JPS6156264A - High strength and high ductility ultrathin steel wire - Google Patents

Abstract

PURPOSE:To manufacture the titled wire, by cold drawing at a specified working ratio, a wire material having a specified chemical compsn. and complex structure in which phases formed by low temp. transformation are distributed uniformly in ferrite by a specified volume fraction. CONSTITUTION:Steel billet composed of, by wt%, 0.02-0.15 C, 0.01-1.2 Si, 0.1- 2.5 Mn and the balance Fe with inevitable impurities is subjected to necessary hot working, heat treatment, to obtain the structure composed of bainite, martensite of <=about 35mu precedent austenite grain diameter, or mixed fibers thereof. Said material is heated to Ac1-Ac3 to regulate austenitized fraction to >=about 20%. Next, the rolled steel material is cooled to normal temp.-about 500 deg.C at 40-150 deg.C/sec average rate to obtain wire material in which phases formed by low temp. transformation composed of needle martensite, bainite or mixed structure thereof are dispersed uniformly by 15-40% volume fraction against ferrite phase. The wire material is cold drawn by >=90% total reduction in area, to obtain ultrathin steel wire having about 170kgf/mm. <2>strength, >=about 40% reduction of area after rupture and <=2mm. diameter.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-strength, high-ductility ultrafine steel wire and a method for manufacturing the same.

In the present invention, the ultra-fine steel wire refers to a steel wire that has been cold-drawn to a wire diameter of approximately 2 fins or less, preferably 1.5 fins or less, such as rope wire, bead wire, spring steel, port wire, tire cord. , used for inner wires, etc. These ultra-fine copper wire numbers: 1. Conventionally, they are usually manufactured from 5.5 fin wire by wire drawing, but in this case,
The total area reduction C11 is about 90% or more, and the drawing limit W of the conventional conventional 0.6 to 0°8% medium-high carbon patented wire is iL or 4. Since it exceeds r, it is necessary to perform the patenting process once or twice during wire drawing.

Others JJ, pure iron and I1 (according to carbon ferrite/pearlite steel, it is possible to draw the ultra-fine steel wire by adding strength to it, but the increase in strength due to the wire drawing process is small), so the final The strength of the ultra-fine steel wire as a product is low, i.e. 9
The strength is 70 to 130 k even with 5 to 99% strength wire drawing.
gf/mm2, and it is impossible to achieve a strength greater than μ0 kgf/mm2. In addition, even when wire drawing is performed at a processing rate of 99% or more, the strength is 90 kgf/mm.
2 or less. In other words, depending on strong wire drawing of pure iron or low carbon ferrite/pearlite steel, the strength is 240 kgf/m.
It is not possible to obtain an ultra-fine steel wire with a diameter of 30% or more and a diameter of 30% or more.

The present inventors have conducted intensive research to obtain a wire rod that provides ultra-fine steel wire with high strength and high ductility by cold wire drawing with a reduction rate of 90% or higher. By making the structure of a low carbon steel wire rod for wire drawing into a bainite, martensite, or a fine mixed structure thereof which may contain residual austenite in advance, and then transforming the reversely transformed austenite from this under predetermined cooling conditions. , the final structure is a composite structure in which a fine low-temperature transformation product phase consisting of acicular bainite, martensite, or a mixed structure thereof, which may contain some retained austenite, is uniformly dispersed in a ferrite phase. As a result, this low carbon steel wire rod has excellent strong workability of 90% or more as described above, and therefore, according to this wire rod, it is difficult to heat the wire to an AC temperature or higher than -12 during processing. It is possible to perform cold wire drawing processing to 90% or more without any problem, strength μ0kgf/mm or more, rupture area of 4()% or more, in preferable cases. Strength 24
The present invention was achieved by discovering that it is possible to produce a high-strength, high-ductility ultra-fine steel wire with a diameter of 0 kgf/mm" or more and a reduction of area at break of 30% or more.

The high-strength, high-ductility ultrafine steel wire according to the present invention has a weight percentage of C 0.02 to 0.15%, Si 0.01 to 1.2%, Mn 0.1 to 2.5%, and the balance iron and unavoidable impurities. A steel wire having a metal structure in which a low-temperature transformation phase consisting of acicular martensite, paynoite, or a mixed structure thereof is uniformly dispersed in the ferrite phase at a volume fraction of 15 to 40% relative to the ferrite phase. is cold drawn with a total area reduction of 90% or more. In the present invention, as described above, the ultra-fine steel wire refers to a steel wire having a diameter of 2 mm or less, preferably 1.5 nm or less.

In other words, the ultra-fine steel wire according to the present invention has a predetermined chemical composition, and has a unique complex structure never seen before in which a low-temperature transformation product phase is uniformly distributed in ferrite at a predetermined volume fraction. It is obtained by cold drawing a wire rod with excellent workability at a processing rate of 90% or more, and has high strength and high ductility.

The reasons for limiting the chemical components in the wire used in the present invention will be explained below.

In addition, in the present invention, acicular (elongated or a)
"cicular" means that the particles have directionality, and "globular" means that the particles do not have directionality. Also, the equivalent particle diameter of needle-like particles refers to the area of the needle-like particles as a circle. means the diameter when converted to

It is necessary to add C in an amount of 0.01% or more in order for the wire rod before cold drawing used in the present invention to have the final metal structure specified by the present invention, but when it exceeds 0.30%. In this case, the ductility of the low-temperature transformation-generated phase (hereinafter sometimes simply referred to as the second phase) consisting of acicular martensite, bainite, or a mixed structure thereof begins to deteriorate. Therefore, C content It: o, 0'1~. 0.30%
shall be.

Si is effective as a reinforcing element for the ferrite phase, but 1
．． If it exceeds 5%, the transformation temperature will shift significantly to the high temperature side, and the surface of the wire will be more likely to decarburize, so 1.5%
Let be −L limit.

In order to strengthen the cotton material, increase the hardenability of the second phase, and make the shape T1-like, Mn is added at 0°3% or more.
Although it is necessary to add Mn-containing
is set to 0.3 to 2.5%.

In the present invention, in order to refine the metal structure of the wire, at least one element selected from Nb, ■ and Ti can be further added. In order to reduce the i-degradation of this structure, it is necessary to add 0.005% or more of each element. Since this is disadvantageous, the upper limit is set to 0.2% for Nb and 0.3% for each of ■ and Ti.

Furthermore, elements that are inevitably included or may be included in line) A in the present invention will be explained.

S is 0.005 in order to reduce the amount of MnS in the wire.
% or less, thereby improving the ductility of the wire.

Since P is an element with significant grain boundary segregation, its content should be reduced to 0.
．． It is preferable to set it to 0.01% or less.

When N exists in a solid solution state, it is the element that is most easily aged. Therefore, the content is preferably 0.003% or less because it ages during processing and impedes workability, or it ages after processing and deteriorates the ductility of the ultra-fine steel wire obtained by wire drawing.

Aρ forms oxide-based inclusions, and since these oxide-based inclusions are difficult to deform, they may impede the workability of wire) A. Breakage is likely to occur. Therefore, the Al content is usually 0.01
% or less.

On the other hand, it is also preferable to adjust the shape of the MnS inclusions by adding rare earth elements such as Ca and Ce.

Further, solid solution C and N can also be fixed by adding Al1, etc., including the above-mentioned Nb, (2), and Ti. Furthermore, depending on the use of the ultra-fine steel wire according to the present invention, the wire rod for cold drawing may contain 1.0% or less of Cr, CI3 and/or Mo, 6% or less of Ni, and AN and/or P. It is also possible to appropriately add 0.1% or less of each, and 0.02% or less of B.

Next, the production of the ultra-fine steel wire according to the present invention from the above-mentioned wire rod will be explained.

In order to produce the high-strength, high-ductility ultrafine steel wire according to the present invention, first, a steel billet having the above-mentioned chemical composition is subjected to the necessary hot working and heat treatment to change its structure to a prior austenite grain size. After forming a bainite, martensite, or a mixed structure of these with a particle size of 35μ or less, this is converted into Ac+~A(
When heated to 3 temperature ranges, the austenitization fraction is approximately 20%.
The austenitization is progressed as described above, and then the rolled steel material thus obtained is cooled at an average cooling rate of 40 to 1.
By cooling at 50° C./sec from room temperature to 500° C., a wire rod for cold wire drawing with strong workability used in the present invention is manufactured. Next, this wire has a total area reduction rate of 90%.
After cold drawing, the diameter is 21 mm depending on the purpose and use.
Obtain the following ultra-fine steel wire.

According to the present invention, during the cold wire drawing described in -, the total area reduction rate is 90% without preheating the wire above the Ac temperature.
By cold drawing as follows, the strength μ0kgf/m
It is possible to obtain ultra-fine @ wires with a strength of 240 kgf/mm2 and a reduction in area of 30% or more by cold drawing with a total area reduction of 99% or more. You can get the line.

That is, first, the metal 3 in the wire used in the present invention
．． ■In order to make the second phase in the weave a fine needle-like structure,
Before the steel billet having the predetermined N1 composition is heated to a temperature range of ACl to AC3, it is subjected to a treatment including hot working under predetermined conditions to change its structure to a structure containing some retained austenite. Prior austenite grain size may be 35
μ or less, preferably 20 tr, heinite under Iu, martensite, or a fine mixture of these x12 textures (hereinafter, these may be simply referred to as pre-textures). By refining the previous structure in this manner, the final structure is refined by 31 mm, improving the ductility and toughness of the steel, thus imparting the required strength to the steel.

In order to adjust the prior austenite grain size to 35μ or less, when hot working the steel slab obtained by ingot formation or continuous casting, the temperature range where austenite recrystallization and grain growth progress is extremely slow, that is, 980° C or below, eyes, two,
It is necessary to perform hot working at an area reduction rate of 30% II in a temperature range of "Ar 3 or higher". Hot treatment quality is 9
This is because when the temperature exceeds 80° C., austenite tends to recrystallize and grow grains, and when the working area reduction rate is less than 30%, the austenite grain size cannot be reduced. Furthermore, in order to obtain austenite fine l:i of about 10 to 20μ, in addition to the above processing conditions, it is necessary to set the final processing bath to 900°C or less, and 5
In order to obtain ultrafine grains of about 10 to 10 mm, it is necessary to perform the final processing at a strain rate of 300/sec or more.

Note that after the hot working described above for adjusting the prior austenite grain size, a desired shape can be obtained by adding cold working, but in this case, the processing rate of cold working is up to 40%.

-F When cold working greater than 40% is applied to the above structure, recrystallization of martensite occurs during heating to the Ac+ to Ac3 temperature range, which will be described later, and the desired final 3. ■ texture can be obtained. I can't.

Next, the following method can be used to make the previous structure a bainite, martensite, or a mixed structure thereof.

The first method is to obtain the required pre-structure during the rolling process, by subjecting the billet to controlled rolling or hot rolling followed by accelerated cooling. The cooling rate needs to be 5° C./second or more. This is because at a cooling rate lower than this, a normal ferrite I...pearlite structure is formed.

The second method for obtaining the front &; Fl weave is to heat-treat the rolled steel anew, in which the steel is heated to an austenite region of Ac3 or higher and then adjusted and cooled. In the case of this method as well, the heating temperature is preferably in the range of Aca to Ac, +150° C., as described in the first method.

In this way, before heating to Ac+~Ac, 3 range! 11■
A rolled steel material with a low-temperature transformation phase consisting of martensite, bainite, or a mixed structure thereof, which may contain residual austenite, instead of the conventional ferrite/pearlite structure, is heated to the ACl to AC3 range. By doing so, a large number of initial austenite grains are generated using the retained austenite or cementite existing at the lath boundary of the low-temperature transformation generation phase as a core, and grow along the lath boundary in -I-.

The martensite or bainite that transforms from this austenite by cooling under specified conditions then becomes acicular and has good consistency with the surrounding ferrite phase, thus making it more acicular than the conventional ferrite-pearlite structure. As a result, the second phase particles are significantly reduced in size by 1iTh. Therefore, heating and cooling to the A c 1-A C3 region ar+
The following conditions are important. That is, depending on the conditions, the second phase may become lumpy, or lumpy particles may be mixed in the second phase, impairing strong workability.

To explain in more detail, the reverse transformation occurs when a single weave is heated to an austenite region before forming a fine bainite, martensite, or mixed structure of these. Until the austenite fraction is about 20%, the former austenite grain boundary It starts with the formation of massive austenite from the inside of the grains, and the formation of acicular austenite from within the grains. From this state, for example, from 150 to Obtain 11 minutes of tissue. Therefore, the finer the prior austenite, the higher the frequency of formation of massive austenite. When the austenitization further progresses by about 40% or more, the acicular austenite particles coalesce with each other and change to massive austenite. When rapidly cooled from this state, a mixture of ferrite and a magnificent massive low-temperature transformation phase forms H1
Form a weave. Furthermore, if the austenitization progresses by about 20% or more, the massive austenite will coalesce and grow, and the austenitization will be completed, so if the austenitization is rapidly cooled from this state, the structure will be mainly composed of the low-temperature transformation product phase.

Therefore, in the present invention, when the steel adjusted to the previous structure is heated to the Ac+ to AC3 range, the austenitization is made so that the austenitization fraction is about 20% or more, and from this state, the average cooling rate is 40-b. By fractionating ferrite and acicular austenite from massive austenite during the transformation process during cooling, and transforming this acicular austenite into a low-temperature transformation phase, even if some residual austenite is contained. A final metal structure is obtained in which a fine phase formed by low-temperature transformation consisting of good acicular bainite, martensite, or a mixed structure thereof is uniformly distributed in the ferrite phase.

The average cooling rate is limited as described above. cooling rate is 4
When the cooling rate is slower than 0"C/sec, polygonal ferrite is formed from the massive austenite and the remaining massive austenite particles are transformed into the massive second phase, while when the cooling rate is faster than 150°C/sec, This is because a lumpy second phase is generated as described above.In addition, in the present invention, the volume fraction of the second phase in the ferrite phase is 15 to 4.
The range is 0%. When the volume fraction of the second phase is within this range, the second phase particles are acicular and have an average equivalent particle diameter of 3μ or less, and the resulting wire has a unique composite structure that has never existed before. Because of this, it has excellent strong workability. Further, when the volume fraction of the second phase is outside the above range, the lumpy second phase is likely to be mixed into the final structure even by cooling under the above conditions.

The cooling stop temperature is from room temperature to 500°C. This is to obtain bainite, martensite, or a mixed structure of these as a phase formed by low-temperature transformation, and also to temper the generated second phase by slowing down or stopping the cooling rate within this temperature range. This is because it can also be done.

As described below, the wire rod used in the present invention is made by making the structure of low carbon steel into bainite, martensite, or a fine mixture thereof, and then transforming the massive austenite that is reversely transformed from this under predetermined cooling conditions. , has an unprecedentedly unique fine composite structure in which an acicular low-temperature transformation phase is uniformly dispersed in a ferrite phase at a volume fraction of 15 to 40%. Ultra-fine wire with high strength and high ductility due to cold drawing with a processing rate of over 90% without heating! 1iil cotton can be obtained, for example, with a total area reduction rate of 90 to 99%, an ultra-fine steel wire with a strength μ0 kgf#a+a'' and a breakage area of 40% or more, and a total area reduction rate of 99% or more, Strength 24
0 kgf/n+n+", and an ultra-fine steel wire with a breaking area of 30% or more can be obtained.

Examples As shown in Table 1, steels A and B having the chemical composition specified in the present invention are water-cooled after rolling to have a fine martensitic structure as A1 and B1, respectively, and comparative steels are Steel A is air-cooled after rolling and the front &F1m has a ferrite/pearlite structure, which is referred to as A2.

The prior austenite grain size is 20μ or less in all cases.

Next, the above A1 and B1 were heated and held in the AC3 to AC3 region for 3 minutes so that they had different austenite fractions Z-1
Cooling was performed to room temperature at various average cooling rates. FIG. 1 shows the morphology of second phase particles and their volume fractions with respect to heating temperature and cooling rate. The solid line has a uniform mixed structure of ferrite and an acicular second phase, and the broken line has a mixed structure of ferrite and a lumpy second phase, or a mixture of ferrite 1 and an acicular or lumpy second phase.
l fM is shown.

According to the invention, an average cooling rate of 125°C/sec or 80°C
No. 241-I JE41 of steel 4A when cooled at /sec.
The second phase is needle-shaped, and the structure is formed by uniformly dispersing this second phase in the ferrite phase.
The volume fraction of the phase remains constant regardless of the heating temperature. On the other hand, even if the weave is the same, when the average cooling rate is 0°C/sec or more, the second phase form becomes lumpy or a mixture of lumps and needles; The phase fraction increases as the heating temperature increases.

Figure 2 shows the relationship between the second phase volume fraction contained in the final structure and the average converted particle diameter of the second phase particles.
1 and B1, and ferrite/pearlite front &111
1iiiA2 and B2 are shown respectively. Here, the average converted particle diameter means the average diameter when the area is converted into a circle as described above for any form.

For any steel material, the particle size of the second phase particles increases as the second phase volume fraction increases, but when the second phase fraction is the same, pre-martensitic N, The particle size of the obtained particles is significantly smaller than that of the particles obtained from 11 m before ferrite/pearlite. That is,
Even for steel pieces with the same K11I structure, by adjusting the previous structure from ferrite/pearlite to martensitic I,N weave, the second phase (11") can be significantly refined. Due to two-phase grain (0)jjIQ fine 1hi,
Although the ductility of the steel material has been significantly improved, it does not necessarily mean that it will have strong workability. That is, ;1 invention 1.71ir- By setting the volume fraction of the second phase in the range of 15 to 40%, the second phase mainly has a L;l:tl shape,
■First, the second phase consists of fine acicular particles with an average equivalent particle diameter of 3μ or less, and furthermore, because these fine acicular second phases are uniformly distributed in the ferrite, it has strong workability. It is excellent. Of course, the above also applies when the second phase is acicular bainite or a mixed structure of acicular bainite and martensite.

Next, Table 2 shows the heating and cooling conditions, final N1 weave i1i, and mechanical properties of the invention steel A1 and comparative steel A2. A where the former NIIH & is fine martensite
1 to A so that the austenitization fraction is 20% or more
Steel No. 3, 4, 5 and 6 obtained by heating to C, -A, 3 region and cooling at 125° (7 seconds) have ris & fine 31-shaped martensite ( It is clear that the strength and ductility balance is significantly skewed when the second phase) is uniformly mixed and dispersed within the volume fraction range of 15 to 40%. be.

On the other hand, comparative steel A2, whose previous structure is ferrite/pearlite, has a secondary structure regardless of the heating and cooling conditions.
Steel No. 10, 11 or 12 whose phase morphology is blocky
Well, all of these have a poor balance of strength and ductility. On the other hand, the previous f: [I it is martensite,
Steel No. 1 has a too slow cooling rate after heating to the AC1 to AC3 range, and Steel No. 2 has an austenitization fraction of 16% when heated to the A, 1 to Ac3 range.
In both cases, the structure is a fine mixed N1 weave of ferrite and massive and acicular martensite, and the above steel number 10
Although the balance between strength and ductility is worse than that of steel materials 12 to 12, it is clear that they are inferior to the steel materials according to the present invention.

In addition, steel numbers 7 to 9 all have a mixed structure of ferrite and massive martensite, and are inferior in strength and 1[1)1 balance.

Next, a 6.4 m diameter wire rod having a second phase morphology of 5'I: was subjected to severe working by cold wire drawing. 1 after this processing
-1 quality is shown in Table 3. According to the wire rod of the present invention of Steel No. 1, it is possible to obtain an ultra-fine steel wire with a tensile strength of 90 kgf/mm2 and a diameter of 2 at breakage of 58% at a processing degree of 90%,
Ultra-fine steel IJ+ with a diameter of 0°7-mm with even higher steel strength due to the processing rate of 99%! can be obtained. On the other hand, according to the comparison steel wire of steel number 2 having a lumpy second phase, the ductility deteriorated rapidly as the working degree increased, and wire breakage occurred at about 90% working degree. Comparison of Steel No. 3 Although the steel wire rod has a finer structure than Steel No. 2 and has better strong workability than Steel No. 2, the properties after processing deteriorate significantly compared to Steel No. 1. .

FIG. 3 shows changes in properties when the wire rod of the present invention, steel number 4 in Table 2, was heat treated at a temperature of 300° C. for a predetermined period of time. Strength·
The change in ductility is relatively small, especially the yield ratio is 3 at 300°C.
It shows a low value even after holding for 0 minutes. This is also related to the fact that the wire rod of the present invention has low solid solution C and N in the as-cooled state. On the other hand, if a similar heat treatment is applied after 11 working hours, the yield ratio will be significantly higher, and a combination of processing and low-temperature heat treatment can be used depending on the purpose.

Next, as shown in Table 1, steels B and C having the chemical composition specified by the present invention were prepared into uniform 111 (5,5 mm wires having a fine composite LTL weave) of ferrite and acicular martensite according to the present invention. The diameter wire + A.These are respectively B1 and C1.The mechanical properties of B1 and C1 and the drawing of the ultra-fine steel wire with a wire diameter of 1.0 or less (mechanical properties of A) ('I quality is shown in Table 4.

Both B1 and C1 have high ductility and can be subjected to strong processing of 99.9%, and the ultra-fine steel wire made in this way also has high strength and high ductility. In addition, steel 01 was processed at a processing rate of 97
% to make a wire rod (wire diameter 0.95 mm), and this
The mechanical properties after low/grade annealing at temperatures of 00 to 400°C are also shown in Table 4. It is clear that the ductility of the wire is improved by low-temperature annealing. No decrease in strength was observed. Therefore, the ductility of the ultra-fine steel wire according to the present invention can be improved by low-temperature annealing heat treatment, and the ductility of the final drawn wire material can be further increased by combining low-temperature annealing with the mid-drawing process of the wire according to the present invention. can. Furthermore, low-temperature annealing can also be applied as a diffusion heat treatment of the plating layer performed during the wire drawing process or after the final wire drawing.

[Brief explanation of drawings]

Figure 1 shows steels with compositions specified in the present invention, AC1 to A
A graph showing the relationship between the morphology of the low-temperature transformation product phase and its volume fraction in the ferrite phase with respect to the heating temperature and average cooling rate when heating to Ca region and cooling.
A graph showing the relationship between the volume fraction of the phase, the morphology of the second phase, and the average equivalent particle diameter of tζi. 2 is a graph showing changes in physical properties of .

Claims (4)

[Claims] (1) Consisting of 0.02 to 0.15% of C, 0.01 to 1.2% of Si, 0.1 to 2.5% of Mn by weight, the balance being iron and unavoidable impurities, including acicular martensite, bainite, or these A wire having a metal structure in which a low-temperature metamorphic phase consisting of a mixed structure is uniformly dispersed in the ferrite phase at a volume fraction of 15 to 40% with respect to the ferrite phase has a total area reduction rate of 90% or more. A high-strength, high-ductility ultra-fine steel wire with a diameter of 2 mm or less, which is produced by cold drawing. (2) The high-strength, high-ductility ultra-fine steel wire according to claim 1, wherein the particles of the low-temperature transformation-generated phase have an average equivalent particle diameter of 3 μ or less. (3) In weight% (a) C0.02-0.15%, Si0.01-1.2%, Mn0.1-2.5%, and (b) Nb0.005-0.20%, V0 At least one element selected from the group consisting of 0.005% to 0.30% Ti, and 0.005% to 0.30% Ti, the balance consisting of iron and inevitable impurities, consisting of acicular martensite, bainite, or a mixed structure thereof A wire rod having a metal structure in which a low-temperature metamorphic phase is uniformly dispersed in a ferrite phase at a volume fraction of 15 to 40% of the ferrite phase is cold drawn with a total area reduction of 90% or more. A high-strength, high-ductility ultra-fine steel wire with a diameter of 2 mm or less. (4) The high-strength, high-ductility ultrafine steel wire according to claim 3, wherein the particles of the low-temperature transformation phase have an average particle size of 3 μm or less.