JP2938101B2 - Manufacturing method of steel for cold forging - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing steel for cold forging having low deformation resistance during cold forging.

<Conventional Technology> Cold forging is a processing method widely used in the manufacture of various machine parts including bolts and nuts because of its high production efficiency, excellent material yield, and excellent finish dimensional accuracy.

Conventionally, as a steel material for cold forging, C of 0.45% by weight or less (hereinafter referred to as%) is used, for example, JIS S45C, which is subjected to spheroidizing annealing to be softened and then subjected to cold forging. Was customary.

However, in recent years, examples of using a steel material whose C is higher than 0.45% as a steel for cold forging are increasing. This is because the surface height after quenching and tempering has been required to be higher than before in order to satisfy the function as a mechanical part. As is well known, the surface hardness after quenching and tempering, in other words, the hardness of tempered martensite depends on the C content.

However, an increase in the amount of carbon not only increases the deformation resistance during cold forging and extremely shortens the life of the cold forging die, but also increases the forging load to exceed the capacity of the forging machine. However, such a forging machine has a problem that processing is difficult.

To solve such a problem, Japanese Patent Application Laid-Open No. 61-113744 discloses that the chemical composition of Si, Mn, Cr, S, P, N, and O is restricted to reduce deformation resistance and improve deformability. Is being planned.

However, according to the study of the present inventors, the above method is based on the premise that spheroidizing annealing of carbide is performed before cold forging, but even if the chemical composition is adjusted in the spheroidized structure, it is possible to reduce the deformation resistance. Is limited and the deformation resistance is still high.

<Problems to be Solved by the Invention> An object of the present invention is to solve the above problems and provide a method of obtaining a steel material having low deformation resistance during cold forging and having good deformability by short-time annealing. It is something to offer.

<Means for Solving the Problems> The present inventors have conducted intensive studies to achieve the above object, and as a result, have obtained the following knowledge. That is, by adjusting the chemical components and controlling the microstructure before annealing to a specific range of the hot rolling conditions and the cooling conditions after rolling, a fine ferrite + pearlite structure or bainite, martensite or a mixed structure thereof is obtained. Thereby, the graphitization of the oxide is promoted, and the cold forgeability is dramatically improved.

That is, in the present invention, C: 0.4 to 1.1%, Si: 0.
6 ~ 1.5%, Mn: 0.2 ~ 0.9%, S: 0.001 ~ 0.03%, B: 0.0005 ~
0.005%, Al: 0.01-0.1%, P: 0.
02% or less, N: 0.007% or less, O: 0.003% or less, and Cr: 0.10%
The steel is substantially limited to Fe,
After heating to a temperature range of 0 ° C and forming a steel bar by hot rolling, the steel is cooled from the rolling end temperature to 500 ° C at a cooling rate of 0.1 to 30 ° C / s, and then annealed at a temperature of ₁ point or less of Ac. Of the steel for cold forging, characterized in that the rolling end temperature is in a temperature range of 850 to 650 ° C.

<Operation> The present inventors have found that a graphitized structure composed of graphite + ferrite graphitized from spherical cementite is more graphitized than a conventional spheroidized structure composed of spherical cementite and ferrite.
The present invention has been made based on the finding that it is effective in reducing cold deformation resistance.

Incidentally, the graphitization of carbides in steel has been proposed for the purpose of improving machinability, for example, as disclosed in
816, JP-A-49-67817, JP-A-49-103817
However, according to the study of the present inventors, these steels have an inferior cold deformability and require an extremely long time to be graphitized, It is difficult to implement on an industrial scale.

Further, the composition and the hot rolling conditions are different from the present invention, and are clearly different from the present invention.

As described above, the present invention adjusts the chemical composition in order to promote graphitization, controls the microstructure before annealing to a specific range of the hot rolling conditions and the cooling conditions after rolling to a fine ferrite + pearlite structure. Or, it is bainite, martensite or a mixed structure thereof. These increase the transformation nuclei at the time of γ → α transformation, particularly by low-temperature finishing of hot rolling and / or accelerated cooling after rolling, and refine the structure.

 Next, the reasons for limiting the component composition will be described first.

C: It is an important element in securing the strength as a mechanical part, but if it is less than 0.4%, the effect of applying the present invention is small, so it is made 0.4% or more. Further, if added in excess of 1.1%, the deformation resistance during hot rolling increases and hot rolling becomes difficult, so the content is set to 1.1% or less.

Si: Actively used because it is an element that promotes graphitization and is also useful for deoxidation. However, if it is less than 0.6%, its effect is small, and if it exceeds 1.5%, its effect is saturated. Limited to 0.6-1.5%.

Mn: It is a useful element for securing hardenability, so it is used positively. However, if its addition is less than 0.2%, its effect is small, so the lower limit is made 0.2%. Further, if added in excess of 0.9%, the graphitization is inhibited, so the addition should be 0.9% or less.

S: It is an element that improves machinability, so it is positively added, but if it is less than 0.001%, its effect is small, so at least 0.001% or more is necessary. However, if the content exceeds 0.03%, the deformability during cold forging is degraded, so the content is set to 0.03% or less.

P: It is desirable to reduce as much as possible because it deteriorates cold forgeability and also inhibits graphitization, but it is allowable up to 0.02%.

B: Actively used because it improves the hardenability by adding a small amount, but if less than 0.0005%, the effect is small, and 0.005%
%, The effect saturates even if it is contained more than 0.0005%.
The range is 0.005%.

Al: an element effective for deoxidation and an element effective for promoting graphitization. Further, in order to sufficiently exert the effect of improving the hardenability of B, it is a useful element, so it is positively added, but if it is less than 0.01%, the effect is small, and 0.1% or less.
If the content exceeds 0.01%, the effect is saturated.
% Range.

0.007% for N, O and Cr as other impurities
Hereinafter, it is limited to 0.003% or less and 0.01% or less, and the reason is described below.

N: It is an element that causes dynamic strain aging during cold forging and increases deformation resistance, and also reduces the effect of improving the hardenability of B, so it is desirable to reduce it, but up to 0.007% is allowable.

O: Oxide-based inclusions are increased and the deformability at the time of cold forging is deteriorated, so it is desirable to reduce them, but up to 0.003% is allowable.

Cr: A strong carbide-forming element that inhibits graphitization and should be reduced as much as possible, but is allowed up to 0.10%.

Next, hot rolling and cooling conditions after rolling will be described.

The heating temperature at the time of hot rolling is set to 900 ° C. or higher because, if it is lower than this temperature, the deformation resistance at the time of hot rolling becomes excessive and hot rolling becomes difficult. Further, at a temperature exceeding 1150 ° C., the γ particle size at the time of heating becomes too coarse, and it becomes difficult to make the γ particle size before transformation difficult to obtain fine particles, and it is difficult to obtain a target fine structure. Therefore, the upper limit is set to 1150 ° C. .

The reason why the cooling rate to 500 ° C. after the end of the rolling is set to 0.1 to 30 ° C./s is that even if cooling at a cooling rate of less than 0.1 ° C./s, the effect of promoting graphitization is not recognized. If the cooling rate exceeds 30 ° C./s, the hardness is too high, and the cutting property is deteriorated. The cooling rate is preferably 0.1-5 ° C / s.

The reason why the cooling stop temperature is set to 500 ° C. is that at temperatures higher than this, the transformation is not completed and the desired microstructure cannot be obtained, so that the effect of promoting graphitization is insufficient.

Further, to the annealing temperature than Ac ₁ point is the temperature range over _a point Ac partially γ formation proceeds, or less Ac ₁ point because inhibits graphitization.

Further, the rolling end temperature is preferably set to 650 to 850 ° C. This is because the effect of promoting graphitization is small in a temperature range exceeding 850 ° C., whereas at a temperature lower than 650 ° C., the deformation load during hot rolling becomes high and rolling becomes difficult.

<Example> Hereinafter, the present invention will be described with reference to examples.

Steel having the chemical composition shown in Table 2 was melted in a 180-t converter, then made into a bloom by continuous vacuum degassing casting, and then hot-rolled into a 150 mmφ billet. Further, these billets were subjected to a hot-rolling condition and a cooling condition shown in Table 2 by 50 mm.
φ steel bar.

These steel bars have an Ac of ₁ point or less at 700 ° C for 5 to 15 hours.
After annealing, a cylindrical test piece of 15 mmφ × 22.5 mmH was prepared, and a compression test was carried out under the condition of complete constraint of the end face to determine the cold deformation resistance and the critical compression ratio during processing. here,
The critical compression ratio was defined as the compression ratio at which cracks began to occur in the test piece. Further, the microstructure after annealing was observed, the number of graphite particles and the number of carbides were counted by an image analyzer, and the number of graphite particles / (the number of graphite particles + the number of carbides) × 100 (%) was quantified as the graphitization rate.

The results are shown in Table 2. As is clear from Table 2, the steels of the components A to E that conform to the present invention are graphitized rapidly by annealing for 5 to 15 hours by using the steel bars according to the hot rolling conditions and the cooling conditions of the present invention. On the other hand, in the steels F to J, the graphitization does not progress at all even if the steel bars are annealed under the hot rolling conditions and the cooling conditions of the present invention and subjected to annealing.

Further, as a result, it is understood that the graphitized material has a deformation resistance at the time of cold working that is about 10% or more lower than that of the graphitized material at the same C amount, and has a higher critical compressibility and higher deformability. .

Table 3 shows the results of rolling and cooling in the range of the present invention using B steel, and rolling and cooling under conditions other than the present invention, followed by annealing. When the conditions deviate from the conditions of the present invention, graphite is used. It is understood that the progress of the formation is remarkably slow.

<Effect of the Invention> According to the present invention, it is possible to obtain a steel material for cold forging having low cold deformation resistance and excellent deformability by short-time annealing, which contributes to production of machine parts by cold forging. Is big.

Continuation of front page (56) References JP-A-2-111842 (JP, A) JP-A-64-25946 (JP, A) JP-A-49-103817 (JP, A) (58) Fields investigated (Int .Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] (1) C: 0.4 to 1.1%, Si: 0.6 to 1.5%, M
n: 0.2 to 0.9%, S: 0.001 to 0.03%, B: 0.0005 to 0.005%, Al:
Contains 0.01-0.1%, P as impurities: 0.02% or less, N:
After restricting 0.007% or less, O: 0.003% or less and Cr: 0.10% or less, the remainder substantially made of Fe is heated to a temperature range of 900 to 1150 ° C, and after being hot-rolled into a bar, it is rolled. Cool from the end temperature to 500 ° C at a cooling rate of 0.1 to 30 ° C / s,
A method for producing steel for cold forging, characterized by subsequently annealing at a temperature of ₁ point or less of Ac to graphitize carbides. 2. The method for producing cold forging steel according to claim 1, wherein the rolling end temperature is in a temperature range of 850 to 650 ° C.