JP3819529B2 - Case-hardened steel with excellent cold workability - Google Patents

Description

【００３４】
ここで、フェライト面積率およびフェライトの平均結晶粒径は、光学顕微鏡を用いて倍率４００倍で任意に４視野観察した平均値である。硬さは、２０ｋｇｆの荷重をかけて測定したヴィッカース硬度であり、各試料４点ずつ測定した値の平均値である。変形抵抗および表面粗さは、上記試験片を各試料３個ずつ使用し、冷間にて圧縮率６０％まで一気に鍛造した際の値で、３個の平均値である。変形能は、上記試験片を各試料１０個ずつ使用し、図１に示す様な深さ０．３ｍｍの溝加工を試料に施し、冷間にて圧縮率３５％まで一気に鍛造した後、２．５％刻みで圧縮率を増加させながら冷間鍛造を繰り返し、下記▲１▼〜▲３▼のいずれかに該当する割れが１０個の試験片のいずれにも発生しない加工率である。
▲１▼幅０．０１ｍｍ以上、長さ０．５ｍｍ以上
▲２▼幅０．０２ｍｍ以上
▲３▼長さ１．０ｍｍ以上
これらの結果を表２に示す。
【００３５】
【表２】

【００３６】
試料Ｎｏ．１〜１１は、本発明の要件を満足するものであり、いずれも変形抵抗が低く、且つ優れた変形能を有している。これらに対して、Ｎｏ．１２は、熱間圧延時の冷却速度が速いため、金属組織が硬質のベイナイト主体となり、硬度および変形抵抗が高く、変形能は低い。Ｎｏ．１３は、熱間圧延時の冷却速度が遅いため、フェライトの結晶粒径が大きくなり、変形能が低く、表面が粗い。Ｎｏ．１４は、熱間圧延時の仕上温度が高いため、フェライトの結晶粒径が大きくなり、変形能が低く、表面が粗い。Ｎｏ．１５〜１７は鋼の化学組成と熱間圧延条件のバランスがよくなかったため、フェライト面積率が低くなり、硬さおよび変形抵抗が高く、変形能が低い。
【００３７】
【発明の効果】
本発明は以上説明してきた通り、熱間圧延後の鋼の金属組織中に占めるフェライト面積率が９０％以上であって、且つ、フェライトの平均結晶粒径が４０μｍ以下である様に構成されているので、熱間圧延ままの状態で優れた冷間加工性を有すると共に、高加工部品の製造に際しても冷間加工前に球状化焼きなまし処理を施す必要がなく、また、冷間鍛造ままで仕上加工した場合にも表面肌荒れが生じることなく、更に、冷間加工後の表面硬化処理の効果が十分に発揮される様な肌焼鋼を提供することが可能となった。
【図面の簡単な説明】
【図１】実験例１の変形能測定に用いた試験片の形状を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a case-hardened steel excellent in cold workability having high deformability and low deformation resistance in a hot-rolled state.
[0002]
[Prior art]
Conventionally, machine parts used in automobiles, construction machines, various industrial machines, and the like have been manufactured by a process in which cast steel is roughly processed by hot forging and then formed into a desired shape by cutting. When manufacturing machine parts in this way, a considerable cutting margin is required at the time of cutting after the rough machining, and the steel yield is about 50 when the amount of steel is compared after the rough machining of the machine parts and after the cutting. %.
[0003]
In recent years, due to the growing interest in global environmental issues and demands for cost reduction, there has been a demand for energy reduction, abolishment / reduction of machining, and yield improvement in the manufacture of the machine parts. In order to cope with these problems, attention has been paid to a net shaping technique in which steel after rolling or normalizing is formed as it is into a desired shape by cold working.
[0004]
However, since the steel after rolling or normalizing has a large forming load and low deformability, the ability to cold work as it is is limited to low-worked products. For parts that require high processing, the spheroidizing annealing treatment that reduces the hardness by spheroidizing the carbides in steel is performed at least once, but this heat treatment is a manufacturing cost. This leads to an increase in the number of products and damages the benefits of net shaping technology.
Further, when finishing is performed in the cold forging state without performing processing such as cutting, the surface skin of the machine part becomes rough, which may cause a problem in terms of part accuracy.
[0005]
Further, if the alloying element content is excessively reduced with emphasis on cold workability, the effect of surface hardening treatment such as carburizing or carbonitriding performed after the working may be insufficient.
[0006]
[Problems to be solved by the invention]
The present invention has been made paying attention to the circumstances as described above, and its purpose is to provide a case-hardened steel that is excellent in cold workability in a rolled state. There is no need to perform spheroidizing annealing before cold working during manufacturing, and there is no surface roughness even when finishing with cold forging, and the effect of surface hardening treatment after cold working It is to provide a case-hardened steel that can be fully demonstrated.
[0007]
[Means for Solving the Problems]
The case-hardened steel excellent in cold workability according to the present invention, which can achieve the above-mentioned problems,
Steel components are C: 0.05 to 0.35% (mass%, the same applies hereinafter), Si: 1.0% or lower (not including 0%), Mn: 2.0% or lower (including 0%) Not included), P: 0.030% or less (excluding 0%), Al: 0.015% to 0.06%, N: 0.005 to 0.03%, the balance being Fe and inevitable The ferrite area ratio in the metal structure of the steel after hot rolling is 90% or more and the average crystal grain size of ferrite is 20 μm or more and 40 μm or less. is there.
[0008]
Further, the steel component contains one or more elements selected from the group consisting of Cr, Mo, V, Nb and Ti so that the value of A obtained by the following formula (1) is 3.0 or more. And the content of each element constituting the group is
Cr: 1.98% or less (excluding 0%),
Mo: 2.0% or less (excluding 0%),
V: 2.0% or less (excluding 0%),
Nb: 2.0% or less (excluding 0%),
Ti: 2.0% or less (excluding 0%),
It is one of the preferable embodiments of the present invention to satisfy the above.
A = [Cr] +3 [Mo] +5 [V] +4 [Nb] +4 [Ti] (1)
(However, [X] indicates mass% of element X contained in steel)
[0009]
Furthermore,
Ni: 2.5% or less (excluding 0%),
It is also preferable that it contains,
Cu: 1.0% or less (excluding 0%),
It is further more preferable that it contains.
Still further, the components of the steel are
Ca: 0.01% or less (excluding 0%),
Zr: 0.08% or less (excluding 0%),
Pb: 0.30% or less (excluding 0%),
Sb: 0.10% or less (excluding 0%),
It is also recommended as one of the preferred embodiments of the present invention that it contains one or more selected from the group consisting of.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have advanced research and development for the purpose of improving the cold workability of hot rolled material. As a result, the conventional hot-rolled material has produced a hard phase of pearlite, bainite, martensite, etc. (hereinafter sometimes referred to as pearlite) in an area ratio of 30% or more in the hot-rolled state, Therefore, it was found that the deformation resistance was high and the deformability was low. And in order to improve the deformability during cold working and reduce deformation resistance, in the metal structure of the steel after hot rolling, the pearlite etc. should be reduced as much as possible to make the soft ferrite phase the main component. The inventors have found that it is effective to reduce the average crystal grain size of the ferrite phase and have completed the present invention.
[0011]
That is, it is a case-hardened steel in which the ferrite area ratio in the metal structure of the steel after hot rolling is 90% or more and the average crystal grain size of ferrite is 40 μm or less. By doing in this way, even if it is hot-rolling, it can be set as the deformability and deformation resistance equivalent to steel which performed the spheroidizing annealing process.
[0012]
Here, if the ferrite area ratio in the metal structure is less than 90%, the hardness increases due to pearlite or the like appearing in the remainder, and the deformation resistance increases. On the other hand, when the average crystal grain size of ferrite exceeds 40 μm, the deformability is lowered and the metal structure becomes rough, resulting in a problem of surface roughness of the product.
[0013]
The ferrite area ratio is preferably 95% or more, and more preferably 97% or more. If the average crystal grain size of ferrite is 40 μm or less, the effect of the present invention can be exhibited, and the lower limit is not particularly limited, but surface hardening treatment such as carburizing or carbonitriding performed after cold working. In order to further exhibit the above effect, the average crystal grain size is preferably 20 μm or more, more preferably 25 μm or more. The preferable upper limit of the average crystal grain size is 35 μm.
[0014]
In the metal structure of conventional case-hardened steel, the hot rolling conditions are usually as follows, so that 30% or more of pearlite or the like was generated.
Finishing temperature: 950 ° C. or more Cooling rate: 10 ° C./second or more Finishing rate: 20 to 50%
In order to produce the case hardening steel according to the present invention, the hot rolling conditions must satisfy at least the following conditions.
Finishing temperature: 950 ° C. or less Cooling rate: 0.2 to 5 ° C./sec Finishing rate: 30% or more The detailed hot rolling conditions depend on the steel composition, thickness, etc. Need to be adjusted accordingly.
[0015]
Further, the steel component contains one or more elements selected from the group consisting of Cr, Mo, V, Nb, and Ti so that the value of A obtained by the formula (1) is 3.0 or more. And the content of each element constituting the group is
Cr: 1.98% or less (excluding 0%),
Mo: 2.0% or less (excluding 0%),
V: 2.0% or less (excluding 0%),
Nb: 2.0% or less (excluding 0%),
Ti: 2.0% or less (excluding 0%),
It is one of the preferable embodiments of the present invention to satisfy the above.
[0016]
Cr, Mo, V, Nb, and Ti are all elements that generate carbonitrides, and have the effect of agglomerating and reducing hard pearlite, improving the deformability of hot-rolled case-hardened steel. In addition, the effect of reducing deformation resistance is improved. Although the effect of these elements can be obtained even by adding a small amount, it is desirable that the value of A calculated by the above formula (1) is 3.0 or more in order to exhibit it more effectively. In addition, if excessively contained, the cold workability is adversely affected and the effect of the surface hardening treatment applied thereafter may be insufficient, so the upper limit as described above is required for each element. It is. The A value is preferably 10.0 or less.
[0017]
In carrying out the present invention,
Ni: 2.5% or less (excluding 0%),
It is also preferable to contain. Ni further improves the effect of surface hardening treatment such as carburizing or carbonitriding performed after cold working, and refines the structure of the hot-rolled material to improve toughness and improve cold workability. However, these effects saturate at about 2.5%, and it is economically wasteful to contain more than that, so the upper limit was made 2.5%.
[0018]
Furthermore,
Cu: 1.0% or less (excluding 0%),
It is also preferable to contain. Cu, like Ni, further improves the effect of surface hardening treatment such as carburizing or carbonitriding performed after cold working. Furthermore, Cu also has an effect of improving the corrosion resistance, but the effect is saturated at about 1.0%, and conversely if excessively contained, the workability at the time of hot rolling may be deteriorated. Was 1.0%. However, if Cu is contained alone, it tends to affect the hot workability, so it is desirable to contain it simultaneously with Ni. It is because it will become difficult to generate | occur | produce the influence on hot workability when both are included simultaneously.
[0019]
Furthermore,
Ca: 0.01% or less (excluding 0%),
Zr: 0.08% or less (excluding 0%),
Pb: 0.30% or less (excluding 0%),
Sb: 0.10% or less (excluding 0%),
It is also preferable to contain one or more selected from the group consisting of: These elements all improve the machinability of the case hardening steel according to the present invention. However, since there is a reason that excessive inclusion of any element is not preferable, the upper limit as described above is set. Hereinafter, the upper limit value of each element will be described in detail.
[0021]
Ca has an effect of wrapping hard inclusions with soft inclusions, and as a result, is an element that improves machinability, but since this effect is saturated at about 0.01%, it is economical to contain more than that It is useless.
[0022]
Zr has the effect of enhancing machinability by spheroidizing MnS and improving anisotropy, but this effect is also saturated at about 0.08%, so the further inclusion is economically wasteful.
Pb generates PbS and contributes to the improvement of machinability. However, if excessively contained, it causes deterioration of fatigue strength, so it is necessary to make it 0.30% or less.
[0023]
Sb also contributes to the improvement of machinability. However, even if it is excessively contained, not only the effect is saturated, but also coarse non-metallic inclusions may be generated and become the starting point of surface destruction, so 0.10% Is the upper limit.
[0024]
The case-hardened steel that is the subject of the present invention can obtain excellent cold workability as long as it is configured as described above, but the steel composition depends on the use of the machine parts produced from the case-hardened steel. Needs to be selected. In the case of mechanical parts such as shafts, gears, drums, spiders, etc., in which case-hardened steel is generally used, it is recommended to use steel containing the following elements.
C: 0.05 to 0.35%,
Si: 1.0% or less (excluding 0%),
Mn: 2.0% or less (excluding 0%),
P: 0.030% or less (excluding 0%),
Al: 0.015% to 0.06%,
N: 0.005 to 0.03%
The reasons for limiting the content of each component will be described in detail below.
[0025]
C is an element indispensable for securing the internal strength as a mechanical part as described above, and if it is less than 0.05%, sufficient strength cannot be obtained. However, if it is contained excessively, toughness and machinability are reduced and workability is impaired, so 0.35% is made the upper limit. If it is 0.30% or less, it is more preferable.
[0026]
Si is an element necessary as a deoxidizing element at the time of steel melting, but if contained excessively, cold workability deteriorates, so it must be made 1.0% or less. 0.35% or less is more preferable, and 0.15% or less is still more preferable.
[0027]
Mn is contained as a deoxidizing element in the same manner as Si, but if it is contained excessively, the cold workability deteriorates, so it must be made 2.0% or less. 1.0% or less is more preferable.
[0028]
P is an element that segregates at the grain boundaries and lowers the toughness. In order not to cause such an effect, the content must be suppressed to 0.03% or less. More preferably it is 0.02% or less, more preferably 0.01% or less. Unlike other elements, the content may be 0, but it is an element inevitably mixed in the steel manufacturing process, and it is difficult to make the content 0, so the above upper limit Just keep it inside.
[0029]
Al needs to be 0.015% or more for deoxidation, but if it is too much, AlN aggregates locally and may cause abnormal growth of crystal grains. If the AlN crystal grains become too large, the crystal grains may cause adverse effects such as cracking starting points. Therefore, the upper limit is 0.06%.
[0030]
N is an element that improves the toughness by forming a nitride. It is necessary to contain 0.005% or more in order to effectively exhibit this effect. However, if it is excessively contained, the cold workability deteriorates, so 0.03% is made the upper limit.
[0031]
【Example】
Hereinafter, the present invention will be described in more detail by way of examples. However, the present invention is not limited by the following examples, and any modifications made in accordance with the gist of the preceding and following descriptions are within the technical scope of the present invention. included.
[0032]
(Experimental example 1)
Steel materials having chemical components of steel types A to L shown in Table 1 are hot-rolled at a finishing temperature of 900 to 1050 ° C., a cooling rate of 0.1 to 10 ° C./second, and a finishing rate of 40%, and have a diameter of 20 mm and a length of 30 mm. Specimens (Sample Nos. 1 to 17) were prepared, and the structure observation, hardness measurement, surface roughness measurement after finishing, deformation resistance, and deformability were measured.
[0033]
[Table 1]

[0034]
Here, the ferrite area ratio and the average crystal grain size of the ferrite are average values obtained by arbitrarily observing four visual fields using an optical microscope at a magnification of 400 times. The hardness is a Vickers hardness measured by applying a load of 20 kgf, and is an average value of values measured for four samples. Deformation resistance and surface roughness are values when three specimens are used for each of the above specimens and forged at a stretch to a compression rate of 60% in the cold, and are average values of the three specimens. Deformability is determined by using 10 specimens for each of the above specimens, subjecting the specimen to a groove with a depth of 0.3 mm as shown in FIG. It is a processing rate in which cold forging is repeated while increasing the compression rate in increments of 5%, and cracks corresponding to any of the following (1) to (3) do not occur in any of the ten test pieces.
(1) Width 0.01 mm or more, Length 0.5 mm or more (2) Width 0.02 mm or more (3) Length 1.0 mm or more These results are shown in Table 2.
[0035]
[Table 2]

[0036]
Sample No. Nos. 1 to 11 satisfy the requirements of the present invention, and all have low deformation resistance and excellent deformability. On the other hand, no. No. 12 has a high cooling rate during hot rolling, so that the metal structure is mainly composed of hard bainite, has high hardness and deformation resistance, and has low deformability. No. No. 13 has a slow cooling rate during hot rolling, so the crystal grain size of ferrite is large, the deformability is low, and the surface is rough. No. No. 14 has a high finishing temperature during hot rolling, so the crystal grain size of ferrite is large, the deformability is low, and the surface is rough. No. Since Nos. 15 to 17 had a poor balance between the chemical composition of steel and the hot rolling conditions, the ferrite area ratio was low, the hardness and deformation resistance were high, and the deformability was low.
[0037]
【The invention's effect】
As described above, the present invention is configured such that the ferrite area ratio in the metal structure of steel after hot rolling is 90% or more and the average crystal grain size of ferrite is 40 μm or less. Therefore, it has excellent cold workability in the state of hot rolling, and it is not necessary to perform spheroidizing annealing before cold working when manufacturing high-working parts. Even when processed, it is possible to provide a case-hardened steel that does not cause surface roughening and that sufficiently exhibits the effect of surface hardening after cold working.
[Brief description of the drawings]
FIG. 1 is a diagram showing the shape of a test piece used for deformability measurement in Experimental Example 1. FIG.

Claims (5)

Steel composition is
C: 0.05 to 0.35% (mass%, the same shall apply hereinafter)
Si: 1.0% or less (excluding 0%),
Mn: 2.0% or less (excluding 0%),
P: 0.030% or less (excluding 0%),
Al: 0.015% to 0.06%,
N: 0.005 to 0.03%,
And the balance consists of Fe and inevitable impurities,
Excellent in cold workability, characterized in that the ferrite area ratio in the metal structure of steel after hot rolling is 90% or more and the average crystal grain size of ferrite is 20 μm or more and 40 μm or less Case-hardened steel. The steel component further includes one or more elements selected from the group consisting of Cr, Mo, V, Nb, and Ti so that the value of A obtained by the following formula (1) is 3.0 or more. And the content of each element constituting the group is
Cr: 1.98% or less (excluding 0%),
Mo: 2.0% or less (excluding 0%),
V: 2.0% or less (excluding 0%),
Nb: 2.0% or less (excluding 0%),
Ti: 2.0% or less (excluding 0%),
The case-hardened steel according to claim 1, wherein:
A = [Cr] +3 [Mo] +5 [V] +4 [Nb] +4 [Ti] (1)
(However, [X] indicates mass% of element X contained in steel)   The case hardening steel according to claim 1 or 2, wherein the steel component further contains Ni: 2.5% or less (not including 0%).   The case hardening steel according to any one of claims 1 to 3, wherein the steel component further contains Cu: 1.0% or less (not including 0%). The steel component
Ca: 0.01% or less (excluding 0%),
Zr: 0.08% or less (excluding 0%),
Pb: 0.30% or less (excluding 0%),
Sb: 0.10% or less (excluding 0%),
The case-hardened steel according to any one of claims 1 to 4, which contains at least one selected from the group consisting of:

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