KR101281236B1 - Method for Manufacturing Pb-Free Free-Cutting Steel Wire Rod - Google Patents

Abstract

The present invention relates to a method for manufacturing an environment-friendly lead-free free-cutting steel wire, by controlling the wire rolling conditions of the lead-free free-cutting steel billet appropriately to suppress the occurrence of surface defects during wire-rolling to produce a lead-free free-cutting steel wire with excellent surface quality To provide.

      The present invention is a method for producing a wire rod by heating, rough rolling, intermediate rough rolling, intermediate sand rolling and finishing rolling in a wire-free free-cut steel billet process,

Controlling the amount of rolling down at the final stand such that a difference in material flow rate (ΔV = V _Exit -V _Entry ) at the final stand of the finishing mill during finishing rolling of the wire rolling process is less than 4.7 (m / s); And

A method of manufacturing a lead-free free-cutting steel wire, characterized in that the rolling reduction of the finishing mill is controlled so that the absolute value of the cumulative strain per second during finishing rolling in the wire rod rolling process is less than 4.1.

Lead free, Free cutting steel, Surface defect, Wire rod

Description

Method for manufacturing lead-free free-cutting steel wire {Method for Manufacturing Pb-Free Free-Cutting Steel Wire Rod}

The present invention relates to a method for manufacturing an environmentally friendly lead-free free-cutting steel wire, and more particularly, to a method of manufacturing a lead-free free-cutting steel wire that can produce wire with excellent surface quality by suppressing occurrence of surface defects in the wire-rolling process of lead-free free-cutting steel billets. It is about.

   Free-cutting steels basically have excellent machinability, and in order to impart such machinability, a non-metallic inclusion is usually distributed in the steel, or a composite nitride, a composite carbide, a metal inclusion, a low melting point element alone, etc. Is distributed.

  A typical nonmetallic inclusion used in free cutting steel is MnS, and machinability is obtained by controlling the size, shape, distribution, and the like of MnS.

In particular, Pb-free free-cutting steel is a steel grade with the addition of low melting point metals Bi and Sn to improve processing quality and chip processing during cutting, and free sulfur-free cutting without adding environmentally harmful element Pb. It has better cutting workability.

   However, while lead-free free cutting steel shows excellent cutting processability, when wire rods are manufactured using lead-free free cutting steel, surface defects are caused by these inclusions and low melting point metals in the wire rolling process.

  Accordingly, there is a demand for a wire rod rolling technology capable of sufficiently suppressing surface defects generated during the production of wire rods using lead-free free cutting steel.

   The present invention provides a method for producing a lead-free free-cutting steel wire rod that can produce lead-free free-cutting steel wire having excellent surface quality by appropriately controlling wire rolling conditions of lead-free free steel billets.

   Hereinafter, the present invention will be described.

   The present invention is a method for producing a wire rod by heating, rough rolling, intermediate rough rolling, intermediate sand rolling and finishing rolling in a wire-free free-cut steel billet process,

Controlling the amount of rolling down at the final stand such that a difference in material flow rate (ΔV = V _Exit -V _Entry ) at the final stand of the finishing mill during finishing rolling of the wire rolling process is less than 4.7 (m / s); And

The present invention relates to a method for manufacturing a lead-free free cutting steel wire, characterized in that the rolling reduction of the finishing mill is controlled so that the absolute value of the accumulated strain per second during the rolling of the wire rod is less than 4.1.

   Lead-free free-cutting steel billets that can be preferably applied to the present invention

By weight% Carbon (C): 0.03-0.30%, Silicon (Si): 0.01-0.30%, Manganese (Mn): 0.2-2.0%, Phosphorus (P): 0.02-0.10%, Sulfur (S): 0.06- 0.45%, Bismuth (Bi): 0.04-0.20%, Tin (Sn): 0.04-0.20%, Boron (B): 0.001-0.015%, Nitrogen (N): 0.001-0.010%, Oxygen (T [O] ): 0.002% to 0.025%, aluminum (Al): 10 ppm or less and calcium (Ca): 10 ppm or less, lead-free free-cutting steel billets composed of residual Fe and unavoidable impurities.

According to the present invention, it is possible to suppress the occurrence of surface defects in the wire rod rolling process by appropriately controlling the wire rolling conditions of the lead-free free-cutting steel billet, it is possible to more economically provide an eco-friendly lead-free free-cutting steel wire with excellent surface quality.

The present invention will be described in detail.

The present inventors have conducted research and experiment to suppress the surface defects generated in the wire rod rolling process of lead-free free-cutting steel billet, the surface defects in the wire rod rolling process is the material outflow in the final stand of the finishing mill in the wire rod rolling process of lead-free free steel Velocity difference (ΔV) (ΔV = V _Exit -V _Entry ; V _Exit : material exit speed, V _Entry : material entrance speed) and the absolute value of the cumulative strain per second during finishing rolling are below a predetermined value, i.e. It is confirmed that it does not occur in the case, and completed the present invention in consideration of the results of these studies and experiments.

Although the composition of the lead-free free cutting steel billet applied to the present invention is not particularly limited, the composition of the lead-free free cutting steel that can be preferably applied to the present invention will be described.

Carbon (C): 0.03 ~ 0.30 wt%

Carbon should be added at least 0.03% by weight to ensure surface roughness and mechanical properties. However, when the content of carbon exceeds 0.30% by weight, the increase in light pearlite structure causes a decrease in machinability.

Therefore, the content of carbon (C) is preferably limited to 0.03 to 0.30% by weight.

Silicon (Si): 0.01-0.30 wt%

Silicon should be added at least 0.01% by weight to act as a deoxidizer to produce SiO ₂ and form a low melting complex oxidative inclusion that can minimize tool wear due to thermal diffusion during high speed cutting. However, when the content of silicon exceeds 0.30% by weight, high melting point inclusions or SiO ₂ alone inclusions are formed, rather the wear rate of the tool is significantly increased.

Therefore, the content of silicon (Si) is preferably limited to 0.01-0.30% by weight.

Manganese (Mn): 0.2-2.0 wt%

Manganese forms MnS inclusions to prevent red brittleness due to sulfur (S), and also acts as a deoxidizer to form MnO which also acts as a nucleus of MnS inclusions. It is preferable to add.

However, if the content of manganese exceeds 2.0% by weight, the ferrite is hardened, resulting in a decrease in machinability.

Therefore, the content of manganese is preferably limited to 0.2-2.0% by weight.

Phosphorus (P): 0.02-0.10 wt%

Phosphorus is segregated at grain boundaries to improve machinability. For this purpose, phosphorus is preferably contained in an amount of 0.02% by weight or more, but should not exceed 0.10% by weight in order to secure mechanical properties and cold workability.

Therefore, the content of phosphorus (P) is preferably limited to 0.02-0.10% by weight.

Sulfur (S): 0.06-0.45 wt%

Sulfur forms MnS inclusions, which inhibits the creation of built up edges during cutting operations, thereby reducing wear of cutting tools and improving the surface finish of the workpiece.

Sulfur should be added at least 0.06% by weight for this purpose. However, when the amount of sulfur is large, it should not exceed 0.45% by weight because FeS of low melting point is easily produced and high temperature ductility is degraded, making hot rolling difficult.

Therefore, the content of sulfur is preferably limited to 0.06-0.45% by weight.

Bismuth (Bi): 0.04-0.20 wt%

When bismuth is added to steel, it exists as a metal inclusion alone or adheres to MnS inclusions. It is easily melted by the processing heat during cutting to improve cutting characteristics, and acts as a lubricating film between chips and cutting tools to reduce friction and reduce cutting tools. It acts to suppress wear.

If the content of bismuth is less than 0.04% by weight, the machining effect is inferior. On the other hand, if the content of bismuth is more than 0.20%, the content of bismuth is preferably limited to 0.04-0.20% by weight because it is not good for castability and rollability.

Tin (Sn): 0.04-0.20 wt%

Tin is an element that can play a role similar to lead.

That is, tin may play the same role as liquid metal embrittlement, in which lead is one of the mechanisms for improving the machinability of steel.

Specifically, this phenomenon is shown by tin being segregated to ferrite grain boundaries and facilitating grain boundary fracture by lowering grain boundary binding energy.

Therefore, in order to obtain the machinability improvement effect by tin, 0.04 weight% or more of tin should be added.

However, the content of tin is preferably limited to 0.04-0.20% by weight since the content exceeding 0.20% by weight may cause harmful effects on casting and rolling properties.

Boron (B): 0.001-0.015 wt%

Boron segregates at the austenite grain boundaries to strengthen grain boundaries and improve high temperature ductility. In addition, it is known that steel containing graphite is excellent in machinability. When boron reacts with nitrogen in the steel to form boron nitride (BN) having a crystal structure and physical properties similar to that of graphite, graphite The same machinability improvement effect as steel containing can be expected.

When the boron is less than 0.001% by weight, the effect of addition is insufficient, and thus it is necessary to add more than 0.001% by weight.On the contrary, when it is added in excess of 0.015% by weight, the effect can not be expected to increase any more, and the boron nitride at the austenite grain boundary Due to the precipitation of grain boundary strength may be lowered and hot workability may be lowered, the boron content is preferably limited to 0.001-0.015% by weight.

Nitrogen (N): 0.001-0.010 wt%

Nitrogen should be added at least 0.001% by weight to form BN with boron.

However, if it exceeds 0.010% by weight, the amount of effective boron segregated in the austenite grain boundary is reduced, which lowers the grain boundary strengthening effect.

Therefore, the content of nitrogen is preferably limited to 0.001-0.010% by weight.

Oxygen (T [O]): 0.002-0.025 wt%

Oxygen is required to be added at least 0.002% by weight in order to prevent machinability deterioration due to MnS inclusion stretching during hot rolling. However, in order to ensure the plastic deformation of the MnS inclusions during cutting, it should not exceed 0.025% by weight.

Therefore, the content of total oxygen (T [O]) is preferably limited to 0.002-0.025% by weight.

Aluminum (Al) and Calcium (Ca): Less than 10ppm each

Aluminum and calcium are required for the formation of the low melting composite oxidative inclusions formed in the steel in the present invention, but need not be added intentionally, and an amount naturally contained in slag or the like is sufficient. Such aluminum and calcium are preferably present at 10 ppm or less.

Hereinafter, the method of manufacturing a lead-free free cutting steel wire rod using a lead-free free cutting steel billet will be described.

In the present invention, the billet is heated, rough rolling, intermediate rough rolling, intermediate sand rolling and finishing rolling to produce a lead-free free-cutting steel wire in the wire rod rolling process.

The size of the billet which can be applied to the present invention can be any size as long as it is commonly used, and examples of the representative billet include a billet having a size of 160 mm (width) ⅹ 160 mm (thickness) ⅹ 10200 mm (length).

The billet is re-heated at a high temperature of, for example, 1200 ° C. or higher, preferably 1200-1300 ° C. in the wire rod rolling process, and then the final wire rod product is subjected to continuous rolling of rough rolling, intermediate rough rolling, intermediate sand rolling, and finishing rolling. Produced as

The rough rolling, the intermediate rough rolling, and the intermediate sand rolling of the billet described above are not particularly limited, and may be performed according to a conventional method.

As a preferable example of rough rolling, intermediate rough rolling and intermediate sand rolling of the billet, rough rolling is rolled 2 to 10 passes at a reduction rate of 10 to 40% per pass, and intermediate rough rolling is performed at a reduction ratio of 10 to 40% per pass. Rolling of ~ 10 pass, and intermediate sand rolling is 4 to 10 pass rolling with a reduction rate of 10 to 40% per pass.

The finishing rolling is rolled 2 to 10 passes at a reduction rate of 10 to 40% per pass.

In the finishing rolling, the difference in material flow rate in the final stand of the finishing mill (ΔV = V _Exit -V _Entry ; V _Exit : material exit speed, V _Entry : material entrance speed) is less than 4.7 (m / s). The amount of reduction in the final stand shall be controlled.

The material flow rate difference (ΔV = V _Exit -V _Entry ) can be expressed as V _Exit {1−1 / (1 + Reduction Rate / 100)}.

In the present invention, the relationship between the reduction rate at the final stand, the rolling speed condition, and the occurrence of surface defects in the filament rolling step of the wire rod rolling step of the lead-free free cutting steel billet was examined, and an example of the result is shown in FIG. 1.

In FIG. 1, the area under the dotted line is an area where no surface defects occur, and the area above the dotted line is an area where the surface defects occur.

As shown in Figure 1, it can be seen that the occurrence of surface defects during the wire rolling is affected by the reduction rate and the rolling speed at the final stand.

The present inventors have recognized that the occurrence of surface defects during the rolling of the wire is related to the tensile force generated during the rolling process. Could.

Since the tensile force at the time of rolling is generated by the difference between the exit and exit speeds during rolling, in the present invention, the difference (ΔV) between the exit and exit speeds of the final stand at the wire rod is less than 4.7 (m / s). Control the amount of rolling down on the final stand.

As described above, if the difference (ΔV) between the material exit speed and the entrance speed at the final stand during the wire rod is less than 4.7 (m / s), the tensile force generated during the wire rod rolling can suppress the occurrence of surface defects. The surface defects are suppressed.

That is, in the present invention, it is possible to prevent the occurrence of surface defects by controlling the amount of reduction so that the difference in material flow rate (ΔV) in the final stand of the finishing mill is less than 4.7 (m / s).

In addition, the rolling reduction of the finishing mill must be controlled so that the absolute value of the cumulative strain per second during the finishing rolling is less than 4.1.

 In case of high inclusions such as free cutting steel and high alloying steel, grain boundary fracture may occur due to grain boundary segregation of inclusions and alloying elements due to high reduction / high speed rolling in the finishing rolling section. The threshold value of the cumulative strain is derived. If the absolute value of the cumulative strain per second exceeds 4.1, a defect occurs.

Therefore, in order to prevent defects in finishing rolling, the rolling reduction of the finishing mill should be controlled so that the absolute value of cumulative deformation per second is less than 4.1.

Hereinafter, the present invention will be described more specifically by way of examples.

(Example)

By weight, carbon (C): 0.07%, silicon (Si): 0.05%, manganese (Mn): 1.4%, phosphorus (P): 0.05%, sulfur (S): 0.4%, bismuth (Bi): 0.1 %, Tin (Sn): 0.05%, Boron (B): 0.005%, Nitrogen (N): 0.007%, Oxygen (T [O]): 0.015%, Aluminum (Al): 1 ppm and Calcium (Ca): 160mm (width) ⅹ 160mm (thickness) ⅹ 10200mm (length) lead-free free-cut steel billets of 1ppm, balance Fe and unavoidable impurities are heated at a temperature of 1250 ℃ or more, 5 pass rough rolling with 21% reduction per pass, 6 pass intermediate rough rolling with 28% reduction rate per step, 6 pass intermediate rolling with 22% reduction rate per pass, followed by 2 pass, 4 pass, 6 pass and 8 pass finishing rolling, respectively, as shown in Table 1 below. , Surface defects of wire rods of up to 12 mmφ, 10 mmφ and 8 mmφ

It was investigated whether the occurrence and the results are shown in Table 1 below.

Table 1 shows the cumulative deformation amount per second, and the occurrence of surface defects was marked as good when no surface defects occurred, and marked as defects when the defects occurred.

Stand number (pass number) Stand distance (mm) Wire diameter (mmφ) Cumulative strain per second Surface Defects 18 800





- -3.0 Good


19 15 -3.2 20 - -3.8 21 12 -4.0 22 - -4.5 Defect


23 10 -4.8 24 - -5.3 25 8 -5.8

As shown in Table 1, when the absolute value of the cumulative strain per second during finishing rolling is less than 4.1, which is the scope of the present invention, that is, the absolute value of the wire rod 15 mmφ and the absolute value of the cumulative strain per second In the case of manufacturing the wire rod of 12mmφ which is 4.0, it can be seen that no surface defect occurs.

In addition, the above-mentioned lead-free free-cut steel billets of 160 mm (width) ⅹ 160 mm (thickness) ⅹ 10200 mm (length) are heated at a temperature of 1250 ° C. or higher and roughly rolled in 10 passes with a 23% reduction rate per pass, and 21% reduction ratio per pass. 4 pass intermediate rough rolling, and 6 pass intermediate rolling with 16% reduction rate per pass, and then 2 pass, 4 pass, 6 pass and 8 pass finishing rolling, respectively, as shown in Table 2 below, and the diameters of 15mmφ, 12mmφ, 10mmφ and A wire rod of 8 mm phi was prepared and the occurrence of surface defects was investigated and the results are shown in Table 2 below.

Table 2 shows the cumulative deformation amount per second, and the occurrence of surface defects is indicated as good when no surface defects are generated and marked as defects when a defect is generated.

Stand number (pass number) Stand distance (mm) Wire diameter (mmφ) Cumulative strain per second Surface Defects 21 920

- -2.3 Good

22 15 -2.8 23 - -3.3 24 12 -3.7 25 - -3.9 26 10 -4.0 27 - -4.6 Defect
28 8 -5.0

As shown in Table 2 above, when the absolute value of the cumulative strain per second during finishing rolling is less than 4.1, which is the scope of the present invention, that is, the wire rod having a diameter of 15 mmφ and the absolute value of the cumulative strain per second is 3.9. It can be seen that surface defects do not occur when a wire having a diameter of 12 mmφ and a wire having a diameter of 10 mmφ having an absolute value of cumulative strain per second are 4.0.

In addition, after manufacturing the wire by adjusting the rolling conditions in the final pass (stand) of finishing rolling in the manufacture of the wire having the wire diameters of Tables 1 and 2 as shown in Table 3, and investigates the occurrence of surface defects It is shown in Table 3 below.

fairyland
(mmφ) Reduction rate
(%) Inlet Speed (m / s) Entrance area (mm ² ) Outlet speed (m / s) Outlet area (mm ² ) Flow rate (mm ² xm / sec) Speed difference (m / s) Defect Table 2 Manufacturing Process
12 19.5 13.4 140 16.6 113.1 1877 3.24 Good 10 18.6 20.4 96 25.0 78.5 1963 4.65 Good 8 18.9 31.8 62 39.2 50.3 1970 7.41 Occur Table 1 Manufacturing Process 12 18.6 19.5 139 24.0 113.1 2714 4.46 Good 10 17.9 28.7 96 35.0 78.5 2749 6.27 Occur

As shown in Table 3, it can be seen that the surface defect does not occur when the difference in material flow rate (ΔV) in the final stand of the finishing mill is less than 4.7 (m / s) of the present invention.

1 is a graph showing an example of the bond generation area against the rolling speed and reduction rate at the final stand;

Claims (3)

As a method for producing a wire rod by heating, rough rolling, intermediate rough rolling, intermediate sand rolling and finishing rolling in a wire-free free cutting steel billet, The difference in material flow rate in the final stand of the finishing mill during finishing rolling of the wire rod rolling process (ΔV = V _Exit -V _Entry ; V _Exit : material exit speed, V _Entry : material entrance speed) is less than 4.7 (m / s) Control the amount of reduction in the final stand so that; And And a rolling reduction of the finishing mill so that the absolute value of the cumulative strain per second during finishing rolling of the wire rolling process is less than 4.1. The lead-free free-cutting steel billet according to claim 1, wherein the lead-free free-cut steel billet is in weight percent, carbon (C): 0.03-0.30%, silicon (Si): 0.01-0.30%, manganese (Mn): 0.2-2.0%, phosphorus (P): 0.02 ~ 0.10%, sulfur (S): 0.06-0.45%, bismuth (Bi): 0.04-0.20%, tin (Sn): 0.04-0.20%, boron (B): 0.001-0.015%, nitrogen (N): 0.001 ~ 0.010%, Oxygen (T [O]): 0.002 to 0.025%, Aluminum (Al): 10 ppm or less and Calcium (Ca): 10 ppm or less Way. According to claim 1 or 2, wherein the heating temperature of the billet is 1200 ~ 1300 ℃, the rough rolling is rolled 2 to 10 passes at a reduction rate of 10 to 40% per pass, the intermediate rough rolling is 10 per pass 4-10 passes are rolled at a reduction rate of ˜40%, and the intermediate sand rolling is 4-10 passes at a reduction rate of 10-40% per pass, and the finishing rolling is performed at a reduction rate of 10-40% per pass. A method for producing a lead-free free cutting steel wire, characterized by rolling in a ~ 10 pass.

Images