JPH06306483A - Production of high strength steel wire rod excellent in cold wire drawability - Google Patents

Abstract

PURPOSE:To produce high strength steel wire rod excellent in cold wire drawability and free from deterioration of ductility in the process of wire drawing by forming the structure of steel wire rod having a specified compsn. constituted of C, Si, Mn and Fe into a one consisting essentially of martensite or bainite and thereafter executing specified heat treatment. CONSTITUTION:The structure of steel wire rod contg., by weight, 0.03 to 0.3% C, <=2% Si and 0.3 to 2.5% Mn, and the balance Fe with inevitable impurities is formed into a one consisting essentially of martensite or bainite or into a mixed one of those and ferrite. This structure can be obtd. by accelerated cooling after controlled rolling or rolling. After that, this steel wire rod is heated to a ferrite-austeite region. At this time, it is heated in the temp. range of less than AC1-20 deg.C at 10 to 100 deg.C/sec average rate and in the temp. range of AC1-20 deg.C or above at 0.1 to 2 deg.C/sec average rate, and after that, cooling is executed. In this way, the mixed structure in which the low temp. transformation formed phase of martensite or bainite partially contg. retained austenite is regulated to 15 to 30% and the lumpy phase is regulated to <=30%, and the balance acicular low temp. transformation formed phase is formed.

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 high-strength steel wire rod which is excellent in cold drawability and can impart high strength to an ultrafine steel wire obtained by drawing. Is.

[0002]

2. Description of the Related Art A drawn steel wire has a P
It is used in various applications such as C wire, various spring wire, rope wire, bead wire, tire cord wire, high pressure hose reinforcing wire, switching wire, corona wire, and dot printer wire.

Among the steel wires, ultrafine steel wires are particularly high carbon steels.
Manufactured by cold drawing for several times while using a rolled wire with a diameter of 5 mm and performing patenting treatment several times in the middle to prevent the toughness of the drawn wire from degrading after each wire drawing. Is common. However, such a manufacturing method has a problem that a large number of manufacturing steps are required and the manufacturing cost is inevitably high.

On the other hand, with pure iron and low carbon ferritic / pearlitic steel wire rods, it is possible to wire-draw ultrafine wires by heavy working, but since the increase in strength by wire drawing is small, the final product is There is a drawback that the strength of the ultrafine wire is low. That is, even in the case of 95-99% heavy working wire drawing, the strength is about 70-130 kgf / mm ^2.
The strength of 0kgf / mm ² or more cannot be achieved, and the strength is 190kgf / mm even if the drawing rate is 99% or more.
² or less.

Further, a wire having a tempered martensite structure which is hardened and tempered by heat treatment is also known. However, since this wire cannot obtain good workability as it is hardened, it is hardened by tempering. The strength of the steel wire is greatly reduced to obtain workability, and therefore a high strength and high ductility steel wire cannot be obtained. On the other hand, if it is left as it is, it is unavoidable that surface cracks are likely to occur in the pickling step as a pre-drawing treatment and the ductility itself is insufficient.

[0006]

DISCLOSURE OF THE INVENTION The present inventors have been conducting research for some time in order to obtain high-strength and high-ductility wire rods that can replace the above-mentioned various conventional wire rods. As a result, while having a predetermined chemical component, a fine low-temperature transformation forming phase consisting of needle-like bainite or needle-like martensite which may partially contain retained austenite or a mixed structure thereof is uniformly present in the ferrite phase. It has been found that a wire having a dispersed composite structure has excellent workability. However, even in this wire, it was found that the ductility was deteriorated during wire drawing and the wire was broken in some cases. Such ductile deterioration depends on the shape of the tissue,
Although it does not occur in the case of needle-like tissue, it is a problem that occurs when a large number of massive low-temperature production phases are present.

The present invention has been made in order to solve these technical problems, and its purpose is to adjust the metal structure so that the needle-like structure is as large as possible, and to have excellent cold wire drawing workability. It is an object of the present invention to provide a method for obtaining a high-strength steel wire rod without deterioration of ductility during wire drawing.

[0008]

Means for Solving the Problems The present invention capable of achieving the above-mentioned object is that C: 0.03 to 0.3% by weight, Si: 2% by weight or less, Mn: 0.3 to 2.5% by weight. %, And the balance Fe and unavoidable impurities in the steel wire rods, after forming a structure mainly composed of martensite or bainite,
Alternatively, after the mixed structure of martensite or bainite and ferrite is heated to a ferrite-austenite region, a temperature range lower than (A _c1 −20 ° C.) is average speed: 10 to 100 ° C./sec, (A _c1 −20) (° C.) Or higher temperature range at an average rate of 0.1 to 2 ° C./sec, and then cooling from the ferrite / austenite range, whereby martensite or bainite which may partially contain retained austenite, or these The low-temperature transformation-generating phase consisting of the mixed structure of 15 to 40% is a volume fraction of 15 to 40%, the bulk low-temperature transformation-generating phase is 30% or less, and the rest is the acicular low-temperature transformation-generating phase. This is a method for producing a high-strength steel wire rod, which has a gist in that it has a mixed structure in which it is uniformly dispersed.

[0009]

The present inventors examined the manufacturing conditions from various angles in order to obtain a metal structure having excellent cold drawability and less deterioration of ductility during wire drawing. As a result, a steel wire rod having a predetermined chemical composition, after having a structure mainly composed of martensite or bainite, or after having a mixed structure of martensite or bainite and ferrite, when heating to a ferrite-austenite region, a predetermined speed By heating and then cooling, acicular martensite or bainite that may partially contain retained austenite, or a fine low-temperature transformation-forming phase consisting of a mixed structure of these, is dispersed uniformly in ferrite in an appropriate amount. The present invention was completed by finding that a desired wire rod can be obtained by finally adjusting the composite structure thus obtained.

The wire produced according to the present invention has a unique chemical structure which has a predetermined chemical composition and which has a low temperature transformation generation phase uniformly dispersed and distributed in ferrite at a predetermined volume fraction. Have. The average reduced particle size of the acicular particles in the low temperature transformation-generated phase is 3 μm or less,
Remarkably fine. Therefore, not only the ductility but also the strong workability is excellent, for example, 99% or more (preferably 9% or more).
It is possible to perform wire drawing at a work ratio of 9.8% or more), and the drawn wire obtained has not only high strength and high ductility but also excellent fatigue properties.

In the present invention, needle-shaped (elongat)
ed or acicular) means that the particles have directionality, and the term “globulal” means
It means that the particles do not have directionality. The reduced particle size of the acicular particles means the diameter of the cross section of the acicular particles at right angles to the circle. The reasons for limiting the range of chemical components in the wire rod targeted by the present invention are as follows.

C: 0.03 to 0.3 wt% Since C has the final metal structure defined in the present invention, C must be contained in an amount of 0.03 wt% or more. However, if the content of C exceeds 0.3% by weight, the ductility of the low temperature transformation-producing phase (hereinafter sometimes simply referred to as the second phase) composed of acicular martensite or acicular bainite or a mixed structure thereof Will deteriorate. Therefore, the content of C is specified to be 0.03 to 0.3% by weight.

Si: 2 wt% or less Si is effective as a strengthening element for the ferrite phase,
If the Si content exceeds 2% by weight, the transformation temperature is significantly shifted to the high temperature side, and decarburization of the surface of the steel material is likely to occur, so the upper limit was 2% by weight.

Mn: 0.3 to 2.5 wt% Mn is effective for strengthening the steel, enhancing the hardenability of the second phase, and making the morphology acicular, and achieves that effect. In order to achieve this, it is necessary to contain 0.5% by weight or more. However, even if the content exceeds 2.5% by weight, the effect is saturated, so the upper limit was 2.5% by weight.

The steel wire rod according to the present invention has the above-mentioned elements as basic components and the balance Fe and unavoidable impurities. S, P, N, etc. in the unavoidable impurities should be adjusted as follows. good. First, S is preferably 0.005 wt% or less from the viewpoint of reducing the amount of MnS in the steel, which improves the ductility of the steel. Further, since P is an element with significant grain boundary segregation, its content is 0.0
It is preferably 1% by weight or less. N is an element most easily aged when it exists in a solid solution state. Therefore, it ages during processing to impede workability, or ages after processing,
Since the ductility of the processed material is deteriorated, it is preferably 0.003% by weight or less. On the other hand, it is also preferable to adjust the shape of the MnS inclusions by adding a rare earth element such as Ca or Ce.

Next, the manufacturing method of the present invention will be described. The high-strength low-carbon steel wire rod excellent in strong workability has a structure of steel having the above-described chemical composition as a needle-like bainite or a needle-like martensite having a former austenite grain size of 35 μm or less, or a mixed structure thereof. After that, A _c1 ~
When heated to the A _c3 temperature range, the austenite fraction is about 20.
%, The austenitization is allowed to proceed, and then the average cooling rate is 40 to 150 ° C./sec.
It is manufactured by cooling to ℃. That is, in order to finally form the second phase in the metal structure into a fine needle-shaped structure, before the steel having the predetermined chemical composition is heated to the A _{c1 to} A _c3 temperature range, the structure is Part of the former austenite which may contain retained austenite is 35
The bainite or martensite having a size of μm or less (preferably 20 μm or less) or a mixed structure thereof (hereinafter, may be simply referred to as a front structure) is used. By refining the front structure in this way, it is possible to refine the final structure and improve the ductility and toughness of the steel, thus imparting the required strength to the steel.

In order to adjust the former austenite grain size to 35 μm or less, a temperature range in which recrystallization of austenite or grain growth is remarkably slow in hot working of a steel obtained by ingot casting or continuous casting, that is, 980 Area reduction of 30% in the temperature range below ℃ and above Ar ₃ point
It is necessary to perform hot working as described above. When the hot working temperature is higher than 980 ° C, austenite easily recrystallizes or grows grains, and when the work area reduction ratio is less than 30%, the austenite grain size cannot be reduced. . Furthermore, in order to obtain austenite fine particles of about 10 to 20 μm, in addition to the above processing conditions, the final processing pass must be 900 ° C. or lower. To obtain ultrafine particles of about 5 to 10 μm, The strain rate in processing needs to be 300 / sec or more.

After the hot working for adjusting the prior austenite grain size, cold working can be added to obtain a desired shape. In this case, the cold working rate is 40%.
It is good to set up to%. When cold working of more than 40% is applied to the preceding structure, recrystallization of martensite occurs at the time of heating to the A _{c1 to} A _c3 temperature range described later, and the desired final structure cannot be obtained.

Next, a method for making the pre-structure into acicular bainite or acicular martensite or a mixed structure thereof will be described. The first method is a method of obtaining the above-mentioned pre-structure during the rolling process, which is achieved by controlled rolling of steel or hot rolling followed by accelerated cooling. The cooling rate at the time of accelerated cooling is required to be 5 ° C./sec or more, and at a cooling rate lower than this,
It has a normal ferrite / pearlite structure. The second method for obtaining the pre-structure is a method in which the rolled steel is heat treated again, and the steel is heated to the austenite region of A _c3 point or more and then adjusted and cooled. The heating temperature in this method is preferably in the range of A _c3 to (A _c3 + 150 ° C.).

As described above, the structure before heating to the A _{c1 to} A _c3 region is replaced with the conventional ferrite / pearlite structure, and is composed of martensite or bainite which may contain residual austenite, or a mixed structure thereof. By heating the steel used as the low temperature transformation forming phase to the A _{c1 to} A _c3 region, a large number of initial austenite grains are generated with the retained austenite or cementite existing at the lath boundary of the low temperature transformation forming phase as the priority nucleus. It grows along the lath boundary.

Then, the martensite or bainite transformed from this austenite by cooling under a predetermined condition is made into a needle shape so as to have a good conformity with the surrounding ferrite phase, and thus the conventional ferrite / pearlite prestructure. The second phase particles can be remarkably miniaturized as compared with the above. Therefore, the conditions of heating and cooling in the A _{c1 to} A _c3 range are important. That is, depending on the conditions, the second
This is because the phases are agglomerated or the agglomerated particles are mixed in the second phase, which impairs the strong workability.

More specifically, the reverse transformation when heating a pre-structure consisting of fine bainite or martensite or a mixed structure thereof to an austenite region is
If the austenite fraction is up to about 20%, massive austenite is generated from the old austenite grain boundaries, and acicular austenite is generated from the inside of the grain, so from this state, for example, 150 to 200 ° C / sec or more. By rapidly cooling at a cooling rate of, the acicular and lumpy low-temperature transformation-producing phases become a structure in which ferrite is dispersed. Therefore, the finer the grains of old austenite, the higher the frequency of formation of massive austenite. About 4 more austenitic
When it progresses to 0% or more, the acicular austenite particles are united with each other and change into massive austenite, so when rapidly cooled from this state, a mixed structure of ferrite and a coarse massive low-temperature transformation-forming phase is formed. Further, if the austenitization progresses by about 90% or more, the massive austenites grow together and the austenitization is completed. Therefore, if the material is rapidly cooled from this state, the structure mainly composed of the low temperature transformation forming phase is formed.

Therefore, in the method of the present invention, when the steel adjusted to the preceding structure is heated to the A _{c1 to} A _c3 region, a structure mainly containing martensite or bainite or a mixed structure of martensite or bainite and ferrite is used in advance. Then, when heating to the ferrite / austenite range, the average speed is in the temperature range of ( _Ac1 −20 ° C.) or higher:
It is necessary to heat at 0.1 to 2 ° C / sec. The reason for this is
It is preferable that the low-temperature generation phase in the metal structure of the wire is mainly acicular, and it is preferable that the volume ratio of the acicular phase in the entire low-temperature generation phase is 80% or more. The growth rate is high, and the formation of acicular austenite and the growth of the massive austenite formed in the former austenite grain boundary are prioritized over the growth, and the structure is mainly composed of the massive austenite, and the ductility deterioration and disconnection during wire drawing are likely to occur. If it is shorter than that, the heat treatment time becomes too long, and industrially the productivity is lowered. Therefore, when heating to the ferrite-austenite range, the average speed in the temperature range of ( _Ac1-20 ° C) or higher is 0.1
Specified as ~ 2 ° C / sec. However, in the temperature range below (A _c1 −20 ° C.), heating at an average rate of about 10 to 100 ° C./sec is sufficient. If the heating rate is lower than that, the intragranular carbides, which are the starting points for the formation of acicular austenite, decrease, and as a result, the agglomerates increase. If it is higher than this, investment such as introduction of new equipment such as high-frequency heating is required, and the grain size becomes too fine, so that massive austenite increases. The austenite fraction is set to about 20% or more, and by cooling from this state to a temperature of room temperature to 500 ° C. at an average cooling rate of 40 to 150 ° C./sec, the austenite is converted from massive austenite in the transformation process during cooling. By separating ferrite and acicular austenite, and transforming this acicular austenite into a low temperature transformation forming phase, acicular martensite or acicular bainite which may partially contain retained austenite or a mixed structure thereof. Thus, the final microstructure in which the fine low-temperature transformation forming phase is uniformly dispersed in the ferrite phase is obtained.

When the average cooling rate is lower than 40 ° C./sec, polygonal ferrite is produced from the agglomerated austenite, and the remaining agglomerated austenite particles are transformed into the agglomerated second phase, while the cooling rate is 150 ° C. / If it is faster than a second, a massive second phase is produced as described above. Further, in the steel wire rod of the present invention, the volume fraction of the second phase in the ferrite phase needs to be in the range of 15 to 40%. When the volume fraction of the second phase is within this range, the second phase particles are needle-shaped, and the average reduced particle size thereof is 3 μm or less. Thus, the steel wire rod of the present invention has a unique composite structure that has never existed before. Therefore, it has excellent strong workability. When the volume fraction of the second phase is out of the above range, the massive second phase is likely to be mixed in the final structure even by cooling under the above conditions.

The cooling stop temperature must be room temperature to 500 ° C. This is to obtain bainite, martensite, or a mixed structure thereof as a low-temperature transformation-forming phase, and to slow down or stop the cooling rate within this temperature range to temper the second phase produced. This is because it can also be combined with.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and the following. It is included in the technical scope.

[0027]

EXAMPLE A steel having the chemical composition shown in Table 1 was used as 5.5.
After rolling to a wire having a diameter of mm, intermediate wire drawing was performed to a wire diameter of 2.5 mm. The wire rod was reheated and then rapidly cooled to make the previous structure a martensite structure. Subsequently, it was heated in the A _c1 -A _c3 region and then rapidly cooled to obtain a structure in which martensite was dispersed in the ferrite material. At this time, the heating rate was changed variously as shown in Table 2. The tensile strength, fracture drawing, volume fraction of the low-temperature transformation-forming phase, and bulk martensite fraction of the low-temperature transformation-forming phase of the wire obtained at that time are shown in Table 2 together. The agglomerated martensite refers to martensite particles having a short diameter of 3 μm or more, and the area ratio of the agglomerated martensite in all martensite is defined as the agglomerated martensite fraction.

[0028]

[Table 1]

[0029]

[Table 2]

FIGS. 1 (A) and 1 (B) show the metal structures of the wire rod of the present invention [No. 2: FIG. 1 (A)] and the wire rod of the comparative example [No. 5: FIG. 1 (B)]. It is a drawing substitute micrograph shown. Martensite particles appear white and the ferrite matrix appears dark. (A _c1 -20 ℃)
The difference in the shape of the martensite particles due to the difference in the heating rate is obvious, and most of the martensite of wire rod No. 2 is needle-shaped, but the mass of martensite of wire rod No. 5 is very large. I understand.

Next, the wire rod No. 1 of the present invention and the wire rod N of the comparative example
The drawing result of o.4 is shown in FIG. Wire rod No. 4 is wire drawing strain 4
The degree of ductility decreased from the degree, and the increase in strength reached the ceiling. However, it is understood that the wire rod No. 1 of the present invention is capable of wire drawing with a wire drawing strain of 6 or more, and further, a high strength steel ductile wire drawing material of 300 kgf / mm ² or more can be obtained.

Next, the wire rod No. 2 of the present invention and the wire rod N of the comparative example
Drawing results of o. 5 and No. 7 are shown in FIG. Wire No.
No. 5 shows a decrease in ductility from a wire drawing strain of about 4, and No. 7 shows a decrease in ductility from a wire drawing strain of about 5, and the increase in strength has reached a ceiling. However, it can be seen that the wire rod No. 2 of the present invention is capable of wire drawing with a wire drawing strain of 6 or more, and further, a high strength and ductile wire drawing material of 350 kgf / mm ² or more can be obtained.

Next, the wire rod No. 3 of the present invention and the wire rod of the comparative example
The wire drawing result of No. 6 is shown in FIG. Wire rod No. 6 is wire drawing strain 3
The degree of ductility decreased from the degree, and the increase in strength reached the ceiling. However, it can be seen that the wire rod No. 3 of the present invention is capable of wire drawing with a wire drawing strain of 6 or more, and that a high strength steel ductile wire drawing material of 400 kgf / mm ² or more is obtained.

[0034]

As described above, according to the present invention, the structure of the low carbon steel having a predetermined chemical composition is previously made into bainite, martensite or a fine mixed structure thereof, and then ferrite / ferrite at a predetermined heating rate is used. By heating to the austenite region and then cooling, a structure in which the needle-like low-temperature generation phase, which is excellent in strong cold drawing, is finely dispersed in the ferrite phase, and deterioration of ductility and disconnection during strong cold drawing It is possible to obtain an ultrafine steel wire with high strength and high ductility without causing

[Brief description of drawings]

1 is a drawing-substituting micrograph showing the metal structures of a wire rod No. 2 of the present invention [FIG. 1 (A)] and a wire rod No. 5 of a comparative example [FIG. 1 (B)].

FIG. 2 is a graph showing the wire drawing results of wire rod No. 1 of the present invention and wire rod No. 4 of a comparative example.

FIG. 3 is a graph showing the wire drawing results of wire rod No. 2 of the present invention and wire rods No. 5 and No. 7 of comparative examples.

FIG. 4 is a graph showing the wire drawing results of wire rod No. 3 of the present invention and wire rod No. 6 of a comparative example.

Claims (1)

[Claims] 1. C: 0.03 to 0.3% by weight, Si: 2
% Or less, Mn: 0.3 to 2.5% by weight, respectively, and the steel wire rods composed of the balance Fe and unavoidable impurities after being made into a structure mainly composed of martensite or bainite, or with martensite or bainite When a ferrite / austenite region is heated after forming a mixed structure of ferrite, the temperature range below (A _c1 −20 ° C.) is an average speed: 10 to 100 ° C./sec, and the temperature range above (A _c1 −20 ° C.). Is heated at an average rate of 0.1 to 2 ° C./sec and then cooled from the ferrite-austenite region, so that a low temperature of martensite or bainite which may partially contain retained austenite or a mixed structure thereof is obtained. The transformation generation phase has a volume fraction of 15 to 40%, and the bulk low temperature transformation generation phase is 30% or less, and the rest is acicular low temperature transformation. A production phase, the method of producing a high strength steel wire rod having excellent cold drawing property, characterized in that they are a mixed structure composed are uniformly dispersed in the ferrite phase.