JP2014098177A - Steel wire and method of manufacturing steel wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel wire capable of being welded satisfactorily and having high strength after a weldment, and to provide a method of manufacturing the steel wire capable of manufacturing such steel wire at good productivity.SOLUTION: A steel wire contains, by mass%, C of 0.10% to 0.30%, Si of 0.20% to 2.00%, Mn of 0.30% to 2.50%, Cr of 0.20% to 2.00%, Mo of 0.01% to 0.30%, and the balance Fe with inevitable impurities and has a bainite structure of 95 vol.% or more. As the content of C is low, no hardening structure is substantially generated during a weldment and the weldment is easy to be conducted. As carbide generating element is contained, it can be highly reinforced by precipitation strengthening. It has high strength because it is substantially constituted by the bainite structure, and the structure substantially does not change before and after the weldment and the high strength can be maintained after the weldment.

Description

  The present invention relates to a steel wire and a manufacturing method thereof. In particular, the present invention relates to a steel wire that is excellent in strength and can be welded well, and a method of manufacturing a steel wire that can manufacture such a steel wire with high productivity.

  Steel wires are used in various fields. Among them, high carbon steel wires with high carbon content such as piano wires and hard steel wires represented by steel grades SWP-A and SWP-B, and oil tempered wires that have been quenched and tempered have strength and toughness. It has excellent fatigue resistance and is used in parts such as automobiles and household electrical appliances.

  It may be used by welding steel wires or welding a steel wire and another member. In this case, the steel wire is annealed by heat during welding, and carbide spheroidization occurs, and the strength in the vicinity of the welded portion may decrease. Patent Document 1 discloses a high-strength steel wire in which a welded portion also has high tensile strength by performing quenching and tempering after welding the hot-rolled materials.

Japanese Patent No. 2954771

  There is a demand for the development of a steel wire that can be welded satisfactorily, maintains high strength after welding, and has excellent productivity.

  The above-mentioned high carbon steel wire and oil tempered wire are high in strength but have high hardenability due to the high carbon content.When welding is performed, the hardened structure (martensite) is heated and cooled during welding. Phase) is produced, toughness is reduced, and cracking may occur. So-called burning cracks occur and cannot be used. Even if it can be welded, a heat treatment such as annealing is necessary to improve the toughness of the quenched structure, resulting in a decrease in productivity. Moreover, even if it can weld, the intensity | strength of a welding part vicinity may fall rather than places other than a welding part with the heat at the time of welding as mentioned above.

  If the carbon content is reduced, the formation of the above-described hardened structure can be suppressed. However, the strength of the steel wire itself is lowered. In addition, when a steel wire with reduced carbon is simply welded, the strength in the vicinity of the welded portion is further lowered and cannot be used.

  In order to improve the strength when the carbon content is reduced, for example, it is conceivable to add additive elements such as Al, Ti, and V. However, an increase in additive elements leads to a decrease in workability of the wire. For this reason, there is no problem with a hot rolled material with a large diameter, but when a hot wire is subjected to wire drawing, for example, to produce a thin wire, wire breakage is likely to occur and the productivity of the thin wire is inferior. Or a thin wire rod cannot be manufactured substantially. Further, an increase in additive elements causes an increase in material cost.

  If quenching and tempering is performed after welding as described in Patent Document 1, the strength of the welded portion can be increased. However, quenching and tempering generally results in an increase in manufacturing cost and, in turn, a decrease in productivity.

  Therefore, one of the objects of the present invention is to provide a steel wire that can be welded well and has excellent strength after welding. Another object of the present invention is to provide a method of manufacturing a steel wire that can be welded satisfactorily and can manufacture a steel wire that is excellent in strength even after welding with high productivity.

  The inventors of the present invention studied to produce a high-strength steel wire in which a decrease in toughness due to quenching during welding does not substantially occur and a decrease in strength after welding is small. As a result, the inventors have found that it is preferable to reduce the carbon content, to contain a specific carbide-forming element in a specific range, and to make the parent phase a bainite structure. Moreover, the knowledge that the steel wire comprised from this specific composition and a specific structure can be manufactured by performing the specific heat processing for forming a specific structure was acquired. In particular, it has been found that a steel wire with higher strength can be produced by performing wire drawing after the specific heat treatment. The present invention is based on the above findings.

  The steel wire of the present invention has a mass% of C of 0.10% to 0.30%, Si of 0.20% to 2.00%, Mn of 0.30% to 2.50%, Cr of 0.20% to 2.00%, and Mo of 0.01%. % And 0.30% or less, the balance is composed of Fe and inevitable impurities, and the bainite structure is 95% by volume or more.

  Since the steel wire of the present invention has a low C (carbon) content of 0.30% by mass or less, a hardened structure (martensite phase) is not substantially generated by heat during welding. Therefore, the steel wire of the present invention can suppress a decrease in toughness due to heat during welding, and can be used well after welding. That is, the steel wire of the present invention is excellent in weldability. In addition, the steel wire of the present invention contains a solid solution strengthening element such as Si and Mn, and contains a carbide forming element called Cr in a specific range, thereby achieving high strength by solid solution strengthening and precipitation strengthening. be able to. In particular, by containing Cr in a specific range, the effect of increasing the strength by precipitation strengthening is great. Furthermore, the steel wire of this invention can acquire stably the improvement effect of the intensity | strength by the production | generation of a carbide | carbonized_material by containing Mo in the specific range.

  In addition, the steel wire of the present invention has substantially the same strength and fatigue limit as a steel wire composed of a pearlite structure by being composed substantially of a bainite structure. Further, the steel wire of the present invention substantially composed of a bainite structure can maintain the bainite structure without being transformed by heating during welding (the recrystallized structure is also a bainite structure). As described above, the steel wire of the present invention has high strength even before welding, and also has high strength after welding because the structure does not substantially change before and after welding.

  The steel wire of the present invention is excellent in strength even if it does not contain many various additive elements, and can maintain high strength even after welding. Therefore, by using the steel wire of the present invention, it is possible to construct a member having a welded portion and a welded portion having a high strength in the vicinity thereof. In addition, by using the steel wire of the present invention, it is not necessary to separately perform heat treatment for increasing the strength after welding. Therefore, it is expected that the steel wire of the present invention can be suitably used as a material for an arbitrary member having a welded portion.

  Further, as a result of investigations by the present inventors, it was found that the time required for transformation of the bainite phase was very short as compared with the case where transformation to the pearlite phase was achieved with the above-mentioned specific composition. Therefore, the steel wire of the present invention is excellent in strength and fatigue limit, but may have a short manufacturing time (heat treatment time) for obtaining a specific structure and is excellent in productivity.

When manufacturing the steel wire of the present invention that has been subjected to wire drawing in particular, wire drawing may be performed after performing the above-described specific heat treatment. As a concrete manufacturing method, the manufacturing method of the following steel wire of this invention is mentioned, for example. The method for manufacturing a steel wire of the present invention includes the following preparation process, heat treatment process, and wire drawing process.
Preparatory process In mass%, C is 0.10% to 0.30%, Si is 0.20% to 2.00%, Mn is 0.30% to 2.50%, Cr is 0.20% to 2.00%, Mo is 0.01% to 0.30%. The process of preparing the hot-rolled material which contains and the remainder is comprised from Fe and the inevitable impurity.
Heat treatment step A step of obtaining a heat treatment material having a bainite structure of 95% by volume or more by subjecting the hot-rolled material to a heat treatment.
Wire drawing step A step of drawing the heat treated material to obtain a wire drawing material.
The above heat treatment is performed with austenite at a temperature of 850 ° C. to 1100 ° C., a holding time of 10 seconds to 10 minutes, and then a constant temperature transformation temperature of 400 ° C. to 500 ° C., and a holding time of 10 seconds to 10 minutes. As a bainite structure is formed.

  By performing the above heat treatment step, the steel wire of the present invention has the above-mentioned specific composition and specific structure, can perform welding well, and has excellent strength after welding without adding an excessive additive element. Can be manufactured. Further, this heat treatment has a relatively short holding time and a short manufacturing time. Therefore, the manufacturing method including the heat treatment step can manufacture the steel wire of the present invention with high productivity. In particular, by using the steel wire manufacturing method of the present invention including a wire drawing step after the heat treatment step, it is possible to manufacture the steel wire of the present invention which is excellent in strength and can be welded satisfactorily. The reason for this is that when plastic processing such as wire drawing is performed after transformation into the bainite phase, the degree of strength improvement by work hardening is large and the strength can be further increased. And the bainite phase is maintained even after wire drawing. Therefore, the method for producing a steel wire of the present invention can perform welding well and can produce a thin steel wire having a higher strength and a deformed steel wire with high productivity.

  As one form of the steel wire of the present invention, a form in which the ratio of the long diameter to the short diameter of the steel wire is a deformed wire of 1.5 or more and 10 or less is mentioned.

  The present inventors have obtained the knowledge that the effect of improving the strength by work hardening is greater when processing that imparts processing strain in the shear direction is performed. The deformed line is formed by performing processing that imparts processing strain in the shear direction. Therefore, the above form is more excellent in strength.

As one form of the steel wire of this invention, the form whose cross-sectional area of the said steel wire is 30 mm < ² > or less is mentioned.

  The above form is typically a relatively thin steel wire having a wire diameter of 6.0 mmφ or less. Such a thin steel wire is obtained by, for example, drawing, and an effect of improving the strength by work hardening of the drawing can be expected. From this, the said form is excellent in intensity | strength by the strength improvement effect by work hardening.

  As one form of the steel wire of this invention, when welding the said steel wires, the form whose tensile strength of the welded wire is 1300 MPa or more is mentioned.

  As described above, the steel wire of the present invention is excellent in strength even after welding, and can have a high strength of 1300 MPa or more as in the above embodiment. Therefore, a member having a welded portion and a tensile strength of 1300 MPa or more can be constructed by using the steel wire of the above form.

  As one form of the manufacturing method of the steel wire of the present invention, the wire drawing is performed with a total wire drawing degree of 50% or more, and the wire drawing material is subjected to plastic working that applies processing strain in the shear direction of the wire drawing material, Examples include a step of obtaining a deformed wire having a ratio of the major axis to the minor axis of 1.5 to 15 inclusive.

  The said form can manufacture the steel wire whose intensity | strength was raised by work hardening by performing a wire drawing process with a high total workability. In addition to the wire drawing, the above-mentioned form also provides plastic working that adds work strain in the shearing direction, so that the effect of improving the strength by work hardening can be further obtained. Wire).

  The steel wire of the present invention can perform welding well and is excellent in strength even after welding. The method for producing a steel wire of the present invention can perform welding well, and can produce a steel wire excellent in strength even after welding with high productivity.

It is sectional explanatory drawing explaining the cross-sectional shape of a deformed line, (A) shows a flat wire, (B) shows the cross-sectional trapezoid square wire.

  Hereinafter, the present invention will be described in more detail. In the following description, the contents of “composition” are all “mass%”.

[composition]
The steel wire of the present invention is made of steel containing C, Si, Mn, Cr, and Mo as essential elements, and the balance being Fe and inevitable impurities. Inevitable impurities include P (phosphorus), S (sulfur), and the like.

(C: 0.10% to 0.30%)
C (carbon) is a strengthening element of steel. When the C content is 0.10% or more, an effect of improving the strength can be obtained. The more C, the higher the strength. However, when C is too much, a hardened structure is formed at the time of welding, and quench cracks may occur. Therefore, in the present invention, the C content is set to 0.30% or less in order to prevent defects (burning cracks, etc.) particularly during welding. One feature of the steel wire of the present invention is that it is thus low carbon. A more preferable content of C is 0.10% or more and 0.20% or less.

(Cr: 0.20% to 2.00%)
In the present invention, Cr (chromium) is contained as a carbide generating element. In addition, Cr is an element that contributes to the refinement of the structure. When the Cr content is 0.20% or more, the effect of precipitation strengthening and the refinement of the structure can be obtained. The more Cr, the easier it is to precipitate carbide and the strength is increased. However, when there is too much Cr, a martensite phase is generated at the time of welding, and a reduction in toughness due to the martensite phase may occur. Considering suppression of toughness deterioration after welding, the Cr content is set to 2.00% or less. A more preferable Cr content is 0.50% or more and 1.50% or less.

(Si: 0.20% to 2.00%)
Si (silicon) is an element contributing to solid solution strengthening and heat resistance improvement. Si is also used as a deoxidizer during melting and refining. When the Si content is 0.20% or more, a solid solution strengthening effect, a heat resistance improving effect, and a deoxidizing effect can be obtained. Although the effect is enhanced as the amount of Si increases, the toughness decreases as the amount of Si increases. Considering suppression of toughness reduction, the Si content is set to 2.00% or less. A more preferable Si content is 1.00% or more and 2.00% or less.

(Mn: 0.30% to 2.50%)
Mn (manganese) is used as a deoxidizing agent during melting and refining in the same manner as Si. Mn also has the effect of improving strength. When the Mn content is 0.30% or more, a deoxidizing effect and an effect of improving the strength can be obtained. These effects are enhanced as the amount of Mn increases. However, when the amount is too large, a martensite phase is generated during welding, and a reduction in toughness due to the martensite phase may occur. Considering the suppression of the decrease in toughness after welding, the Mn content is set to 2.50% or less. A more preferable Mn content is 0.30% or more and 1.50% or less.

(Mo: 0.01% or more and 0.30% or less)
In the present invention, one of the characteristics is that Mo is also contained as a carbide forming element. When the Mo content is 0.01% or more, the effect of carbide precipitation strengthening can be stably obtained, and this effect can be obtained as the amount of Mo increases. Moreover, heat resistance can also be improved by containing Mo. However, if there is too much Mo, it will be difficult to obtain a bainite structure industrially, leading to a decrease in productivity. In consideration of industrial productivity, the Mo content is set to 0.30% or less. A more preferable Mo content is 0.03% or more and 0.10% or less.

[Organization]
One feature of the steel wire of the present invention is that it is substantially composed of a bainite structure. Quantitatively, it is comprised from the structure | tissue containing 95 volume% or more of bainite. Since bainite has a high hardness equivalent to pearlite, it can have substantially the same strength and fatigue limit as the pearlite structure by being substantially composed of a bainite structure. Excellent. In addition, by using the bainite structure as a main component, the transformation time may be shorter and the productivity is excellent as compared with the case where the pearlite structure is a main component. The content of bainite can be adjusted by the heat treatment conditions described later, and can be, for example, 97% by volume or more, and further 99% by volume or more. Examples of the remaining structure include retained austenite, martensite, ferrite, and cementite. In the range of 5% by volume or less, it is allowed to contain martensite, which is generally a brittle phase.

[Cross-sectional shape]
The steel wire of the present invention can take various shapes. A typical example is a round wire having a circular cross-sectional shape. In addition, various deformed lines can be used. For example, as a steel wire 10 shown in FIG. 1 (A), a rectangular wire whose cross-sectional shape is a rectangular shape, and a square wire whose cross-sectional shape is a trapezoidal shape like a steel wire 20 shown in FIG. It is done. In addition, the cross-sectional shape includes a wire rod having convex portions on the front and back of a flat wire, and a wire rod having a cross-sectional shape of an ellipse, a race track, or the like. In particular, deformed wires formed by processing (typically deformed processing) in which processing strain is applied in a direction perpendicular to the axial direction of the wire (shear direction) are typically subjected to wire drawing using a drawing die. Thus, the degree of improvement in strength by work hardening is larger than the round line formed, and the strength tends to be higher. That is, when the steel wire of the present invention is a deformed wire such as a flat wire, it is more excellent in strength.

[aspect ratio]
Taking the cross section of the steel wire of the present invention, the ratio of the major axis to the minor axis `` major axis / minor axis '' is referred to as the aspect ratio (sometimes referred to as the major axis x minor axis ratio), the aspect ratio is 1.5 or more Higher strength. This is because a steel wire having a large aspect ratio is typically formed by being subjected to processing to which the processing strain in the shear direction described above is applied. The greater the aspect ratio, the greater the processing degree of the above processing, and the steel wire is further excellent in strength. Considering the processing limit, the aspect ratio is preferably 15 or less. The minor axis and the major axis are two lengths perpendicular to each other in the cross section of the steel wire. In a round wire having a circular cross section, the minor axis = major axis = diameter, and the aspect ratio is major axis / minor axis = 1.0. In the steel wire 10 (flat wire) shown in FIG. 1 (A), the width w ₁₀ is the major axis and the thickness t ₁₀ is the minor axis. In the steel wire 20 (trapezoidal square wire) shown in FIG. 1 (B), the width w ₂₀ (trapezoidal height) is the major axis, and the minimum thickness t ₂₀ (the trapezoidal shorter side) is the minor axis.

[Cross sectional area]
The cross-sectional area of the steel wire of the present invention can take various sizes. After performing the specific heat treatment as described above, the cross-sectional area can be further reduced by performing a wire drawing process or a deforming process. It is preferable to adjust the degree of processing (such as the degree of wire drawing) so as to obtain a desired cross-sectional area. For example, (in the case of round wire, the following diameter diameter: 6 mm) cross-sectional area 30 mm ² or less, further (in the case of round wire, the following diameter 3 mm.phi) cross-sectional area 7 mm ² or less, particularly in the case of 1 mm ² or less (round wire, wire The diameter can be 1.1 mmφ or less. In the case of a flat wire, the width is about 0.5 mm to 20 mm and the thickness is about 0.3 mm to 5 mm. As the cross-sectional area is smaller, the strength improvement effect by work hardening of wire drawing or deforming is obtained, and the strength tends to be excellent.

[Tensile strength]
Since the steel wire of the present invention is excellent in strength, it has high tensile strength, and examples thereof include those satisfying 1300 MPa or more, further 1500 MPa or more, 1800 MPa or more. As described above, the smaller the cross-sectional area, the higher the tensile strength. Further, the steel wire of the present invention, even when welding (typically arc welding or spot welding), since the degree of decrease in strength of the welded portion is small, even when a member having a welded portion is constructed. High strength. For example, when the steel wires of the present invention are welded to each other, one in which the tensile strength of the welded wire rod (member having a welded portion) satisfies 1300 MPa or more can be mentioned. Depending on the size and shape of the cross-sectional area of the steel wire, the tensile strength of the welded wire can be 1500 MPa or more, and more preferably 1800 MPa or more.

[Production method of steel wire]
The steel wire of this invention can be manufactured with the manufacturing method which provides the process similar to manufacturing processes, such as a general piano wire, for example. Specifically, it can be manufactured through the steps of raw material preparation → melting / casting → hot rolling → heat treatment (→ appropriate drawing, → appropriately processing into a deformed wire). Especially the steel wire of this invention can follow the conventional manufacturing process by making the said heat processing into specific conditions and providing this heat processing process. That is, a step of preparing a material having at least a specific composition and a step of performing a specific heat treatment on the material may be performed.

(Preparation of material)
A material used for the heat treatment typically includes a hot-rolled material manufactured through the above-described steps. Specifically, a steel containing C, Si, Mn, Cr, and Mo in a specific range was prepared, and this raw steel was melted and cast based on conventional manufacturing conditions and known manufacturing conditions. Then, a hot rolled wire is produced. The size of the hot-rolled wire can be selected as appropriate and includes a diameter of 5.5 mmφ to 13 mmφ.

(Heat treatment)
The obtained material (typically hot-rolled wire) must first be subjected to a homogenization treatment of the metal structure. By this treatment, the plastic workability can be improved and the wire drawing can be easily performed. In particular, even when a wire drawing with a high degree of work is performed (for example, when the total degree of wire drawing is 50% or more, and further 75% or more), the wire drawing can be performed satisfactorily. Alternatively, even when a slight wire drawing process for correcting the shape is performed after the heat treatment (for example, the total wire drawing degree is about 5% or more and about 25% or less), it is easy to obtain a wire with excellent dimensional accuracy and shape accuracy.

  In general, the homogenization of the metal structure of low-carbon steel (C content of about 0.10% to 0.40%) is to suppress the generation of hardened structure (martensite) by gradually cooling after austenization. Thus, a mixed phase structure of ferrite (bainite) and pearlite is obtained. For the homogenization treatment for obtaining this mixed phase structure, methods such as stealmore (air cooling), salt bath cooling, boiling water cooling, etc. are usually employed as direct heat treatment immediately after hot rolling.

  And the wire (mainly one form of the steel wire of this invention) mainly comprised from a bainite structure is obtained by performing bainite-izing to the above-mentioned direct heat treatment and the wire which performed wire drawing after this direct heat treatment. Alternatively, a wire rod mainly composed of a bainite structure (one form of the steel wire of the present invention) can also be obtained by directing bainite during heat treatment. In this case, bainite can be performed continuously to austenite. As described above, after hot rolling, by performing austenitization and bainite formation at an appropriate time (not necessarily continuous), a wire mainly composed of a bainite structure can be obtained. Here, a bainite structure is inferior to workability compared with the above-mentioned mixed phase structure. Therefore, for example, after performing the above homogenization treatment on the hot-rolled wire, after processing (for example, wire drawing) close to a predetermined size and shape, after the bainite as the final heat treatment, Processing (for example, wire drawing) can be performed up to the size and shape of the final product. In this case, it is considered that the manufacturing method is easy to adjust the strength, size, and shape of the steel wire.

  The conditions for austenitizing are a temperature of 850 ° C. to 1100 ° C., a holding time of 10 seconds to 10 minutes, and a bainite condition is a constant temperature transformation temperature of 400 ° C. to 500 ° C., and a holding time of 10 seconds to 10 seconds. Less than minutes. That is, the constant temperature transformation is performed in the temperature range of the upper bainite. In the range of the steel component specified in the present invention, the holding time can be shortened and the productivity can be improved, particularly in the range where the isothermal transformation temperature is 400 ° C. or higher and 500 ° C. or lower, preferably 450 ° C. or lower. When the temperature is out of this temperature range, it is necessary to lengthen the holding time, resulting in a decrease in productivity. The austenitizing temperature is more preferably 900 ° C. or higher and 950 ° C. or lower, the bainite forming constant temperature transformation temperature is 420 ° C. or higher and 450 ° C. or lower, and the holding time is preferably 10 seconds or longer and 3 minutes or shorter.

  In addition, the steel which consists of the above-mentioned specific composition can also form a pearlite structure | tissue substantially by adjusting heat processing conditions, as shown in the test example mentioned later. However, the steel having a substantially pearlite structure has a large degree of strength reduction due to the heat effect during welding. Therefore, the heat treatment conditions are adjusted so that the steel wire of the present invention has a substantially bainite structure, and the steel wire manufacturing method of the present invention substantially forms a bainite structure.

(Wire drawing)
A steel wire having a smaller cross-sectional area (one form of the steel wire of the present invention) can be manufactured by appropriately drawing (drawing) the heat-treated material subjected to the heat treatment. The drawing degree (drawing area reduction ratio) can be selected so that a desired cross-sectional area (or wire diameter) is obtained. By comprising a specific composition as described above, when wire drawing is performed after the heat treatment, an effect of improving strength by work hardening can be obtained, and a steel wire with higher strength can be manufactured. Moreover, the effect of improving the strength by work hardening is comparable to that of a general high carbon steel wire (piano wire), and the strength can be rapidly increased as the degree of wire drawing increases. Therefore, when further improvement in strength is desired, the total wire drawing degree is preferably 50% or more, and more preferably 75% or more. For wire drawing, a hole die (drawing die) is typically used.

(Deformation processing)
Furthermore, the above-mentioned appropriate deformed wire can be manufactured by applying a deforming process to the wire drawing material. For the profile processing, a known plastic processing means such as a hole die or a roller die can be used.

[Test Example 1]
Steel wires of various compositions were prepared and the tensile strength was measured. Moreover, the obtained steel wires were welded together, the tensile strength after welding was measured, and the degree of strength reduction before and after welding was investigated.

  Here, steels having various compositions shown in Table 1 (the content of each element is mass%, the balance is Fe, and Ceq is the carbon equivalent specified in JIS G 0203 (2009)) are melted in a vacuum melting furnace. Then, hot forging and hot rolling were sequentially performed to produce a hot rolled material (round wire with a wire diameter of 7 mmφ). The obtained hot rolled material was subjected to heat treatment. The heat treatment was first austenitized (heating temperature: 900 ° C.), and in the cooling process from the austenitizing heating temperature, patenting (560 ° C. × 20 seconds) or bainite (420 ° C. × 20 seconds) was performed. . After the heat treatment, a cross section of the obtained heat treated material was taken, and after polishing this cross section, the structure was examined by X-ray diffraction on the polished surface. Here, for each sample, a region near the surface of the heat treatment material (wire) (region within 100 μm from the surface), a region of D / 4 (D is the wire diameter, and the region of D / 4 is the surface of the wire. The area of the intermediate point with the center of the wire.For the flat wire and the wire of Test Example 2 (deformed wire) to be described later, the case where D is the major axis and the minor axis is performed)), the central area of the wire, Examine the organization of multiple areas. A cross section was taken so that the structure of each region could be observed, and the structure was examined for each cross section. And, for each cross-sectional field of n ≧ 3 for each sample, when the volume ratio of bainite in all regions is 95% by volume or more, it is evaluated that it is substantially composed of a bainite structure. Show. Similarly, when the volume ratio of pearlite is 95% by volume or more, it is evaluated that the pearlite is substantially composed of a pearlite structure, and “P” is shown in Table 2. Here, except for sample No. 1-141, in all samples of each composition, when the heat treatment for bainite was performed, the transformation was completed and a hardened structure was not generated. Therefore, it can be said that each composition can satisfactorily obtain a bainite structure. On the other hand, when a pearlite heat treatment (patenting) is performed, a transformation structure is insufficient and a hardened structure is often generated. The reason for this is that these samples contain a large amount of carbide-forming elements (here, especially Cr contains 0.2% by mass or more, and further 0.5% by mass or more), so that the transformation start time is delayed and the transformation time is prolonged. It is thought that this happened. The pearlite transformation has been completed for Samples No. 1-111 and No. 1-112 having the same composition as Samples No. 1-11 and No. 1-12.

  Each heat-treated material obtained was drawn to produce a drawn material. For wire drawing, multi-pass processing was performed and the total wire drawing degree was set to 50%. The tensile strength TS (50%) (MPa) of the obtained wire drawing material (round wire with a wire diameter of 4.95 mmφ) was examined. The results are shown in Table 2. The tensile strength was measured at room temperature (here, about 20 ° C.) by performing a tensile test based on the metal material tensile test method of JIS Z 2241 (1998).

  As shown in Table 2, it can be seen that the strength increases as the content of any additive element increases. However, Sample No. 1-141 with a very large amount of Mo could not form a bainite structure even when it was subjected to bainite heat treatment under the above heat treatment conditions. From this, it can be said that the Mo content is preferably less than 0.50 mass%, particularly preferably 0.30 mass%.

  The structure after welding was evaluated. Here, instead of actually performing welding, a heating test simulating heating during welding was performed to examine the structure after heating. Specifically, the obtained wire rod was rolled into a 1 mm thick rectangular wire, and the rectangular wire was heated to about 1000 ° C. and then allowed to cool naturally. Then, after each rectangular wire reached about room temperature, the surface was polished, and X-ray diffraction was performed on the polished surface to examine the amount of martensite (α ′) produced. It can be said that the greater the amount of martensite produced, the easier it is to produce martensite during welding and the more likely to cause cracking. The case where the amount of martensite produced is 1% by volume or less is evaluated as ◯, the case where it exceeds 1% by volume and 5% by volume is evaluated as Δ, and the case where it exceeds 5% by volume is evaluated as ×. Table 2 shows the evaluation results. When the structure observation of the wire drawing material before rolling and the structure observation of the flat wire after rolling were performed as described above, they were substantially the same as the structure of the heat-treated material (bainite structure or pearlite structure).

  The end surfaces of the obtained wire drawing materials having respective compositions were butted and welded to produce a member having a welded portion, and the tensile strength (TS, MPa after welding) of the obtained member was examined. The tensile strength was performed in the same manner as in the case of the wire drawing material described above. In addition, the degree of decrease in tensile strength before and after welding was examined. The degree of reduction was evaluated by obtaining {1-TS / TS after welding (50%)} × 100 = TS reduction rate (%). These results are shown in Table 2. Note that this welding was not performed for samples in which a large amount of martensite was generated during welding (samples in which evaluation of α ′ during welding in Table 2 is x).

  As shown in Table 2, C is 0.10% to 0.30% by mass, Si is 0.20% to 2.00%, Mn is 0.30% to 2.50%, Cr is 0.20% to 2.00%, and Mo is 0.01%. It can be seen that the steel wire containing 0.30% or less and substantially composed of a bainite structure has high strength, high tensile strength after welding, and small decrease in strength before and after welding (weldability) The judgment is ○). Specifically, the tensile strength after welding satisfies 1300 MPa or more and the TS reduction rate satisfies 5.5% or less. Some of the samples satisfy the tensile strength after welding of 1350 MPa or more and 1400 MPa or more, and the samples satisfy the TS decrease rate of 5% or less, 4.5% or less, and further 4% or less. The reason for this is considered that martensite is not substantially generated at the time of welding, and there is substantially no burning crack, and it is difficult to break even if it has a welded portion.

  In addition, the steel wire that contains C, Si, Mn, Cr, Mo in the specific range described above and is substantially composed of a bainite structure may be high strength by being drawn. I understand. Specifically, there is a sample satisfying TS (50%) of 1400 MPa or more, further 1500 MPa or more, and in particular 1600 MPa or more. From this, steel containing C, Si, Mn, Cr, and Mo in the above specified range is subjected to heat treatment under specific conditions, and then subjected to plastic working such as wire drawing, resulting in work hardening. It can be said that the strength improvement effect can be obtained more greatly. In other words, steel wires that contain C, Si, Mn, Cr, Mo in the specific range described above and are substantially composed of a bainite structure are those that have undergone wire drawing processing (typically wire drawing materials). Then, it can be said that it can be welded well and is superior in strength.

  In the above-described observation of the structure after welding, the sample in which the amount of martensite produced (the sample with an evaluation of ○) was substantially composed of a bainite structure. From this, it can be said that the structure is substantially maintained before and after welding (the bainite structure is maintained). Thus, since the structure does not substantially change before and after welding, the strength of the bainite structure itself can be maintained, and it is considered that high strength was maintained even after welding.

  Furthermore, even if it is composed of a bainite structure that is generally inferior in plastic workability, the degree of workability (here, the total wire drawing work degree) is 50% or more by controlling the structure by performing a specific heat treatment as described above. It can be seen that such processing can be performed satisfactorily. In addition, it has confirmed that the structure | tissue after a process is also a bainite structure.

  On the other hand, when the additive element is outside the above specific range (too much), it can be seen that martensite is easily formed during welding. From this, it can be said that the increase of the additive element is effective in improving the strength, but the welding cannot be performed well (the weldability is judged as x).

  As described above, from Test Example 1, the steel wire containing C, Si, Mn, Cr, and Mo in the specific range described above and substantially composed of a bainite structure can be welded satisfactorily. It was confirmed that the strength after welding was also excellent. Moreover, it was confirmed that by performing a bainite process by performing a specific heat treatment, the workability is excellent and the strength improvement effect by work hardening is large. It has also been confirmed that this work hardening rate is equal to or higher than that of piano wire type B.

[Test Example 2]
Produced deformed wires with different aspect ratios (major axis x minor axis ratio), and examined TS (50%) (MPa), TS (MPa) after welding, and TS reduction rate (%) in the same manner as in Test Example 1. .

  Here, steel with the same composition as sample No. 1-2 (mass%, C: 0.20%, Si: 1.00%, Mn: 0.30%, Cr: 1.00%, Mo: 0.03%, Ceq: 0.4992) is prepared. Then, in the same manner as in Test Example 1, a hot-rolled wire (round wire with a wire diameter of 7 mmφ) was produced, and heat treatment was performed under the same conditions as in Test Example 1 (austenite → bainite: 420 ° C. × 20 seconds). . The heat treated material was subjected to wire drawing (total wire drawing degree: 50%, round wire with a wire diameter of 4.95 mmφ), and further subjected to rolling to produce a rectangular wire. In the rolling process for forming a rectangular wire, the cross-sectional area is hardly changed before and after the process, and thus the apparent cross-sectional reduction rate does not change. When the width of the flat wire was taken as the major axis and the thickness as the minor axis, the ratio of the major axis to the minor axis (major axis / minor axis) was taken as the aspect ratio, and the rolling conditions were adjusted so that the aspect ratio was the value shown in Table 3. The larger the aspect ratio, the more the processing strain in the shear direction (the direction perpendicular to the axial direction of the wire drawing material) is applied to the wire drawing material.

  Table 3 shows the tensile strength (TS (50%), MPa) of the prepared rectangular wire. Further, the end faces of the produced rectangular wires were butted and welded to produce a member having a welded portion, and the tensile strength (TS, MPa after welding) of the obtained member was examined. The results are also shown in Table 3. Each tensile strength was determined in the same manner as in Test Example 1. Further, Table 3 also shows the TS reduction rate (%) obtained in the same manner as in Test Example 1.

  By comparing sample No. 1-2 in Table 2 and sample No. 2-1 in Table 3, in addition to wire drawing, further processing strain is applied in the shear direction (here, rectangular wire) It can be seen that the strength after the processing can be further increased by performing the rolling process for forming the. Moreover, it turns out that the fall of the strength after welding can further be suppressed. In particular, it can be seen that the strength after processing can be greatly increased by increasing the aspect ratio. Here, a sample having a tensile strength after processing of 1700 MPa or more, further 1800 MPa or more, and further 2000 MPa or more was obtained. In addition, as shown in Table 3, it can be seen that the tensile strength after processing is high, the decrease in strength after welding is small, and high strength is maintained even after welding. Here, a sample was obtained in which TS after welding satisfied 1650 MPa or more, further 1750 MPa or more, further 1900 MPa or more, and TS decrease rate was 4.0% or less, and further 3.0% or less. From this fact, it can be said that the effect of improving the strength by work hardening can be obtained more greatly by performing the processing in which processing strain is applied in the shear direction. In addition, when the structure of each sample after rolling was observed in the same manner as in Test Example 1, it was confirmed that the sample was substantially composed of a bainite structure.

  Further, from Test Example 2, the steel wire containing C, Si, Mn, Cr, and Mo in the above-described specific range and substantially composed of a bainite structure has an aspect ratio of 1.5 or more, and further 2 or more. It was confirmed that, when the deformed wire has a large ratio, the welding can be performed well and the strength is further improved.

  Note that the present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention. For example, a wire diameter, a cross-sectional shape, etc. can be changed.

  The steel wire of the present invention can be suitably used for various members, for example, automobile parts (for example, a seat frame, etc.), parts for household electric products, and the like. In addition, since the steel wire of the present invention can form an annular body by welding, it contributes to the improvement of the material yield by using it as a material for various annular members that have been conventionally manufactured by punching. Expected to be able to. The manufacturing method of the steel wire of this invention can be utilized suitably for manufacture of the steel wire of the said invention.

  10,20 steel wire

Claims (6)

% By mass
C from 0.10% to 0.30%,
Si is 0.20% or more and 2.00% or less,
Mn 0.30% to 2.50%,
Cr is 0.20% or more and 2.00% or less,
Mo is contained 0.01% or more and 0.30% or less, the balance is composed of Fe and inevitable impurities,
Steel wire with a bainite structure of 95% by volume or more.   2. The steel wire according to claim 1, wherein the steel wire is a deformed wire having a ratio of a major axis to a minor axis of 1.5 to 15 inclusive. The steel wire according to claim 1 or 2, wherein a cross-sectional area of the steel wire is 30 mm ² or less.   The steel wire according to any one of claims 1 to 3, wherein when the steel wires are welded to each other, the tensile strength of the welded wires is 1300 MPa or more. Containing 0.10% to 0.30% C, Si 0.20% to 2.00%, Mn 0.30% to 2.50%, Cr 0.20% to 2.00%, Mo 0.01% to 0.30% by mass% A preparatory step of preparing a hot rolled material in which the balance is composed of Fe and inevitable impurities;
A heat treatment step of performing a heat treatment on the hot-rolled material to obtain a heat-treated material having a bainite structure of 95% by volume or more;
A wire drawing step of drawing the heat treatment material to obtain a wire drawing material,
In the heat treatment step, after austenitizing with a temperature of 850 ° C. to 1100 ° C. and a holding time of 10 seconds to 10 minutes, the isothermal transformation temperature is 400 ° C. to 500 ° C., and the holding time is 10 seconds to 10 minutes. The manufacturing method of the steel wire which forms a bainite structure as follows. The wire drawing process has a total wire drawing degree of 50% or more,
6. The production of a steel wire according to claim 5, further comprising a step of subjecting the wire drawing material to plastic working that applies processing strain in a shear direction of the wire drawing material to obtain a deformed wire having a ratio of a major axis to a minor axis of 1.5 to 15. Method.

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