JP2003321712A - Method of producing steel product for induction hardening having excellent grain size property and machinability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of producing a steel product which has excellent machinability in addition to excellent grain size properties without performing additional heat treatment stages such as annealing by prescribing the chemical components in the steel and heat history in the production of the steel. <P>SOLUTION: In the method of producing a Ti-containing steel product for induction hardening, steel having a composition containing, by mass, 0.10 to 0.45% C, 0.03 to 1.0% Si, 0.2 to 2.0% Mn, 0.05 to 0.2% Ti, 0.005 to 0.05% Al and ≤0.01% N, and the balance Fe with inevitable impurities is subjected to thermal history in a series of working stages including a rolling stage from an ingot or a bloom into a slab, a rolling stage into a bar steel wire rod, and a forging stage into a product, the steel is reheated to a temperature range of 800 to 1,100°C after it is once heated to ≥1,250°C and is cooled to ordinary temperature. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a steel product for induction hardening, which has excellent grain size characteristics and machinability.

[0002]

2. Description of the Related Art Conventionally, in many cases, medium- and high-carbon steels are induction-hardened and tempered and used for automobile parts and parts for industrial machines. Coarsening of crystal grains is one of the factors that reduce the mechanical properties of these induction-hardened parts.
In order to prevent the coarsening of the crystal grains, it is important not to raise the temperature of the steel too much during austenitization, but induction hardening causes a temperature gradient from the surface to the inside in order to heat the steel in a short time, The temperature near the surface of steel tends to increase. In particular, when the quenching depth is made deeper, the temperature in the vicinity of the surface becomes even higher because the inside must also be heated to a temperature sufficient for austenitizing. As described above, induction hardening is a heat treatment method in which coarse particles are likely to be generated particularly near the surface, and therefore, induction hardening steel is required to have a high coarsening effect.

On the other hand, Japanese Patent Laid-Open No. 8-283910 discloses TiN and AlN, and further Nb carbonitride,
A steel material for induction hardening in which coarsening of crystal grains is suppressed by finely depositing V carbonitrides to pin the crystal grain boundaries is disclosed. However, since these precipitates cannot be said to be sufficiently fine, the effect of coarsening during induction hardening is not sufficient.

The inventors of the present invention conducted extensive studies in order to realize a steel having improved fatigue strength by improving the coarsening resistance effect during induction hardening and refining the crystal grains. I got the knowledge of.

By adding 0.05% or more of Ti to steel and precipitating a Ti compound and a Ti carbonitride, it is possible to improve the coarsening resistance effect. On the other hand, however, in such a Ti-added induction hardening steel, a large amount of fine Ti compound precipitates during cooling after hot working to increase the hardness, which makes subsequent cutting difficult. However, regarding the influence of the thermal history during hot working, there are some that focus on the grain size characteristics, but few that focus on the machinability. As an example, there is one disclosed in Japanese Patent Laid-Open No. 2000-80446. This is because a large amount of fine Ti compound precipitates during cooling after hot working to increase the hardness, which makes subsequent cutting difficult, An annealing process is performed before cutting. The technology that is excellent in machinability as hot working is not yet known.

[0006]

As described above, Ti-added steel has been developed as an induction hardening steel having excellent grain size characteristics during induction heating. However, depending on the heat history during production, a fine Ti compound may be produced. Precipitation may occur in large amounts, resulting in high hardness and poor machinability. The problem to be solved by the present invention is that by defining the chemical composition of the steel material and the heat history at the time of manufacturing, it is possible to obtain not only excellent grain size characteristics but also cutting without an additional heat treatment step such as annealing. The present invention provides a method for manufacturing steel having excellent properties.

[0007]

The means of the present invention for solving the above-mentioned problems is, in the invention of claim 1, mass%,
C: 0.25 to 0.70%, Si: 0.03 to 1.0
%, Mn: 0.2 to 2.0%, Ti: 0.05 to 0.2
%, Al: 0.005-0.05%, N: 0.01% or less, and the balance Fe and unavoidable impurities,
In the thermal history of a series of processing processes, including rolling process from steel ingot or bloom to steel slab, rolling process to bar steel wire rod and forging process to products, once heated to 1250 ° C or higher and cooled to normal temperature After 800 to 1100
It is a method for producing a steel product for induction hardening containing Ti, which comprises reheating to a temperature range of ℃.

In the invention of claim 2, in addition to the steel components in the manufacturing method of the means of claim 1, B: 0.00 in mass%.
A steel containing 0.05 to 0.0050% and the balance Fe and unavoidable impurities was included from a steel ingot or bloom into a billet rolling step, a rolling step into a bar steel wire rod and a forging step into a product. In the thermal history of a series of processing steps,
Once heated to 1250 ° C or higher and cooled to room temperature,
It is a method of manufacturing a steel product for induction hardening containing Ti, which comprises reheating to a temperature range of 00 ° C to 1100 ° C.

In the invention of claim 3, in addition to the steel components in the manufacturing method of the means of claim 1 or 2, C in mass%
r: 2.0% or less, Ni: 3.0% or less, Mo: 1.5
%, Steel containing 1 or 2 or more selected from the following, consisting of the balance Fe and unavoidable impurities, from a steel ingot or bloom to a billet rolling step, and then a rolling step to a bar steel wire rod and a product. In the heat history of a series of processing steps including the forging step, a high frequency containing Ti characterized by being heated to 1250 ° C. or higher once, cooled to room temperature, and then reheated to a temperature range of 800 ° C. to 1100 ° C. It is a method of manufacturing a steel product for quenching.

According to the invention of claim 4, in addition to the steel components in the manufacturing method of the means of any one of claims 1 to 3, V: 0.02 to 0.30% in mass%, Nb: 0. 02 ~
Steel containing 1 or 2 selected from 0.10% and the balance Fe and unavoidable impurities, from a steel ingot or bloom to a billet rolling step, and then to a bar steel wire rod rolling step and product. In the thermal history of a series of processing steps including the forging step, it contains Ti, which is characterized by being heated to 1250 ° C. or higher once, cooled to room temperature, and then reheated to a temperature range of 800 ° C. to 1100 ° C. It is a method of manufacturing a steel product for induction hardening.

According to the invention of claim 5, in addition to the steel components in the manufacturing method of the means of any one of claims 1 to 4, in mass% Pb: 0.3% or less, Bi: 0.3% or less. , S:
Steel containing 0.2% or less, Ca: 0.01% or less, one or two or more, and the balance Fe and unavoidable impurities from a steel ingot or bloom to a steel slab, followed by a steel bar In the thermal history of a series of processing processes including the rolling process for wire rods and the forging process for products, 1250 once
After heating above ℃ and cooling to room temperature, from 800 ℃ to 11
It is a method for producing a steel product for induction hardening containing Ti, which comprises reheating to a temperature range of 00 ° C.

According to the sixth aspect of the present invention, by hot working including rolling and forging, the number of Ti compounds dispersed and precipitated is 10000 or less per 1 μm 2, and the Ti compound dispersed and precipitated during the subsequent high frequency heating is 1 square. The method for producing a steel product for induction hardening containing Ti according to any one of claims 1 to 5, wherein the number is 50 or more per μm.

According to the invention of claim 7, in the final cooling process of the product after hot working, the average cooling rate from 800 ° C. to 500 ° C. is set to 5 ° C./sec or less, so that the hardness after hot working is reduced. The method for producing a steel product for induction hardening containing Ti according to any one of claims 1 to 6, wherein the thickness is 25 HRC or less.

The reasons for limiting the upper and lower limits of each steel component and the reasons for limiting each condition in the above means of the present invention will be described.

The means of claim 1 is such that the basic component is mass%,
C: 0.25 to 0.70%, Si: 0.03 to 1.0
%, Mn: 0.2 to 2.0%, Ti: 0.05 to 0.2
%, Al: 0.005 to 0.05%, N: 0.01% or less, and the balance Fe and unavoidable impurities in the heat history of processing, once heated to 1250 ° C. or more, Ti A Ti compound such as carbide or Ti carbonitride is completely dissolved. At 1250 ° C. or less, the Ti compound does not completely form a solid solution, and the Ti compound deposited in a coarse size of 1 μm or more during casting remains and the effective Ti amount becomes insufficient. Per 50
The number is less than the number and the grain size characteristics are impaired. After heating to a temperature range of 1250 ° C or higher and cooling to room temperature, 1100 ° C to 1
When heating, holding, and processing (rolling and / or forging) in the temperature range of 250 ° C, and then not again heating to 1250 ° C or higher, a coarse Ti compound of 100 nm or more precipitates / remains and is effective. The amount of Ti is insufficient. for that reason,
The number of TiC compounds per square μm at the time of high frequency heating becomes 50 or less, and the grain size characteristics are impaired. Furthermore, this 110
When heated to a temperature range of 0 ° C to 1250 ° C, solid solution of TiC starts, and after cooling, a large amount of fine Ti compounds with a size of 10 nm or less that contributes to hardness increase by coherent precipitation and precipitation is formed. The number of Ti compounds is 10 per square μm
It will be over 000. Therefore, the hardness becomes high and the machinability is impaired.

After being heated to a temperature range of 1250 ° C. or higher and cooled to room temperature, if it is not reheated to a temperature range of 800 ° C. or higher, a large amount of fine Ti compounds having a size of 10 nm or less that contributes to hardness increase by coherent precipitation. And the number of Ti compounds is 1
Since the number is 10,000 or more per square μm, the hardness becomes high and the machinability is impaired.

C: 0.25 to 0.70% C is an element necessary to secure the hardness after induction hardening as a component for machine structure, and if it is less than 0.25%, its effect is sufficiently obtained. On the contrary, if it exceeds 0.70%, the induction hardening hardness is saturated and the workability is deteriorated. Therefore, the content is set to 0.25 to 0.70%.

Si: 0.03 to 1.0% When Si is less than 0.03%, a sufficient deoxidizing effect cannot be obtained, and when it is excessively contained, workability is deteriorated. Therefore, the upper limit is 1.0% and the content is 0.03 to 1.0%.
And

Mn: 0.2-2.0% Mn is an element necessary to secure hardenability and is 0.2%.
If it is less than 2.0%, the effect cannot be sufficiently obtained, and if it exceeds 2.0%, it segregates in the steel to deteriorate the workability. Therefore, the content is 0.2 to 2.0%.

Ti: 0.05 to 0.2% Ti is a Ti carbide, a composite carbonitride containing Ti, T
It is an element necessary for suppressing coarsening of austenite crystal grains during high frequency heating by finely depositing i-nitride, and its effect is not sufficient if it is less than 0.05% and exceeds 0.2%. If so, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, the content is 0.05-0.2%
And

Al: 0.005-0.05% Al is an element used as a deoxidizing agent, and is 0.005
If it is less than 0.1%, the effect is not sufficient, and if it exceeds 0.05%, the amount of alumina-based oxide increases, and the fatigue properties and workability are deteriorated. Therefore, the content is set to 0.005 to 0.05%.

N: 0.01% or less When N exceeds 0.01%, TiN increases,
In addition to adversely affecting fatigue characteristics, it becomes impossible to secure the Ti amount necessary to suppress coarsening of crystal grains. Therefore, the content is set to 0.01% or less.

In order to improve hardenability, strength and toughness, the means of claim 2 is the basic component of the steel according to claim 1, wherein B is B.
0.0005 to 0.0050% is added.

B: 0.0005 to 0.0050% B is an element that remarkably improves the hardenability of steel by the addition of an extremely small amount, and segregates at grain boundaries to suppress grain boundary fracture, resulting in strength and strength. Greatly improves toughness, but 0.0
If less than 005%, the effect is not sufficient, and 0.0050
If it exceeds%, the hot workability is deteriorated. Therefore, the content is set to 0.0005 to 0.0050%.

The means of claim 3 further improves the hardenability and the strength and toughness of the steel composition according to claim 1 or 2 by adding Cr: 2.0% or less by mass% and Ni: 3.0% by mass. %
Hereinafter, one or more selected from Mo: 1.5% or less is added.

Cr: 2.0% or less, Ni: 3.0% or less, Mo: 1.5% or less Cr, Ni and Mo are elements effective in improving hardenability and toughness and are selective. However, if too much, the effect will be saturated. One or two or more selected from Cr: 2.0% or less, Ni: 3.0% or less, and Mo: 1.5% or less may be appropriately used depending on the use of the material.

According to the means of claim 4, in order to assist the effect of suppressing Ti crystal grain coarsening, in the steel according to any one of claims 1 to 3, V in mass% V: 0.02 to 0 .30
%, Nb: One or two selected from 0.02 to 0.10% is added.

V: 0.02 to 0.30%, Nb: 0.0
2 to 0.10% V and Nb have an effect of suppressing coarsening of austenite crystal grains similarly to Ti by forming a carbide, and if less than 0.02%, the effect cannot be obtained and V is 0.30%. If Nb exceeds 0.10%, the amount of precipitates becomes excessive and the workability is deteriorated. Therefore V: 0.02-0.30
%, Nb: 0.02 to 0.10%, or 1 type or 2 types may be appropriately used depending on the use of the material.

According to a fifth aspect of the present invention, in order to improve machinability, the steel according to any one of the first to fourth aspects,
Pb: 0.3% or less by mass%, Bi: 0.3% or less,
One or two or more of S: 0.2% or less and Ca: 0.01% or less is added.

Pb: 0.3% or less Pb is an element effective for improving the chip disposability and the machinability without substantially deteriorating the anisotropy of mechanical properties. If it is contained in excess of 1.0, the effect will be saturated and it is necessary to consider environmental issues.
%.

Bi: 0.3% or less Like Pb, Bi is an element effective for improving the chip disposability and the machinability without substantially deteriorating the anisotropy of mechanical properties. Even if the content exceeds 0.3%, the effect is saturated and it is necessary to consider environmental issues.
The upper limit is 0.3%.

S: 0.2% or less S is an element effective for improving the machinability, but it is 0.
If the content exceeds 2%, the effect is saturated and the anisotropy of mechanical properties becomes large. Therefore, the upper limit is 0.2
%.

Ca: 0.01% or less Ca is an element effective for improving the machinability,
If the content exceeds 0.01%, the effect is saturated, the amount of oxide inclusions increases, and the mechanical properties deteriorate. Therefore, the upper limit is made 0.01%.

According to a sixth aspect of the present invention, as a result of hot working (rolling and / or forging), when the amount of Ti compounds dispersed and precipitated is 10,000 or more per 1 μm, the hardness is increased due to the effect of precipitation strengthening. Machinability is impaired. On the other hand, if the number of Ti compounds dispersed / precipitated during high-frequency heating is 50 or less per 1 μm, the number of precipitates that suppress coarsening of crystal grains will be insufficient during high-frequency heating, and the grain size characteristics will be impaired. The number is 10000 or less per square μm, and the number of Ti compounds dispersed and precipitated during the subsequent high frequency heating is 50 or more per 1 μm 2.

The means of claim 7 is 8 after the final hot working.
Average cooling rate from 00 ℃ to 500 ℃ is 5 ℃ / sec
When it exceeds, the hardness after hot working exceeds 25HRC due to the formation of bainite and fine pearlite,
The machinability may be impaired. So 800 ° C to 500
By setting the average cooling rate up to ° C to 5 ° C / sec or less, the hardness after hot working is set to 25 HRC or less.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION In Table 1, "inventive steel" refers to steel as a constituent of the method of the present invention. Inventive steel 1 is in the SC system, inventive steel 2 is in the SC-B system, Inventive Steel 3 is a steel in which Ti is added to each SCR system, Comparative Steel 4 shown in comparison with the invention steel is an SC steel, and Comparative Steel 5 is a steel in which a small amount of Ti is added to the SC system. Yes, comparative steel 6
Is a steel in which a large amount of N is added to the SCR system.

[0037]

[Table 1]

Sample steels made of the invention steels and comparative steels having the chemical composition shown in Table 1 were melted in a vacuum melting furnace, and a steel ingot made of these test steels was hot-worked into a billet and cooled to room temperature. After that, it was reheated as bar steel rolling, hot rolled to φ65 and cooled to room temperature. After that, hot forging up to φ30 was performed as part forging. The heating temperature during each hot working is shown in Table 2.
The cooling after hot working was air cooling. No. In No. 4, fan cooling was performed after hot forging to φ30. After that, the number of Ti compounds after hot working was counted by a transmission electron microscope using a hot forged product of φ30 as a test material.
Regarding the number of Ti compounds at the time of high frequency heating, the hot forged product of φ30 was subjected to high frequency heating at 1000 ° C. for 25 seconds for quenching, and then counted with a transmission electron microscope. Carburizing grain size No. For hot forging to φ30
After induction hardening for 5 seconds, the former austenite crystal grains were observed with saturated picric acid, and the grain size No. Was calculated. The hardness was measured by measuring the hardness of a hot forged product of φ30. In order to evaluate the machinability, a drill drilling test was conducted, and the drilling time when drilled under each condition is shown in Table 2. The value obtained by dividing by the drilling time of 11 was shown as the drilling property index.

[0039]

[Table 2]

In Table 2, No. 1 and No. 5 and N
o. Since No. 8 is a condition in which both the chemical composition and the hot working condition are good, it is excellent in both grain size characteristics and machinability. On the other hand, No. 2, No. 6, No.
In No. 9, since the heating temperature at the time of billet rolling is low, the number of Ti compounds at the time of high frequency heating is insufficient and the grain size characteristics are poor. N
o. 3, No. 7, No. In No. 10, since the heating temperature at the time of forging parts is high, a large amount of Ti compound precipitates after hot working, resulting in high hardness and poor machinability. No. In No. 4, since the cooling rate after forging the parts is high, bainite is generated and the hardness becomes high, resulting in poor machinability. No. 11, No. In No. 12, since the Ti addition amount is small, the Ti compound is insufficient and the grain size characteristics are poor. No. In No. 13, since the amount of N added is large, a large amount of TiN, which does not form a solid solution in the solid phase, precipitates, so the effective Ti amount becomes insufficient, and the number of Ti compounds during high frequency heating becomes insufficient, resulting in poor crystal grain size characteristics.

[0041]

As described above, according to the present invention, the induction hardening steel having a specific steel composition is rolled from a steel ingot or bloom into a billet, and then a rolling process into a bar steel wire rod and a product. In a thermal history of a series of processing steps including a forging step, a Ti compound that is dispersed and precipitated by heating once to 1250 ° C. or higher and cooling to normal temperature and then reheating to a temperature range of 800 ° C. to 1100 ° C. 1 square μm
In addition to 10000 or less per unit, the Ti compound dispersed and precipitated during the subsequent high frequency heating is 50 per square μm.
By making the number of pieces or more, it is possible to obtain a steel product for induction hardening which is excellent in grain size characteristics and machinability, and the average cooling rate from 800 ° C to 500 ° C after the final hot working is 5 ° C / sec or less. With this, the hardness after hot working can be set to 25 HRC or less, and an excellent effect such as impairing machinability can be achieved.

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Claims (7)

[Claims] 1. C: 0.25 to 0.70% by mass%,
Si: 0.03 to 1.0%, Mn: 0.2 to 2.0%,
Ti: 0.05-0.2%, Al: 0.005-0.0
Steel containing 5% and N: 0.01% or less and the balance Fe and unavoidable impurities is rolled from a steel ingot or bloom into a billet, a rolling process into a bar steel wire rod, and forging into a product. In the thermal history of a series of processing steps including the steps, once heated to 1250 ° C. or higher, cooled to room temperature, and then reheated to a temperature range of 800 ° C. to 1100 ° C., induction hardening containing Ti. Steel product manufacturing method. 2. In addition to the steel components in the manufacturing method according to claim 1, B: 0.0005 to 0.0050% in mass%.
Of the steel containing the balance Fe and unavoidable impurities, from a steel ingot or bloom to a steel billet rolling step, followed by a bar steel wire rod rolling step and a product forging step In history, once heated above 1250 ℃ and cooled to normal temperature, then 800 ℃ to 1100 ℃
The method for producing a steel product for induction hardening containing Ti, which comprises reheating to the temperature range of 1. 3. In addition to the steel components in the manufacturing method according to claim 1, Cr: 2.0% or less by mass%, N
1 selected from i: 3.0% or less and Mo: 1.5% or less
A series of steels containing 1 or 2 or more kinds and the balance Fe and unavoidable impurities, including a rolling process from a steel ingot or bloom to a billet, a rolling process to a bar steel wire rod and a forging process to products. In the thermal history of the processing step, once heated to 1250 ° C or higher and cooled to room temperature, 800
A method for producing a Ti-containing steel product for induction hardening, which comprises reheating to a temperature range of ℃ to 1100 ℃. 4. In addition to the steel components in the manufacturing method according to any one of claims 1 to 3, V: 0.02 in mass%.
~ 0.30%, Nb: 0.02 to 0.10% selected from one or two kinds of steel, the balance Fe and unavoidable impurities steel, from the steel ingot or bloom rolling process to billet In the thermal history of a series of processing steps including the subsequent rolling step for steel bar wire rods and forging step for products, after heating once to 1250 ° C. or higher and cooling to room temperature, 800
A method for producing a Ti-containing steel product for induction hardening, which comprises reheating to a temperature range of ℃ to 1100 ℃. 5. In addition to the steel components in the manufacturing method according to any one of claims 1 to 4, Pb: 0.3 in mass%.
% Or less, Bi: 0.3% or less, S: 0.2% or less, C
a: a steel containing 0.01% or less of one kind or two kinds or more, and a balance of Fe and unavoidable impurities from a steel ingot or a bloom into a steel slab, a rolling step to a steel bar wire rod thereafter, and In the thermal history of a series of processing steps including the forging step for products, Ti is characterized by being heated to 1250 ° C or higher once, cooled to room temperature, and then reheated to a temperature range of 800 ° C to 1100 ° C. A method for producing a steel product for induction hardening containing the same. 6. The Ti compound dispersed and precipitated by hot working including rolling and forging is 1000 per 1 μm 2.
The Ti-containing high frequency wave according to any one of claims 1 to 5, characterized in that the number of Ti compounds is 0 or less, and the Ti compound dispersed and precipitated during the subsequent high-frequency heating is 50 or more per 1 µm. Manufacturing method of steel products for quenching. 7. The average cooling rate from 800 ° C. to 500 ° C. is 5 ° C. in the cooling process of the product after the final hot working.
/ Sec or less, the hardness after hot working is 2
5HRC or less, The manufacturing method of the steel product for induction hardening containing Ti of any one of Claims 1-6 characterized by the above-mentioned.