JPS605821A - Production of parts steel for cold forging - Google Patents

Abstract

PURPOSE:To obtain a steel having excellent cold workability and strength and toughness after forming without requiring a spheroidizing heat treatment by hot rolling and cooling a specifically composed carbon steel or low allow steel at a prescribed temp. then repeating rolling at the limited temp. and draft and cooling the rolled steel. CONSTITUTION:A steel contg., by weight %, 0.10-0.60 C, 0.01-0.70 Si, 0.20- 2.00 Mn and 0.01-0.10 sol Al, contg. >=1 kind of <=0.15 Nb, Ti and Zr respectively and 0.001-0.030 B according to need and the balance Fe is heated to a temp. of the Ac3 point +200 deg.C or above and is hot-rolled. The hot-rolled steel is then cooled down to a temp. of the Ar1 point or below and in succession the steel is heated to a temp. region of the Ac3 point -Ac3 point +200 deg.C and is then rolled at a temp. region of the Ar3 point or above at >=10% draft per pass. The rolled steel is allowed to cool or is forcibly cooled in the range where the hardened structure is not formed right after finish rolling to >=10% draft at the Ar3 point or below.

Description

[Detailed description of the invention] This invention provides cold forging component steel with excellent cold workability.
The present invention relates to a highly efficient manufacturing method without requiring a long spheroidizing annealing process.

Until now, various steel types have been used for mechanical structural parts made by cold forging, such as bolts, nuts, and various types of shaft tilting, depending on the required strength, etc., and JI
Even in the 'S standard, carbon steel C8C material (G 405.1
) 3g chromium steel (SCr material (G4104)), chromium molybdenum steel (SCM material (+o+1o5)), nickel chromium molybdenum steel [sNcM material (G4103)]
Many component steels for cold forging are specified depending on their composition and use.

By the way, these conventional cold forging component steels require spheroidizing annealing to prevent cracking during the cold wire drawing process and to improve cold forgeability. Furthermore, even after cold forging, quenching and tempering treatments were indispensable to improve mechanical properties. In particular, for the spheroidizing annealing, 7oo to 80.0
A holding process of nearly 10 hours in a temperature range of 10°C was required, which was a major obstacle in the production of steel parts for cold forging.

Therefore, in the production of parts for cold forging, it is possible to improve work efficiency by omitting the long heat treatment process as described above or by shortening the time in this process.
Therefore, reducing product costs has traditionally been the biggest challenge.

From the above-mentioned viewpoint, the present inventors have developed a material that exhibits excellent cold workability without requiring long-term spheroidizing heat treatment.
In order to provide a part steel for cold forging that also exhibits good mechanical properties without quenching or tempering after cold forging,
As a result of many studies, experiments, and studies, we found that carbon steel or low-alloy steel with a specific composition is heated to a temperature of (Ar3 transformation point + 200℃) or higher and then hot rolled. A9N contained in steel.

When carbonitride precipitates such as NbN, NbC, 'I' iC, and TiN, which are effective in refining austenite particles, are uniformly dispersed and completely dissolved in the matrix, when cooled to below the Ar1 transformation point, The austenite fine-grained precipitates are again precipitated uniformly and finely. Then, if you heat this to an extremely low temperature range of 3 AC points or higher,
The austenite grains that are produced are extremely fine, but by rolling with a reduction of more than a certain level in each pass in this temperature range, only recrystallization is promoted while preventing abnormal grain growth. Furthermore, if the rolling is finished in a temperature range below the A'r3 transformation point and left to cool as is, or rapid cooling is continued within a range that does not result in a quenched structure, not only will cold workability be excellent, but strength and toughness after forming will be improved. (b) that an ultrafine-structured steel exhibiting extremely good values can be obtained as is;
In this case, if the secondary rolling is followed by continuous cooling or forced cooling to just below the Arc transformation point and then slow cooling, or if the cooling or forced cooling is followed by tempering at a temperature below the Act transformation point, We have come to the knowledge shown in (a) and (b) above that the elongation of the resulting steel is recovered and the cold workability is further improved.

This invention was made based on the above findings. C: 0.10-0.60% (hereinafter, the ratio representing the component ratio will be referred to as the weight ratio).

Si: 0.01~O,'70%, Mn: 0.20
~2.00%.

SOmuAQ: O, O], ~0.10%.

In addition to containing Nb: 0.15
% or less, Ti 20, 15% or less.

Zr: 0.15% or less, B: 0.001 to 0.0
30%.

It also contains one or more of the following: Fe and inevitable impurities: Residue.

Steel with a composition consisting of (Ac3 transformation point + 200℃)
After heating to a temperature above and hot rolling, it is cooled in a trench to a temperature below the Arz transformation point, and then rolled from the Ac3 transformation point to [Ac3
After heating to a temperature range of [transformation point + 200 ° C], the rolling reduction per pass is 10% or more, and the subsequent rolling reduction is 10% The upper surface is subjected to finish rolling to a temperature below the Ar3 transformation point, and then immediately left to cool or forcedly cooled, or the cooling is carried out until it becomes below the Arc transformation point and then slowly cooled, or the first step is left to cool or forcedly cooled after rolling. By tempering at a temperature below the Ac1 transformation point after completion of the work, we can produce cold forging component steel that has excellent cold workability and also exhibits good mechanical properties after cold working without quenching. It is characterized by being realized in a relatively short time and at low cost.

Note that "forced cooling" in the method of this invention means rapid cooling within a range that does not result in a hardened structure.

Next, the reason why the chemical composition of the steel and the rolling and heat treatment conditions are limited as described above in the method for producing steel parts for cold forging of the present invention will be explained.

A. Chemical composition of steel■ C C component is indispensable for imparting a certain strength to steel, but if its content is less than 0.10%, the desired strength cannot be obtained. On the other hand, if the C content exceeds 0.6.0%, cold workability and toughness will deteriorate, so the C content is set at 0.10 to 0.60%.

■ The 5I Si component is an effective element as a deoxidizing agent for steel, and also has the effect of strengthening steel, but if its content is less than 0.01%, the desired effect cannot be obtained,
On the other hand, if the content exceeds 0.70%, the toughness will be significantly lowered, so the S1 content was set at 0.01 to O,'70%.

■ Mn The Mn component has the effect of forming a solid solution in the base steel and strengthening it, as well as improving strength and toughness by refining fine grains, but if its content is less than 0.20%. In the case of 2., the desired effect of improving toughness cannot be obtained.
If the Mn content exceeds OO%, cold workability deteriorates, so the Mn content was set at 0.20 to 2.00%.

■ Sob, Ae sot, and Al components have excellent grain refining effects and are extremely important elements, especially in the method of this invention that performs primary rolling to obtain fine precipitates such as AAN. . However, if the content is less than 0.0I, the desired grain refining effect cannot be obtained, and on the other hand, if the content exceeds 0.10%, the amount of nonmetallic inclusions will increase rapidly and the machinability will be impaired. Since the sot and Alf content is reduced to 0.0
It was set at 1 to 0.10%.

■Nb, Ti, and Zr All of these components have the effect of refining the steel structure and increasing strength and ductility through the formation and precipitation of carbides. Each species contains 0.1 or more seeds.
Even if the Nb content exceeds 4%, it will not be possible to obtain any further strength-increasing effect, but it will also lead to deterioration of toughness.

The Ti and Zr contents were each set at 0.15% or less.

■ B The B component has the effect of improving the hardenability of steel and providing a microstructure to the rolled material, so it can be included as necessary, but if its content is less than 0.0 OL %, the above-mentioned It is not possible to obtain the desired effect in the action, and on the other hand, 0.
Even if the B content exceeds 0.030%, no further improvement in the above-mentioned action can be obtained and it may lead to deterioration of the toughness, so the B content was set at 0.030% to 0.030%. .

B. Rolling/Heat Treatment Conditions - Next Rolling Conditions The solidification structure of steel with the above chemical composition is generally
Precipitates such as MN, NbN, NbC, TiC, and TiN, which are effective in refining austenite, are dispersed non-uniformly, and some of them are coarse precipitates. Even if heated just above the transformation point, it is extremely difficult to form a uniform fine austenite structure. However, when this is heated to a temperature higher than (Ac3 transformation point + 200°C) and then subjected to primary hot rolling to an intermediate size, the austenite refining component completely dissolves into the base material, and then By once cooling this to a temperature below the Arc transformation point, the fine austenite precipitates can be uniformly and finely dispersed and precipitated.

If the heating temperature during this second rolling is less than (AC3 transformation point + 200°C), it will be extremely difficult to achieve complete solid solution of the austenite refining component, and the final microstructure will become finer and more uniform. This becomes an impediment to the development of

In addition, primary hot rolling at a heating temperature below the Ac3 transformation point causes significant grain growth, making it difficult to achieve the desired grain refinement.
It is preferable to carry out low-temperature rolling at a finishing temperature of A r 3 transformation point or lower, since this makes the austenite grains even finer after reheating.

■ Secondary rolling conditions Secondary hot rolling is particularly important in order to obtain the final objective of steel with fine α grains, high formability, high strength, and high toughness.

That is, after heating to a temperature in the range of AC3 transformation point to [Ac3 transformation point + 200 ° C.], the reduction rate per pass is set to 10% or more to suppress abnormal growth of γ grains, and the temperature is below the Ar3 transformation point. After finishing by rolling under the condition of adding 10% or more processing, and then letting it cool or rapidly cooling it without forming a hardened structure, the growth of α grains can be effectively prevented, resulting in high toughness. can be obtained.

If the heating temperature during secondary rolling is less than the Ac3 transformation point, it will not be possible to obtain uniformly fine austenite grains, while on the other hand, the austenite grains heated to a temperature exceeding (AC3 transformation point + 2oo°C) will become coarse, and eventually However, the desired final structure of fine grains cannot be obtained.

Furthermore, if the rolling reduction per bath above the Ar3 transformation point is less than 10%, abnormal growth of γ grains will occur, and if the rolling reduction is less than 10 inches below the Ar3 transformation point, the fineness of crystal grains will occur. As a result, the desired high toughness cannot be achieved due to insufficient granulation and particle size regulation. By performing natural cooling or forced cooling after the secondary cooling, the growth of α grains is prevented and the refinement of the structure is ensured.

■ Slow cooling or tempering treatment conditions after the completion of secondary rolling When cooling after the completion of secondary rolling, do slow cooling occur after the steel material temperature falls below the Arc transformation point, or do you perform slow cooling after the completion of cooling?
Tempering at a temperature below the transformation point recovers the elongation of the copper phase after rolling and further improves cold workability, but if the slow cooling start temperature is above the Arc transformation point or the tempering temperature is A
If the temperature is higher than the ct transformation point, the steel H strength decreases and desired mechanical properties cannot be obtained.

Steel with fine-grained 7-elite and pearlite colonies manufactured under the above conditions not only has excellent deformability required during cold forging, but also exhibits good values for strength and toughness, so it is superior to conventional steel. Long-time spheroidizing heat treatment process like steel parts for cold forging and quenching after cold forging.
It becomes possible to obtain a cold forged part having the desired strength and toughness without requiring a tempering process.

Next, the present invention will be explained using examples and comparing with comparative examples.

Example First, carbon steel and low alloy steel having the compositions shown in Table 1 were melted using a normal melting method, and then a diameter of 1.
It was made into a billet of 807nfL.

Next, this billet was subjected to rolling and heat treatment under the conditions shown in Table 2 to produce a wire rod with a diameter of 25 mm.

Next, using the wire rod obtained in this way, after performing cold upsetting forging in the process shown in Fig. Evaluate,
Furthermore, a test piece was cut out from the product shaft and subjected to a tensile test, and the results are also shown in Table 2.

From the results shown in Table 2, it is clear that all of the steel materials obtained by the method of the present invention have excellent workability and also exhibit good mechanical properties, whereas any of the manufacturing conditions It is clear that steel materials obtained by comparative methods outside the scope of the present invention are inferior in these properties.

As described above, according to the present invention, excellent cold workability can be exhibited without performing a spheroidizing heat treatment that requires a long time, and strength and strength can be improved even without a quenching treatment after cold-open forming. Industrially useful effects are brought about, such as the ability to produce cold-forging component steel with excellent toughness at low cost without requiring special equipment.

[Brief explanation of drawings]

FIG. 101 is a process diagram of cold upsetting forging employed in an example of the present invention. Applicant: Sumitomo Metal, Gens Kogyo Co., Ltd. Agent: Tomi 1) Kazuo and 1 other person

Claims (3)

[Claims] (1) Weight percentage: C: 0.10-060%. Si:O, O2N2.70%. Mn: 0.20-2.00%. sob-M2 0.01-0.10%. and, if necessary, further Nb: 0.15
% or less, Ti: 0.15% or less. Zr: 0.15% or less, B2 0.001 to 0.03
0%. It also contains one or more of the following: Fe and unavoidable impurities: residue. Steel with a composition consisting of [Ac3 transformation point + 200℃]
After heating to a temperature above and hot rolling, it is cooled to a temperature below the Arc transformation point, and then the AC3 transformation point ~ (Ac3
After heating to a temperature range of transformation point +20C)C, ], rolling in a temperature range of Ar3 transformation point or higher, where the rolling reduction per pass is 10% or more, and the subsequent rolling reduction is 10%.
A method for producing cold forging component steel with excellent workability, characterized by subjecting it to the above-described finish rolling below the Ar3 transformation point, and then immediately allowing it to cool or forcedly cooling. (2) C: 0.10 to 0.60% by weight. Si: 0.01~O,'70%. Mn: 0.20-2. O0%. soL, AQ: 0.01-0.10%. and, if necessary, further contains Nb: Oyl
5% or less, Ti: 0.15% or less. Zr: 0.15% or less, B: 0.001 to 0.03
0%. It also contains one or more of the following: F'e and unavoidable impurities: the remainder. Steel with a composition consisting of [Ac3 transformation point + 200 ° C]
After heating to a temperature above and hot rolling, it is cooled to a temperature below the Ar□ transformation point, and then the AC3 transformation point ~ (Ac3
After heating to a temperature range of transformation point + 200 ° C], rolling in a temperature range of Ar3 transformation point or below, where the rolling reduction per pass is 10% or more, and the subsequent rolling reduction of 10%. Part steel for cold forging with excellent workability, characterized by subjecting it to the above finish rolling below the Ar3 transformation point, followed by immediate cooling or forced cooling to below the Arc transformation point, and then gradual cooling. manufacturing method. (3) C: 0.10 to 0.60% by weight. Si: 0.01-0.70%. Mn: 0.20-2.00%. SOmuAg: o, o1-0.10%. and, if necessary, further Nb: 0.15
% or less, Ti: 0.15 or less. Zr: 0.15% or less, B: 0.001 to 0.03
0%. Also contains one or more of the following: Fe and unavoidable impurities: remainder. Steel with a composition consisting of (Ac3 transformation point + 200℃)
After heating to a temperature above and hot rolling, it is cooled to a temperature below the Arc transformation point, and then the Ac3 transformation point ~ ('Ac
3Ac3 transformation 00°C], then rolling in a temperature range of Ar3 transformation point or higher where the rolling reduction per bath is 10q6 or higher, followed by rolling with a rolling reduction of 10q or higher per bath.
A cold-forged part with excellent workability, characterized in that it is subjected to finish rolling at a temperature of 6 or higher and below the Ar3 transformation point, then immediately left to cool or forcedly cooled, and then tempered at a temperature below the Ac4 transformation point. Method of manufacturing steel.