JPH05339676A - Steel for machine structure excellent in cold workability and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a steel having excellent cold workability as hot-rolled by specifying the contents of C, Si, Mn, Al, Ti and B in steel, subjecting it to finish rolling and cooling under prescribed conditions to regulate the size of ferritic grains and dispersing carbides therein. CONSTITUTION:Steel constituted of, by weight, 0.1 to 0.5% C, <=0.3% Si, 0.2 to 1.7% Mn, 0.01 to 0.1% Al, 0.002 to 0.03% Ti and 0.0003 to 0.007% B is melted. This steel is subjected to finish rolling at TC+30 deg.C to TC-30 deg.C (where TC( deg.C)=865-155XC%+11.5XSi%-33.8XMn%-8'Cr%+2.8XM%-21.8XNi%+13.7XV%) and at 70 to 95% total reduction of area. Next, it is gradually cooled in the temp. range from the start of ferritic transformation to the completion of pearlitic transformation at <=15 deg.C/min cooling rate. Then, the average grain size of ferritic grains and pearlitic grains over the wire size (mm)X0.1 to the wire size (mm)X0.3 from the surface layer is regulated to <=10mum, and carbides are dispersed in the ferritic grains.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention does not require heat treatment when manufacturing mechanical parts such as bolts, nuts, gears and the like by cold plastic working such as cold forging, cold forging and cold rolling. The present invention relates to a steel material for machine structure having excellent cold workability as hot rolled and a method for producing the same.

[0002]

2. Description of the Related Art Cold working has higher productivity than that of hot working and mechanical cutting, has excellent dimensional accuracy of products, and has a good yield of steel materials, so that cold working of bolts, shafts, gears and other mechanical parts Widely applied in manufacturing. The steel material used in such cold working needs to have excellent cold workability, that is, low tensile strength and high ductility (drawing). This is because if the tensile strength of the steel material is high, the tool life in cold working is shortened, and if the ductility is low, cracking is likely to occur during cold working, resulting in poor economic efficiency. Therefore, conventionally, spheroidizing annealing is generally performed before cold working in order to reduce the tensile strength of the steel material and enhance the ductility. However, since the spheroidizing annealing process requires 10 to 20 hours,
From the viewpoint of improving productivity or saving energy, there has been a demand for the development of a hot-rolled material having excellent cold workability capable of omitting the spheroidizing annealing treatment.

On the other hand, as conventional knowledge, for example,
Japanese Patent Publication No. 61-37333, Japanese Patent Publication No. 61-3733
No. 4, JP-A-58-107416, JP-B-1
No. 12815 discloses that the controlled rolling in which the hot finish rolling conditions are regulated makes the austenite grains finer and introduces a deformation zone to promote the ferrite / pearlite transformation, and to improve the cold workability as it is in the hot rolling. An excellent method of manufacturing a steel material has been proposed. Although these manufacturing methods are effective in preventing the formation of bainite or martensite, which has a high tensile strength generated in the cooling process after hot rolling, it is cold workable in comparison with the spheroidized material. Since it is inferior, it is difficult to apply it to a machine part having a severe cold working degree, and it cannot be said to be a satisfactory solution.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and provides a steel material for machine structure having excellent cold workability as hot rolled and a method for producing the same. The purpose is.

[0005]

[Means for Solving the Problems] In order to solve the above problems, the present inventors have analyzed in detail the controlling factors of the cold workability of a hot rolled material having a ferrite-pearlite structure, If the C content is the same, the ferrite / pearlite grain size of the surface layer and the width from the surface layer have a great influence on the cold workability, and in order to improve the cold workability, a fine ferrite / pearlite grain size is required. In addition, we confirmed that it is necessary to optimize the width with a finer ferrite / pearlite grain size, and that if the carbide is dispersed in the ferrite of the surface layer, the cold workability is significantly improved. In order to make the pearlite grain size finer and to disperse the carbide in the ferrite grains,
The present invention was constructed by obtaining completely new knowledge that the steel composition, hot rolling finishing conditions, and cooling conditions after hot rolling should be optimally selected.

That is, the present invention is based on the above findings, and the gist thereof is as follows. (1) C: 0.10 to 0.50%, Si: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.10%, Ti:
0.002 to 0.030%, B: 0.0003 to 0.0070%, the balance Fe and unavoidable impurities, ferrite from the surface layer over the minimum wire diameter (mm) × 0.1 and the maximum wire diameter (mm) × 0.3 The average grain size of grains and pearlite grains is 1
A steel material for machine structural use having excellent cold workability, characterized in that carbides are dispersed in ferrite grains of 0 μm or less.

(2) C: 0.10 to 0.50% by weight, S
i: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.
10%, Ti: 0.002-0.030%, B: 0.0003-0.0070%
In addition, Cr: 0.10 to 2.00%, Mo: 0.05 to 1.00
%, Ni: 0.10 to 2.00%, V: 0.05 to 1.00%, Nb: 0.
005 to 0.10% of 1 type or 2 types or more, and the balance is F
e and unavoidable impurities, and the average particle diameter of ferrite particles and pearlite particles is 10 from the surface layer to the minimum wire diameter (mm) × 0.1 and the maximum wire diameter (mm) × 0.3.
A steel material for machine structural use having excellent cold workability, which is characterized in that carbides are dispersed in ferrite grains having a size of not more than μm.

(3) C: 0.10 to 0.50% by weight, S by weight
i: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.
10%, Ti: 0.002-0.030%, B: 0.0003-0.0070%
When the steel containing Fe and the unavoidable impurities in the balance is hot-rolled, after finishing rolling with a total area reduction rate of 70 to 95% in the temperature range of Tc + 30 ° C. to Tc-30 ° C., at least ferrite transformation is performed. A method for producing a steel material for machine structural use having excellent cold workability, which comprises gradually cooling a temperature range from the start to the end of pearlite transformation at a cooling rate of 15 ° C./minute or less.

Tc (° C.) = 865−155.0 × C% + 11.5 ×
Si% -33.8 x Mn% -8.0 x Cr% +2.8 x Mo% -21.8 x Ni
% + 13.7 x V% (4) Weight%, C: 0.10 to 0.50%, Si: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.10%, Ti:
0.002 to 0.030%, B: 0.0003 to 0.0070%, Cr: 0.10 to 2.00%, Mo: 0.05 to 1.00%, Ni: 0.
10-2.00%, V: 0.05-1.00%, Nb: 0.005-0.10%
1 or 2 or more of, and the balance Tc + 3 when hot rolling steel consisting of Fe and unavoidable impurities.
The total area reduction rate is 70 in the temperature range of 0 ° C to Tc-30 ° C.
After finishing rolling at ~ 95%, at least the temperature range from the start of ferrite transformation to the end of pearlite transformation is 15
A method for producing a steel material for machine structural use having excellent cold workability, which comprises gradually cooling at a cooling rate of not more than ° C / minute.

Tc (° C) = 865 -155.0 x C% +11.5 x
Si% -33.8 x Mn% -8.0 x Cr% +2.8 x Mo% -21.8 x Ni
% + 13.7 x V% The present invention will be described in detail below. First, in the present invention, steel having excellent cold workability means that the limit upsetting rate Hc (%) determined by the C content in the as-hot-rolled state is not less than the value represented by the following formula. There is.

Hc (%) = 87.7-81.7 × C (%) + 62.7
× C (%) ² Here, it is shown that the higher the Hc, the better the cold workability. This formula shows that if the structure of the hot-rolled material is a ferrite-pearlite structure, the critical upsetting rate is C if the amount of C and the effect of alloying elements on the critical upsetting rate of the hot-rolled material are analyzed. Uniquely determined by the content, S
Since it became clear that it did not depend on the type and amount of alloying elements such as i, Mn, and Cr, it was regressed by the C content. The critical upsetting ratio is obtained according to the standard of cold upsetting test method established by the Japan Society for Plasticity Processing.

Next, the reasons for limiting the composition of the steel to which the present invention is applied will be described. C: C is an essential element for imparting the required tensile strength to the product in the quenching and tempering treatment after cold working,
If it is less than 0.10%, the required tensile strength cannot be obtained, while 0.5
If it exceeds 0%, the limit upsetting rate becomes low with hot rolling, which makes cold working difficult. Therefore, the content is limited to 0.10 to 0.50%.

Si: Si enhances the tensile strength of the rolled material by the solid solution hardening action, so that the effect of the solid solution hardening action is less than 0.30%. Preferably it is 0.10% or less. Mn: Mn is an element effective not only for deoxidation and desulfurization but also for improving the hardenability during heat treatment after cold working and the temper softening resistance. On the other hand, if it exceeds 1.70%, the growth rate of ferrite and pearlite after hot rolling decreases, and the temperature at which ferrite and pearlite transformation ends are lowered, so bainite, which is harmful to cold workability, is likely to occur. However, the tensile strength is increased with the increase in temperature, so the range was limited to 0.20 to 1.70%.

B: B is an essential element for precipitating carbides in the ferrite grains of the hot rolled wire rod, and the use of B is one of the features of the present invention. That is, by the present inventors, when the steel containing B is hot-rolled under normal conditions and then subjected to a slow cooling treatment at a cooling rate of 15 ° C./min or less, ferrite / austenite is obtained. Very fine boron carbide (Fe ₂₃ (CB) ₆ ) precipitates at the interface, which promotes ferrite transformation and pearlite transformation, and has the effect of lowering tensile strength, but further optimizes the finishing conditions for hot rolling. It has been newly found that, when selected, the precipitation of boron carbide is promoted and at the same time, fine boron carbide in ferrite becomes nuclei and becomes cementite (Fe ₃ (CB)) containing B. Further, it was revealed as a new finding that cold forgeability can be remarkably improved when cementite is dispersed in ferrite grains. Further, B also has an effect of improving hardenability during heat treatment after cold forging. The effect of B shown above is
If less than 0.0003%, the effect cannot be exhibited, while 0.0070%
The effect is saturated even if added in excess of 0.0003 to 0.0
Limited to the range of 07%.

Al: Al is added to prevent B from reacting with N to form a nitride (BN) by forming AlN by combining with N. When B reacts with N, the solid solution B is reduced, so that the effect of B shown above is reduced. Furthermore, Al also has an effect of preventing coarsening of austenite grains during heat treatment after deoxidation and cold working, but if it is less than 0.01%, these effects are not exhibited,
Even if it exceeds 0.10%, the effect will be saturated, so 0.01 to 0.10%
Limited to.

Ti: Ti, like Al, combines with N to form T
By forming i (CN), B is prevented from becoming a nitride and the austenite crystal grains after hot rolling are refined to increase the nucleation rate of ferrite and pearlite, thereby increasing the tensile strength of the rolled material. Has the effect of reducing
Further, it has an effect of preventing coarsening of austenite crystal grains during heat treatment, but if it is less than 0.002%, the effect is insufficient, while if it exceeds 0.030%, it is harmful to cold workability. Since Ti (CN) is generated, 0.002 ~
The range was limited to 0.003%.

The above are the basic components of the steel to which the present invention is applied. In the present invention, however, Cr: 0.10-2.00% Mo: 0.05-1.00% Ni: 0.10-2.00% V: 0.05- 1.00% Nb: 0.005 to 0.10% of one kind or two or more kinds may be contained.

Cr: Cr is an element effective for increasing the growth rate of ferrite and pearlite at high temperatures, and is extremely effective for improving the hardenability and the temper softening resistance during the heat treatment after cold forging. If less than 0.10%, the effect cannot be fully exerted, while if over 2.00%, the ferrite / pearlite growth rate decreases, the ferrite and pearlite transformation end temperature decreases, and the tensile strength of the hot rolled material decreases. Since it becomes higher, the content is limited to 0.10 to 2.00%.

Mo: Mo has a strong temper softening resistance,
It is an element effective for increasing the tensile strength after heat treatment, but if it is less than 0.05%, its effect is small, while if it exceeds 1.00%, bainite which has a high tensile strength even if annealed after hot rolling is applied. Since this is apt to occur, the upper limit is set. Ni: Ni is added to improve toughness and hardenability during heat treatment to increase tensile strength, but if less than 0.10%, its effect is small, while 2.00%
Even if it exceeds the above, the effect of matching the added amount cannot be exerted, so
The range was limited to 0.10 to 2.00%.

V: V has the effect of increasing the temper softening resistance during tempering and the effect of refining the austenite grains after hot rolling and during heat treatment. However, if it is less than 0.05%, the above effect is obtained. On the other hand, the effect is saturated even if it exceeds 1.00%, so it was limited to 0.05 to 1.00%. Similar to V, Nb: Nb is also an effective element for refining austenite grains after hot rolling, but if it is less than 0.005%, its effect is insufficient, while if it exceeds 0.10%, this effect is obtained. Since it saturates, it was limited to 0.005 to 0.10%.

Although P and S are not particularly limited,
0.02% because all elements deteriorate cold workability.
The following is preferable. Next, the ferrite pearlite crystal grain size and its width of the steel surface layer which is an important point for improving the cold forging property in the as-hot-rolling as the object of the present invention,
The reasons for limiting the dispersion of carbide in ferrite grains will be described.

Ferrite / pearlite grain size: When the ferrite / pearlite grain size of the steel surface layer exceeds 10 μm, the critical upsetting ratio is lowered and the cold workability is deteriorated.
Below. Fig. 1 shows that the composition system is 0.34% C-0.03% Si-0.38.
% Mn-1.31% Cr-0.28% Mo-0.044% Al- 0.008% Ti-
The wire diameter of 0.0031% B finished under various hot rolling conditions is 11
An example of analysis of the relationship between the ferrite / pearlite grain size and the critical upsetting ratio of a hot-rolled wire of mm is shown. Here, the particle size of ferrite / pearlite is 2.2 mm from the surface layer of the wire.
The average particle size is between the two, and the critical upsetting ratio is obtained according to the Japan Plastics Society standards. The higher the critical upsetting ratio, the better the cold workability. As is clear from the figure, the particle size of ferrite / pearlite is 10μ.
It can be seen that when the value exceeds m, the critical upsetting ratio deteriorates rapidly. The grain size of ferrite / pearlite of the usual hot rolled material is about 15 to 25 μm.

Width of ferrite / pearlite grain size of 10 μm or less; if the width of ferrite / pearlite grain size of 10 μm or less from the surface layer is less than the wire diameter × 0.1, the effect of improving cold workability is small, while the line from the surface layer is small. Even if the diameter exceeds 0.3, not only is the effect of improving cold workability saturated, but the effect of refining the ferrite grains also increases tensile strength, reducing the tool life for cold work during cold work. 10μ to do
The width of ferrite particles and pearlite particles of m or less was set to a minimum wire diameter × 0.1 and a maximum wire diameter × 0.3 from the surface layer. FIG. 2 shows an example of analyzing the influence of the ratio of the wire diameter to the width from the surface layer of the wire having a ferrite / pearlite particle size of 10 μm or less on the critical upsetting ratio. The composition of the steel used is 0.34% C-0.03% Si-0.38% Mn-1.31% Cr-0.28%
Mo-0.044% Al-0.008% Ti-0.0031% B. It is clear that when the ratio of the ferrite / pearlite grain region of 10 μm or less to the wire diameter is less than 0.1, the effect of improving the critical upsetting ratio is small, and when it exceeds 0.3, the effect is saturated.

Carbides in ferrite grains: In the structure of a normal hot rolled material, carbides are not precipitated in ferrite grains as shown in FIG. 3, but as shown in FIG. 4, ferrite / pearlite grain size is 10 μm or less. In addition, since the cold workability is significantly improved when the structure is such that the carbides are dispersed in the ferrite grains, the reason why the carbides are dispersed in the ferrite grains is the limiting reason. FIG. 5 shows an example of analysis of the effect of carbide in ferrite grains on the relationship between the upsetting rate and the cracking rate. The particle size and width of ferrite / pearlite are compared under the same conditions. Steel in which carbide is dispersed in ferrite grains (marked with ● in the figure) is not dispersed in steel (○ in the figure)
It is clear that even when the upsetting rate is higher than that of (1), cracks are less likely to occur due to cold working and the cold workability is improved. Fig. 6 shows the effect of ferrite / pearlite grain size and carbide in the ferrite grain on the critical upsetting rate.
An example of comparison with a conventional hot rolled material is shown. The steel types are those in which the contents of C and alloy elements are variously changed. Steel with ferrite / pearlite grain size of 10 μm or less and carbide dispersed in ferrite grains (marked with ● in the figure) is a steel with ferrite / pearlite grain size of 10 μm or less but no carbide dispersed in ferrite grains. (○ in the figure), a steel manufactured by the conventional method with a ferrite / pearlite grain size of around 20 μm and no carbide dispersed in the ferrite grain (□ in the figure)
It can be seen that the marginal upsetting ratio is markedly higher than that of (). In addition, in order to disperse the carbide in the ferrite grains, the addition of B is essential as a component.

Next, from the surface layer, the minimum wire diameter (mm) × 0.
1. The reasons for limiting the manufacturing method for dispersing the carbides in the ferrite grains in which the average grain size of the ferrite grains and the pearlite grains are 10 μm or less over the maximum wire diameter (mm) × 0.3 are described. Temperature range of finish rolling: The temperature range of finish rolling has a great influence on the grain size and width of ferrite grains and pearlite grains, and the precipitation behavior of carbides in ferrite grains, so it is important together with the total reduction rate of finish rolling. Is a factor. As a result of detailed analysis of the effect of the finish rolling temperature on the ferrite / pearlite grain size and its width, and the precipitation of carbides in the ferrite grains, under the condition that the total area reduction rate is 70 to 95%, the finish rolling temperature range is C and alloy. Tc ± 30 determined by the kind and amount of element
It was clarified that when the temperature was set to 0 ° C., a grain size of ferrite / pearlite of 10 μm or less excellent in cold workability and a structure in which carbide was dispersed in the ferrite grain were obtained. Tc is Si, Mn, Cr, Mo, Ni, in addition to the C content in steel.
It changes depending on the V content (% by weight) and is represented by the following formula.

Tc (° C.) = 865−155.0 × C% + 11.5 ×
Si% -33.8 x Mn% -8.0 x Cr% +2.8 x Mo% -21.8 x Ni
% + 13.7 x V% The above formula shows that the average grain size of ferrite grains and pearlite grains is 10 μm or less from the surface layer to the minimum wire diameter (mm) x 0.1 and the maximum wire diameter (mm) x 0.3. In addition, the results of detailed and wide-range analysis of the influence of alloying elements for dispersing carbides in ferrite grains were obtained by performing multiple regression analysis. When the finishing temperature is higher than Tc + 30 ° C., carbide does not precipitate in the ferrite grains,
The width of the ferrite / pearlite structure having a ferrite / pearlite particle size of more than 10 μm or 10 μm or less from the surface layer is less than the wire diameter × 0.1, and it is difficult to obtain a structure excellent in cold workability. Therefore, the upper limit of the finishing temperature is limited to Tc + 30 ° C. Also Tc-3
When finish rolling is performed at a low temperature of less than 0 ° C., the width of the ferrite / pearlite structure having a grain size of 10 μm or less easily exceeds wire diameter × 0.3, and the tensile strength increases due to the grain refining effect of ferrite, Further, since the deformation resistance during rolling increases and there are cases where rolling becomes difficult, the lower limit of the finishing temperature is limited to Tc-30 ° C.

Total reduction rate of finish rolling; finish rolling temperature is Tc
If the total area reduction rate of finish rolling in the range of -30 ° C to Tc + 30 ° C is less than 70%, the grain size of ferrite / pearlite exceeds 10 µm, and since carbides do not precipitate in the ferrite grains, cold workability is low. Cannot be made into excellent steel,
On the other hand, the effect is saturated even if the finish rolling is performed under the condition of the total area reduction ratio exceeding 95%, and the width of the ferrite / pearlite grain size of 10 μm or less easily exceeds the wire diameter × 0.3, and the tensile strength is increased. Due to the increase, the range was limited to 70 to 95%.

Cooling conditions after hot rolling: First, the cooling rate after hot rolling is 15 ° C./min.
If cooled faster than min, since the ferrite transformation and pearlite transformation start temperature decreases, the pearlite fraction increases and the cementite interval becomes finer and the tensile strength of the rolled material increases, and the tensile strength is higher, This is because there is a risk that bainite, which has low ductility, will be generated.
The slower the cooling rate, the more advantageous it is to reduce the tensile strength of the rolled material. However, considering the practical point in terms of equipment and productivity, the cooling rate of 3 to 10 ° C./min is lower than that of the rolled material. This is a preferable cooling rate range that achieves both reduction in tensile strength and production. Although the slow cooling start temperature may be slow cooled at the above cooling rate immediately after the hot rolling, the productivity is extremely deteriorated. Since the gradual cooling start temperature is sufficient if the gradual cooling is started from the temperature at which ferrite transformation occurs, the minimum temperature is set as the ferrite transformation start temperature. The gradual cooling stop temperature was set to the pearlite transformation end temperature because low temperature transformation pearlite or bainite having high tensile strength and low ductility is formed in the subsequent cooling process when gradual cooling is stopped before the pearlite transformation is completed.

Next, the effects of the present invention will be described more specifically by way of examples.

[0030]

[Examples] Table 1 shows the chemical components of the test materials. A wire diameter of 8 to 16 mm was finished by hot rolling using these test materials.
The conditions of finish rolling temperature, total area reduction of finish rolling, and cooling rate after rolling are shown in Tables 2 and 3 (continued from Table 2). The evaluation of the hot rolled material is carried out by measuring the ferrite / pearlite grain size and the ratio of the width of the ferrite / pearlite grain size of 10 μm or less to the wire diameter, and observing the structure of the carbide in the ferrite grain (the circle indicates that the carbide is dispersed. The symbol X indicates that the carbide is not dispersed), the mechanical test of tensile strength and drawing, and the cold upsetting test. The test results are also shown in Tables 2 and 3. Test No. in both tables 2,
8, 11 to 13, 15 to 20 (marks) are examples of the present invention, and others are comparative examples. As is clear from both tables, in all the examples of the present invention, the structure in which the width of the ferrite / pearlite grain size of 10 μm or less is between 0.1 and 0.3 with respect to the wire diameter and the carbide is dispersed in the ferrite grain Has become. As a result, the tensile strength can be lowered and the drawing can be increased with the hot rolling as it is, and the critical upsetting ratio is the target Hc (%).
The level is higher than the above.

On the other hand, in Comparative Example No. No. 1 is Japanese Industrial Standard SWRCH35K, No. 6 is SCM42
However, since neither of them contained B, carbides were not dispersed in the ferrite grains, and the critical upsetting ratio did not reach the target level. In addition, No. 1
No. 0 is an example in which bainite having a high tensile strength and a low ductility was formed because ferrite-pearlite transformation was delayed because the content of B was contained but the content of Mn was too large. The formation of bainite lowers the upset marginal rate.

No. which is a comparative example. Nos. 5, 7, and 21 were performed under the hot rolling conditions of the conventional method, and are examples in which the finish rolling temperature was higher than Tc + 30 ° C. If the finish rolling temperature is high, it is impossible to obtain a ferrite / pearlite grain size of 10 μm or less, and it is not possible to obtain a structure in which carbide is dispersed in the ferrite grain. As a result, the critical upsetting ratio is less than the target Hc. Conversely, No. Three
Is an example in which the finish rolling temperature is lower than Tc-30 ° C. Since the finish rolling temperature is low, the ratio of the width of the ferrite / pearlite grain size of 10 μm or less to the wire diameter exceeds 0.3. As a result, the No. Compared with No. 2, the effect of grain refinement was greater and the tensile strength was higher.

No. 3 which is a comparative example. Nos. 4 and 14 are examples in which the total area reduction rate of finish rolling is less than 70%. When the total area reduction rate is low, the ratio of the particle diameter of 10 μm or less to the wire diameter is less than 0.1, and it becomes difficult to obtain a structure in which carbides are dispersed in ferrite grains. Therefore, the critical upsetting ratio is also less than the target Hc. Furthermore, No. In No. 9, bainite was generated because the cooling rate after hot rolling was high,
In this example, the tensile uptake was high and the drawing was low, and the critical upsetting ratio was poor.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

As is clear from the above examples, according to the present invention, the chemical composition, the temperature range of finish rolling, the total area reduction rate and the cooling rate after rolling are optimally selected to obtain the ferrite / pearlite grain size. Of 10 μm or less and carbides dispersed in the ferrite grains, and a structure with good critical upsetting characteristics can be obtained, and a steel material having excellent cold workability as hot-rolled and its production were made possible. Is something
The industrial effect is extremely remarkable.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of analysis of a relationship between a ferrite / pearlite grain size and a critical upsetting ratio.

FIG. 2 is a diagram showing an example of analysis of a relationship between a critical upsetting ratio and a ratio of a width of a ferrite-pearlite structure having a grain diameter of 10 μm or less to a wire diameter.

FIG. 3 is an optical microscope metallographic photograph showing an example of a rolled material structure manufactured by a conventional method.

FIG. 4 is an optical microscope metallographic photograph showing an example of the structure of a rolled material produced by the method of the present invention.

FIG. 5 is a diagram showing an example of analysis of influence of carbides in ferrite grains on cold upsetting characteristics.

FIG. 6 is a diagram showing an example in which the effects of ferrite / pearlite grain size and carbides in ferrite grains on the critical upsetting rate are sorted by C content.

Claims (4)

[Claims] 1. By weight%, C: 0.10 to 0.50%, Si: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.10%, Ti: 0.002 to 0.030%, B: 0.0003 to 0.0070%. Included, the balance consisting of Fe and unavoidable impurities, minimum wire diameter (mm) x 0.1 from the surface layer, maximum wire diameter (mm)
A steel material for machine structural use having excellent cold workability, characterized in that the average grain size of ferrite grains and pearlite grains is 0.3 μm or less over 0.3 and carbides are dispersed in the ferrite grains. 2. In% by weight, C: 0.10 to 0.50%, Si: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.10%, Ti: 0.002 to 0.030%, B: 0.0003 to 0.0070%. In addition, Cr: 0.10 to 2.00%, Mo: 0.05 to 1.00%, Ni: 0.10 to 2.00%, V: 0.05 to 1.00%, Nb: 0.005 to 0.10%, and one or more kinds, and the balance is Fe and unavoidable impurities, and the minimum wire diameter (mm) x 0.
1. Excellent cold workability, characterized in that the average grain size of ferrite grains and pearlite grains was 10 μm or less over a maximum diameter (mm) × 0.3 and carbides were dispersed in the ferrite grains. Steel material for machine structure. 3. In weight%, C: 0.10 to 0.50%, Si: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.10%, Ti: 0.002 to 0.030%, B: 0.0003 to 0.0070%. At the time of hot-rolling steel consisting of Fe and unavoidable impurities, the balance of which is at least ferrite transformation after finishing rolling at a total area reduction of 70 to 95% in a temperature range of Tc + 30 ° C to Tc-30 ° C. A method for producing a steel material for machine structural use, which is excellent in cold workability, characterized by gradually cooling the temperature range from to the end of pearlite transformation at a cooling rate of 15 ° C./minute or less. Tc (° C) = 865 -155.0 x C% +11.5 x Si% -33.8 x
Mn% −8.0 × Cr% + 2.8 × Mo% −21.8 × Ni% + 13.7 × V
% 4. In weight%, C: 0.10 to 0.50%, Si: 0.30% or less, Mn: 0.20 to 1.70%, Al: 0.01 to 0.10%, Ti: 0.002 to 0.030%, B: 0.0003 to 0.0070%. In addition, Cr: 0.10 to 2.00%, Mo: 0.05 to 1.00%, Ni: 0.10 to 2.00%, V: 0.05 to 1.00%, Nb: 0.005 to 0.10%, and one or more kinds, and the balance is When hot rolling steel consisting of Fe and inevitable impurities, Tc + 3
The total area reduction rate is 70 in the temperature range of 0 ° C to Tc-30 ° C.
After finishing rolling at ~ 95%, at least the temperature range from the start of ferrite transformation to the end of pearlite transformation is 15
A method for producing a steel material for machine structural use having excellent cold workability, which comprises gradually cooling at a cooling rate of not more than ° C / minute. Tc (° C) = 865 -155.0 x C% +11.5 x Si% -33.8 x
Mn% −8.0 × Cr% + 2.8 × Mo% −21.8 × Ni% + 13.7 × V
%