JP2012132077A - Steel excellent in pitting-resistant strength, bending fatigue-resistant strength and torsional fatigue-resistant strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel excellent in pitting-resistant strength, bending fatigue-resistant strength and torsional fatigue-resistant strength, by improving core strength at low cost.SOLUTION: The steel includes, by mass%, 0.10-0.35% of C, 0.40-1.50% of Si, 0.10-1.50% of Mn, 0.030% or less of P, 0.030% or less of S, 0.50-3.0% of Cr, 0.020-0.200% of Al and 0.01-0.03% of N, with the remainder being Fe and unavoidable impurities. Furthermore, in the steel, the content of solid-solution Al obtained from the content of Al and the content of N satisfies 0.020% or more in a value obtained by subtracting 27/14 of the mass% of N from the mass% of Al, and the formation of a slack quenching structure in a core of a product obtained after quenching/tempering performed in a heat pattern shown in the figure when the steel is formed into the product is suppressed. The steel is excellent in pitting-resistant strength, bending fatigue-resistant strength and torsional fatigue-resistant strength.

Description

  The present invention can improve the pitching strength, bending fatigue strength, and torsional fatigue strength, which are the main characteristics of gears and shafts that transmit power, by performing low-cost gas carburizing and tempering. Concerning Hadake steel.

  A power transmission gear for automobiles is mainly required to have excellent pitting resistance of the tooth surface and impact strength of the tooth root, and the shaft portion is required to have torsional fatigue resistance. For this reason, car power transmission gears and shafts are often used by carburizing and tempering hardened steel such as SCr420 and SCM420, which are JIS standard steels. However, in recent years, there has been an increase in the prevention of global warming, and there is a growing need for improving fuel efficiency for reducing the carbon dioxide emissions of automobiles. For this reason, there is an increasing need for miniaturization and weight reduction of gears and shafts, and sufficient strength cannot be obtained with the steel of the JIS standard.

  Conventionally, in case-hardened steel for gears, when trying to improve the pitting resistance and bending fatigue strength at the same time, the Si amount is reduced compared to the general JIS SCr420 and SCM420, and the carburizing abnormal layer depth is reduced. Has been proposed, and an alloy element such as Cr and Mo is added in an increased amount to improve the temper softening resistance characteristics (see, for example, Patent Document 1). However, since this technique reduces Si, the temper softening resistance characteristic is not so high, and it cannot be said that the pitting resistance is sufficient.

  On the other hand, compared to the conventional JIS stipulated hardened steel, the amount of Si is increased, the temper softening resistance characteristics are improved, the carburizing abnormal layer depth is reduced, and the impact strength and bending fatigue strength are not reduced. Hardened steels for gears with improved pitching properties have been proposed. (Patent Document 2).

  In addition, elements that improve the temper softening resistance characteristics and reduce the carburizing abnormal layer depth have the same effect on Cr and Mn as well as Si, and the components Si and Cr in steels containing these components And, by making the steel satisfying the relational expression of 7Si + 3Cr + Mn ≧ 7.0 with Mn% by mass, there has been proposed a case-hardened steel with improved pitting resistance without reducing impact strength and bending fatigue strength. (Patent Document 3). However, although it is stated that this technology affects the carburizing abnormal layer depth, and Cr and Mn improve temper softening resistance characteristics as well as Si, core hardness is important for torsional fatigue strength. There is no mention of the effects of

JP 2000-297347 A JP 7-258793 A JP 2009-68065 A

  The problem to be solved by the present invention is to provide steel having improved core strength at low cost in addition to the above-described technology, and excellent in pitting strength, bending fatigue strength, and torsional fatigue strength.

  The means of the present invention for solving the above-mentioned problems is, in the first means, in mass%, C: 0.10 to 0.35%, Si: 0.40 to 1.50%, Mn: 0.10 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 3.0%, Al: 0.020 to 0.200%, N: 0.01 to 0 0.03% steel, the balance being Fe and inevitable impurities. Moreover, in this steel, the content of the solid solution Al obtained from the Al content and the N content of the above composition is 0.00 by the value obtained by subtracting 27/14 of the mass% of the N from the mass% of the Al. It is steel excellent in pitching strength, bending fatigue strength, and torsional fatigue strength that satisfies 020% or more and suppresses the incompletely quenched structure of the core.

  In the second means, in addition to the composition of the first means, the composition further contains one or two kinds of Ni: 3.0% or less and Mo: 1.0% or less, and the balance Fe and inevitable impurities. It is made of steel. Moreover, in this steel, the content of the solute Al obtained from the Al content and the N content of the composition of the first means is 27/14 from the mass% of Al to the mass% of N of the first means. Is a steel excellent in pitching strength, bending fatigue strength, and torsional fatigue strength that satisfies 0.020% or more in terms of the value obtained by subtracting the above and suppresses the incompletely quenched structure of the core.

  In the third means, in addition to the composition of the first means, one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% are further contained by mass%. Further, the steel is composed of the remaining Fe and inevitable impurities. Moreover, in this steel, the content of the solute Al obtained from the Al content and the N content of the composition of the first means is 27/14 from the mass% of Al to the mass% of N of the first means. Is a steel excellent in pitching strength, bending fatigue strength, and torsional fatigue strength that satisfies 0.020% or more in terms of the value obtained by subtracting the above and suppresses the incompletely quenched structure of the core.

  In the fourth means, in addition to the composition of the second means, one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% are further contained by mass%. Further, the steel is composed of the remaining Fe and inevitable impurities. Moreover, in this steel, the content of the solute Al obtained from the Al content and the N content of the composition of the first means is 27/14 from the mass% of Al to the mass% of N of the first means. Is a steel excellent in pitching strength, bending fatigue strength, and torsional fatigue strength that satisfies 0.020% or more in terms of the value obtained by subtracting the above and suppresses the incompletely quenched structure of the core.

  In the fifth means, C: 0.10 to 0.35% by mass, Si: 0.40 to 1.50%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.50 to 3.0%, and in place of Al and N in the first means, Al: 0.020 to 0.100%, N: 0 .01% or less, Ti: 0.10 to 0.20%, the balance being Fe and inevitable impurities. Moreover, in this steel, a value obtained by subtracting 27/14 of the mass% of N from the mass% of Al in the fifth means by the content of solid solution Al obtained from the Al content and the N content of the above composition. The steel is excellent in pitching strength, bending fatigue strength, and torsional fatigue strength, satisfying 0.020% or more and suppressing the incompletely quenched structure of the core.

  In the sixth means, in addition to the composition of the fifth means, in addition to mass, one or two of Ni: 3.0% or less and Mo: 1.0% or less are contained, the remaining Fe and inevitable impurities It is made of steel. Moreover, in this steel, the content of solute Al obtained from the Al content and the N content of the composition of the fifth means is 27/14 from the mass% of Al to the mass% of N of the fifth means. Is a steel excellent in pitching strength, bending fatigue strength, and torsional fatigue strength that satisfies 0.020% or more in terms of the value obtained by subtracting the above and suppresses the incompletely quenched structure of the core.

  In the seventh means, in addition to the composition of the fifth means, Nb: 0.02 to 0.20%, V: 0.02 to 0.20%, or one or two kinds in mass% is further contained. Further, the steel is composed of the remaining Fe and inevitable impurities. Moreover, in this steel, the content of solute Al obtained from the Al content and the N content of the composition of the fifth means is 27/14 from the mass% of Al to the mass% of N of the fifth means. Is a steel excellent in pitching strength, bending fatigue strength, and torsional fatigue strength that satisfies 0.020% or more in terms of the value obtained by subtracting the above and suppresses the incompletely quenched structure of the core.

  In the eighth means, in addition to the composition of the sixth means, one or two kinds of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% are further contained by mass%. Further, the steel is composed of the remaining Fe and inevitable impurities. Moreover, in this steel, the value obtained by subtracting 27/14 of N% from the Al% of the fifth means, the content of the solid solution Al obtained from the Al content and the N content of the composition of the fifth means The steel is excellent in pitching strength, bending fatigue strength, and torsional fatigue strength, satisfying 0.020% or more and suppressing the incompletely quenched structure of the core.

  The reasons for limiting the steel components in the above means will be described below. In addition,% shows the mass%.

  C: C is an element necessary for imparting strength, but if it is less than 0.10%, the core strength after carburizing and quenching cannot be secured, and if it exceeds 0.35%, the toughness decreases. At the same time, the hardness of the material increases and the workability deteriorates. Therefore, C is set to 0.10 to 0.35%, preferably 0.10 to 0.25%.

  Si: Si is an element effective for deoxidation of steel, imparts necessary strength and hardenability to steel, improves temper softening resistance characteristics, and reduces carburizing abnormal layer depth by adding more than a certain amount. It is an effective element to do. However, if Si is less than 0.40%, the temper softening resistance characteristic is low, and the carburizing abnormal layer depth during gas carburizing becomes large. On the other hand, if Si exceeds 1.50%, the material hardness increases and the workability deteriorates. Therefore, Si is 0.40 to 1.50%, preferably 0.50 to 1.0%.

  Mn: Mn is an element that improves the hardenability of the steel, but if it is less than 0.10%, deoxidation is insufficient, and if it exceeds 1.50%, workability deteriorates. Therefore, Mn is 0.10 to 1.50%, preferably 0.10 to 1.00%.

  P: P is an element that segregates at the grain boundary to lower toughness and fatigue strength, and as a result, lowers bending fatigue strength. Therefore, P is set to 0.030% or less.

  S: S is an element that improves the machinability of the material by being present as MnS in the steel. However, if it exceeds 0.030%, grain boundary embrittlement is caused by grain boundary segregation, and cold workability and Degradation of toughness. Therefore, S is set to 0.030% or less.

  Cr: Cr is an element necessary for improving the hardenability, toughness and temper softening resistance characteristics of steel. If the amount of Cr is too small, the temper softening resistance characteristics will be low, and if it is too large, the workability will be reduced and the carburizing property will be reduced. Therefore, Cr is 0.50 to 3.0%, preferably 1.3% to 3.0%.

  Ni: Ni is an element effective for improving the hardenability and toughness of steel. If Ni exceeds 3.0%, the hardness of the material will increase too much and the workability will decrease, and the steel material cost will increase. Therefore, Ni is set to 3.0% or less.

  Mo: Mo is an element necessary for improving the hardenability, toughness and temper softening resistance of steel. However, if there is too much Mo, workability will be reduced and the steel material cost will rise. Therefore, Mo is set to 1.0% or less.

  Al: Al is an important element in the present invention, and is an element effective for deoxidation of steel, and reacts with N in the steel to form AlN, thereby suppressing the coarsening of crystal grains. Further, when the mass% of the solid solution Al is {mass% of Al− (27/14) × N mass%} ≧ 0.020%, the hardenability is improved and the incompletely quenched structure of the core is suppressed. Therefore, Al is 0.020 to 0.200%, preferably 0.040 to 0.200%.

  N: N reacts with Al in the steel to form AlN and has the effect of preventing coarsening of the austenite crystal grains during carburizing. However, if N is less than 100 ppm, the effect of preventing coarsening of the crystal grains is obtained. If it is too small and too much, nitrides increase and fatigue strength and workability decrease. Therefore, N is set to 0.010 to 0.030%.

  The fifth to eighth means are steels obtained by further adding Ti to the steel components of the first to fourth means. When Ti is added in this way, N is defined as follows. Further, the upper limit of Al is also changed in accordance with the change in the definition of N, and Al is set to 0.020 to 0.100%, preferably 0.040 to 0.100%.

  N: When N is too much, TiN is excessively generated to reduce the fatigue strength, and the workability is further reduced. Therefore, when adding Ti, N is specified to be 0.010% or less.

  Ti: Ti has an effect of preventing coarsening of crystal grains, but the effect is small when the content is less than 0.10%, and the workability is degraded when the content exceeds 0.20%. Therefore, Ti is set to 0.10 to 0.20%, preferably 0.12 to 0.17%.

  Nb: Nb has an effect of preventing the coarsening of crystal grains, but if less than 0.02%, the effect is small, and if it exceeds 0.20%, the effect tends to saturate and inhibits carburization. . Therefore, Nb is 0.02 to 0.20%, preferably 0.03 to 0.10%.

  V: V has an effect of preventing coarsening of crystal grains, but if less than 0.02%, the effect is small, and if it exceeds 0.20%, the effect tends to saturate and the workability decreases. . Therefore, V is 0.02 to 0.20%, preferably 0.05 to 0.10%.

  In the first means, the first means improves the tempering softening resistance characteristics of the carburized parts and improves the hardenability and suppresses the incompletely quenched structure of the core part, thereby preventing the pitting resistance, the bending fatigue resistance, A case-hardened steel with improved torsional fatigue strength characteristics was achieved.

  In the second means, in addition to the first means, by containing one or two kinds of Ni: 3.0% or less and Mo: 1.0% or less in mass%, pitting resistance, bending resistance A case-hardened steel with improved fatigue strength and torsional fatigue strength characteristics was obtained.

  In 3rd means, in addition to 1st means, by mass%, it contains 1 type or 2 types of Nb: 0.02-0.20%, V: 0.02-0.20%, A case-hardened steel with improved pitting resistance, bending fatigue strength and torsional fatigue strength characteristics has been achieved.

  In the fourth means, in addition to the composition of the second means, one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% are contained by mass%. Thus, a case-hardened steel with improved pitting resistance, bending fatigue resistance, and torsional fatigue resistance characteristics was obtained.

  In the fifth means, in addition to the composition of the first means, by mass, N: 0.01% or less, Ti: 0.10 to 0.20% are contained, and further Al is 0.100% or less. As a result, a case-hardened steel with improved characteristics of pitting strength, bending fatigue strength, and torsional fatigue strength was obtained.

  In the sixth means, in addition to the composition of the fifth means, by mass containing 1 type or 2 types of Ni: 3.0% or less and Mo: 1.0% or less, the anti-pitting strength, A case-hardened steel with improved bending fatigue resistance and torsional fatigue strength characteristics was achieved.

  In the seventh means, in addition to the composition of the fifth means, one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% are contained by mass%. Thus, a case-hardened steel with improved pitting resistance, bending fatigue resistance, and torsional fatigue resistance characteristics was obtained.

  In the eighth means, in addition to the composition of the sixth means, one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% are contained by mass%. Thus, a case-hardened steel with improved pitting resistance, bending fatigue resistance, and torsional fatigue resistance characteristics was obtained.

  Embodiments of the present invention will be described below. The steel of the embodiment of the present invention having the chemical composition shown in Table 1 except for Fe and the steel of the comparative example having the chemical composition shown in Table 2 are melted in a 100 kg vacuum melting furnace, respectively, and cast into an ingot. Steel slabs. The steel piece was heated to 1250 ° C. and held for 5 hours to form a solution, and then forged into a steel bar. On the other hand, the rotating bending fatigue test piece was forged to φ20 mm, and the roller pitching test piece and the torsional fatigue test piece were forged to φ32 mm.

  As seen in Table 1, in the examples of the present invention, solid solution Al is present at 0.020% or more. On the other hand, as can be seen in Table 2, in the comparative example, no solid solution Al is present so that the column of solid solution Al is indicated by a hyphen. In Tables 1 and 2, the columns of Ti, Nb, and V indicated by hyphens indicate that these elements are not added.

  Next, these steel bars were heated to 900 ° C., held for 1 hour, air cooled and normalized, and then a rotating bending fatigue test piece having a 2 mmV notch shown in FIG. 1, a roller pitching test shown in FIG. A torsional fatigue test piece shown in FIG. 3 was prepared, and carburizing quenching and tempering by gas carburizing were performed under the carburizing and quenching conditions shown in FIG. In FIG. 1 to FIG. 3, the numerical values are shown with the unit mm omitted.

In this case, the carburizing and tempering treatment by gas carburizing is performed by heating a steel member manufactured by machining using the above steel bar as a raw material to 800 to 1000 ° C., and then carburizing gas containing CO or CH ₄ in a heating furnace. The steel was kept in the atmosphere for 1 to 5 hours, and carbon was diffused and infiltrated from the surface of the steel member to a depth of about 1 mm and carburized. The steel member that had been carburized was quenched in water or oil, further tempered to 150 to 200 ° C. and air-cooled. The heat pattern of the heat treatment for quenching in oil is shown in FIG. 4 as described above.

  Next, the rotating bending fatigue test, the roller pitching test, and the torsional fatigue test of FIGS. 1 to 3 described above were carried out, respectively, and the resulting solid solution Al amount, rotating bending fatigue strength, roller of the examples and comparative examples of the present invention Table 3 shows the pitching life and torsional fatigue strength.

  When the above-mentioned rotational bending fatigue strength, roller pitching life and torsional fatigue strength are shown as a ratio when the value of No. 16 of the comparative example is 1.0, the bending fatigue strength is 1.4 or more and the pitching life is 2 0.0 or more and torsional fatigue strength satisfying any one of 1.4 or more satisfy the present invention, and all of No. 1 to No. 15 in the examples of Table 3 satisfy the above conditions. In addition, the torsional fatigue strength that has the most influence on the hardness of the core is satisfied by all the examples of No1 to No15, and as seen in Table 1 above, In all of No. 15, the solid solution Al was 0.020% or more, and thus the incompletely quenched structure of the core part was suppressed. On the other hand, no. 16 to 26 all had a bending fatigue strength of less than 1.4, a pitching life of less than 2.0, and a torsional fatigue strength of less than 1.4.

The shape of a rotating bending fatigue test piece is shown, (a) is a front view, (b) is an enlarged view of a portion A which is a notch portion surrounded by a circle in (a), and the unit of each numerical value is mm. . The shape of a roller pitching test piece is shown, and the unit of each numerical value is mm. The shape of the torsional fatigue test piece is shown, and the unit of each numerical value is mm. It is an example of carburizing quenching and tempering by gas carburizing, (a) shows the heat pattern of carburizing and quenching, (b) shows the heat pattern of tempering.

Claims (8)

  In mass%, C: 0.10 to 0.35%, Si: 0.40 to 1.50%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030 % Or less, Cr: 0.50 to 3.0%, Al: 0.02 to 0.20%, N: 0.01 to 0.03%, with the balance being Fe and inevitable impurities, and the Al Pitting resistance, characterized in that the solid solution Al shown by the value obtained by subtracting 27/14 of the mass% of N from the mass% of N satisfies 0.020% or more and suppresses the incompletely quenched structure of the core. Steel with excellent strength, bending fatigue strength, and torsional fatigue strength.   In addition to the composition of claim 1, further containing, by mass%, one or two of Ni: 3.0% or less and Mo: 1.0% or less, the balance comprising Fe and inevitable impurities, and claim 2 The solid solution Al shown by the value obtained by subtracting 27/14 of the mass% of N from the mass% of 1 Al satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core part. Steel with excellent resistance to pitting, bending fatigue, and torsional fatigue.   In addition to the composition of claim 1, further contains 1% or 2 kinds of Nb: 0.02 to 0.20%, V: 0.02 to 0.20% in mass%, with the balance being Fe and inevitable impurities The solid solution Al expressed by the value obtained by subtracting 27/14 of the mass% of N from the mass% of Al in claim 1 satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core. A steel with excellent pitting resistance, bending fatigue resistance, and torsional fatigue strength.   In addition to the composition of claim 2, the composition further contains one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% in mass%, with the balance being Fe and inevitable impurities. The solid solution Al expressed by the value obtained by subtracting 27/14 of the mass% of N from the mass% of Al in claim 1 satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core. A steel with excellent pitting resistance, bending fatigue resistance, and torsional fatigue strength.   C: 0.10 to 0.35% by mass, Si: 0.40 to 1.50%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% Hereinafter, Cr: 0.50 to 3.0%, Al: 0.02 to 0.10%, N: 0.01% or less, Ti: 0.10 to 0.20%, with the balance being Fe and It is composed of inevitable impurities, and the solid solution Al shown by the value obtained by subtracting 27/14 of the N% from the Al% satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core. Steel with excellent pitting resistance, bending fatigue strength, and torsional fatigue strength.   6. In addition to the composition of claim 5, further containing, by mass%, one or two of Ni: 3.0% or less and Mo: 1.0% or less, the balance comprising Fe and inevitable impurities, and claim 6 The solid solution Al shown by the value obtained by subtracting 27/14 of 5% by mass of Al from 5% by mass of Al satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core part. Steel with excellent resistance to pitting, bending fatigue, and torsional fatigue.   In addition to the composition of claim 5, the composition further contains one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% by mass, with the balance being Fe and inevitable impurities. The solid solution Al expressed by the value obtained by subtracting 27/14 of the mass% of N from the mass% of Al in claim 5 satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core. A steel with excellent pitting resistance, bending fatigue resistance, and torsional fatigue strength.   In addition to the composition of claim 6, the composition further contains one or two of Nb: 0.02 to 0.20% and V: 0.02 to 0.20% by mass, with the balance being Fe and inevitable impurities. The solid solution Al expressed by the value obtained by subtracting 27/14 of the mass% of N from the mass% of Al in claim 5 satisfies 0.020% or more, and suppresses the incompletely quenched structure of the core. A steel with excellent pitting resistance, bending fatigue resistance, and torsional fatigue strength.

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