JP2003034839A - High-strength medium carbon steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve strength of stele parts compose of a high-strength medium carbon steel including Ti, by making the metal structure martensitic with ultra fine crystal grains. SOLUTION: The high-strength medium carbon steel includes, by mass%, 0.30-0.80% C, 0.03-0.50% Si, 0.20-2.0% Mn, more than 0.1% to 0.2% Ti, 0.01% or less N, 0.005-0.050% Al, and the balance Fe with unavoidable impurities, and the martensitic structure having old austenitic grains which have been fined to sizes of No.11 prescribed in JIS G0551. The parts of high-strength medium carbon steel include the martensitic structure having fined grains to No.11 in JIS G0551 or finer by double quenching the above steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to parts made of medium carbon steel for machine structures, for example, automotive steel parts such as gears and CVJs, which require high strength.

[0002]

2. Description of the Related Art In automobile parts such as gears and CVJs,
Higher strength is required due to smaller size and lighter weight. So 1)
Alloying elements such as Ni steel such as SNCM are added to increase the strength. However, when alloy elements are added to increase the strength in this way, the material cost increases and cold workability is poor, so cold forging cannot be performed, and furthermore, after hot forging, it cannot be cut as it is. There is a problem that heat treatment such as annealing is required.

2) Further, quenching is carried out twice to make the crystal grains finer to enhance the strength. However, even in this case, for example, when general steel such as S53C is used, there is a limit to the refinement of crystal grains, and there is a problem that sufficient high strength cannot be achieved.

3) Alternatively, the strength is increased by making the crystal grains finer by processing. However, also in this case, since the molding and the heat treatment are combined, there is a problem that the shape of the part is limited such that it cannot be applied to those difficult to mold.

The method disclosed in Japanese Patent Laid-Open No. 2000-63979 prevents coarsening of γ grains during induction hardening.

[0006]

The problem to be solved by the present invention is to solve the above-mentioned problems of the prior art, and the former austenite grains are positively removed by double hardening or hardening from a bainite structure. The purpose is to make it ultra-fine and improve its strength.

[0007]

Means for Solving the Problems The means for solving the problems according to the present invention is as follows.
0.30 to 0.80%, Si: 0.03 to 0.50%,
Mn: 0.20 to 2.0%, Ti: more than 0.1% to 0.2
%, N: 0.015% or less, Al: 0.005-0.0
It contains 50% and consists of the balance Fe and unavoidable impurities. The former austenite grain size is specified by JIS G 0551 No. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the invention of claim 2, in addition to the steel composition of the means of claim 1, in mass%, Cr: 2.0% or less, M
o: 2.0% or less and Ni: 3.0% or less, one or more of them are contained, and the balance Fe and unavoidable impurities are included, and the former austenite grain size is defined by JIS G 0551. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the invention of claim 3, in addition to the steel component of the means of claim 1, V: 0.02 to 0.10.
%, Nb: 0.02 to 0.10% of 1 type or 2 types, the balance Fe and unavoidable impurities, and the former austenite grain size of No. 1 specified in JIS G 0551. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the invention of claim 4, in addition to the steel component of the means of claim 1, in mass%, Cr: 2.0% or less, M
o: 2.0% or less, Ni: 3.0% or less, one or more kinds are contained, and V: 0.02 to 0.1.
0%, Nb: 0.02 to 0.10% of 1 type or 2 types, the balance Fe and unavoidable impurities, and the former austenite grain size is defined by JIS G 0551. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the invention of claim 5, in addition to the steel component of the means of any one of claims 1 to 4, B% in mass%:
0.0005 to 0.005% is contained, the balance is Fe and inevitable impurities, and the former austenite grain size is J
No. specified in IS G0551. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the sixth aspect of the invention, C: 0.
30 to 0.80%, Si: 0.03 to 0.50%, M
n: 0.20 to 2.0%, N: 0.020%, especially 0.
015% or less, Al: 0.005-0.05% is contained, Ti: 0.05-0.2%, V: 0.02-
0.10%, Nb: 0.02 to 0.1%, containing 1 or 2 or more, balance Fe and inevitable impurities, and former austenite grain size is JIS G0551.
No. specified in. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

In the invention of claim 7, in addition to the steel composition of the means of claim 6, in mass%, Cr: 2.0% or less, M
o: 2.0% or less and Ni: 3.0% or less, one or more of them are contained, and the balance Fe and unavoidable impurities are included, and the former austenite grain size is defined by JIS G 0551. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the invention of claim 8, in addition to the steel composition of the means of claim 6 or 7, B: 0.0005 in mass%.
.About.0.005%, balance Fe and unavoidable impurities, former austenite grain size is JIS G05
No. 51 specified in No. 51. It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

According to the invention of claim 9, in order to improve the machinability, the steel composition of the means of any one of claims 1 to 8 is further mass% S: 0.01 to 0.2%. , Pb: 0.0
1 to 0.3%, Bi: 0.01 to 0.3%, Te: 0.
01-0.2%, Se: 0.01-0.2%, Ca:
0.0003-0.01%, Mg: 0.0003-0.
01%, Sn: 0.01 to 0.2%, one or more selected from, and the balance Fe and unavoidable impurities. The former austenite grain size is JIS G055.
No. 1 specified in No. 1 It is a high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more.

In a tenth aspect of the invention, a steel part made of a high-strength case hardening steel having a martensitic structure according to any one of the first to ninth aspects is quenched twice to obtain JIS G05.
51 No. It is a high-strength medium carbon steel part characterized by having a martensite structure refined to 11 or more.

In accordance with JIS G 0551 No. 3, by quenching from the bainite structure instead of the double quenching in the invention of claim 10 described above. It is also possible to obtain a high-strength medium carbon steel part having a martensite structure refined to 11 or more.

The reasons for limiting the steel components contained in the medium carbon steel of the present invention will be described. In addition,% shows the mass%. C: C is an element necessary to secure the core strength as a machine structural part, but if it is less than 0.30%, the hardness at the time of quenching becomes low, and if it exceeds 0.80%, at the time of quenching. Hardness is saturated and the workability is lowered. Therefore, the content is 0.
It was set to 30 to 0.80%.

Si: If Si is less than 0.03%, a sufficient deoxidizing effect cannot be obtained, and if it is excessively contained, the workability is deteriorated, so the upper limit was made 0.50%. Therefore, the content is set to 0.03 to 0.50%.

Mn: Mn is an element necessary for ensuring the hardenability, and if it is less than 0.20%, its effect is not sufficiently obtained, and if it exceeds 2.0%, it segregates in the steel and the workability is deteriorated. Lower. Therefore, the content is set to 0.20 to 2.0%.

Ti: Ti fixes free-N in steel and improves the effect of B on the hardenability, and at the same time Ti carbide,
By finely precipitating a Ti-containing composite carbide or Ti nitride, quenching and tempering, or suppressing coarsening of austenite crystal grains during induction hardening and tempering.
If the content of Ti is less than 0.05%, the effect is not sufficient.
It is desirable to exceed 1%. However, if it exceeds 0.2%, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, in the means of claim 1, the content is more than 0.1% to 0.2.
%, And the content is 0.05 to 0.2 by the means of claim 6.
%.

If the content of N: N: N exceeds 0.015%, especially 0.020%, TiN increases and the fatigue characteristics are adversely affected. Therefore, in the invention of claim 6, the content is 0.020%, and particularly in the invention of claim 1,
It was set to 0.015% or less.

Al: Al is an element used as a deoxidizing agent. If it is less than 0.005%, its effect is not sufficient, and if it exceeds 0.050%, the amount of alumina-based oxide increases to improve fatigue properties and workability. Lower. Therefore, the content is 0.0
It was set to 05 to 0.050%.

Cr, Mo, Ni: Cr, Mo and Ni are elements effective for improving hardenability and bending strength, and can be selectively added, but if the amount is too large, the effect is saturated. C
r: 2.0% or less, Mo: 2.0% or less, Ni: 3.0
% Or less, one or two or more may be appropriately used depending on the use of the material.

V: V forms a carbide and has the effect of suppressing coarsening of the austenite grain size, similar to Ti. In particular, nano-order VC finely dispersed in steel suppresses the growth of crystal grains. If V is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.10%, the amount of precipitates becomes excessive and the workability is deteriorated. Therefore, the content is set to 0.02 to 0.10%.

Nb: Nb forms a carbide or a nitride and, like Ti, has the effect of suppressing coarsening of the austenite grain size. In particular, nano-order NbC finely dispersed in steel suppresses the growth of crystal grains. Nb is 0.02
If it is less than 0.1%, the effect cannot be obtained, and if it exceeds 0.10%, the amount of precipitates becomes excessive and the workability is deteriorated. Therefore, the content is set to 0.02 to 0.10%.
The means of claim 3 can contain one or two kinds of V and Nb, and one of Ti, V and Nb in the invention of claim 6
It may contain one species or two or more species.

B: B is an element that remarkably improves the hardenability of steel by the addition of a very small amount, is an element that can be selectively added arbitrarily, and has the bending property after quenching and tempering or induction hardening and tempering. Greatly improve. But,
If less than 0.0005%, the effect is not sufficient, and 0.0
On the contrary, if it exceeds 05%, the hardenability is deteriorated. Therefore, the content is set to 0.0005 to 0.005%.

As elements for improving machinability, S:
0.01-0.2%, Pb: 0.01-0.3%, B
i: 0.01 to 0.3%, Te: 0.01 to 0.2%,
Se: 0.01-0.2%, Ca: 0.0003-0.
01%, Mg: 0.0003 to 0.01%, Sn: 0.
One or more selected from 01 to 0.2% can be contained. Below these lower limits, these elements have no sufficient machinability-improving effect, and above their upper limits conversely lower toughness, so they are made below the upper limit.

The operation of the present invention will be described. In the present invention, in order to improve the strength of the steel, the former austenite grains in the medium carbon steel to which Ti is added have the former austenite grain No. defined by JIS G 0551. A medium carbon steel with a martensite structure extremely refined to 11 or more is used. As a result, the fracture starting point of steel shifts from intergranular fracture to intragranular fracture, and bending strength and impact properties are improved.

It is particularly important that the steel part made of the above-mentioned medium carbon steel of the present invention is further hardened twice, and the crystal grains do not become ultra-fine when the normal hardening (including induction hardening) is left as it is. . If it is once hardened and then hardened with the previous structure being martensite (or bainite), it becomes ultra-fine.

Here, martensite (or bainite) once becomes austenite at the time of heating for re-quenching, but since there are many austenite formation sites, austenite grains are formed everywhere. Then, the TiC finely dispersed the austenite grains are pinned,
Stop the growth of crystal grains. Since quenching is performed from that state, an ultrafine martensite structure is formed.

That is, a medium carbon steel containing Ti (a steel developed by the applicant and disclosed in Japanese Patent Application Laid-Open No. 2000-63979) has a former austenite grain size of JIS G0.
No. stipulated in No. 551. A medium carbon steel having a martensite structure refined to 11 or more is used, and a steel part made of this medium carbon steel is utilized, in particular, by quenching twice to make the crystal grains ultrafine. General steel (S53C etc.) is also refined by quenching twice, but grain size No. There is a limit of less than 11 and mixed particles are likely to occur.

In the case of medium carbon steel to which Ti is added, Japanese Patent Laid-Open No.
In the invention disclosed in No. 00-63979, the coarsening of the crystal grains is prevented and the strength becomes high as in the case of S53C described above, but the grain size is No. It is less than about 11. On the other hand, in the present invention, the former austenite grain size is JIS
No. specified in G0551. A medium carbon steel composed of a martensite structure refined to 11 or more
By quenching the steel part made of medium carbon steel with i added twice, the nano-order TiC is finely dispersed and the growth of crystal grains is stopped (pinning effect). As a result, the grain size No. of the region that could not be achieved with general steel was obtained. It becomes possible to achieve ultrafine graining exceeding 11, and by further miniaturizing the crystal grains, higher strength can be achieved.

As a result, the mechanical properties (flexural strength, toughness, etc.) can be greatly improved without using expensive elements such as Ni. Since it is not a thermomechanical treatment, it can be applied to all parts.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION The medium carbon steels according to the embodiments of claims 1 to 9 of the present invention are No. 1 of the invention steels of Tables 1 and 2. 9 ~
No. In the steel composition shown in No. 33, the former austenite grains are No. No. specified in JIS G0551. It is a medium carbon steel with a martensite structure extremely refined to 11 or more, and Ti addition medium carbon steel in which nano-order TiC is finely dispersed. The steel component according to the embodiment of claim 10 is
After carburizing the steel parts made of the above-mentioned Ti-added medium carbon steel,
Re-quenched to obtain crystal grains according to JIS G0551 No. 11
It is assumed to be composed of the martensite structure that has been refined to the above.

[0036]

[Examples] Steel pieces prepared by melting the test materials of the comparative steels and the invention steels shown in Tables 1 and 2 in a 100kg vacuum melting furnace were heated to 1250 ° C and forged into φ32mm steel bars. In this case, in order to precipitate fine Ti, Nb, and V carbides, the steel slab is rolled at 1250 to 1400 ° C. Further, when rolling into a bar wire, rolling is performed at Ac _{3 to} 1050 ° C.
Next, finish forging at 950 ° C and further φ14mm
After being processed into a cylindrical test piece of × 21 mmH, it is held at 850 ° C. for 0.5 hours to be quenched twice, and then held at 180 ° C. for 1.5 hours to be tempered. Next, the ratio of the hardened layer (distance from the surface to the point where 500 HV is reached / part radius) is 0.25
After induction hardening as described above and tempering at 150 ° C., a bending test was performed. Evaluation was made by the ratio of the breaking load.

[0037]

[Table 1]

[0038]

[Table 2]

The crystal grains of the test piece obtained above were measured according to JIS.
It was measured according to the rules of GO551, and the results are shown in Tables 3 and 4. The steel of the present invention No. 9 to 33 are N of comparative steels
o. The particle size No. Is high, and both are No. 1
It can be seen that when it is 1 or more, the crystal grains are ultrafine grains.

Further, in order to evaluate the crack initiation load and the fracture load, a static three-point bending test was carried out, and the results are shown as comparative steel No. The ratio to 1 is shown in Table 3. The static three-point bending test was performed at a distance between spans of 50 mm by providing a 2 mm V notch on a test piece having a corner of 10 mm and a length of 70 mm that has been quenched twice. No. of the test material of the present invention steel Nos. 9 to 33 are sample materials of comparative steels. Excellent in initial cracking load and cracking load as compared with Nos. 1 to 8, and particularly No. It is understood that the initial crack initiation load of 2 times or more as compared with 1 is excellent.

[0041]

[Table 3]

In order to further evaluate the impact value, 2
1 time on a test piece of 10 mm square and 55 mm long that has been tempered.
0RC 2mm notch was provided and a Charpy impact test was performed, and the result was compared with Comparative Steel No. The ratio to 1 is shown in Table 4. No. of the test material of the present invention steel Nos. 9 to 33 are sample materials of comparative steels. Compared to 1-8, it is much superior in impact value,
Especially No. It can be seen that the impact value is more than twice that of 1.

[0043]

[Table 4]

[0044]

As described above, according to the present invention, the medium carbon steel to which Ti is added has the former austenite grain size of JI.
No. stipulated by SG0551. A high-strength medium carbon steel composed of a martensite structure refined to 11 or more is made into a steel part from this medium carbon steel, and the grain size is determined according to JIS G0551 No. 2 by quenching twice. 1
One having a martensite structure refined to 1 or more can be obtained, and a high-strength medium carbon steel part which has never been obtained as an automobile part can be obtained.

Claims (10)

[Claims] 1. C: 0.30 to 0.80% by mass%,
Si: 0.03 to 0.50%, Mn: 0.20 to 2.0
%, Ti: more than 0.1% to 0.2%, N: 0.015% or less, Al: 0.005 to 0.050%, and the balance F.
e and unavoidable impurities and the former austenite grain size is defined by JIS G 0551 No. 11
A high-strength medium carbon steel characterized by comprising a martensite structure refined to the above. 2. The steel composition according to claim 1, further comprising Cr: 2.0% or less, Mo: 2.0% or less, and N in mass%.
i: One or more of 3.0% or less is contained, the balance is Fe and unavoidable impurities, and the former austenite grain size is No. specified by JIS G0551.
A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 3. The steel composition according to claim 1, further comprising V: 0.02 to 0.10% and Nb: 0.02 in mass%.
0.10% of 1 type or 2 types is contained, the balance is Fe and unavoidable impurities, and the former austenite grain size is J
No. specified in IS G0551. A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 4. The steel composition according to claim 1, further comprising Cr: 2.0% or less, Mo: 2.0% or less, and N in mass%.
i: One or more of 3.0% or less, and V: 0.02 to 0.10%, Nb: 0.02
.About.0.10% of 1 type or 2 types, the balance Fe and unavoidable impurities, and the former austenite grain size of No. 1 specified in JIS G 0551. A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 5. The steel composition according to claim 1, further comprising B: 0.0005 to 0.0 in mass%.
No. No. specified by JIS G 0551, the former austenite grain size is defined by JIS No. 05551. A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 6. In mass%, C: 0.30 to 0.80%,
Si: 0.03 to 0.50%, Mn: 0.20 to 2.0
%, N: 0.020% or less, Al: 0.005-0.0
5%, Ti: 0.05-0.2%, V:
0.02 to 0.10%, Nb: 0.02 to 0.1% of one or more of 0.02 to 0.1%, and the balance Fe and unavoidable impurities. The former austenite grain size is JIS.
No. specified in G0551. A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 7. The steel composition according to claim 6, further comprising Cr: 2.0% or less, Mo: 2.0% or less, and N in mass%.
i: One or more of 3.0% or less is contained, the balance is Fe and unavoidable impurities, and the former austenite grain size is No. specified by JIS G0551.
A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 8. The steel composition according to claim 6 or 7, further containing B: 0.0005 to 0.005% by mass%, the balance being Fe and inevitable impurities, and having a former austenite grain size. The No. specified in JIS G0551. A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 9. The steel composition according to claim 1, further comprising S: in mass% to improve machinability.
0.01-0.2%, Pb: 0.01-0.3%, B
i: 0.01 to 0.3%, Te: 0.01 to 0.2%,
Se: 0.01-0.2%, Ca: 0.0003-0.
01%, Mg: 0.0003 to 0.01%, Sn: 0.
It contains one or more selected from 01 to 0.2%, consists of the balance Fe and unavoidable impurities, and has a former austenite grain size of No. 1 specified in JIS G 0551. A high-strength medium carbon steel characterized by comprising a martensite structure refined to 11 or more. 10. A steel part made of a high-strength case hardening steel having a martensitic structure according to claim 1,
Quenching was performed twice, and JIS G0551 No. A high-strength medium carbon steel part characterized by comprising a martensite structure refined to 11 or more.