JP2006118014A - Method for producing case hardening steel having excellent cold workability and crystal grain coarsening resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a case hardening steel having excellent cold workability in which, even when being subjected to working at a high reduction of area in wire drawing, coarsening of crystal grains after heat treatment is suppressed, and which is optimum for producing a bolt or the like. <P>SOLUTION: A steel having a composition satisfying, by mass, 0.10 to 0.25% C, ≤0.5% (exclusive of 0%) Si, 0.3 to 1.0% Mn, ≤0.03% (including 0%) P, ≤0.03% (including 0%) S, 0.3 to 1.5% Cr, 0.02 to 0.1% Al, 0.005 to 0.02% N, and the balance iron with inevitable impurities is used and is subjected to hot finish rolling or hot finish forging at 700 to <850°C, is thereafter cooled to 600°C at a cooling rate of ≤0.5°C/sec, and is successively made to cool to a room temperature, thus the reduction of area in wire drawing performed after that is suppressed to <20%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a case hardening steel that is preferably used for cold work parts such as bolts and gears, and in particular, a skin excellent in cold workability and grain coarsening resistance. It relates to a method useful for producing a steel for baking.

  Conventionally, cold-worked parts with complex shapes such as bolts and gears, for example, hot rolled or hot forged steel for case hardening, then softened and then drawn, and then into a predetermined shape It is obtained by cold working or cutting, and finally carburizing treatment (heat treatment) and hardening the surface, but in order to efficiently obtain steel parts such as bolts and gears that exhibit excellent characteristics, in the manufacturing process There are several points to be improved.

  In the carburizing process, the steel material is exposed to a high temperature for a long time, so that the crystal grains are likely to be coarsened, and there is a concern about the deterioration of the toughness of steel parts. In order to solve such problems, for example, in Patent Document 1, the Al amount and the N amount are defined, and the steel is heated to a predetermined temperature before hot rolling, and the rolling end temperature and the subsequent cooling rate are controlled. Thus, it has been shown that the structure of the rolled material may be a ferrite-pearlite structure.

  Patent Document 2 specifies the composition of the steel material to be used, specifies the final finish processing rate and final finish finish temperature in hot rolling, and further defines the cooling rate after hot rolling, thereby providing a post-rolling The case hardening steel has stable characteristics that have little variation in quality and do not generate abnormal structures such as coarsening of grains or mixed grains even in surface hardening at high temperatures such as carburizing or carbonitriding. Is shown to be obtained.

  In Patent Document 3 and Patent Document 4, in order to obtain a steel material that has a fine structure in the state of being hot-rolled and in which coarsening of crystal grains is suppressed even during subsequent carburizing, the component composition of the steel material to be used is It has been proposed to regulate and control the hot rolling conditions and the subsequent cooling rate.

  However, as proposed in these documents, only by specifying the component composition, the hot working and the subsequent cooling conditions, the amount of crystal grains depends on the processing amount when cold working into the part shape and the conditions of the subsequent heat treatment. Since coarsening may occur, it seems that further improvement is required to reliably suppress the coarsening of crystal grains.

  In addition, in the conventional method, softening treatment is often performed before molding the part, but this reduces deformation resistance during cold working to reduce the cost of the mold and suppresses the coarsening of crystal grains. The purpose is to do. However, in recent years, it has been required to omit this softening process from the viewpoint of cost reduction.

Patent Document 1 discloses that the heat treatment before cold working can be omitted or simplified and cold working can be performed even while rolling. As a specific method, the rolling temperature is lowered and the rolling structure is refined. Has been shown to do. However, in the method of Patent Document 1, it is difficult to say that the rolling temperature is sufficiently low, and it seems that excellent cold workability that can be cold worked as it is after wire drawing is not realized. In addition, the above Patent Documents 2 to 4 do not take into account the securing of cold workability in addition to the suppression of crystal grain coarsening.
Japanese Patent Laid-Open No. 2-43319 JP-A-56-9326 JP-A-5-163525 JP-A-10-152754

  This invention is made | formed in view of the said situation, The objective is to provide the manufacturing method of the steel for case hardening excellent in cold work property and a crystal grain coarsening characteristic.

The method for producing a case hardening steel excellent in cold workability and crystal grain coarsening characteristics according to the present invention,
C: 0.10 to 0.25% (meaning mass%),
Si: 0.5% or less (excluding 0%),
Mn: 0.3 to 1.0%
P: 0.03% or less (including 0%),
S: 0.03% or less (including 0%),
Cr: 0.3 to 1.5%,
Al: 0.02 to 0.1%,
N: 0.005 to 0.02%
The steel is made of iron and inevitable impurities, and the hot finish rolling or hot finish forging is performed at a temperature below 700 to 850 ° C., and then cooling to 600 ° C. is performed at 0.5 ° C./sec. It is characterized in that it is carried out at the following cooling rate, subsequently allowed to cool to room temperature, and then the area reduction rate of the subsequent wire drawing is suppressed to less than 20%.

  One or more selected from the group consisting of Mo: 0.05 to 0.5% and / or Ni: 0.05 to 1.2%, and V, Nb, Ti and W as the steel material In addition, a steel material containing 0.5% or less in total and B: 0.0005 to 0.003% may be used.

  If steel parts for machine structures such as bolts and gears are manufactured using the steel for case hardening obtained by the manufacturing method of the present invention, the softening treatment performed before cold working (part forming) is omitted. As a result, it is possible to reduce the cost, and even when heat treatment such as carburizing is performed at a high temperature after forming the part into a complicated shape, a steel part is obtained in which coarsening of crystal grains is suppressed and toughness is ensured. be able to.

  The present inventors can perform cold working well without increasing the deformation resistance during cold working without performing softening treatment before cold working into a component shape, and have a complex shape steel. Even when heat treatment such as carburizing is performed at high temperature after the parts are cold-formed, the coarsening of the crystal grains is suppressed over the entire area (hereinafter, such characteristics are referred to as “crystal grain coarsening characteristics”). The method for producing the case hardening steel, which is the raw material, was studied so that steel parts could be obtained.

  The inventors first investigated the relationship between the wire drawing area reduction rate (degree of processing) and crystal grain coarsening by heat treatment, and confirmed that the grain coarsening becomes significant when the wire drawing area reduction rate is close to zero. did. Further, it was confirmed that the grain coarsening becomes remarkable in proportion to the wire drawing area reduction rate even when the wire drawing area reduction rate exceeds a certain value.

  Therefore, based on this tendency, we examined a method that can suppress the coarsening of crystal grains and can perform a good cold work without increasing the deformation resistance during the cold work. As well as controlling the chemical composition of the steel material used, the heating conditions during hot rolling or hot forging and the subsequent cooling rate are controlled as manufacturing conditions, and further the wire drawing performed after hot rolling or hot forging is controlled. It was found that the area reduction rate should be controlled.

  First, the reason why the chemical composition of the steel material used in the present invention is specified will be described in detail.

<C: 0.10 to 0.25%>
C is an element necessary for securing the hardenability and strength of the steel, and is contained in an amount of 0.10% or more in the present invention. Preferably it is 0.13% or more, More preferably, it is 0.15% or more. However, when the C content is excessive, not only the toughness of the steel material is lowered but also the cold workability is deteriorated, so that the softening process cannot be simplified or omitted. Therefore, the upper limit of the C amount is 0.25%. Preferably it is 0.2% or less, More preferably, it is 0.18% or less.

<Si: 0.5% or less (excluding 0%)>
Si is an element that acts as a deoxidizer, and may be contained in an amount of 0.15% or more. However, if the amount of Si is excessive, the cold workability tends to decrease, so the upper limit of Si is 0.5. %. Preferably, it is 0.3% or less, more preferably 0.2% or less, and if it is important to ensure cold workability superior to the deoxidation effect by Si, it is further suppressed to 0.1% or less. preferable.

<Mn: 0.3 to 1.0%>
Mn is a hardenability improving element and is an extremely useful element for increasing the strength. Therefore, it is contained in an amount of 0.3% or more. Preferably it is 0.4% or more, More preferably, it is 0.5% or more, More preferably, it is 0.6% or more. However, if Mn is contained in a large amount, the transformation is promoted during cooling after hot rolling, the cold workability deteriorates, and the softening process cannot be simplified or omitted. Therefore, the Mn content is 1.0% or less. Preferably it is 0.85% or less, More preferably, it is 0.8% or less, More preferably, it is 0.7% or less.

<P: 0.03% or less (including 0%)>
P is an element that causes grain boundary segregation and degrades the toughness of steel. Therefore, it is desirable to reduce as much as possible. In the present invention, the P content is suppressed to 0.03% or less. Preferably it is 0.025% or less, more preferably 0.02% or less, still more preferably 0.015% or less, and particularly preferably 0.01% or less.

<S: 0.03% or less (including 0%)>
S is an element that adversely affects the deformability during cold working by forming MnS in steel. Therefore, in the present invention, the amount of S is suppressed to 0.03% or less. Preferably it is 0.025% or less, More preferably, it is 0.02% or less, More preferably, it is 0.015% or less, Most preferably, it restrains to 0.01% or less.

<Cr: 0.3 to 1.5%>
Cr is an element useful for increasing hardenability and achieving high strength, and since such an effect can be exhibited while ensuring cold forgeability (particularly deformability), the present invention. Then, it is made to contain 0.3% or more. Preferably it is 0.7% or more, More preferably, it is 0.85% or more. However, if excessively contained, carbides in the steel are stabilized and the cold workability is lowered, so the upper limit of Cr content is set to 1.5%. Preferably it is 1.1% or less, More preferably, it is 1.0% or less.

<Al: 0.02-0.1%>
Al combines with N to form AlN and exhibits the effect of refining crystal grains. In addition, the refinement of crystal grains contributes to improved delayed fracture resistance. In order to sufficiently exhibit such an effect, it is necessary to contain Al by 0.02% or more. Preferably it is 0.03% or more, More preferably, it is 0.035% or more. However, if it is excessively contained, oxide inclusions are generated and the toughness of the steel material is lowered, so the upper limit of the Al content is set to 0.1%. Preferably it is 0.05% or less, More preferably, it is 0.045% or less.

<N: 0.005 to 0.02%>
N combines with Al as described above to form AlN and exhibits the effect of refining crystal grains. In order to exhibit such an effect, it is necessary to contain N 0.005% or more. Preferably it is 0.007% or more, More preferably, it is 0.01% or more. However, when the amount of N becomes excessive, cracks occur on the surface during continuous casting or ingot rolling, so the upper limit was set to 0.02%. Preferably it is 0.016% or less.

  The elements defined in the present invention are as described above, and the remaining component is substantially Fe, but inevitable impurities brought into the steel material depending on the situation of raw materials, materials, manufacturing equipment, etc. in addition to those described above Furthermore, in order to impart further characteristics within a range that does not adversely affect the achievement of the object of the present invention, the present invention also includes Mo, Ni, V, Nb, Ti, W, and B as described below. Included in the steel used.

<Mo: 0.05 to 0.5% and / or Ni: 0.05 to 1.2%>
Mo is an element useful for improving the hardenability, and the effect is effectively exhibited by containing 0.05% or more, more preferably 0.1% or more. In order to further improve the hardenability, the Mo content is more preferably 0.35% or more, and particularly preferably 0.4% or more. However, if a large amount of Mo is contained, fine carbides are precipitated and the cold workability is remarkably lowered. Therefore, the Mo amount is preferably suppressed to 0.5% or less. In the case where emphasis is placed on ensuring cold workability even better than securing hardenability with Mo, the Mo content is more preferably 0.3% or less, and even more preferably 0.25% or less.

  Ni is also an element useful for improving the hardenability, and the effect is effectively exhibited by containing 0.05% or more. Preferably it is 0.1% or more, More preferably, it is 0.35% or more, More preferably, it is 0.4% or more. However, if Ni is contained in a large amount, the cold workability deteriorates, so the upper limit is preferably made 1.2%. More preferably, it is 0.8% or less, More preferably, it is 0.75% or less, Most preferably, it is 0.7% or less.

<One or more selected from the group consisting of V, Nb, Ti and W:
0.5% or less in total (excluding 0%)>
V, Nb, Ti, and W are effective elements for forming fine nitrides, carbides, and carbonitrides to refine crystal grains. In order to exert such an effect, it is preferable to contain 0.02% or more in total of one or more selected from the group consisting of V, Nb, Ti and W, more preferably 0.05% in total. That's it. However, if it is excessively contained, cold workability is impaired, so even if these elements are contained, it is preferable to keep the total to 0.5% or less, more preferably 0.2% or less. More preferably, it is 0.15% or less.

<B: 0.0005 to 0.003%>
B is a useful element that can improve the hardenability of steel during heat treatment without increasing the strength of the rolled material. In order to sufficiently exhibit such an effect, 0.0005% or more of B is preferably added, and more preferably 0.001% or more. However, excessive addition of B inhibits toughness, so it is limited to 0.003% or less. Preferably it is 0.0025% or less.

  In the present invention, by using a steel material satisfying the above component composition and producing a steel for case hardening so as to satisfy the following conditions, it is possible to combine cold workability and grain coarsening resistance.

<Finishing (rolling or forging) temperature during hot rolling or hot forging: 700 to less than 850 ° C.>
The finishing (rolling or forging) temperature during hot rolling or hot forging is set to be 700 ° C to less than 850 ° C. If the temperature is too low, the deformation resistance at the time of hot rolling or hot forging increases, the processing load increases, and it becomes impossible to roll or forge into an appropriate shape. Preferably it is 750 degreeC or more, More preferably, it is 800 degreeC or more. On the other hand, if the temperature is too high, AlN is dissolved, and the crystal grain size is increased during the subsequent heat treatment (carburizing), so the temperature is kept below 850 ° C. Preferably it is 820 degrees C or less.

<Cooling rate to 600 ° C. after hot rolling or hot forging: 0.5 ° C./sec or less>
When the cooling rate after hot rolling or hot forging is increased, a bainite or martensite structure is partially formed and cold workability is lowered. Therefore, in the present invention, the cooling rate to 600 ° C. is gradually cooled to 0.5 ° C./sec or less. The cooling is preferably 0.3 ° C./sec or less, more preferably 0.2 ° C./sec or less, and particularly preferably 0.1 ° C./sec or less. From the viewpoint of productivity, it is desirable to cool at 0.2 ° C./sec or more.

<Drawing after hot rolling or hot forging: Drawing area reduction rate of less than 20%>
The wire drawing performed after hot rolling or hot forging and before component forming is performed in a range of wire drawing area reduction ratio of less than 20%. If the wire drawing area reduction ratio is higher than this, the deformation resistance becomes high during cold working, and not only can cold working not be performed well, but also the crystal grains during the subsequent heat treatment at locations where the strain caused by cold working is high. This is because remarkably coarsening occurs. Preferably, the wire drawing should be performed while keeping the wire drawing area reduction rate to 15% or less, more preferably 10% or less. If the drawing area reduction is extremely low, the die will come into contact with each other at the time of drawing and it will not become a perfect circle. Therefore, the drawing is performed so that the area reduction is preferably 5% or more, more preferably 7% or more. Processing should be done.

  The steel for case hardening of the present invention is obtained by performing hot rolling as described above and then performing wire drawing at the above-mentioned area reduction rate, but does not prescribe up to production conditions other than the above, General manufacturing conditions can be adopted for melting and casting. In addition, when manufacturing complex shaped parts, it is recommended to perform softening treatment after hot rolling or hot forging and before wire drawing, but in this case as well, softening treatment may be performed under general conditions, Specifically, it may be gradually cooled at 10 to 30 ° C./hr after heating and holding at 600 to 800 ° C. for 1 to 10 hours.

  To obtain steel parts, use the case-hardening steel and perform cold working to obtain a predetermined part shape, but omit the softening process between wire drawing and cold working as described above. In addition, cold working can be satisfactorily performed without particularly limiting the working conditions. After the part molding, it is subjected to surface hardening treatment such as carburizing and nitriding, but the treatment conditions at that time are not limited, for example, about 2 at about 850 to 950 ° C. in a general carburizing atmosphere or nitriding atmosphere. Even when the surface is hardened by holding for ˜6 hours, the grain coarsening resistance is exhibited, and a steel part having high strength and excellent toughness can be obtained.

  The steel for case hardening obtained by the production method of the present invention can be obtained in various sizes such as a linear shape or a rod shape. The obtained case hardening steel can be used in the manufacture of various machine structural parts. For example, in addition to automobile parts such as bolts and gears, other automobiles such as gears, sliding parts, shafts, bearings, etc. It can also be applied to steel parts in industrial parts, construction machinery, industrial machinery and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Using test steels having the chemical composition shown in Table 1, after rolling to φ15 mm under the rolling conditions shown in Table 2, as shown in Table 2, the first drawing was performed while changing the drawing area reduction ratio. A wire drawing material having a diameter of φ13.0 to φ14.3 mm was obtained. And cold workability was evaluated using these wire drawing materials.

Evaluation of cold workability was performed as follows. That is, the ratio of the wire diameter (D) to the height (H): a test piece having a shape with an H / D of 1.5 was cut out (for example, φ14.3 mm × H21.45 mm), and the reduction rate was measured in the end face constrained compression test. The deformation resistance when 80% compression processing was performed was determined. And it evaluated that the case where this deformation resistance was 750 N / mm < ² > or less was excellent in cold workability.

The evaluation of the crystal grain coarsening resistance was performed only for those having excellent cold workability with the deformation resistance of 750 N / mm ² or less. Using the drawn wire material of φ13.0 to φ14.3 mm that has been drawn, the part forming process was simulated, and the drawing area reduction rate was 0% (as steel wire), about 10%, about 25%, about 50 % Or about 75% for the second time, and after heating for 3 hours at 880 ° C. assuming a heat treatment and water cooling, the size of the crystal grains is measured to determine the degree of grain coarsening. Asked. In addition, assuming a heat treatment at a high temperature, the size of the crystal grains was measured by measuring the size of the crystal grains when heated at 930 ° C. for 3 hours and then cooled with water. Specifically, as the degree of crystal grain coarsening (crystal grain coarsening rate), the crystal grain size in the wire drawing section is No. The area ratio (%) of crystal grains that were less than 5 was determined. And when the crystal grain coarsening rate was all 5% or less when it was drawn at various wire drawing processing rates, it was evaluated that the crystal grain coarsening characteristics were excellent. These results are also shown in Table 2. In this example, the effect on the properties was investigated by changing the finish rolling temperature during hot rolling, but also when hot forging was performed by changing the forging finishing temperature, The same tendency as in the case of.

  Table 1 and Table 2 can be considered as follows (note that the following No. indicates the experiment No. in Table 2). No. 1 to 3 and 12 to 21 produced steel so as to satisfy the requirements specified in the present invention, so that the obtained steel has low deformation resistance during cold working and does not undergo softening treatment after hot working. Both can be cold worked well. Even when the second wire drawing is performed at a high area reduction rate, coarsening of the crystal grains is suppressed not only when the heat treatment is performed at 880 ° C. but also when the heat treatment is performed at a high temperature of 930 ° C.

  In contrast, no. Since Nos. 4 to 11 and 22 to 24 do not satisfy any of the requirements defined in the present invention, the obtained steel is inferior in cold workability or coarsened crystal grains.

  That is, no. In 4, 5, 22, and 23, the first wire drawing area reduction rate exceeded the specified upper limit, so that cold workability was inferior or crystal grains were coarsened. In particular, no. 4 shows that when heat treatment is performed at 880 ° C., coarsening of crystal grains is suppressed regardless of the area reduction rate of the second wire drawing, whereas when heat treatment is performed at 930 ° C., the second wire drawing is performed. When the surface area reduction ratio was high, coarsening of the crystal grains was confirmed.

  No. Nos. 6, 7, and 9 had a high finish rolling temperature during hot rolling and a high cooling rate after rolling, and thus were inferior in cold workability or coarsened crystal grains. In particular, no. 9 shows that when heat treatment is performed at 880 ° C., coarsening of the crystal grains is suppressed regardless of the area reduction ratio of the second wire drawing, whereas when heat treatment is performed at 930 ° C., the coarsening of the crystal grains is suppressed. You can see it happening. No. No. 8 is inferior in cold workability because the cooling rate after rolling is high.

  No. Nos. 10, 11 and 24 have problems because the steel materials used do not satisfy the component composition defined in the present invention. No. No. 10 has insufficient N amount. Since No. 11 has an insufficient amount of Al, the crystal grains are becoming coarser. In particular, no. Nos. 10 and 11 show that when heat treatment is performed at 880 ° C., coarsening of crystal grains occurs only when the area reduction of the second wire drawing is as high as 75%, whereas when heat treatment is performed at 930 ° C. In the case where the area reduction ratio of the second wire drawing is 0%, 10% or 50%, the crystal grains are coarsened. No. It can be seen that No. 24 has an excessive amount of C, and therefore has high deformation resistance during processing and poor cold workability.

Even when a steel material satisfying the component composition defined in the present invention is used, when the finishing temperature in hot rolling or hot forging is extremely low as less than 700 ° C., during hot rolling or hot forging. The deformation resistance increased, making machining difficult.

Claims (4)

% By mass (the same applies below)
C: 0.10 to 0.25%,
Si: 0.5% or less (excluding 0%),
Mn: 0.3 to 1.0%
P: 0.03% or less (including 0%),
S: 0.03% or less (including 0%),
Cr: 0.3 to 1.5%,
Al: 0.02 to 0.1%,
N: 0.005 to 0.02%
The steel is made of iron and inevitable impurities, and the hot finish rolling or hot finish forging is performed at a temperature below 700 to 850 ° C., and then cooling to 600 ° C. is performed at 0.5 ° C./sec. Skin with excellent cold workability and grain coarsening resistance, characterized by the following cooling rate, followed by cooling to room temperature, and then reducing the area reduction rate of wire drawing performed to less than 20% A method of manufacturing steel for baking. As other ingredients,
Mo: 0.05-0.5% and / or Ni: 0.05-1.2%
The manufacturing method of the steel for case hardening of Claim 1 using the steel materials containing this. As other ingredients,
The manufacturing method of the steel for case hardening of Claim 1 or 2 using the steel materials which contain 1 or more types selected from the group which consists of V, Nb, Ti, and W in total 0.5% or less. As other ingredients,
B: The manufacturing method of the steel for case hardening in any one of Claims 1-3 using the steel materials containing 0.0005 to 0.003%.