JP2010242209A - Steel for machine structural use excellent in crystal grain coarsening resistance and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for machine structural use in which crystal grains are not easily coarsened, when it is processed by cold forging or cold working and optional cutting work followed by carburizing treatment, by limiting chemical components, a lamellar pearlite area rate after spheroidizing annealing, and spheroidizing annealing conditions. <P>SOLUTION: The steel for machine structural use excellent in crystal grain coarsening resistance includes, by mass%, 0.10 to 0.25% C, 0.05 to 2.0% Si, 0.1 to 1.5% Mn, 0.030% or less P, 0.030 or less S, 1.8 to 3.0% Cr, 0.005 to 0.050% Al, and 0.030% or less N; further includes one or more of 0.25 to 3.0% Ni and 0.05 to 1.0% Mo; and further includes one or more of less than 0.050% Ti, 0.02 to 0.10% Nb, and 0.0010 to 0.0050% B, with the balance being Fe and unavoidable impurities, wherein the lamellar pearlite area rate after the spheroidizing annealing shown in the figure is 3% or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  For example, a machine structural steel for carburized parts having excellent crystal grain coarsening characteristics used for power transmission in automobiles and the like, and a method for producing the same.

  Cold methods such as cold forging and cold working are advantageous methods for reducing the cost of manufacturing parts such as automobile drive system parts. However, when parts are manufactured by carburizing directly after cold working, the effect of cold working is that fine austenite grains are formed in the initial stage of carburizing, which makes it easier to coarsen the grains instead of carburizing. Has a problem. Since the strength of a part may decrease when the crystal grains become coarse, it is essential to suppress the coarsening of the crystal grains. Due to this problem, the cost merit of the cold work method cannot be fully utilized. In the process of heating the parts to the carburizing temperature after cold working, the ferrite undergoes a fine recrystallization stage due to the effects of strain during cold working and then transforms to austenite, which forms fine austenite grains at the beginning of carburizing. Urging. Therefore, as a conventional technique, annealing is performed in the process of heating to the carburizing temperature after cold working, and the strain energy that becomes the driving force of the above-mentioned ferrite recrystallization is released, thereby suppressing grain coarsening during carburizing. There is a method (for example, refer nonpatent literature 1).

  However, since a new process is added by this, it is hard to use from a viewpoint of cost reduction of components. On the other hand, the inventors have a non-uniform microstructure with respect to spheroidizing annealing performed for the purpose of improving workability prior to cold working with respect to steel for machine structural use for carburizing treatment used as a component material. It has been found that this promotes grain coarsening during carburizing. In general, when spheroidizing annealing is applied to machine structural steel for carburizing treatment, partial lamellar pearlite is inevitably generated, and the resulting spheroidizing annealing structure is non-uniform. The part of pearlite that contains carbide in lamellar form in the steel is hard and hard to deform compared to the steel of the parent phase, so locally uneven strain concentrates around the lamellar pearlite during the cold forging process of the part. It's easy to do. As a result, a process in which ferrite is recrystallized particularly finely around lamellar pearlite when cold forging or cold working, and cutting to the required shape by heating to carburizing temperature. Therefore, austenite grains at the initial stage of carburizing are formed particularly finely. Due to this influence, crystal grains grow and become coarser during carburizing.

K. C. Evanson, G.M. Krauss and D.C. K. Matlock: Grain Growth in Polycrystalline Materials III, ed. by H.M. Weiland, B.M. L. Adams and A.M. D. Rollet, TMS, Warrendale, PA (1993), 599. J. et al. Jpn. Soc. Technol. Plast, 22 (1981), p. 139. "Cold Forging Subcommittee Standard" edited by Cold Forging Subcommittee Materials Research Group

  Therefore, the problem to be solved by the present invention is to limit the chemical components, limit the area ratio of lamellar pearlite after spheroidizing annealing, and limit the conditions of spheroidizing annealing, thereby performing cold forging or cold working. In addition, it is an object of the present invention to provide a machine structural steel that is less likely to cause crystal grain coarsening when a carburizing process is performed after cutting into a predetermined shape by performing cutting as required, and a method for manufacturing the same. To do. By providing this technology, the effect of suppressing grain coarsening due to fine precipitates such as AlN, NbC, and Nb (C, N) that pin the crystal grain boundaries contained in machine structural steels for carburizing treatment applications. Together, a machine structural steel that exhibits excellent grain coarsening resistance is obtained.

  Means for solving the above-mentioned problems are, in the invention of claim 1, mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 0.1%. 1.5%, P: 0.030% or less, S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less Is a steel for machine structural use that is excellent in grain coarsening resistance, characterized by comprising Fe and unavoidable impurities, and the area ratio of lamellar pearlite after spheroidizing annealing is 3% or less .

  In the invention of claim 2, in mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% Hereinafter, S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ni: The area ratio of lamellar pearlite after spheroidizing annealing, containing one or more of 0.25 to 3.0%, Mo: 0.05 to 1.0%, the balance being Fe and inevitable impurities Is a steel for machine structure excellent in crystal grain coarsening characteristics, characterized by being 3% or less.

  In the invention of claim 3, by mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% Hereinafter, S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ni: Containing one or more of 0.25 to 3.0%, Mo: 0.05 to 1.0%, and further by mass, Ti: less than 0.050%, Nb: 0.02-0 .10%, B: 0.0010 to 0.0050% of one type or two or more types, balance Fe and inevitable impurities, and the area ratio of lamellar pearlite after spheroidizing annealing is 3% or less It is a steel for machine structural use, which is characterized by having excellent grain coarsening resistance.

  In the invention of claim 4, in mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% Hereinafter, S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ti: Contains less than 0.050%, Nb: 0.02 to 0.10%, B: 0.0010 to 0.0050%, or one or more of the remaining Fe and inevitable impurities. It is a steel for machine structural use having excellent grain coarsening resistance, characterized in that the area ratio of later lamellar pearlite is 3% or less.

  In the invention of claim 5, the steel for machine structural use is subjected to spheroidizing annealing, cold forging or cold working and cutting as required to form a predetermined shape, and then used for machine parts that are carburized. In steel for machine structure, in mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less , S: 0.030% or less, Cr: 1.8-3.0%, Al: 0.005-0.050%, N: 0.030% or less, and the balance Fe and inevitable impurities After heating to 740-810 ° C. and holding for a predetermined time, it is cooled to 720-650 ° C. at a cooling rate of 8-40 ° C./Hr, and then subjected to spheroidizing annealing that is air-cooled. This is a method for producing a steel for machine structural use having excellent crystal grain coarsening characteristics.

  In the invention of claim 6, the steel for machine structure is subjected to spheroidizing annealing, and then cold forging or cold working and cutting according to need is performed to obtain a predetermined shape, which is then used for machine parts to be carburized. In steel for machine structure, in mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less , S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further Ni: 0. Containing one or more of 25 to 3.0%, Mo: 0.05 to 1.0%, using steel consisting of the balance Fe and inevitable impurities, heating to 740 to 810 ° C. and holding for a predetermined time Later, it is cooled to 720-650 ° C. at a cooling rate of 8-40 ° C./Hr, and then subjected to spheroidizing annealing for air cooling. It is a method for manufacturing a steel for machine structural use excellent in resistance to grain coarsening properties characterized by.

  In the invention of claim 7, the steel for machine structural use is subjected to spheroidizing annealing, cold forging or cold working and cutting as required to form a predetermined shape, and then used for machine parts to be carburized. In steel for machine structure, in mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less , S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further Ni: 0. Containing one or more of 25-3.0%, Mo: 0.05-1.0%, and further by mass, Ti: less than 0.050%, Nb: 0.02-0.10 %, B: 0.0010 to 0.0050% of one type or two or more types, with the balance Fe and inevitable impurities used, 74 Heat resistance to 810 ° C., hold for a predetermined time, cool to 720-650 ° C. at a cooling rate of 8-40 ° C./Hr, and then subject to spheroidizing annealing to air cooling, This is a method for producing mechanical structural steel having excellent properties.

  In the invention of claim 8, the steel for machine structure is subjected to spheroidizing annealing, and then cold forging or cold working and cutting as required are processed into a predetermined shape and then used for machine parts that are carburized. In steel for machine structure, in mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less , S: 0.030% or less, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further by mass%, Ti: 0 Less than 0.050%, Nb: 0.02 to 0.10%, B: 0.0010 to 0.0050% of one kind or two or more kinds, and the balance of Fe and unavoidable impurities are used. After heating to 810 ° C and holding for a predetermined time, cooling to 720-650 ° C at a cooling rate of 8-40 ° C / Hr A method for producing a subsequent anti-coarsening properties superior mechanical structural steel, characterized in that performing the spheroidizing annealing to cooling.

  The reasons for limiting the components of the mechanical structural steel and the area ratio of lamellar pearlite of the present invention and the heat treatment conditions of the method for manufacturing the mechanical structural steel will be described. In addition,% of a component is the mass%.

C: 0.10 to 0.25%
C is an element necessary for ensuring the core strength after carburizing treatment of steel as a machine structural component. However, if C is less than 0.10%, the effect cannot be sufficiently obtained, and if it exceeds 0.25%, workability is lowered and toughness is lowered. Therefore, C is set to 0.10 to 0.25%.

Si: 0.05-2.0%, desirably 0.05-1.0%
Si is an element necessary for deoxidation. However, if Si is less than 0.05%, sufficient deoxidation cannot be obtained, and if it exceeds 2.0%, the workability is lowered. Therefore, Si is set to 0.05 to 2.0%, preferably 0.05 to 1.0%.

Mn: 0.1 to 1.5%, desirably 0.2 to 0.8%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.1%, the effect of hardenability cannot be obtained sufficiently, and if it exceeds 1.5%, workability is lowered. Therefore, Mn is set to 0.1 to 1.5%, more preferably 0.2 to 0.8%.

P: 0.030% or less P is an unavoidable element contained in scrap, and segregates at the austenite grain boundary to lower toughness such as impact strength and bending strength. Therefore, P is set to 0.030% or less.

S: 0.030% or less S is an element that improves machinability. However, it produces MnS, which is a non-metallic inclusion, and lowers the toughness and fatigue strength in the lateral direction. Therefore, S is set to 0.030% or less.

Cr: 1.8-3.0%, desirably 2.0-2.5%
Cr is an indispensable element for suppressing the generation of lamellar pearlite during spheroidizing annealing and obtaining a homogeneous spheroidizing annealing structure mainly composed of spherical carbides. In order to obtain this effect, Cr needs to be 1.8% or more. On the other hand, if Cr is added excessively, the workability is impaired and the carburizing property is inhibited, so the content is made 3.0% or less. Therefore, Cr is 1.8 to 3.0%, preferably 2.0 to 2.5%. By adding 1.8% or more of Cr, the formation reaction of spherical carbides at the ferrite-austenite interface is promoted during the slow cooling process of spheroidizing annealing, so that a homogeneous spheroidizing annealing structure is obtained. Brought about.

Ni: 0.25 to 3.0% or less Ni is an element that improves hardenability and toughness, and in order to obtain the effect, it is necessary to add 0.25% or more. However, if Ni exceeds 3.0%, the workability is remarkably lowered and the cost is increased. Therefore, Ni is set to 0.25 to 3.0%.

Mo: 0.05-1.0%
Mo is an element that improves hardenability and toughness, and it is necessary to add 0.05% or more to obtain the effect. However, if the Mo content exceeds 1.0%, the workability is lowered. Therefore, Mo is set to 0.05 to 1.0%.

Al: 0.005 to 0.050%, desirably 0.015 to 0.050%
Al is an element used as a deoxidizing material, and also binds to N and precipitates as AlN as will be described later, thereby bringing about an effect of suppressing grain coarsening. In order to obtain this effect, it is necessary to add 0.005% or more of Al. On the other hand, if the Al content exceeds 0.050%, the alumina-based oxide increases, and the fatigue characteristics and workability deteriorate. Therefore, Al is made 0.005 to 0.050%, preferably 0.015 to 0.050%.

N: 0.030% or less, desirably 0.025% or less, less than 0.010% for steel materials containing Ti and / or B N is finely precipitated in the steel as AlN or Nb nitrides, resulting in grain coarsening It has the effect of preventing. However, if it exceeds 0.030%, nitrides increase, and fatigue strength and workability decrease. Therefore, in the invention of claim steel 1 to claim steel 8, N is 0.030% or less, preferably 0.025% or less. Note that, in a steel material containing Ti, if the amount of N is large, TiN is excessively generated and workability and fatigue strength are impaired. Further, in a steel material containing B, when N is contained in an amount of 0.010% or more, BN of the compound is generated and solid solution B is reduced, and the effect of improving hardenability is hindered. Therefore, among steels according to claims 3 and 4 or claims 7 and 8, N is particularly less than 0.010% in steels containing Ti and / or B.

Nb: 0.02 to 0.10%, desirably 0.02 to 0.08%
Nb forms carbides or nitrides, and has the effect of preventing grain coarsening. In particular, nano-order NbC or Nb (C, N) finely dispersed in steel suppresses the growth of crystal grains. If Nb is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.10%, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, Nb is 0.02 to 0.10%, preferably 0.02 to 0.08%.

Ti: Less than 0.050% Ti can contribute to improving hardenability by fixing free-N in steel and suppressing B from becoming compound BN. In order to obtain the effect, it is necessary to add Ti at less than 0.050%.

B: 0.0010 to 0.0050%
B is selectively contained as an element that remarkably improves the hardenability of the steel when contained in a very small amount. When obtaining the effect, the effect of improving the hardenability is small if it is less than 0.0010%, while the strength is lowered if it exceeds 0.0050%. Therefore, B is 0.0010 to 0.0050%.

The area ratio of lamellar pearlite after spheroidizing annealing is 3% or less, preferably 2% or less. In the steel, the part of pearlite where lamellar carbides are present is hard and less deformable than the parent phase steel. In the process of cold forging parts, uneven strain tends to concentrate locally around the lamellar pearlite. As a result, it undergoes a process in which ferrite is recrystallized particularly finely around lamellar pearlite when it is heated to carburizing temperature after cold forging or cold working and cutting as necessary to a predetermined shape. Therefore, austenite grains in the initial stage of carburization are formed particularly finely. Thereby, the coarsening of the crystal grains during carburization is likely to occur. In order to avoid this, the area ratio of lamellar pearlite after spheroidizing annealing needs to be 3% or less, preferably 2% or less.

Spheroidizing annealing conditions: after heating to 740-810 ° C, preferably 760-800 ° C and holding for a predetermined time, cooling to 720-650 ° C, 8-40 ° C / hr, preferably 10-20 ° C / hr Cooling at a speed and then air cooling In addition to the steel components described in the claims, the formation of lamellar pearlite during spheroidizing annealing is suppressed by performing spheroidizing annealing under the above conditions. A homogeneous spheroidized annealing structure can be obtained. As a result, when the carburizing process is performed after cold forging or cold working and cutting as necessary to form a predetermined shape, the crystal grains are difficult to coarsen. In order to obtain the effect, after heating to 740 to 810 ° C., preferably 760 to 800 ° C. and holding for a predetermined time (the holding time varies depending on the processing amount and the characteristics of the furnace, it is not particularly limited), then 720 It is necessary to perform spheroidizing annealing by cooling to 650 ° C. at a cooling rate of 8-40 ° C./hr, desirably 10-20 ° C./hr, and then air cooling.

  In the above-mentioned means of the present invention, cold forging or cold cooling is performed following spheroidizing annealing by limiting the steel components, limiting the area ratio of lamellar pearlite after spheroidizing annealing, and limiting spheroidizing annealing conditions. When the carburizing process is performed after performing the intermediate processing and cutting as required, the area ratio of lamellar pearlite after spheroidizing annealing is 3% or less, preferably 2% or less Since the crystal grains are difficult to coarsen, it becomes possible to reduce the manufacturing cost of driving system parts such as gears and shafts of automobiles, construction machines, machine tools, etc. using the cold work method. There is an excellent effect that has never existed before.

It is a figure which shows an example of the heat processing conditions of spheroidization annealing. It is a figure which shows a microstructure, (a) shows the comparative steel 1, (b) shows the invention steel 1. FIG.

  Embodiments for carrying out the present invention will be described with reference to tables and drawings.

  Comparative steels 1 to 7 and invention steels 1 to 16 in Table 1 and comparison steels 8 to 10 and invention steels 17 to 29 in Table 2 to which Nb was added were melted in a vacuum induction melting furnace to obtain a 100 kg steel ingot. It was. First, the molten steel ingot was forged into a steel bar having a diameter of 65 mm after heating at 1250 ° C. for 18 ks. Next, after holding at 10.8 ks at 900 ° C., spheroidizing annealing was performed following normalization by air cooling. In this example, spheroidizing annealing was carried out under the conditions shown in FIG. 1, which are within the condition range of the present invention. After spheroidizing annealing, the specimen was mirror polished and corroded with 5% nital, and then the microstructure was observed with an optical microscope to measure the lamellar pearlite area ratio in the field of view.

  The micrograph of FIG. 2 illustrates the spheroidized annealed structures of the comparative steel 1 and the inventive steel 1 obtained by the above treatment. While many lamellar pearlites are observed within the field of view in the comparative steel 1 in the above-described microscope observation, almost no lamellar pearlite is observed in the inventive steel 1.

  Next, a cylindrical test piece having a diameter of 14 mm and a length of 21 mm was produced by cutting from the vicinity of the middle circumference of the 65 mm diameter steel bar subjected to the spheroidizing annealing. The length direction of the specimen was matched with the forging direction of the base material. Using a universal testing machine, the test piece was cold upset 70% in height ratio. In the present invention, the cold working rate is not particularly limited to 70%. By the way, the shape of the test piece and the cold upsetting method were performed in accordance with the standards of the cold forging subcommittee of the Japan Society for Technology of Plasticity in Non-Patent Document 2.

  Next, a pseudo carburizing test was performed in order to confirm the grain coarsening temperature during carburizing. This test is a conventional method that simulates only the heat curve of carburizing treatment and actually observes the grain size without carburizing. First, the above-mentioned 70% cold upsetting test piece was divided into four parts, and each piece was heated to 300 ° C./Hr at each temperature and held at 10.8 ks, followed by water cooling. After this heat treatment, the cross section of the test piece was mirror-polished and corroded with a saturated picric acid solution to reveal prior austenite grain boundaries, and this test piece was observed with an optical microscope. The temperature at which coarse grains corresponding to the grain size number 3 were observed by observation with an optical microscope was defined as the crystal grain coarsening temperature.

  Tables 3 and 4 show the measurement results of the lamellar pearlite area ratio after spheroidizing annealing and the measurement results of the crystal grain coarsening temperature in the pseudo carburization test. In comparison between Comparative Steels 1 to 7 and Inventive Steels 1 to 16 in which Nb is not added, Inventive Steels 1 to 16 have improved grain coarsening temperatures compared to Comparative Steels 1 to 7. In addition, even when Nb having an effect of preventing grain coarsening is added, in comparison between the comparative steels 8 to 10 and the inventive steels 17 to 29, the inventive steels 17 to 29 are coarser than the comparative steels 8 to 10. The conversion temperature is improved.

Claims (8)

  In mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030 Hereinafter, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, comprising the remainder Fe and inevitable impurities, after spheroidizing annealing Machine structural steel with excellent crystal grain coarsening characteristics, characterized in that the area ratio of lamellar pearlite is 3% or less.   In mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030 %: Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ni: 0.25 to 3.0 %, Mo: 0.05 to 1.0%, or one or more of the remaining Fe and inevitable impurities, and the area ratio of lamellar pearlite after spheroidizing annealing is 3% or less A machine structural steel with excellent crystal grain coarsening characteristics.   In mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030 %: Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ni: 0.25 to 3.0 %, Mo: 0.05-1.0%, or one or two or more kinds, and further by mass, Ti: less than 0.050%, Nb: 0.02-0.10%, B: 0 100% to 0.0050% of one kind or two or more, comprising the balance Fe and inevitable impurities, and the area ratio of lamellar pearlite after spheroidizing annealing is 3% or less Machine structural steel with excellent grain coarsening properties.   In mass%, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030 Hereinafter, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ti: less than 0.050%, Nb: It contains one or more of 0.02 to 0.10%, B: 0.0010 to 0.0050%, consists of the balance Fe and inevitable impurities, and the area ratio of lamellar pearlite after spheroidizing annealing is A machine structural steel excellent in crystal grain coarsening characteristics, characterized by being 3% or less.   Machine structural steel used for machine parts that are subjected to carburizing treatment after cold forging or cold working and machining as required after machined steel is spheroidized and annealed. %, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030 or less , Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and using steel consisting of the remainder Fe and inevitable impurities, 740 to 810 ° C Heated and held for a predetermined time, cooled to 720 to 650 ° C. at a cooling rate of 8 to 40 ° C./Hr, and then subjected to spheroidizing annealing that is air-cooled, and was excellent in grain coarsening resistance characteristics Manufacturing method of steel for machine structural use.   Machine structural steel used for machine parts that are subjected to carburizing treatment after cold forging or cold working and machining as required after machined steel is spheroidized and annealed. %: C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030% Hereinafter, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ni: 0.25 to 3.0% , Mo: containing 0.05 to 1.0% of one or more kinds, steel using balance Fe and inevitable impurities, heating to 740 to 810 ° C. and holding for a predetermined time, 720 to 650 ° C. Spheroidizing annealing, which is cooled at a cooling rate of 8 to 40 ° C./Hr, and then air-cooled. Excellent production method for steel for machine structural use in grain coarsening properties.   Machine structural steel used for machine parts that are subjected to carburizing treatment after cold forging or cold working and machining as required after machined steel is spheroidized and annealed. %: C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030% Hereinafter, Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further in mass%, Ni: 0.25 to 3.0% , Mo: 0.05 to 1.0%, or one or more of them, and in terms of mass%, Ti: less than 0.050%, Nb: 0.02 to 0.10%, B: 0.0. It contains one or more of 0010 to 0.0050%, and uses steel consisting of the remaining Fe and inevitable impurities, and is heated to 740 to 810 ° C. Then, after holding for a predetermined time, it is cooled to 720 to 650 ° C. at a cooling rate of 8 to 40 ° C./Hr, and then subjected to spheroidizing annealing that is air-cooled, and has a mechanical structure excellent in grain coarsening resistance Steel manufacturing method.   Machine structural steel used for machine parts that are subjected to carburizing treatment after cold forging or cold working and machining as required after machined steel is spheroidized and annealed. %, C: 0.10 to 0.25%, Si: 0.05 to 2.0%, Mn: 0.1 to 1.5%, P: 0.030% or less, S: 0.030 or less , Cr: 1.8 to 3.0%, Al: 0.005 to 0.050%, N: 0.030% or less, and further by mass, Ti: less than 0.050%, Nb: 0 0.02 to 0.10%, B: 0.0010 to 0.0050% of one type or two or more types, and using steel consisting of the remainder Fe and inevitable impurities, heated to 740 to 810 ° C. for a predetermined time After holding, it is cooled to 720 to 650 ° C. at a cooling rate of 8 to 40 ° C./Hr, and then air cooled. Method for producing a machine structural steel excellent in resistance to grain coarsening properties, characterized by performing annealing.