 0°30%. However, as the amount of C increases, the tensile strength in the annealed state increases and the cold forgeability decreases significantly, so the upper limit is set to 0.
It was set at 40%.

Si: 0.30% or lessSi is an element that is effective in deoxidizing, but it is also a strong ferrite-forming element, which increases strength through solid solution strengthening and reduces cold forgeability. , it is necessary to reduce its content as much as possible, and the upper limit is 0.30%.
And so.

Mn: 0.40% or less Mn is an element that is effective in deoxidizing and is a strong austenite forming element, but on the other hand, it is an element that increases strength due to solid solution action and significantly reduces cold forgeability. Therefore, it is necessary to reduce the content as much as possible, and the upper limit should be set at 0.
It was set at 40%.

S: 0.005% or less S is an element that generates MnS, which becomes a starting point for cracks, during cold open forging, and significantly reduces cold forgeability, as well as deteriorating corrosion resistance.The upper limit is 0.005%. did. In addition,
Furthermore, in order to improve cold forgeability, the content is preferably 0.002% or less.

Cr;12. OO~14. OO% C'' is a basic element that imparts corrosion resistance to stainless steel, and to increase this effect it is necessary to contain it at 12.00% or more, so the lower limit was set at 12.00%.On the other hand, C'' is also a strong ferrite-forming element and impairs hardenability, so its upper limit was set at 14.00%.

Mo: 1.5% or less, Ti: 0.20% or less, V: 0.
15% or less, Nb, 0.10% or less Mo, Ti, V, and Nb are elements that contribute to improving the tensile strength in the tempered state, but on the other hand, they are strong ferrite-forming elements and reduce hardenability. Therefore, the upper limit of Mo is 1.
．． 5%, Ti is 0.20%, ■ is 0°15%, Nb is 0
．． It was set at 10%.

In the manufacturing method of the present invention, the heating temperature is 800 to 950.
℃ because if the heating temperature is less than 800℃, it will have to be held for 34 hours to obtain the desired crystal grain size, which will significantly reduce productivity.
This is because if the temperature exceeds 50°C, the crystal grain size becomes too large. In addition, the grain size was set to 7 to 10 because if the grain size is 7 or less, the product texture after cold forging will deteriorate, and if the grain size is 10 or more, the tensile strength in the annealed state will deteriorate. This is because the strength cannot be guaranteed to be 55 kgf/am.

[Function] In particular, the first invention steel of the present invention has a Si content of 0.30% or less,
By setting the Mn content to a low value of 0.40% or less and the S content to a low value of 0.005% or less, and the synergistic effect of these and the optimal range of C and Cr, cold forgeability, which is a drawback of conventional steel, has been improved. It is something. In addition, the grain size of conventional steel is fine (approximately 14).
By considering the heat treatment conditions in relation to the above composition range, the grain size was set to 7 to 10, and the carbides were made spheroidized, thereby converting the conventional steel 5US420.
It has improved cold forgeability while maintaining mechanical strength equivalent to J.

Therefore, according to the first invention, the tensile strength in the annealed state is 55 kgf / +ua2 or less, the cold forgeability can be improved, and the quenched hardness is -1v6
00 or more, and can improve tensile strength and strength in a tempered state, and further improve corrosion resistance.

Furthermore, due to the reduction in Si content, the toughness in the tempered state can be significantly improved compared to conventional steels.

Therefore, the steel of the present invention is a martensitic stainless steel suitable as structural steel for home appliances, roof plate fixing bins, screws, shafts, and the like.

Moreover, the second invention is characterized in that the first invention contains a specific amount of one or more of Mo, Ti, V, and Nb.
In addition to obtaining the same effect as the invention, strength and corrosion resistance are further improved.

The method for producing martensitic stainless steel for cold forging of the present invention includes the first invention steel or the second invention steel having a
Heat treatment at a temperature of 50°C for 2 to 16 hours is performed once or twice to reduce the grain size to 7 to 10 and to make the carbide spheroidal, so it maintains the same quenched hardness as conventional steel. It is possible to produce steel with excellent cold forgeability.

[Example] Next, while comparing the characteristics of the present invention with conventional steel and comparative steel,
This will be clarified by examples.

Table 1 shows the chemical composition of these test steels. In Table 1, A- is the steel of the present invention, A-D is the first invention steel, and E- is the second invention steel.

In addition, L-P steel is a comparative steel, L is a comparative steel whose C content is higher than the composition range of the present invention, M is a comparative steel whose C content is lower than the composition range of the present invention, and N is a Si content. 0 is a comparative steel whose Mn content is higher than the composition range of the present invention, P is a comparative steel whose Cr content is lower than the composition range of the present invention. Q and R are conventional steels, with Q having a composition corresponding to 5US410 and R having a composition corresponding to 5US420J2, respectively.

(Left below) The test steels in Table 1 were annealed under the conditions of holding at the temperature and time shown in Table 2, furnace cooling at a cooling rate of 25°C/Hr to 600°C, and then air cooling. The tensile strength, area of area, and crystal grain size were measured, and the results are shown in Table 2.

In addition, Table 2 shows the hardness of the product that was oil-quenched at 1000°C for 1 hour, and the hardness, tensile strength, and corrosion loss of the product that was IHr tempered at 750°C after the above-mentioned quenching. It is. Regarding corrosion loss, 5% Na
The corrosion loss was measured after 24 hours of immersion in a Cl-2% 820240°C aqueous solution.

Note that the tensile strength and area of area were measured by creating a JIS No. 4 test piece.

(Left below) As is clear from Table 2, in the comparative example in which the C content was higher than the composition range of the present invention, the tensile strength in the annealed state was 57.9 to 59.2 kgf/2. Therefore, cold forgeability in the annealed state is poor. In addition, in Comparative Example M, in which the C content was lower than the composition range of the present invention, the tensile strength in the annealed state was about 53 kgf/m theory 2 and excellent cold forgeability, but the quenched hardness was Hv574. , inferior in mechanical strength.

On the other hand, in Comparative Examples N and O in which the content of Si or Mn was higher than the composition range of the present invention, the tensile strength in the annealed state was 57.3 to 60.3 kgf/am'', and the cold forgeability was Comparative example P, which had a low Cr content, had excellent cold forgeability and quenching hardness, but
Corrosion loss is 8.9 g/m"Hr, which is poor in corrosion resistance.

Conventional steel Q equivalent to 5US410 has a tensile strength in the annealed state of 51 kg4/-m" and has excellent cold forgeability, but has an insufficient mechanical strength with a quenching hardness of Hv422. Conventional steel R, which corresponds to SUS 420 Jz, has a tensile strength of 58.5 to 59.7 kg in the annealed state.
f/--2, and poor cold forgeability.

On the other hand, steel A~, which is the steel of the present invention, has a desired grain size of 8.4 to 9.5 and a tensile strength of 53.6 to 57.6 kgf/am'' in the annealed state. It has excellent cold forging properties and has a quenched hardness of Hv615.
674, which showed excellent mechanical strength, and the effect of the present invention could be confirmed.

In addition, when the microstructure of the carbides in A-, which is the steel of the present invention, was examined in the annealed state, the carbides were spheroidized, and the heat treatment of the method of the present invention made the carbides spheroidal.
It was also confirmed that the desired cold forgeability was obtained. In addition, when the impact values of the inventive steel and the conventional steel were measured in the annealed state, the inventive steel had a lower impact value of approximately 8 k compared to the conventional steel.
gf/am” was found to be improved.

[Effects of the Invention] The martensitic stainless steel for cold forging of the present invention and its manufacturing method are particularly advantageous in that the Si content is 0.30% or less and the Mn content is 0.30% or less.
By setting the S content to a low value of 0.40% or less and the S content to a low value of 0.005% or less, and considering the synergistic effect between these and the optimum range of C and Cr, and the heat treatment conditions in relation to the above composition range, According to the present invention, which has a crystal grain size of 7 to 10 and improved cold forgeability and mechanical strength, the tensile strength in annealed state is 53.6 to 57.6 kgf/mm.
It is possible to improve the cold forgeability, the quenching hardness is Hv615 to 674, and the strength in the tempered state can be improved, and the corrosion resistance can be further improved. Therefore, the steel of the present invention is extremely useful as a martensitic stainless steel used as structural steel for home appliances, roof plate fixing bins, screws, shafts, and the like.

Claims (1)

[Claims] (1) C; 0.30 to 0.40% by weight, Si;
0.30% or less, Mn; 0.40% or less, S; 0.00
Contains 5% or less, Cr; 12.00 to 14.00%,
A martensitic stainless steel for cold forging, characterized in that the remainder consists of Fe and impurity elements. (2) C: 0.30-0.40%, Si: 0.30% by weight
Hereinafter, Mn: 0.40% or less, S: 0.005% or less,
Contains Cr; 12.00 to 14.00%, and further contains Mo
; 1.5% or less, Ti; 0.20% or less, V; 0.15
% or less, Nb; 0.10% or less, and the remainder consists of Fe and impurity elements. (3) C: 0.30 to 0.40%, Si;
0.30% or less, Mn; 0.40% or less, S; 0.00
Contains 5% or less, Cr; 12.00 to 14.00%,
Steel consisting of the balance Fe and impurity elements is heated to 800 to 9
A cold workpiece characterized by having been subjected to heat treatment at a temperature of 50°C for 2 to 16 hours once or twice or more to obtain a crystal grain size of 7 to 10 and to spheroidize carbides to improve cold forgeability. A method for manufacturing martensitic stainless steel for forging. (4) C: 0.30 to 0.40%, Si;
0.30% or less, Mn; 0.40% or less, S; 0.00
Contains 5% or less, Cr; 12.00 to 14.00%,
Furthermore, Mo: 1.5% or less, Ti: 0.20% or less, V
0.15% or less, Nb; 0.10% or less, and the remainder is Fe and impurity elements. Heat treatment in which steel is heated at a temperature of 800 to 950°C for 2 to 16 hours. A method for producing martensitic stainless steel for cold forging, characterized in that cold forgeability is improved by applying the following steps once or twice or more to obtain a crystal grain size of 7 to 10 and to spheroidize carbides.