JPH0699743B2 - Method for manufacturing softened rods / wires - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method of manufacturing a softened wire rod and a softened steel bar.

[Prior Art] As is well known, among cold-forged wire rods and steel bars, those that are hard and have poor formability are subjected to spheroidizing annealing in order to lower their hardness and improve their deformability in advance. It is generally done. Since this annealing is usually a very long treatment at a temperature of A1 point or higher, reducing gas containing CO or H ₂ is used as atmosphere gas to prevent decarburization of the steel surface by oxidizing gas. There is. However, when scale is present on the steel surface, reducing gas reacts with the scale to generate oxidizing gas, and decarburization is rather promoted due to an increase in oxidizing gas concentration. Therefore, before the spheroidizing annealing of the wire rod and the steel bar by the conventional method, a chemical scale removing step by pickling is essential.

On the other hand, in Japanese Examined Patent Publication No. 59-18447, this scale removing step is omitted, and the scale remains attached to the surface of the wire rod, and it is not an expensive reducing gas but an inexpensive inert gas such as N _2. A method of performing spheroidizing annealing with a gas is disclosed. In this method, the wire rod is wound at a temperature of 850 ° C or higher after hot rolling, allowed to stand for 8 seconds or longer, and then cooled at a rate of 4 ° C / second or higher to form a scale of 8μ or more on the surface of the steel material, and The method of making the inside into a quenched structure (sorbite, bainite, martensite) is adopted.

[Problems to be Solved by the Invention] However, according to the above method, since the wire rod structure after rolling becomes a quenched structure such as bainite or martensite, there is a possibility that breakage or misplacement cracking may occur during handling after rolling. Furthermore, when this is subjected to spheroidizing annealing,
Since the carbide becomes extremely fine, it has excellent cold workability, but has a high hardness and a short die life during cold forging.

Therefore, the main object of the present invention is not to decarburize even if annealed in an inexpensive inert gas without requiring the above scale removal step, and it does not cause breakage or set cracks after rolling, and hardens after annealing. Another object of the present invention is to provide a method for manufacturing a softened wire or the like, which can be appropriately reduced and can prolong the life of a die during cold forging.

[Means for Solving the Problems] The present invention for solving the above problems controls the finish rolling end temperature to 850 ° C. or lower when hot rolling a steel material containing 1.5% or less C. While finishing rolling, after finishing, 500 ℃ at a cooling rate of 0.5 ℃ / sec or more and less than 4 ℃ / sec.
After cooling to room temperature and then to room temperature, spheroidizing annealing is performed in an atmosphere containing 90% or more of an inert gas with the scale on the surface of the product adhered.

[Operation] In the present invention, the temperature control in finish rolling is performed as described above, and the subsequent cooling rate is within a predetermined range. Therefore, the steel structure after rolling is rapidly cooled to sorbite, bainite, martensite, or the like. Instead of a microstructure, a mixed structure of ferrite / pearlite or ferrite / pearlite / spherical cementite can be used, and the hardness after annealing can be reduced appropriately, so breakage etc. after rolling can be prevented, and cold forging The mold life can be extended.

Further, in the present invention, since the structure and thickness of the scale can be adjusted within a predetermined range, decarburization during annealing can be effectively prevented.

In the present invention, it is not necessary to use an expensive reducing gas at the time of annealing, and the manufacturing cost is reduced accordingly.

Further, in the present invention, since the scale can be annealed with the scale attached, the decarburizing scale step can be omitted and the steps can be simplified.

[Specific Configuration of the Invention] Next, the reasons for limiting each numerical value in the present invention will be described in detail. First, the amount of C contained in steel is limited to 1.5% or less. Generally, when so-called pro-eutectoid cementite that precipitates in a mesh-like shape is present in steel, surface cracks occur from the pro-eutectoid cementite as a starting point during cold forging. If the C content exceeds 1.5%, it is impossible to prevent the formation of the above pro-eutectoid cementite under any conditions of rolling and cooling, and it is impossible to eliminate it by subsequent annealing. Because there is.

Next, controlling the finish rolling end temperature to 850 ° C or lower is
There are two reasons. First, as shown in FIG.
When the 850 ° C. or less, the composition of the surface scale products, effective Fe ₂ O ₃ to prevent decarburization of spheroidizing annealing in an inert gas in the next step, Fe ₃ O ₄ (in particular, Fe ₃ This is because the composition has a large proportion of O ₄ ). Secondly, when the finish rolling finish temperature exceeds 850 ° C, the scale becomes thicker,
This is because the scale is likely to be peeled off during handling after rolling, and the peeled off portion is decarburized during spheroidizing annealing in an inert gas atmosphere.

In addition, as shown in FIG.
If it exceeds 20 μm, it tends to occur rapidly, but as shown in FIG. 3, when the finishing rolling end temperature is 850 ° C. or lower, the cooling rate thereafter is 0, 5 ° C./sec or more,
Since the scale thickness is 20 μm or less, the cooling rate after finishing rolling is 0.5 ° C./sec or more. Also, the cooling range is 5
The reason why the temperature is set to 00 ° C is that there is almost no change in the scale thickness and composition at 500 ° C or less. In addition, the cooling rate was set to less than 4 ° C / sec. When the cooling rate was higher than 4 ° C / sec, the product structure became a quenched structure such as bainite and martensite, and after rolling, the product was placed before spheroidizing annealing. This is because there is a high possibility that cracking or breakage during handling will occur, and, as already mentioned, the hardness will be high even if spheroidizing annealing is performed. On the other hand, if it is less than 4 ° C./second, the structure of the product does not become a rapidly cooled structure and the above problems do not occur. From the above, the cooling rate after finish rolling was set to 0.5 ° C / sec or more and less than 4 ° C / sec.

Next, the reason why the wire rod and the steel bar rolled by the above method are subjected to spheroidizing annealing in an atmosphere containing 90% or more of an inert gas with the scale attached to the surface will be described.

As shown in Fig. 4, when the wire rod and the steel bar are annealed with the scale adhered to the surface, when the content of the inert gas (N ₂ etc.) in the annealing atmosphere becomes lower than 90%, the surface layer portion is abruptly increased. Since the decarburization of No. 1 proceeds, it was specified that annealing should be performed in an atmosphere containing 90% or more of an inert gas. In this case, an inexpensive inert gas such as N ₂ can be used as the inert gas, and it is not necessary to use an expensive reducing gas.

It should be noted that, in addition to the specific conditions adopted in FIGS. 1 to 4 referred to in the above description, FIG.
FIG. ₃ shows the finish rolling temperature and the composition ratio of Fe ₃ O ₄ and Fe ₂ O ₃ contained in the scale after rolling when a wire rod having a diameter of 35 and 18 mm was manufactured. The cooling rate after rolling is 2 ° C./sec.

Figure 2 shows SCM435, 18 mm with scales of various thickness
It shows the scale thickness and scale peeling rate (% by weight) when a wire of φ is pulled in the axial direction and a plastic deformation of 2% is applied.

FIG. 3 shows the relationship between the finish rolling finish temperature and the scale thickness after rolling when the SCM435, 18 mmφ wire rod was manufactured, for each cooling rate after finish rolling.

Figure 4 is, N ₂ content in the spheroidizing annealing atmosphere when the scale thickness is subjected to annealing heat pattern shown in FIG. 5 to SCM435,18mmφ wire 14 [mu] m (% by volume) and surface de-after annealing This figure shows the relationship of coal seam depth.

The steel types used in FIGS. 1 to 4 are all SCM435, but the present invention can be applied to other steel types.

[Example] Next, an example will be described.

Two-ton steel pieces of S45C, SCM435, and SUJ2 were manufactured into 18 mmφ wire rods by changing the finish rolling finish temperature and the cooling rate after finish rolling. After the wire rod was manufactured, its structure was observed, and these wire rods were cut into a length of 500 mm to prepare 1000 samples, which were left for 7 days to confirm the occurrence rate of cracking. Next, with the surface scale remaining on the remaining wire rod, for S45C, use the heat pattern in Fig. 6 for SC
Spheroidal annealing was performed for M435 with the heat pattern shown in FIG. 5 and for SUJ2 with the heat pattern shown in FIG. The N ₂ content in the spheroidizing annealing atmosphere is 97% in all cases.

After spheroidizing annealing, each hardness and the surface decarburized layer depth were measured. The results are shown in Table 1.

In Table 1, Nos. 1 to 6, 11 to 16, and 21 to 26 are wire rods manufactured according to the method of the present invention, and No. 7 to 10, 17 to 20,
27 to 30 are wire rods manufactured by a method other than the present invention. The winding temperatures of these wire rods are within the range of the finish rolling finish temperature or higher and the finish rolling finish temperature of -20 ° C or lower.

As is clear from Table 1, in the wire rods manufactured under the conditions within the scope of the present invention, the structures after finish rolling and cooling are not quenching structures (bainite, martensite),
The rate of occurrence of misplacement cracks before spheroidizing was 0%,
The depth of the surface decarburized layer after spheroidizing is 0.10 μm or less, and the hardness is sufficiently low.

On the other hand, the wire rods of Comparative Examples are inferior in any of the occurrence rate of placement cracking before spheroidizing annealing, the surface decarburized layer depth after spheroidizing annealing, and the hardness.

[Effects of the Invention] As described above, according to the present invention, no scale removal step is required, decarburization does not occur even when annealed in an inexpensive inert gas, and breakage or set crack does not occur after rolling. It is possible to provide a method for manufacturing a softened rod / wire that can sufficiently reduce the hardness after annealing and prolong the life of a die during cold forging.

[Brief description of drawings]

FIG. 1 shows the relationship between the finish rolling finish temperature and the scale composition, FIG. 2 shows the relationship between the scale thickness and the scale peeling rate, and FIG. 3 shows the relation between the finish rolling finish temperature and the scale thickness. FIG. 4 is a diagram showing the relationship between the N ₂ gas content and the surface decarburized layer depth, and FIGS. 5 to 7 are diagrams showing various heat patterns.

Claims (1)

[Claims] 1. When hot rolling a steel material containing 1.5% or less of C, finish rolling is performed while controlling the finish rolling end temperature to be 850 ° C. or lower, and after finishing, 0.5 ° C./sec or more,
After cooling to 500 ° C. at a cooling rate of less than 4 ° C./sec and then to room temperature, spheroidizing annealing is performed in an atmosphere containing 90% or more of an inert gas with the product surface scale attached. A method for manufacturing a softened rod / wire, which is characterized in that