JPH08291362A - Steel with excellent cold workability - Google Patents

Abstract

(57) [Summary] (Corrected) [Purpose] A steel material with excellent cold workability, which is used for various machinery, automobile parts, etc. by cold working such as forging and rolling, and heat treatment. And a method of manufacturing the same. [Structure] C, Si, Mn, SoL. AL, B, N, N
In the steels in which i and Ca are specified, the structure consists of ferrite, spheroidized cementite and graphite particles, and 20 to 75% of the C content is graphitized, and there is no graphite particle with a diameter of more than 20 μm in the cross-section observation. , And diameter 3 to 20 μm
Particles of C weight% x 100 / mm ² or more, C weight% x 1000
Steel materials with excellent workability that are distributed at a frequency of less than 1 piece / mm ² . The manufacturing method is to complete processing in the range of 800 to 1000 ° C, cool to 500 ° C or less at a cooling rate of 10 to 100 ° C / min, and then anneal at 600 to 720 ° C for 4 to 50 hours. Or, after hot working, cold working with a working ratio of 3 to 40% and annealing at 600 to 720 ° C for 1 to 24 hours.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a steel which is used as a component of various machines, automobiles, etc. by cold working such as drawing, forging, rolling, etc., and heat-treated. For improved steel.

[0002]

2. Description of the Related Art The processing of a steel material that is cold forged or rolled is a highly vigorous processing method, although it has higher production efficiency, higher material yield and higher dimensional accuracy than cutting. For this reason, while high performance of tools and lubrication is strictly required, it is also necessary that the workability of the work material is small in deformation resistance and large in deformability. In general, steel has increased deformation resistance and deteriorated deformability as the carbon content increases. Therefore, such cold working has been mostly applied to steel materials having a relatively low carbon content of 0.25% C or less.

In steels for which high strength is to be obtained by heat treatment such as quenching and tempering after working, the carbon content is usually 0.30% or more. In that case, spheroidizing annealing before working is performed to ensure cold workability, but the carbon content of the steel material still used is up to about 0.45% C, and if the carbon content is higher than that, cold forging or the like is not applicable. Have difficulty.

For applications requiring wear resistance such as gears, not only high hardness but also high C content is required, and cold forging and rolling are used for forming the low carbon steel material. When applied, it is carburized after processing to increase the amount of carbon in the contact portion and improve wear resistance and fatigue resistance. However, since the carburizing method is a heat treatment that requires a long time at a high temperature, the productivity is poor, and distortion due to quenching after carburizing is likely to occur. A surface hardening method for improving wear resistance similar to carburizing,
There are induction hardening method and flame hardening method, and the treatment can be carried out efficiently in a short time, but a certain amount of C is required in the base material to obtain the required hardness and wear resistance.

For such applications, graphitized steel has attracted attention as a steel excellent in cold workability even with a higher C content. This is a steel that if carbon in steel is graphitized instead of cementite, the strength will be further reduced and workability will be improved even if the same amount of C is subjected to spheroidizing annealing of cementite. For example, Japanese Patent Application Laid-Open No. 3-146618 proposes an invention in which deformation resistance and deformability are improved.

Graphitization of cementite certainly improves the cold workability even in high carbon steel, but in order to obtain the necessary quenching hardness in the heat treatment after working, C is added when heated to the austenite region. It must be sufficiently re-dissolved. In the case of conventional steel in which C exists in the form of cementite, which is a compound with iron, C easily re-dissolves, whereas
If graphite is used, re-dissolution requires heating for a longer period of time, and there is a problem that sufficient quenching hardness cannot be obtained especially by short-time heating such as induction hardening.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a steel for machine structural use which is excellent in cold workability such as cold forging and rolling, and also has good heat-treatability of short-time heating such as induction hardening, and its production. Concerning the law.

If a working method such as cold forging, which has been frequently used for low carbon steel, can be applied to a steel having a higher carbon, which is conventionally formed mainly by a cutting method, a significant improvement in yield can be achieved. is there. Alternatively, it is also possible to rationalize the manufacturing method in which low carbon steel is cold worked and then carburized for surface hardening to a method in which high carbon steel is cold worked and induction hardened. Parts such as automobile gears, for example, are usually carburized after gear cutting, but the cutting can be changed to cold forging and the surface hardening can be changed from carburizing and induction hardening to obtain products of the same performance.

[0009]

[Means for Solving the Problems] To this end, the present inventors have made various studies on graphitization of carbon in steel, which may have sufficient cold workability in high carbon steel. It was As a result, it was found that the hardness decreases as the graphitization progresses, but the deformability deteriorates if it progresses excessively. If the precipitated graphite particles are too small, they are not very effective in decreasing the hardness, and if too large, they become the starting point of cracking during deformation. Therefore, in order to improve cold workability, it is necessary to graphitize a specific amount of C in steel and then control the particle size distribution of precipitated graphite.

Further, as a heat treatment after working, induction hardening was examined, and it was found that sufficient hardness could not be obtained when excessive graphitization or coarsening of graphite particles was caused, as compared with the hardening hardness due to short-time heating. It was found that proper control of graphitization is important from the viewpoint of solid solution of carbon. And
In order to obtain a steel material having a graphitized structure morphology suitable for such cold working and heat treatment, the present invention has been completed by knowing that the control of components and manufacturing conditions is important, but the gist thereof is It is as follows.

(1) C: 0.30 to 0.70% by weight, S
i: 0.20 to 0.60%, Mn: 0.05 to 0.60%, sol.Al: 0.
05 to 0.50%, B: 0.0005 to 0.0050%, N: 0.002 to 0.01
0%, Ni: 2.00% or less, and Ca: 0.01% or less, the balance being substantially Fe and unavoidable impurities, and the steel structure consisting of ferrite, spheroidized cementite and graphite particles, and having a C content of 20-75% is graphitized, and there are no graphite particles with a diameter of more than 20 μm in the cross-section observation.
And 100 particles having a diameter of 3~20μm is C wt% × / mm ² or more, formability excellent steel which is characterized in that distributed in C wt% × 1000 pieces / mm ² or less frequently.

As the manufacturing method thereof, (2) when the steel having the composition shown in (1) above is heated and hot-worked, the working is completed within the range of 800 to 1000 ° C.
After cooling to below 500 ° C at a cooling rate of ° C / min,
The method for producing a steel product according to (1) above, which comprises performing annealing at 00 to 720 ° C for 4 to 50 hours.

Or (3) hot working the steel having the composition shown in (1) above, and then cold working at a working rate of 3 to 40%, at 600 to 720 ° C.
The method for producing a steel material according to the above (1), characterized in that annealing is performed for 1 to 24 hours.

The degree of graphitization specified in the present invention is
It can be determined by a chemical analysis in which graphite is extracted by acid dissolution, and the particle size and distribution of graphite particles can be measured by observing a cross section after polishing with an optical microscope or a scanning electron microscope.

[0015]

In the present invention, the reason for limiting the range of graphitization and distribution of graphite particles in steel, the component composition, and the manufacturing conditions thereof is as follows.

(1) Graphitization and Graphite Particles Graphitization of cementite causes a remarkable decrease in softening and deformation resistance because the amount of graphite is 1/5 of the total amount of carbon contained in steel, that is, 20%. It is beyond. As the graphitization progresses further, the hardness and deformation resistance decrease, but when the deformability was examined, graphite contained 3/4 of the total carbon,
That is, if it exceeds 75%, it deteriorates. Especially for this deformability, the distribution of graphite particles also has an effect, and even if the graphitization ratio is 20 to 75% of the total carbon amount, when large graphite particles are present or the number is too large. Deformability decreases.

Even if there are many small graphite particles, that is, particles having a size of less than 3 μm, the deformation resistance does not decrease significantly, so it is necessary to pay attention to graphite particles having a size of 3 μm or more. It is not clear whether the size or number of these graphite particles directly affects the cold workability, but during the annealing process for graphitization, spheroidization of precipitated cementite is also progressing at the same time, so graphite particles of 3 μm or more The existence of a certain amount or more indicates that the spheroidization is sufficiently advanced. On the other hand, when large graphite particles appear,
Although the deformation resistance is low, the deformability is deteriorated, and when the size exceeding 20 μm is observed, remarkable deterioration is observed. When the graphite particles are too large or too large as described above, the cold workability, especially the deformability is deteriorated because the frequency of occurrence of cracks originating from the graphite particles increases.

When the relationship between the graphite particles and the cold workability was investigated, the diameter of the precipitated graphite particles was 3 μm by observing the section with a microscope.
C% x 100 pieces / mm ² for sizes between m and 20 μm
To C% × 1000 pieces / mm ² , the deformation resistance was low and good deformability was exhibited.

Next, regarding the quenching hardness in the case of short-time heating such as induction hardening, when the graphitization proceeds too much, insufficient hardness becomes conspicuous. Further, even if the degree of graphitization is the same, quenching hardness becomes insufficient when the number of large particles increases. The graphitized state in which the quenching hardness is insufficient and the graphitized state in which the deformability is deteriorated are almost common, and the degree of graphitization and the distribution of the graphite particles in the range in which the deformability is not deteriorated In this case, quenching hardness will not be insufficient.

As described above, in a steel material having excellent cold workability such as cold forging and rolling, and having good heat treatability by short-time heating such as induction hardening, graphitization of C in steel As a state, 20 to 75% of the total C content in the steel is graphitized, and no microscopic observation of the cross section shows graphite particles with a diameter of 20 μm or more, and particles with a diameter of 3 to 20 μm are C weight%.
It is regulated that the distribution is × 100 pieces / mm ² or more and C weight% × 1000 pieces / mm ² or less.

Next, the action of each component and the reason for limiting the range will be described.

(2) C content The lower the C content is, the better it is, considering only the workability, but at least 0.30% or more is necessary to adjust the strength to the required strength by heat treatment such as quenching and tempering after working. Further, considering the easiness of graphitization and the case where the wear resistance of the final product is required, the larger the content of C, the better. However, even if the graphitization is sufficiently performed, it is preferable that the workability be low in C, and as will be described later, the addition of B compensates for the decrease in the hardenability due to the decrease in Mn. Then, the effect of improving the hardenability of B is lost, so the upper limit is made 0.70%. That is, the C content is limited to 0.30 to 0.70%.

(3) Si content Since it is a component that promotes graphitization, it is preferable that it is present in a certain amount or more, but if the amount increases, the base becomes hard and the deformation resistance increases. Therefore, the content range is 0.20 to 0.60%.

(4) Mn content In order to prevent hot embrittlement due to S and to secure hardenability, it is an essential element to add to steel, but since it hinders graphitization, it should be kept as low as possible. If it is less than 0.05%, the damage of S cannot be prevented, and if it exceeds 0.60%, graphitization is significantly hindered, so the content is limited to 0.05% to 0.60%.

(5) sol.Al amount It is necessary as a deoxidizing agent in casting, and 0.01% or more is usually added to prevent surface flaws of a high carbon steel slab. Moreover,
Since it has an effect of promoting graphitization, it is added in a larger amount than the usual deoxidizing agent in the present invention, and the content is made 0.05% or more. However, if the amount is too large, the base material becomes hard and the toughness of the product after heat treatment deteriorates, so the upper limit is 0.50%. That is, the range of the content of sol.Al (acid-soluble Al)
0.05 to 0.50%.

(6) B content Since Mn cannot be added in a large amount due to inhibition of graphitization, it is added to ensure hardenability. Further, the addition of B also has the effect of promoting graphitization. If the addition amount is too small, there is no effect, and if the addition amount is too large, the effect is saturated.
Limited to 05-0.0050%.

(7) N content N is an impurity element that is unavoidably contained in steel and has the effect of workability hindrance and deterioration of toughness. Since it coexists with Al and has an effect of preventing crystal grain coarsening, it is contained in the range of 0.002 to 0.010%.

(8) P and S amounts These are unavoidable impurity elements and hinder graphitization, so the smaller the better. As a limit at which the effect of inhibition is not significant, 0.02% is acceptable in all cases, but 0.005% or less is desirable in each case.

(9) Ni content It is effective in promoting graphitization and improving hardenability.
If it is not added sufficiently, the effect is not remarkable and it is expensive, so it is not necessary to add it. Since the base material is not hardened as much as Si, it is added as necessary when the hardenability is insufficient due to the shape of the molded part for the final use. When added, it is desirable to contain 0.1% or more, but if the content increases, it becomes harder, so the content is limited to 2.0%.

(10) The amount of Ca does not have to be added, but it has the effect of promoting graphitization and changes the form of the sulfide to improve the workability and toughness, so depending on the situation of the part for the final use . When added, the lower limit content is preferably 0.001% or more. However, if it exceeds 0.01%, inclusions increase, so the content is
Limited to 0.01% or less.

Next, the process conditions for producing the steel of the present invention will be described.

(11) Graphitization after hot rolling A method for obtaining a steel having a steel structure, graphitization of contained carbon and distribution of graphite particles within a predetermined range by using a raw material steel having the components specified in the present invention One is graphitization after hot working.

It is performed by heating a raw material steel having a prescribed composition and performing hot rolling to obtain a desired shape, finishing in a temperature range of 800 to 1000 ° C., and cooling at a cooling rate of 10 to 100 ° C./min.
After cooling to below ℃, it is a method of annealing at 600 to 720 ℃ for 4 to 50 hours.

The heating temperature of the material steel is not particularly limited as long as it is a temperature suitable for hot working. However, if it is too low, the deformation resistance during hot working is large, and if it is too high, there is a risk of decarburization, so 950 to 1200 ° C is desirable. The temperature for hot working is
In order to facilitate the subsequent graphitization and disperse the graphite particles in the range defined by the present invention, a lower value is preferable, but if it is too low, the deformation resistance increases and transformation starts, so the temperature range is 800 to 1000 ° C. And After processing, in order to accelerate graphitization and bring the graphite particles into a preferable dispersion state, the material is cooled to a temperature of 500 ° C or lower at a cooling rate of 10 ° C / min or more. In that case, if cooling is too fast, martensitic transformation or bainite transformation will occur and harden more than necessary.
Cool down to 500 ° C or less at a cooling rate of 00 ° C / min.

After cooling, annealing for graphitization is performed.
If the temperature is too low, graphitization does not proceed easily, and above the transformation point, the transformation precedes the graphitization, so 600-720
℃. If the time is short, the graphitization is insufficient and the existence of graphite particles of the desired size is insufficient, and if it is too long, the required graphitization range is exceeded, and coarse graphite particles are likely to be formed. 50 hours.

(12) Graphitization after cold working: The steel material of the present invention is manufactured by hot rolling to a predetermined size using a steel billet having a prescribed composition, and after cold working such as cold drawing, graphitization. It is also possible to perform annealing for.

The method is as follows. After hot working a raw material steel with specified components, cold working at 3 to 40% to obtain a desired shape, and annealing at 600 to 720 ° C for 1 to 24 hours. It is intended to produce steel having graphitization and graphite particle distribution defined in the invention.

The hot working conditions are not particularly limited as long as they have a ferrite / pearlite structure that allows cold working after cooling. In cold working, cementite with a pearlite structure is finely divided by plastic deformation of the base material,
It has the effect of promoting graphitization during annealing and dispersing graphite particles within the regulated range. This effect is insufficient if the amount is less than 3%, and if the amount exceeds 40%, the effect is not only saturated but also difficult to process. Annealing for graphitization does not proceed to facilitate the graphitization temperature is too low, since transformation precedes the progress of graphitization is lower than the transformation point of the temperature, from 600 to 720 below Ac ₃ transformation point ℃. The annealing time for graphitization can be shortened by performing cold working, but if it is less than 1 hour, graphitization is insufficient, and if it exceeds 24 hours, it progresses excessively and the amount of graphite is too large. Coarse graphite particles are created. The target graphite particle range is from 1 to
24 hours.

[0039]

【Example】

[Example 1] Ten kinds of steel shown in Table 1 were melted in a converter to form a billet having a diameter of 200 mm, and then hot-rolled into a steel bar having a diameter of 30 mm. Finishing temperature of hot rolling is 920 ℃ and after rolling 5
The average cooling rate up to 00 ° C is 50 ° C / min. This steel bar was annealed for graphitization under the conditions shown in Table 2. After the annealing, the degree of graphitization and the cross-sectional microscopic observation were performed to examine the graphitization state such as the number of precipitated graphite particles and the particle size. For the workability investigation, the hardness was measured, a test piece having a diameter of 20 mm and a height of 30 mm was prepared by cutting, and a compression test was performed by a crank press to measure the deformation resistance. 28mm diameter and 50mm length
Of the test piece was prepared and subjected to a forward extrusion test with a die angle of 30 ° and a surface reduction rate of 10% / pass by lubrication with Bonderite Bonda Leube (product name: Nippon Parka) to determine the processing limit of internal cracking. Examined. Also, in order to confirm the surface hardness by induction hardening, a rod-shaped test piece with a diameter of 28 mm was used.
Surface temperature of 950 ℃ by high frequency induction heating of 40kHz 5
After soaking for s, quenching was performed, tempering was performed at 180 ° C for 45 minutes, and the surface hardness was measured. The results are shown together in Table 2.

[0040]

[Table 1]

[0041]

[Table 2]

Test Nos. 1 and 2 show excellent workability, but the surface hardness after quenching is insufficient because the C content is less than the range specified by the present invention. In Test No. 14, the deformation resistance is lowered even if C is high, but since C exceeds the range specified in the present invention, graphitization excessively proceeds and internal cracking easily occurs. There is. Further, from the test numbers 3 to 5 or 6 to 7, it is possible to compare cases in which the same steel type but different graphitization conditions are used. Even if the steel composition is within the range defined by the present invention, if the graphitization conditions are inappropriate, the state of the graphitized structure defined by the present invention cannot be obtained, and a steel material having low hardness and good workability cannot be obtained. I understand.

Example 2 C and I of the steels shown in Table 1
Using a billet with a diameter of 200 mm, the rolling end temperature shown in Table 3 and the cooling rate after rolling were hot-rolled to a diameter of 30 mm.
mm rods were further annealed for graphitization.
The cross section of the obtained steel bar was examined for graphitization status and hardness, and the deformation resistance at 50% compression was measured using a 20 mmφ × 30 mm L test piece. The results are also shown in Table 3. Also,
In the same manner as in Example 1, the working limit of occurrence of internal cracks by the forward extrusion test and the surface hardness by induction hardening were measured. Table 3 shows the results.

[0044]

[Table 3]

Both Steel C and Steel I are steels having chemical compositions within the ranges specified in the present invention. However, if the rolling conditions or the graphitization annealing conditions are insufficient, the graphitized structure morphology defined in the present invention cannot be obtained. That is, when the ratio of graphitization is low, softening is insufficient and deformation resistance is large, and when the number of precipitated particles of graphite is too large, sufficient surface hardness cannot be obtained during induction hardening, and large graphite particles are generated. Internal cracks are more likely to occur and the working limit is lowered.

[Example 3] Using a steel billet having a diameter of 200 mm of C, F and I shown in Table 1, the finishing temperature of hot rolling was 920 ° C, and the average cooling rate up to 500 ° C after rolling was 50 ° C.
A steel bar with a diameter of 30 mm was used as the min. These steels are listed in Table 4.
After cold drawing, as shown in (4), annealing for graphitization was performed. Regarding the obtained steel bar, the morphology and structure of graphitization, the hardness, the deformation resistance and the like were investigated in the same manner as in the above examples. With respect to the working limit of the generation of internal cracks in the forward extrusion and the surface hardness due to induction hardening, the same method as in Example 1 was performed after cutting and forming a steel bar having a diameter of 20 mm.

[0047]

[Table 4]

The results are also shown in Table 4. In order to obtain a steel material having a graphitized structure morphology within the ranges defined by the present invention such as the degree of graphitization, the number of precipitated grains and the diameter of the maximum grain, the cold workability and It is understood that it is necessary to appropriately select the subsequent annealing conditions.

[0049]

EFFECTS OF THE INVENTION The steel material of the present invention has excellent cold workability such as drawing, forging and rolling, and can easily obtain a required strength by heat treatment such as quenching and tempering. By applying this steel material to the manufacture of processed parts having complicated shapes such as parts of various machines and automobiles, the manufacturing process can be rationalized and the yield of steel materials can be improved.

Claims (3)

[Claims] 1. A weight ratio of C: 0.30 to 0.70%, Si: 0.
20-0.60%, Mn: 0.05-0.60%, sol.Al: 0.05-0.
50%, B: 0.0005 to 0.0050%, N: 0.002 to 0.010%,
Ni: 2.00% or less and Ca: 0.01% or less, the balance being substantially Fe and unavoidable impurities, the steel structure consisting of ferrite, spheroidized cementite and graphite particles, and having a C content of 20-75. % Is graphitized, cross-section observation shows no graphite particles with a diameter of 20 μm, and
Graphite particles of up to 20 μm are C weight% x 100 pieces / mm ² or more, C
A steel material with excellent workability characterized by being distributed at a frequency of less than 1000% by weight / mm ² . 2. A weight ratio of C: 0.30 to 0.70%, Si: 0.
20-0.60%, Mn: 0.05-0.60%, sol.Al: 0.05-0.
50%, B: 0.0005 to 0.0050%, N: 0.002 to 0.010%,
When steel containing Ni: 2.00% or less and Ca: 0.01% or less and the balance being substantially Fe and unavoidable impurities is heated and hot-worked, the working is completed in the range of 800 to 1000 ° C. The steel material according to claim 1, wherein the steel material is cooled at a cooling rate of 10 to 100 ° C / min to 500 ° C or less and then annealed at 600 to 720 ° C for 4 to 50 hours. Method. 3. A weight ratio of C: 0.30 to 0.70%, Si: 0.
20-0.60%, Mn: 0.05-0.60%, sol.Al: 0.05-0.
50%, B: 0.0005 to 0.0050%, N: 0.002 to 0.010%,
Steel containing Ni: 2.00% or less and Ca: 0.01% or less, the balance being substantially Fe and unavoidable impurities, is hot-worked and then cold-worked at a workability of 3 to 40%.
The method for producing a steel product according to claim 1, wherein annealing is performed at 0 to 720 ° C for 1 to 24 hours.