JPH0617190A - Bainitic wire rod or steel wire for wire drawing and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide bainitic wire rod or steel wire excellent in wire drawability and to provide the manufacturing method thereof. CONSTITUTION:The bainitic wire rod or steel wire for wire drawing has characteristics of having a compsn. contg., by weight, 0.70 to 1.20% C, 0.30 to 0.90% Mn and 0.15 to 1.00% Si, contg., as alloy components, one or two kinds of 0.006 to 0.100% Al and 0.01 to 0.35% Ti, contg., at need, 0.10 to 0.50% Cr, in which each content of P and S is limited to <=0.02% and <=0.01%, and the balance Fe with inevitable impurities as well as having tensile strength and reduction of area prescribed by the following inequalities (1) and (2): TS<=85X(C)+60... (1) and RA>=-0.875X(TS)+158... (2); where C: carbon content (wt.%), TS: tensile strength (kgf/mm<2>) and RA: reduction of area (%).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bainite wire or steel wire for wire drawing and a method for manufacturing the wire. In the present invention, a wire rod as a product means a wire rod for direct drawing by subjecting a steel slab to a heat treatment after being rolled into a wire rod, and a steel wire as a product is a wire rod before or after hot rolling, It means a steel wire that has been heat-treated to be subjected to wire drawing, and a steel wire that has been subjected to a first drawing process by hot working and then cold working and then a heat treatment for a second drawing process.

[0002]

2. Description of the Related Art Usually, wire rods or steel wires are drawn according to the use of various end products. Prior to this wire drawing, the wire rods or steel wires are suitable for wire drawing. Need to be kept. Conventionally, for high carbon steel wire or steel wire, it is necessary to make the structure uniform and a mixed structure of fine pearlite and a small amount of proeutectoid ferrite before wire drawing.Therefore, a heat treatment unique to wire or steel wire called patenting is required. Is given. This is a heat treatment method in which a wire or steel wire is heated to an austenitizing temperature and then cooled at an appropriate cooling rate to complete the pearlite transformation and form a mixed structure of fine pearlite and a small amount of proeutectoid ferrite.

In the method for producing a wire rod disclosed in Japanese Examined Patent Publication No. 60-56215, a wire rod at an austenitizing temperature is immersed in a molten salt, and a cooling rate between 800 and 600 ° C. is 15 to
By setting the temperature to 100 ° C./sec, a heat treatment method for forming a mixed structure of fine pearlite and a small amount of proeutectoid ferrite is performed. However, in the pearlite structure, deterioration of ductility at a high area reduction rate in the wire drawing process and occurrence of cracks in the twisting test (hereinafter referred to as delamination) pose problems.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a wire rod or a steel wire having excellent wire drawability which does not cause the above-mentioned problems in the wire drawing step and a manufacturing method thereof. .

[0005]

The above-mentioned problems of the present invention are as follows.
According to the present invention, specific amounts of C, Mn, Si and Al or Ti
1 or 2 of any of the above, further containing a specific amount of Cr as required, and having a chemical composition with a limited upper limit of the amount of P and S, and having a specified tensile strength and drawing value. A solution is provided by providing a wire or steel wire having a bainite structure.

Further, the object of the present invention is to cool the wire rod after hot rolling, or in the heat treatment of the steel wire after heating to the austenitizing temperature, by increasing the cooling rate to the nose position in the TTT diagram, the pearlite The present invention aims to solve the problem by providing a bainite wire or steel wire obtained by preventing the formation of a structure and then isothermally holding at 350 to 500 ° C. That is, after rolling the wire or heating the steel wire, 1100 to 755 ° C
From the above temperature range to a temperature range of 350 to 500 ° C. at a cooling rate of 60 to 300 ° C./sec, and holding at this temperature for a certain period of time or more to suppress the generation of a micro martensite structure, thereby improving wire drawability. A wire or steel wire having an excellent bainite structure can be obtained, and thus a wire or steel wire excellent in wire drawing workability can be obtained even at a high area reduction rate.

That is, the gist of the present invention is as follows. (1) C: 0.70 to 1.20% by weight%, Mn: 0.30 to 0.90%, Si: 0.15 to 1.00% are contained, and Al: 0. 006-0.100%, Ti: 0.01-0.35%, containing any one or two kinds, P: 0.02% or less, S: 0.01% or less, balance Fe And inevitable impurities,
A bainite wire rod or steel wire for wire drawing, which has a tensile strength and a drawing value defined by the following formulas (1) and (2).

TS ≦ 85 × (C) +60 ─ (1) RA ≧ −0.875 × (TS) +158 ─ (2) where C: carbon content (wt%) TS: tensile strength (kgf / mm ² ) RA: drawing (%) (2) As an alloy component, Cr: 0.10-0.
50% is contained, The bainite wire rod for wire drawing or steel wire of the preceding clause 1 characterized by the above-mentioned.

(3) The upper bainite structure has an area ratio of 8
The bainite wire rod or steel wire for wire drawing according to item 1 or 2 above, which has a microstructure of 0% or more and Hv of 450 or less. (4) C: 0.70 to 1.20% by weight%, Mn: 0.30 to 0.90%, Si: 0.15 to 1.00% are contained, and Al: 0. 006-0.100%, Ti: 0.01-0.35%, containing any one or two kinds, P: 0.02% or less, S: 0.01% or less, balance Fe After rolling a steel slab having a composition consisting of unavoidable impurities into a wire rod, 350 to 5 at a cooling rate of 60 to 300 ° C./sec from a temperature range of 1100 to 755 ° C.
A method for producing a bainite wire rod for wire drawing, which comprises cooling to a temperature range of 00 ° C. and holding in this temperature range for a time Y seconds or more defined by the following formula (3).

Y = exp (19.83-0.0329 × T)-(3) where T: holding temperature (° C.) (5) The starting steel piece is an alloy component, and Cr: 0.
The method for producing a bainite wire rod for wire drawing according to the preceding item 4, which contains 10 to 0.50%. (6) C: 0.70 to 1.20% by weight%, Mn: 0.30 to 0.90%, Si: 0.15 to 1.00% are contained, and Al: 0. 006-0.100%, Ti: 0.01-0.35%, containing any one or two kinds, P: 0.02% or less, S: 0.01% or less, balance Fe After heating a steel wire having a composition consisting of unavoidable impurities and a temperature range of 1100 to 755 ° C., 350 to 50 at a cooling rate of 60 to 300 ° C./sec.
Cool to a temperature range of 0 ° C, and then use the following formula (3) in this temperature range.
The method for producing a bainitic steel wire for wire drawing, characterized in that the time is defined as Y seconds or more.

Y = exp (19.83-0.0329 × T)-(3) where T: holding temperature (° C.) (7) The starting steel wire is an alloy component, and Cr: 0.
The method for producing a bainitic steel wire for wire drawing according to the above item 6, which contains 10 to 0.50%.

[0012]

The reason for limiting the constituent features of the present invention will be described.
The reasons for limiting the chemical composition of the starting billet and the steel wire are as follows. C is a basic element that controls the strength and ductility of steel, and the higher the carbon content, the higher the strength. The lower limit of the C content is 0.70 wt% to ensure hardenability and strength, and the upper limit is 1.20 w to prevent the occurrence of pro-eutectoid cementite.
It was set to t%.

Si is added as a deoxidizing agent in an amount of 0.15 wt% or more. In addition, Si is an element that solid-solution strengthens steel and is an element that can reduce relaxation loss of the steel wire. However, it reduces the amount of scale generation, deteriorates the mechanical descaling property, and slightly lowers the bonder lubrication property of the wire. Therefore, the upper limit of the amount of Si is 1.00 wt.
%.

Mn is added as a deoxidizing agent in an amount of 0.30 wt% or more. Further, Mn is an element that forms a solid solution in steel and strengthens it.
If the amount of addition is increased, segregation is likely to occur in the central part of the wire. Since the segregated part has improved hardenability and the transformation end time shifts to the long side, the untransformed part becomes martensite, which leads to disconnection during wire drawing. Therefore, the upper limit of Mn is 0.9
It was set to 0 wt%.

Al has the deoxidizing action and is the most economical element for fixing N in steel to make fine-grained austenite, but when N is low, Al is not an essential element.
The upper limit is 0.100 wt% considering the increase of non-metallic inclusions
And The lower limit was set to 0.006 wt% at which the effect of Al appears. At present, Ti is already used for controlling the austenite grains of Ti deoxidized steel, mainly of ordinary carbon steel.
The upper limit was set to 0.35 wt% in order to suppress the increase of Ti inclusions and to suppress the formation of solute carbonitride in the steel. The lower limit is set to 0.01 wt% where these effects effectively appear.

The wire rod and steel wire of the present invention are made of Al or T
It contains one or two of both elements i.

P and S precipitate at the grain boundaries and deteriorate the properties of the steel, so they must be kept as low as possible. The upper limit of P is 0.02 wt% and the upper limit of S is 0.01 w.
It was set to t%. Cr is an element that increases the strength of steel,
It can be added as needed. Although the strength is increased by the addition of Cr, the hardenability is also improved and the transformation end line moves to the longer side. Since this increases the time required for heat treatment, the upper limit was made 0.50 wt% and the lower limit was made 0.10 wt% to increase the strength.

Next, rolling conditions and heat treatment conditions for obtaining the bainite wire rod and steel wire of the present invention will be described. The cooling start temperature or steel wire heating temperature after wire rod rolling is 755 to 1
The reason for limiting the temperature to 100 ° C. is that 755 ° C. is the lower limit of the austenite transformation point, while if it exceeds 1100 ° C., abnormal growth of austenite grains occurs.

After the start of cooling the wire or steel wire, 350 to 5
The cooling rate up to the constant temperature holding temperature of 00 ° C is 60 to 300.
The upper limit of the critical cooling rate for forming the upper bainite structure is 60 ° C./sec.
This is because 0 ° C./sec is the upper limit of the industrially possible cooling rate. The reason why the isothermal holding temperature after cooling is set to 350 to 500 ° C. is that 350 ° C. is the lower limit temperature of the upper bainite structure formation, while 500 ° C. is the upper limit temperature of the upper bainite structure formation.

The time required to maintain a constant temperature in the temperature range of 300 to 500 ° C. is obtained from the transformation end line of the TTT diagram, but if the immersion time in the cooling tank is insufficient, martensite will occur, It may cause wire breakage during wire drawing. Therefore, since it is necessary to maintain the transformation end time or more, 350-
The lower limit of the time to keep in the temperature range of 500 ℃ is as follows (3)
The time defined by the formula was Y seconds.

Y = exp (19.83-0.0329 × T)-(3) However, T: heat treatment temperature (° C.) Next, the reason for limiting the characteristics of the wire rod and the steel wire of the product of the present invention will be described. Since the tensile strength strongly depends on the C content, it is given by the relationship with the C content as shown in formula (1). A wire or steel wire having a bainite structure has coarser precipitation of cementite than a wire or steel wire having a conventional pearlite structure, and therefore has a low tensile strength in the same composition. In the wire drawing process, the lower the initial tensile strength, the better the wire drawing workability, and the wire can be drawn to a high area reduction rate. Therefore, the tensile strength is limited as in formula (1) as a limit that does not deteriorate wire drawing workability. If the upper limit is exceeded, wire drawability deteriorates, causing wire breakage and delamination during wire drawing.

The drawing value is an important factor showing the ease of working during wire drawing. Even with the same tensile strength, the higher the drawing value, the lower the work hardening rate during wire drawing, and the wire drawing can be performed up to a high area reduction rate. A wire rod having a bainite structure has coarser precipitation of cementite as compared with a wire rod having a conventional pearlite structure, so that the drawing value becomes high even at the same tensile strength. Therefore, the aperture value is limited as in Expression (2) so that the wire drawing limit does not deteriorate. If the lower limit is not reached, wire drawing workability deteriorates, causing wire breakage and delamination during wire drawing.

The wire or steel wire having the bainite structure of the present invention has the tensile strength and the drawing value defined as described above, and the upper bainite structure has an area ratio of 80.
%, And having Hv of 450 or less, the wire drawing workability is further improved.

[0024]

【Example】

Example 1 Table 1 shows the chemical composition of the test steel. A to D in Table 1 are examples of the steels of the present invention, and E and F are examples of comparative steels. E steel has an amount of C exceeding the upper limit, and F steel has an amount of Mn exceeding the upper limit.

These test steels were placed in a continuous casting facility for 30
0x500mm slab and 122 by slab rolling
A steel piece having a mm-square cross section was manufactured. After manufacturing these billets by billet rolling into billets, they were rolled into wire rods with the diameters shown in Table 2,
DLP (Direct Lead Patentin
g) Cooled. These wire rods were drawn to 1.00 mmφ with an average surface reduction rate of 17%, and subjected to a tensile test and a twist test.

The tensile test was carried out by using the JISZ2201 No. 2 test piece according to the method described in JISZ2241. In the twist test, the test piece was cut to a length of 100d + 100, and then rotated at a distance between chucks of 100d and a rotation speed of 10 rpm until it was broken. d represents the diameter of the steel wire.

The characteristic values thus obtained are also shown in Table 2. No. 5 to No. 10 is a comparative example. No. In No. 5, since the cooling rate was too slow, pearlite was generated, wire drawability was deteriorated, and wire breakage occurred during wire drawing.

No. In No. 6, pearlite was generated because the isothermal transformation temperature was too high, the wire drawing workability was deteriorated, and wire breakage occurred during wire drawing. No. In No. 7, martensite was generated because the isothermal transformation treatment time was short, wire drawability was degraded, and wire breakage occurred during wire drawing. No. In No. 8, since the cooling start temperature was too low, the bainite structure did not occur, wire drawability deteriorated, and wire breakage occurred during wire drawing.

No. In No. 9, since the C content was too high, pearlite was generated and wire drawability was deteriorated. No. In No. 10, since the amount of Mn was too high, micro martensite was generated due to center segregation, and wire drawability was deteriorated.

[0030]

[Table 1]

[0031]

[Table 2]

Example 2 Table 3 shows the chemical composition of the test steel wire. Tables A to D are examples of the present invention, and E and F are comparative examples. For E steel, the C content exceeds the upper limit,
The F steel has an Mn content exceeding the upper limit. These steel wires were austenitized under the conditions shown in Table 4, heat-treated, and then drawn to 1.00 mmφ with an average surface reduction rate of 17%, and subjected to a tensile test and a twist test.

The tensile test was carried out by using the JISZ2201 No. 2 test piece according to the method described in JISZ2241. In the twist test, the test piece was cut to a length of 100d + 100, and then rotated at a distance between chucks of 100d and a rotation speed of 10 rpm until it was broken. d represents the diameter of the steel wire. The characteristic values thus obtained are also shown in Table 4.

No. 1-No. Up to 4 are examples of the present invention, and all the heat treatment conditions of the present invention are satisfied, so that delamination does not occur even after 1.0 mmφ after drawing and wire drawing is possible. In addition, No. 5 to No. 10 is a comparative example. No. In No. 5, since the cooling rate was too slow, pearlite was generated, wire drawability was deteriorated, and wire breakage occurred during wire drawing.

No. In No. 6, since the isothermal transformation temperature was too high, pearlite was generated, wire drawability was deteriorated, and wire breakage occurred during wire drawing. No. In No. 7, martensite was generated because the isothermal transformation treatment time was short, wire drawability was degraded, and wire breakage occurred during wire drawing. No. In No. 8, the heating temperature was too low, the bainite microstructure ratio became zero, the wire drawing workability deteriorated, and wire breakage occurred during wire drawing.

No. In No. 9, since the C content was too high, pearlite was generated and wire drawability was deteriorated. No. 10 is Mn
Since the amount was too high, pearlite was generated and the aperture value was also low, so the wire drawability was deteriorated.

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

As described above, the wire rod or steel wire produced according to the present invention can be drawn to a much higher area reduction rate than the conventional method and the delamination resistance is also improved. Therefore, according to the present invention, it is possible to provide a bainite wire or steel wire having excellent wire drawing workability.

[Brief description of drawings]

FIG. 1 is a diagram showing a heat treatment pattern of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Nishida Century 1 Kimitsu, Kimitsu City, Chiba Prefecture Nippon Steel Corporation Kimitsu Steel Co., Ltd.

Claims (7)

[Claims] 1. By weight%, C: 0.70 to 1.20%, Mn: 0.30 to 0.90%, Si: 0.15 to 1.00% are contained, and as an alloy component, Al: 0.006 to 0.100%, Ti: 0.01 to 0.35%, containing any one or two kinds, P: 0.02% or less, S: limited to 0.01% or less, The balance consists of Fe and inevitable impurities,
A bainite wire rod or steel wire for wire drawing, which has a tensile strength and a drawing value defined by the following formulas (1) and (2). TS ≤ 85 x (C) + 60- (1) RA ≥ -0.875 x (TS) + 158-(2) where C: carbon content (wt%) TS: tensile strength (kgf / mm ² ) RA : Aperture (%) 2. As an alloy component, Cr: 0.10 is further added.
The bainite wire rod or steel wire for wire drawing according to claim 1, wherein the bainite wire rod or steel wire contains 0.5 to 0.50%. 3. The bainite wire rod or steel wire for wire drawing according to claim 1, wherein the upper bainite structure has a microstructure having an area ratio of 80% or more and Hv of 450 or less. 4. C: 0.70 to 1.20% by weight%, Mn: 0.30 to 0.90%, Si: 0.15 to 1.00% are contained, and Al: 0.006 to 0.100%, Ti: 0.01 to 0.35%, containing any one or two kinds, P: 0.02% or less, S: limited to 0.01% or less, After rolling a steel slab having a composition consisting of the balance Fe and unavoidable impurities into a wire rod, 350 to 5 at a cooling rate of 60 to 300 ° C / sec from a temperature range of 1100 to 755 ° C.
A method for producing a bainite wire rod for wire drawing, which comprises cooling to a temperature range of 00 ° C. and holding in this temperature range for a time Y seconds or more defined by the following formula (3). Y = exp (19.83-0.0329 × T)-(3) where T: holding temperature (° C) 5. The starting steel billet is used as an alloy component, and further C
The method for producing a bainite wire rod for wire drawing according to claim 4, which contains r: 0.10 to 0.50%. 6. By weight%, C: 0.70 to 1.20%, Mn: 0.30 to 0.90%, Si: 0.15 to 1.00% are contained, and as an alloy component, Al: 0.006 to 0.100%, Ti: 0.01 to 0.35%, containing any one or two kinds, P: 0.02% or less, S: limited to 0.01% or less, After heating a steel wire having a composition consisting of the balance Fe and unavoidable impurities to a temperature range of 1100 to 755 ° C., 350 to 50 at a cooling rate of 60 to 300 ° C./sec.
Cool to a temperature range of 0 ° C, and then use the following formula (3) in this temperature range.
The method for producing a bainitic steel wire for wire drawing, characterized in that the time is defined as Y seconds or more. Y = exp (19.83-0.0329 × T)-(3) where T: holding temperature (° C) 7. The starting steel wire as an alloy component, further comprising C
The method for producing a bainitic steel wire for wire drawing according to claim 6, which contains r: 0.10 to 0.50%.