JPH10211506A - Manufacture of composite sulfur free-cutting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method about a method for preventing surface defect and improvement of machinability as to the method for manufacturing composite sulfur free-cutting steel from a middle cross-sectional bloom. SOLUTION: This method is for manufacturing the composite sulfur free- cutting steel and, after taking the shape of the cast bloom as a square or a rectangle, continuously casting and cutting, blooming rolling is executed to the bloom for bar steel by 2-4 passes so that an elongation ratio becomes 1.6-2.8 and, after that, rolled into steel bars and wire rods. In the relation with the thickness of the cast bloom, after adjusting T.O in molten steel so as to satisfy the equation: 0.22D+68<=T.O<=0.22D+118 (In the equation, the thickness of the cast bloom is expressed by Dmm and the concentration of total oxygen in the molten steel by T.O ppm.), and executing continuous casting, the bloom is cut, the blooming rolling is executed and, after that, the bloom is rolled into the steel bars and wire rods.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a sulfur composite free-cutting steel by continuous casting of a medium-section bloom, and provides a method of preventing surface flaws in a bar or a wire rod and improving the machinability.

[0002]

2. Description of the Related Art Low-carbon S free-cutting steels such as SUM22 and AISI1213 and low-carbon S-Pb free-cutting steels such as AISI12L14 have excellent machinability. It is used in large quantities in cutting parts.

[0003] Several continuous casting methods for these sulfur composite free-cutting steels have been reported. As examples of reports on improvement of machinability, JP-A-62-207547 and JP-A-62-207548 disclose limiting the specific water volume in continuous casting and reducing the cooling rate of a slab in a continuous casting machine. Thus, a method of increasing the size of MnS to be crystallized and improving machinability is described.

Japanese Patent Application Laid-Open No. 2-155548 discloses a method of improving the machinability by controlling the superheat of molten steel in a tundish to 10 ° C. or more and controlling the cooling rate of a slab to a predetermined value or less. Further, JP-A-7-173574 describes C,
A method is described in which Si and other chemical components are regulated and the amount of oxide-based inclusions of 53 μm or more is regulated.

As a report on a method for preventing surface flaws,
For example, Japanese Patent Publication No. 59-19182 discloses [% S] / [%
A method is described in which the ratio of [C] / [% O] is regulated to suppress the occurrence of blowholes, limit the Mn concentration, secure hot workability, and prevent rolling cracks.

[0006]

[0003] Generally, as a billet for a strip steel to be subjected to rolling of a bar or a wire rod, a steel strip of 115 mm square is used.
A 180 mm square size is used, but these materials have recently been used as blooms manufactured by continuous casting.
Billet slabs are widely used, and about 4
It is roughly divided into a large-section bloom having a cross-sectional area of up to 15 times or a small-section billet of the same size as a billet.

[0007] In the method of producing a billet by slab rolling from a large-section bloom, the heating time is as long as about 2 to 3 hours due to the large slab size, and the number of rolling passes is about 10 to 2 hours.
Since the number of times is as large as 0 times, there is a problem that the energy consumption required for hot rolling increases, the rolling yield decreases, and the manufacturing cost increases significantly.

[0008] On the other hand, in the case of a billet having a small cross section, lumping and rolling are omitted, which is advantageous in terms of cost. However, since the billet with the cast structure is rolled into a steel bar or a wire rod, the grain boundary during solidification is reduced. The sulfides and oxides crystallized on the surface cause embrittlement to cause deep surface flaws during rolling, or high-speed casting impairs the levitation of oxide-based inclusions and reduces machinability. There are problems such as lowering. For these reasons, the fact is that sulfur composite free-cutting steels are actually manufactured from large cross-section blooms, which are more expensive to manufacture than before.

As described above, there is a problem in both large section bloom continuous casting and small section billet continuous casting, and if it is possible to produce from a medium section bloom having a draw ratio in the range of 1-4, The above problems are drastically improved.

[0010] The first problem in the production of medium-section blooms of sulfur composite free-cutting steel is to establish a method for preventing surface flaws when rolling slabs slab-rolled at a low elongation ratio into bars or wires. The second problem is to improve buoyancy of oxide-based inclusions during high-speed casting and to reduce machinability due to downsizing of MnS due to increase in slab cooling rate due to medium sectioning.

[0011]

The first object of the present invention is to provide a method for preventing surface flaws. That is, C
≦ 0.15%, Si ≦ 0.05%, Al ≦ 0.003
%, Mn: 0.50 to 1.50%, P: 0.020 to
0.100%, N: 20 to 150 ppm, S: 0.10
0 to 0.350%, Pb: 0 to 0.300%, total oxygen T.O. O: A method for producing a sulfur composite free-cutting steel containing 100 to 250 ppm as a basic component, wherein the cast slab is continuously cast into a square or rectangular shape, cut into a predetermined length, and then heated to draw ratio. Is slab-rolled in two to four passes on a steel slab for bar steel so that the value becomes 1.6 to 2.8, and then the steel slab is rolled into a bar or a wire rod.

Next, claim 2 of the present invention relates to measures for improving machinability. That is, C ≦ 0.15% by mass,
Si ≦ 0.05%, Al ≦ 0.003%, Mn: 0.5
0 to 1.50%, P: 0.020 to 0.100%, N:
20-150 ppm, S: 0.100-0.350%,
Pb: 0 to 0.300%, total oxygen T.P. O: 100-25
A method for producing a sulfur composite free-cutting steel containing 0 ppm as a basic component, comprising: After adjusting the O concentration so as to satisfy the formula (1) in relation to the slab size and continuously casting and cutting the slab, the slab is subjected to slab rolling to obtain a steel slab. It is to be rolled into wire.

0.22D + 68 ≦ T. O ≦ 0.22D + 118 (1) where D is the thickness (mm) of a square or rectangular slab.
O is the total oxygen concentration (ppm) of the molten steel.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In a sulfur composite free-cutting steel targeted by the present invention, S as a machinability improving element has a content of 0.100 to 0.100.
0.350%, Pb is added in an amount of 0 to 0.300%,
S solidifies to form spindles by crystallizing and rolling MnS at the time of solidification, and Pb is uniformly dispersed as Pb particles having a diameter of several μm in steel.

Both MnS and Pb are fine soft inclusions,
This has the effect of improving the cutting performance by acting as a source of internal stress concentration during the cutting process, increasing the lubricating action between the tool and the chip, reducing the cutting resistance, and improving the chip processing performance.

[0016] Further, in the sulfur composite free-cutting steel, S or P
Components other than b also affect the machinability, so the content is defined as follows. That is, C is set to 0.15% or less in order to extend tool life, and Si and Al form 0.05% or less and 0.
003% or less, Mn is added in an amount of 0.50 to 1.50% so as to be greater than the stoichiometric ratio with S in MnS, and P and N are dissolved in steel to reduce the finished surface roughness. 0.020 to 0.100% and 20 to 1 respectively to improve
50 ppm is added.

O in molten steel is basically close to the concentration equilibrium with C, Mn, etc., and exists as MnO—SiO ₂ system or other oxides due to the reaction between molten steel and refractory or slag.
Its content is T. total oxygen. 100 to 250 ppm as O
It is.

First, the reason why the draw ratio (slab slab cross-sectional area / steel slab cross-sectional area) is specified to be 1.6 to 2.8 in the present invention will be described below. In order to prevent the occurrence of surface cracks when rolling steel strip for sulfur composite free-cutting steel into bar or wire rods, the embrittlement factors due to sulfides, oxides and Pb inclusions crystallized at grain boundaries are considered. Need to be removed.

However, since the middle section bloom has a smaller draw ratio from the slab to the slab than the large section bloom, it is necessary to refine the crystal grains of the slab by slab rolling to remove the cause of embrittlement. , Increase the rolling reduction per pass to allow the rolling force to fully penetrate from the surface of the slab to the center,
It is necessary to destroy the casting structure.

According to the investigation by the inventors, there is a close relationship between the drawing ratio and the number of passes in the bulk rolling and the austenite grain size of the obtained steel slab, and the drawing ratio is set to 1.6 as shown in FIG. It has been found that a fine structure of 5 or more can be obtained stably when 2-4 pass rolling is performed.

As is apparent from the figure, when the stretching ratio is less than 1.6 and rolling is performed in two to four passes, or when the stretching ratio is 1.6 or more, the rolling reduction per pass is small and multi-pass rolling is performed in five or more passes. When 5 is performed, 5 or more cannot be obtained stably.

For austenite grain size number 5 or more,
Cast structure is destroyed and refined to reduce sulfide, oxide and Pb
Since the inclusions are finely and uniformly dispersed, grain boundary embrittlement can no longer occur, and hot workability equivalent to that of a slab rolled from a large-section bloom can be ensured. For the above reasons, in the present invention, the draw ratio from the slab to the steel slab is specified to be 1.6 or more, and rolling is performed in 2 to 4 passes.

Next, the size of the billet for the bar is 1 as described above.
15 mm square to 180 mm square is generally used. Table 1 is obtained by obtaining a typical example of a slab size that can roll these steel slabs in 2 to 4 passes.

Here, the prerequisites are as follows: the maximum area reduction ratio per pass is within 30%, the axial ratio (the dimensional ratio of the long side / short side of each pass) is 1.8 or less, and the roll diameter of the bulk rolling is 1100m
In addition to the value of mφ or less, the width expansion due to the reduction was calculated by approximating to 1/3 of the reduction amount. If the area reduction ratio or the axial ratio is larger than these values, the biting angle becomes excessively large, causing slipping or twisting or falling of the steel slab, which lowers the rolling workability. Value.

According to Table 1, a 115 mm square slab is drawn from a 145 mm square slab having a draw ratio of 1.6 in two passes with a draw ratio of 2.0.
The four 180 mm square slabs can be rolled in four passes. A 180 mm square steel slab has a draw ratio of 1.8 and 24
300mm with draw ratio 2.8 in 2 passes from 0mm square slab
Each can be rolled from a square slab in four passes. And
If the slab size exceeds 300 mm square, that is, if the stretching ratio exceeds 2.8, the reduction in area exceeds 30%, and the axial ratio exceeds 1.8, making molding impossible in four passes.

For the reasons described above, in the present invention, the draw ratio from the slab to the steel slab is specified to be 1.6 to 2.8, and the slab rolling is performed in 2 to 4 passes. It is to be noted that a square or rectangular cast slab is more advantageous for manufacturing a steel slab with fewer passes than a circular or the like.

[0027]

[Table 1]

In the present invention, T.P. The reason why O is adjusted so as to satisfy the expression (1) in relation to the slab size will be described. In continuous casting, the casting speed (Vc) changes according to the size of the slab, and the smaller the slab size, the higher the speed of drawing.

For example, depending on the continuous casting machine and operating conditions, 14
For 5 mm square cast slabs, Vc = 2.0-3.0 m / min, 2
For a slab of 00 mm square, Vc = 1.5 to 2.0 m / min,
Vc = 0.6-1.2m / min for 300mm square cast slab
Cast in.

In the sulfur composite free-cutting steel, in addition to MnO—SiO ₂ -based inclusions as deoxidation products, a small amount of SiO ₂ and Al ₂ O ₃ unavoidably mixed from refractories and slag, and others Is harmful to machinability.

In particular, as the slab size becomes smaller, the casting speed becomes higher as described above, so that the levitation of inclusions is hindered and the machinability decreases. From this point of view, the inventors have found that T.V. The relationship between O and the number of large inclusions in the slab was investigated by the slime extraction method.

According to the results, T.M. When O is the same, the number of inclusions in the slab is clearly larger when the slab size is smaller. Therefore, in order to reduce the number of inclusions,
T. in molten steel O needs to be adjusted to an appropriate range within the standard according to the slab size. The slab size and T.M. in molten steel measured by the inventors. FIG. 2 shows the relationship between O and machinability.

The machinability was evaluated by the finished surface roughness (Rz) of plunge cut of 80 mmφ steel bar. Assuming that the finished surface roughness is less than 20 μm as a good region, and obtaining this range from the measurement results, the formula (1) is obtained. 0.22D + 68 ≦ T. O ≦ 0.22D + 118 (1) where D is the thickness (mm) of a square or rectangular cast piece,
T. O is the total oxygen concentration (ppm) of the molten steel.

The machinability is determined by T.S. In the region where the O concentration is higher than the right side of the equation (1), the oxide inclusions are too large,
z becomes 20 μm or more, which is defective. On the other hand, T. Even when the O concentration is lower than the left side of the equation (1), Rz is 20 μm or more and the machinability is poor.

As a result of investigating the reason, T.P. When O was low, it turned out that MnS becomes easy to expand by rolling and machinability falls. Then, within the range defined by the expression (1), MnS also exhibits an appropriate spindle shape, has good machinability without adverse effects of oxide-based inclusions, and holds for any slab size and casting speed. Make sure that.

FIG. 3 shows an example of the embodiment of the present invention. 1
Is a ladle, 2 is a tundish, 3 is a mold, 4 is a secondary cooling zone, 5 is a guide roll, 6 is a cutting machine, 7 is a slab, 8 is a slab heating furnace, 9 is a slab rolling mill, 10 Denotes a billet, 11 denotes a billet heating furnace, 12 denotes a wire rolling mill, and 13 denotes a wire coil.

An example of the first embodiment of the present invention will be described below. Using the mold 3, a bloom slab for sulfur composite free-cutting steel having a cross-sectional shape of, for example, 145 mm square to 300 mm square is cast, and the cut slab 7 is heated. 115mm square ~ 180 in ~ 4 passes
It is formed into a steel slab 10 of mm square, and a wire rod 13 of 7 mmφ is manufactured from the steel slab. In this case, the draw ratio from the cast slab to the steel slab is 1.6 to 2.8. With respect to the obtained wire, a favorable surface quality level is secured as a sulfur composite free-cutting steel.

Next, in the second embodiment of the present invention, the T.P. in the molten steel in the ladle 1 and the tundish 2 is set. O is adjusted according to the slab thickness so as to satisfy the expression (1), and the slab 7 is adjusted.
After casting, the slab is slab-rolled into a steel slab 10 and then rolled from the slab into a wire coil 13. The obtained wire material secures good machinability and material as a sulfur composite free-cutting steel.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail below. 0.08% C- in 270 ton converter
0.01% Si-1.05% Mn-0.075% P-
0.285% S-80 ppm N-280 ppm After smelting molten steel of O, (% Ca
O), (% SiO ₂ ), (% FeO), (% MnO), etc., and then P in molten steel in the ladle by the injection method.
b powder was blown and adjusted to 0.250% Pb.

Next, the bloom continuous caster shown in FIG.
The medium section bloom of 45 mm square to 300 mm square has a superheat degree (TD-SH) of molten steel in a tundish of 20 to 40 ° C, a secondary cooling specific water amount of 0.41 / kg, and a casting speed corresponding to each slab size. After casting, the slab is cut to a predetermined length and the cross-sectional average temperature is 1100 to 11 in a slab heating furnace.
After heating to 50 ° C for about 1 hour, roll diameter 700m
It was formed into a 115 mm square to 180 mm square steel slab by 2 to 4 pass rolling using an HV type bulking mill of mφ to 900 mmφ.

As an example according to the first aspect of the present invention, Table 2 shows the slab size, the slab rolling conditions, and the surface flaw results of the wire rod along with comparative examples. In addition, the surface flaw results were evaluated by scoring according to the depth and length of the 7.0 mmφ wire.

First, a 145 mm square slab was formed into a 115 mm square steel slab by two-pass rolling at a rolling reduction of 45 mm per pass, a maximum area reduction rate of 24%, an axial ratio of 1.6 and a draw ratio of 1.6. . A 300 mm square slab was formed into a 180 mm square slab by 4-pass rolling at a rolling reduction of 90 mm per pass, a maximum area reduction of 30%, an axial ratio of 1.8 and a draw ratio of 2.8.

Similarly, from a square or rectangular slab between 145 mm square and 300 mm square, a 115 mm square, 1 mm square
50 mm square or 180 mm square steel slabs with a maximum area reduction of 30
%, The axis ratio is 1.8 or less, and the stretching ratio is 1.6 to 2.8.
It was formed by pass rolling.

Here, the shape of the bloom slab for a rod or wire is generally a square or a rectangle having an aspect ratio (dimension ratio of long side to short side) of 1.7 or less. Indicates that the rectangular cast slab has an aspect ratio of about 1.5 to 1.6.

Here, a 115 mm square steel piece is 145 mm long.
It is also possible to roll in two passes from a slab of less than an angle, but in this case, the draw ratio is less than 1.6 and the casting structure is refined (securing an austenite grain size number of 5 or more).
145 mm square was set as the lower limit of the slab size.

Further, when a 180 mm square slab is rolled from a slab exceeding 300 mm square, the maximum reduction in area in four-pass rolling exceeds 30% and the axial ratio exceeds 1.8, so that
The 00 mm square was set as the upper limit of the slab size. As is clear from Table 2, in the examples according to the present invention, the surface flaw rating of the wire is as good as 0 to 1, and the quality level as a sulfur composite free-cutting steel is sufficiently satisfied.

On the other hand, in the comparative example, when a slab of 130 mm square was formed into a slab of 115 mm square by two-pass rolling, a slab of 210 mm square was rolled to 180 m by four-pass rolling.
In the case of forming into a m-square steel slab, and further performing direct wire rolling without slab rolling from a 150 mm slab,
In each case, since the stretching ratio was less than 1.6, surface flaws occurred in the wire.

When a 165 mm square steel slab was formed from a large cross-section bloom of 400 mm × 600 mm by a usual sizing method, the surface flaw rating of the wire was as good as 1, but the stretching ratio was 8.8. Because of this, it took about 3 hours to heat the slab and 25 passes for slab rolling, and the production cost related to slab rolling increased significantly.

[0049]

[Table 2]

As an embodiment according to the second aspect of the present invention, the slab size, the T.M. Table 3 shows the O, casting conditions, the number of slime extraction inclusions having a size of 53 μm or more in the obtained slabs, and the results of machinability investigations, together with comparative examples.

T. in molten steel O is basicity in slag on pot
(% SiO ₂ ) / (% CaO) and oxidation degree (% FeO +% Mn)
O), which is generally known to be closely related to O), in this embodiment, the amount of slag on the pot, (% CaO),
(% SiO ₂ ) was measured, and if necessary, quicklime or the like was added to the pot to control the basicity and the degree of oxidation, thereby obtaining each T.V. in accordance with the slab thickness specified by the equation (1). Adjusted to O value.

The machinability was determined by rolling 8 from the obtained slab.
Plunge cutting was performed on the 0 mmφ steel bar, and the finished surface roughness Rz (JIS) was evaluated. The plunge cutting conditions are shown below. Test material: 80mmφ steel bar, Tool: SKH57, Cutting speed: 80m / min Feeding speed: 0.05mm / rev, 2sec cutting / 5sec non-cut

[0053]

[Table 3]

As apparent from Table 3, in the method according to the present invention, the number of slime-extracted inclusions having a size of 53 μm or more is as small as less than 50 / kg at any slab size and casting speed. The finished surface roughness (Rz) is also less than 20 μm, and the machinability is good.

On the other hand, a 145 mm square or 3
00 mm square bloom in molten steel In the case of casting at 180 ppm O, the number of slab inclusions exceeded 50 pieces / kg, so that the finished surface roughness exceeded 20 μm and the machinability was poor.

In addition, 145 mm square and 300 mm square blooms were used in T.M. In the comparative example in which the castings were adjusted to 70 ppm and 100 ppm, respectively, the number of slab inclusions was significantly reduced, but as described above, T.O. Since O was too low, MnS was stretched by rolling, and similarly, the finished surface roughness exceeded 20 μm and the machinability was reduced.

[0057]

According to the present invention, a steel slab is formed by rolling two to four passes from a medium-section bloom, thereby significantly reducing the cost of lumping and preventing the occurrence of surface flaws in a bar or wire rod. Can be prevented. In addition, T.M. By appropriately adjusting O in accordance with the thickness of the slab, it is possible to produce a sulfur composite free-cutting steel having excellent machinability, and these industrial application effects are extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the draw ratio, the number of passes, and the austenite grain size of a slab.

Fig. 2 Slab thickness, T. in molten steel. Diagram showing the relationship between O and machinability

FIG. 3 is a diagram showing an example of the embodiment;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ladle, 2 ... Tundish, 3 ... Mold, 4 ... Secondary cooling zone, 5 ... Guide roll, 6 ...
..Cutting machines, 7 cast slabs, 8 cast slab heating furnaces, 9.
..Bloom rolling mill, 10 ... Slab, 11 ... Slab heating furnace, 12 ... Wire rod rolling machine, 13 ... Wire coil

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koichi Isobe 12 Murakami-cho, Muroran-shi, Hokkaido Nippon Steel Corporation Muroran Works

Claims (2)

[Claims] 1. C ≦ 0.15% by mass, Si ≦ 0.05
%, Al ≦ 0.003%, Mn: 0.50 to 1.50
%, P: 0.020 to 0.100%, N: 20 to 150
ppm, S: 0.100 to 0.350%, Pb: 0
0.300%, total oxygen T.O. O: A method for producing a sulfur composite free-cutting steel containing 100 to 250 ppm as a basic component, wherein the cast slab is continuously cast into a square or rectangular shape, cut into a predetermined length, and then heated to draw ratio. Is 1.6 to 2.8
2. A method for producing a sulfur composite free-cutting steel, comprising: performing slab rolling on a steel slab for two or more passes in two to four passes, and then rolling the steel slab into a bar or a wire. 2. C ≦ 0.15% by mass, Si ≦ 0.05 by mass
%, Al ≦ 0.003%, Mn: 0.50 to 1.50
%, P: 0.020 to 0.100%, N: 20 to 150
ppm, S: 0.100 to 0.350%, Pb: 0
0.300%, total oxygen T.O. O: A method for producing a sulfur composite free-cutting steel containing 100 to 250 ppm as a basic component, wherein T.O. After adjusting the O concentration so as to satisfy the formula (1) in relation to the slab size and continuously casting and cutting the slab, the slab is subjected to slab-rolling to obtain a steel bar for a bar steel, and thereafter, from the steel slab. A method for producing a sulfur composite free-cutting steel, characterized by rolling to a bar or a wire rod. 0.22D + 68 ≦ T. O ≦ 0.22D + 118 (1) where D is the thickness (mm) of a square or rectangular slab.
O is the total oxygen concentration (ppm) of the molten steel.