JPH0754041A - Method for manufacturing cold forging steel - Google Patents

Abstract

(57) [Summary] [Purpose] Eliminate the steps of annealing, quenching / tempering, and straightening to manufacture long small parts of cold forging steel with high efficiency and at low cost. [Composition] C: 0.05 to 0.25%, Si: 0.30% or less, Mn:
0.50 to 2.00%, P: 0.025% or less, S: 0.025% or less, N: 0.0060% or less, V: 0.05 to 0.30%, Al: 0.0
10 to 0.100%, Cu: 0.10 to 0.50%, Ti: 0.005 to 0.050
%, The balance Fe and unavoidable impurities after heating the steel slab to a temperature range of 1000-1150 ℃,
It is hot-rolled and finished in a temperature range of 800 to 900 ° C., cooled from the finishing temperature to 500 ° C. at a cooling rate of 1 to 5 ° C./sec, and then cold drawn.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention is suitable for use in small and long parts such as tie rods, screws, spark plugs, etc., and enables simplification and omission of heat treatment steps, bending correction steps and the like. The present invention relates to a method for manufacturing cold forging steel with improved cold forgeability.

[0002]

2. Description of the Related Art Carbon steel for machine structural use is used in the production of the above-mentioned small and long parts. However, in the conventional carbon steel for machine structure, since cold forgeability is not sufficient, in the production of small-sized long parts, as shown as a conventional example in FIG. 1, an annealing treatment is performed between hot rolling and drawing. Since cold forging is followed by quenching / tempering and bending correction work after quenching / tempering, the production efficiency is low and the manufacturing cost is high. There has also been a strong demand for improvement in die life.

[0003]

SUMMARY OF THE INVENTION The object of the present invention is to provide a highly efficient cold forging steel which enables the omission of the steps of annealing, quenching / tempering, and straightening in the production of small and long parts. And to provide an inexpensive manufacturing method.

Specifically, an object of the present invention is to obtain a tensile strength of 7 as a drawing material by hot rolling and cold drawing.
The object is to provide a material having excellent cold forgeability of 35 to 830 N / mm ² , a reduction of 55 to 65%, and a sliding number of 200 or more at a dynamic friction coefficient μ = 0.2 in a Bowden tester.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted various studies in order to achieve the above-mentioned objects, and as a result, obtained a fine ferrite and pearlite structure by adjusting the steel composition and hot rolling, and then drawing. The inventors have completed the present invention, knowing that the strength can be secured by processing, excellent ductility and toughness can be realized, and that they have excellent cold forgeability as they are rolled.

In the present invention, C: 0.05 to 0.25%, Si: 0.30% or less, Mn: 0.50 to 2.00, in weight%.
%, P: 0.025% or less, S: 0.025% or less, N: 0.0060
% Or less, V: 0.05 to 0.30%, Al: 0.010 to 0.100%, Cu:
A steel slab consisting of 0.10 to 0.50%, Ti: 0.005 to 0.050%, the balance Fe and unavoidable impurities is heated to a temperature range of 1000 to 1150 ° C, then hot-rolled and finished in a temperature range of 800 to 900 ° C. The method for producing cold forging steel is characterized by cooling from the finishing temperature to 500 ° C. at a cooling rate of 1 to 5 ° C./sec and then performing cold drawing.

[0007]

Next, the reason why the steel composition and working conditions are defined in the above range in the present invention will be explained. In this specification, all "%" mean "% by weight".

C: C is preferably low in terms of ductility and toughness of the final steel part to be obtained, and the upper limit is 0.25%. The lower limit was set to 0.05% to give the steel a predetermined strength.

Si: Si is an effective component for deoxidizing steel,
If deoxidation is performed with Al, Si addition is not always necessary,
Moreover, if the content exceeds 0.30%, the deformability will be adversely affected, so the upper limit was made 0.30%.

Mn: Mn is an element necessary for deoxidizing and desulfurizing steel, and its lower limit must be 0.50% or more in order to impart predetermined strength and cold forgeability to steel. On the other hand, if it exceeds 2.00%, the strength of the material becomes high, and it is necessary to perform cold forging after annealing in the manufacturing process, resulting in high cost.

P: P is mixed as an impurity in the steel from the raw material, but if it is too much, the deformation strength and ductility of the material are impaired, so the upper limit was made 0.025% or less.

S: S, which is mixed as an impurity in the steel like the P component, deteriorates cold forgeability.
The upper limit was made 0.025% or less.

N: N is an element that causes blue heat embrittlement, and it is desirable that the amount is as small as possible. 0.0060% or less because increasing N content not only increases the amount of Al and Ti necessary to fix N as a nitride but also increases the strength of the material and reduces cold forgeability. And

V: V is an extremely effective element for imparting strength. Further, in the case of the present invention, it is also an effective element for obtaining a fine structure of ferrite and pearlite as it is rolled. If the V content is less than 0.05%, the above effect is not sufficiently exhibited, while if it exceeds 0.30%, the effect is saturated, so the content was made 0.05 to 0.30%.

Al: Al has the effect of improving cold forgeability and hardenability without increasing the strength of steel. That is,
Al fixes N to suppress age hardening during cold forging and makes the lubricating coating uniform, thereby improving cold forgeability. Moreover, Al is effective not only for deoxidizing steel, but also for preventing coarsening of austenite crystal grains during heating of steel before hot rolling.

If Al: less than 0.010%, the above effect cannot be exhibited. On the other hand, when Al is contained in excess of 0.100%, Al ₂ O ₃ -based inclusions increase, and during cold forging, the deformability becomes insufficient and cracking easily occurs.

Cu, Ti: These elements have the effect of improving cold forgeability and hardenability without increasing the strength of steel. That is, Cu forms a concentrated layer on the steel surface to facilitate the formation of film crystal nuclei and makes the zinc phosphate crystals formed during the chemical conversion treatment fine, and Ti fixes N and cools it. Cold forgeability is improved by suppressing age hardening during hot forging and making the lubricating coating uniform. Moreover, since Ti has the effect of promoting the action effect of Cu and homogenizing the crystals of the lubricating film, the improvement effect is remarkable due to the synergistic action effect of simultaneous addition of Ti and Cu.

The content of these is Cu: less than 0.10%, Ti: 0.0
If less than 05%, the above effect cannot be exhibited. On the other hand, Cu is 0.50
If it is contained in excess of%, the ductility decreases and the workability is impaired.
When Ti exceeds 0.050%, carbonitrides coarsen and the number increases, which causes deterioration of hot workability.

FIG. 1 shows, as an example of the present invention, the difference in the degree of processing from the above-mentioned conventional example. In the present invention, the steps of hot rolling → air cooling (cold) → cold drawing are performed. Steel for cold forging is manufactured. According to the present invention, the annealing step is not required prior to cold drawing, and the cold forging has sufficient strength and dimensional accuracy, so that quenching strengthening and bending straightening, which have been conventionally required, are also unnecessary.

The conditions for hot rolling include heating at a temperature of 1000 to 1150 ° C. in order to refine the initial grain size during rolling heating, to form a solid solution and fine precipitation of carbonitride for precipitation hardening. After that, hot rolling is performed. If the heating temperature is less than 1000 ° C, not only the hot deformability is insufficient, but also the solid solution of carbonitride is not sufficient, while if it exceeds 1150 ° C, the austenite crystal grains (initial crystal grains) become coarse. Therefore, the heating temperature before hot rolling is set to 1000 to 1150 ° C.

The operation itself of hot rolling is not particularly limited except for the finishing temperature. For example, ordinary hot rolling may be used. The finishing temperature range is 800 to 900 ° C, but if the finishing temperature exceeds 900 ° C, the crystal grains will become coarse and
The desired sufficient miniaturization cannot be realized, the toughness and ductility deteriorate, and the cold forgeability decreases. On the other hand, if the temperature is lower than 800 ° C, non-uniform cooling occurs in the cross section of the hot rolled material, not only the variation in product quality becomes large, but also it becomes impossible to secure a sufficient cooling rate for performing fine precipitation of carbonitride. Is.

Then, controlled cooling is performed at a cooling rate of 1 to 5 ° C./sec from the hot rolling finishing temperature to 500 ° C. to achieve fine precipitation of carbonitrides. At a cooling rate lower than this, such as less than 1 ° C./sec, dislocations serving as precipitation sites disappear, the number of precipitates becomes small and coarse, and the strengthening effect becomes small. On the other hand, at a cooling rate higher than 5 ° C./sec, bainite appears to cause variations in strength, and further, toughness and ductility are deteriorated, and cold forgeability deteriorates.

Cooling at such a cooling rate is at least 500
It can be up to ℃, but it allows the pearlite transformation to take place sufficiently, and has a uniform grain size and fine ferrite +
In order to obtain a pearlite transformation structure, in other words, the cooling rate in the temperature range of less than 500 ° C. is not particularly limited. It is because the perlite transformation has already been completed.

After cooling, cold drawing is subsequently performed to obtain a steel for cold forging. However, even if this cold drawing is carried out in the cooling process subsequent to the above-mentioned cooling to 500 ° C., or once it reaches room temperature. You may perform cold drawing after cooling.

The conditions for the cold drawing process are not particularly limited in the present invention, but if one example is taken, the drawing process may be carried out at room temperature with a surface reduction rate of 25 to 35% after the phosphorylation treatment. Good. Conventionally, an annealing treatment was performed at the time of cold drawing, but according to the present invention, a carbonitride is finely precipitated in a pearlite + ferrite structure, so that the annealing treatment is not particularly performed. Can perform cold drawing.

Thus, according to the present invention, a cold forging steel having a material capable of forming a sound lubricating coating having a good balance between strength and toughness can be obtained by an efficient and inexpensive means. FIG. 1 shows a comparison of each manufacturing process between the conventional method and the method of the present invention in the method of manufacturing a small-sized long component.

As can be seen from the above, the cold drawing after the hot rolling can be followed by the cold drawing, and the material obtained by the cold drawing is given a predetermined shape by the cold forging. As described above, machining can be performed immediately without going through the steps of quenching, tempering, and straightening. Next, the specific operation of the present invention will be described in more detail with reference to its examples.

[0028]

EXAMPLE A steel type whose composition is limited according to the present invention shown in Table 1 and a steel type for comparison which deviates from the composition range of the present invention are respectively melted by a vacuum melting method, and a process chart of the example of the present invention shown in FIG. According to the above, hot rolling was performed under the rolling conditions shown in Table 2, cooling to room temperature, and then drawing processing with a surface reduction rate of 25 to 30% was performed. The conventional example follows that of FIG. The steel compositions of Nos. 12 to 14 in Table 1 are within the scope of the present invention, but the processing conditions shown in Table 2 are deviated.

The cold forgeability is the adhesion of the lubricating coating by the Bowden tester (the number of sliding times when the dynamic friction coefficient μ = 0.2).
Also, the die life when the actual cold forged parts were manufactured was evaluated.

The results are shown in FIG. 2 and Table 2. Nos. 1 to 7, which are examples of the method of the present invention, have more sliding times at the dynamic friction coefficient .mu. = 0.2 of the obtained steel than those of Comparative Examples 8 to 15, and are actually cold forged parts manufactured from them. It can be seen that the die life is long and the cold forgeability is improved in the example of the present invention. In addition, the bending of the actual final part is small, and the strength is comparable to conventional steel.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described above, the steel for cold forging manufactured by the method according to the present invention has the strength ensured without impairing the ductility and toughness by the combination of the fine ferrite and pearlite structure and the drawing process. In addition, the cold forgeability of the material is improved by improving the adhesion of the lubricating coating, and hot working to machining can be continuously performed. As a result, the processes of annealing between hot rolling and drawing, the quenching / tempering after cold forging, and the straightening, which have been performed conventionally, are no longer required, and production efficiency is improved and production cost is reduced. Is possible, and its practical value is extremely high.

[Brief description of drawings]

FIG. 1 is a comparison diagram of manufacturing processes of a conventional method and a method of the present invention.

FIG. 2 is a graph showing the relationship between the number of slides and die life by the Bowden tester.

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[Procedure amendment]

[Submission date] September 29, 1993

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/04

Claims (1)

[Claims] 1. By weight%, C: 0.05 to 0.25%, Si: 0.30% or less, Mn: 0.50 to 2.
00%, P: 0.025% or less, S: 0.025% or less, N: 0.0060%
Below, V: 0.05 to 0.30%, Al: 0.010 to 0.100%, Cu: 0.10 to 0.50%, Ti: 0.005 to 0.050%, a steel slab consisting of the balance Fe and unavoidable impurities is heated to a temperature range of 1000 to 1150 ° C. After doing
Hot rolling, finishing in a temperature range of 800 to 900 ° C, cooling from the finishing temperature to 500 ° C at a cooling rate of 1 to 5 ° C / sec, and then cold drawing. Method for manufacturing hot forging steel.