JP6211784B2 - Manufacturing method of automotive machine parts having excellent fatigue strength and automotive machine parts by the method - Google Patents

Description

  The present invention relates to a method for manufacturing automotive machine parts such as pinions and side gears constituting a differential gear of an automobile, and an automotive machine part by the method.

  Cold methods such as cold forging and other cold working methods are advantageous for reducing the manufacturing cost of parts such as automobile drive system parts. However, when manufacturing parts by carburizing directly after cold working, the problem is that fine austenite grains are formed at the initial stage of carburizing due to cold working, so that the crystal grains tend to be coarsened instead of during carburizing. Have. Since the strength of parts 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 fine austenite grains in the initial stage of carburizing may be transformed to austenite once after the stage of fine recrystallization of ferrite due to the influence of strain during cold working. Encourage formation.

  Therefore, as a prior art, a method of suppressing grain coarsening during carburization by releasing heat and strain energy that becomes the driving force of ferrite recrystallization at the stage where the ferrite is recrystallized by performing heat treatment after cold working. (For example, refer nonpatent literature 1). However, since a new process is added by this method, it is difficult to use from the viewpoint of cost reduction of parts.

  By the way, by limiting the chemical composition, limiting the area ratio of lamellar pearlite after spheroidizing annealing, limiting spheroidizing annealing conditions, cold forging or cold working is performed, and further cutting is performed as necessary When a carburizing process is performed after processing and processing into a predetermined shape, a steel for mechanical structures that hardly causes crystal grain coarsening and a manufacturing method thereof have been proposed (for example, see Patent Document 1). .

  Furthermore, many techniques aiming at preventing coarsening of crystal grains by controlling the precipitates of the second phase in the matrix have been developed, and many proposals have been made (for example, see Patent Documents 2 to 14). However, when the direct carburization at high temperature is performed after cold forging or other cold working as proposed above, it is difficult to stably maintain the fine grain after carburizing. Therefore, it is necessary to limit those conditions.

JP 2010-242209 A Japanese Patent Laid-Open No. 04-247848 Japanese Patent Laid-Open No. 08-199303 JP 09-59745 A Japanese Patent Laid-Open No. 10-81938 Japanese Patent Application Laid-Open No. 2000-63943 Japanese Patent Laid-Open No. 2001-20038 JP 2001-279383 A Japanese Patent Laid-Open No. 04-176816 Japanese Patent No. 2716301 JP-A-10-130720 JP-A-10-152754 Japanese Patent Laid-Open No. 11-50191 JP 2001-303174 A

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.

  The problem to be solved by the present invention is a carburizing part that is carburized and quenched at 950 ° C. or higher without cold-forging and then performing normalizing and annealing, and stably crystallizing during carburizing treatment. An object of the present invention is to provide a method for manufacturing a machine part for an automobile capable of preventing the coarsening of the grains and a machine part for an automobile by this production method.

  The means of the present invention for solving the above-mentioned problems is, in the first means, in mass%, C: 0.13 to 0.20%, Si: 0.30 to 0.70%, Mn: 0.00. 20 to 0.50%, P: 0.030% or less, S: 0.030% or less, Cr: 2.00 to 2.50%, Al: 0.010 to 0.050%, Ti: 0.02 -0.08%, Nb: 0.02-0.08%, B: 0.0005-0.0035%, N: 0.020% or less, with the balance being Fe and inevitable impurities After heating to 1250 ° C. or higher and rolling into steel slabs, cooling to room temperature, heating the steel slabs to a temperature range of 800 to 1050 ° C., further rolling into steel materials, and then softening heat treatment of the steel materials After that, it is formed into the shape of a machine part for automobiles by cold forging, and then normalization or annealing is performed. Without carburizing at 950 ° C. or higher without subjecting the shaped body to quenching and tempering, the austenite grain size number specified in JIS G 0551 is 7 or more. .

The softening heat treatment in the first means is to maintain the maximum temperature of the steel material in a temperature range of 750 to 800 ° C., and then cool the temperature range from this maximum temperature to 650 ° C. at a cooling rate of 50 ° C. or less per hour. Features.

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In the second means, by mass%, C: 0.13 to 0.20%, Si: 0.30 to 0.70%, Mn: 0.20 to 0.50%, P: 0.030% or less , S: 0.030% or less, Cr: 2.00-2.50%, Al: 0.010-0.050%, Ti: 0.02-0.08%, Nb: 0.02-0. Austenite grain size number defined in JIS G 0551, which is 08%, B: 0.0005 to 0.0035%, N: 0.020% or less, the balance being Fe and inevitable impurities. The automotive machine part made of the steel, which is 0 or more, is a carbide containing Nb in the composition of precipitates present in the steel, and less than 0.05 precipitates having a diameter of 100 nm or more per 1 μm ^2. This is a machine part for automobiles.

In the third means, a motor vehicle mechanical parts of the second means mentioned above, this part is the second means automotive mechanical components, characterized in that the pinion gears and the side gears constituting the differential gear .

  By adopting the above means, the steel for machine structural use of the present invention has excellent crystal grain size characteristics, excellent fatigue strength, and is extremely excellent as a machine part for automobiles such as pinions and side gears constituting an automobile differential gear. Parts can be manufactured.

  In describing the mode for carrying out the present invention, first, the reasons for limiting the chemical components of steel in the means of the above claims will be described. In addition,% of each element in a chemical component is shown by mass%.

C: 0.13-0.20%
C is an element necessary for securing the strength after quenching and tempering of a steel material or the core strength after carburizing, quenching and tempering as a machine part for automobiles. If C is less than 0.13%, the strength cannot be secured, and if it exceeds 0.20%, the hardness of the material increases and the workability decreases. Therefore, C is set to 0.13 to 0.20%.

Si: 0.30 to 0.70%
Si is an element necessary for deoxidation, imparts necessary strength and hardenability to steel, and has an effect of shallowing the carburizing abnormal layer depth by addition of a certain amount or more. In order to obtain the effect of Si, addition of 0.30% or more is necessary. On the other hand, if Si exceeds 0.70%, the hardness of the material is increased, so that the workability is lowered. Therefore, Si is 0.30 to 0.70%.

Mn: 0.20 to 0.50%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.20%, the effect on hardenability cannot be sufficiently obtained. On the other hand, if Mn exceeds 0.50%, the machinability deteriorates and at the same time coarsening of crystal grains during carburization tends to occur. Therefore, Mn is set to 0.20 to 0.50%.

P: 0.030% or less P is an unavoidable element contained in scrap, but segregates at the grain boundary and lowers properties 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, but produces MnS that is a non-metallic inclusion and lowers the toughness and fatigue strength in the transverse direction. Therefore, S is set to 0.030% or less.

Cr: 2.00 to 2.50%
Cr is an element necessary for ensuring hardenability and preventing coarsening of crystal grains during carburizing. However, when Cr is less than 2.00%, these effects cannot be obtained sufficiently. On the other hand, when Cr exceeds 2.50%, carburization is inhibited, and the material hardness is increased to lower the machinability. Therefore, Cr is set to 2.00 to 2.50%.

Al: 0.010 to 0.050%
Al is an element used as a deoxidizing material. In order to obtain this effect, Al needs to be added in an amount of 0.010% or more. On the other hand, if Al is added in an amount exceeding 0.050%, large alumina inclusions are formed, and fatigue characteristics and workability are deteriorated. Therefore, Al is made 0.010 to 0.050%.

Ti: 0.02 to 0.08%
Ti forms carbides and has the effect of preventing crystal grain coarsening. Further, Ti bonds to N, thereby preventing B from binding to N and becoming BN. In order to obtain the effect, Ti needs to be added by 0.020% or more. On the other hand, if Ti is added over 0.08%, the machinability is impaired. Therefore, Ti is made 0.02 to 0.08%.

Nb: 0.02 to 0.08%
Nb forms carbides and has an effect of preventing grain coarsening, and particularly nano-order-sized NbC or NbTiC finely dispersed in steel suppresses the growth of crystal grains. However, if Nb is less than 0.02%, the effect cannot be obtained. On the other hand, if Nb exceeds 0.20%, the amount of precipitates becomes excessive and workability is lowered. Therefore, Nb is made 0.02 to 0.08%.

B: 0.0005 to 0.0035%
B is an element that significantly improves the hardenability of steel when contained in a very small amount, and since it can reduce the amount of other alloy elements added, it is an element effective in reducing the cost of steel materials. is there. However, if B is less than 0.0005%, the effect of improving hardenability is small. On the other hand, when B exceeds 0.0035%, the strength is lowered. Therefore, B is set to 0.0005 to 0.0035%.

N: 0.020% or less B exhibits the effect described above by dissolving in steel, but if free-N is present when B is added, BN is generated and the effect cannot be exhibited. Therefore, it is necessary to add Ti before addition of B and fix the free-N in the steel in the form of TiN. However, when N is too much, the amount of TiN increases, and fatigue strength, impact strength, and workability deteriorate. Therefore, N is set to 0.020% or less.

The reason why the steel cast material is heated to 1250 ° C or higher, rolled into a steel slab, cooled, and then heated to a temperature range of 800 ° C to 1050 ° C. The steel cast material is heated to 1250 ° C or higher. By heating Nb carbide and NbTi carbide to a solid solution, and then heating the cooled steel piece to a temperature range of 800 ° C. to 1050 ° C., in the stage where it is subjected to carburizing treatment at 950 ° C. Many fine Nb carbides and NbTi carbides are dispersed. If it is less than 1250 degreeC, Nb carbide | carbonized_material and NbTi carbide | carbonized_material will not fully dissolve, but coarse Nb type carbide | carbonized_material of 100 nm or more which precipitated at the time of casting will remain. When coarse Nb carbide of 100 nm or more remains, Nb carbide effective for preventing grain coarsening decreases, and at the same time, Ostwald growth occurs during heating at 800 ° C. to 1050 ° C. or carburization, and the surrounding fine particles By reducing Nb-based carbides, the dispersion state becomes non-uniform and the crystal grain size characteristics deteriorate. Therefore, the heating temperature is 1250 ° C. or higher, preferably 1270 ° C. or higher. Furthermore, when heated to a temperature range of 1050 ° C. or higher after steel slab rolling, the Nb-based carbide grows and the effect of preventing the coarsening of crystal grains is reduced. On the other hand, when rolling after heating to 800 ° C. or less after steel slab rolling, the crystal grains after rolling become smaller and the grain size characteristics at the time of carburizing are deteriorated. Therefore, the steel slab is heated to a temperature range of 800 ° C to 1050 ° C.

The reason why the temperature range from the maximum temperature to 650 ° C is cooled at a rate of 50 ° C or less after being held in the temperature range of 750 ° C to 800 ° C. To improve the grain size characteristics, the microstructure after the softening heat treatment should be uniform. It is necessary to make a spheroidized structure. In order to obtain a uniform spheroidized structure, it is necessary to cool from a temperature range of 750 ° C. to 800 ° C. to a temperature range of 650 ° C. at a rate of 50 ° C. or less per hour. When cooled from a temperature range below 750 ° C. and above 800 ° C., a uniform spheroidized structure cannot be obtained. This is because a uniform spheroidized structure cannot be obtained even when cooled at a rate exceeding 50 ° C per hour.

The reason why the diameter of carbides containing Nb is 100 nm or more is less than 0.05 per 1 μm ^{2 When the} number of precipitates of 100 nm or more is 0.05 or more per 1 μm ² , prevention of grain coarsening The Nb-based carbides that are effective for carbon dioxide are insufficient, and Ostwald growth is performed during heating or carburizing at 800 ° C. to 1050 ° C. to reduce the surrounding fine Nb-based carbides. Deteriorates. On the other hand, when the number of precipitates of 100 nm or more is less than 0.05 per 1 μm ^2, Nb-based carbide effective for preventing grain coarsening can be secured, and at the same time, excellent crystals can be dispersed. This is because particle size characteristics can be obtained.

  No. of comparative steel shown in Table 1. A-No. 8 types of H and No. of invention steel. I-No. Steels containing each of the five chemical components of M and consisting of the remainder Fe and inevitable impurities were melted in an electric furnace, cast after refining, and manufactured into a steel ingot or bloom.

The steel ingot or bloom produced above was heated to the range of rolling temperature of each steel slab rolling shown in Table 2, and then the steel slab was rolled by hot rolling and cooled to room temperature as it was. Subsequently, it heated to the range of the heating temperature of steel bar rolling shown in Table 2, and bar steel rolling was performed, and it manufactured to the steel bar. Furthermore, after cutting these steel bars, they were heated to the maximum softening heat treatment temperature shown in Table 2, and softening heat treatment was performed from this temperature to 650 ° C. at the cooling rate shown in Table 2. Further, after forming into pinions and side gears by cold forging, carburizing and quenching and tempering were performed at 950 ° C. without performing normalization or annealing. With respect to the pinion and the side gear manufactured by these, the crystal grain size of the tooth root part of the gear was observed. Table 2 shows the number of Nb dispersed particles of 100 nm or more per 1 μm ² . Further, a fatigue test of the pinion and the side gear was conducted, and the strength ratio between the 10 ⁴ cycle and the 10 ⁵ cycle (comparison with the No. 1 for the pinion and the No. 2 for the side gear) was investigated, and Table 2 also shows Indicated.

  In Table 2, no. No. 12 is steel type I of the invention steel. 13 and no. 16 is J of the invention steel. No. 20 is the steel type K of the invented steel. 24 is a steel type L of the invention steel. 26 is a steel type M of the invention steel. These are the conditions according to all claims of the present invention, that is, the conditions shown in Table 2, steel slab rolling, bar rolling, softening heat treatment temperature, grain size No. And Nb dispersed particles are all satisfied. In contrast, no. 14, no. 15, no. 17, no. 18, no. 19, no. 21, no. 22, no. 23, no. 25 is a steel type showing the component composition shown in Table 1 of the invention steel of the present application, the conditions according to all claims of the present invention, that is, the steel slab rolling, bar rolling, softening heat treatment temperature shown in Table 2, Particle size No. And the number of Nb dispersed particles do not satisfy any of the conditions, and these unsatisfied conditions are indicated by shading.

On the other hand, in Table 2, No. Steel types A to H of Nos. 1 to 11 are the chemical components shown in Table 1 of the steel of the present invention, as shown by the shaded area in Table 1. Steel slab rolling, steel bar rolling, softening heat treatment temperature, grain size No. Or the number of dispersed Nb particles is out of the conditions of the present invention. Steel types A to H of Nos. 1 to 11 have a particle size No. Is less than 7.0 and is not sufficiently miniaturized.

  As described above, each steel of the examples of the present invention is excellent in grain size characteristics, and in the gear fatigue test, as shown in Table 2, the fatigue strength is high and excellent. It was confirmed that these were machine parts for automobiles such as pinions and side gears constituting the differential gear.

Claims (3)

% By mass, C: 0.13 to 0.20%, Si: 0.30 to 0.70%, Mn: 0.20 to 0.50%, P: 0.030% or less, S: 0.030 % Or less, Cr: 2.00 to 2.50%, Al: 0.010 to 0.050%, Ti: 0.02 to 0.08%, Nb: 0.02 to 0.08%, B: 0 .0005 to 0.0035%, N: 0.020% or less, and the balance of Fe and unavoidable impurities is heated to 1250 ° C. or higher after casting, rolled into steel slab, cooled, and then The steel slab is heated to a temperature range of 800 to 1050 ° C. and then rolled into a steel material , and the maximum temperature is maintained at a temperature range of 750 to 800 ° C. as a softening heat treatment of the steel material. after cooling per hour 50 ° C. less cooling rate, molded into the shape of automotive mechanical parts with cold forging Then, after carburizing the shaped body at 950 ° C. or higher without performing normalization or annealing, quenching and tempering are performed to obtain an austenite grain size number of 7.0 or higher as defined in JIS G 0551. The manufacturing method of the machine component for motor vehicles characterized by these. % By mass, C: 0.13 to 0.20%, Si: 0.30 to 0.70%, Mn: 0.20 to 0.50%, P: 0.030% or less, S: 0.030 % Or less, Cr: 2.00 to 2.50%, Al: 0.010 to 0.050%, Ti: 0.02 to 0.08%, Nb: 0.02 to 0.08%, B: 0 .0005 to 0.0035%, N: 0.020% or less, the balance being Fe and inevitable impurities, steel having an austenite grain size number of 7.0 or more as defined in JIS G 0551 The automotive machine part is a carbide containing Nb in the composition of precipitates present in the steel, and the precipitate having a diameter of 100 nm or more is 1 μm. ² A machine part for automobiles characterized by being less than 0.05 per unit. The automotive machine part according to claim 2, wherein the automotive machine part is a pinion gear and a side gear constituting a differential gear .