JP2548067C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has high toughness, high strength, hardenability and machinability used for parts for driving and driving industrial vehicles. It relates to a low-alloy steel having excellent properties. [0002] Track links, crankshafts, and other parts for driving and driving industrial vehicles.
There are steering nuts and the like. For example, the large truck link 1 shown in FIGS. 2 and 3 has a large cross section with a minimum cross section of 25 mm or more. After being formed by hot forging, a pair of bush holes 2 are finished by cutting. A bush 4 is press-fitted into a pair of bush holes 2 via a washer 3 and a pin 5 is inserted and fixed in the bush 4 to form a chain, and then attached to a shoe 8 with bolts 6 and nuts 7. The shoe rotates integrally to run the industrial vehicle. These parts for vehicles are subjected to loads such as high bending stress, torsional, tensile and compressive stresses during running, and have high toughness and strength to steel used for such running and driving parts. In addition, in order to obtain a hardened structure up to the core of the component having a large cross section, it is required to have excellent hardenability. Further, since the industrial vehicle works even in a cold region of −50 ° C. or lower, it is required that the above-mentioned properties do not decrease so much even in an extremely low temperature region, and that the material be particularly excellent in low-temperature toughness. . [0004] Conventionally, nickel chromium-molybdenum steel such as SNCM431 has been used as a steel for running and driving parts of industrial vehicles requiring the above characteristics. Although SNCM431 contains 1.8% Ni, it is excellent in low-temperature toughness. On the other hand, since it contains 1.8% Ni, it takes a long time for annealing after hot forging, and In addition, the machinability is inferior and the steel contains a large amount of Ni, resulting in expensive steel. Further, conventional steels such as SNCM431 have a structure mainly composed of martensite during quenching, and quenching cracks are easily generated by applying a large transformation stress and thermal stress at the time of quenching. Had been oil quenched. However, oil quenching has the disadvantage that quenching hardness varies if quenching oil is not adequately managed.For example, when quenching is performed using deteriorated quenching oil, the cooling rate in each part of the material to be quenched is reduced. Is changed, the quenched structure becomes a structure in which martensite and bainite are unevenly distributed, and the quenching hardness varies, and there is a problem that a desired hardness cannot be obtained. Since quenching hardness cannot be obtained, conventional quenching requires quenching at high temperature and tempering at low temperature.The oil quenched material has low toughness. However, there has been a problem that cracks occur near the bush hole during use. [0005] The present invention provides a quenching hardness and a tempering hardness equal to or higher than those of a nickel-chromium-molybdenum steel such as SNCM431, which is a conventional steel. An object of the present invention is to provide a high-toughness low-alloy steel which has superior tensile strength and toughness as compared with conventional steel and enables water quenching. That is, according to the study of the present inventors, in conventional Ni-containing steel, quenching cracking occurs when water quenching is performed, because the Ni-containing steel has a wide austenite region, and therefore, substantially hardened martensite during quenching. It has been found that a combination of a single-phase structure and a high C content causes a large transformation stress and thermal stress to be applied to the material to be quenched during water quenching. Therefore, in the present invention, in order to reduce the transformation stress, the precipitation of martensite is suppressed, and the structure after quenching is a mixed structure of martensite and bainite. It is a steel which can be water-quenched by being contained in an amount. In addition, in the present invention, sufficient quenching hardness is obtained by enabling water quenching, and quenching can be performed at a lower temperature than conventional steel, and can be tempered at a high temperature. A steel having excellent toughness compared to conventional steel can be provided. Further, according to the present invention, the strength and toughness are further improved by including an appropriate amount of one or both of V and Nb as needed, and the steel coating is further improved by including one or both of S and Pb. High toughness, high strength, high hardenability, excellent hardenability, machinability, etc. To provide. [0006] The first invention steel according to claim 1 has a weight ratio of C: 0.32 to 0.40%, S
i: 0.15 to 0.65%, Mn: 0.65 to 1.20%, Cr: 0.90
2.00%, Mo: containing 0.36 to 0.49%, and the balance Fe and impurity elements, organization Ri is Do from mixed structure of martensite and tempered bainite tempering, cracking baked during water quenching No toughness, high strength and excellent hardenability
It has . The second invention steel according to claim 2 is one or more of V: 0.05 to 0.30% and Nb: 0.05 to 0.30% in addition to the chemical composition of the first invention steel. The low-temperature toughness is further improved by containing seeds and refining the crystal grains. The third invention steel according to the third aspect is characterized in that, in addition to the chemical composition of the first invention steel, one or two of S: 0.035% or less and Pb 0.15% or less are contained to cut the steel. It is an improvement in the performance. The steel of the present invention has a mixed structure of tempered martensite and tempered bainite by quenching and tempering the steel of each component range described above, and is compared with the quenched and tempered structure of SNCM431 of the conventional steel. This is a structure having better toughness while maintaining the same or higher strength. The operation of the components of the steel of the present invention is as follows. By ensuring that C contains 0.32 to 0.40%, the strength required for steel as a part for driving and driving industrial vehicles is secured. By containing 0.15 to 0.65% of Si, the steel has a deoxidizing effect during steel making, improves the strength of the base by being solid-dissolved in the geology of the steel, and has a tempering softening resistance. . When Mn contains 0.65 to 1.20%, it has a deoxidizing effect in the same way as Si during steelmaking, and improves the hardenability of the steel. By adding 0.90 to 2.0% of Cr, the hardenability of steel is increased as in Mn, and a carbide is formed to improve wear resistance. By containing 0.36 to 0.49 % of Mo, the transformation stress of the steel of the present invention is relaxed, excellent hardenability enabling water quenching, and toughness and tempering softening resistance are increased. Generates fine carbides to improve wear resistance. By containing one or both of V and Nb in an amount of 0.05 to 0.30%, carbon and nitride are formed in the steel, the crystal grains are refined, and the strength and toughness are improved. By containing 0.035% or less of S, MnS is generated in steel to improve machinability. By containing 0.15% or less of Pb, it forms a single compound or a compound in steel. Notch effect during cutting and cutting resistance is reduced by lubrication to significantly improve tool life and chip crushability. I do. The steel of the present invention has a weight ratio of C: 0.32 to 0.40%, Si: 0.15 to
0.65%, Mn: 0.65 to 1.20%, Cr: 0.90 to 2.00%, M
o: containing 0.36 to 0.49%, Ri Do the balance Fe as well as consists of the impurity element organization is tempered mixed structure of martensite and tempered bainite, water quenching
It is a high toughness low alloy steel with high toughness , high strength, and excellent hardenability , sometimes without quenching cracks . V: 0.05 to 0.30%, Nb: 0.05 to 0.30%
One or two of which are included to further improve the low-temperature toughness by refining the crystal grains, or S: 0.035% or less, Pb: 0.1
Machinability is improved by containing 5% or less. The reasons for limiting the components of the steel of the present invention will be described. C is an element necessary to secure the strength required for parts for driving and driving industrial vehicles, and it is necessary to contain at least 0.32% or more of C in order to obtain a desired hardness. And the lower limit was set to 0.32%. However, when the content exceeds 0.40%, the toughness is reduced and strain is easily generated by heat treatment.
. 40%. Si is an element which has a deoxidizing effect and forms a solid solution in the geology to improve the strength of the base material and further has a resistance to tempering and softening. To obtain these effects, it is necessary to contain 0.15% or more. The lower limit was set to 0.15. However, if Si is contained more than necessary, the machinability is impaired, so the upper limit was made 0.65%. Mn is used as a deoxidizing agent similarly to Si, and is an element for improving hardenability. Even in a traveling part having a large cross section such as a large truck link, Mn is required to have a hardened structure up to the core. .65% or more.
. 65%. However, if it is contained more than necessary, cracks easily occur in water quenching, so the upper limit was made 1.20%. Cr is an element that enhances hardenability similarly to Mn and forms carbides to improve wear resistance.To obtain the hardenability and wear resistance required for traveling parts,
The content must be 0.90% or more, and the lower limit is set to 0.90%. However, when a large amount is contained, the carbides are coarsened and hard carbides are formed, which impairs hardenability and wear resistance. Therefore, the upper limit is set to 2.0%. [0010] Mo reduces the transformation stress of the steel of the present invention, enhances the hardenability that enables water quenching, increases the toughness and temper softening resistance, and further generates fine carbides to improve wear resistance. It is the most important element in the present invention. To obtain these effects, it is necessary to contain at least 0.36 % or more, and the lower limit is set to 0.36 %. bainite nose is retracted, cracking baked at the time of water quenching is likely to occur, and since Mo is an expensive element, the upper limit 0.49
%. V and Nb are elements that form carbonitrides, refine crystal grains, and improve strength and toughness. To obtain these effects, it is necessary to contain 0.05% or more, and the lower limit is set to 0.1%. 05%. However, if both V and Nb are contained in excess of 0.30%, the effect is not significantly improved, so the upper limit is set to 0.30%. S is an element that forms MnS to improve machinability, and Pb forms a single compound or forms a compound in steel, reduces notch effect during cutting, and reduces cutting resistance by lubricating action to reduce tool life. Is an element that significantly improves the friability of chips. But,
Since S and Pb impair the mechanical properties when the content thereof is large,
S regulated 0.035%, Pb: 0.15% and the upper limit. [0011] Table 1, the chemical component of the embodiment of a high toughness low alloy steel according to the present invention, there is provided a table described in conjunction with a chemical component of the comparative steel versus, A steel test steel is the first invention steel , B steel is the second invention steel, C steel, and D steel are the third invention steel. Of the comparative steels, W steel is a steel based on SNCM431, and X is Mo content as compared with the present invention steel. Low SCM4
The steel based on No. 30 and the Y steel are C and Mo in the present invention in an amount not more than the lower limit of the C and Mo contents.
The comparative steel and Z steel to which Mo is added are comparative steels to which Mn and Mo are added in amounts not more than the lower limits of the Mn and Mo contents in the present invention. The amounts of Ni described in the columns of the steels A to D of the steel of the present invention and the steels X to Z of the comparative steels indicate the amounts of Ni normally contained as impurities in the electric furnace melting. In the present invention, the amounts of S described in the columns of W to Z steels of the steels A, B, and C and the comparative steels are also the amounts of S contained as impurities in the electric furnace melting. [Table 1] After casting a steel having the chemical components shown in Table 1 above, the rolling ratio is 50 or more and the diameter is 50 mm.
Hot-rolled into a test steel, quenched and tempered, and measured for mechanical properties such as hardness, tensile strength, elongation, drawing, and Charpy impact value during quenching and tempering. . Table 2 shows the results. Note that the quenching and tempering temperatures of the conventional steel in the comparative steel, W steel, were heated to 880 ° C. based on the prior art to perform oil quenching, then tempered at 480 ° C., and the steel of the present invention and The remaining comparative steels were heated to 850 ° C. for water quenching, and then tempered at 560 ° C. The structures after quenching and tempering of the present invention were all mixed structures of tempered martensite and tempered bainite. Here, the cooling rate during quenching of the X steel, Y steel and Z steel in the steel of the present invention and the comparative steel was 29 ° C./sec , and the cooling rate during quenching of the W steel was 5 ° C./sec . For tensile strength, elongation, and drawing,
The measurement was performed using a JIS No. 4 test piece, and the impact value was measured using a JIS No. 3 test piece. As is apparent from Table 2, the steels of the present invention, quenching hardness HB475～ 495, within the tempering hardness is HB363～ 377, a tensile strength of 115 to 124 kg ·
f / mm ² , elongation in the range of 12.4 to 15.8, and aperture of 41.9 to
While the Charpy impact value is within the range of 7.7 to 10.1 kg · fm / cm ² within the range of 46.3, the conventional steel W (SNCM431) is quenched at 880 ° C.
As a result of high temperature quenching and low temperature tempering at 480 ° C., the quenching hardness was HB485, the tempering hardness was HB370, and the tensile strength was 119 kg · f /
mm ² and excellent strength, but Charpy impact value is 6.9 kg · f
−m / cm ² , low in toughness. Further, in the comparative steels, the conventional steel X steel (SCM430) and other comparative steels Y steel and Z steel were water-quenched at 850 ° C. and tempered at 560 ° C., and the quenching hardness was HB385 to 446. Temper hardness HB 297-342, lower than W steel, and tensile strength is also 72-101 kg · f / mm ² , lower than W steel, and strength as compared with W steel and the present invention steel. Is also inferior. In contrast to the conventional steel and the comparative steel, the steels A to D, which are the steels of the present invention, are all water-quenched at a temperature as low as 850 ° C., and have a quenching hardness of HB475 even when tempered at a temperature as high as 560 ° C. ９５ 495 , and the temper hardness was HB 363 to 377 .
It has hardness equal to or higher than W steel, which is a conventional steel containing as much as 8% Ni, and has an excellent tensile strength of 115 to 124 kg · f / mm ^2, which is superior to W steel. It has. In addition, the steel of the present invention has a Charpy impact value of 7.7 to 10.1 kg · fm / cm ² , which is significantly superior to that of the W steel, as a result of being subjected to high-temperature tempering. It can be said that it has high strength and high toughness. [Table 3] Table 3 shows the results of measuring the hardenability of the test steels shown in Table 1. These measurement values were obtained by preparing a Jominy test specimen from the hot-rolled specimen, and measuring 1. The hardness was measured at each distance of 5 mm, 3 mm, 5 mm, 8 mm, 11 mm, 13 mm, 15 mm, and 25 mm. As is clear from Table 3, the conventional steels X and the comparative steels Y and Z have lower quenching hardness than the conventional steel W, whereas the steels A to D according to the present invention. It is clear that the steel has a hardness equal to or higher than that of the W steel, indicating that the steel of the present invention is also excellent in hardenability. Next, FIG. 1 shows a comparison of the low-temperature toughness of the steel A in the steel of the present invention together with the result of measurement together with the conventional steel W. The low temperature toughness is obtained by preparing a JIS No. 4 test piece and measuring the Charpy impact value of the test steel in a low temperature range of 20 ° C to -80 ° C. As is clear from FIG. 1, the Charpy impact value at −40 ° C. of the conventional steel W is 5.2 kg · fm / cm ² , whereas the steel A of the present invention is 7.3. kg
-Fm / cm ² , which is much higher than that of W steel, indicating that the steel of the present invention is also excellent in low-temperature toughness. Next, the machinability of the steel of the present invention is compared with the machinability of the conventional steel W for the steels A, C and D. In this comparison test, a SKH9 straight drill having a diameter of 5 mm was prepared by preparing a material having a diameter of 40 mm and a length of 10 mm from each of the test steels that had been quenched and tempered, and fixed on a platen. , The rotational speed is 1140 rpm,
The drill piercing property was measured by a thrust of 30 kg (free weight drop method). In Table 4, an index is shown with the drilling pierceability of W steel, which is a conventional steel, as 100. [Table 4] As is clear from Table 4, all of the steels of the present invention have excellent machinability as compared with the conventional steel W, and particularly contain the free-cutting elements S and Pb. C steel
The steels D and D have significantly improved machinability as compared with the steel W. According to the present invention, in a high-toughness low alloy steel used for running and driving parts of industrial vehicles, C is used to secure required strength, and to reduce the toughness by heat treatment. Si is added in an amount of 0.32 to 0.40% in a range where the generation of strain is prevented, and in a range in which Si is improved in order to improve the strength of the base material, to have a softening resistance by tempering, and not to impair the machinability.
15 to 0.65%, Mn is used to form a quenched structure up to the core of a forged part having a large cross section, and 0.65 to 1.20 in a range that prevents the occurrence of cracks during quenching.
%, Cr, in order to obtain hardenability of a forged part having a large cross section and to form fine carbides to obtain wear resistance, and to reduce the amount of Cr in the range not to coarsen the carbides.
In addition to containing 90 to 2.0%, Mo reduces the transformation stress, enhances the hardenability to enable water quenching, and enhances the toughness and tempering softening resistance. And 0.36 to 0.49 % of the range in which the bainite nose recedes and does not cause quenching cracks during water quenching. In the presence of a high amount of C, large transformation stress and heat Without adding Ni, which adds stress to the material to be quenched, actively, the amount of impurities was stopped to about the amount of impurities, thereby alleviating the transformation stress during quenching and enabling water quenching. 810-88 lower than SNCM431 used for running and driving parts
Heating to a temperature of 0 ° C., quenching at a cooling rate of 25 ° C./sec or more from this temperature by water quenching,
It has the same or higher hardness and mechanical properties as conventional steel, and has the effect of greatly improving toughness. On the other hand, the X steel and the Y and Z steels in the comparative steels contain Ni only in the amount of impurities, but have a lower Mo content than the steel of the present invention, and the Y steel has a C content. Quantity, Z
The steel has a low Mo content, and as a result, is significantly inferior in strength and hardness as compared with the W steel which is SNCM431. Therefore, the steel of the present invention is a conventional steel of SNCM431.
It can be said that the steel has more excellent toughness while securing strength equal to or higher than. Further, in the present invention, V: 0.05 to 0.30%, Nb: 0.05
By adding one or two kinds of ０．３0.30%, carbonitrides can be formed and crystal grains can be refined, whereby the low-temperature toughness can be further improved, or S: 0.035% Hereinafter, by containing one or two of Pb: 0.15% or less, the cutting resistance can be reduced by the notch effect and the lubricating effect at the time of cutting, and the machinability can be improved. Further, the mixed structure of tempered martensite and tempered bainite of the steel of the present invention is obtained by melting and forming a steel containing C, Si, Mn, Cr, and Mo in the above-mentioned predetermined amount, and then forming, for example, a heat treatment furnace such as an electric furnace. To 810 to 880 ° C. by quenching at a cooling rate of 25 ° C./sec or more by means such as water quenching to obtain a mixed structure of martensite and bainite, and then tempering at 540 to 560 ° C. And the structure obtained by quenching and tempering is not a one-phase structure of martensite as in the case of the conventional steel SNCM431, so that the steel of the present invention has excellent toughness and This is a steel in which the transformation stress is relaxed because precipitation of is suppressed, and the occurrence of quenching cracking is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the test temperature indicating the low-temperature toughness and the Charpy impact value of the steel of the example of the present invention and the comparative steel. FIG. Exploded view of track link [Fig. 3] Assembly view of track link [Explanation of reference numerals] 1 track link 2 bush hole 4 bush 5 pin 8 shoe

Claims (1)

Claims: C: 0.32 to 0.40%, Si: 0.15 to 0 by weight ratio
. 65%, Mn: 0.65 to 1.20%, Cr: 0.90 to 2.00%, Mo:
Containing 0.36 to 0.49%, and the balance Fe and impurity elements, Ri organization Do a mixed structure of martensite and tempered bainite tempering, during water quenching
High toughness, low alloy steel characterized by high toughness , high strength and excellent hardenability without quenching cracks . 2. C: 0.32 to 0.40%, Si: 0.15 to 0.
65%, Mn: 0.65 to 1.20%, Cr: 0.90 to 2.00%, Mo: 0
. 36 to 0.49 %, V: 0.05 to 0.30%, Nb: 0.05 to 0%
. It one free of 30% and contains two, and the balance Fe and impurity elements, Ri organization Do a mixed structure of martensite and tempered bainite tempering, water
A high toughness low alloy steel characterized by being free from quenching during quenching, having high toughness, high strength, and excellent hardenability . 3. A weight ratio of C: 0.32 to 0.40% and Si: 0.15 to 0.
65%, Mn: 0.65 to 1.20%, Cr: 0.90 to 2.00%, Mo: 0
. 36 to 0.49 %, one or more of S: 0.035% or less and Pb: 0.15% or less, the balance being Fe and impurity elements, and the structure is tempered martensite. Ri Do from mixed structure of tempered bainite, during water quenching
A high toughness low alloy steel characterized by high toughness , high strength and excellent hardenability without quenching cracks .