JPH11236616A - Production of nickel-containing case hardening steel excellent in workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the method for producing a case hardening steel excellent in workability by which the formation of residual austenite is evaded to form a ferritic-pearlitic structure, the treating time is shortened, and the industrial productivity is increased as a softening method for an Ni-contg. case hardening steel. SOLUTION: A steel having a compsn. contg., by weight, 0.10 to 0.35% C, 0.03 to 0.35% Si, 0.20 to 2.0% Mn, 0.003 to 0.30% S, 0.010 to 0.05% Al, 0.10 to 2.0% Cr, 0.03 to 0.8% Mo, 0.30 to 3.0% Ni and 0.010 to 0.025% N, and the balance Fe with inevitable impurities is heated at 830 to 900 deg.C in accordance with a temp. pattern shown in the fig. and is cooled at a cooling rate of <=35 deg.C/h with the temp. interval of 700 to 550 deg.C as a slow cooling temp. region to form a two phase structure of ferrite and pearlite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a case hardened steel having excellent cold workability or cutting workability, which is used as steel for machine structural use.

[0002]

2. Description of the Related Art In many cases, case hardening steel is cold worked and carburized. Therefore, conventionally, in order to improve workability, spheroidizing treatment is often performed after rolling or forging. This spheroidizing process is disclosed in
As described in No. 27, the heating is performed by heating to a temperature range of Ac ₁ to Ac ₃ and gradually cooling to a temperature of Ar ₁ or less. But,
In the case of Ni-containing case hardening steel, if the steel is gradually cooled from the two-phase region of ferrite and toustenite, residual austenite remains after heat treatment as seen in the micrograph of FIG. 2, which significantly impairs workability. In addition, heating to Ar ₃ or more,
In the case of annealing in which cooling is performed gradually thereafter, the normal cooling rate is about 50 ° C. to 100 ° C. due to the problem of industrial productivity. However, at this cooling rate, in the case of the Ni-containing case hardening steel, the structure after the treatment becomes a ferrite + pearlite + martensite structure as seen in the micrograph of FIG.
For this reason, workability is significantly impaired.

[0003]

Therefore, in the present invention,
As a method for softening Ni-containing case hardening steel, 830 ° C. to 90 ° C.
By heating to 0 ° C. and cooling at a cooling rate of 35 ° C./h or less in a temperature range of 700 ° C. to 550 ° C. as a slow cooling temperature range, generation of residual austenite is avoided, as shown in the micrograph of FIG. Ferrite + pearlite structure. Further, it is another object of the present invention to reduce the processing time by defining the annealing temperature section, to increase industrial productivity, and to enable the production of case hardened steel having excellent workability.

[0004]

According to the first aspect of the present invention, there is provided a method for solving the above-mentioned problems.
10 to 0.35% by weight, Si: 0.03 to 0.35% by weight, Mn: 0.20 to 2.0% by weight, S: 0.003 to
0.30% by weight, Al: 0.010 to 0.05% by weight,
Cr: 0.15 to 2.0% by weight, Mo: 0.03 to 0.
8% by weight, Ni: 0.30 to 3.0% by weight, N: 0.0
A steel containing 10 to 0.025% by weight and the balance consisting of Fe and unavoidable impurities is heated to 830 ° C to 900 ° C, and the temperature range of 700 ° C to 550 ° C is set to 35 ° C / This is a method for producing a case hardened steel excellent in workability, characterized in that a two phase structure of ferrite and pearlite is obtained by cooling at a cooling rate of not more than h.

[0005] In the invention of claim 2, in addition to the chemical composition of steel in the means of claim 1, Nb: 0.01-0.
In steel containing 10% by weight, the balance being Fe and unavoidable impurities, the steel was heated to 830 ° C. to 900 ° C.
A temperature range of 0 ° C to 550 ° C is set as a slow cooling temperature range of 35 ° C /
This is a method for producing a case hardened steel excellent in workability, characterized in that a two phase structure of ferrite and pearlite is obtained by cooling at a cooling rate of not more than h.

According to the third aspect of the present invention, in addition to the chemical composition of the steel of the first aspect, Pb: 0.01 to 0.30% by weight, Bi: 0.01 to 0.20% by weight, Te: 0% .00
Steel containing 1 to 0.05% by weight, Ca: 0.001 to 0.003% by weight, Se: 0.03 to 0.05% by weight, the balance being Fe and unavoidable impurities At 830 ° C. to 900 ° C., 700 ° C.
Production of case hardening steel excellent in workability characterized by having a two-phase structure of ferrite and pearlite by cooling at a cooling rate of 35 ° C./h or less in a temperature range of up to 550 ° C. as an annealing temperature range. Is the way.

According to a fourth aspect of the present invention, in addition to the chemical composition of the steel of the second aspect, Pb: 0.01 to 0.30% by weight, Bi: 0.01 to 0.20% by weight, Te: 0. .00
Steel containing 1 to 0.05% by weight, Ca: 0.001 to 0.003% by weight, Se: 0.03 to 0.05% by weight, the balance being Fe and unavoidable impurities At 830 ° C. to 900 ° C., 700 ° C.
Production of case hardening steel excellent in workability characterized by having a two-phase structure of ferrite and pearlite by cooling at a cooling rate of 35 ° C./h or less in a temperature range of up to 550 ° C. as an annealing temperature range. Is the way.

According to the present invention, as described above, the conventional Ni-containing steel for machine structural use is rolled or hot forged at 830 ° C.
By heating to about 900 ° C. and cooling at a cooling rate of 35 ° C./h or less in a temperature range of 700 ° C. to 550 ° C., a two-phase structure of ferrite and barite can be obtained. Can produce case hardened steel with excellent properties.

The reasons for limiting the chemical composition of steel in the present invention will be described. However, the unit indicating the amount of addition is% by weight.

[0010] C is an element necessary for securing the core strength after carburizing as a component for a mechanical structure.
%, The effect is not sufficiently obtained.
%, The toughness of the core decreases. Therefore, the content of C is set to 0.10 to 0.35%.

[0011] Si is added for deoxidation, but 0.0
If the content is less than 3%, a sufficient deoxidizing effect cannot be obtained. If the content is excessive, the workability is reduced, and the formation of a grain boundary oxide layer during carburization is promoted, and the fatigue characteristics are also reduced. And

Mn is an element necessary for ensuring hardenability, but if it is less than 0.20%, its effect cannot be sufficiently obtained. Reduce the nature. Therefore, the content of Si is set to 0.20 to 2.0%.
And

S becomes MnS and is an element for improving machinability. However, if it is less than 0.003%, its effect cannot be sufficiently obtained, and if it exceeds 0.30, cold workability is remarkably reduced. Therefore, the content of S is set to 0.003 to 0.30%.

Although Al is a deoxidizing agent and forms AlN precipitates, it has the effect of preventing coarsening of the crystal grain size during carburization, but if it is less than 0.010%, the effect is not sufficient.
If it exceeds 0.05%, the hot workability is significantly reduced.
Therefore, the content of Al is set to 0.010 to 0.05%.

Cr is an element that improves hardenability, but its effect is not sufficient if it is less than 0.10%.
When the content exceeds 0%, carbides are formed in the carburized layer, and mechanical properties and fatigue properties are reduced. Therefore, the content of Cr is set to 0.10 to 2.01%.

Mo is an element which improves the hardenability and toughness, but its effect is not sufficient if it is less than 0.03%, and if it exceeds 0.8%, the content of bainite or martensite after rolling or forging is increased. It becomes a structure and significantly reduces workability. Therefore, the content of Mo is set to 0.8% or less.

Ni is an element that improves the hardenability and toughness, and if its content is less than 0.30%, its effect is not sufficient, and even if it exceeds 3.0%, its effect is small. Therefore, the content of Ni is set to 0.30 to 3.0%.

N forms a carbonitride with Al and Nb,
It has the effect of preventing the crystal grains from becoming coarse during carburization.
%, The effect is not sufficient, and if it is contained at 0.025% or more, the hot workability is significantly reduced. Therefore, the content of N is set to 0.010% to 0.025%.

Nb forms carbide or nitride,
Like Ti, it has the effect of suppressing coarsening of austenite grain size, but if it is less than 0.01%, the effect cannot be obtained. If it exceeds 0.10%, the amount of precipitates becomes excessive and workability becomes poor. Lower. Therefore, the content of Nb is set to 0.02 to 0.10%.

Pb is 0.01 to 0.30%, Bi:
0.01 to 0.20%, Te: 0.001 to 0.05
%, Ca: 0.001 to 0.003%, Se: 0.00
These elements, which are contained in an amount of 3 to 0.05%, are elements that improve machinability, and can be added in the above component range when machinability is required. The effect is insufficient when the content is less than the lower limit of each element, and when the content is more than the upper limit, the cold workability is remarkably reduced.

If the heating temperature is lower than 830 ° C., a two-phase region of ferrite + austenite is obtained, and residual austenite remains after cooling. If it exceeds 900 ° C., nitrides, carbides and carbonitrides of Al or Nb increase, Austenite grains are likely to become coarse during carburization. Therefore, the heating temperature was set to 830 ° C to 900 ° C.

In the slow cooling section, a large amount of ferrite starts to be formed at 700 ° C., and at a cooling rate of 35 ° C./h or less, 550 ° C.
Then the pearlite transformation ends. Therefore, in order to shorten the processing time as much as possible, the most effective 700 ° C. to 550 ° C.
The temperature range of ° C. was defined as a slow cooling section. Further, the cooling rate in a section other than the slow cooling temperature section, that is, a section between the above-mentioned heating temperature and 700 ° C. or a section between 550 ° C. and normal temperature is not specified and may be an arbitrary rate. From the viewpoint of industrial productivity, it is preferable to increase the speed as much as possible.

The slower the cooling rate in the slow cooling section, the slower the amount of ferrite produced, the lower the hardness and the better the workability. However, if the cooling rate exceeds 35 ° C., a martensite structure is formed,
It significantly reduces workability. Therefore, the cooling rate is 35
° C / h or less.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Test steels having the chemical composition shown in Table 1 were melted in a vacuum melting furnace, cast pieces were rolled into １６167 mm steel slabs, and then rolled to ３０30 mm.

[0025]

[Table 1]

Each steel in Table 1 is a steel for machine structural use containing Ni. No. 1 is a steel No. of SNCM420. No. 2 is a steel obtained by adding 0.03% of Nb to SNCM420. No. 3 is a steel obtained by adding 0.15% of Pb to SNCM415. No. 4 is a steel obtained by adding 0.04% of Bi to SNCM415. No. 5 is a steel obtained by adding 0.0015% of Te to SNCM220. No. 6 is a steel obtained by adding 0.0023% of Ca to SNCM220. No. 7 is a steel obtained by adding 0.08% of Se to SNCM220.

These steels were rolled as described above, and the obtained φ30 mm steel was subjected to a temperature pattern shown in FIG.
After heating to 0 ° C. to 900 ° C. and holding in that temperature range, the cooling rate is not limited from this heating temperature to 700 ° C., but is cooled as fast as possible to improve productivity. Then 7
Cooling is performed at a cooling rate of 35 ° C./h or less by setting a slow cooling section from 00 ° C. to 550 ° C. Then, it is cooled by air cooling from 550 ° C. Thus, the formation of retained austenite is avoided, and a case hardened steel excellent in workability of the two-phase structure of ferrite and pearlite is obtained.

[0028]

EXAMPLES Test steels having the chemical composition shown in Table 1 were melted in a vacuum melting furnace, cast pieces were rolled into φ167 mm steel pieces, then rolled to φ30 mm, and annealed. First, no.
After holding at 850 ° C. for 1 hour using steel of chemical composition 1
Cooling rate 150 ° C in the temperature range from 50 ° C to 700 ° C
/ H, and cool from 700 ° C to 500 ° C at 100 ° C /
h, 50 ° C / h, 40 ° C / h, 35 ° C / h, 20 ° C / h
After cooling at 10 ° C./h, air cooling was performed from 500 ° C. Then, these microstructures and hardness were examined. Table 2 shows the results. At a cooling rate of 35 ° C./h or less, no martensite structure is observed, the structure is a ferrite + pearlite structure, and the hardness is low.

[0029]

[Table 2]

Next, No. Using steel of chemical composition of 1, 8
After holding at 50 ° C. for 1 hour, the temperature range from 850 ° C. to 700 ° C. was cooled at a cooling rate of 150 ° C./h, the slow cooling end temperature was changed to 500 ° C., 550 ° C., 600 ° C., and then air-cooled. The microstructure and hardness of the obtained steel were examined. However, the cooling rate from 700 ° C. to each slow cooling end temperature is 3
The temperature was set to 5 ° C./h, and thereafter, air cooling was performed as described above. Table 3 shows the results. At the annealing end temperature of 550 ° C. or lower, no martensite structure is observed, and a ferrite + pearly structure is obtained, and the hardness is low.

[0031]

[Table 3]

Next, No. After using steel having a chemical composition of 1 to 7 and holding at 850 ° C. for 1 hour, the temperature range from 850 ° C. to 700 ° C. is cooled at a cooling rate of 150 ° C./h, and 700
After cooling the temperature section from 35 ° C to 550 ° C at 35 ° C / h,
After air cooling, the microstructure and hardness of the obtained steel were examined. Table 4 shows the results. No. In all of the steels 1-7,
No martensite structure was observed, and a ferrite + pearlite structure was observed as seen in the micrograph of FIG.

[0033]

[Table 4]

[0034]

As described above, according to the present invention, after rolling or hot forging a steel for machine structural use containing Ni,
Since it was heated to 830 ° C. to 900 ° C. and gradually cooled from 700 ° C. to 550 ° C. as a slow cooling section at a cooling rate of 35 ° C./h, the formation of a retained austenite was avoided and the two-phase structure of ferrite and pearlite was formed. It is possible to obtain steel excellent in workability or cutting workability.

[Brief description of the drawings]

FIG. 1 is a view showing a heat treatment pattern according to the present invention.

FIG. 2 is a micrograph of a steel in which retained austenite is observed.

FIG. 3 is a micrograph of a steel in which a martensite structure is observed.

FIG. 4 is a micrograph of a steel according to the method of the present invention in which the ferrite + pearlite structure of the present invention is recognized.

Claims (4)

[Claims] 1. C: 0.10 to 0.35% by weight, Si:
0.03 to 0.35% by weight, Mn: 0.20 to 2.0% by weight, S: 0.003 to 0.30% by weight, Al: 0.0
10 to 0.05% by weight, Cr: 0.10 to 2.0% by weight, Mo: 0.03 to 0.8% by weight, Ni: 0.30 to 0%
3.0% by weight, N: 0.010-0.025% by weight, balance of Fe and unavoidable impurities,
Workability characterized by forming a two-phase structure of ferrite and pearlite by heating to 900 ° C. and cooling at a cooling rate of 35 ° C./h or less in a temperature range of 700 ° C. to 550 ° C. in a slow cooling temperature range. Method for producing case hardened steel that is excellent. 2. The steel of claim 1, further comprising N
b: steel containing 0.01 to 0.10% by weight at 830 ° C.
To a 900 ° C. to 550 ° C. temperature range and gradually cooling at a cooling rate of 35 ° C./h or less to form a two-phase structure of ferrite and pearlite. Method for producing case hardened steel with excellent heat resistance. 3. The steel of claim 1 further comprising P
b: 0.01-0.30% by weight, Bi: 0.01-0.
20% by weight, Te: 0.001 to 0.05% by weight, C
a: 0.001 to 0.003% by weight, Se: 0.003
A steel containing at least one of two or more of -0.05% by weight to 830 ° C to 900 ° C;
By cooling at a cooling rate of 35 ° C./h or less with the temperature zone of
A method for producing a case hardened steel having excellent workability, characterized by having a phase structure. 4. The steel of claim 2 further comprising P
b: 0.01-0.30% by weight, Bi: 0.01-0.
20% by weight, Te: 0.001 to 0.05% by weight, C
a: 0.001 to 0.003% by weight, Se: 0.003
A steel containing at least one of two or more of -0.05% by weight to 830 ° C to 900 ° C;
By cooling at a cooling rate of 35 ° C./h or less with the temperature zone of
A method for producing a case hardened steel having excellent workability, characterized by having a phase structure.