KR101944599B1

KR101944599B1 - High-carbon steel wire having superior wire drawing properties and method for producing same

Info

Publication number: KR101944599B1
Application number: KR1020167023858A
Authority: KR
Inventors: 도시유키 마나베; 다츠세이 다다; 다이스케 히라카미; 아라타 이소; 스스무 사하라
Original assignee: 신닛테츠스미킨 카부시키카이샤
Priority date: 2014-03-06
Filing date: 2015-03-06
Publication date: 2019-01-31
Also published as: KR20160114697A; WO2015133614A1; CN105980589B; CN105980589A; JP5900710B2; EP3115478A4; JPWO2015133614A1; EP3115478A1; EP3115478B1

Abstract

The present invention provides a wire having excellent wire drawing characteristics stably in actual manufacture and a method of manufacturing the wire. Wherein the wire material comprises 0.7 to 1.2% of C, 0.1 to 1.5% of Si, and 1.0% of Mn, in an amount of not more than 0.005 ppm of N and the balance of Fe and inevitable impurities And the remainder is a bainite structure, the half width of the ferrite phase (211) crystal face in the structure of the cross section of the wire is 0.6 占 or less, and the tensile strength TS (1) and the following formula (2), and the standard deviation of the hardness distribution in the cross section is less than 6 in terms of Vickers hardness (Hv).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high-carbon steel wire rod,

The present invention relates to a high carbon steel wire material or an aluminum conductor steel reinforcement (ACSR), a high carbon steel wire material for a rope, and a method of manufacturing the same, which require a primary drawing prior to final patenting or oil tempering.

In the secondary machining of the wire rod, mainly a drawing process is utilized, and generally, a pearlite steel which is heat-treated by a stell-mower or a lead-panteting is used. Particularly, in an extremely fine line of STC (Steel Cord) or a small diameter rope, intermediate pattening is performed to reduce the diameter to a predetermined line diameter, or the diameter of the rolled line is reduced, Lt; / RTI >

On the other hand, it is known to utilize a low-strength pearlite structure or bainite structure as a devise for improving the straightening deformation itself.

These structures are suppressed to a low initial tensile strength due to the initial strength of the wire rod or the drawing process, and superiority in terms of processing intensity and material is expected from the viewpoints of reducing the pulling force at the time of drawing and controlling the heat generation amount of the work. A method for producing a bainite wire material by a single transformation has been proposed (for example, see Patent Documents 1 to 3).

However, the bainitic wire has been disclosed in the heat treatment for controlling the tissue rate, but the tissue factor for stably lowering the strength is not clear.

Japanese Patent Application Laid-Open No. 06-330240 Japanese Patent Application Laid-Open No. 06-73502 Japanese Patent Application Laid-Open No. 06-73501

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a wire having excellent wire drawing characteristics stably in actual manufacture and a method for manufacturing the wire.

All of the inventions disclosed in Patent Documents 1 to 3 are maintained at a temperature of 350 ° C. to 500 ° C. for a predetermined time to initiate some bainite transformation from the supercooled austenite structure and then increase the temperature until complete bainite transformation And the rough bainite structure of cementite precipitation is generated by the correction. That is, all the inventions disclosed in Patent Documents 1 to 3 are characterized in that the upper bainite structure is softened during the two-stage heat treatment, and the bainite transformation by the first-stage heat treatment is not intended to be completed.

The inventors of the present invention have studied a softening mechanism by two-stage cooling in order to obtain good drafting characteristics on a bainite line, and (i) correcting the bainite transformation until the bainite transformation is completed in the first- (Ii) Even if a single bainite structure having a hard initial structure is used, the effect of annealing by heating in the two-stage cooling can be improved to a desired strength of the wire rod And (iii) a tissue fraction capable of lowering the fresh work hardening rate without being influenced by the bibbynite structure. The present invention has been accomplished based on this finding.

The present invention has been made based on the above knowledge, and its gist is as follows.

(1) A steel sheet comprising (1) a steel sheet having a composition of 0.7 to 1.2% of C, 0.1 to 1.5% of Si and 1.0% or less of Mn and having a composition of N: 0.005% or less and Fe and inevitable impurities , The remainder being at least 80% of the bainite structure in the cross section of the wire, the residual structure being a bainite structure, the half width of the ferrite phase (211) crystal face in the cross section of the wire rod being 0.6 占 or less, ) And the cross sectional shrinkage ratio RA (%) satisfy the following formulas (1) and (2) and the standard deviation of the hardness distribution in the cross section is less than 6 in Vickers hardness (Hv) Pre-existing.

Here, [C], [Mn] and [Cr] represent mass% of C, Mn and Cr, respectively.

(2) The steel according to the above item (1), wherein the composition of the above-mentioned components is 1.0% or less of Cr, 1.0% or less of Ni, 0.1% or less of Cu, 0.1% or less of V, 0.1% or less, Al: 0.1% or less, Ca: 0.05% or less, and B: 0.005% or less.

(3) A steel strip having the composition described in the above item (1) or (2) is hot-rolled into a wire rod and then rolled into a coil shape at 850 to 1050 캜, followed by immersion in molten salt or molten lead at 300 to 475 캜 (1) or (2) above, characterized in that the bainite fraction is made 80% or more by completing the bainite transformation and then immersed in a molten salt or molten lead of 550 to 650 DEG C for 1 second or more A method of producing a wire having excellent workability.

(4) The wire material having the composition described in the above item (1) or (2) is heated to 850 DEG C or higher and then dipped in a sand, molten salt, or molten lead at 300 to 475 DEG C to perform faceting treatment, (1) or (2), wherein the bainite structure is 80% or more in the cross section and then heated at 550 to 650 캜 for 1 second or more by sand, molten salt, molten lead, (2). &Lt; / RTI >

According to the present invention, it is possible to provide a wire rod excellent in drafting characteristics based on the softening mechanism of bainite and the knowledge on the structure fraction capable of reducing the work hardening rate.

1 is a view showing an example of a relationship between a tensile strength TS (MPa) and a C amount (mass%).

Hereinafter, the present invention will be described.

The present invention provides a wire rod excellent in drawability (hereinafter sometimes referred to as " present wire rod ") having a composition of 0.7 to 1.2% of C, 0.1 to 1.5% of Si, 1.0% And the remainder is made of Fe and inevitable impurities, at least 80% of the bainite structure in the cross section of the wire is a bainite structure, and the remainder of the ferrite phase (211) crystal face (1) and (2) satisfy the following formulas (1) and (2) and the standard deviation of the hardness distribution in the cross section of the wire rod is Vickers hardness (Hv) of less than 6.

Further, the cross section of the wire rod means a cross section perpendicular to the longitudinal direction of the wire rod.

Here, [C], [Mn] and [Cr] represent mass% of C, Mn and Cr, respectively.

First, the reason for limiting the composition of the wire of the present invention will be described. Herein,% means mass%.

C: 0.7 to 1.2%

C is an element that increases strength by increasing cementite fraction, number density and dislocation density of bainite structure. If it is less than 0.7%, it becomes difficult to secure the bainite fraction due to the ferrite transformation at the time of the heat treatment, so that it is set to 0.7% or more. It is preferably 0.9% or more. On the other hand, if it exceeds 1.2%, crud stone cementite will precipitate and the drawability will be deteriorated, so that it will be 1.2% or less. And preferably 1.0% or less.

Si: 0.1 to 1.5%

Si is an element that deoxidizes and also strengthens the ferrite. If it is less than 0.1%, the generation of the alloy layer at the time of galvanizing is not stable, so it is set to 0.1% or more. It is preferably 0.4% or more. On the other hand, when it exceeds 1.5%, decarburization during heating is promoted, mechanical descalability is deteriorated, and carbide precipitation at the time of bainite transformation is also retarded, so that it is 1.5% or less. And preferably 1.2% or less.

Mn: 1.0% or less

Mn is a deoxidizing element and also an element for improving the quenching property. The generation of ferrite at the time of heat treatment is suppressed. If it exceeds 1.0%, transformation may be delayed and a non-transformed structure may be formed. Therefore, the content should be 1.0% or less. Preferably 0.7% or less. The lower limit is not particularly limited, but is preferably 0.2% or more, and more preferably 0.3% or more, from the viewpoint of increasing the structure ratio of bainite.

The wire according to the present invention may contain one or more of Cr, Ni, Cu, V, Mo, Ti, Nb, Al, Ca and B in addition to the above- It may contain an amount.

Cr: not more than 1.0%

Cr is an element improving the quenching property, and is an element that acts to suppress ferrite transformation and pearlite transformation at the time of heat treatment. If it exceeds 1.0%, not only the transformation end time is lengthened but also the mechanical descalability is deteriorated, so that it is 1.0% or less. Preferably 0.7% or less. The lower limit includes 0%, but 0.05% or more is preferable in order to reliably obtain the effect of addition.

Ni: not more than 1.0%

Ni is an element that improves the quenching property and suppresses ferrite transformation to increase the structure of bainite. If it exceeds 1.0%, the transformation end time becomes longer, so it is set to 1.0% or less. Preferably 0.7% or less. The lower limit includes 0%, but 0.05% or more is preferable in order to reliably obtain the effect of addition.

Cu: not more than 0.1%

Cu is an element for improving corrosion resistance. If it exceeds 0.1%, CuS is segregated in the austenite grain boundaries by reacting with S, which causes scratches on the steel ingot or the wire rod in the wire rod manufacturing process. Preferably 0.07% or less. The lower limit includes 0%, but it is preferably 0.01% or more in order to reliably obtain the effect of addition.

V: not more than 0.1%

V is an element that acts to retard ferrite transformation in an employment state. If it exceeds 0.1%, a nitride is formed in the austenite, the quenching property is lowered, and the carbide is precipitated at the time of heating after the transformation, and the toughness of the wire is deteriorated. , Preferably 0.05% or less, more preferably 0.03% or less. The lower limit includes 0%, but it is preferably 0.01% or more in order to reliably obtain the effect of addition.

Mo: 0.5% or less

Mo is an element that improves the quenching property, suppresses ferrite transformation and pearlite transformation, and improves the structure of bainite. If it exceeds 0.5%, not only the transformation end time is prolonged but also carbide is generated at the time of heating after transformation to secondary curing, so that it is 0.5% or less. And preferably 0.3% or less. The lower limit includes 0%, but 0.1% or more is preferable in view of ensuring the effect of addition.

Ti: not more than 0.05%

Ti is an element contributing to improvement of ductility by making the? Grain size finer and making the structure formed thereafter finer. If it exceeds 0.05%, the effect of addition becomes saturated, so it is made 0.05% or less. It is preferably 0.02% or less. The lower limit includes 0%, but it is preferably 0.005% or more in order to reliably obtain the effect of addition.

Nb: not more than 0.1%

Nb is an element for improving the quenching property, and the nitride serves as a pinning particle, contributing to the transformation time and grain size control during the heat treatment. If it exceeds 0.1%, the transformation end time becomes longer, so it is set to 0.1% or less. Preferably 0.07% or less. The lower limit includes 0%, but it is preferably 0.005% or more in order to reliably obtain the effect of addition.

Al: 0.1% or less

Al is an element effective as a deoxidizing element. If it exceeds 0.1%, a hard inclusion is generated and drawing workability is lowered, so that it is set to 0.1% or less. Preferably 0.07% or less. The lower limit includes 0%, but it is preferably 0.02% or more in order to reliably obtain the effect of addition.

Ca: not more than 0.05%

Ca is a deoxidizing element and an element effective for controlling the shape of inclusions in the steel. If it exceeds 0.05%, coarse inclusions are produced, so the upper limit is set to 0.05% or less. It is preferably 0.02% or less. The lower limit includes 0%, but it is preferably 0.001% or more in order to reliably obtain the effect of addition.

B: not more than 0.005%

B is an element that is segregated in grain boundaries in the solid solution B state and inhibits ferrite formation. If it exceeds 0.005%, M ₂₃ (C, B) ₆ is precipitated in the grain boundary and the freshness is lowered, so that it is 0.005% or less. It is preferably 0.002% or less. The lower limit includes 0%, but it is preferably 0.0003% or more in order to reliably obtain the effect of addition.

N: 0.005% or less

Nitrogen (N) combines with nitride-forming elements such as Al and Ti to form precipitates in the steel and acts as pinning particles in the austenite grain boundary. Further, N existing as a solid solution element lowers the cross-sectional shrinkage ratio in the tensile test. If the amount of N exceeds 0.005%, the austenite grain boundary becomes finer, the aimed bainite structure becomes difficult to obtain, and the cross-sectional shrinkage ratio of the wire rod decreases, so that the upper limit is set to 0.005%.

Next, the structure of the wire rod of the present invention will be described.

The structure of the wire according to the present invention is characterized in that, in the cross section of the wire, the bainite structure is 80% or more in area ratio and the remainder is a bibbynite structure and the half width of the (211) crystal face of the ferrite phase in the cross- °.

In order to increase the brittleness of the bainite, it is necessary to suppress the ferrite transformation and the pearlite transformation (both diffusion transformation) as far as possible from the heated austenite state and to cool it to a predetermined temperature. However, in the case of an alloy component having a large wire diameter and a low quenching property, it is difficult to make a structure, and it is difficult to make the organization rate of bibenite 0% in actual production.

Therefore, the inventors of the present invention have extensively studied the range in which the bibbynite structure does not affect the strength of the entire wire or wire after drawing. As a result, it has been found that if the bainite structure is less than 20%, the entire wire and the wire strength after drawing are not affected. Based on this knowledge, the bainite structure was defined as 80% or more in the wire section.

The fraction of the bainite structure is determined by taking a sample with a cross section perpendicular to the longitudinal direction of the wire rod as an observation surface, polishing the observation surface, etching away it, and if necessary, referee etching and subjecting it to an optical microscope, Ray diffraction method. An image analysis is performed by binarizing a microstructure photograph obtained by an optical microscope or an electron microscope into white and black, and the area ratio of bainite can be obtained. The tissue fraction was obtained by photographing a sample taken from an arbitrary position of the steel sheet in a range of 300 x 300 mu m with a magnification of 1/4 of the plate thickness direction at 1000 times, . The bainite structure and the bibenite structure may be discriminated by analyzing the crystal orientation measurement data of the electron diffraction pattern obtained by EBSD (Electron Backscatter Diffraction) with the KAM method (Kernel Average Misorientation).

The bainite structure is composed of a carbide of granular cementite and a ferrite phase. The fraction of the bainite structure of the wire rod of the present invention is substantially determined by the bainite transformation step comprising heating and cooling after the winding step described later.

Further, by carrying out a heat treatment step to be described later for heating the wire after completion of the bainite transformation, the half width of the ferrite phase (211) crystal plane in the structure of the cross section of the wire rod decreases and the wire rod strength Is obtained.

Further, the half-value width means the width of the angle at the half of the peak height in the diffraction peaks of any crystal plane measured by X-ray diffraction. Since the pearlite structure contains a lot of elastic deformation, the half-value width at the production step is increased, and even if heating is performed, the half-width is not as low as the bainite. As a result, the higher the pearlite fraction is, the higher the half-value width is, and therefore, it is suitable as an evaluation index of the produced tissue.

The (211) crystal face of the ferrite phase in the structure of the cross section of the wire member is closely related to the dispersed state of the carbide of the granular cementite in the structure of the cross section of the wire and the content of pearlite. Therefore, the half-value width is a parameter for determining the bainite fraction of the wire, the dispersed state of the carbide of granular cementite in the bainite structure, and the content ratio of pearlite. In fact, the half-value width has a tendency to decrease as the bainite fraction increases. Further, the half-value width decreases with the uniformity of the dispersed state of cementite, increases with an increase in the content of pearlite, which is a bainite structure, and tends to decrease with decrease in the strength of the wire rod.

Next, the mechanical characteristics of the wire rod of the present invention will be described.

The wire rod of the present invention is characterized in that the tensile strength TS (MPa) and the cross-sectional shrinkage ratio RA (%) satisfy the following formulas (1) and (2), respectively.

Here, [C], [Mn] and [Cr] represent mass% of C, Mn and Cr, respectively.

The tensile strength TS and the cross-sectional shrinkage RA of the bainite wire rod depend on the average inter-cementite distance, the dislocation density and the block grain size. In particular, in the wire of the present invention, it depends on the amount of carbon corresponding to the cementite fraction. The present inventors investigated the relationship between the tensile strength TS and the amount of carbon ([C]) within the specified range of the bismuth structure and the half-width of the ferrite phase, and found that the cross-sectional shrinkage ratio RA was 100-46 x [ 18 x [Mn] -10 x [Cr] ".

"100-46 × [C] -18 × [Mn] -10 × [Cr]" is an index for evaluating the overall effect by multiplying the amount of representative elements inhibiting the section shrinkage by the influence coefficient. By specifying the lower limit of this index, the mechanical properties of the wire of the present invention can be characterized.

Fig. 1 shows the results of examining the relationship between the tensile strength TS and the amount of carbon ([C]). It can be seen that the tensile strength satisfies the above formula (1). As to the cross-sectional shrinkage ratio RA, the present inventors have found that satisfying the above formula (2) is preferable.

The hardness distribution in the cross section also affects the freshness characteristics. It has been found that a wire material with good drawing characteristics can be obtained by making the standard deviation of the hardness distribution in the wire rod cross section less than 6 as Vickers hardness (Hv).

Next, a method of manufacturing the wire rod of the present invention will be described.

A method for manufacturing a wire rod according to the present invention is a method for producing a wire rod according to the present invention, comprising the steps of hot-rolling a steel strip having the composition of the wire rod of the present invention into a wire rod at a temperature of 850 to 1050 캜, , The bainite transformation is completed to make the bainite fraction 80% or more, and then immersed in the molten salt or molten lead at 550 to 650 占 폚 for at least 15 seconds.

The wire material temperature at the time of rolling the steel strip having the composition of the present invention into a coil shape after hot rolling the wire material is important in the adjustment of the austenite grain size. The winding temperature of the wire rod is changed according to the hardness of the steel sheet. However, if the temperature exceeds 1050 deg. C, it is difficult to physically treat the wire rod. Preferably not higher than 1000 캜.

On the other hand, if the coiling temperature is less than 850 캜, the austenite grain size becomes finer and the quenching is lowered, and the decarburization of the two-phase region of the surface layer progresses, so that it is set to 850 캜 or higher. Preferably 900 DEG C or more.

Further, in the method of manufacturing the wire rod of the present invention, the wire rod having the composition of the wire rod of the present invention is heated at 850 ° C or higher, then immersed in a sand, molten salt, or molten lead at 300 to 475 ° C, Characterized in that the bainite structure is 80% or more in the cross section of the wire and then heated at 550 to 650 캜 for 1 second or more by sand, molten salt, molten lead, energizing, or induction heating.

The heating temperature in the case of bainite transformation of the once-cooled wire rod influences the hardenability of the steel material. If the heating temperature is lower than 850 占 폚, the austenite grain size becomes finer, the quenching property is lowered, the fraction of bainite is not improved, and the decarburization of the two-phase region in the surface layer progresses. Preferably 900 DEG C or more.

The heating temperature is set in accordance with the amount of the alloying element in order to control the particles which peen the austenite grains. The upper limit of the heating temperature is not specifically defined, but is preferably 1150 占 폚 or less from the viewpoint of economy. More preferably 1100 占 폚 or less.

The temperature (i.e., the coolant temperature) of the wire after the hot rolling of the billet, or the sand, molten salt, or molten lead for immersing the wire after reheating the wire once cooled is influenced by the bainite transformation temperature and the cooling rate of the wire . When the coolant temperature exceeds 475 DEG C, the cooling rate is lowered and pearlite transformation occurs, which makes it difficult to bainitize the entire cross section of the wire rod. Preferably 450 DEG C or less.

On the other hand, when the refrigerant temperature is less than 300 ° C, the bainite transformation takes a long time, so it is set to 300 ° C or higher. Preferably 350 DEG C or more.

In the present invention, after the wire material once cooled is reheated to 850 DEG C or higher, the wire material is corrected in a temperature range of 300 to 475 DEG C to advance the bainite transformation of the wire material texture, It can be made uniform. This is because, in the amount of carbon, the bainite structure is mainly generated at a temperature of about 300 ° C to about 500 ° C, but the size of the bainite structure is influenced by the temperature at the time of formation of the bainite structure.

The bainite structure of the wire rod can be made uniform by correcting the wire rod at a temperature range of 300 to 475 DEG C until the bainite transformation is completed. However, long-time correction is not preferable from the viewpoint of manufacturing cost.

On the other hand, if the wire rod is heated to a temperature higher than 475 DEG C for a predetermined period of time before completion of the bainite transformation, the bainite transformation is completed, but the bainite structure becomes uneven and the distribution of the hardness of the wire rod section becomes uneven.

Therefore, in the present invention, the wire rod is corrected in the temperature range of 300 to 475 占 폚 until the bainite structure in the structure within the wire rod cross section becomes 80% or more, and thereafter, Second or more.

The correction time until the bainite transformation is completed or the correction time until the bainite fraction becomes 80% or more may be determined in advance under predetermined experimental conditions. For example, the relationship between the composition of the wire, the correction time by the immersion treatment or the patenting treatment in the molten salt or the molten lead, the temperature in the immersion treatment or the patenting treatment, and the bainite fraction are investigated in advance, The correction time may be determined based on the result. In this case, it is necessary to judge the degree of bainite transformation by strictly corresponding to the measured value. It is also possible to carry out interpolation or extrapolation on the basis of the relationship between the adjacent production conditions and the fraction of the bainite structure in the production conditions, The correction time may be determined in anticipation of the fraction of the bainite structure of the drawn wire. Alternatively, a test piece may be manufactured under the same manufacturing conditions as the production conditions to be carried out, and the wire rods may be manufactured while confirming the fraction of the bainite structure during the production process of the wire rods.

A heat treatment step of heating the wire after completion of bainite transformation is performed. The heating temperature in the heat treatment process affects the recovery and softening of the bainite wire. If the heating temperature is less than 550 占 폚, a sufficient softening effect can not be obtained. Therefore, the heating temperature should be 550 占 폚 or higher. Preferably 570 DEG C or more. When the temperature exceeds 650 ° C, the osteoplastic growth of the cementite proceeds and the ductility of the wire is lowered. Preferably 630 占 폚 or less.

The heating time after completion of the bainite transformation is adjusted according to the heating temperature, but it is set to 1 second or more in order to promote the softening. If the heating time is too long, the osmosis growth of the cementite proceeds and the ductility is lowered. However, the upper limit is not particularly set because it can be appropriately adjusted within the heating temperature range. The time until the heating temperature is reached or the rate of temperature rise until the heating temperature is reached is not particularly limited.

The heating may be carried out by immersion in a sand, molten salt, or molten lead at a predetermined temperature, or by energization or induction heating.

Next, an embodiment of the present invention will be described. The conditions in the embodiment are examples of conditions employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to this one conditional example. As long as the object of the present invention is achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

Example

The bainite transformation was completed by correcting the wires of the component compositions A to O shown in Table 1 for a predetermined time at a predetermined temperature shown in " cooling conditions " shown in Table 2-1. The wire after completion of the bainite transformation was subjected to heat treatment for heating to a predetermined temperature indicated by " heat treatment conditions after completion of bainite transformation " and correcting the wires at the predetermined temperature for a predetermined time. The results of measuring the tensile strength TS (MPa) and the sectional shrinkage percentage (%) of the wire after the heat treatment, and the results of measuring the bainite structure and the half-width of the ferrite phase in the bainite structure and the distribution of the hardness of the cross- 2-2. The transformation time of the bainite is suitably changed in the upper limit of 300 seconds in the case of directly heat treating the wire after hot rolling, and when the wire after reheating is faceted, the upper limit of 1800 seconds is appropriately changed .

In each of Examples 4 to 7 and Comparative Examples 1 to 6, a wire rod obtained by hot-rolling a steel strip having the composition shown in Table 1 under the conditions shown in Table 2-1 was used. Each of the wire rods of Inventive Examples 1 to 3 and Comparative Example 7 was manufactured by cooling the wire rods having the component compositions shown in Table 1 once and then reheating the wire rods at the heating temperatures shown in Table 2-1 &Lt; / RTI >

The composition of the steel type K in Table 1 corresponds to the composition of the steel wire in Patent Document 3. The bainite transformation of the wire of Comparative Example 6 was proceeded until the bainite transformation was completed by holding the wire having these compositions at a predetermined temperature of "cooling condition" shown in Table 2-1 for a predetermined time. Then, the wire of Comparative Example 7 was heated to a predetermined temperature indicated by "heat treatment conditions after completion of bainite transformation", and subjected to a heat treatment for correcting the wire at the predetermined temperature for a predetermined time to complete the bainite transformation.

Electron beam backscattering diffraction (EBSD) was used to measure the bainite organization rate. A region of 300 占 퐉 占 180 占 퐉 or more at the center of the wire rod was measured and a region where crystal rotation did not occur was defined as a bainite structure by a Kernel Average Misorientation (KAM) method to calculate a bainite fraction.

An X-ray diffraction apparatus was used for the half-value width of the ferrite phase, and a Cr reference was used for the X-ray source. The measurement surface was a (211) plane, and time measurement was performed so that the maximum count number became 3000 or more, and the half value width was taken as a measurement value.

For each of the wire rods of Inventive Examples 1 to 7 and Comparative Examples 1 to 7, the relationship between the production conditions such as the composition of the steel species and the heat treatment, and the fraction of the bainite structure was examined in advance. Based on the results of the investigation, the progress of the bainite transformation of the wire rods was judged, and the initiation and completion of the bainite transformation of the wire rods of Inventive Examples 1 to 7 and Comparative Examples 1 to 7 were judged.

The distribution of the hardness of the cross section of the wire rod was measured by using a Vickers hardness tester and a score of 100 points was measured at a load of 1 kgf against the longitudinal cross section of the obtained structure. The standard deviation was defined as a deviation of hardness.

[Table 1]

[Table 2-1]

[Table 2-2]

Inventive Examples 1 to 7 are examples of the present invention, and as shown in Table 2-2, a bainite wire material excellent in drawing processing characteristics is obtained.

In Comparative Example 1, the ferrite transformation progresses from the time of winding to the time of winding until the coiling temperature is low and the desired bainite structure fraction is not obtained. In addition, the tensile strength TS does not satisfy the formula (1).

In the comparative examples 2 and 3, since the quenching-improving elements Si and Mn exceeded the specified ranges and the quenching became too high, the transformation at the first cooling stage was not completed. In Comparative Example 4, since the temperature at the first stage of cooling exceeded the specified range, the cooling was slowed and a large amount of pearlite was generated, so that the aimed bainite structure fraction was not obtained.

In Comparative Example 5, since the second stage of cooling was not performed, the half-value width exceeded a prescribed value, and the tensile strength TS did not satisfy the formula (1). In Comparative Example 6, C exceeded the specified range, cementite was formed during cooling from austenite, and the cross-sectional shrinkage ratio RA did not satisfy the formula (2).

In Comparative Example 6, since the wire rod was heated before completion of the bainite transformation, the bainite structure was uneven, and the hardness distribution of the wire rod section was uneven. Therefore, in Comparative Example 6, the cross-sectional shrinkage ratio RA does not satisfy the formula (2), and the ductility of the wire rod is lowered, and the drawing machining characteristic is lowered.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it is possible to provide a wire rod excellent in drafting characteristics based on the softening mechanism of bainite and the knowledge on the structure ratio capable of reducing the work hardening rate. Therefore, the present invention is highly available in the wire rod manufacturing industry.

Claims

A ferritic stainless steel having a composition of 0.7 to 1.2% by mass, 0.1 to 1.5% by mass Si, 0.2 to 1.0% by mass Mn, 0.005% by mass or less of N and a balance of Fe and inevitable impurities , The remainder being a bainite structure, a half-width of a crystal plane of the ferrite phase (211) in the structure of the cross section of the wire is 0.6 占 or less, and a tensile strength TS ( (1) and the following formula (2), and the standard deviation of the hardness distribution in the cross section is less than 6 in terms of Vickers hardness (Hv). This excellent wire rod.

Here, [C], [Mn] and [Cr] represent mass% of C, Mn and Cr, respectively.

The method according to claim 1,
Wherein said composition comprises, by mass%, at most 1.0% of Cr, at most 1.0% of Ni, at most 0.1% of Cu, at most 0.1% of V, at most 0.5% of Mo, at most 0.05% , Al: not more than 0.1%, Ca: not more than 0.05%, and B: not more than 0.005%.

A steel strip having the composition described in claim 1 or 2 is hot rolled into a wire rod and then rolled into a coil shape at 850 to 1050 캜 and then immersed in a molten salt or molten lead at 400 to 475 캜, A process for producing a biaxially stretched polyimide film according to any one of claims 1 to 3, wherein the biaxite content is 80% or more by completing the transformation and then immersed in a molten salt or molten lead at 550 to 650 캜 for at least 15 seconds. A method of manufacturing a wire rod.

A wire material having the composition described in claim 1 or 2 is heated to 850 DEG C or higher and then dipped in a sand, molten salt, or molten lead of 400 to 475 DEG C to carry out a patenting treatment, % Or more of bainite structure and then immersing in a sand, molten salt, or molten lead at 550 to 650 占 폚 for 15 seconds or more, or heating at 550 to 650 占 폚 for 1 second or more by energization or induction heating The method of producing a wire according to any one of claims 1 to 3, wherein the wire has excellent drawability.