KR100354213B1 - Manufacturing method of ferritic 430 stainless steel wire with excellent cold workability - Google Patents

Abstract

The present invention provides a method for manufacturing a ferritic 430 stainless steel wire having excellent cold workability by the component, the drawing rate and heat treatment of the ferritic 430 stainless steel wire manufactured by bolts or screws. Its purpose is to.

According to the present invention, a ferritic 430 stainless steel wire composed of 0.12 wt% or less of C, 1.0 wt% or less of Si, 1.0 wt% or less of Mn, 16-18 wt% of Cr, balance Fe and other unavoidable impurities The manufacturing method includes a cold drawing process and an annealing heat treatment step. In the cold drawing process, the wire is manufactured by wire rolling by hot rolling and heat-treated heat-treated wire or brightly annealed wire after primary drawing. Cold drawing with 7-16% freshness reduction rate, and in the annealing heat treatment step, ferrite having excellent cold workability, characterized in that annealing the wire or wire after the cold drawing processing step at 770-820 ° C. for 20-40 seconds. Provided is a method of making a 430 stainless steel wire.

Description

Manufacturing method of ferritic 430 stainless steel wire with excellent cold workability

The present invention relates to a method for manufacturing a ferritic 430 stainless steel wire manufactured by bolts, screws, etc. by a cold rolling process, and in particular, by controlling the cold drawing rate of wire and heat treatment conditions of bright annealing, suitable for cold rolling process It relates to a method for producing a ferritic 430 stainless wire having excellent workability.

A method of manufacturing a conventional ferritic 430 stainless wire will be described in detail.

In general, the material used in the cold rolling process is made of wire rod in the wire rolling mill.

Then, in the hot rolling, the carbides and surface defects existing in the wire rod are removed, and the homogenization treatment is performed in a solid solution heat treatment furnace to homogenize grains and mechanical properties.

After pickling the homogenized wire rod to be suitable for the cold rolling process, it is immediately used for cold rolling, or it is first cold drawn to make a small size of cold rolled wire bolts and screws. Manufactured by lines of size.

The wires produced in the sizes of bolts and screws are usually subjected to annealing in a bright annealing furnace which is heat-treated at 800 to 850 ° C. for 20 to 40 seconds, and the annealing heat-treated wire is used for cold pressing.

The wire or wire manufactured in the above manufacturing process is a material having homogenized crystal grains, and cracks are generated in the header of the bolt or the screw when cold pressing the bolt or the screw in the cold pressing process.

These cracks are caused by the cold workability of the material.

On the other hand, Japanese Patent (JP-55138057) adds bronze (B) which improves cold workability as a method for suppressing cracks generated during cold pressing, and coarsening of surface grains in solid solution heat treatment or bright annealing. Niobium (Nb), which is added for the purpose of grain homogenization, is suppressed and precipitated as carbon and nitride.

However, this method leads to abrasion of the header of the cold press bolt and screw due to the increase in the material strength due to the finely precipitated niobium carbonitride or boron precipitates, resulting in reduced productivity and reduced corrosion resistance by the precipitates. Caused.

In addition, in Japanese Patent (JP-89264596), niobium (Nb), titanium (Ti), zirconium (Zr), and the like are added to refine the crystal grains on the basis that the refinement of the crystal grains improves the cold workability.

However, the corrosion resistance by the precipitate was prevented by adding copper (Cu), nickel (Ni), molybdenum (Mo) and the like. The austenitic forming element added to suppress the deterioration of corrosion resistance increases the amount of austenite formation at high temperatures. As a result, the interface between ferrite and austenite increases.

As the interface between ferrite and austenite increases, cracking increases at the interface between ferrite and austenite during hot rolling, causing surface cracking.

Therefore, it is inevitable to reduce the real error rate of the material and the process load in the subsequent process, and in particular, copper (Cu) is a representative element that causes high temperature embrittlement, which causes a large amount of cracks during material rolling and causes a decrease in productivity. There is this.

The present invention is provided to solve the problems of the prior art as described above, after the hot-rolled ferrite-based 430 stainless produced by continuous casting to manufacture a wire rod, after the solidified wire rod or primary wire processing It is an object of the present invention to provide a method for producing a ferritic 430 stainless wire having excellent cold workability in which fine grains are formed on the surface of the annealed wire and coarse grains are formed in the center.

1 is a graph showing the cold workability according to the difference in grain size between the surface portion and the center of the wire manufactured by the method of manufacturing a ferritic 430 stainless wire having excellent cold workability according to an embodiment of the present invention,

2 is a graph showing a grain distribution according to cold drawing rate.

According to the present invention for achieving the object as described above, with 0.12wt% C, 1.0wt% Si, 1.0wt% Mn, 16-18wt% Cr, balance Fe and other unavoidable impurities In the method of manufacturing a ferritic 430 stainless steel wire, the method includes a cold drawing process and an annealing heat treatment step, wherein the cold drawing process includes a wire rod or primary heat-treated heat-treated after being manufactured by wire rolling by hot rolling. After drawing, the cold-annealed wire is cold drawn to a total reduction rate of 7 to 16%, and in the annealing heat treatment step, the annealing heat-treated wire or wire is subjected to an annealing at 770 to 820 ° C. for 20 to 40 seconds. Provided is a method of manufacturing a ferritic 430 stainless wire having excellent cold workability.

Hereinafter, with reference to the accompanying drawings a preferred embodiment of a method for producing a ferritic 430 stainless wire having excellent cold workability according to the present invention will be described in detail.

1 is a graph showing the cold workability at the time of cold working according to the difference between the grains of the surface portion and the central part as a work hardening index, and FIG. 2 is a graph showing the grain distribution according to the cold drawing rate.

It comprises a component composition step of the ferritic 430 stainless steel, cold drawing processing step, annealing heat treatment step,

The components of the ferritic 430 stainless steel are composed of 0.12 wt% or less of C, 1.0 wt% or less of Si, 1.0 wt% or less of Mn, 16-18 wt% of Cr, balance Fe, and other unavoidable impurities.

The ferritic 430 stainless steel thus formed is continuously cast and hot rolled to manufacture the wire. The manufactured wire is divided into solid solution wire and brightly annealed wire after primary drawing.

Cold drawing is a step of producing a thinner diameter wire diameter.

The solidified wire rod and brightly annealed wire are machined so that the overall freshness reduction rate is 7% to 16%. Here, the freshness reduction ratio is the ratio of the diameter of the wire before the wire is processed and the diameter of the wire after the wire is processed. Therefore, Equation 1 is as follows.

The annealing heat treatment step is a cold heat-treated line heat treatment step.

Heat treatment is annealed at about 770 ~ 820 ℃ 20 ~ 40 seconds in a bright annealing furnace.

In the annealing heat treatment step, the texture of the surface of the line forms fine grains, and the structure of the center of the line forms huge grains.

As a result, the wire rod is formed of a wire rod that forms uniform crystal grains as a whole, or a wire rod having better cold pressure composition than the wire.

On the other hand, if the total freshness reduction rate is less than 7%, the cold reduction of the surface portion is uneven due to the lack of reduction rate of the material, and the recovery of the surface portion occurs first rather than recrystallization due to the lack of recrystallization driving force during bright annealing.

Therefore, the amount of deformation formed during cold working is not sufficiently removed, which adversely affects the cold pressure composition and does not cause grain refinement of a desired surface portion.

If the freshness reduction rate is 16% or more, it is cold processed to the center of the material. If the annealing temperature is low, the deformation due to the unrecrystallization of the center remains, which weakens the cold press composition. It is difficult to control the behavior of recrystallization, resulting in uniform grains with little difference in grain size.

Therefore, it is one of the preferred embodiments to control the overall cold freshness reduction rate from 7% to 16%.

In addition, it is preferable to control the bright annealing temperature of the cold drawn wire or wire to 770 ~ 820 ℃, because when the heating temperature is lower than 770 ℃, recovery occurs more than recrystallization of the material, thereby eliminating the work hardened region. It remains unaffected and adversely affects the cold pressure composition. If the heating temperature is 820 ℃ or higher, recrystallization on the surface causes grain growth.

Particularly, in the case of non-uniform deformation, very uneven grains in which coarse grains and fine grains are mixed on the surface due to abnormal grain growth appear to deteriorate cold workability, and in particular, deformation is concentrated on the coarse grains during cold deformation, resulting in cracking. It acts as a source of occurrence.

Therefore, the freshness reduction rate is 7% to 16%, and annealing at about 770 to 820 ° C. for 20 to 40 seconds is the best example.

As described in detail above, the work hardening index and grain size of the wire rod according to the manufacturing method of the ferritic 430 stainless wire rod having excellent cold workability were analyzed.

Table 1 is a table showing the change in the structure of the ferritic 430 stainless steel wire according to cold draw reduction rate and bright annealing temperature.

Cold freshness reduction rate (%) Bright Annealing Temperature (℃) Recrystallization: O, Unrecrystallization: ×, Grain Growth: G 5% 750 ℃ Surface part: × 800 ℃ Surface part: × 830 ℃ Surface part: × 8% 750 ℃ Surface part: × 7800 ℃ Surface part: O 800 ℃ Surface part: O 830 ℃ Surface part: G 13% 750 ℃ Surface part: × 780 ℃ Surface part: O 800 ℃ Surface part: O 830 ℃ Surface part: G 17% 750 ℃ Surface: O, Center: × 800 ℃ Surface part: G, center part: O 830 ℃ Surface part: G, Center part: G

Table 1 shows the recrystallization behavior of the material subjected to the cold draw reduction step and the annealing heat treatment step of the heat-treated heat-treated material to have a uniform grain after wire-rolled ferritic stainless 430 steel in the form of wire coil.

The holding time at the bright annealing temperature was heat treated from 20 seconds to 40 seconds. As shown in Table 1, when the cold drawing reduction ratio was 5%, recrystallization did not occur at the cold drawn surface in the range of the annealing temperature investigated.

After annealing at 13% freshness and annealing temperature of 750 ° C, recrystallization did not occur in the cold-worked surface layer, and in the case of annealing at 830 ° C, grain growth occurred after the surface layer was recrystallized to coarse grains on the surface. Done.

When the cold drawing reduction ratio is increased to 17%, the material is deformed to the surface and the center portion, and when the annealing temperature is low, recrystallization occurs at the surface portion, but there is a problem that the cold deformation remains at the center portion.

When the annealing temperature is 800 ° C. or more, recrystallization occurs in the center portion, and the grain becomes fine, but the cold pressure composition is lowered due to grain growth after recrystallization on the surface portion.

Figure 1 shows the cold workability during cold working according to the difference in grain between the surface portion and the central portion as a work hardening index.

The difference in grain size is obtained by the difference in grain size between the surface portion and the center by the American Society for Testing Materials (ASTM) number, and means that the difference in grain size increases as the number difference increases.

In addition, the cold workability of the material used a high-speed compression deformation tester that can simulate the cold pressing process. At this time, the deformation rate was about 3000 (1 / sec), which is similar to the strain rate at the time of cold pressing, and the cold workability was evaluated by using the work hardening index.

In high-speed compression deformation, cracks occur where local deformation is concentrated. Therefore, localized strain concentration at the time of high-speed cold pressing is mainly concentrated on the surface of the material to generate surface cracks during machining of bolts and screws. Therefore, suppressing strain concentration at the surface portion is a method of improving workability during high-speed compression deformation. An effective method for suppressing local strain concentration is to increase the work hardening rate at high strain rates.

At high speed deformation, the work hardening rate increases as the material grains become finer. Grain refinement of the material, in particular grain refinement of the surface portion, greatly improves cold workability at high speed deformation.

As shown in Figure 1, it can be seen that the cold work hardening index increases as the grain size difference of the material increases than the conventional material of the uniform grain with little difference of grains at high deformation.

In particular, it can be seen that the work hardening rate increases sharply when the size difference of the grains is 2.5 or more as the ASTM number. As a result, the grain size difference between the optimum surface and the center should be 2.5 or more.

2 is a graph showing a grain distribution according to cold drawing rate.

After ferritic stainless 430 steel is rolled in the form of wire coil, and after solidifying heat treatment to have uniform grains, cold wire is processed by changing the cold drawing reduction ratio so that the wire surface is fine and the center has coarse grain size. .

In the bright annealing furnace at 800 ° C., a normal heat treatment time of 20 seconds to 40 seconds is maintained.

In Figure 2, the size difference of the grains of the line made by cooling the ferritic 430 stainless steel is shown as a difference in ASTM number.

As shown in FIG. 2, the grain difference increases and then decreases as the cold freshness reduction rate increases.

When the cold drawing reduction ratio exceeds the threshold value, it is deformed to the center of the material, the center of the material is recrystallized upon annealing, so that the grain size decreases, and grain growth occurs on the surface portion.

Accordingly, cracks occur in the coarse grains in the surface portion during cold pressing, which adversely affects the cold pressing composition. Therefore, the cold work hardening rate shows a grain difference of 2.5 or more, which is rapidly increasing, and the range of freshness reduction rate with fine surface and coarse core should be 7% ~ 16%.

Although the technical idea of the method for manufacturing a ferritic 430 stainless steel wire having excellent cold workability according to the present invention has been described together with the accompanying drawings, this is by way of example and not by way of limitation. . In addition, it is obvious that any person skilled in the art can make various modifications and imitations without departing from the scope of the technical idea of the present invention.

Claims (1)

In a method for producing a ferritic 430 stainless steel wire composed of 0.12 wt% or less of C, 1.0 wt% or less of Si, 1.0 wt% or less of Mn, 16-18 wt% of Cr, balance Fe, and other unavoidable impurities, Cold drawing process and annealing heat treatment step, In the cold drawing process, the wire is manufactured by hot rolling and then heat-treated and heat-treated heat-treated wire or brightly annealed wire after the primary drawing is cold drawn to 7 ~ 16% of the total reduction rate, In the annealing heat treatment step is a method of producing a ferrite-based stainless steel wire having excellent cold workability, characterized in that the annealing heat treatment for the wire or wire passed through the cold drawing step for 20 to 40 seconds at 770 ~ 820 ℃.