JP5868099B2 - Steel with excellent toughness and wear resistance - Google Patents

Description

  The present invention relates to 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. A steel material mainly composed of a martensite structure by quenching treatment 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, higher hardness, on the other hand, lowers toughness and causes cracking when an impact is applied, so steel materials are 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, in the tempering process after quenching, if alloy components such as Mn, Ni, and Cr are included, carbides such as Mn, Ni, and Cr are precipitated at the prior austenite grain boundaries, causing intergranular fracture. When Mo is added, Mo carbide precipitates with the dislocations 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. Steel with excellent wear resistance has 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 the amount of Mo and making finer by adding Nb, and further adding Nb, Cr, and Mo In order to remarkably increase the temper softening resistance of steel, a high strength, high toughness and wear resistant steel with improved toughness and good toughness and wear resistance by adopting a high tempering temperature has been proposed ( For example, see 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 the dephosphorization operation at the steel making stage to be performed more excessively than the normal operation. That is, high hardness and high toughness cannot be achieved by the prior art.

  Therefore, the problem to be solved by the present invention is to provide a steel material having both high hardness and high toughness and excellent wear resistance under the conditions of low temperature tempering after quenching in order to keep the hardness high. It is.

  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. This steel contains 0200% or less, O: 0.0030% or less, with the balance being Fe and inevitable impurities. Moreover, the steel has a value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn, and Cr of the above composition satisfying 6.00% or more, and the prior austenite grain size is 8.0 μm or less. Steel with high hardness, high toughness and excellent wear 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. It is a steel containing one or more of 0.10 to 0.100%, the balance being Fe and inevitable impurities. Moreover, the steel has a value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn, and Cr of the above composition satisfying 6.00% or more, and the prior austenite grain size is 8.0 μm or less. Steel with high hardness, high toughness and excellent wear resistance.

  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 generate TiN, N is fixed to ensure effective B that contributes to improving hardenability. Furthermore, since Ti combines with C to produce TiC, it is effective for 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%
Since Nb produces Nb carbonitride, it is effective for suppressing grain coarsening due to the pinning effect of Nb carbonitride. For this purpose, it is necessary to add 0.010% or more of Nb. On the other hand, if Nb exceeds 0.100%, coarse Nb precipitates are produced and the toughness is lowered. Therefore, Nb is made 0.010 to 0.100%.

Value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn, and Cr: ≧ 6.00%
The steel in 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 must be minimized. Therefore, the value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn, and Cr is set to 6.00% or more.

  By using the above-described means of the present invention, a steel material having both high hardness and high toughness can be obtained under the conditions of low temperature tempering after quenching 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. The relationship between hardness and impact value is shown. Shows the relationship between hardness and wear.

  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. It should be noted that the shaded portions in Table 1 are outside the claims of the present invention.

  After that, after the above 40 mm × 40 mm square steel is processed into a rough shape of a 2 mm-U notch Charpy impact test piece shown in FIG. 1, after quenching for 20 minutes at 850 ° C., oil quenching is performed. Tempering was carried out by holding at 180 ° C. for 90 minutes and air cooling, and in the case of twice quenching, holding at 850 ° C. for 20 minutes followed by oil quenching followed by holding at 550 ° C. for 90 minutes and water cooling. Subsequent to these quenching and quenching twice, oil quenching was further performed at 850 ° C. for 20 minutes, followed by tempering by holding at 180 ° C. for 90 minutes and air cooling, and repeated quenching and tempering treatment. Thereafter, the rough shape was further processed into a 2 mm-U notch Charpy impact test piece shown in FIG. A Charpy impact test was performed using the 2 mm-U notch Charpy impact test piece. Furthermore, the prior austenite particle size was calculated | required by performing hardness measurement and optical microscope observation using said test piece.

  About the above-mentioned 1 time quenching and 2 times quenching, in order to control the prior austenite particle size to 8.0 micrometers or less, it used properly.

In addition, when 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. A tempering process was performed for 90 minutes. Moreover, when performing quenching twice, after holding at 850 degreeC for 20 minutes and performing oil hardening, the tempering process which hold | maintains at 550 degreeC for 90 minutes and water-cools was performed. Furthermore, after performing oil quenching by holding at 850 ° C. for 20 minutes, tempering by holding at 180 ° C. for 90 minutes and air cooling was performed, and repeated quenching and tempering treatments were 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 roller pitching test was stopped, and the sliding contact portion of the test piece The amount of wear 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.

In Table 2, No. of the comparative steel. 16-No. The shaded portion 27 has a particle size larger than 8.0 μm, an impact value of less than 50 J / cm ² , and a wear amount larger than 15 μm. On the other hand, No. of invention steel. 1-No. Since No. 15 has an impact value of 50 J / cm ² or more and an abrasion amount of 15 μm or less, it has excellent toughness and wear resistance.

  Further, the relationship between hardness and impact value, and hardness and wear amount is shown in FIGS. No. of invention steel. 1-No. No. 15 is a comparative steel No. 15. 16-No. It can be seen that the impact value and the wear amount at the same hardness are superior to 27.

  In addition, No. of invention steel. No. 2 and No. of comparative steel. 25, No. of invention steel. 11 and No. of comparative steel. 26, No. 26 of invention steel. No. 14 and No. of comparative steel. 27 is the same component. No. of invention steel. 2, no. 11, no. No. 14 exhibited excellent toughness with a prior austenite grain size of 8.0 μm or less as a result of being quenched twice. 25, no. 26, no. No. 27 is a one-time quenching, so that the prior austenite grain size becomes larger than 8.0 μm, and No. 27 of the invention steel. 2, no. 11, no. Compared to 14, toughness decreased.

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 Furthermore, the steel satisfies the value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn, and Cr of the above composition satisfying 6.00% or more, and the prior austenite grain size is 8.0 μm or less. Steel with excellent toughness and wear resistance.   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% One or more of them are contained, the balance is made of Fe and inevitable impurities, and the steel has a value of 4Si + 3Cr-Mn calculated from the contents of Si, Mn and Cr having the above composition of 6.00%. Toughness characterized by satisfying the above and having a prior austenite grain size of 8.0 μm or less. Wear resistance excellent steel.

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