JP6192519B2 - Method for producing steel for machine structure capable of stably controlling generation of coarse grains, and steel for machine structure comprising the method - Google Patents

Description

  The present invention relates to a method for manufacturing a carburized steel of a carburized part formed by cold forging, and a steel material comprising the manufacturing method, and in particular, no additional normalizing treatment after cold forging or further carburizing. Furthermore, the present invention relates to a method for manufacturing a steel material for machine structure capable of obtaining stable grain size characteristics and the steel material.

  Cold methods such as cold forging and cold working are advantageous methods for cost reduction in the production of parts such as automobile drive system parts. However, when parts are manufactured by directly carburizing after cold working, the grains become coarser than carburizing due to the effects of cold working or the formation of fine austenite grains at the beginning of carburizing. Has the problem of being easy. That is, when the crystal grains are coarsened, the strength of the parts may be lowered, and thus it is essential to suppress the grain coarsening. Due to this problem, the cost merit of the cold work method cannot be fully utilized.

Conventionally, cold forging or cold working, and cutting as required, by limiting chemical components, limiting the area ratio of lamellar pearlite after spheroidizing annealing, and limiting spheroidizing annealing conditions A steel for machine structural use and a method for producing the same are proposed that are less likely to cause crystal grain coarsening when carburizing is performed after being processed into a predetermined shape (see, for example, Patent Document 1).
On the other hand, many technologies have been developed and applied for controlling the coarsening of crystal grains by controlling the precipitates as the second phase (see, for example, Patent Documents 2 to 14). However, in these techniques, when carburizing directly at a high temperature after cold forging or cold working, it is difficult to stably maintain the fine grain after carburizing, and it is necessary to limit those conditions.

  By the way, in the process of heating the parts to the carburizing temperature after cold working, the ferrite is transformed into austenite once through the stage of fine recrystallization due to the influence of strain during cold working. It encourages the formation of grains. Therefore, as a conventional technique, there is a method of suppressing grain coarsening during carburization by performing heat treatment after cold working and releasing the strain energy that becomes the driving force of the above-mentioned ferrite recrystallization (for example, non-patent literature) 1). However, since a new process is added by this method, this method is difficult to use from the viewpoint of cost reduction of parts.

JP 2010-242209 A JP-A-4-247848 JP-A-8-199303 JP-A-9-59745 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 JP-A-4-176816 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. byH. 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 method for producing a steel for machine structural use that can be carburized and quenched at 950 ° C. or higher without performing normalization or annealing after cold forging, and a steel material comprising the method. Then, it is providing the manufacturing method of the steel material for machine structures which can prevent the coarsening of a crystal grain stably at the time of a carburizing process, and the steel material for machine structures which consists of this method.

Means for solving the above-mentioned problems are, in the first means, in mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0 .60%, P: 0.030% or less, S: 0.030% or less, Cr: 0.80-2.00%, Al: 0.010-0.050%, Nb: 0.02-0. The steel containing 08%, N: 0.010 to 0.020%, the balance being Fe and inevitable impurities is cast, the cast steel is heated at 1250 ° C. or more for 2 hours or more, and then rolled into a steel slab. Then, the steel slab is heated to a temperature range of 800 ° C. to 1050 ° C. and rolled, and the number of Al nitrides and Nb carbonitrides having a diameter of 100 nm or more is less than 0.05 / μm ^2. In the carburizing and quenching process at 950 ° C. to 1000 ° C. after cold forging, the particle size number is 6 or less. This is a method for producing a steel for machine structure capable of stably controlling the generation of coarse grains.

In the second means, by mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less , S: 0.030% or less, Cr: 0.80-2.00%, Al: 0.010-0.050%, Nb: 0.02-0.08%, Ti: 0.02-0. A steel containing 08%, B: 0.0005 to 0.0035%, N: 0.008% or less, the balance being Fe and inevitable impurities is cast, and the cast steel is heated at 1250 ° C. or more for 2 hours or more. after, and rolled steel strip was cooled, then rolling the steel piece after heating to a temperature range of 800 ° C. to 1050 ° C., 0.05 or the number of Nb-based carbide is not less than 100nm diameter / [mu] m ² In the carburizing and quenching process at 950 ° C. to 1000 ° C. after the cold forging, the particle size number is 6 This is a method for producing a steel material for machine structure capable of stably controlling the generation of coarse grains having a size of less than that.

In the third means, by mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less , S: 0.030% or less, Cr: 0.80-2.00%, Al: 0.010-0.050%, Nb: 0.02-0.08%, N: 0.010-0. A steel material containing 020%, the balance being made of steel consisting of Fe and inevitable impurities, the number of which is less than 0.05 pieces / μm ² , and the diameter of which is 100 nm or more of Al nitride and Nb carbonitride It is a steel for machine structure capable of stably controlling the generation of coarse grains having a grain size number of 6 or less even in a carburizing and quenching process at 950 ° C. to 1000 ° C. after cold forging.

In the fourth means, by mass, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less , S: 0.030% or less, Cr: 0.80-2.00%, Al: 0.010-0.050%, Nb: 0.02-0.08%, Ti: 0.02-0. The diameter of which is 08%, B: 0.0005 to 0.0035%, N: 0.008% or less, the balance being made of steel consisting of Fe and inevitable impurities, the number of which is less than 0.05 / μm ² In a carburizing and quenching process at 950 ° C. to 1000 ° C. after cold forging, characterized by being a steel material containing Nb-based nitride of 100 nm or more in a stable manner. Steel material for machine structure that can be controlled.

In the steel material of the present invention, the number of Al nitride and Nb carbonitride having a diameter of 100 nm or more of the third means is less than 0.05 / μm ² by adopting the manufacturing method of the first means. And even if carburizing is performed, the grain size number is stable to 7 or more, and a steel material for mechanical structure with fine crystal grains can be obtained accurately. Further, by adopting the manufacturing method of the second means, the number of Nb-based carbides having a diameter of 100 nm or more of the fourth means is less than 0.05 / μm ² , and even if carburizing treatment is performed, the particle size number However, it is possible to accurately obtain a steel material for mechanical structure having fine crystal grains stably at 7 or more. Moreover, the mechanical structural steels of the third means and the fourth means obtained by the manufacturing method of the first means and the second means are both the case-hardening for mechanical structure having excellent crystal grain coarsening resistance. It is a steel material.

  First, the reasons for limiting the chemical components of the steel material in the present invention will be described below. In addition,% of each chemical component element is the mass%.

C: 0.12-0.27%
C is an element necessary for ensuring the strength after quenching and tempering as a steel material for machine structural parts, or the core strength after carburizing and quenching and tempering. If the range of C is less than 0.12%, the strength cannot be ensured. If it exceeds 0.27%, the hardness of the material increases and the workability decreases. Therefore, C is set to 0.12 to 0.27%.

Si: 0.30 to 0.70%
Si is an element necessary for deoxidation, imparts the necessary strength hardenability to the 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, Si needs to be added in an amount of 0.30% or more. 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 set to 0.30 to 0.70%.

Mn: 0.10 to 0.60%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.10%, the effect on hardenability cannot be obtained sufficiently, and if it exceeds 0.60%, the machinability deteriorates and at the same time coarsening of crystal grains occurs during carburizing. It becomes easy. Therefore, Mn is set to 0.10 to 0.60%.

P: 0.030% or less P is an inevitable element contained from scrap, but if P is more than 0.030%, it segregates at the grain boundary and deteriorates 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. However, if S exceeds 0.030%, MnS, which is a non-metallic inclusion, is generated to reduce the toughness and fatigue strength in the transverse direction. To do. Therefore, S is set to 0.030% or less.

Cr: 0.80 to 2.00%
Cr is an element necessary for ensuring hardenability and preventing coarsening of crystal grains during carburizing. However, if Cr is less than 0.80%, these effects cannot be obtained sufficiently, and if it exceeds 2.00%, coarse carbides are generated during carburizing, leading to deterioration of fatigue characteristics, and increasing the material hardness. Reduces workability. Therefore, Cr is made 0.80 to 2.00%.

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 excess of 0.050%, large alumina inclusions are formed, and fatigue characteristics and workability are deteriorated. Therefore, Al is made 0.010 to 0.050%.

Nb: 0.02 to 0.08%
Nb forms carbides and carbonitrides, and has the effect of preventing grain coarsening. In particular, nano-order sized Nb carbides or Nb carbonitrides finely dispersed in steel suppress the growth of crystal grains. If Nb is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.08%, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, Nb is made 0.02 to 0.08%.

In the first means, N: 0.010 to 0.020% or less, and in the second means, N: 0.0080% or less. N reacts with Al or Nb in the steel to react with Al nitride or Nb system It forms carbonitrides and has an effect of preventing coarsening of austenite crystal grains during carburization. However, if N is less than 0.010%, the effect of preventing crystal grain coarsening is small, and if it is more than 0.020%, nitrides increase and fatigue strength and workability deteriorate. Therefore, in the first means, N is set to 0.010 to 0.020%.
The second means is steel obtained by further adding B and Ti to the steel component of the first means. Thus, when B and Ti are added, it is necessary to define N as follows. That is, when N is more than 0.0080%, TiN is excessively generated and the fatigue strength is lowered, and the workability is further lowered. Therefore, when adding Ti by the second means, N is set to 0.080% or less.

Ti: 0.02 to 0.08%
Ti forms carbides and has the effect of preventing crystal grain coarsening. Furthermore, Ti combines with N to prevent B from becoming BN. In order to obtain the effect, 0.02% or more of Ti needs to be added. On the other hand, the addition of Ti exceeding 0.08% impairs machinability, so it is made 0.08% or less. Therefore, Ti is made 0.02 to 0.08%.

B: 0.0005 to 0.0035%
B is an element that remarkably improves the hardenability of the steel when contained in a very small amount. By adding B, the amount of other alloy elements added can be reduced, which is effective in reducing the steel material cost. If B is less than 0.0005%, the effect of improving hardenability is small, whereas if it exceeds 0.0035%, the strength is lowered. Therefore, B is set to 0.0005 to 0.0035%.

The reason why the steel by casting of the present invention is heated at 1250 ° C. or more for 2 hours or more, then rolled into a steel slab and cooled, and then the steel slab is heated to a temperature range of 800 ° C. to 1050 ° C. The steel by casting is 1250 ° C. By heating for 2 hours or longer as described above, Al nitride and Nb-based carbonitride are dissolved in the first means, or Nb-based carbide is dissolved in the second means, and then 800 ° C. to 1050 ° C. By heating to 950 ° C., a large number of nano-order to several tens of nano-order Al nitride, Nb-based carbonitride, or Nb-based carbide is dispersed finely at the time of carburizing at 950 ° C. If the time of heating below 1250 ° C. or holding at 1250 ° C. or higher for less than 2 hours is less than 2 hours, Al nitride, Nb-based carbonitride, or Nb-based carbide does not sufficiently dissolve and precipitates during casting Coarse Al nitride, Nb carbonitride, or Nb carbide of 100 nm or more remains. If coarse Al nitride, Nb carbonitride, or Nb carbide of 100 nm or more remains, the amount of Al nitride, Nb carbonitride, or Nb carbide effective for preventing grain coarsening decreases. Ostwald growth during heating at 800 ° C. to 1050 ° C. or carburizing to reduce fine Al nitride, Nb-based carbonitride, or Nb-based carbide in the periphery, resulting in non-uniform dispersion and crystal grain size The characteristics deteriorate. Therefore, in the present invention, the heating temperature is 1250 ° C. or more and the holding time is 2 hours or more. Furthermore, when rolling after heating to a temperature range exceeding 1050 ° C. after rolling the steel slab, Al nitride, Nb-based carbonitride, or Nb-based carbide grows, and the effect of preventing coarsening of the crystal grains is small. Become. When steel slab rolling and cooling and then rolling to a temperature range of less than 800 ° C., the rolled crystal grains become smaller and the grain size characteristics during carburization are deteriorated. Therefore, in the present invention, cast steel is heated at 1250 ° C. or more for 2 hours or more, then rolled into a steel slab and cooled, and then the steel slab is heated to a temperature range of 800 ° C. to 1050 ° C.

Reason why the number of Al nitrides and Nb-based carbonitrides or Nb-based carbides having a diameter of 100 nm or more is less than 0.05 pieces / μm ^{2 When the} number of precipitates of 100 nm or more is 0.05 pieces / μm ² or more In addition, Al nitride, Nb-based carbonitride, or Nb-based carbide effective for preventing grain coarsening are insufficient, and Ostwald growth occurs during heating at 800 ° C. to 1050 ° C. or carburizing, and the surrounding fine Al By reducing nitride, Nb-based carbonitride, or Nb-based carbide, the dispersion state becomes non-uniform, and the crystal grain size characteristics deteriorate. On the other hand, when the number of precipitates of 100 nm or more is less than 0.05 / μm ^2, Al nitride, Nb-based carbonitride, or Nb-based carbide effective for preventing grain coarsening can be secured, and at the same time, Since Al nitride, Nb carbonitride, or Nb carbide can be uniformly dispersed, excellent grain size characteristics can be obtained. Therefore, the number of Al nitrides and Nb carbonitrides or Nb carbides having a diameter of 100 nm or more is less than 0.05 / μm ² .

Reason why the carburizing and quenching conditions after cold forging are 950 ° C. to 1000 ° C. When the conditions of the carburizing and quenching treatment are less than 950 ° C., the diffusion rate of carbon becomes slow, and there is a possibility that the carburizing and quenching is not sufficiently performed. Further, when the condition is 1000 ° C. or higher, the Ostwald growth of austenite grains is promoted, and the Ostwald growth of Nb-based carbides is promoted to reduce the pinning force, which may result in a mixed grain structure. . Therefore, the carburizing and quenching conditions after cold forging are set to 950 ° C to 1000 ° C.

  Each steel slab rolling heating temperature shown in Table 2 is a steel ingot which is a steel material by casting consisting of chemical components of comparative steel and invention steel shown in Table 1 and the balance consisting of Fe and inevitable impurities. The steel strip was rolled by heating to room temperature and cooled to room temperature as it was, and further heated to the bar rolling temperature shown in Table 2 to perform bar rolling to produce a bar steel. Furthermore, after cutting, these steel bars are subjected to spheroidizing annealing and then 70% cold upsetting, and simulated carburizing and quenching is performed at a carburizing temperature of 950 ° C. or higher without normalizing or annealing. And crystal grains were observed. Furthermore, the number of particles of Al nitride and Nb carbonitride or Nb carbide of 100 nm or more was measured with an electron microscope.

  In the present invention, the steels of Table 1 are heated to 1250 ° C. or higher and rolled into steel slabs, cooled to room temperature, further heated to 800 ° C. to 1050 ° C. to perform bar steel rolling, and 950 ° C. to 1000 ° C. No. of Examples shown in Table 2 in which pseudo carburizing and quenching was performed at the temperature. 12, no. 13, no. 16, no. 17, no. 20, no. 23, no. 25 and No. Each steel of No. 26 has a maximum grain size of No. It was confirmed that it was excellent as 7 or more.

Claims (4)

In mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less, S: 0.030 %: Cr: 0.80-2.00%, Al: 0.010-0.050%, Nb: 0.02-0.08%, N: 0.010-0.020%, Cast the steel consisting of Fe and unavoidable impurities as the balance, heat the cast steel at 1250 ° C. or more for 2 hours or more, then roll and cool the steel slab, and then heat the steel slab in the temperature range of 800 ° C. to 1050 ° C. And after the cold forging, the number of Al nitrides and Nb carbonitrides having a diameter of 100 nm or more is less than 0.05 / μm ^2. A steel for machine structural use that can stably control the generation of coarse grains with a grain size number of 6 or less in the charcoal quenching process. Production method. In mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less, S: 0.030 % Or less, Cr: 0.80 to 2.00%, Al: 0.010 to 0.050%, Nb: 0.02 to 0.08%, Ti: 0.02 to 0.08%, B: 0 .0005 to 0.0035%, N: 0.008% or less, and the balance is made of Fe and inevitable impurities, and the cast steel is heated at 1250 ° C. or more for 2 hours or more. Rolled and cooled, and then rolled after heating the steel slab to a temperature range of 800 ° C. to 1050 ° C., so that the number of Nb-based carbides having a diameter of 100 nm or more was less than 0.05 / μm ^2. In the carburizing and quenching process at 950 ° C. to 1000 ° C. after the cold forging, A manufacturing method of steel for machine structure that can stably control generation. In mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less, S: 0.030 %: Cr: 0.80-2.00%, Al: 0.010-0.050%, Nb: 0.02-0.08%, N: 0.010-0.020%, A cold steel characterized in that the balance is made of a steel composed of Fe and inevitable impurities, and is a steel material containing Al nitride and Nb carbonitride having a diameter of less than 0.05 / μm ² and a diameter of 100 nm or more. A steel for machine structure capable of stably controlling the generation of coarse grains having a grain size number of 6 or less even in a carburizing and quenching process at 950 ° C. to 1000 ° C. after hot forging. In mass%, C: 0.12 to 0.27%, Si: 0.30 to 0.70%, Mn: 0.10 to 0.60%, P: 0.030% or less, S: 0.030 % Or less, Cr: 0.80 to 2.00%, Al: 0.010 to 0.050%, Nb: 0.02 to 0.08%, Ti: 0.02 to 0.08%, B: 0 Nb system having a diameter of 100 nm or more and comprising 0.05 to 0.0035%, N: 0.008% or less, the balance being made of steel consisting of Fe and inevitable impurities, the number of which is less than 0.05 pieces / μm ² A steel material for machine structure capable of stably controlling the generation of coarse grains having a particle size number of 6 or less even in a carburizing and quenching process at 950 ° C. to 1000 ° C. after cold forging, characterized by being a steel material containing nitride .