JP2004068064A - Mechanical structural steel excellent in cold forgeability after spheroidizing annealing and method for producing the same - Google Patents

Abstract

Kind Code: A1 A steel for machine structural use excellent in cold forgeability after spheroidizing annealing, in which spheroidized carbide is uniformly dispersed even in spheroidizing annealing for a relatively short time and sufficient cold forgeability is ensured, and production thereof. Suggest a method.
SOLUTION: The chemical composition is represented by mass ratio of C: 0.3 to 0.6%, Mn: 0.2 to 1.5%, Si: 0.05 to 2.0%, Cr: 0.04. 2.0%, balance: iron and unavoidable impurities. In the metal structure, the average crystal grain size of prior austenite is 100 μm or more, and the ferrite fraction is 20% or less.
[Selection diagram] None

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 applications]
The present invention relates to a machine structural steel excellent in cold forgeability used for automobile parts and parts of industrial machines and a method for producing the same, particularly a machine structural steel excellent in cold forgeability after spheroidizing annealing and its production method. It relates to a manufacturing method.
[0002]
[Prior art]
Materials for parts of automobiles and the like manufactured by cold-forming hot-rolled bars or wires are required to have high cold-formability, and therefore, as described in, for example, JP-A-6-33190. Spheroidizing annealing. In general, this spheroidizing annealing is performed by holding the wire near the Ar ₁ point for a long time, for example, for 20 to 30 hours, and thus requires a special annealing furnace and consumes a large amount of heat energy. Energy saving and equipment cost reduction.
[0003]
Furthermore, even if such a long-time spheroidizing annealing is performed, the metal structure of the obtained product, particularly, the dispersion of the spheroidized cementite in the ferrite matrix is not uniform, so that the target forgeability is sufficiently secured. There is a problem that can not be.
[0004]
[Problems to be solved by the invention]
The present invention has been made in view of the above situation, even after a relatively short spheroidizing annealing, after spheroidizing annealing can sufficiently ensure cold forgeability by uniformly dispersing the spheroidized carbide. It is an object of the present invention to propose a mechanical structural steel excellent in cold forgeability and a method for producing the same.
[0005]
[Means for Solving the Problems]
In the steel for machine structural use of the present invention, the chemical composition is represented by 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: composed of iron and unavoidable impurities, the average grain size of old austenite is 100 μm or more in the metal structure, and the ferrite fraction is 20% or less, whereby spheroidization is achieved. It has the property of being excellent in cold forgeability after annealing. Here, the average grain size of the prior austenite (hereinafter, referred to as “old austenite grain size”) refers to the average grain size of the parent austenite that forms a transformed phase such as proeutectoid ferrite, pearlite, bainite, and martensite. The ferrite fraction refers to the proportion of proeutectoid ferrite in the metal structure of the product steel material.
[0006]
In order to manufacture the steel for machine structural use, hot rolling is performed on a material having the above chemical composition so that the end temperature thereof is 970 ° C. or higher, and after the end of the hot rolling, from 970 ° C. It is preferable to adopt a step of cooling at an average of 2 ° C./min or more between 500 ° C.
[0007]
[Embodiment of the present invention]
Hereinafter, the chemical composition and metal structure of the steel for machine structural use having excellent cold forgeability after spheroidizing annealing according to the present invention and the method for producing the same will be specifically described.
[0008]
The steel for machine structural use of the present invention has the following chemical composition. Hereinafter, the unit (%) of the content of each element is based on the mass ratio.
[0009]
C: 0.3-0.6%
C is an element necessary for securing strength. If it is less than 0.3%, it is difficult to secure predetermined strength and fatigue strength, and it is not possible to sufficiently reduce the ferrite fraction, and after spheroidizing annealing, Cementite cannot be dispersed uniformly. On the other hand, if it exceeds 0.6%, the cold forgeability is significantly reduced, and defects such as cracks are liable to occur when heat treatment is performed after forging.
[0010]
Mn: 0.2-1.5%
Mn is an element necessary for deoxidation. Further, 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 is significantly reduced.
[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 is significantly reduced, and it is also difficult to reduce the ferrite fraction to 20% or less.
[0012]
Cr: 0.4 to 2.0%
Cr promotes the formation of cementite and contributes to spheroidization of cementite to improve forgeability. The effect appears above 0.04%. However, if the content exceeds 2.0%, the effect is not improved, but rather adversely affects mechanical properties such as fatigue strength and ductility.
[0013]
The balance: iron and inevitable impurities Inevitable impurities include P, S, O, etc., as well as tramp elements. The smaller these are, the better. In particular, P and S segregate at the grain boundaries of the steel and embrittle the steel. Therefore, each of P and S is preferably limited to 0.03% or less.
[0014]
The steel for machine structural use according to the present invention is required to have a ferrite fraction of 20% or less. Even if the material is 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 then the spherical shape In the annealing, the dispersion of the cementite becomes more uniform. This ferrite fraction can be determined by measuring the area of the pro-eutectoid ferrite phase in the visual field from a photograph of 10 visual fields with a magnification of 400 obtained by an optical microscope.
[0015]
Such a ferrite fraction can be reduced by increasing the size of the austenite grains as the matrix, thereby improving the uniformity of pearlite, bainite or martensite. In the hypoeutectoid steel, the austenite grain boundary, which is the parent phase, mainly functions as a precipitation site for proeutectoid ferrite, and when the austenite grains are small, preferential precipitation of proeutectoid ferrite proceeds. Conversely, when the austenite grains are large, such preferential precipitation of proeutectoid ferrite is hindered, and the ferrite fraction decreases. This tendency is particularly remarkable when the austenite particle size is 100 μm or more on average, and the total fraction of pearlite, bainite, and martensite is 80% or more in the composition range of the steel of the present invention. Further, if the austenite grain size as the parent phase is too small, the forgeability decreases due to the refinement and strengthening of the steel structure. Therefore, in the present invention, it is necessary that the average grain size of the matrix austenite, that is, the prior austenite grain size is 100 μm or more.
[0016]
The measurement of the prior austenite particle size is performed by corroding a sample cut from a hot-rolled product as a product by the following method and observing the sample under a microscope.
{Circle around (1)} In the case of ferrite-pearlite structure: After corrosion with nitric acid alcohol, the average crystal grain size is measured with pro-eutectoid ferrite surrounding the pearlite grains.
{Circle around (2)} In the case of a structure mainly composed of martensite or bainite: Corrosion is performed with a solution in which ferric chloride and hydrochloric acid are dissolved in ethyl alcohol, and the average crystal grain size is measured.
Note that the method described in JIS G 0551 (test method for austenite grain size of steel) was used to determine the grain size.
[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. Thereby, austenite crystal grains after hot rolling are sufficiently grown, and the grain size reaches 100 μm or more on average. The austenite crystal grains after the hot rolling are the former austenite grains referred to in the present invention, and the grain boundaries are observed by the method described above, and the prior austenite grain size is determined thereby.
[0018]
After the end of the hot rolling, cooling is performed at an average cooling rate of 970 to 500 ° C. of 2 ° C./min or more. As a result, pearlite, bainite and martensite can be produced in a state where their total fraction is high from coarse matrix austenite having a particle diameter of 100 μm or more produced by increasing the above-mentioned hot rolling end temperature. When the cooling rate is slower than 2 ° C./min, a large amount of proeutectoid ferrite is formed, and it is difficult to obtain a uniform structure by the subsequent spheroidizing annealing.
[0019]
【Example】
(Example 1)
A steel ingot having the composition shown in Table 1 was produced, heated to 1200 ° C., and hot-rolled into a round bar having a diameter of 60 mm at a finish rolling temperature of 970 ° C. After the completion of hot rolling, the product was cooled at a temperature of 970 to 500 ° C at a rate of 5 ° C / min. 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 old austenite grain size and ferrite fraction).
[0020]
The obtained product was subjected to simple spheroidizing annealing at 745 ° C. for 5 hours, and a tablet test piece having a diameter of 15 mm and a height of 22.5 mm shown in FIG. 1 was adjusted so that the height direction matched the rolling direction of the product. And subjected to a cold forging test. In the cold forging test, compression was performed for each of ten test pieces while changing the rolling reduction, and the presence or absence of cracks was examined. In the evaluation of cold forgeability, the relationship between the rate of occurrence of compression cracking and the compressibility was plotted on a graph, and the compressibility at which 50% (5 pieces) of the test pieces broke was defined as the cold forgeability evaluation value.
[0021]
In addition to the above test, the product (a round bar having a diameter of 60 mm) was further hot-rolled to a thickness of 20 mm, and then subjected to a spheroidizing treatment at 745 ° C. for 5 hours. Also, cut out each Ono type rotating rolling bending fatigue test piece in a direction perpendicular to the rolling direction and rolling direction, under the rotational speed 3000 rpm, to determine the fatigue limit reaching repeated several 10 ⁷ times. These test results are shown in Table 1 together with the results of the structure observation.
[0022]
[Table 1]

[0023]
From the above results, it can be seen that a product having excellent 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 by mass ratio of C: 0.48%, Si: 0.21%, Mn: 0.85%, P: 0.010%, S: 0.004%, Cr: 0.11% Was hot-rolled under the conditions shown in Table 2 to obtain a steel bar having a diameter of 60 mm. A test piece was cut out from the obtained steel bar in a direction perpendicular to the rolling direction, and observed with a microscope. The obtained steel bars were subjected to a spheroidizing annealing treatment at 745 ° C. for 5 hours. A tablet test piece was cut out from the spheroidized annealed steel bar in the same manner as in Example 1 so as to match the rolling direction, and subjected to a cold forging test. The test results are also shown in Table 2.
[0025]
[Table 2]

[0026]
Test No. with appropriate hot rolling conditions. 21 and test no. 23, 26, and 27 have a low ferrite fraction, an appropriately large austenite grain size, and high cold forgeability. However, the test No. having a small average cooling rate. In the case of No. 22, although the prior austenite grain size is large, the ferrite fraction is high, and the cold forgeability sharply decreases. Test No. 24, test no. In the case of 25, the former austenite grain size is small and the ferrite fraction is high, so that the forgeability is low. Test No. In the case of 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 a low ferrite fraction, and even in a relatively simple spheroidizing annealing, the spheroidized carbide is uniformly dispersed, and furthermore, the prior austenite particle size , The forgeability can be prevented from deteriorating due to the refinement of the structure, and excellent cold forgeability can be obtained.
[Brief description of the drawings]
FIG. 1 shows a perspective view of a cold forged test piece and a state of occurrence of compression cracking.

Claims (2)

Chemical composition is C: 0.3-0.6%, Mn: 0.2-1.5%, Si: 0.05-2.0%, Cr: 0.04-2.0 by mass ratio. %, Balance: composed of iron and unavoidable impurities, characterized in that the average grain size of the prior austenite is 100 μm or more and the ferrite fraction is 20% or less in the metal structure, and the cold forgeability after spheroidizing annealing is characterized in that Excellent machine structural steel. Chemical composition is C: 0.3-0.6%, Mn: 0.2-1.5%, Si: 0.05-2.0%, Cr: 0.04-2.0 by mass ratio. %, Balance: hot rolling is performed on a steel material composed of iron and unavoidable impurities so that the ending temperature is 970 ° C. or more, and after the end of the hot rolling, an average temperature between 970 ° C. and 500 ° C. A method for producing steel for machine structural use having excellent cold forgeability after spheroidizing annealing, wherein the steel is cooled at 2 ° C./min or more.

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