JP5332517B2 - Manufacturing method of carburizing steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel material to be carburized which is preferably employed as a material for manufacturing such a high-strength gear and the like as to show a higher bending fatigue strength at a dedendum when worked to be a gear, for instance, than that in a conventional gear, and further shows superior contact-pressure fatigue characteristics, and besides can be mass-produced. <P>SOLUTION: This manufacturing method includes the steps of: preparing a base steel material that has a composition comprising, by mass%, 0.1 to 0.35% C, 0.01 to 1.1% Si, 0.5 to 2.0% Mn, 0.5 to 2.5% Cr, 0.018% or less P, 0.02% or less S, 0.015 to 0.05% Al, 0.008 to 0.025% N, 0.002 to 0.02% Sb, 0.0015% or less O so as to satisfy following expressions (1) and (2), and the balance Fe with unavoidable impurities, and has a structure in which a maximum diameter of oxide-based non-metallic inclusions is 19 &mu;m or smaller; heating the material to 1,100 &deg;C or higher; then finishing rolling on conditions that the total rolling reduction is 70% or more and a rolling reduction in a temperature range of 800 to 950&deg;C is 30% or more; and cooling the rolled material at a rate of 0.1 to 1.0&deg;C/s in a temperature range of 800 to 500&deg;C. (1) 3.0&ge;ä([%Si]/2)+[%Mn]+[%Cr]}&ge;2.4; and (2) 2.5&ge;[%Al]/[%N]&ge;1.7. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for producing carburizing steel suitable for use in automobiles and various industrial equipment.

  In recent years, gears used in automobiles and the like have been required to be smaller in size as the weight of the vehicle body has been reduced due to energy saving, and the load on the gears has increased as the output of the engine has increased. Yes. The durability of the gear is mainly determined by the bending fatigue failure of the tooth root and the surface pressure fatigue failure of the tooth surface.

  Conventionally, gears have been manufactured by preparing gear materials using case-hardened steel defined by JIS SCM420H, SCM822H, etc., and subjecting the gear materials to surface treatment such as carburization. However, since such gears cannot withstand use under high stress, the tooth root bending fatigue strength and pitting resistance can be reduced by changing steel materials, heat treatment methods, surface work hardening, etc. I tried to improve the sex.

For example, in Patent Document 1, while reducing Si in steel and controlling Mn, Cr, Mo and Ni, the grain boundary oxide layer on the surface after carburizing heat treatment is reduced, and the occurrence of cracks is reduced. Moreover, by suppressing the generation of an incompletely hardened layer, the reduction of surface hardness is suppressed to increase the fatigue strength, and further Ca is added to control the elongation of MnS that promotes the generation and propagation of cracks. Is disclosed.
Patent Document 2 discloses a method for increasing the temper softening resistance using a steel material to which Si is added in an amount of 0.25 to 1.50%.
Japanese Patent Publication No. 07-122118 Japanese Patent No. 2945714

However, both of Patent Documents 1 and 2 described above have the following problems.
According to Patent Document 1, since the grain boundary oxide layer and the incompletely hardened layer are reduced by reducing Si, it is possible to suppress the occurrence of bending fatigue cracks at the tooth root. However, by simply reducing Si, the resistance to temper softening decreases, and the occurrence of fracture shifts from the root to the tooth surface. As a result, temper softening due to frictional heat on the tooth surface cannot be suppressed. Since the surface is softened, there is a problem that pitching is likely to occur.

  The present invention has been developed in view of the above circumstances, and is suitable as a material for high-strength gears, etc., which has a higher bending fatigue strength at the root than that of conventional gears, and is excellent in surface fatigue properties, and is also cooled. The object is to propose an advantageous manufacturing method of carburizing steel which is excellent in hot forging and can be mass-produced.

As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
a) By optimizing the amount of Si, Mn, and Cr in the steel material, the resistance to temper softening is increased, and if the softening due to heat generation at the gear contact surface is suppressed, the generation of cracks in the tooth surface that occurs during gear driving is suppressed. be able to.
b) For grain boundary oxide layers that can be the starting point of bending fatigue and fatigue cracks, by adding more than a certain amount of Si, Mn, Cr, the growth direction of the grain boundary oxide layer is changed from the depth direction to the surface density increasing direction. change. Therefore, since there is no oxide layer grown in the depth direction to be a starting point, it becomes difficult to be a starting point for bending fatigue and fatigue cracks.
c) As described in the above a and b, Si, Mn, and Cr are effective for improving the temper softening resistance and controlling the grain boundary oxide layer. In order to achieve both of these effects, Si, Mn, It is necessary to strictly control the content of Cr.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.1 to 0.35%, Si: 0.01 to 1.1%, Mn: 0.5 to 2.0%, Cr: 0.5 to 2.5%, P: 0.018% or less, S: 0.02% or less, Al: 0.015 to 0.05% , N: 0.008 to 0.025%, Sb: 0.002 to 0.02%, and O: 0.0015% or less within the range satisfying the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities In addition, after heating a steel material having a structure in which the maximum diameter of oxide-based nonmetallic inclusions is 19 μm or less to 1100 ° C. or higher, the total rolling reduction: 70% or more and the rolling reduction in the temperature range of 800 to 950 ° C .: A method for producing carburizing steel, characterized by cooling the temperature range of 800 to 500 ° C. at a rate of 0.1 to 1.0 ° C./s after rolling under conditions of 30% or more.
Record
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (2)
However, [% M] is the content of element M (% by mass)

2. In mass%, C: 0.1 to 0.35%, Si: 0.01 to 1.1%, Mn: 0.5 to 2.0%, Cr: 0.5 to 2.5%, P: 0.018% or less, S: 0.02% or less, Al: 0.015 to 0.05% , N: 0.008 to 0.025%, Sb: 0.002 to 0.02%, and O: 0.0015% or less within the range satisfying the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities In addition, after heating a steel material having a structure in which the maximum diameter of oxide-based nonmetallic inclusions is 19 μm or less to 1100 ° C. or higher, the total rolling reduction: 70% or more and the rolling reduction in the temperature range of 800 to 950 ° C .: After rolling under the condition of 30% or more, the 800-500 ° C temperature range is cooled at a rate of 0.1-1.0 ° C / s, and the heating temperature and hot rolling temperature , or the heating temperature and hot forging temperature are set. A method for producing a carburizing steel, characterized by cooling a temperature range of 800 to 500 ° C at a rate of 0.1 to 1.0 ° C / s after setting the temperature to 1050 to 800 ° C.
Record
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (2)
However, [% M] is the content of element M (% by mass)

  According to the present invention, for example, when carved into a gear, carburizing steel not only with the bending fatigue characteristics of the tooth root but also with the surface pressure fatigue characteristics of the tooth surface is mass-produced in a process involving cold forging. Can be obtained at

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel slab is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.1-0.35%
In order to increase the hardness of the central part by quenching after carburizing treatment, 0.1% or more of C is required, but if the content exceeds 0.35%, the toughness of the core part decreases, so the C content is 0.1 to 0.35% It was limited to the range. Preferably it is 0.1 to 0.3% of range.

Si: 0.01-1.1%
Si is an element that enhances the softening resistance in the temperature range of 200 to 300 ° C. that the gears and the like are supposed to reach during rolling, and at least 0.01% addition is indispensable in order to exert the effect. Preferably 0.03% or more is added. However, Si, on the other hand, is a ferrite stabilizing element, and excessive addition raises the Ac ₃ transformation point, and ferrite tends to appear in the core with a low carbon content in the normal quenching temperature range, resulting in a decrease in strength. Invite. Excessive addition also has the disadvantage of hardening the steel material before carburizing and degrading the cold forgeability. In this respect, when the Si content is 1.1% or less, the above-described adverse effects do not occur, so the Si content is limited to a range of 0.01 to 1.1%. Preferably it is 0.03 to 0.7% of range.

Mn: 0.5-2.0%
Mn is an element effective for hardenability, and requires addition of at least 0.5%. However, Mn tends to form an abnormal carburizing layer, and excessive addition causes an excessive amount of retained austenite and leads to a decrease in hardness, so the upper limit was made 2.0%. Preferably it is 0.5 to 1.5% of range.

Cr: 0.5-2.5%
Cr is an element effective for improving not only hardenability but also temper softening resistance. However, if the content is less than 0.5%, the effect of addition is poor. On the other hand, if it exceeds 2.5%, the effect of increasing softening resistance is The Cr content is limited to the range of 0.5 to 2.5% because it becomes saturated and rather easily forms a carburized abnormal layer.

P: 0.018% or less P is segregated at the grain boundary and lowers the toughness of the carburized layer and the core. Therefore, the lower the content, the better, but 0.018% is acceptable. Preferably it is 0.016% or less.

S: 0.02% or less S is an element that exists as sulfide inclusions and is effective in improving machinability. However, excessive addition causes a decrease in fatigue strength, so the upper limit was made 0.02%.

Al: 0.015-0.05%
Al is an element that combines with N to form AlN and contributes to the refinement of austenite crystal grains. To obtain this effect, 0.015% or more, preferably 0.02% or more is required. When the content exceeds 0.05%, Al ₂ O ₃ inclusions harmful to fatigue strength are promoted, so the Al content is limited to a range of 0.015 to 0.05%.

N: 0.008-0.025%
N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains. Therefore, the appropriate addition amount is determined by the quantitative balance with Al, but 0.008% or more of addition is necessary to exert the effect. However, if added in excess, bubbles are generated in the steel ingot during solidification and deterioration of forgeability is caused, so the upper limit is made 0.025%. Preferably it is 0.010 to 0.020% of range.

Sb: 0.002 to 0.02%
Sb has the effect of reducing the abnormal layer during carburization, so it must be added at least 0.002%, but the effect is saturated even if it exceeds 0.02%, so the amount of Sb is in the range of 0.002 to 0.02%. Limited to. Preferably it is 0.005 to 0.015% of range.

O: 0.0015% or less O is an element that exists as an oxide inclusion in steel and impairs fatigue strength. A lower value is preferable, but 0.0015% is acceptable.

Although the proper composition range of the basic component of the present invention has been described above, in the present invention, it is not sufficient that each element simply satisfies the above range. For Si, Mn, Cr, Al, and N, It is important to satisfy the relationship of equations (1) and (2).
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (2)
However, [% M] is the content of element M (% by mass)

The above formula (1) is a factor affecting the hardenability and temper softening resistance.If the formula (1) is less than 2.4, the improvement effect of the hardenability and temper softening resistance is not sufficient, while 3.0 If it exceeds, not only the above improvement effect is saturated, but also the workability is deteriorated.
The above formula (2) is a factor that affects the refinement of austenite crystal grains.If the value of formula (2) is less than 1.7, the effect of miniaturization is poor. In addition to coarsening, the workability is lowered due to the solid solution Al and N.

In the present invention, it is also important to control the size of oxide inclusions present in the material.
In other words, even if the amount of oxygen in the steel is reduced, if the maximum diameter of the oxide in the steel is large, improvement in surface fatigue strength cannot be expected, so the maximum diameter of oxide inclusions present in the material should be 19 μm or less. Restrict.
In order to adjust the size of the oxide inclusions within the above range, the RH degassing step is important, and the treatment time at this time is preferably 50 minutes or more.

Next, the manufacturing conditions of the present invention will be described.
In the present invention, the steel material having the preferred component composition described above is heated to 1100 ° C. or higher, and then rolled under the conditions of a total rolling reduction of 70% or more and a rolling reduction of 800% to 950 ° C .: 30% or more. After completion, the temperature range of 800 to 500 ° C. can be cooled at a rate of 0.1 to 1.0 ° C./s , and further the heating temperature and hot rolling temperature , or the heating temperature and hot forging temperature can be set to 1050 to after a 800 ° C., it is necessary to cool the temperature range of 800 to 500 ° C. at a rate of 0.1 to 1.0 ° C. / s.
Hereinafter, the reason why each processing condition is limited as described above will be described.

Steel material heating temperature: 1100 ° C or higher In the present invention, since it is necessary to sufficiently dissolve AlN before hot working, the steel material is heated to a temperature of 1100 ° C or higher prior to hot working. .

Total reduction ratio in hot working: 70% or more If the total reduction ratio is small, the grains become coarse, the ferrite fraction after cooling decreases, and not only does coarse grains easily form during carburizing, 70% or more because hardness increases.

Rolling ratio in the temperature range of 800 to 950 ° C: 30% or more If the rolling reduction ratio in this temperature range is less than 30%, the crystal grains become coarse, the ferrite fraction after cooling decreases, and is coarse during carburizing Not only is it easy to generate grains, but also the hardness of the rolled material is increased, so 30% or more.
In addition, this rolling reduction means the reduction rate of thickness when the steel material obtained by hot working is a plate, and the area reduction rate when it is a steel bar or wire.

Cooling rate in the temperature range of 500 to 800 ° C: 0.1 to 1.0 ° C / s
In the cooling process after hot working, if the cooling rate in the temperature range of 800 to 500 ° C is less than 0.1 ° C / s, the ferrite particle size becomes large, and the particle size during carburization also becomes coarse, while 1.0 ° C If it exceeds / s, the ferrite fraction after cooling will decrease and not only coarse grains will be generated easily during carburizing, but also the hardness of the rolled material will increase, so the cooling rate in this temperature range is 0.1 to 1.0 ° C Limited to / s range.

In the present invention, after the above cooling treatment, hot rolling or hot forging can be further performed. In that case, it is preferable to perform the following conditions.
Heating temperature and hot rolling temperature , or heating temperature and hot forging temperature: 1050 to 800 ° C
If the reheating temperature is too high, fine AlN cannot be obtained, and coarse grains are likely to be generated during carburizing. On the other hand, if the temperature is too low, ferrite and carbide remain and the structure of the rolled material becomes non-uniform, so the heating temperature is limited to the range of 1050 to 800 ° C. The same applies to hot rolling or hot forging.

Cooling rate in the temperature range of 800-500 ° C: 0.1-1.0 ° C / s
The cooling rate in this temperature range is limited to the range of 0.1 to 1.0 ° C./s as described above.
About the processing rate at the time of this reheating rolling or reheating forging, even if it carries out on what conditions, the effect of this invention is not lost.

  Steels having various component compositions shown in Table 1 were melted in a 100 kg vacuum melting furnace, and the slab was rolled under the hot rolling conditions and cooling conditions shown in Table 2 to obtain bar steel. In Table 2, when the second rolling condition is blank, it means that the finishing is performed by one heating rolling. In the case of performing the second rolling, the rolling reduction was 30% and the rolling temperature was 800 ° C. or higher.

The obtained steel bar was evaluated for cold workability, carburized part characteristics and fatigue characteristics under the following conditions.
(1) Evaluation method of cold workability Cold workability was evaluated by two items of deformation resistance and limit upsetting rate.
For deformation resistance, test pieces with a diameter of 10 mm and a height of 15 mm were taken from the 1 / 4D position of the rolled steel bar, and the compressive load when 70% upsetting was applied using a 300 t (9.8 kN) press. It measured and calculated | required using the deformation resistance measuring method by the end surface restraint compression which the Japan Society for Technology of Plasticity advocated.
The limit upsetting rate was defined as the upsetting rate when compression processing was performed by a method of measuring deformation resistance and a crack occurred at the end.
If the deformation resistance value is 918 MPa or less and the limit upsetting rate is 76% or more, it can be said that the cold workability is good.
(2) Evaluation method of carburized part characteristics The carburized part characteristics were evaluated by two items: presence or absence of coarse grains in the carburized part and grain boundary oxidation depth.
In the carburized portion, the case where no coarse particles were generated was marked with ◯, and the case where coarse particles were formed was marked with x.
Grain boundary oxidation behavior was evaluated by observing the surface of the test piece after carburizing treatment with an optical microscope and measuring the grain boundary oxidation depth. That is, an optical microscope observation was performed at a magnification of 400 times, the maximum grain boundary oxidation depth in each field of view was obtained, and the average value of 10 fields of view was defined as the grain boundary oxidation depth.
If there is no generation of coarse grains in the carburized part and the grain boundary oxidation depth is 10 μm or less, it can be said that the carburized part characteristics are excellent.
(3) Evaluation Method of Fatigue Properties Fatigue properties were evaluated by two items: a rotating bending fatigue test piece and surface fatigue strength.
That is, a rotating bending fatigue test piece and a roller pitching test piece for evaluating surface fatigue strength were processed from a rolled steel bar and subjected to the test. These test pieces were carburized under conditions of 930 ° C., 7 hours, carbon potential: 0.8%, and then subjected to heat tempering treatment at 180 ° C. for 1 hour.
The rotating bending fatigue test was carried out at a rotational speed of 1800 rpm and evaluated with a strength of 10 ⁷ times.
The roller pitching test was evaluated with a strength of 10 ⁷ times under the conditions of slip ratio: 40% and oil temperature: 80 ° C.
If the rotating bending fatigue strength is 860 MPa or more and the surface fatigue strength is 3120 MPa or more, the fatigue strength is good.
The obtained results are shown in Table 3.

  As shown in Table 3, all of the inventive examples obtained in accordance with the present invention are excellent in cold workability, shallow in grain boundary oxidation depth, and no coarse grains are generated in the carburized portion. It can be seen that the rotational bending fatigue strength and the contact pressure fatigue strength are superior.

Claims (2)

In mass%, C: 0.1 to 0.35%, Si: 0.01 to 1.1%, Mn: 0.5 to 2.0%, Cr: 0.5 to 2.5%, P: 0.018% or less, S: 0.02% or less, Al: 0.015 to 0.05% , N: 0.008 to 0.025%, Sb: 0.002 to 0.02%, and O: 0.0015% or less within the range satisfying the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities In addition, after heating a steel material having a structure in which the maximum diameter of oxide-based nonmetallic inclusions is 19 μm or less to 1100 ° C. or higher, the total rolling reduction: 70% or more and the rolling reduction in the temperature range of 800 to 950 ° C .: A method for producing carburizing steel, characterized by cooling the temperature range of 800 to 500 ° C. at a rate of 0.1 to 1.0 ° C./s after rolling under conditions of 30% or more.
Record
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (2)
However, [% M] is the content of element M (% by mass) In mass%, C: 0.1 to 0.35%, Si: 0.01 to 1.1%, Mn: 0.5 to 2.0%, Cr: 0.5 to 2.5%, P: 0.018% or less, S: 0.02% or less, Al: 0.015 to 0.05% , N: 0.008 to 0.025%, Sb: 0.002 to 0.02%, and O: 0.0015% or less within the range satisfying the following formulas (1) and (2), with the balance being the composition of Fe and inevitable impurities In addition, after heating a steel material having a structure in which the maximum diameter of oxide-based nonmetallic inclusions is 19 μm or less to 1100 ° C. or higher, the total rolling reduction: 70% or more and the rolling reduction in the temperature range of 800 to 950 ° C .: After rolling under the condition of 30% or more, the 800-500 ° C temperature range is cooled at a rate of 0.1-1.0 ° C / s, and the heating temperature and hot rolling temperature , or the heating temperature and hot forging temperature are set. A method for producing a carburizing steel, characterized by cooling a temperature range of 800 to 500 ° C at a rate of 0.1 to 1.0 ° C / s after setting the temperature to 1050 to 800 ° C.
Record
3.0 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.4 --- (1)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (2)
However, [% M] is the content of element M (% by mass)