JP3237990B2 - Cold forging steel with excellent cold workability and hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold forging steel excellent in cold workability and hardenability, and relates to a material for machine parts which is heat treated after cold forging.

[0002]

2. Description of the Related Art The steel for cold forging which is currently most widely used will be described. Normal cold forging steel is annealed offline for a long time after rolling. Although the microstructure after rolling is a two-phase structure of ferrite + pearlite, it has a spherical cementite structure after annealing. The steel types are mainly carbon steel for machine structure and alloy steel for machine structure. For example, in the case of a steel having a carbon content of 0.55%, the hardness of the ferrite + pearlite structure is about HV200, but the hardness becomes approximately HV155 in the case of the spherical cementite structure, so that the cold workability is excellent. . The steel for cold forging is often used after quenching and tempering after cold working. Spherical cementite is easily dissolved in austenite during quenching and heating, and has excellent hardenability. However, the limit value of softening is about HV155, and it is expected that a microstructure that further reduces the hardness will be found.

On the other hand, when ferrite + pearlite in steel has a two-phase structure of ferrite + graphite, its hardness is HV11.
It is well known that it is reduced to about 0 and the cold workability is remarkably improved. For example, Journal of the Japan Institute of Metals, No. 2, v
ol. 53 (1989); In the research paper No. 206, when the graphite ratio is increased, it is reported that the cold workability is superior to that of the sulfur free-cutting steel, and a performance having both machinability and cold workability can be obtained. Industrially
No. 9580 discloses that when the microstructure is made into two phases of ferrite and graphite, the workability is remarkably improved.

[0004] However, as disclosed in Japanese Patent Application Laid-Open No. 2-111842, when two phases of ferrite and graphite are formed, the cold workability is improved, but the hardenability is reduced. Cementite + graphite 3
It is desirable to have a phase structure. Graphite has the disadvantage of low quenching hardness because it has a slow decomposition rate during quenching heating and does not dissolve sufficiently in austenite. In addition, when graphite is decomposed, pores remain, so that the mechanical properties deteriorate. That is, it is said that hardenability is inferior. It is said that controlling the graphitization of cementite and leaving a part of the cementite structure is effective for achieving both cold workability and hardenability. As an industrial method for controlling graphitization, it has been proposed to adjust the annealing time after hot rolling. Adjusting the annealing time increases the operating cost of the annealing furnace. There is a strong demand from the industry to develop a steel having a three-phase structure of ferrite + cementite + graphite by adjusting the chemical composition of the steel under the conditions of constant annealing time.

[0005]

SUMMARY OF THE INVENTION The present invention has developed a steel having a three-phase structure of ferrite + cementite + graphite by adjusting the chemical composition of the steel under the condition of a constant annealing time, and has developed a steel having a cold workability. It is an object of the present invention to provide a steel for cold forging having excellent hardenability.

[0006]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is that the chemical component value is expressed by weight%.
As display, C: 0.30 to 0.70%, Si: 0.
30 to 1.1%, Mn: 0.3 to 0.8%, P: ≦ 0.
020%, S: 0.025%, Al: 0.01-0.
05%, B: 0.001 to 0.005%, Pb: 0.0
And the balance consists of Fe and unavoidable impurities, and has a three-phase structure of ferrite + cementite + graphite, and the ratio of graphite in the three-phase structure (the amount of carbon precipitated as graphite). / Carbon content in steel x 100) is 2
Cold forging steel excellent in cold workability and hardenability of 0 to 70%. C: 0.30 to 0.70%, Si: 0.30 to 1.1
%, Mn: 0.3 to 0.8%, P: ≦ 0.020%,
S: ≦ 0.025%, Al: 0.01 to 0.05%,
B: 0.001 to 0.005%, Pb: 0.005 to
0.05%, Mo: 0.01 to 0.03%, the balance being Fe and unavoidable impurities, and having a three-phase structure of ferrite + cementite + graphite, graphite occupying the three-phase structure The ratio of (carbon content precipitated as graphite / carbon content in steel × 100) is 20 to 70%, and is a cold forging steel excellent in cold workability and hardenability.

That is, the present inventors have made various studies and found that a trace amount of lead is effective as an inhibitor to suppress graphitization in the process of decomposing cementite and depositing graphite during annealing. By controlling the lead content without changing the annealing time, the ratio of graphite in the three-phase structure of ferrite + cementite + graphite was successfully controlled, and the present invention was made.

[0008]

The present invention will be described below in detail. In the first invention, the C content must be 0.30% or more after quenching / tempering to secure the strength as a part. The upper limit is 0.70% in order to prevent the occurrence of burning cracks
And Si is an essential element because it has an effect of promoting graphitization by increasing the carbon activity in steel, and its lower limit must be 0.30%. If it exceeds 1.1%, it forms a solid solution with the ferrite phase, so that the hardness of the base material increases and the cold workability deteriorates. Therefore, the upper limit is limited to 1.1%.
Mn is required to add an amount necessary for fixing and dispersing sulfur in steel as MnS and an amount necessary for dissolving it in a matrix to secure strength after quenching,
The lower limit is 0.3%. The upper limit was set to 0.8% because the hardness of the base material with an increased Mn content increases and the cold workability decreases.

P precipitates as a phosphorus compound at the grain boundaries in steel and becomes a nucleus generation site for graphite, which promotes graphitization, but increases the average graphite particle size.
Must be 20%. S combines with Mn to form Mn
Present as S inclusions. The upper limit was set to 0.025% from the viewpoint of cold workability.

[0010] Al is required to be 0.01% or more in order to deoxidize steel and prevent surface flaws at the time of rolling. The deoxidizing effect is saturated at 0.05%, and alumina-based inclusions increase. Therefore, the upper limit was set to 0.05%. B reacts with N and precipitates as BN at the austenite grain boundaries. BN has the same hexagonal crystal structure as graphite, and serves as a precipitation nucleus of graphite. In order to deposit graphite, its lower limit must be 0.001%. Since the effect of depositing graphite is saturated at 0.005%, the upper limit is made 0.005%.

Although the mechanism of Pb is not clear, Pb has an effect of suppressing graphitization in the process of depositing graphite with a very small amount of addition. That is, it is a very effective inhibitor for graphitization. There are other elements that suppress graphitization, such as Cr and Mn, but they are difficult to use because they affect other properties such as workability. Pb does not affect other properties such as workability, mechanical properties and hardenability because Pb is added in a small amount and exists as inclusions. When the Pb content is less than 0.005%, the effect of suppressing graphitization is not recognized, so the lower limit was made 0.005%. When 0.05% is added, graphitization is suppressed by 100%. Therefore, the upper limit is set to 0.05%.

The reason why the ratio of graphite in the three-phase structure of ferrite + cementite + graphite (the amount of carbon precipitated as graphite / the carbon content in steel × 100) will be described. When the proportion of graphite is less than 20%, the cold workability significantly decreases, so the lower limit is set to 20%. Also, 7
If the content exceeds 0%, the hardenability is significantly reduced.
0%.

Next, the reasons for limiting the components of the second invention will be described. C, Si, Mn, P, S, Al, B, P
b is exactly the same as in the first invention. Mo is an element having a high hardenability factor and not inhibiting graphitization. Mo is an element that easily forms carbides and lowers the activity of carbon, so it is considered to be an element that inhibits graphitization. However, in the component system of the claims, it does not inhibit graphitization, although the reason is unknown. In order to increase the strength after quenching, it is necessary to set the lower limit to 0.01%. Since the saturation point of the hardenability effect is 0.03%, the upper limit is set to 0.03%.
And

Here, means for producing the steel of the present invention will be described. The steel of the present invention can be easily manufactured by ordinary manufacturing methods and rolling steps. Immediately after the end of rolling, water is evenly sprayed on the steel surface by a water cooling device installed on the line and rapidly cooled,
Thereafter, annealing is performed in a heating furnace. The cooling start temperature after rolling is 780 ° C, the cooling end temperature is 200 ° C, and the average cooling rate is 8 ° C / s. The annealing temperature is 680 ° C.

[0015]

Next, the effects of the present invention will be described more specifically by way of examples. Table 1 shows the chemical components of the test steel, and Table 2 shows the microstructure, cold workability and quenching hardness.

[0016]

[Table 1]

[0017]

[Table 2]

The ratio of graphite in the three-phase structure of ferrite + cementite + graphite was determined by dividing the amount of carbon precipitated as graphite by the carbon content in steel. The amount of carbon deposited as graphite can be quantified by a method of electrolytically extracting graphite and chemical carbon (cementite). Cold forgeability was evaluated by a cold upsetting test. The test conditions are as follows. Test piece shape: 20mmφ × 30mmh, condition of test piece:
Ferrite-graphite structure, compressibility: 5% from 40 to 65%
The maximum compressibility at which the crack occurrence rate was less than 50% was defined as the critical compressibility, and the quality of the cold forgeability was judged based on the magnitude of the value. The hardenability was determined by quenching a round bar on which graphite was precipitated at 825 ° C. for 60 minutes and measuring the hardness. The critical compressibility showing the cold forgeability of the steel of the present invention is almost equal to that of the comparative steel, and the quench hardness is about HV40-HV60, which is a high cold workability steel with excellent cold workability and hardenability. is there.

[0019]

According to the present invention, as is apparent from the above embodiments, it is possible to provide a steel for cold forging which is remarkably excellent in both properties of cold workability and hardenability. Yes, the industrial effects are very significant.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-311546 (JP, A) JP-A-4-214839 (JP, A) JP-A-2-111842 (JP, A) JP-A-63-1988 216952 (JP, A) JP-A-4-228519 (JP, A) JP-A-5-339676 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 301 C22C 38/60

Claims (2)

(57) [Claims] 1. C: 0.30 to 0.70% Si: 0.30 to 1.1% Mn: 0.3 to 0.8% P: ≦ 0.020% S: ≦ 0.025% Al : 0.01 to 0.05% B: 0.001 to 0.005% Pb: 0.005 to 0.05%, with the balance being Fe and unavoidable impurities, and of ferrite + cementite + graphite Cold forging excellent in cold workability and hardenability in which the ratio of graphite in the three-phase structure (the amount of carbon precipitated as graphite / the carbon content in steel) is 20-70%. For steel. 2. C: 0.30 to 0.70% Si: 0.30 to 1.1% Mn: 0.3 to 0.8% P: ≦ 0.020% S: ≦ 0.025% Al : 0.01 to 0.05% B: 0.001 to 0.005% Pb: 0.005 to 0.05% Mo: 0.01 to 0.03%, with the balance being Fe and unavoidable impurities And has a three-phase structure of ferrite + cementite + graphite, and the ratio of graphite in the three-phase structure (the amount of carbon precipitated as graphite / the carbon content in steel) is 20 to 70%. Cold forging steel with excellent hardenability.