JPS6320419A - Low alloy steel wire rod permitting quick spheroidization treatment and its production - Google Patents

Abstract

PURPOSE:To produce a titled wire rod by subjecting a specifically composed low alloy steel to rough rolling and intermediate rolling, then subjecting the steel to finish rolling, coiling and cooing under specified conditions and specifying the ferrite grain size thereof. CONSTITUTION:This wire rod contains, by weight %, 0.13-0.43 C, 0.15-0.35 Si, 0.30-1.20 Mn, 0.75-1.20 Cr, <=0.10 Al, and <=0.020 S, consists of the balance Fe and has the fine ferrite pearlite structure having <=5mu ferrite grain size. The above-mentioned wire rod is obtd. by subjecting the stock having said compsn. to the rough rolling and intermediate rolling, then to the finish rolling at over 700-900 deg.C, and cooling the rolled steel down to 700-680 deg.C immediately thereafter, coiling the cooled rod and cooling the coil at 0.1-0.5 deg.C/s cooling rate down to 600 deg.C. The spheroidization annealing time as a pretreatment before the cold forging of the wire rod is considerably shortened by the above- mentioned method.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a low-alloy steel wire rod that can be quickly spheroidized and a method for manufacturing the same, and in particular to a low-alloy steel wire rod for cold forging. This is intended to make it possible to shorten the processing time in the process annealing as a pretreatment prior to the process.

(Prior Art) Low-alloy steel used for automobile bolts, nuts, rods, etc. is subjected to spheroidizing annealing before being formed into the various shapes described above by cold forging. This is because low-alloy steel as described above, in its hot-rolled state, lacks deformability to withstand cold working and at the same time has high deformation resistance, resulting in cracking and reduced tool life. This is to improve the deformability of the material through spheroidizing annealing, extend tool life, and at the same time prevent the occurrence of cracks.

However, such heat treatment requires holding at a high temperature for a long time, and therefore requires heat treatment equipment such as a heating furnace and a large amount of thermal energy. In addition, scale loss and decarbonization were unavoidable, resulting in large losses in terms of resources, energy, costs, and productivity.

As a solution to this problem, for example,
No. 45 and No. 55-31165 disclose a method in which the steel wire rod is immediately quenched after hot rolling to form an intermediate-stage composite fiber, and then annealed at a temperature range of 650°C or higher, A, or lower than the transformation point. Disclosed.

All of the techniques disclosed in the above-mentioned publications are aimed at appropriately controlling the microstructure of the steel wire rod by controlling cooling after hot rolling, and advantageously realizing the subsequent spheroidizing annealing. It is.

(Problems to be Solved by the Invention) However, in the above method, although the deformability after spheroidization treatment is improved, the deformation resistance is still high. Requires. This tendency is particularly pronounced in the low-alloy steel mentioned above, so long-term annealing is required to simultaneously improve deformability and reduce deformation resistance, which ultimately results in lower energy costs and lower heat treatment processes as a whole. The problem remained that there was not much difference in productivity compared to the conventional method.

(Means for Solving the Problems) The inventors have conducted intensive research to solve the above problems, and as a result, they have obtained the following knowledge.

First, the inventors investigated the relationship between the microstructure of a steel wire before spheroidizing annealing and the mechanical properties after spheroidizing annealing.

The results are shown in FIGS. 1 and 2 as the relationships between tensile strength, critical compressibility, and spheroidizing annealing time, respectively.

As shown in the figure, when the microstructure of the steel wire before annealing is a low-temperature transformed structure of martensite or bainite, higher deformability can be obtained by annealing for a shorter time than in the case of a ferrite/pearlite structure. It is possible. However, deformation resistance is difficult to decrease. On the other hand, when the structure before spheroidizing annealing is ferrite/pearlite, the deformation resistance is lower than that of a low-temperature transformed structure, but the deformability is also lower, and as a result, the same degree of heat treatment as conventional methods is required. I understand.

As mentioned above, the purpose of spheroidizing annealing is severe cold annealing.

The relationship between high deformability that can withstand machining and tool life requires low deformation resistance, and in order to enable rapid spheroidization, it is not enough to simply change the microstructure before annealing to the above-mentioned microstructure. It turned out to be inadequate.

Therefore, the inventors further investigated this point, and as a result, the microstructure of the steel wire before spheroidizing annealing was changed to ferrite.
Even in the case of a pearlite structure, we have found that the deformability after spheroidizing annealing can be dramatically improved by controlling the ferrite grain size to a certain size or less.

In other words, ■) Refinement of ferrite grains increases the grain boundary area and increases C
(2) Further, the refinement of ferrite grains simultaneously achieves refinement of pearlite grains, promoting fragmentation and agglomeration of cementite in pearlite parts during spheroidizing annealing. , (2) and as a result of these acting synergistically, it was discovered that the spheroidizing annealing properties, especially the deformability, were dramatically improved.

Figure 3 shows the specific experimental results.
The figure shows the relationship between the ferrite grain size and the critical compressibility after annealing for 5 hours with the heat pattern shown in the figure. By setting the ferrite grain size before annealing to 5 μm or less, the critical compressibility can be It has improved significantly.

Next, the inventors investigated the rolling and cooling conditions for obtaining the above-mentioned micro-assembly. The results are described below.

In order to obtain a fine ferrite/pearlite structure, a good (
As is known, it is necessary to keep the austenite grain size fine before transformation. This is because the austenite grain boundaries act as locations where transformation nuclei occur during the γ→α transformation. For this purpose, the grain boundary area is increased by making the austenite grain finer, and the number of transformation nuclei increases. As a result, the ferrite grain size after transformation becomes finer. The results of a survey regarding this relationship are shown in FIG. The figure is
When samples with various austenite grain sizes before transformation were transformed at the same cooling rate (0.1°C/s),
This figure shows the change in ferrite grain size.

From the same figure, it was found that in order to obtain the target ferrite grain size of 5 μm, it is necessary to make the austenite grain size before transformation smaller than 9.5 in ASTM Na.

Incidentally, the grain size before transformation has a close relationship with the rolling conditions, particularly the finishing temperature of rolling, and the cooling rate after rolling. In other words, during high-strain deformation such as wire rolling, dynamic recrystallization occurs, but this dynamic recrystallization grain is uniquely determined by the finish rolling temperature, and the higher the finish rolling temperature, the coarser the grain. This is because grain growth after recrystallization progresses more rapidly as the steel temperature increases, but the faster the cooling rate after rolling, the more austenite grain growth is suppressed.

These relationships are shown in FIG. FIG. 6 shows the relationship between the rolling end temperature, the post-rolling cooling stop temperature (coiling temperature), and the austenite grain size at the start of adjustment cooling, that is, immediately before the start of transformation. The lower the rolling end temperature and the lower the coiling temperature after rolling, the finer the grains become. temperature to 90
It can be seen that it is necessary to control the winding temperature to 0°C or lower and the winding temperature to 700°C or lower.

Furthermore, the ferrite grain size after transformation is closely related to the cooling rate after rolling. This is because the cooling rate during transformation is slow (as the cooling rate becomes slower, the growth of pro-eutectoid ferrite grains during γ→α transformation is promoted. If the cooling rate is slow, it becomes difficult to obtain a fine ferrite/pearlite structure.

Figure 7 shows that samples made of fine-grained austenite of ASTMNα9.5 were cooled at various cooling rates to change γ→α.
The relationship between the cooling rate and the ferrite grain size during transformation is shown.

As is clear from the figure, the ferrite grain size increases as the cooling rate becomes slower, and in the cooling rate range slower than 0.1 t/S, the ferrite grain size after transformation becomes larger than 5 μm. In such a structure, rapid spheroidization becomes difficult.

This invention was completed based on the above findings.

That is, in the invention, C: 0.13 to Q, 43wt%
(hereinafter simply expressed in %), Si: 0.15 to 0.35
%, Mn: 0.30-1.20%, Cr: 0.7
5 to 1.20%, Al: 0.10% or less, S: 0.020% or less, and the remainder is substantially Fe.
This is a steel wire rod which can be rapidly spheronized, and has a fine ferrite/pearlite structure with a ferrite grain size of 5 μm or less.

In addition, this invention has C: 0.13 to 0.43%, Sl: 0
．． 15-0.35%, Mn: 0.30-1.20%
, Cr: 0.75-1.20%, Al: 0.10%
As shown on the right below, a low alloy steel wire material containing 0.020% or less of S:0,000 with the remainder being substantially Fe is subjected to rough rolling and intermediate rolling, and then subsequently heated to a temperature of over 700°C to 900°C. Finish rolling is performed in the temperature range, and then immediately rolled at 700
A low-alloy steel wire rod capable of rapid spheroidization, which consists of cooling to a temperature in the range of ~680°C, winding, and then cooling to 600°C at a cooling rate of 0.1 to 0.5°C/s. This is a manufacturing method.

(for production) This invention will be specifically explained below.

First, in this invention, the reason why the material components are limited to the above range will be explained.

C: O, 13-0.43% If C is less than 0.13%, sufficient strength as a structural property cannot be obtained, while if it exceeds 0.43%, deformation resistance increases and cold workability deteriorates. Therefore, it was limited to a range of 0.13 to 0.43%.

Si: 0.15-0.35% Si has poor deoxidizing effect when it is less than 0.15%;
．． If it exceeds 35%, it is an opposition! i! 0.
It was limited to a range of 15 to 0.35%.

Mn: 0.30-1.20% Mn effectively contributes to improving hardenability, but 0.3%
If it is less than 0.3%, the effect of addition is poor, while if it exceeds 1.2%, the strength will increase and cold workability will decrease.
It was limited to a range of 0 to 1.2%.

Cr: 0.75-1.20% Cr not only improves hardenability but also improves ferrite and
Although it is a useful element in making the pearlite structure fine, if it is less than 0.75%, its addition effect is poor; on the other hand, 1.
If it exceeds 20%, the strength will increase and cold workability will deteriorate, so it was limited to a range of 0.75 to 1.20.

Al: 0.10% or lessAl has the effect of suppressing the growth of austenite grains, so it is a useful element for obtaining a fine ferrite/pearlite structure, but if it exceeds 0.10%, AI-based inclusions may occur. 0.10% because it increases and causes deterioration of cold workability.
It was decided to contain it in the following range.

S: 0.020% or less S is a useful element that improves machinability, but 0.020% or less
If it exceeds 20%, MnS-based inclusions increase and cold workability deteriorates, so it was limited to a range of 0.020% or less.

In addition, the microstructure of the steel wire has a ferrite grain size of 5 μm.
The reason for the fine ferrite/pearlite structure of less than m is as follows.
This is because if the ferrite particle size exceeds 5 μm, spheroidization progresses slowly and rapid processing cannot be performed. The ferrite/pearlite structure is used because bainite, martensite, or a mixed structure thereof has high deformation resistance after spheroidizing annealing (quick spheroidizing treatment cannot be performed).

Next, the reasons for limiting the rolling and cooling conditions will be explained.

The reason why the finish rolling temperature was set to over 700°C to 900°C was 7.
This is because if the temperature is below 00°C, the deformation resistance increases and rolling becomes difficult, while if it exceeds 900°C, the austenite grain size increases and the target fine ferrite-pearlite structure cannot be obtained.

Also, the winding temperature was set at 680 to 700°C.
When the temperature drops below ℃, a hardened structure occurs on the surface of the steel wire;
This is because if the temperature exceeds 00°C, the effect of suppressing grain growth of austenite grains becomes insufficient.

Furthermore, the cooling rate after winding was set to 0.1 to 0.5°C/s because if it is less than 0.1°C/s, pro-eutectoid ferrite will grow and it will be difficult to obtain the desired fine ferrite/pearlite structure.
On the other hand, if it exceeds 0.5°C/s, bainite fibers will be mixed in, which will increase the deformation resistance after spheroidizing annealing, making it difficult to perform a rapid spheroidizing process. The reason why the cooling stop temperature is set at 600° C. is that if it exceeds 600° C., bainite is generated, leading to an excessive increase in strength and making it difficult to soften.

(Example) Embodiments of the present invention will be described below in comparison with conventional examples.

Steel wire materials having the compositions shown in Table 1 were rolled and cooled to produce wire rods under the conditions shown in Table 2 according to the conventional method and the method of this invention. The specimens were subjected to spheroidizing annealing according to the heat pattern shown in FIG. 8, and the strength after the spheroidizing annealing was investigated. The results are shown in FIG. 9, and the results of the cold forgeability test are shown in FIG.

As is clear from the results shown in FIGS. 9 and 10, the steel wire rod obtained according to the present invention can easily obtain low deformation resistance and high deformability by short-time spheroidizing annealing, and can be quickly spheroidized. processing has been achieved.

(Effects of the Invention) Thus, according to the present invention, it is possible to significantly shorten the spheroidizing time for low alloy steel wire rods, which conventionally required a long spheroidizing process prior to cold forging. This results in a reduction in energy costs and a dramatic improvement in productivity.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between spheroidization annealing time and tensile strength when changing the microstructure before annealing. Figure 2 is a graph showing the relationship between spheroidization annealing time and tensile strength when changing the microstructure before annealing. Figure 3, a graph showing the relationship between annealing time and critical compressibility, shows the relationship between ferrite grain size and critical compressibility when the ferrite grain size of the ferrite-pearlite structure is changed and spheroidizing annealing is performed for 5 hours. Figure 4 is a graph showing the relationship between the austenite grain size before ° transformation and the ferrite grain size of the ferrite-pearlite structure. Figure 6 is a graph showing the relationship between the finishing rolling temperature and winding temperature and the austenite grain size just before the start of transformation. Figure 7 is the relationship between the cooling rate and the ferrite grain size of the ferrite-pearlite structure. Figure 8 is a graph showing the relationship with the diameter, and Figure 8 is a heat pattern diagram of the spheroidizing annealing of the steel wire rod produced in the example. A graph showing the relationship between spheroidizing annealing time and tensile strength, Fig. 10 shows the relationship between spheroidizing annealing time and critical compressibility when spheroidizing annealing was performed on the steel wire produced in the example. It is a graph. Figure 1 Figure 2! 1 large it diagonal -5rJ (h) Fig. 3 Ferrite O stop 1 (, μm) Fig. 7 Rei 1 pigeon a (”e/s) Fig. 8 As ding M No. Ferrite 1 flea (voice fn) MRfL1fi #, ) %, nvI % -V~ - g1 Zhang Pengsaku Kt Buddha m2)

Claims (1)

[Claims] 1. C: 0.13 to 0.43 wt% Si: 0.15 to 0.35 wt% Mn: 0.30 to 1.20 wt% Cr: 0.75 to 1.20 wt% Al: 0.10 wt% or less and S: 0.020 wt% or less, the remainder is substantially Fe, and has a fine ferrite/pearlite structure with a ferrite grain size of 5 μm or less. Steel wire rod that can be spheroidized. 2, c: 0.13 to 0.43 wt% Si: 0.15 to 0.35 wt% Mn: 0.30 to 1.20 wt% Cr: 0.75 to 1.20 wt% Al: 0.10 wt% or less and A low-alloy steel wire material containing 0.020 wt% or less of S, with the remainder being essentially Fe, is subjected to rough rolling and intermediate rolling, and then
Finish rolling is performed at a temperature range of over 00°C to 900°C, then immediately cooled to a temperature in the range of 700 to 680°C and coiled, and then rolled up to 600°C by 0.1 to 0.5
A method for producing a low-alloy steel wire rod capable of rapid spheroidization treatment, characterized by cooling at a cooling rate of °C/s.