JPH01104718A - Manufacture of bar stock or wire rod for cold forging - Google Patents

Abstract

PURPOSE:To efficiently manufacture a bar stock or wire rod for cold forging having superior quality at a low cost by forming a steel of a specific C content into a mixed structure of fine ferrite and pearlite by means of final finish rolling at a specific temp. and then applying annealing to the above for a specific short time. CONSTITUTION:A steel containing 0.1-0.8% C is hot rolled into a bar steel or a wire rod. At this time, the final finish rolling of the steel stock is carried out at 700-800 deg.C, by which the steel stock is formed into a mixed structure of ferrite and pearlite of grain size No.8 or above. Subsequently, this steel stock is reheated to (Ac1+15+ or -10) deg.C at a heating rate, preferably, of >=300 deg.C/h and held for 5-60min. The steel stock is cooled, preferably, from this temp. down to 720 deg.C at >=30 deg.C/h cooling rate, and then cooled from 720 deg.C to 680 deg.C at a cooling rate lower than 10-30 deg.C/h. By this method, the bar stock or wire rod for cold forging having superior characteristics, such as hardness and degree of spheroidizing, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method of manufacturing rods and wires to be subjected to cold forging.

f Conventional technology] Cold forging is often used as a method for forming mechanical structural parts from steel materials, but due to problems such as short tool life and easy occurrence of processing cracks, spheroidizing annealing is performed before forging. Processing is taking place.

This heat treatment usually requires 10 hours or more, and shortening this time has been a long-standing challenge from the standpoint of productivity and energy conservation.

There are two ways to shorten the spheroidizing annealing time: improving the steel material and improving the annealing method. As an example of improved steel materials, JP-A-60
-152827, Jikaiame No. 80-255922, etc.

After rolling these steel materials, they are rapidly cooled to form a mixed structure of fine ferrite-pearlite and bainite or martensite, thereby increasing the rate of spheroidization of cementite.

However, in this method, the cementite is excessively fine and the hardness is considerably higher than that in the conventional method, so that the intended purpose of spheroidizing annealing cannot be achieved.

On the other hand, improvements in the annealing method are disclosed in Japanese Patent Publication No. 80-22050 and Japanese Patent Publication No. 81-15930. These are made of rods and wires in the form of strands and heated to a temperature of 30°C or more higher than the A c 1 transformation point using a special heating device for 300°C.
Rapid heating at 0°C/h or higher, holding for a short time, and slow cooling for 3
By performing cooling at a rate of 0° C./h or more, the annealing time is shortened to within 60 minutes.

However, this method not only requires modification of equipment such as special heating equipment, but also tends to result in insufficient spheroidization, and also has the problem that variations in the material properties of the annealed materials are carried over after annealing, increasing the variation in the properties of the annealed materials. Ta.

As described above, all of the conventional methods for shortening the spheroidizing annealing time have problems and have not been put to practical use.

[Problems to be solved by the invention] When cold forging steel rods and wire rods for machine structural use, spheroidizing annealing is performed for 10 hours or more in order to spheroidize and soften the cementite, but this does not improve productivity or save energy. Therefore, shortening the annealing time has been a long-standing challenge.

The present invention is for cold forging, which combines increasing the ferrite fraction through controlled rolling, which was previously considered unrelated, and improving the material through grain refinement, shortening the holding time, and improving the annealing method to slowly cool only a specific temperature range. The present invention provides a method for manufacturing rods and wires.

[Means for Solving the Problems] In order to solve the problems of the prior art, the present inventors conducted research on shortening the spheroidizing annealing time and obtained the following findings.

(1) In the spheroidizing annealing process, the slow cooling process is the most important among the heating and holding to a temperature just above the A c 1 transformation point and the slow cooling process, especially in a specific range centered on the temperature at which cementite reprecipitates. When the cooling rate is increased, the spheroidization of cementite becomes significantly worse and the hardness increases.

(2) However, by refining the ferrite-pearlite crystal grains of the material, which was conventionally considered to be almost unrelated, it is effective to improve the degree of spheroidization even if the slow cooling rate during annealing is increased. be.

(3) When such fine-grained steel is annealed, the heating rate to just above the A C1 transformation point should be moderately fast, and the A C1
The degree of spheroidization can be further improved by appropriately shortening the holding time at a temperature just above the transformation point.・(4) A, c After maintaining the temperature above the transformation point, the cooling rate of A r to just above the transformation point is increased to speed up the precipitation of cementite in the subsequent slow cooling process that crosses one point of A r , it is possible to improve the degree of spheroidization.

(5) Controlled rolling not only refines the crystal grains of the steel material, but also increases the ferrite microstructure fraction by 0. If the above-mentioned suitable short-time annealing is applied to a material that has been spheroidized, it can be annealed well, with not only the degree of spheroidization but also the hardness comparable to that of the conventional spheroidization annealing method that involves long-term retention and slow cooling. wood is obtained.

Based on these findings, we have come to invent the present invention. That is, in the present invention, when hot rolling steel containing 0.1 to 0.8% of CO into a steel bar or wire rod, the final finishing rolling is performed at 700%.
The A c
1+ Reheat to 15±10℃ and hold for 5 to 60 minutes, then heat the temperature range from 720℃ to 880℃ or less by 10℃
A method for producing a rod and wire rod for cold forging, characterized by cooling at a rate of from 30° C./h to less than 30° C./h.
The gist is that the heating rate for reheating to 1+15±10°C is 300°C/h or more, and the cooling rate from this temperature to 720°C2 is 30°C/h or more.

[Function] The reasons for the steel composition, rolling conditions, and annealing conditions limited in the present invention will be explained below.

If C is less than 0.1%, sufficient strength as a mechanical structural component cannot be obtained, and the softening effect of spheroidizing annealing is also reduced. On the other hand, if it exceeds 0.6%, the hardness will be high even after spheroidizing annealing, making it unsuitable for cold working.
% range.

Elements other than C do not need to be particularly limited when applying the present invention, but are preferably within the following ranges.

Sl is effective for increasing product strength and hardenability, but it has a large effect of increasing the hardness of annealed materials, and the upper limit is set at 0o.
It should be 35%.

The lower limit of Mn needs to be 0.15% for hardenability and prevention of hot embrittlement due to S, but the upper limit is set to 1.5% to increase the hardness of the annealed material.

Although S is effective in improving machinability, it increases inclusions that cause hot embrittlement and processing cracks, so the upper limit should be set at 0.
10%.

Cr, Mo, and N1 are effective in increasing the hardenability, but in order to increase the hardness of the annealed material, the upper limit of each is set to 1.
．． 2%, 0.5% and 0.5%.

Although a small amount of B is effective in increasing hardenability, it hardly increases the hardness of the annealed material. Therefore, Mn.

It is an element useful for reducing hardness after spheroidizing annealing by replacing it with hardenability increasing elements such as Cr and Mo.

However, if excessively added, the product toughness deteriorates, so to prevent this, the upper limit should be set to 0.003%.

Next, we will discuss the rolling conditions and structure of the steel material.

The reason why the upper limit of the final finish rolling was set at 800℃ is that by performing the final rolling at a temperature below this temperature, the austenite crystal grains are refined, the ferrite and pearlite that are transformed from these are refined, and the transformation temperature is reduced. This is because by raising the temperature, it is possible to increase the amount of ferrite and soften it.

Therefore, if the temperature exceeds 800°C, ferrite and pearlite will become coarse and such an effect cannot be expected.

On the other hand, if it is less than 700°C, the rolling load will be significantly large, making it difficult to carry out finish rolling, and in order to keep the steel material temperature below 700°C during finish rolling, cooling treatment such as water cooling must be performed during rolling. First, 700°C was set as the lower limit in order to increase the hardness of the annealed material by generating a lower transformed structure such as hard bainite in the surface layer.

It is desirable to add slow cooling to this controlled rolling at a cooling rate of 0.5°C/see or less after rolling, as this will increase the ferrite fraction of the material and further reduce the hardness of the annealed material. .

Note that in low-alloy steel containing large amounts of Cr, Mo, N1, etc., the generation of bainite etc. can be greatly suppressed by lowering the rolling temperature, but slow cooling after rolling is required to create a structure consisting only of ferrite and pearlite. Required.

The reason for limiting the grain size to No. 8 or higher is that when the spheroidizing annealing time is shortened as described below, the spheroidization of cementite is insufficient in conventional steel materials, but by setting the grain size to No. 8 or higher, This is because a good spheroidal structure can be obtained.

Next, the annealing conditions will be described.

The heating rate from room temperature to the holding temperature above the Ac1 transformation point is:
Although the effect on the hardened material is relatively small, in the case of fine-grained steel with a grain size of 8 or more, the degree of spheroidization is improved by setting it to 301)'C/h or more. However, if the heating rate is too high by 1, the spheroidization will be insufficient, so it is preferable that the heating rate be within 2504)'C/h.

Next, if the holding temperature is lower than Acm+15-10℃, undissolved cementite remains and the degree of spheroidization deteriorates, resulting in A CL +
On the contrary, if the temperature exceeds 15 + 10°C, most of the cementite becomes solid solution, producing recycled pearlite and significantly deteriorating the degree of spheroidization.

The time to hold at this temperature was limited to 5 to BO minutes because if it was less than 5 minutes, the solid solution of cementite would not be sufficient, and if it exceeded 60 minutes, it would be excessively dissolved, but in order to shorten the annealing time, , preferably within 5 to 30 minutes.

Slow cooling in the 720-680°C range is most important for cementite spheroidization, and if the slow cooling starts below 720°C or slow cooling ends above 68°C (1'C), the precipitation of spheroidal cementite will be hindered and the degree of spheroidization will be markedly reduced. Due to deterioration, the slow cooling section was
The range was limited to the temperature range from 0°C to 880'C or less.

During this time, if the slow cooling rate is 38℃/h or more, recycled pearlite will be generated, the degree of spheroidization will deteriorate, and the hardness will increase.
The temperature was less than ℃/h. Also, the slower the slow cooling rate, the better.
If the annealing time is less than 10°C/h, the effect is small even if the annealing time is further delayed, and the annealing time is unnecessarily extended, so the lower limit is set to 10°C/h.

When using the hot rolled material of the present invention, by setting the cooling rate from the holding temperature to the slow cooling start temperature to 30°C/h or more, it is possible to improve the degree of spheroidization without increasing hardness. .

[Example] Examples according to the present invention will be specifically described below in comparison with comparative examples according to the conventional method.

Table 1 shows the chemical composition of the sample materials.

All of the sample materials satisfy the composition standards for mechanical structural carbon steel and low alloy steel specified by JIS.

All of these were cast by continuous casting after melting in a converter furnace. 1
After blooming into a 20 square steel billet, it was rolled under the rolling conditions shown in Table 2.
2 It was rolled into a round steel bar.

After rolling, the steel was left to cool on a cooling bed (steel material cooling rate 0.8℃
/5ee) and one that was covered with a heat insulating cover immediately after rolling and slowly cooled (steel material cooling rate 0.3°C/5ee) were manufactured.

Table 2 also shows JIS GO552 of this rolled material.
The grain size, structure, and ferrite structure fraction based on

Furthermore, the Ac1 transformation temperature of each steel material, spheroidizing annealing conditions, and material evaluation results of the spheroidizing annealing material are also shown.

Evaluation of spheroidized annealed material is based on hardness and JIS G353
Two points of the degree of spheroidization as defined in No. 9 were examined.

The target material quality achieved by spheroidizing annealing is hardness (Hv of 105
(%C+%St/3+%Mn/6+%Cr/19
) +72.6 (points) or less, degree of spheroidization is NQ
, 2 or less.

In Table 2, the level No. is marked with a circle: 6 and 11
-15, 17-19-21 is an example of the present invention, and compared with the example using the conventional method (long-term retention + long-time slow cooling) shown in level N (Ll, 3, 1B, 18, 20), Even if the total annealing time (1) is reduced to 1/2 to 1/'3 of the conventional method, the hardness and degree of spheroidization are equal to or higher than those obtained by the conventional method, and both of the softening targets mentioned above are achieved. I can see that you are satisfied.

The nine examples shown in levels 5, 7 to io, and 12 to 14.22 are examples in which the softening target could not be achieved.

Nα5 is an example in which the material was annealed for a short time without performing controlled rolling during rolling, and the spheroidization of cementite was poor and the hardness was somewhat high.

No. 7 is an example in which the holding temperature (TH) was set below the lower limit of the range of the present invention, and the degree of spheroidization was poor due to insufficient solid solution of cementite, and the hardness was also extremely high.

On the other hand, No. 8 is an example where the value is set above the upper limit, and cementite is dissolved excessively, resulting in recycled pearlite.
The degree of sphericity is poor and the hardness is somewhat high.

No. 9 has a retention time (1, .) shorter than the range of the present invention, and like No. 7, the degree of spheroidization is poor due to insufficient solid solution due to cementite, and the hardness is also slightly high.

On the other hand, Nα10 is an example where the retention time (tH) was too long, and No.
, 8, cementite is excessively dissolved in solid solution and the degree of spheroidization is poor. In Nα12, the slow cooling start temperature (T1) is lower than the lower limit of the range of the present invention, pearlite is precipitated without spheroidal cementite precipitated, and both hardness and degree of spheroidization are extremely poor.

N[Ll3 is an example in which the cooling rate (v2) during slow cooling was faster than the upper limit of the range of the present invention, and in this case, most of the cementite did not become spheroidized but precipitated as pearlite, and the hardness/spheroidization degree Both are defective.

No. 14 is an example in which the slow cooling end temperature (T2) was set above the upper limit of the range of the present invention, and because the slow cooling was finished before the precipitation of spherical cementite was completely completed, some pearlite formed and the hardness decreased.・High degree of spheroidization.

In addition, No. 22 is an example in which low alloy steel was not slowly cooled after rolling, and bainite was generated in the material structure.
In short-time annealing, the degree of spheroidization is good, but the hardness is extremely high.

Note that Nα6 is an example in which the heating rate (V, ) is as slow as the conventional method, and it satisfies the targets for both hardness and spheroidization, but compared to Nα4, the total annealing time (t) is longer and the hardness The degree of spheroidization is also somewhat poor.

NG, 11 is an example where the cooling rate (V t,) until the start of slow cooling is slow, and although this also satisfies the target, compared to No, 4, the total annealing time (1) is long, but it becomes hard and spheroidized. The level has not improved.

No. 15 is slowly cooled immediately after rolling (cooling rate 0, 3
"C/5ee)", and the hardness is further reduced after annealing compared to Nα4 which was not slowly cooled.

[Effects of the Invention] As explained above, the present invention solves the long-standing problem of shortening the spheroidizing annealing time of rods and wires for cold forging by improving and annealing materials that were previously considered unrelated. A cold forging rod that has an annealed material that is equal to or better than the material annealed by the conventional method, even if the spheroidizing annealing, which conventionally required 10 hours or more, is shortened to 1/3 to 1/2 by combining improvements in methods. Established a method for manufacturing wire rods.

As a result, it is of extremely high industrial value as it can significantly improve productivity and energy costs.

[Brief explanation of the drawing]

FIG. 1 is a diagram of spheroidizing annealing conditions.

Claims (2)

[Claims] (1) When hot rolling steel containing 0.1 to 0.6% C into steel bars or wire rods, the final finish rolling is performed at a temperature range of 700 to 800°C, and ferrite and pearlite with a grain size of No. 8 or higher are removed. A steel material with a mixed structure is
After reheating to c_1+15±10℃ and holding for 5 to 60 minutes, heat the temperature at 10℃ from 720℃ to 680℃ or less.
A method for producing a rod and wire rod for cold forging, characterized by cooling at a rate of from /h to less than 30°C/h. (2) Heating rate up to Ac_1+15±10℃ 300
The method for producing a rod and wire rod for cold forging according to claim 1, wherein the cooling rate from this temperature to 720°C is 30°C/h or more.