JP2003321710A - Method of producing steel product for carburization 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 stipulating chemical components in the steel and thermal history in the production of the steel. <P>SOLUTION: In the method of producing a steel product for carburization, 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 once heated to ≥1,250°C, is cooled to ordinary temperature, and is thereafter reheated to a temperature range of 800 to 1,100°C in thermal history of a series of working stages including a rolling stage from an ingot or a bloom into a slab, the subsequent rolling stage into a bar steel wire rod and a forging stage into a product. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a carburizing steel product having excellent grain size characteristics and machinability.

[0002]

2. Description of the Related Art Carburizing steel is generally used for hot working, cold working,
900 ° C after the product has a predetermined shape due to cutting, etc.
The above carburizing quenching and tempering process is performed. However, coarsening of crystal grains often occurs during carburization due to a combination of the previous structure and carburizing conditions. To prevent that, A
In order to utilize precipitates such as 1N and NbCN as pinning particles, the composition of Al, Nb, N, etc. may be adjusted in the steel material. The Ti-added carburizing steel is also a steel material added with Ti for the purpose of preventing crystal grain coarsening. By the way, in such a Ti-added carburizing steel, a large amount of fine Ti compounds are precipitated 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,
000-80446 is disclosed. However, since a large amount of fine Ti compound precipitates upon cooling after hot working to increase hardness, it becomes difficult to perform subsequent cutting, so that before cutting. The annealing step is performed. The technology that is excellent in machinability as hot working is not yet known.

[0003]

As described above, Ti-added steel has been developed as a carburizing steel having excellent grain size characteristics during carburization. However, depending on the heat history during production, fine Ti may be used.
A large amount of compound may be precipitated, resulting in high hardness and reduced machinability. The problem to be solved by the present invention is to define the chemical composition of the steel material and the thermal history during manufacturing,
It is intended to provide a method for producing steel which is excellent not only in grain size characteristics but also in machinability, without any additional heat treatment step such as annealing.

[0004]

The means of the present invention for solving the above-mentioned problems is, in the invention of claim 1, mass%,
C: 0.10 to 0.45%, 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 carburizing containing Ti, which comprises reheating to a temperature range of ° C.

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 for producing a steel product for carburizing containing Ti, which comprises reheating to a temperature range of 00 ° C to 1100 ° C.

[0006] 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 a thermal history of a series of processing steps including a forging step, carburizing containing Ti, characterized by heating once to 1250 ° C or higher, cooling to normal temperature, and then reheating to a temperature range of 800 ° C to 1100 ° C. It is a manufacturing method of steel products for use.

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 carburizing.

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, Pb: 0.3% or less and Bi: 0.3% or less in mass%. , 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 carburizing 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, and the Ti compound dispersed and precipitated during the subsequent carburization is 1 μm 2. The number is 50 or more per one, 1 in any one of Claims 1-5 characterized by the above-mentioned.
It is a method for producing a steel product for carburizing containing Ti according to the means of the item.

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 carburizing steel product containing Ti according to any one of claims 1 to 6, characterized in that the thickness is 100 HRB 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: 0.10 to 0.45%, 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. The number is 50 or less 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 1250 ° C
If it is heated, held and processed (rolled and / or forged) in the temperature range of ℃, and then not heated again to 1250 ℃ or higher, coarse Ti compounds of 100 nm or more will precipitate and remain, and effective Ti Insufficient amount. Therefore, the number of TiC compounds during carburization is 50 or less per 1 μm 2, and the grain size characteristics are impaired. Furthermore, from this 1100 ℃ 12
When it is heated to a temperature range of 50 ° C., solid solution of TiC starts, and after cooling, it precipitates by coherence and contributes to increase in hardness. 10
A large amount of fine Ti compounds having a size of nm or less is deposited, and the number of Ti compounds after hot working becomes 10,000 or more per 1 μm 2. Therefore, the hardness becomes high and the machinability is impaired.

After heating to a temperature range of 1250 ° C. or higher and cooling to room temperature, if it is not reheated to a temperature range of 800 ° C. or higher, a large amount of fine Ti compounds 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: 0.10 to 0.45% C is an element necessary for ensuring the strength of the core portion after carburizing as a machine structural part, and if it is less than 0.10%, its effect is sufficient. If it is not obtained, on the contrary, if it exceeds 0.45%, the toughness of the core part is deteriorated. Therefore, the content should be 0.10
Was set to 0.45%.

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, the workability is deteriorated and the grain boundary oxide layer at the time of carburization is formed. It also promotes the formation of the alloy and reduces the fatigue property. Therefore, the upper limit is 1.0% and the content is 0.
It was set to 03 to 1.0%.

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

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 the 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 the grain boundaries to suppress the 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%.

According to a third aspect of the present invention, in order to further improve the hardenability, strength and toughness, the steel composition according to the first or second aspect contains the Cr content of 2.0% or less by mass% and the Ni content of 3.0. %
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 Bi, like Pb, is an element effective for improving the chip disposability and 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), if 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, when the amount of Ti compounds dispersed / precipitated during carburization is 50 or less per 1 μm, the number of precipitates that suppress the coarsening of crystal grains during carburization becomes insufficient and the grain size characteristics are impaired.
The number of compounds is 10000 or less per 1 μm, and the number of Ti compounds dispersed and precipitated during the subsequent carburization 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
If it exceeds, the hardness after hot working exceeds 100HRB due to the formation of bainite and fine pearlite, which may impair the machinability. So from 800 ℃ 5
By setting the average cooling rate up to 00 ° C to 5 ° C / sec or less, the hardness after hot working is set to 100HRB or less.

[0033]

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 an SCR system, and inventive steel 2 is an SCM-B system. Comparative steel 3 which is a steel to which Ti is added and which is shown in comparison with the invention steel is an SCR type steel, Comparative steel 4 is a SCR type steel to which a small amount of Ti is added, and Comparative steel 5 is an SCM type steel. It is a steel with a large amount of N added.

[0034]

[Table 1]

Sample steels made of the invention steels and comparative steels having the chemical composition shown in Table 1 are melted in a vacuum melting furnace, and a steel ingot made of these test steels is hot-rolled into billets 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 carburizing, a hot forged product having a diameter of φ30 was pseudo-carburized at 1000 ° C. for 2 hours and quenched, and then counted with a transmission electron microscope. Carburizing grain size N
o. After carrying out pseudo carburization of the hot forged product to φ30 at 1000 ° C. for 2 hours, the former austenite crystal grains were observed with saturated picric acid, and the carburized 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 numerical value divided by the drilling time of SCR420 of 7 was shown as a drilling property index.

[0036]

[Table 2]

In Table 2, No. 1 and No. Since No. 5 is a condition in which both the chemical composition and hot working conditions are good, it is excellent in both grain size characteristics and machinability.
On the other hand, No. 2, No. In No. 6, since the heating temperature during rolling of the billet is low, the number of Ti compounds during carburization is insufficient and the grain size characteristics are poor. No. 3, No. In No. 7, since the heating temperature during forging of 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. 8, No. 9 for T
Since the amount of i added is small, the Ti compound is insufficient and the grain size characteristics are poor. No. In No. 10, 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, resulting in an insufficient amount of effective Ti, and the number of Ti compounds during carburization becomes insufficient, resulting in poor grain size characteristics.

[0038]

As described above, according to the present invention, a carburizing steel having a specific steel composition is rolled from a steel ingot or bloom into a billet rolling step, and then into a bar steel wire rod rolling step and a product. In the thermal history of a series of processing steps including the forging step, once, after heating to 1250 ° C. or higher and cooling to room temperature, re-heating in the temperature range of 800 ° C. to 1100 ° C. during forging of parts causes dispersed precipitation. A carburizing steel product excellent in grain size characteristics and machinability is obtained by controlling the Ti compound content to 10000 or less per 1 μm and the Ti compound content to be dispersed and precipitated during the subsequent carburization to 50 or more per 1 μm. The average cooling rate from 800 ° C to 500 ° C after the final hot working is 5 ° C / se.
By setting the hardness to c or less, the hardness after hot working is 100H.
It can be RB or less, and has an excellent effect such as impairing machinability.

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Claims (7)

[Claims] 1. C: 0.10 to 0.45% 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 heat history of a series of processing steps including steps, for carburizing containing Ti, which is characterized by heating once to 1250 ° C or higher, cooling to normal temperature, and then reheating to a temperature range of 800 ° C to 1100 ° C. Manufacturing method of steel products. 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 ℃
A method for producing a steel product for carburizing containing Ti, which comprises reheating to a 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 carburizing steel product, 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 carburizing steel product, 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 carburization containing. 6. The Ti compound dispersed and precipitated by hot working including rolling and forging is 1000 per 1 μm 2.
For carburizing containing Ti 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 carburizing is 50 or more per 1 μm. Manufacturing method of steel products. 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 1
It is below 00HRB, The manufacturing method of the steel product for carburizing containing Ti of any one of Claims 1-6 characterized by the above-mentioned.