JP3591467B2 - Carbon steel for machine structural use with excellent cold forgeability, hardenability and scale peelability - Google Patents

Description

【００３２】
得られた上記各供試鋼材について、冷間鍛造性、焼入れ性及びスケール剥離性を評価した。
【００３３】
（１）冷間鍛造性
冷間鍛造用試験用として、前記鋼材から機械加工により１５ｍｍφ×２２．５ｍｍの円柱状試験片を採取した。冷間鍛造試験は端面完全拘束の条件で、逐次圧縮を行い、限界圧縮率、圧縮率７０％の変形抵抗を求めた。冷間鍛造試験は１鋼種あたり１０個の試験片を用い、試験片の５０％が割れを発生する圧縮率を限界圧縮率とする。
【００３４】
（２）焼入性
高周波焼入試験用として、前記鋼材から機械加工により１２ｍｍφ×１００ｍｍの試験片を採取した。これに、周波数１５ｋＨｚの高周波焼入装置を用いて焼入れし、１５０℃×６０ｍｉｎの焼戻しを施して、表面硬さ、有効硬化深さを測定した。ここで、有効硬化深さとはビッカース硬さＨｖ：３９２以上となる表面からの距離をいう。
【００３５】
（３）スケール剥離性
上記脱スケール処理を行わず、上記各供試鋼から機械加工によりＪＩＳ Ｚ ２２０１ ５号引張り試験片を採取し、所定の引張り歪を付与した後、残存スケール量を測定し、スケール剥離性を評価した。
これらの評価結果を表２に示す。
【００３６】
【表２】

【００３７】
表２に示す評価結果から、本発明鋼はいずれも、冷間鍛造性、焼入れ性及びスケール剥離性に優れていた。
一方、鋼中のＳｉ含有量が０．０５％以下である比較鋼５は、スケール剥離性が劣っており、また、鋼中のＳｉ含有量が０．１０％を超える比較鋼６は、冷間鍛造性（変形抵抗）および高周波焼入性（有効硬化深さ）が劣っていた。
【００３８】
【発明の効果】
本発明は、Ｓｉ含有量を適正範囲に限定したことによって、優れた冷間鍛造性及び焼入れ性を保持しつつ、特に、冷間鍛造工程で行う熱処理時に発生するスケール量を少なくして、スケール剥離性を飛躍的に向上させることができる。
また、Ａｌ含有量を０．５％超えとすることによって、取鍋精錬法による精錬を行わなくても、Ｏ：０．００１５％以下の低酸素鋼を安定して製造する事が可能となり、この結果、製造コストの低減等が図れる。
【図面の簡単な説明】
【図１】鋼中Ｓｉ含有量と低温焼鈍（ＬＡ）後のスケール厚さとの関係を示す図である。
【図２】付与した引張り歪と残存スケール量の関係に及ぼす鋼中Ｓｉ含有量の影響を示す図である。
【図３】ＲＨ法による真空精錬のみをを行い取鍋精錬を行わなかった場合の、鋼中ＡｌとＯの含有量の関係を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a carbon steel for machine structural use which is excellent in cold forgeability, hardenability and scale releasability in a processing step.
[0002]
[Prior art]
As the carbon steel for machine structural use having excellent cold forgeability and hardenability, the component systems described in JP-A-2-129341 and JP-A-61-174323 are known.
[0003]
However, in all of the steels described in the above-mentioned publications, the amount of scale generated in the heat treatment performed in the cold forging step is increased, and sufficient scale releasability may not be obtained in some cases.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to provide a carbon steel for machine structural use which is excellent in cold forgeability, hardenability and scale releasability in a working process.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have conducted detailed studies on the improvement of scale releasability, and as a result, have found that the quality of scale releasability is closely related to the amount of Si in steel.
That is, in all of the steels described in the above publication, the amount of Si in the steel was 0.05% by mass or less. It has been found that when the content is within the range of 1% by mass or less, carbon steel for machine structural use having significantly improved scale releasability can be obtained.
[0006]
The gist of the present invention is as follows.
(1) In mass%, C: 0.40 to 0.60%, Si: more than 0.05% and 0.10% or less, Mn: 0.20 to 0.65%, Cr: 0.30% or less, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0030%, Al: contains 0.05% to 0.07% or less and Mo: 0.05 to 0.20%, and limits the impurity elements S, O and N to 0.02% or less, 0.0015% or less and 0.0070% or less, respectively, with the balance being Fe and A carbon steel for machine structural use that is excellent in cold forgeability, hardenability and scale releasability, characterized by being composed of unavoidable impurities.
(2) The cold forging according to the above (1), further comprising one or two selected from V: 0.01 to 0.5% and Nb: 0.005 to 0.05% in mass%. Carbon steel for machine structure with excellent heat resistance, hardenability and scale peelability.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the reason for limiting the steel composition of the present invention to the above range will be described. In addition, mass% is simply shown as% below.
[0008]
C: 0.40 to 0.60%
C is an effective element for ensuring surface hardening and effective hardening depth during induction hardening, and is actively utilized, but if less than 0.40%, the strength required for mechanical parts is secured. Is difficult to do. If the content exceeds 0.60%, the deformation resistance at the time of cold forging becomes excessively large, and the desired low deformation load cannot be achieved. Therefore, the upper limit of the content is set to 0.60%.
[0009]
Si: more than 0.05% and not more than 0.10% Si is the most important element in the present invention, and if its content exceeds 0.05%, the amount of scale generated in the processing step decreases, and accordingly, In addition, the scale releasability can be significantly improved. On the other hand, if the Si content exceeds 0.10%, the deformation resistance during cold forging increases. Therefore, in the present invention, the Si content is set to be more than 0.05% and 0.10% or less.
[0010]
As an example, FIG. 1 shows the results of measuring the scale thicknesses of the inventive steel having a Si content of 0.08% and the comparative steel having a Si content of 0.04%. FIG. 2 is a graph in which a work strain of 0 to 5% is applied to each steel having the above scale, and the residual scale amount of the steel to which the work strain is applied at this time is plotted against the work strain.
From FIG. 1 and FIG. 2, it can be seen that the invention steel has a smaller scale thickness than the comparative steel, and accordingly, the amount of residual scale when work strain is applied is small.
[0011]
Mn: 0.20 to 0.65%
Mn is an element useful for securing hardenability, but partially forms a solid solution in carbide during spheroidizing annealing. For this reason, if it is less than 0.20%, hardenability will be insufficient at the time of induction hardening, so it is made 0.20% or more. On the other hand, if it exceeds 0.65%, the deformation resistance increases, so the content was made 0.65% as the upper limit.
[0012]
Cr: 0.30% or less Cr in the spheroidized annealing state is solid-dissolved in the carbide and renders the carbide hardly soluble, thus deteriorating the induction hardenability. On the other hand, the addition of a small amount has the effect of improving the spheroidized structure and improving the deformability. Therefore, the upper limit of the amount of added Cr is set to 0.30% from the balance between the two. Preferably, the effect is further enhanced when the Cr content is 0.01 to 0.13%.
[0013]
Ti: 0.005 to 0.05%
Ti has a strong affinity with C and N, and forms a nitride and a carbide with each other, and reduces solid solution C and N in ferrite. Therefore, since the strain aging is suppressed, the deformation resistance at the time of cold forging can be reduced, and the effect of improving the hardenability of B described later can be effectively exerted. Is small, and when it exceeds 0.05%, a huge nitride is formed. These are extremely detrimental to rolling fatigue life. For the above reasons, the amount of Ti added is in the range of 0.005 to 0.05%.
[0014]
B: 0.0003-0.0030%
B is an element useful for hardenability, so it is positively added. However, if it is less than 0.0003%, its effect is small, and if it exceeds 0.0030%, its effect is saturated. The amount added was limited to 0.0003 to 0.0030%.
[0015]
Al: more than 0.05% and 0.07% or less
Al is not only useful as a deoxidizing agent, but also has a strong affinity with N to form AlN. For this reason, it is added positively to effectively exert the effect of improving the hardenability of B. However, if the content is less than 0.05%, the effect is small.On the other hand, if it exceeds 0.07%, the deformation resistance is increased by solid solution strengthening. Its content was limited to more than 0.05% and 0.07% or less.
[0016]
As described in, for example, Japanese Patent No. 1553792, the Al content in steel is 0.050% or less, and the oxygen level in the steel is lower (specifically, In general, molten steel is usually smelted by a ladle refining method typified by the LF (Ladle Furnace) method. This is not preferable because it leads to an increase in manufacturing cost.
[0017]
Thus, the present inventors have conducted studies to suppress the oxygen level to a low level without performing refining by the LF method. When the Al content in the steel is set to be more than 0.05%, the LF method is performed. It has been found that it is possible to stably produce a carbon steel having a low oxygen level without performing refining by the method.
[0018]
Therefore, in the present invention, the Al content is set to be more than 0.05% and 0.07% or less in order to suppress the oxygen level in the steel to a low level (O: 0.0015% or less) without performing refining by the LF method.
[0019]
FIG. 3 is an example in which the Al content in steel manufactured without refining by the LF method is changed and the change in the O content in the steel at that time is plotted.
From the results shown in FIG. 3, it can be seen that when the Al content in the steel is more than 0.05%, the O content in the steel is 0.0015% or less.
[0020]
In general, as the Al content in the steel increases, the amount of generated Al inclusions also tends to increase. However, if the Al content is in the range of more than 0.05 to 0.07%, forging is performed. However, it has already been confirmed by experiments that no problem arises and the material properties are not adversely affected.
[0021]
Mo: 0.05 to 0.20%
Mo is an element that improves the hardenability, and if the addition amount is less than 0.05%, the effect on the improvement of the hardenability is small, so it is necessary to add 0.05% or more. , The work hardening is increased and the deformation resistance during cold forging is increased. Therefore, the upper limit of the content is set to 0.20%.
[0022]
S: 0.02% or less Since S forms MnS-based inclusions and deteriorates cold deformability, it is desirable to reduce S. The upper limit of the content is set to 0.02%.
[0023]
O: 0.0015% or less O forms oxides such as Al, Mn, and Si in steel and deteriorates cold forgeability and rolling fatigue life characteristics. Therefore, the upper limit of the content is 0.0015%. %.
[0024]
N: 0.0070% or less N forms a solid solution in ferrite to cause strain aging, increases deformation resistance, and reacts with B to form BN, thereby reducing the effect of improving the hardenability of B. It is desirable to reduce as much as possible, and the upper limit of the content is set to 0.0070%.
[0025]
In the present invention, it is essential to specify the steel composition components, but other components, for example, V: 0.01 to 0.5%, Nb: 0.005 to 0.05% One or two selected from them may be contained as necessary.
[0026]
V: 0.01 to 0.5%, Nb: 0.005 to 0.05%, which further contains one or two kinds selected from V. V increases the strength of steel by precipitation hardening, In addition, it is an element that increases tempering softening resistance and increases torsional strength. However, if the addition is less than 0.01%, the increase in strength is small, while if the addition exceeds 0.5%, no increase in strength commensurate with the added amount is observed. Therefore, it is preferable that the amount of V added be in the range of 0.01 to 0.5%.
[0027]
Nb is an element that increases the strength of the steel by precipitation hardening and also increases the tempering softening resistance, and increases the torsional strength. However, if the addition is less than 0.005%, the increase in strength is small, while if it exceeds 0.05%, the increase in strength is saturated. Therefore, it is preferable that the amount of Nb added be in the range of 0.005 to 0.05%.
[0028]
Next, an example of the method for producing the carbon steel for machine structure of the present invention will be described.
The carbon steel for machine structural use of the present invention is obtained by melting molten steel in a converter, and further performing vacuum refining or other secondary refining by a RH method or the like in a ladle as required, and then performing continuous casting. Into an ingot or a billet by an ingot casting method or a rheocasting method, and, if necessary, subject it to ingot rolling or billet rolling to produce a billet of a required size. It can be manufactured by performing cold rolling to obtain a steel bar. Then, after cutting to a predetermined length and performing a heat treatment for spheroidizing carbides in the steel, a shot bonde for descaling (mechanical peeling using a lubricant after performing shot blasting) After performing cold forging a plurality of times with a low-temperature annealing and a shot bonder in between, cutting, and then quenching (IQT) to obtain a component for machine structure. it can.
[0029]
The present invention is not limited to the above-described manufacturing method, and various modifications can be made.
[0030]
【Example】
Next, a trial production of a round bar of carbon steel for machine structure of the present invention and evaluation of its performance will be described below.
Each steel material having the chemical composition shown in Table 1 was melted to form a 100 kg steel ingot, which was then hot-rolled into a round bar having a diameter of 50 mm. Next, after performing spheroidizing annealing treatment at 745 ° C. × 7 hours, gradually cooling, performing chemical conversion treatment, performing cold working from a diameter of 50 mm to a diameter of 30 mm, and then performing low temperature annealing (LA) at 725 ° C. × 1 hour. ) And descaling treatment to produce a carbon steel material for machine structure.
[0031]
[Table 1]

[0032]
Each of the obtained test steel materials was evaluated for cold forgeability, hardenability and scale peelability.
[0033]
(1) Cold Forging Property For the test for cold forging, a cylindrical test piece of 15 mmφ × 22.5 mm was sampled from the steel material by machining. In the cold forging test, compression was sequentially performed under the condition of complete constraint of the end face, and the limit compression ratio and the deformation resistance at a compression ratio of 70% were determined. The cold forging test uses ten test pieces per steel type, and the compression rate at which 50% of the test pieces crack is defined as the critical compression rate.
[0034]
(2) Hardenability For the induction hardening test, a test piece of 12 mmφ × 100 mm was sampled from the steel material by machining. This was quenched using an induction hardening device with a frequency of 15 kHz, tempered at 150 ° C. for 60 minutes, and the surface hardness and effective hardening depth were measured. Here, the effective hardening depth refers to a distance from the surface where the Vickers hardness Hv is 392 or more.
[0035]
(3) Scale peelability Without performing the descaling treatment described above, a JIS Z 22015 tensile test piece was sampled from each of the test steels by machining to give a predetermined tensile strain, and the residual scale amount was measured. And the scale releasability were evaluated.
Table 2 shows the evaluation results.
[0036]
[Table 2]

[0037]
From the evaluation results shown in Table 2, the steels of the present invention were all excellent in cold forgeability, hardenability and scale peelability.
On the other hand, the comparative steel 5 in which the Si content in the steel is 0.05% or less is inferior in the scale peeling property, and the comparative steel 6 in which the Si content in the steel exceeds 0.10% is cold. The forging property (deformation resistance) and the induction hardening property (effective hardening depth) were inferior.
[0038]
【The invention's effect】
The present invention, by limiting the Si content to an appropriate range, while maintaining excellent cold forgeability and hardenability, in particular, by reducing the amount of scale generated during the heat treatment performed in the cold forging process, scale The releasability can be dramatically improved.
Further, by setting the Al content to more than 0.5%, low oxygen steel of O: 0.0015% or less can be stably manufactured without performing refining by the ladle refining method, As a result, it is possible to reduce the manufacturing cost.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the Si content in steel and the scale thickness after low-temperature annealing (LA).
FIG. 2 is a diagram showing the effect of the Si content in steel on the relationship between the applied tensile strain and the amount of residual scale.
FIG. 3 is a diagram showing a relationship between Al and O contents in steel when only vacuum refining by the RH method is performed and ladle refining is not performed.

Claims (2)

In mass%, C: 0.40 to 0.60%, Si: more than 0.05% and 0.10% or less, Mn: 0.20 to 0.65%, Cr: 0.30% or less, Ti: 0.005 to 0.05 %, B: 0.0003 to 0.0030%, Al: 0.05 % And 0.07% or less and Mo: 0.05 to 0.20%, and limit the impurity elements S, O and N to 0.02% or less, 0.0015% or less and 0.0070% or less, respectively, with the balance being Fe and unavoidable impurities. A carbon steel for machine structural use having excellent cold forgeability, hardenability and scale releasability, comprising: The cold forgeability and quenching according to claim 1 , further comprising one or two selected from V: 0.01 to 0.5% and Nb: 0.005 to 0.05% by mass%. Steel for machine structural use with excellent heat resistance and scale peelability.

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