JP3783666B2 - Machine structural steel excellent in cold forgeability after spheroidizing annealing and method for producing the same - Google Patents

Description

【００２３】
上記の結果から、鋼組成が本発明の範囲内において疲労強度、冷間鍛造性が優れた製品が得られることがわかる。
【００２４】
（実施例２）
質量比で、C：0.48%、Si：0.21%、Mn：0.85%、P：0.010％、S：0.004%、Cr：0.11%の組成を有する鋼を表２に示す条件で熱間圧延し、直径60mmの棒鋼とした。得られた棒鋼に対し、圧延方向と直角な方向に試験片を切りだし顕微鏡観察を行った。また、得られた棒鋼に対し、745℃で5h保持しする球状化焼鈍処理を施した。球状化焼鈍の施された棒鋼から、実施例1と同様にタブレット試験片を圧延方向に一致するように切り出し、冷間鍛造試験に供した。試験結果を表2に併せて示す。
【００２５】
【表２】

【００２６】
熱間圧延条件が適当な試験No.21および試験No.23、26、27のものでは、フェライト分率が低く、旧オーステナイト粒径も適当に大きくなっており、冷間鍛造性も高い。しかし平均冷却速度が小さい試験No.22のものでは旧オーステナイト粒径は大きいもののフェライト分率が高く、冷間鍛造性は急激に低下している。さらに試験No.24、試験No.25のものでは、旧オーステナイト粒径は小さくフェライト分率が高いため、鍛造性は低い。また、試験No.28のものでは、フェライト分率は20%以下であるものの、旧オーステナイト粒径が小さく鍛造性が悪い。
【００２７】
【発明の効果】
本発明による機械構造用鋼は、フェライト分率が低いために組織が均一であり、比較的簡易な球状化焼鈍を施した状態でも、球状化した炭化物が均一に分散し、さらに旧オーステナイト粒径が大きいため、組織の微細化による鍛造性の劣化を回避でき、優れた冷間鍛造性を有する。
【図面の簡単な説明】
【図１】 冷間鍛造試験片の斜視図及び圧縮割れの発生状況を示す。[0001]
[Industrial application fields]
The present invention relates to a machine structural steel excellent in cold forgeability used for automobile parts and industrial machine parts and a manufacturing method thereof, in particular, steel for machine structure excellent in cold forgeability after spheroidizing annealing and its It relates to a manufacturing method.
[0002]
[Prior art]
Highly cold formability is required for materials for parts such as automobiles and the like that are manufactured by cold forming rods or wires that are hot-rolled. For this reason, for example, as described in JP-A-6-33190 Spheroidizing annealing. In general, this spheroidizing annealing is performed by holding the wire in the vicinity of the Ar ₁ point for a long time, for example, for 20 to 30 hours. Therefore, a special annealing furnace is required and a large amount of heat energy is consumed. It is an obstacle to energy saving and equipment cost reduction.
[0003]
In addition, even after such a long spheroidizing annealing, the metal structure of the resulting product, especially the spheroidized cementite, is not evenly distributed in the ferrite matrix, ensuring sufficient target forgeability. There is a problem that you can not.
[0004]
[Problems to be solved by the invention]
The present invention has been made in view of the situation as described above, and after spheroidizing annealing in which spheroidized carbides are uniformly dispersed and cold forgeability can be sufficiently ensured even in a relatively short spheroidizing annealing. It aims at proposing the machine structural steel excellent in the cold forgeability, and its manufacturing method.
[0005]
[Means for Solving the Problems]
The steel for machine structural use according to the present invention has a chemical composition with a mass ratio of C: 0.3 to 0.6%, Mn: 0.2 to 1.5%, Si: 0.05 to 2.0%, Cr: 0.04 to 2.0%, balance: iron and inevitable It consists of impurities, and the average grain size of prior austenite in the metal structure is 100 μm or more, and the ferrite fraction is 20% or less, and this has the properties of excellent cold forgeability after spheroidizing annealing. Here, the average particle size of prior austenite (hereinafter referred to as “old austenite particle size”) refers to the average particle size of the parent phase austenite that generates a transformation phase such as proeutectoid ferrite, pearlite, bainite, and martensite. The ferrite fraction means the proportion of pro-eutectoid ferrite in the metal structure of the product steel.
[0006]
In order to produce the steel for machine structural use, the material having the chemical composition is subjected to hot rolling so that its end temperature is 970 ° C. or more, and after the end of the hot rolling, from 970 ° C. It is better to adopt a process of cooling between 500 ℃ at an average of 2 ℃ / min or more.
[0007]
[Embodiments of the Invention]
Hereinafter, the chemical composition and metal structure of the steel for machine structure excellent in the cold forgeability after spheroidizing annealing according to the present invention and the production method thereof will be specifically described.
[0008]
The steel for machine structure of the present invention has the following chemical composition. Hereinafter, the unit (%) of the content of each element depends on the mass ratio.
[0009]
C: 0.3-0.6%
C is an element necessary for ensuring the strength, and if it is less than 0.3%, it is difficult to ensure the predetermined strength and fatigue strength, and the ferrite fraction cannot be lowered sufficiently, and cementite is not added after spheroidizing annealing. It cannot be uniformly dispersed. On the other hand, if it exceeds 0.6%, the cold forgeability is remarkably lowered or defects such as cracks are likely to occur when heat treatment is performed after forging.
[0010]
Mn: 0.2-1.5%
Mn is an element necessary for deoxidation. In addition, mechanical properties and fatigue properties are improved by solid solution strengthening. In order to obtain these effects, it is necessary to contain 0.2% or more. However, when it exceeds 1.5%, the cold forgeability deteriorates remarkably.
[0011]
Si: 0.05% to 2.0%
Si is an element necessary for deoxidation, and is added so that 0.05% remains. However, if it exceeds 2.0%, the cold forgeability deteriorates remarkably, and it becomes difficult to make the ferrite fraction 20% or less.
[0012]
Cr: 0.4-2.0%
Cr promotes the formation of cementite and contributes to spheroidization of cementite to improve forgeability. The effect appears at 0.04% or more. However, even if the content exceeds 2.0%, the effect is not improved, but adversely affects mechanical properties such as fatigue strength and ductility.
[0013]
The rest: iron and unavoidable impurities In addition to P, S, O, etc., there are trump elements. The smaller the better, the better. In particular, P and S segregate at the grain boundaries of the steel and cause the steel to become brittle.
[0014]
The mechanical structural steel according to the present invention is required to have a ferrite fraction of 20% or less. Even if the raw material is still hot-worked, if the ferrite fraction is low, the fraction of the structure other than ferrite in the structure, that is, the pearlite fraction, the bainite fraction, and the martensite fraction increase, and the subsequent spherical shape In chemical annealing, the dispersion of cementite becomes more uniform. This ferrite fraction can be obtained by measuring the area of the pro-eutectoid ferrite phase in the field of view from a photograph of 10 fields of magnification of 400 times obtained by an optical microscope.
[0015]
Such a ferrite fraction can be lowered by enlarging the austenite grains that are the parent phase, whereby the uniformity of pearlite, bainite, or martensite can be increased. In hypoeutectoid steel, the austenite grain boundary, which is the parent phase, mainly functions as a precipitation site for pro-eutectoid ferrite, and preferential precipitation of pro-eutectoid ferrite proceeds when the austenite grains are small. Conversely, when austenite grains are enlarged, such preferential precipitation of pro-eutectoid ferrite is hindered and the ferrite fraction is lowered. This tendency is particularly prominent when the austenite grain size is 100 μm or more on average, and the fraction of pearlite, bainite, and martensite is 80% or more in total in the component range of the steel of the present invention. Furthermore, if the austenite grain size, which is the parent phase, is too fine, the forgeability decreases due to the refinement of the steel structure and the strengthening thereby. Therefore, in the present invention, the average particle size of the parent phase austenite, that is, the prior austenite particle size, is required to be 100 μm or more.
[0016]
The prior austenite particle size is measured by corroding a sample cut out from a hot rolled product as a product by the following method and observing it under a microscope.
(1) In the case of ferrite-pearlite structure: After corroding with nitrate alcohol, the average grain size is measured by pro-eutectoid ferrite surrounding the pearlite grains.
(2) When the structure is mainly composed of martensite or bainite: Corrosion is conducted with a solution of ferric chloride and hydrochloric acid in ethyl alcohol, and the average crystal grain size is measured.
The crystal grain size was determined using the method described in JIS G 0551 (Austenite grain size test method for steel).
[0017]
In order to obtain the coarse austenite grains as described above, it is desirable to perform the hot rolling conditions for the steel material so that the end temperature is 970 ° C. or higher. As a result, the austenite crystal grains are sufficiently grown after hot rolling, and the average grain size reaches 100 μm or more. The austenite crystal grains after hot rolling are the prior austenite grains referred to in the present invention, and the grain boundaries are observed by the above-described method, thereby determining the prior austenite grain size.
[0018]
After completion of the hot rolling, cooling is performed at an average cooling rate between 970 and 500 ° C. at 2 ° C./min or more. As a result, pearlite, bainite and martensite can be produced in a state in which their total fraction is high from coarse parent phase austenite having a particle size of 100 μm or more produced by raising the hot rolling end temperature. When the cooling rate is slower than 2 ° C./min, a large amount of pro-eutectoid ferrite is generated, and it is difficult to obtain a uniform structure by subsequent spheroidizing annealing.
[0019]
【Example】
Example 1
Steel ingots having the composition shown in Table 1 were produced, heated to 1200 ° C., and then hot rolled to a round bar having a diameter of 60 mm at a finish rolling temperature of 970 ° C. After the hot rolling was completed, a temperature range of 970 to 500 ° C. was cooled at 5 ° C./min to obtain a product. A test piece was cut out in a direction perpendicular to the rolling direction of the obtained product, and the structure was observed by microscopic observation (measurement of prior austenite grain size and ferrite fraction).
[0020]
The obtained product was subjected to simple spheroidizing annealing that was held at 745 ° C for 5 hours, and the tablet test piece with a diameter of 15 mm and a height of 22.5 mm shown in Fig. 1 was aligned with the rolling direction of the product. Cut out and subjected to a cold forging test. In the cold forging test, each of the 10 test pieces was compressed by changing the rolling reduction ratio and examined for cracks. The evaluation of cold forgeability was made by plotting the relationship between the occurrence rate of compression cracking and the compression rate on a graph, and taking the compression rate at which 50% (5 pieces) of the test piece was cracked as the cold forgeability evaluation value.
[0021]
In addition to the above test, the product (round bar with a diameter of 60 mm) was further hot-rolled to a thickness of 20 mm and then subjected to spheronization treatment at 745 ° C. for 5 hours. In addition, Ono-type rotating rolling fatigue test pieces were cut out from the rolling direction and the direction perpendicular to the rolling direction, respectively, and the fatigue limit reaching 10 ⁷ repetitions at 3000 rpm was determined. These test results are shown in Table 1 together with the results of the tissue observation.
[0022]
[Table 1]

[0023]
From the above results, it can be seen that a product excellent in fatigue strength and cold forgeability can be obtained when the steel composition is within the range of the present invention.
[0024]
(Example 2)
Steel having a composition of C: 0.48%, Si: 0.21%, Mn: 0.85%, P: 0.010%, S: 0.004%, Cr: 0.11% by mass ratio under the conditions shown in Table 2, The steel bar was 60 mm in diameter. With respect to the obtained steel bar, a test piece was cut out in a direction perpendicular to the rolling direction and observed with a microscope. Further, the obtained steel bar was subjected to spheroidizing annealing that was held at 745 ° C. for 5 hours. A tablet test piece was cut out from the steel bar subjected to spheroidizing annealing so as to coincide with the rolling direction in the same manner as in Example 1, and subjected to a cold forging test. The test results are also shown in Table 2.
[0025]
[Table 2]

[0026]
Test No. 21 and Test No. 23, 26, and 27 with appropriate hot rolling conditions have a low ferrite fraction, an appropriately large prior austenite grain size, and high cold forgeability. However, in the test No. 22 having a small average cooling rate, the ferrite fraction is high although the prior austenite grain size is large, and the cold forgeability is rapidly decreased. Further, those of Test No. 24 and Test No. 25 have low forgeability because the prior austenite grain size is small and the ferrite fraction is high. In Test No. 28, although the ferrite fraction is 20% or less, the prior austenite grain size is small and the forgeability is poor.
[0027]
【The invention's effect】
The steel for machine structural use according to the present invention has a uniform structure due to its low ferrite fraction, and even in a state where a relatively simple spheroidizing annealing is performed, the spheroidized carbides are uniformly dispersed, and the prior austenite grain size is further reduced. Therefore, deterioration of forgeability due to refinement of the structure can be avoided, and excellent cold forgeability is achieved.
[Brief description of the drawings]
FIG. 1 shows a perspective view of a cold forged test piece and the occurrence of compression cracking.

Claims (2)

The chemical composition is, by mass ratio, C: 0.3-0.6%, Mn: 0.2-1.5%, Si: 0.05-2.0%, Cr: 0.04-2.0%, the balance: iron and inevitable impurities. A machine structural steel having excellent cold forgeability after spheroidizing annealing, wherein the average grain size of the steel is 100 μm or more and the ferrite fraction is 20% or less. The chemical composition is, by mass ratio, C: 0.3-0.6%, Mn: 0.2-1.5%, Si: 0.05-2.0%, Cr: 0.04-2.0%, the balance: heat against steel material consisting of iron and inevitable impurities Spheroidizing annealing, characterized in that the rolling is performed so that the end temperature is 970 ° C. or higher, and after the hot rolling is completed, cooling between 970 ° C. and 500 ° C. is performed at an average of 2 ° C./min or higher. A method for manufacturing steel for machine structural use, which has excellent cold forgeability later.

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