JP2013072105A - Method for manufacturing steel having excellent toughness and wear resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel product having high hardness and high toughness after quenching and low-temperature tempering.SOLUTION: Steel has a composition containing, by mass%, 0.30-0.65% C, 0.20-1.00% Si, 0.20-0.60% Mn, ≤0.030% P, ≤0.030% S, 1.00-3.00% Cr, 0.005-0.200% Al, ≤0.0200% N, ≤0.0030% O and further, one or more kinds of 0.50-2.00% Ni, 0.05-1.00% Mo, 0.0005-0.0050% B, 0.010-0.200% Ti and 0.010-0.100% Nb, and the balance Fe with inevitable impurities. In the method for manufacturing the high hardness and high toughness steel, the steel where 4Si+3Cr-Mn satisfies ≥6.00% is subjected to quenching at 800-900°C repeatedly two or more times and then is subjected to tempering at 400-650°C, and further, is subjected to quenching at 800-900°C and then is subjected to tempering at 100-250°C, and the high hardness and high toughness steel is produced.

Description

  The present invention relates to a method for producing steel having excellent toughness and wear resistance among mechanical structural steels used for parts such as automobiles and various industrial machines.

  In general, steel used for parts that require wear resistance, fatigue characteristics, etc. among parts of automobiles and various industrial machines is generally hardened by quenching treatment. The steel material mainly composed of a martensite structure by the quenching process has a hardness determined by the C content in the steel component, and the hardness of the steel material can be increased by increasing the C content. However, increasing the hardness of a steel material, on the other hand, lowers the toughness and causes cracking when an impact is applied, so the steel material is required to have a balance between hardness and toughness.

  As a conventional technique for dealing with these, the steel component is characterized by containing Si, Nb, Cr, Mo, V, and Cr, Mo, V having V as a core during use by specific rolling and heat treatment. Steels having excellent wear resistance and toughness in which composite precipitates are formed have been proposed (see, for example, Patent Document 1). Furthermore, when alloy components such as Mn, Ni, and Cr are included in the tempering process after quenching, carbides such as Mn, Ni, and Cr are precipitated at the prior austenite grain boundaries and cause grain boundary fracture. When Mo is added, the carbide of Mo precipitates with dislocations existing in the prior austenite grains as nuclei, so the precipitates are finely dispersed and precipitated in the prior austenite grains and do not cause grain boundary fracture. Steels excellent in impact resistance and wear resistance have been proposed (see, for example, Patent Document 2). Moreover, grain boundary segregation is reduced by lowering P and lowering S, grain boundary strengthening by lowering Mn, increasing toughness by increasing Mo and fine graining by adding Nb, and Nb, Cr, Mo combined addition is steel In order to remarkably increase the tempering softening resistance of steel, high strength, high toughness and wear-resistant steels with high toughness and good toughness and wear resistance have been proposed by adopting high tempering temperatures (for example, And Patent Document 3).

Japanese Patent Laid-Open No. 10-102185 Japanese Patent Publication No. 5-37202 Japanese Patent No. 3360687

  However, in order to form the composite precipitates of Cr, Mo, and V in the above-described prior art documents, it is necessary to perform the tempering temperature at 200 to 550 ° C., so that the predetermined hardness may not be obtained. Moreover, the toughness improvement by addition of Mo is under high-temperature tempering conditions, and the effect is not clear when performing low-temperature tempering to ensure hardness. Further, the reduction in P requires problems of productivity and cost increase because it is necessary to perform dephosphorization operation at the steelmaking stage in excess of normal operation. That is, high hardness and high toughness cannot be achieved by the prior art.

  Accordingly, the problem to be solved by the present invention is to provide a method for producing a steel material that achieves both high hardness and high toughness under the conditions of low temperature tempering after quenching in order to keep the hardness high.

  The means of the present invention for solving the above problem is that in the means of claim 1, C: 0.30 to 0.65%, Si: 0.20 to 1.00%, Mn: 0 in mass%. 20 to 0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 1.00 to 3.00%, Al: 0.005 to 0.200%, N: 0.00. The value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn and Cr of the above composition of steel containing 0200% or less, O: 0.0030% or less and the balance being Fe and inevitable impurities is 6.00. % Of steel satisfying at least 100% is heated in the range of 800 to 900 ° C and quenched, then tempered at 400 to 650 ° C once or twice, and further heated to the range of 800 to 900 ° C and quenched. And then toughness characterized by performing a tempering treatment at 100 to 250 ° C. A method for producing a steel excellent in abrasion resistance.

  In the means of claim 2, by mass%, C: 0.30 to 0.65%, Si: 0.20 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% Hereinafter, S: 0.030% or less, Cr: 1.00 to 3.00%, Al: 0.005 to 0.200%, N: 0.0200% or less, O: 0.0030% or less Further, Ni: 0.50 to 2.00%, Mo: 0.05 to 1.00%, B: 0.0005 to 0.0050%, Ti: 0.010 to 0.200%, Nb: 0.0. The value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn, and Cr of the above composition of steel containing at least one of 0.10 to 0.100% and the balance of Fe and unavoidable impurities is 6 After the steel satisfying 0.000% or more is heated and quenched in the range of 800 to 900 ° C, it is 400 to 650 ° C. The toughness and wear resistance are characterized by performing the tempering treatment once or twice or more, further heating and quenching in the range of 800 to 900 ° C., and then performing the tempering treatment at 100 to 250 ° C. It is a manufacturing method of steel.

  The reason for limiting the chemical composition of steel in the present invention will be described below. In the following, “%” represents mass%.

C: 0.30 to 0.65%
C is an element necessary for ensuring necessary strength and quenching hardness. Therefore, 0.30% or more is necessary in order to ensure the hardness which is the controlling factor of wear resistance. On the other hand, if it exceeds 0.65%, the toughness is lowered and the hardness of the material is increased, so that deterioration of workability and machinability is inevitable. Therefore, C is 0.30 to 0.65%, preferably 0.35 to 0.50%.

Si: 0.20 to 1.00%
Si is an element effective for deoxidation of steel, and is added to impart necessary hardenability to the steel and increase strength. Furthermore, Si improves the temper softening resistance. That is, it is an element that improves the softening resistance due to frictional heat during tempering and use. Therefore, 0.20% or more is necessary. On the other hand, if it exceeds 1.00%, the toughness is lowered and the hardness of the material is increased to deteriorate the workability. Therefore, Si is 0.20 to 1.00%, preferably 0.40 to 0.80%.

Mn: 0.20 to 0.60%
Mn is an element effective for deoxidation of steel. Furthermore, it is added to impart the necessary hardenability to the steel and increase the strength. However, if added in a large amount, the toughness is lowered, and further, it combines with S to form inclusions of MnS, which becomes the starting point of cracking. Therefore, Mn is 0.20 to 0.60%, preferably 0.30 to 0.50%.

P: 0.030% or less P segregates at grain boundaries as an inevitable impurity, and if it exceeds 0.030%, toughness is reduced. Therefore, P is set to 0.030% or less.

S: 0.030% or less S lowers toughness by forming inclusions of MnS as inevitable impurities. Therefore, S is set to 0.030% or less.

Cr: 1.00 to 3.00%
Cr is added to impart the necessary hardenability to the steel and increase the strength. Furthermore, Cr is an element that improves the softening resistance due to frictional heat during tempering and use in order to improve temper softening resistance. Therefore, 1.00% or more is necessary. On the other hand, if it exceeds 3.00%, the toughness is lowered and the hardness of the material is increased to deteriorate the workability. Therefore, Cr is 1.00 to 3.00%, preferably 1.20 to 2.20%.

Al: 0.005 to 0.200%
Al is an element effective for deoxidation of steel, and further binds to N to produce AlN, so that it is effective for suppressing grain coarsening. Therefore, 0.005% or more is necessary. However, if Al is added in a large amount, non-metallic inclusions are generated and become the starting point of cracking. Therefore, Al is made 0.005 to 0.200%, preferably 0.015% to 0.150%.

N: 0.0200% or less N is effective in suppressing grain coarsening because it combines with Al to produce AlN. However, even if there is too much N, the effect is saturated, so N is made 0.0200% or less.

O: 0.0030% or less If O is contained in excess of 0.0030%, an oxide-based inclusion serving as a starting point of cracking is generated. Therefore, in order to suppress the formation of oxide inclusions, O is made 0.0030% or less.

  The above are the basic components of steel in the present invention. In the present invention, one or more of Ni, Mo, B, Ti, and Nb can be added in addition to the above components.

Ni: 0.50 to 2.00%
Ni is an element effective for improving hardenability and toughness. In order to exhibit the effect, 0.50% or more is necessary. However, since Ni is an element that increases the cost, Ni is set to 0.50 to 2.00%.

Mo: 0.05-1.00%
Mo is an effective element for improving hardenability and toughness. In order to exhibit the effect, 0.05% or more is necessary. However, since Mo is an element that increases the cost, Mo is set to 0.05 to 1.00%.

B: 0.0005 to 0.0050%
B is an element effective for improving hardenability by addition of a small amount, further strengthening grain boundaries and improving toughness. Therefore, 0.0005% or more is necessary, but if it exceeds 0.0050%, the effect is saturated, so B is made 0.0005 to 0.0050%.

Ti: 0.010 to 0.200%
Since Ti combines with N to produce TiN, N is fixed to ensure effective B that contributes to improving hardenability. Furthermore, since it combines with C to produce TiC, it is effective in suppressing grain coarsening due to the pinning effect and improving wear resistance. Therefore, 0.010% or more is added. On the other hand, if it exceeds 0.200%, the toughness and workability deteriorate. Therefore, Ti is set to 0.010 to 0.200%.

Nb: 0.010 to 0.100%
Nb produces Nb carbonitride and is therefore effective in suppressing crystal grain coarsening due to the pinning effect. Therefore, 0.010% or more is added. On the other hand, if it exceeds 0.100%, coarse Nb precipitates are produced and the toughness is lowered. Therefore, Nb is made 0.010 to 0.100%.

  The steel in the method of the present invention is a steel in which the value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn and Cr having the above composition satisfies 6.00% or more. As described above, since Si and Cr are elements that improve temper softening resistance, softening due to frictional heat during use is suppressed and effective in reducing the amount of wear. Furthermore, the design of high Si and high Cr components has the effect of strengthening grain boundaries. On the other hand, since Mn is an element that embrittles steel, the addition amount is made the minimum necessary. Therefore, the numerical value that can be derived from the above formula is 6.00% or more.

  Next, the manufacturing method of the steel in this invention is demonstrated.

  First, the steel having the above component composition is heated and held in a temperature range of 800 to 900 ° C. This is to make the steel austenitic. When heated to a temperature lower than 800 ° C., the austenitization of the steel is insufficient, so that a complete martensite structure cannot be obtained after quenching. In addition, when heated to a temperature higher than 900 ° C., austenite coarsens, leading to a decrease in toughness. Therefore, the quenching temperature is set to 800 to 900 ° C.

  The quenched steel is then tempered at 400-650 ° C. This is for the purpose of precipitating carbides that become pinning particles necessary for the fine graining in the subsequent heat treatment. If the temperature is lower than 400 ° C., the precipitation of carbides is insufficient, and the pinning effect cannot be exhibited and the particles are not refined. Moreover, when the temperature is higher than 650 ° C., the solid solution of the carbide in the matrix or the coarsening of the carbide is caused, and the number of pinning particles necessary for the fine granulation is reduced. Therefore, the tempering temperature is 400 to 650 ° C.

  The above quenching and tempering treatment is performed once or twice or more.

  Then, it heats to the temperature range of 800-900 degreeC further. When heated to a temperature lower than 800 ° C., the austenitization of the steel is insufficient, so that a complete martensite structure cannot be obtained after quenching. In addition, when heated to a temperature higher than 900 ° C., austenite coarsens, leading to a decrease in toughness. Therefore, the quenching temperature is set to 800 to 900 ° C.

  The quenched steel is then tempered at 100-250 ° C. This is for improving toughness. When the temperature is lower than 100 ° C., the toughness of the steel is insufficient, and when the temperature is higher than 250 ° C., the hardness is lowered and low temperature temper embrittlement is caused. Therefore, the tempering temperature is 100 to 250 ° C.

  By adopting the method described above, it is possible to obtain a steel material having both high hardness and high toughness under the conditions of low-temperature tempering after the final quenching after the repeated quenching and tempering in order to keep the hardness high.

A 2 mm-U notch Charpy test piece is shown, (a) is a side view, (b) is an end view. A roller pitching test piece is shown.

  Steel having the chemical composition shown in Table 1 was melted in a 100 kg vacuum melting furnace, and the obtained steel was hot forged at 1200 ° C. to produce 40 mm × 40 mm square bar and 32 mm diameter round bar steel. The round steel bar was kept at 870 ° C. for 60 minutes, air cooled, and subjected to a normalizing treatment.

  After that, the above 40 mm × 40 mm square steel was processed into a rough shape of a 2 mm-U notch Charpy impact test piece shown in FIG. 1, held in a temperature range of 800 to 900 ° C. for 20 minutes and subjected to oil quenching, and then 400 A tempering treatment that is held for 90 minutes in the temperature range of ˜650 ° C. and cooled with water is performed once or twice, and further held for 20 minutes in the temperature range of 800 ° C. to 900 ° C., followed by oil quenching, and a temperature of 100 ° C. to 250 ° C. Quenching and tempering treatment was performed in which the temperature was maintained for 90 minutes and air-cooled. Thereafter, this rough shape was further processed into a 2 mm-U notch Charpy impact test piece shown in FIG. Using this 2 mm-U notch Charpy impact test piece, a Charpy impact test was conducted. Furthermore, the prior austenite particle size was determined by performing hardness measurement and optical microscope observation using the above test piece.

Further, the above round bar steel having a diameter of 32 mm is processed into a rough shape of the roller pitching test shown in FIG. Tempering treatment that is held for 90 minutes in the range and water-cooled is performed once or twice, and further, held for 20 minutes in the temperature range of 800 to 900 ° C and then oil-quenched, and then held for 90 minutes in the temperature range of 100 to 250 ° C A quenching and tempering treatment was then performed. Thereafter, this rough shape was further processed into a roller pitching test piece shown in FIG. Using this roller pitching test piece, a roller pitching test was conducted under conditions of a surface pressure of 3.3 GPa and a slip rate of −40%. After reaching 5 × 10 ⁶ cycles, the test was stopped, and the amount of wear at the sliding contact portion of the test piece Was measured.

  Table 2 shows the particle size, hardness, impact value, and wear amount as a result of the Charpy impact test, hardness measurement, optical microscope observation, and roller pitching test. The quenching and tempering conditions are also shown in Table 2.

In Table 2, no. 1-No. 24 shaded parts are those whose heat treatment conditions are outside the scope of claims, those whose particle size is larger than 8.0 μm, those whose impact value is less than 50 J / cm ² , and those whose wear amount is larger than 15 μm It is. In addition, No. 1-No. 8, no. 9-No. 16, no. 17-No. No. 24 has the same steel composition, although the heat treatment conditions are different.

No. 1-No. In one quenching and tempering treatment as in No. 8, the particle size is larger than 8.0 μm, the impact value is less than 50 J / cm ² , and the wear amount is larger than 15 μm. However, no. 9-No. 24, the first or second quenching is performed in a temperature range of 800 to 900 ° C., tempering is performed in a temperature range of 400 to 650 ° C., and further quenching is performed in a temperature range of 800 to 900 ° C. Is carried out in the temperature range of 100 to 250 ° C., it can be seen that the fine particles are reduced to 8.0 μm and the toughness and wear resistance are excellent.

1 2 mm-U notch Charpy test piece 1a Notch part 2 Roller pitching test piece

Claims (2)

  In mass%, C: 0.30 to 0.65%, Si: 0.20 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% or less, S: 0.030 %: Cr: 1.00 to 3.00%, Al: 0.005 to 0.200%, N: 0.0200% or less, O: 0.0030% or less, the balance being Fe and inevitable impurities After heating and quenching steel in which the value of 4Si + 3Cr-Mn calculated from the content of Si, Mn, and Cr of the above-described composition satisfying 6.00% or more is quenched in the range of 800 to 900 ° C., 400 to Toughness and wear resistance characterized by performing tempering treatment at 650 ° C once or twice or more, heating to 800 to 900 ° C and quenching, and then tempering at 100 to 250 ° C. Excellent steel manufacturing method.   In mass%, C: 0.30 to 0.65%, Si: 0.20 to 1.00%, Mn: 0.20 to 0.60%, P: 0.030% or less, S: 0.030 %: Cr: 1.00 to 3.00%, Al: 0.005 to 0.200%, N: 0.0200% or less, O: 0.0030% or less, and Ni: 0.50 -2.00%, Mo: 0.05-1.00%, B: 0.0005-0.0050%, Ti: 0.010-0.200%, Nb: 0.010-0.100% The value of 4Si + 3Cr-Mn calculated from the content of Si, Mn, and Cr of the above composition of the steel containing one or more of them and the balance consisting of Fe and inevitable impurities satisfies 6.00% or more. After heating and quenching the steel in the range of 800-900 ° C, tempering at 400-650 ° C is performed once. Is performed more than once, further 800 to 900 manufacturing method of a steel having excellent toughness and abrasion resistance which is heated to a range of ° C. and performing the tempering treatment of 100 to 250 ° C. After quenching.

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