JPH02217437A - Steel for cold working - Google Patents

Abstract

PURPOSE:To provide a steel for cold working without coarsening gamma grains with quenching after deep cold working by specifying contents of C, Mn, Cr, Al, N, etc., and ratio of Al/N. CONSTITUTION:The composition of the steel for cold working is constituted of 0.30-0.50wt.% C, <=0.50wt.% Si, <=0.50wt.% Mn, 0.1-5.0wt.% Cr, 0.01-0.80wt.% Mo, 0.005-0.20wt.% Nb, 0.005-0.10wt.% Al, <=0.035wt.% N under satisfying 1.00<=Al/N<=2.00 and the balance Fe with inevitable impurities. In the impurities, P and S are regulated to <=0.015wt.% and <=0.010wt.%, respectively. If necessary, further one or more kinds of 0.01-0.30wt.% V, 0.003-0.0050wt.% B and 0.01-2.00wt.% Ni are container. By this method, the excellent steel for machine structural use without any deterioration of toughness and strain of heat treatment, can be obtd.

Description

Detailed Description of the Invention (Industrial Application Field) This invention relates to cold processing used for mechanical structural parts, such as shafts, gears, bolts, etc., which are subjected to quenching and tempering after cold forming. Regarding industrial steel.

(Prior Art) Generally, for example, 0.05 mm is used for mechanical structures.
SCM435 low alloy steel specified in JIS G4105 with a composition of 35%C-1,0%Cr-0,2%Mo, 0.31%C-0,8%Cr-1,8%Ni
- SNCM431 low alloy steel specified in JTS G4103 with a composition of 0.2% Mo, or 0.2% Mo
Boron tone having a composition of C-0.8% Cr-0.02% B is formed by cold working for the purpose of streamlining the manufacturing process and improving finishing accuracy. However, in areas where heavy working is applied during cold working, austenite crystal grains (hereinafter referred to as 1 grain) become coarse during the subsequent quenching, resulting in problems such as deterioration of mechanical properties, especially impact properties, and increased heat treatment distortion. Therefore, it is necessary to prevent one grain from becoming coarse.

In response to this, the following techniques have been conventionally used or proposed as countermeasures.

(1) A method of adding 0.005 to 0.080% Nb and performing a special refining treatment as proposed in Japanese Patent Publication No. 55-38010, and a method of adding 0.005 to 0.080% Nb and performing a special refining treatment, as proposed in Japanese Patent Publication No. 56-75551, A proposed in Publication No. 63-11423
Techniques to prevent coarse grains by limiting the ingredients, such as limiting the ratio of l to N (Ai<χ)IN(χ).

(2) Iron and Steel, Vol. 70 (1984) p. 19
93-2000, a technique of performing annealing or normalizing before hardening treatment.

(This is what the invention attempts to solve!I!!!I) However, the technology for preventing the coarsening of γ grains by limiting the components mentioned in (1) above is difficult to achieve when the reduction rate is reduced during cold processing. When processed by 70% or more, it is difficult to prevent one grain from becoming coarse even if Wb is added or the 8l(χ)IN(χ) ratio is individually limited.
Moreover, the former method is disadvantageous in terms of cost since it may involve special refining treatment. Further, in the technique (2) of performing annealing or normalizing before performing the quenching treatment, it is possible to prevent the γ grains from becoming coarse, but there is a problem in that the manufacturing cost increases.

An object of the present invention is to produce, at low cost, a steel for cold working that does not cause grain coarsening during subsequent quenching even when subjected to strong cold working.

(Means for Solving the Problems) In order to achieve the above object, the present inventors have developed the Nb
As a result of repeated studies on the content of i, the amount of N that reacts with them to form precipitates, and their quantitative balance, we found that the following three points are important in achieving the above objectives. did.

That is, (1) ^l alone cannot prevent grain coarsening.

(2) By containing Nb, it is possible to suppress grain coarsening.

(3) To completely prevent grain coarsening, Nb should be included and the ratio of Ai(χ)IN(χ) should be 1.0θ to 2.00.
It is necessary to

The present invention has been made based on the above findings, and its gist lies in a cold working steel having the following compositions (1) and (2). (Hereinafter, % means weight % in total.) ■ C: 0.30 to 0.50% St: 0.
50% or less Mn; less than 0.50% Cr:
0.1~5.0%Mo: 0.01~0.80%
Nb: 0.005-0.20% 80: To,
005-0.10% N: 0.035% or less (however, 1.00≦^1(χ)IN(χ)≦2.00) Remainder: Fe and inevitable impurities.

■ In addition to the above component ■, V: 0.01-0.30%, B: 0.0003
~0.0050%, Ni: 0.01~2.00% Me - Contains one or more components selected from the three elements.

In any of the above compositions, P as an impurity is 0.015%
Hereinafter, S shall be 0.01% or less.

(Function) Below, the reason for limiting the content of each component constituting the group of the present invention will be described in terms of the function and effect.

C is an effective component for improving the hardenability of steel, increasing its strength, and also for refining crystal grains. Its content is 0.
If it is less than 30%, the effect of improving hardenability is small;
．． If the content exceeds 50%, the susceptibility to quench cracking during quenching increases, and it also causes toughness deterioration in association with other alloy components, so the content was set at 0.30 to 0.50%. .

Si is an effective element for deoxidizing steel and increasing its strength, but if it exceeds 0.50%, it may segregate and deteriorate toughness, so its content was set to 0.50% or less. .

Mn is an effective element for improving hardenability in addition to deoxidizing, but its content is set to less than 0.50% because it causes segregation in grain boundaries and deterioration in toughness.

Cr is an effective element for improving the hardenability of steel, but
If it is less than 0.1%, the effect cannot be obtained, and if it exceeds 5.0%, the toughness deteriorates and the susceptibility to quench cracking increases, so the content was set to 0.1 to 5.0%.

Mo is an effective element for improving the hardenability of steel, but
If the content is less than 0.01%, no effect will be obtained, and if the content exceeds 0.8%, the effect will be saturated and the cost will increase, so the content should be reduced from 0.01 to 0.80. %.

Nb is one of the important elements in the present invention, and is an effective element for improving the strength and toughness of steel, refining crystal grains, and preventing grain coarsening. but,
In order to ensure the effect, it is necessary to contain o, oos% or more.On the other hand, if the content exceeds 0.20%, the effect will be saturated and the cost will only increase. The content was set to o, oos ~ 0.20%.

^l is an effective element for stabilizing, homogenizing, and refining the deoxidation of steel, but if it is less than 0.005%, sufficient effects cannot be obtained; on the other hand, if it is contained more than 0.10%, Even if the content is o, oos ~ 0,1, the effect is saturated.
It was set to 0%.

N is also one of the important elements in the present invention, and Nb, A
It combines with ffi and the like to form precipitates, which contribute to preventing coarsening of crystal grains. However, in low alloy steel, 0.
Since it is difficult to contain more than 0.035%, the content was set to 0.035% or less.

On the other hand, A1. Regarding the balance between the content of This is because, in order to prevent coarsening of the ratio, it is necessary that at least the ratio of A Il, (X)/N (χ) satisfies the above formula.

One of the steels of the present invention consists of the above-mentioned components and the remainder Fθ and unavoidable impurities. It is important to suppress the upper limits of P and S as impurities.

No matter what heat treatment is applied, P cannot completely eliminate its grain boundary segregation and deteriorates toughness, etc., so the upper limit of P is set to 0.015%.

S combines with Mn to become a starting point for cracks, and even when S is used alone, it segregates at grain boundaries and promotes embrittlement, so it is necessary to limit S to as low as possible. Therefore, the upper limit was set at 0.01%.

(2) has the effect of making the steel grain finer and further improving the strength of the steel through precipitation hardening, and these effects become particularly noticeable when combined with Cr, Mo, and Nb. If the content is less than 0.01%, the above effect cannot be obtained, and if the content exceeds 0.30%, the effect is saturated and the toughness may deteriorate. .30%.

B has the effect of further improving the hardenability of steel, so it is added to ensure high strength especially when the dimensions of the product are large, but if the content is less than 0.0003%, the desired effect cannot be obtained. Furthermore, if the content exceeds 0.0050%, the toughness will deteriorate, so the content was set to 0.0003 to 0.0050%.

N1 has the effect of improving the strength and toughness of the rope, but 0.
If it is less than 0.01%, the above effect cannot be obtained, and if it is less than 2.00%
If it is contained in excess of
Therefore, the content was set to 0.01 to 2.00%.

(Example) The following will explain based on examples.

Using the usual method, the component composition shown in Table 1 (symbol A
~■) were melted. Steels A to M in the same table have compositions within the scope of the present invention, and steels N to ■ have compositions outside the scope of the present invention in the points marked with *.

After @A-V with the above-mentioned composition was made into a steel piece by continuous casting method or steel ingot method, it was heated to 1200 to 1250"C and hot rolled to a thickness of 301 mm, and then spheroidized annealed. After applying the material, the surface was cut and removed to give a thickness of 25 mm.This material was cold rolled to a thickness of 7.51 mm (reduction rate of 70%), and then quenched at 900°C. , an austenite grain size test was conducted.

The test results, such as the determination of γ grain coarsening, are also shown in Table 1. The grain size in Table 1 is the grain size measured based on the JIS GO551r steel austenite grain size test method.

As is clear from the table, in the steels A-M of the present invention,
No coarse grains were observed, indicating a healthy structure.

(Hereinafter, the margins) (Effects of the invention) The steel of the present invention has an excellent mechanical structure without the coarsening of a single grain that occurs during quenching after severe cold working, and without deterioration of toughness or heat treatment distortion. It is steel for industrial use. Moreover, this steel can be manufactured at low cost and has extremely high practical value in industry.

Claims (2)

[Claims] (1) In weight%, C: 0.30-0.50% Si: 0.50% or less Mn: less than 0.50% Cr: 0.1-5.0% Mo: 0.01-0.80 % Hb: 0.005-0.
20% Al: 0.005 to 0.10% N: 0.035% or less, and satisfies the following formula: 1.00≦Al(%)/N(%)≦2.00, and the remainder is Fe. and unavoidable impurities, P in the impurities is 0.015% or less, S is 0.01%
A cold working steel characterized by: (2) As component elements, further V: 0.01 to 0.30% B: 0.0003 to 0.0
050% Ni: The steel for cold working according to claim 1, containing one or more of 0.01% to 2.00% Ni.