CN1688733A - Steel material for mechanical structure excellent in suitability for rolling, quenching crack resistance, and torsional property and drive shaft - Google Patents

Abstract

The present invention relates to a steel used by mechanical structures and a transmission shaft. Represented by mass percent, the steel contains C with a mass percent of more than 0.35 percent and less than 0.5 percent, Si with a mass percent of less than 0.15 percent, Mn with a mass percent of more than 0.2 percent and less than 1.1 percent, P with a mass percent of less than 0.02 percent, S with a mass percent of more than 0.005 percent and less than 0.035 percent, Cr with a mass percent of more than 0.1 percent and less than 0.2 percent, Mo with a mass percent of more than 0.05 percent and less than 0.5 percent, Ti with a mass percent of more than 0.01 percent and less than 0.05 percent, Al with a mass percent of more than 0.01 percent and less than 0.05 percent, N with a mass percent of less than 0.01 percent, B with a mass percent of more than 0.0005 percent and less than 0.005 percent, Cu with a mass percent of more than 0.06 percent and less than 0.25 percent and Ni with a mass percent of more than 0.05 percent and less than 0.2 percent, the rest part is composed of Fe and inavoidable impurities, and moreover, the LD value represented by the formula (1) is less than 120. LD equal to 0.569 multiplied by (7.98 multiplied by (C)<1 / 2> multiplied by (1 added by 4.1mn)(1 added by 2.83P)(1 minus 0.62S)(1 added by 0.64Si)(1 added by 2.33Cr)(1 added by 0.52Ni)(1 added by 3.14Mo)(1 added by 0.27Cu)(1 added by 1.5 (0.9 minus C))) added by 52.6 minus (1).

Description

Steel materials for machine structures and propeller shafts with excellent roll formability, quench cracking resistance, and torsional properties

technical field

The invention relates to a steel material for mechanical structure. In particular, it relates to a steel material for machine structural use having excellent roll formability, quench cracking resistance, and torsional properties. To provide steel materials for machine structures manufactured using electric furnace materials instead of blast furnace materials, which do not deteriorate the above-mentioned properties even if inclusion elements such as Cu and Ni are mixed into the steel materials.

Background technique

The required torsional strength is required for mechanical structural members such as automotive propeller shafts and constant velocity joints. In order to ensure torsional strength, in the past, hot-rolled bar steel was generally hot-forged, normalized when necessary, processed into a predetermined shape by cutting, cold forging, etc., and then subjected to induction hardening and tempering.

In recent years, in order to protect the environment, the weight of the vehicle body has been reduced for the purpose of improving the fuel consumption of the vehicle. In order to reduce the weight of automobile parts, it is desired to increase the torsional strength of automobile members. On the other hand, machinability and quench cracking resistance of steel materials are required in the manufacturing process of automobile parts.

In order to improve the torsional strength, it is considered to increase the quenching hardness and quenching depth by induction quenching. However, to increase the quenching hardness and depth, only changing the high-frequency quenching conditions or increasing the amount of alloying elements in the steel will increase the manufacturing cost. In order to simultaneously satisfy the torsional strength, machinability, and quench cracking resistance of automotive components, for example, as shown in Patent Document 1, a technique of limiting the amount of alloy elements has been proposed.

However, there is a problem that the chemical composition is limited only by the above-mentioned technology, and the composition range that satisfies machinability, quench cracking resistance, and torsion characteristics at the same time is extremely narrow. In addition, there is a problem of unstable quality.

Therefore, as a solution to the above-mentioned problems, the present inventors controlled the steel structure while adjusting the composition of the steel material. Steel for mechanical structures.

Patent Document 1: JP-A-4-218641 (Claims)

Patent Document 2: Patent No. 3288563 (Claims)

However, when the steel material for machine structural use disclosed in the aforementioned Patent Document 2 is produced using an electric furnace, desired properties cannot be obtained, and it has been found that the roll formability is significantly lowered. Compared with blast furnace materials, it is unavoidable to mix Cu and Ni and other inclusion elements into the steel smelted by electric furnace. It is considered that this inclusion element deteriorates roll formability.

The present invention aims to solve the above problems. That is, the object is to provide a steel material for machine structural use that effectively prevents deterioration of roll formability even if it is manufactured using an electric furnace material instead of a blast furnace material, and has excellent quench cracking resistance and torsional characteristics. In addition, a propeller shaft formed using the steel material is provided.

Contents of the invention

The inventors of the present invention have obtained the following findings as a result of painstaking studies to achieve the above objects.

(1) In order to reduce the bad influence of inclusion elements, it is effective to increase the amount of Cr;

(2) However, the increase in the amount of Cr leads to a decrease in torsional characteristics and machinability including roll formability;

(3) The reduction in torsional characteristics and machinability associated with the increase in the amount of Cr is eliminated by increasing the amount of Si and reducing the amount of Mn;

(4) The reduction of roll formability is eliminated by controlling the LD value, which is an index of hardness and structure determined by hardenability, within the specified range;

The present invention has been accomplished based on the above knowledge and findings. The specific contents of the invention are as follows.

1. A steel material for machine structures excellent in roll formability, quench cracking resistance and torsional properties, characterized in that it is the following composition: expressed in mass%, when the LD value represented by the following formula (1) satisfies The range below 120 contains

C: 0.35% or more, 0.50% or less,

Si: 0.15% or less,

Mn: 0.2% or more, 1.1% or less,

P: less than 0.020%,

S: 0.005% or more, 0.035% or less,

Cr: more than 0.1%, less than 0.2%,

Mo: 0.05% or more, 0.5% or less,

Ti: 0.01% or more, 0.05% or less,

Al: 0.01% or more, 0.05% or less,

N: 0.01% or less,

B: 0.0005% or more, 0.0050% or less,

Cu: 0.06% or more, 0.25% or less and

Ni: 0.05% or more and 0.2% or less,

The remainder is Fe and unavoidable impurities,

LD＝0.569×{7.98×(C) ^1/2 ×(1+4.1Mn)(1+2.83P)(1-0.62S)(1+0.64Si)

(1+2.33Cr)(1+0.52Ni)(1+3.14Mo)(1+0.27Cu)(1+1.5(0.9-C))}+52.6

---(1)

However, C, Mn, P, S, Si, Cr, Ni, Mo, and Cu in the formula mean the content (mass %) of each element.

2. The steel material for machine structure excellent in roll formability, quench cracking resistance, and torsional properties according to the above item 1, characterized in that the steel material has the following composition: expressed in mass %, and further contains

V: 0.01% or more, 0.30% or less and

Nb: 0.005% or more and 0.050% or less.

3. A transmission shaft, characterized in that it is formed by using the steel material for mechanical structure as described in the above item 1 or 2, and is subjected to high-frequency quenching and tempering to provide a hardened layer.

A brief description of the drawings

Fig. 1 is a graph showing the effect of LD value on roll formability.

Fig. 2 is the result of measuring the static strength of the propeller shaft through the static strength test.

FIG. 3 shows the results of measuring the fatigue strength of the propeller shaft of the inventive example and the propeller shaft of the comparative example by a fatigue strength test.

Detailed ways

The present invention will be specifically described below. The reason why the component composition of the steel material is limited to the above-mentioned range in the present invention will be described. "%" about a component means mass % unless otherwise stated.

C: 0.35% or more and 0.50% or less

C is an element having the greatest influence on induction hardenability, and is an element useful in increasing the hardness and depth of a quenched solidified layer and securing a torsional strength of 1400 MPa or more after induction quenching and tempering. However, if the content is less than 0.35%, the addition effect is insufficient, and if it exceeds 0.50%, the machinability and quench cracking resistance will be reduced, so the amount of C is limited to the range of 0.35% to 0.50%.

Si: 0.15% or less

In addition to functioning as a deoxidizing element, Si is also an element that strengthens steel by solid solution in ferrite to increase the torsional strength, and the added amount is preferably more than 0.05%. However, if the amount of Si exceeds 0.15%, the machinability is remarkably deteriorated, so it is limited to a range of 0.15% or less.

Mn: 0.2% or more and 1.1% or less

Mn is an element useful for improving hardenability and increasing the depth of hardness during induction hardening, thereby contributing to the improvement of torsional strength. However, when the content is less than 0.2%, the effect of adding is insufficient, and when it exceeds 1.1%, not only the roll formability but also the machinability and torsional strength are deteriorated, so the amount of Mn is limited to 0.2% or more and 1.1% or less. scope. Preferably, it is the range of 0.2% or more and 0.8% or less.

P: 0.020% or less

P segregates at the austenite grain boundaries during quenching and promotes the occurrence of quenching cracks, so it is preferable to reduce it as much as possible, and from this point of view, it is prescribed to be suppressed to 0.020% or less.

S: 0.005% or more and 0.035% or less

S has the effect of forming MnS in steel and improving machinability, so it is specified to be contained in an amount of 0.005% or more. However, MnS tends to be the starting point of cracks, leading to a decrease in strength and toughness, so the upper limit of the amount of S is limited to 0.035%. Preferably, it is the range of 0.010% or more and 0.035% or less.

Cr: more than 0.1%, less than 0.2%

Cr is an especially important element in the present invention, and containing Cr can advantageously eliminate the adverse effects of inclusion elements such as Cu and Ni that cause reductions in roll formability, torsional properties, machinability, and the like. However, when the amount of Cr is 0.1% or less, the effect of the addition is lacking, and when it exceeds 0.2%, the roll formability, machinability, and torsional strength decrease, so Cr is specified to be contained in the range of more than 0.1% and 0.2% or less.

Mo: 0.05% to 0.5%

Mo is not only useful for improving hardenability, but also promotes the formation of bainite, and has the effect of improving machinability. For this reason, it is necessary to contain 0.05% or more, but if the content exceeds 0.5%, the machinability will be deteriorated, so the amount of Mo is limited to the range of 0.05% or more and 0.5% or less. Preferably, it is the range of 0.1% or more and 0.5% or less.

Ti: 0.01% to 0.05%

Ti combines with N to form nitrides, and refines austenite grains during high-temperature heating. It is an element necessary to secure solid solution B useful for improving hardenability. For this reason, it is necessary to contain 0.01% or more, but when it exceeds 0.05%, the toughness is impaired, so the amount of Ti is limited to the range of 0.01% or more and 0.05% or less.

Al: 0.01% or more and 0.05% or less

Al is useful as a deoxidizer, so it needs to contain at least 0.01%, but if it exceeds 0.05%, coarse alumina is formed, which becomes the starting point of fatigue fracture and reduces the fatigue strength, so the amount of Al is limited to 0.01% or more and 0.05% or less range.

N: 0.01% or less

N is an element useful for improving fatigue strength by combining with Al or Ti to form nitrides, refining austenite grains during high-temperature heating. However, if the content exceeds 0.01%, the nitrides will coarsen, conversely deteriorating the fatigue strength. In addition, excessive addition of N forms BN, which also has the disadvantage of reducing the amount of solid-solution B effective for hardenability. Therefore, the amount of N is limited to 0.01% or less.

B: More than 0.0005% and less than 0.0050%

B improves the hardenability, increases the quenching depth during induction hardening, and thereby has the effect of increasing the torsional strength. For this reason, it is necessary to add 0.0005% or more, but if it exceeds 0.0050%, the toughness will deteriorate, so B is limited to the range of 0.0005% or more and 0.0050% or less.

Cu: 0.06% or more and 0.25% or less

Cu is an element that is inevitably mixed as an inclusion element. If the content exceeds 0.25%, the roll formability and the like will deteriorate, so it is made 0.25% or less. On the other hand, if it is reduced to less than 0.06%, the production cost will increase, so it is made 0.06% or more.

Ni: 0.05% or more and 0.2% or less

Ni is an element that is inevitably mixed as an inclusion element. If the content exceeds 0.2%, the roll formability and the like will deteriorate, so it is made 0.2% or less. On the other hand, if it is reduced to less than 0.05%, the production cost will increase, so it is made 0.05% or more.

The basic components have been described above, but the present invention may suitably contain elements described below in addition to these.

V: 0.01% or more and 0.30% or less, Nb: 0.005% or more and 0.050% or less

Both V and Nb form carbonitrides, refine the austenite grains, and effectively contribute to the improvement of strength. However, when the amounts of V and Nb are less than 0.01% and less than 0.005%, respectively, the effect of the addition is lacking, and when the amounts exceed 0.30% and 0.050%, respectively, the precipitates are coarsened and the toughness is impaired, so they are limited to V: 0.01. % to 0.30%, Nb: 0.005% to 0.050%.

The appropriate composition ranges of the components have been described above, but in the present invention, it is not sufficient for each component to satisfy the above-mentioned composition ranges alone, and it is necessary to adjust the components so that the LD value represented by the following formula (1) becomes 120 or less.

LD＝0.569×{7.98×(C) ^1/2 ×(1+4.1Mn)(1+2.83P)(1-0.62S)(1+0.64Si)

(1+2.33Cr)(1+0.52Ni)(1+3.14Mo)(1+0.27Cu)(1+1.5(0.9-C))}+52.6

---(1)

However, C, Mn, P, S, Si, Cr, Ni, Mo, and Cu in the formula mean the content (mass %) of each element.

The LD value is an index for determining the hardness and structure by hardenability.

FIG. 1 shows the results of investigating the influence of the above-mentioned LD value on roll formability for the high-Cr and high-Si steel of the present invention. In addition, in this figure, the investigation result about the low-Cr and low-Si steel described in the said patent document 2 is also shown together for comparison.

It should be noted that the roll formability was evaluated by the die life in the rolling test.

As shown in the figure, when the LD value exceeds 120 in any case, the die life decreases rapidly, but the die life in the range of LD value below 120, that is, the roll formability, the high Cr and high Si steel of the present invention is extremely excellent.

Therefore, in the present invention, the components are adjusted so that the above-mentioned LD value is 120 or less.

In the present invention, the steel structure is not particularly limited, but is preferably a structure mainly composed of ferrite and containing a bainite phase with an area ratio of about 5 to 30%.

The steel materials of the present invention described above are most suitable for use in power transmission parts, especially automotive propeller shafts and constant velocity joints. It goes without saying that the processability is excellent, and since the load capacity increases, a great effect of being able to reduce the weight is obtained.

Next, preferred production conditions of the present invention will be described.

The smelting method of the steel material of the present invention may be manufactured according to a conventional method, and is not particularly limited. The steel for mechanical structure provided by the invention has good roll formability even if it contains Cu and Ni which are difficult to remove when smelted in an electric furnace, so it is particularly suitable for smelting in an electric furnace. Vacuum degassing such as RH degassing, refining in a ladle, etc. may also be added. The molten steel is solidified by continuous casting or ingot casting. After solidification, it is hot-rolled or hot-rolled and medium-temperature forged to make a billet of a specified shape. These billets are subjected to intermediate heat treatments such as normalizing, spheroidizing annealing, and softening annealing as required, and are finished into required shapes by cold working such as cutting, forging, and rolling.

In the present invention, hot rolling or hot forging, or further normalizing treatment is carried out, and the product shape is finished. The cooling after austenitization such as normalizing after hot rolling or hot forging is preferably about 0.2 to 2.0° C./s in order to generate an appropriate amount of bainite. In particular, for large-diameter steel bars, accelerated cooling with adjusted cooling is preferable.

In addition, for the final induction quenching and tempering, use an induction quenching device of about 15kHz, heat for about 0.2-1.0 seconds at an output power of about 120kW, and then quench and temper at 170°C for about 30 minutes. Can.

Example

The steel with the composition shown in Table 1 was smelted in a converter, and a steel ingot of 400×540 mm was produced by continuous casting, and then a billet of 150 mm square was produced by hot rolling. Next, after heating the billet to 1030°C, it was hot-rolled into a Ф25mm straight bar, and air-cooled at a cooling rate of 0.7°C/s.

Table 2 shows the results of investigations on the microstructure, roll formability, torsional characteristics, and quench cracking resistance of the steel bars thus obtained.

The evaluation methods for the structure and properties of the steel are as follows.

(1) Organization

The microstructure of the straight bar after cooling was photographed with an optical microscope, and the structure of the steel was analyzed from the photograph, and the area ratio of the bainite phase was measured with an image analyzer.

(2) Roll formability

Roll formability was evaluated by the die life in the rolling test. The die life was evaluated by the number of materials that could be rolled until they could not be rolled due to tooth chipping, surface peeling, tooth wear, etc. of the die.

The mold material is SKD11, and the rack data is as follows.

Tooth shape: involute, modulus: 1.27, pressure angle: 30o, number of teeth: 21, pitch diameter: 26.27mm, major diameter: 28.1mm, minor diameter: 24.88mm, roller diameter (Ф2.5mm pin (pin) ): 30.49mm

(3) Torsion characteristics

Make a parallel part from a straight bar: a Ф20mm smooth round bar torsion test piece, quench it with a high-frequency quenching device with a frequency of 15kHz, perform a tempering treatment at 170°C for 30 minutes, and then perform a torsion test. The depth of hardening after the induction hardening and tempering was set to about 4 mm. In the torsion test, a 4900J (500kgf·m) torsion testing machine was used to obtain the maximum torsional shear strength, which was used as the torsional strength.

(4) Quenching cracking resistance

Quenching cracking resistance, process a round bar (Ф20mm) with an axial V-shaped groove on the surface from the above-mentioned straight bar of Ф25mm, perform the same high-frequency quenching as the above (3), polish and observe the C-section of the round bar The 10 parts are evaluated by the number of cracks.

(5) Machinability

In the machinability test, SKH4 and Ф4mm drills were used to drill holes with a length of 12 mm at a rotational speed of 1500 rpm, and the total hole length (mm) until cutting was not possible was obtained and used as tool life evaluation.

It is clear from Table 2 that the steels obtained according to the present invention have excellent roll formability, torsional properties, quench cracking resistance and machinability.

The static strength and fatigue strength of the propeller shaft manufactured by induction hardening and tempering to form a solidified layer using the steel material of the present invention will be described with reference to FIG. 2 and FIG. 3 , respectively. The propeller shaft of the inventive example was manufactured using No. 2 steel material in Table 1. The propeller shaft of the comparative example was manufactured using No. 18 steel material in Table 1. Fig. 2 is the result of measuring the static strength of the propeller shaft through the static strength test. The so-called static strength test (static strength test) is a test to measure the maximum torque when the propeller shaft breaks and evaluate the static strength. The number of transmission shafts for the test is 1 for the comparative example and 2 for the inventive example. In FIG. 2 , the comparative example, the inventive example 1, and the inventive example 2 are respectively shown. The maximum torque acting when the propeller shaft of the comparative example was broken was denoted as 1, and the maximum torque acting when the propeller shaft of the inventive example was broken was expressed as a ratio thereto. It can be seen that with respect to the transmission shaft of the comparative example, the transmission shaft of the invention example improves

The static strength is about 1.17 times.

FIG. 3 shows the results of measuring the fatigue strength of the propeller shaft of the inventive example and the propeller shaft of the comparative example by a fatigue strength test. The so-called fatigue strength test (fatigue strength test) is a test to measure the fatigue strength when repeated load torque. The specified load torque is repeatedly applied to the propeller shaft, and the number N of load repetitions until fracture is obtained. The ordinate is a value obtained by dividing the load torque by the static strength of the propeller shaft of the comparative example, and is dimensionless. The abscissa is the number of repetitions of the load on the propeller shaft until it breaks. From the test results, it is known that, for example, when the number of repetitions is 10,000 times, the ordinate value of the transmission shaft of the comparative example is about 0.49, and the ordinate value of the transmission shaft of the invention example is about 0.55, and the fatigue strength of the transmission shaft of the invention example is improved. About 10%.

Industrial Applicability

According to the present invention, it is possible to eliminate the adverse effects of inclusion elements from a steel material for machine structural use manufactured using an electric furnace in which inclusion elements such as Cu and Ni are unavoidably mixed. A steel material having excellent roll formability, torsional characteristics, quench cracking resistance, and hardenability can be obtained. When the steel material of the present invention is used for power transmission parts, especially automobile propeller shafts and constant velocity joints, it is excellent in workability, and since it has high strength, it can achieve a great effect of weight reduction.

Table 1 No. Composition (mass%) LD value C Si mn P S Cu Ni Cr Mo Al Ti B N V Nb 1 0.37 0.05 0.83 0.015 0.030 0.08 0.05 0.08 0.19 0.028 0.030 0.0032 0.0051 - - 98.4 2 0.40 0.08 0.45 0.013 0.018 0.16 0.16 0.16 0.17 0.032 0.019 0.0022 0.0044 - - 89.3 3 0.39 0.08 0.25 0.011 0.020 0.16 0.15 0.15 0.16 0.040 0.020 0.0020 0.0038 - - 77.3 4 0.47 0.06 0.56 0.009 0.022 0.12 0.10 0.09 0.17 0.015 0.024 0.0017 0.0076 - - 88.3 5 0.42 0.10 0.65 0.013 0.020 0.11 0.10 0.11 0.25 0.025 0.015 0.0023 0.0041 0.11 - 101.7 6 0.40 0.08 0.95 0.016 0.015 0.16 0.16 0.16 0.12 0.023 0.024 0.0022 0.0045 - 0.020 109.8 7 0.41 0.06 0.70 0.019 0.006 0.11 0.12 0.12 0.36 0.026 0.019 0.0019 0.0048 - - 116.1 8 0.40 0.08 0.60 0.019 0.007 0.22 0.16 0.16 0.35 0.026 0.025 0.0020 0.0052 - - 116.0 9 0.40 0.04 0.88 0.019 0.006 0.06 0.06 0.06 0.35 0.022 0.020 0.0025 0.0044 - - 116.4 10 0.41 0.07 0.60 0.016 0.015 0.11 0.10 0.12 0.15 0.023 0.024 0.0022 0.0045 0.15 0.032 91.1 11 0.40 0.06 0.84 0.018 0.007 0.11 0.11 0.11 0.35 0.025 0.023 0.0020 0.0041 - - 122.2 12 0.41 0.08 0.73 0.019 0.007 0.16 0.16 0.15 0.34 0.023 0.024 0.0023 0.0047 - - 122.6 13 0.42 0.04 0.99 0.018 0.006 0.06 0.06 0.06 0.35 0.024 0.021 0.0018 0.0045 - - 121.5 14 0.61 0.06 0.63 0.012 0.021 0.13 0.12 0.13 0.16 0.022 0.019 0.0019 0.0054 - - 94.2 15 0.31 0.08 0.57 0.011 0.020 0.15 0.16 0.13 0.17 0.031 0.020 0.0018 0.0060 - - 90.9 16 0.40 0.04 1.22 0.014 0.023 0.16 0.15 0.17 0.26 0.034 0.017 0.0018 0.0065 - - 143.0 17 0.42 0.23 0.70 0.025 0.024 0.14 0.11 0.10 0.16 0.025 0.024 0.0016 0.0056 - - 100.8 18 0.39 0.04 0.54 0.014 0.045 0.32 0.26 0.26 0.15 0.022 0.023 0.0022 0.0048 - - 101.2 19 0.40 0.05 0.48 0.014 0.020 0.11 0.10 0.11 0.62 0.024 0.020 0.0025 0.0046 - - 116.0 20 0.38 0.05 0.52 0.013 0.018 0.11 0.11 0.09 0.03 0.025 0.022 0.0017 0.0049 - - 76.4 twenty one 0.37 0.02 1.00 0.015 0.030 0.08 0.05 0.03 0.35 0.021 0.023 0.0032 0.0052 - - 114.3

Table 2 No. organization ^* Bainite ratio (%) Mold life (pieces) Torsional strength (MPa) Number of Quenching Cracks (pieces) Tool life(mm) Remark 1 F+P+B 12 610 1540 15 2430 Invention example 2 F+P+B 14 624 1580 16 2480 " 3 F+P+B 10 642 1570 10 2300 " 4 F+P+B 13 588 1630 19 2390 " 5 F+P+B 18 591 1590 18 1740 " 6 F+P+B 8 525 1570 18 2110 " 7 F+P+B twenty three 493 1580 19 2470 " 8 F+P+B twenty one 469 1590 19 2450 " 9 F+P+B 20 512 1580 20 2440 " 10 F+P+B 10 658 1620 16 2120 " 11 F+P+B twenty one 253 1590 18 2300 comparative example 12 F+P+B 19 218 1580 19 2320 " 13 F+P+B twenty two 226 1600 twenty one 2310 " 14 F+P+B 11 599 1650 45 1120 " 15 F+P+B 7 635 1190 4 2130 " 16 F+P+B 14 151 1430 19 1940 " 17 F+P+B 17 585 1560 twenty four 1290 " 18 F+P+B 16 523 1400 17 1270 " 19 F+P+B 45 514 1600 16 1510 " 20 F+P 0 663 1550 11 1330 " twenty one F+P+B 26 360 1530 17 2400 "

* F: ferrite, P: pearlite, B: bainite

Claims (3)

1. the machine structural steel product of a rollforming, quenching crack resistance and torsional property excellence is characterized in that, be following composition: % represents with quality, is that scope below 120 contains satisfying the LD value of representing with following formula (1),

More than the C:0.35%, below 0.50%,

Below the Si:0.15%,

More than the Mn:0.2%, below 1.1%,

Below the P:0.020%,

More than the S:0.005%, below 0.035%,

Cr: surpass 0.1%, below 0.2%,

More than the Mo:0.05%, below 0.5%,

More than the Ti:0.01%, below 0.05%,

More than the Al:0.01%, below 0.05%,

Below the N:0.01%,

More than the B:0.0005%, below 0.0050%,

Reach more than the Cu:0.06%, below 0.25%

More than the Ni:0.05%, below 0.2%,

Remainder is Fe and unavoidable impurities,

LD＝0.569×{7.98×(C) ^1/2×(1+4.1Mn)(1+2.83P)(1-0.62S)(1+0.64Si)(1+2.33Cr)(1+0.52Ni)(1+3.14Mo)(1+0.27Cu)(1+1.5(0.9-C))}+52.6

---(1)

Wherein, the C in the formula, Mn, P, S, Si, Cr, Ni, Mo, Cu refer to the content (quality %) of each element.

2. the machine structural steel product of rollforming according to claim 1, quenching crack resistance and torsional property excellence is characterized in that, be following composition: % represents with quality, and steel also further contain,

Reach more than the V:0.01%, below 0.30%

More than the Nb:0.005%, below 0.050%.

3. a transmission shaft is characterized in that, adopts claim 1 or 2 described machine structural steel products to form, thereby carries out high-frequency quenching, tempering is provided with cured layer.

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