KR20070023679A - Excellent cold-workability exhibiting high-strength steel wire or steel bar, or high-strength shaped article and process for producing them - Google Patents

Abstract

In the high-strength steel wire or steel bar excellent in cold workability, high-strength molded article, and their manufacturing method, C content is below the solid-solution limit of the ferritic carbon at Ae1 point, and contains no cementite or C content of 0.010 mass% or less. Ingots, castings, slabs or steel semi-finished products of 0.01 to 0.45% by mass are subjected to warm processing in a temperature range of 350 to 800 ° C. to obtain a material having an average grain size of 3 μm or less in a cross section perpendicular to the longitudinal direction. It prepares, and then cold-processes and forms the ferrite structure whose average grain size in the cross section perpendicular | vertical to a longitudinal direction is 500 nm or less.

Description

High-strength steel wire, steel bar or high-strength molded article with excellent cold workability and manufacturing method thereof

The invention of the present application relates to a high-strength molded article such as a screw or bolt using high-strength steel wire or steel bar having excellent cold workability, or the characteristics thereof, and a method for producing the steel wire or bar and high-strength molded article.

Conventionally, cold working of steel wires produced by hot working on screws, bolts, and other high-strength mechanical structural parts that are formed by forming steel wires or bars by cold working such as cold rolling, rolling, and / or cutting. Is processed into a steel wire having a desired linear diameter, and the obtained steel wire is heated at a temperature of about 700 ° C. for a long time from several tens of hours to overnight to form a so-called spheroidizing annealing process for spheroidizing cementite in a metal structure. After softening a material and improving cold workability, such as a cold rolling, it shape-processes into the product shape of various uses. However, since the molded article processed in this way does not satisfy the strength required as the final product by the softening treatment, it is necessary to perform a tempering treatment such as quenching and tempering.

Moreover, it is common to surface-treat etc. suitably after that and to ship as a product. Thus, in the conventional manufacturing process of high strength mechanical structural parts and the like, due to the preliminary softening treatment on the raw material and the tempering treatment on the molded product after cold working, it takes a long time and is complicated, and the loss of thermal energy is large, and the productivity is high. It was low, and there was also a problem in terms of increase in heat treatment cost and delivery date management.

As a measure to solve such a problem, in order to improve the cold rolling performance of the steel wire produced by hot working, there is no case where spheroidization annealing is usually performed on the steel wire which is usually performed, and the cold rolling steel having excellent cold workability ( The method of manufacturing (i) is proposed (for example, patent document 1). This method produces C in the steel as carbides other than Fe3C at a temperature higher than the cementite production temperature, thereby substantially reducing the amount of solid solution C in the steel, thereby suppressing the production of cementite and further pearlite, which inhibits the deformation resistance and the deformation ability. On the other hand, it is intended to greatly increase the amount of cornerstone ferrite, thereby greatly improving cold workability.

However, according to this method, the spheroidizing annealing treatment can be omitted, but since the tensile strength of the steel wire obtained reaches only 500 MPa, when high strength is required as a molded article obtained by cold rolling, such as quenching and tempering. The tempering treatment is necessary. In addition, in order to produce C in the steel as carbides other than Fe 3 C, there is also a problem of causing an increase in cost, such as the addition of V, which is a relatively expensive alloy element.

Moreover, after shaping | molding containing a cold rolling vessel is made into the product shape, the method which does not need to perform temper processing, such as quenching and tempering, is also proposed (for example, patent document 2). In this method, as a raw material to be used, a metal structure has a quenching and tempering structure among steel wires manufactured in the past, and the product of yield strength and work hardening index satisfies a specific condition range and cracks occur in a predetermined compression test. The material is selected so as not to. In this method, however, it is not necessary to perform a spheroidizing annealing treatment that takes a long time to the steel wire that is used as a material for cold pressing in hexagon bolts or the like, but to quench and temper the steel wire before cold rolling. need.

In such a situation, the inventor of the present application also solves all the above problems, develops a technique that can omit the tempering treatment performed after the cold working together with the softening treatment such as the spheroidizing annealing performed before the conventional cold working, and this is new. It is proposed as invention (patent document 3).

In the present invention, caliber rolling in warm is performed in order to introduce a prescribed strain required in a rolling temperature within a range of 350 to 800 ° C for a steel slab or steel material of less than C: 0.45% by mass. Doing.

As a result, steel having a ferrite structure having an average particle diameter of 1 to 2 µm or less in the cross section perpendicular to the rolling direction can be produced, and the area reduction is 70 as the mechanical property without performing quenching or quenching and tempering treatment. It is possible to manufacture steel having an excellent cold rolling property of more than% and a tensile strength of 800 MPa or more. And when this steel is used, it can manufacture molded articles, such as a screw and a bolt excellent in strength, by the cold working containing a cold rolling mill. Based on the technology of the present invention, the inventor of the present application ensures the excellent properties and effects of the steel obtained by this technology, while maintaining the cold workability at a high level, and further measures for improving the strength. A review has been made. At that time, with respect to the characteristics of the mechanical properties of the steel to be manufactured, the target value exceeds the level of 600 MPa or more (preferably 800 MPa or more) of the tensile strength TS targeted in the proposed invention (Patent Document 3). Preferably as much as possible, and also maintaining as much as possible, preferably more than, 65% or more (preferably 70% or more) of the area reduction RA targeted in the same patent application. It was set to. And specifically,

Case 1: TS ≧ 700 MPa, also RA ≧ 65%, where more preferably up to 70% or more with respect to RA,

Case 2: TS≥1000MPa, also RA≥70%,

Case 3: TS≥1500MPa, also RA≥60%

It aimed at obtaining the steel wire or steel bar provided with this. Thus, if the tensile strength TS is a high level and the strength-ductility balance substituted for the tensile strength TS and the area reduction RA is a steel wire or steel rod having both high levels of characteristics, in addition to fastening parts such as screws and bolts, as well as axial flow. In the manufacture of parts which are conventionally formed mainly by cutting, also, forming by cold rolling becomes easy, and a remarkable improvement in molding yield from steel wire or steel bar to high strength axial flow (a conventional level is generally As low as 60 to 65%).

In the course of the examination by the inventor, the material of the component system having a chemical composition of substantially no cementite is used as the material, and the technique of the proposed invention is applied to this material, and this is made of the material (steel wire). By performing appropriate cold work on this, the objective of being able to manufacture the steel wire or the steel bar which is higher strength than the conventional one, and also excellent cold workability, and a high strength molded article was acquired. However, in order to actually make this possible, it is necessary to use steel as a chemical component composition so that steel may not be substantially produced in the standard structure of steel. For example, the refinement | purification process for manufacturing the high purity pure iron for electrical steel sheets, or the steel which reduced C content above this is required. For this purpose, even when any one of a converter and an electric furnace is used as the refining furnace in a steelmaking process, the molten steel outgoing from these refining furnaces is further decarburized by vacuum refining in an appropriate vacuum refining furnace. I) By accelerating the reaction, in addition to refining the ultra-low carbon steel, in the casting process such as continuous casting, measures to secure the cleanliness of the steel by preventing reoxidation of molten steel are also required.

(Patent Document 1) Japanese Patent Application Laid-Open No. 2000-273580

(Patent Document 2) Japanese Patent Application Laid-Open No. 2003-113422

(Patent Document 3) Japanese Patent Application No. 2003-435980

The invention of the present application solves the above-mentioned problems. First, the cold workability is characterized by having a cementite-free ferrite structure with an average particle diameter of 500 nm or less in the vertical section with respect to the longitudinal direction of the steel wire or the steel bar. Provide this excellent high strength steel wire or bar.

Secondly, C content is below the solid solution limit of the ferrite carbon in Ae1, and has a ferrite structure whose average particle diameter in the vertical cross section with respect to the longitudinal direction of steel wire or steel bar is 500 nm or less. Provide high strength steel or bar.

Third, there is provided a high-strength steel wire or steel bar having excellent cold workability, wherein the C content is 0.010% by mass or less and a ferrite structure having an average particle diameter of 500 nm or less in the vertical cross section of the steel wire or steel bar in the longitudinal direction. .

The fourth aspect is to provide a high-strength molded article having a cementite-free ferrite structure in which the average particle diameter in at least one cross section in an arbitrary cross section is 500 nm or less.

Fifth, there is provided a high-strength molded article having a C content of not more than the solid solution limit of carbon in the ferrite phase at the Ae1 point, and having a ferrite structure having an average particle diameter of 500 nm or less in at least one cross section of an arbitrary cross section.

A sixth aspect provides a high-strength molded article having a C content of 0.010 mass% or less and having a ferrite structure having an average particle diameter of 500 nm or less in at least one cross section of an arbitrary cross section.

Seventh, the C content is more than 0.01 to 0.45% by mass, the ferrite structure having a mean particle diameter of 500 nm or less in the vertical cross section of the steel wire or the steel bar in the longitudinal direction as a main phase, the tensile strength is 700MPa or more, and the area is reduced. It provides a high-strength steel wire or bar excellent in cold workability, characterized by having a mechanical property of 65% or more.

Eighth, the C content is more than 0.01 to 0.45% by mass, the ferrite structure having a mean particle diameter of 500 nm or less in the vertical cross section in the longitudinal direction of the steel wire or steel bar as the main phase, the tensile strength is 1500MPa or more, and the area It provides a high-strength steel wire or bar excellent in cold workability, characterized in that the reduction has a mechanical property of 60% or more.

9th, C content is more than 0.01-0.45 mass%, and ferrite structure whose average particle diameter in a vertical cross section with respect to the longitudinal direction of steel wire or steel bar is 500 nm or less, and hardness is 285 or more in Vickers hardness HV. The high-strength steel wire or steel bar excellent in cold workability is provided.

10th, the C content was more than 0.01 to 0.45% by mass, and the ferrite structure having an average particle diameter of 500 nm or less in at least one cross section of an arbitrary cross section as a main phase, and the hardness in at least one cross section of an arbitrary cross section. It provides a high-strength molded article characterized in that the Vickers hardness HV is 285 or more.

The eleventh aspect provides a high-strength molded article having a C content of more than 0.01 to 0.45% by mass, a ferrite structure having an average particle diameter of 500 nm or less in at least one of the cross sections in an arbitrary direction, and a tensile strength TS of 900 MPa or more. do.

A twelfth aspect provides a coil-like steel wire or steel wire, in which the average grain size is micronized to be 10 mu m or less in a region of 90% or more of the area of the C section of the rolled material.

13th, the steel ingot, the casting piece, the steel piece, or the steel semi-finished product which has a cementite-free ferrite structure is warm-processed, and the material whose average grain size in the cross section perpendicular | vertical to a longitudinal direction is 3 micrometers or less is prepared, Then, cold working is performed to form a ferrite structure having an average grain size of 500 nm or less in a cross section perpendicular to the longitudinal direction, thereby providing a method for producing a high strength steel wire or a steel having excellent cold workability.

Fourteenthly, the ingot, the cast piece, the steel piece, or the steel semi-finished product whose C content is equal to or less than the solid-solution limit of the ferritic carbon at the Ae1 point is warmed, and the average grain size in the cross section perpendicular to the longitudinal direction is 3 m. A method for producing a high-strength steel wire or steel bar having excellent cold workability is prepared by preparing a material having the following material, followed by cold working to form a ferrite structure having an average grain size of 500 nm or less in a cross section perpendicular to the longitudinal direction. do.

As a fifteenth aspect, a method for producing a high-strength molded article characterized in that it is produced by cold rolling, cold forging and / or cutting using high strength steel wires or steel bars having excellent cold workability prepared by the manufacturing methods of 13 and 14. To provide.

16th, C. content: The ingot, the casting piece, the steel piece, or the steel semi-finished product which exceeds C1: 0.01 to 0.45 mass% is warm-processed, and the material whose average grain size in the cross section perpendicular | vertical to the longitudinal direction is 3 micrometers or less is used. The present invention provides a method for producing a high strength steel wire or steel bar having excellent cold workability, which is prepared, and then subjected to cold working to form a ferrite columnar structure having an average grain size of 500 nm or less in a cross section perpendicular to the longitudinal direction.

1 is a graph illustrating a relationship between a rolling condition parameter Z and a ferrite average particle diameter.

FIG. 2 is a diagram showing the hole-shaped dimension sites of diamond and square, and elliptical caliber rolls.

3 is an example of a ferrite structure photograph by SEM of the L-direction cross section of the steel (steel wire) after warm working in the process of manufacturing the steel according to the present invention (in Example 3).

FIG. 4 is a graph showing a rise state of the tensile strength TS according to the increase of the cold working rate when the cold working rate is converted into an industrial deformation e, and also shown a difference between the examples and the comparative example at that time. .

FIG. 5 is a graph showing the state of decrease of the area decrease RA caused by the increase of the cold working rate when the cold working rate is converted into the industrial deformation e, and indicated, and the difference between the examples and the comparative example at that time. .

6 is a graph comparing quantification of the level values of tensile strength TS and area reduction RA, and the balance state of the tensile strength TS and area reduction RA with respect to Examples and Comparative Examples.

7 is a graph comparing the level of tensile strength TS with respect to the C content of the steel wire in the Examples and Comparative Examples.

8 is a graph comparing the level of area reduction RA with respect to the C content of the steel wire in the Examples and Comparative Examples.

9 is M1. This is a photograph showing the state of setting the torsional delay fracture test piece of 6 pan head machine screw.

10 is a graph showing the state of rise of the tensile strength TS according to the increase of the cold working rate when the cold working rate is converted into the industrial strain e, and the difference between the examples and the comparative example at that time. .

FIG. 11 is a graph showing the falling state of the area reduction RA caused by the increase in the cold working rate when the cold working rate is converted into the industrial strain e, and is also a graph showing the difference between the examples and the comparative example at that time. .

12 is a graph comparing quantification of the level values of tensile strength TS and area reduction RA and the balance state of the tensile strength TS and area reduction RA with respect to Examples and Comparative Examples.

Fig. 13 is an example (in Example 2) of a ferrite structure photograph by TEM of a cross section in the L direction in the steel (steel wire) after cold working obtained by the manufacturing method according to the present invention.

14 is a graph comparing the level of tensile strength TS with respect to the C content of the steel wire in the Examples and Comparative Examples.

15 is a graph comparing the level of area reduction RA with respect to the C content of the steel wire in the Examples and Comparative Examples.

The invention of the present application is to have the configuration as described above and features related thereto. Then, next, the embodiment of the invention of this application and the reason for limitation of the form in this embodiment are demonstrated.

[1] The main phase of the metal crystal structure is ferrite, the main phase of carbon steel to low alloy steel and metal crystal structure having a C content exceeding 0.01% by mass to 0.45% by mass is substantially cementite free, and the C content is Ae1. Carbon steel or low alloy steel whose C content is 0.010 mass% or less below the solid solution limit of the ferritic carbon in the point.

<1> Regulation and crystal structure of chemical composition

The first characteristic of the high-strength steel wire or the steel bar excellent in cold workability and the chemical composition of the high-strength molded article according to the present invention is that the main phase of the metal crystal structure is ferrite and the C content is more than 0.01% by mass to 0.45% by mass. The second feature of the high-strength steel wire or steel bar, which is carbon steel up to low alloy steel and excellent cold workability according to the invention of the present application, and the chemical composition of the high-strength molded article, is a high-strength steel wire or steel bar excellent in cold workability, and high strength The chemical composition of the molded article is characterized by the fact that the main phase of the metal crystal structure is substantially cementite free, and the carbon content is lower than the solid solution limit of the ferritic carbon at the Ae1 point, or the carbon content is low or less than 0.010 mass%. Apply. Here, in the component design, when other component element contents are given as to what C content is determined, the relationship between the C content and the tensile strength TS described in the specification of the present invention (for example, FIG. 7, FIG. With reference to 8), it performs suitably so that the desired mechanical property etc. may be satisfy | filled for the target use to manufacture.

In addition, in the above, more than 0.01 mass% of lower limits of C content may be made to exceed the solid solution limit of the ferritic carbon in Ae1 point.

The solid-solution limit of C content is a steel wire made of low alloy steel, even when a metal element such as Cr or Mo is used to replace a part of Fe element of Fe 3 C with this element M to form Fe (3-X) MXC. It is because it is approximating the solid solution limit of the ferritic carbon in Ae1 point in a component system of carbon steel as it is content level of the alloying element contained in a steel bar.

In addition, the solid solution limit of the ferritic carbon in Ae1 point can be estimated using well-known calculation software "Thermo-calc", for example ("Thermo-calc" is a calculation in equilibrium state, but actually The cooling conditions at the time of manufacture of are not in equilibrium, and thus cannot be estimated completely. It is also required for the metal structure to have ferrite as the main phase. In any case, the crystal structure of the steel wire having an average particle diameter of 3 µm or less prepared by warm rolling, which is one of the constituent requirements in the method for producing a high strength steel wire or steel bar according to the present invention, is in accordance with the above proposed invention by the inventor. This is because the steel must be ferrite-based steel.

On the other hand, in the definition of the chemical composition, it is not necessary in the invention of the present application to rely on the addition of an alloying element for improving the strength of the material. Here, it is not necessary to deliberately add an element which promotes hardenability improvement, for example, Cr, Mo, other equivalent elements, Cu, Ni which are solid solution strengthening elements, and other equivalent elements. In addition, it is preferable that the alloying element is not added even in view of reducing the production cost.

In addition, in order to more reliably suppress the production of cementite in the material, and not to increase the manufacturing cost due to the addition of a large amount of alloying elements, the Si content is 1.0% by mass or less and the Mn content is 2.0% by mass or less. It is more preferable to limit to. In addition, regarding the definition of the chemical composition in the invention of the present application, C, Si, Mn, Cr, and any of the molded products represented by steel wires or bars, screws and bolts, and steel ingots and steel slabs, etc. Although they do not prescribe | regulate their content about Al, such as Al as a deoxidizer which is a component element other than Ni, Valuable elements, such as Ti, Nb, and V as dispersion precipitation strengthening elements, and P, S, N, etc. which are normally handled as a hazardous impurity, For deoxidation elements, the content of essential levels must be ensured in conventional refining and casting techniques, and the elements to be treated as ordinary impurities must be limited to inevitable incorporation content, and not particularly limited to ultra low content. Although it does not restrict | limit content especially about an outer valuable element, It is not necessary to contain it. This is because the invention of the present application can solve the problem sufficiently.

In particular, in the invention of the present application, it is an important feature that the composition may be a chemical component which does not cause martensite transformation in the quenching treatment. The reason is that the target tensile strength is 800 MPa or more, preferably 900 MPa or more, more preferably 1200 MPa or more, and still more preferably 1500 MPa or more, as long as the constituent requirements of the manufacturing method of the present application are satisfied. This is because steel obtained in this case, and in which the area reduction RA according to these tensile strengths is also maintained at a high level is obtained.

Thus, obtaining the mechanical property which is excellent in the balance of both high strength and high ductility is largely dependent on the fact that hard cementite which is a factor which deteriorates cold workability is not produced substantially.

In addition, in the steel wire, the steel bar, or the molded article which concerns on the invention of this application, determination of whether it is substantially free of cementite is not necessarily easy as a real problem. Here, it can be estimated by the quantitative analysis value of actual C content in daily operation. Herein, in the invention of the present application, from the geological judgment, the C content is prescribed to be equal to or less than the solid solution limit of carbon in the ferrite phase at the Ae1 point. Or in the component system of a normal low alloy steel, it is prescribed | regulated as 0.010 mass% or less as the range of C content which it is thought that no cementite is produced | generated.

In the above, since the solid solution C concentration (mass%) in the ferrite phase at the Ae1 point is less than or equal to, the structure is essentially cementite-free. In either of the carbon steel and the low alloy steel, the C concentration (mass%) in which this cementite-free content is practically obtained can be estimated using, for example, known calculation software "Thermo-calc" ("Thermo -calc "is an equilibrium calculation, but since the cooling conditions at the time of manufacture are not equilibrium, it cannot be said that it can be estimated completely.). As described above, in the invention of the present application, in a steel material having a cementite-free ferrite structure, a material having high strength as described above and also excellent in cold workability (high strength steel having excellent balance between strength and workability) can be designed. Become. Conventionally, the example where the high strength steel wire or the steel bar excellent in the cold workability by such a component design was implement | achieved is not outstanding.

On the other hand, in order to more reliably suppress the production of cementite in the material and not to increase the production cost due to the addition of a large amount of alloying elements, the Si content is 1.0% by mass or less and the Mn content is 2.0% by mass or less. It is more preferable to limit to.

In the invention of the present application, as mentioned above, the primary principle for obtaining high strength characteristics is to focus on being cementite-free steel. Therefore, also in the definition of the chemical composition, it is not necessary to rely on the addition of the alloying element. Here, an element for promoting the hardenability improvement, for example, the addition of Cr or Mo, other equivalent elements, and solid solution It is not necessary to add Cu, Ni, and other similar elements as reinforcement elements on purpose. In addition, it is preferable not to add the said alloying element also in manufacture cost reduction. Therefore, it is preferable that all of the above elements do not have any abnormal content which is unavoidably mixed in the refining and solvent process of steel.

In addition, although not specifically defined in the invention of this application, it is not necessary to add Ti, Nb, and other alloy elements which are effective elements for precipitation strengthening. By the cementite-free component system of the present invention, sufficient tensile strength can be ensured, which also helps to reduce manufacturing costs.

As described above, the C content of the steel (steel wire or bar and molded article) according to the invention of the present application is basically designed to be cementite free. Thus, the standard structure of the steel is always a ferrite structure.

In addition, regarding the definition of the above chemical composition, the deoxidizer which is a component element other than C, Si, Mn, Cr, and Ni also in any of molded products represented by steel wire or steel bars, screws, bolts, etc. Valuable elements such as Ti, Nb, and V as dispersion precipitation enhancing elements, such as Al, and P, S, and N, which are usually treated as hazardous impurities, do not define their contents, but conventional refining for deoxidation elements, The necessary level of content must be ensured in the casting technology, and the elements to be treated as impurities are usually limited to the inevitable mixing content, and in particular, the content is not limited to the ultra low content. For other valuable elements, the content is particularly limited. It is not necessary, but it does not need to be contained. This is because the invention of the present application can solve the problem sufficiently.

<2> Specification of average particle diameter of ferrite, tensile strength TS, area reduction RA

The average particle diameter of the ferrite in the invention of the present application is also defined in any of the molded products represented by the steel wire or the bar, the screw and the bolt according to the invention of the present application. Specifically, it is prescribed | regulated to 500 nm or less in the cross section (C direction cross section) perpendicular | vertical to the longitudinal direction. Thus, the average particle diameter of ferrite is prescribed | regulated in order to ensure the intensity | strength of this steel wire or steel bar, and a molded article more than a desired level. In other words, in the steel wire or the bar, the tensile strength TS is at least 700 MPa, and the tensile strength TS is 1000 MPa or more, more preferably 1500 MPa or more, depending on the application, so as to obtain excellent characteristics of the tensile strength TS. Therefore, in order to secure ductility, all of these whose area reduction RA was also maintained at high level are for obtaining the steel which has the outstanding balance. Here, the balance between the tensile strength TS and the area reduction RA is the balance as shown below:

Case 1: TS ≥ 700 MPa, also RA ≥ 65%, more preferably further improve the level of area reduced RA, TS ≥ 700 MPa, RA ≥ 70%,

Case 2: TS≥1000MPa, also RA≥70%

Case 3: TS≥1500MPa, also RA≥60%

Means. By the combination of each of the tensile strength TS and the area reduction RA, the steel wire or the bar can be supplied to the place of use according to the application.

Such a provision is made in order to enable the improvement of a process acceptance yield and the supply of the molded article of the quality level which was not implement | achieved conventionally in the processing of a molded article. In addition, as for the axial flow, conventionally produced by cutting from steel wires or steel bars, by appropriately supplying the high strength and superior ductility steel wires or steel bars which are the inventors of the present application according to the use, the processing yield is remarkable. To improve.

Further, when the average particle diameter of the ferrite is made fine to 200 nm or less, it is preferable that the combination of the tensile strength TS and the area reduction RA of the steel according to the present invention can be obtained more easily and stably. Moreover, in the molded article represented by a screw and a bolt, it can be considered that the average particle diameter in at least 1 cross section of an arbitrary cross section is substantially the same as the average particle diameter in the C direction cross section in a wire rod or a steel bar.

According to the method of manufacturing high strength steel excellent in cold workability according to the invention of the present application, a material having high strength as described above in low carbon steel or ultra low carbon steel in which the conventionally realized example is not noticeable and also excellent in workability (strength And high strength steel with excellent workability balance) can be designed. Based on such a material design and the possibility of new development of high strength steel excellent in the balance of strength and workability is expected.

<3> hardness regulation

In the steel wire or the steel bar according to the invention of the present application, a rule expressed in hardness is given as a strength characteristic instead of the tensile strength TS. As this hardness, it is preferable that it is 285 or more in Vickers hardness HV. This is because when the Vickers hardness HV is 285 or more, the tensile strength is approximately 900 MPa. On the other hand, in the molded article represented by the screw or the bolt which concerns on this invention, preparation of a tensile test piece may not be easy depending on the shape. In this case, the stipulation of hardness should be sufficient as a mechanical property instead of tensile strength. From this point of view, for molded articles represented by screws or bolts, the definition of hardness as a substitute for tensile strength is of greater importance as an evaluation of the characteristic level of utility products. More preferably, the Vickers hardness HV is 300 or more corresponding to about 1000 MPa in terms of tensile strength TS.

Next, the embodiment of the steel wire or the steel bar according to the invention of the present application having the above-described characteristics, and a method for producing a molded article, and the reason for the limitation thereof will be described in detail.

<4> Basic configuration of the manufacturing method according to the invention of the present application (regulation of the combined process that is hot + cold processing)

The basic feature of the manufacturing method according to the invention of the present application is, first of all, a manufacturing method of a material used for producing a steel wire or a steel bar having excellent cold workability according to the invention of the present application, and the warm processing under appropriate conditions for a predetermined material And fine grain structure steel is prepared by this warming process. Here, it is preferable that the crystal grain diameter of the material obtained is as small as possible, and specifically, it is necessary to be 3 micrometers or less in the average particle diameter in the cross section (C direction cross section) perpendicular | vertical to the longitudinal direction of the material obtained by warm processing. . Subsequently, these materials are subjected to cold working under appropriate conditions. By this cold working, the crystal grains in the cross section (C direction cross section) in the vertical direction in the longitudinal direction of the material after cold working are further refined. To obtain a fine grain tissue steel. Since the microstructure obtained here has a columnar ferrite and is cold worked, the microstructure obtained usually has the shape of a so-called bamboo structure drawn in the cold working direction.

In this way, high strength steel excellent in cold workability is obtained. In that case, in this cold working, when the fine grain structure steel prepared by the said warm working was made into a raw material, although the material strength was remarkably raised, it was discovered that there is very little fall of workability when it is very preferable. This novel knowledge, which was previously difficult to anticipate, forms the basis of the invention of the present application. Thus, just before performing cold working, the reason for performing appropriate cooling processing described below on the material on which fine crystal grains have already been formed is that it is necessary to perform spheroidizing annealing treatment on the obtained steel before forming processing. It is because there is a very big advantage that the molded article obtained by quenching and tempering is not required even after the molding process.

<5> Warm processing conditions (process temperature, plastic deformation, reduction rate)

As an embodiment of the manufacturing process of the said high strength steel wire or steel bar excellent in the cold workability, preferable warm working conditions with respect to predetermined | prescribed ingot, casting piece, or steel piece or steel material should be within the range of 350-800 degreeC of processing temperature. In addition, the plastic deformation introduced in the material at that time must be ensured. This plastic deformation amount can be calculated | required by calculation by a well-known three-dimensional finite element method (it expresses the value as "(epsilon)"), and it is preferable that (epsilon) is 0.7 or more. Such a warm working condition is adopted as a method for realizing high strength of steel without substantially using a reinforcing mechanism due to phase transformation, and for miniaturizing crystal grains. As a result, the inventors have found that the invention as Patent Document 3 is very effective because it is excellent in cold workability such as cold rolling and the like, by making the area reduction RA of the steel to a predetermined level or more. In the above-mentioned warm working conditions, instead of referring to ε as an index, deformation of a material that can be relatively easily demanded in operation (in the specification of the present application, referred to as "industrial deformation" and denoted as "e"). It can be replaced practically. Industrial strain e is a function of the total reduction rate R of the material,

e = -ln (1-R / 100) ·········· (3)

Is represented. However, R is the following formula (1):

R = {(S0-S) / S0} × 100 (1)

Where R is the total reduction rate (%)

   S0: C-section cross-sectional area of the casting piece or steel piece immediately before the start of warm working

    S: Cross-sectional area in the C direction of the material obtained after completion of warm processing

The total reduction rate R is shown.

Using the above formulas (3) and (1), R? 50% is obtained by calculating the value of R corresponding to? Therefore, in the warm working, instead of the plastic strain ε ≥ 0.7, the total reduction ratio R ≥ 50% of the material may be used. On the other hand, the inventor of the present application focuses on the fact that the average particle diameter of the ultrafine particles formed by warm steel processing (large deformation processing by one pass in warm temperature) depends on the processing temperature and the deformation rate, and the rolling conditions. As a parameter, the following formula (4):

Z = 1og [(ε / t) exp {Q / (8.31 (T + 273))}] ········ (4)

However, ε: average plastic strain

    t: time from start of rolling to end (s)

    Q: Integer (254000 J / mol when crystal structure is bcc)

    T: In the case of rolling temperature (° C) and multipass rolling, a Zener-Hollomon parameter represented by a temperature obtained by averaging the rolling temperature of each pass is introduced (but expressed in algebraic form) and determined. The particle size is found to be finer as the rolling condition parameter Z increases. 1 illustrates the relationship between the rolling condition parameter Z and the average ferrite particle diameter. That is, FIG. 1 has shown that the fine grain structure whose average ferrite particle diameter is 1 micrometer or less is obtained by controlling rolling so that Z≥11. Therefore, by controlling the warm rolling temperature to satisfy Z≥11, the average ferrite grain size of the raw material can be made less than 3 µm. As the warm working method, any one of warm rolling and warm forging may be adopted, and at that time, plastic deformation in the material is performed by processing in multiple directions by a plurality of passes (in the case of warm forging, a plurality of forging schedules). It is preferable because the homogenization of is achieved.

(6) Cold working condition (processing temperature, plastic deformation, rule of reduction rate)

Next, as for the preferable cold working conditions which should be performed previously with respect to the material which has the fine grain structure prepared by the warm processing as mentioned above, and is excellent in high strength and workability, it is preferable that cold working temperature is less than 350 degreeC. When the heat generation reaches a higher temperature during cold working, the degree of increase in tensile strength is lowered, which is not preferable. Next, it is necessary to ensure residual strain introduced into the material by cold working in accordance with the desired tensile strength. From such a viewpoint, it is preferable to perform cold working so that plastic deformation (epsilon) calculated by the three-dimensional finite element method may be at least 0.05 or more. As a result, the cold-worked structure of the crystal exhibits a form stretched in the processing direction, and the grain size in the cross section in the C-direction with respect to the processing direction is also atomized, thereby increasing the tensile strength. At that time, the amount of reduction of area reduction RA is suppressed small. In the above cold working conditions, instead of using ε as the index for the machining amount, the total reduction ratio R of the material corresponding to ε ≧ 0.05 is mediated by e, which is an “industrial deformation” described by Equation (3). Is calculated, R≥5% is obtained. Therefore, in cold working, instead of the plastic strain?? 0.05, the total reduction ratio R? 5% of the material may be used.

In the above, as the cold working method, any of the known cold drawing method and cold rolling method may be used. In the cold rolling method, it is preferable to use the known combine roll method. If the form of the steel produced by cold working is steel wire or steel bar, especially in the use of molded products requiring high strength and good cold workability among JIS G 3539 cold-rolled carbon steel wire, or JIS G 3505 hard steel wire, relatively low C content region In spite of being high strength in steel grade. It can provide to the product use which requires favorable cold workability.

[II] Examples <Wide range of carbon steels to low alloy steels whose main phase of the metal crystal structure is ferrite and whose C content is greater than 0.01% by mass to 0.45% by mass>

In Example 1 and Example 2, the manufacturing process of the high-strength steel wire or steel bar which concerns on this invention of this application differs in part, and Example 1 and 2 and Example 3 differ also in a chemical component composition other than the manufacturing process. Therefore, Example 1, 2, and Example 3 demonstrate a test method and a test result, respectively.

[II] <1> Example 1 and Examples

[II] <1> -1) Test common to Examples 1 and 2 (Adhesion test of test material obtained by warm rolling process)

Example 1 and Example 2 were tested as follows. The steel having the chemical composition shown in Table 1 was melted using a vacuum melting furnace and cast into a steel ingot. This chemical component composition contains 0.30 mass% exceeding this with respect to Si content: 0.10 mass% or less in the chemical component composition prescribed | regulated by SWRCH5A which belongs to the cold-rolling carbon steel wire material of JIS G 3507, for example. . However, it is characteristic that C content is low 0.0245 mass%.

Sample source Chemical composition (mass%) C Si Mn P S N sol.Al Example 1 Example 2 0.0245 0.30 0.20 0.010 0.001 0.0018 0.032

The ingot obtained above was formed into a steel bar of 80 mm square by hot forging. The metal structure of these steel bars was a ferrite columnar shape, and the average particle diameter of the ferrite in the C direction cross section was about 20 µm or less. The raw material for rolling was extract | collected from each said steel bar of 80 mm square, it shape | molded to 18 mm square by multipass caliber rolling of the warm direction in warm, water-cooled, and the steel bar was prepared. This warm rolling prepares the raw material for steel wires or steel bars which concerns on this invention of this application, on condition that the average crystal grain size in the cross section perpendicular | vertical to the longitudinal direction of the material obtained by this warm rolling will be 3 micrometers or less. It was done.

As mentioned above, it performed on the following conditions as a method of the warm caliber rolling that an average crystal grain diameter becomes 3 micrometers or less. After heating the 80 mm square rolled material formed by said hot forging at 550 degreeC, as shown in Table 2, in the range of the rolling temperature of 450-530 degreeC, first, a diamond type caliber roll (FIG. 2, upper part) (Refer to the figure), the 19-pass warm rolling of which the reduction ratio of each 1 pass is about 17% was performed, and it shape | molded to 24 mm square. Subsequently, warm rolling was performed by an elliptical caliber roll having a maximum short axis length of 11 mm and a long axis length of 52 mm (a, b in the lower figure, respectively, R = 64 mm Φ in the lower figure), and finally, one pass warm rolling was performed with a square caliber roll. It formed into 18 mm square in 21 passes in total. The total reduction ratio from the warm rolling material (80 mm square) to the 18 mm square is 95%. Table 2 shows the outline of the pass schedule.

Number of passes Caliber Number Caliber Type Caliber Shape One One        Diamond 2 3 2 4 5 3 6 7 4 8 9 5 10 11 6 12 7 13 8 14 9 15 10 16 11 17 12 18 13 19 20 14 Oval 21 15 Diamond

In the one-pass warm rolling by the elliptical caliber roll, since 24 mm rods are rolled by the elliptical caliber roll on all four sides, the cross section in the C direction of the post-rolling material with respect to the stool length of 24 mm in the C direction cross section of the pre-rolling material. The ratio of the maximum short axis length of 11 mm is quite small (11 mm / 24 mm) x 100 = 46%, and the reduction ratio calculated from the hole-shaped dimension at this time is quite large (38%). Therefore, the one-pass warm rolling by the elliptical caliber roll is a condition for further facilitating the miniaturization of the ferrite grain size in the all-round 18 mm steel bar after the end of the warm rolling. In addition, in the rolling process by the diamond-type caliber roll up to the 19th pass, in order to make the cross-sectional shape of the material as close to square as possible, rolling is made to pass through the same caliber roll in successive 2 passes (so-called "double pass"). ), And each double pass was counted as two passes, respectively. In addition, in each pass of rolling, the material was rotated about the longitudinal axis center, the reduction direction was changed, and multipass rolling of the multi-direction was performed. In addition, processing heat is also added, and even in the rolling temperature region of the warm rolling, the amount of heat dissipation is relatively small in the relatively low temperature side region, and there is no necessity of intermediate heating due to the temperature decrease of the material during rolling. Next, the steel bar of 18 mm square prepared by the above-mentioned warm rolling method was reduced in diameter by cutting, and processed into the steel wire of diameter 6.0mm (phi).

Here, the reason for reducing the diameter by cutting from 18 mm to 6.0 mm phi in all directions is as described below. In this embodiment, M1.6 pan head machine screw specified in JIS B1111 as the use of steel wire (the diameter of the effective section of the screw portion) This is because the 1.27 mm phi) is selected, so that a 1.3 mm phi diameter is obtained by cold drawing processing with a target draw rate of 95% or cold rolling with a target total reduction rate of 95%. The M1.6 pan head machine screw is selected because it requires very cold cold forming property in order to pressure-form a cross-shaped recess in the head. M1.6 This is to evaluate whether the crosshead "recess molding test" of panhead machine screws has particularly good cold rolling.

In addition, the particle diameter in the C direction cross section of the said 18 mm steel bar prepared by the warm rolling in the above was equalized over the whole surface.

This 6.0 mm diameter amplification test piece was extract | collected and the following item was tested. In addition, the steel wire material which processed this expansion test material to 6.0 mm (phi) after sampling was provided to the test of Example 1 and Example 2 continuously.

1) Measurement test of tensile strength (TS) and area reduction (RA) by the tension test: In this test, a high level balance in strength and cold workability which is particularly excellent in cold workability as well as excellent in strength. It is an object to obtain basic data for evaluating whether or not the material having a.

2) Hardness measurement test by Vickers hardness tester: It is effective to check the correlation with tensile strength as one of the strength characteristics and when it is difficult to collect the tensile test piece. The process was carried out based on the method defined in JIS Z 2244.

3) Measurement test of ferrite particle diameter (d) by microscope test: Prepare a suitable diameter test piece from each test material, and measure the average particle diameter of the ferrite constituting the columnar phase with the microstructure of the metal crystal in the longitudinal direction of the test material (the above 18mm in all directions). The average ferrite particle diameter of the cross section (cross section in the C direction) perpendicular to the longitudinal direction of the steel bar is measured. At this time, the microstructure in the L direction cross section was actually observed, and the average ferrite particle diameter of the C direction cross section was calculated | required.

Table 3 shows the test results for the warm rolled material.

Sample provider C (mass%) Manufacture process Test piece wire diameter (mmφ) Tensile Strength TS (Mpa) Area reduction RA (%) Vickers Strength Hv (-) Average ferrite particle size d (㎛) at C section Example 1 Example 2 0.0245 Warm rolling 6.0 702 78.6 255 0.7

From the test results in Table 3, the following items can be seen. The steel wire by the warm rolling is a low carbon steel with a C content of 0.0245 mass%, which does not contain any special reinforcing elements, and despite the warm rolling, the high tensile strength TS of 702 MPa and the area are secured. It can be seen that a very high level of characteristics with a reduced RA of 78.6% is obtained, and a balance material having excellent strength and formability is obtained. This is because, according to the conditions within the scope of the invention of the present application, the microstructure of the metal crystal has a ferrite as the main phase, and a fine grain structure steel having a ferrite grain size of 0.7 µm is obtained. In this way, even in low carbon steels that cannot be derived from the commonly used cold-rolled steel wire material having a C content of 0.0245 mass%, the tensile strength achieves a high level of 700 MPa or more, and the area reduction RA is also very high. It is secured.

On the other hand, using the 6.0 mmφ steel wire material after the said 6.0 mmφ amplification test sample was extract | collected, by cold drawing in Example 1, and by cold rolling in Example 2, they all cold-processed from 6.0 mmφ to 1.3 mmφ. To test the steel wire.

[II] <1> -2) Different test between Example 1 and Example 2 (Amplification test of test material obtained by cold working process)

[II] <1> -2)-(a) [Amplification Test of Steel Wire Obtained by Cold Drawn Method in Example 1]

The above-mentioned 6.0 mmφ steel wire material (steel wire material processed into 18 mmφ by warm rolling, and then cut into 6.0 mmφ as mentioned above) at normal temperature is shown in the drawing die of die numbers 1-17 as shown in Table 4. The wires were sequentially drawn to prepare a steel wire of 1.3 mmφ. The material temperature in the fresh wire was less than 200 ° C.

Cold Drawn (6.0 mmφ-> 1.3 mmφ) of Examples 1-6 Die number 1 to 11 12 to 13 14-17 Steel wire diameter after drawing (mmφ) 5.6 *) to 2.1φ 1.9 to 1.8φ 1.6 to 1.3φ

Note *) However, the starting diameter of the fresh material is 6.0 mmφ.

In all the drawing processes of the present Example 1, it was easy to wire from 6.0 mmphi to 1.3 mmphi, without performing integral spheroidization annealing and other softening processes. And the test material for expansion as it was wire | wire was extract | collected from the steel wire of 1.3 mm (phi) total drawing reduction ratio: 95.3%. The amplification test method is as follows, 1) 2) 3) are as described above.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test

2) Hardness test by Vickers hardness tester

3) Measurement test of ferrite particle size (d) by microscope test

4) Recess molding test of machine screw: Pre-formed steel wire with a diameter of 1.3mmφ is preformed by header working in the manufacturing process of M1.6 pan head machine screw specified in JlS B1111, and then Cruciform recesses (grooves such as cruciforms for fastening the screws by means of a driver) are formed by cold pressing. It is a test to observe the situation that a crack occurs in this recess at the time of this shaping | molding with a magnifying glass of 10 times. In general, the occurrence of recess cracks varies greatly depending on the recess shape of the machine screw, but the cross-shaped recess forming of the M1.6 panhead machine screw is very harsh forging, and is a practical test in this specification. It was positioned as an evaluation test of excellent cold rolling. "Good" and micro cracks were confirmed that cracks were not confirmed, but "good cracks" were found to be "good cracks", "good cracks" and cracks were found to be "good cracks".

5) Torsional Torque Test of Machine Screws: From a steel wire of 1.3mm in diameter, M1.6 pan head machine screws are prepared by forming a threaded portion by cold rolling the recessed screw intermediate as described above. Subsequently, when the screw is broken by an appropriate torque measuring device according to the method specified in 5.4 "Torsion Test" of JIS B1060 "Mechanical Properties and Performance of Carburizing Quenching Tempering". Increase the torque until The required torque value (breaking torque (kgf · cm)) until breakage occurred. The purpose of this test is to evaluate "torsional strength", which is one of the characteristics of mechanical properties for fasteners such as screws and bolts. Hereinafter, the same in this specification. M1.6 For panhead machine screws, it is desirable that the breaking torque be at least 3.0 kgf · cm.

Table 5 shows the test results of Example 1.

exam Example 1 C (mass%) 0.0245 Test piece wire diameter (mm) 1.3φ Cold Freshness Total Reduction Rate (%) 95.3 transform 3.06 Tensile Strength TS (Mpa) 1567 Area reduction RA (%) - Vickers Hardness Hv (-) 355 Average ferrite particle size d (nm) in cross section C direction 182 Recess Formability of Machine Screws crack Screw Breaking Torque (kgf × cm) 3.38

From the test results of Table 5, the followings were confirmed. That is, the 1.3 mmφ steel wire obtained in Example 1 is a low carbon steel with a C content of 0.0245 mass%, and no special reinforcing element is added, but no heat treatment such as quenching or tempering or any softening treatment is performed. The tensile strength TS is 1567MPa, which is remarkably high, and the area reduction RA is also quite high, which is 60.2%. This is because, as shown in Table 3, the material has already reached a very high level of tensile strength TS of 702 MPa by warm rolling, Vickers hardness HV of 355, and a high area reduction RA of 78.6%. This is because the fine ferrite structure steel (average ferrite grain size in the C-direction cross section is 0.7 µm) is cold-worked at a total reduction ratio of 95.3% by drawing.

Thus, although the steel wire of Example 1 is a low carbon steel and high strength and high ductility is given to the steel wire after cold work, the crystal grain of this steel wire is comprised by the fine ferrite columnar shape. Specifically, the 1.3 mmΦ steel wire of Example 1 is a ferrite columnar having an average ferrite grain size of 182 nm in the cross section in the C-direction, and a form extending in a bamboo structure shape in the direction of cold drawing.

Here, from the idea that the ferrite grain size in the C-direction cross section after the cold work is controlled by the amount of work strain, the measured value of the grain size before and after cold work is examined. In the case of Example 1, the average ferrite particle diameter in C cross section in the steel wire material (steel wire of cold start start) prepared by the warm rolling was 0.7 micrometer (refer Table 3). On the other hand, when the ferrite grain size of the C-direction cross section of the steel wire material (wire diameter: 6.0 mmΦ) obtained by the warm working is represented by d1, and the total cross-sectional reduction rate by cold drawing to the steel wire is R (%), the cold wire after cold drawing The average ferrite particle diameter d2 of the cross section of the steel wire (wire diameter: 1.3 mm Φ) in the C direction is expressed by the following formula (5):

d2 = (1-R / 100) 1/2 x d _l (5)

Estimate with Since R is 95.3% and d1 was 0.7 µm, it is calculated as d2 = 152 nm. This calculated value 152 nm is in good agreement with the measured value 182 nm.

Therefore, in the manufacturing method of the steel wire or the steel bar which concerns on this invention, when manufacturing a steel wire by cold working the steel wire material of a warm rolling material, it is used as a control means of the ferrite particle diameter in the C direction cross section of this steel wire material. It is effective to use Formula (5) above.

Next, the steel wire according to the invention of the present application manufactured in this way is formed of a recess, which is a molding process in which a very harsh cold press, such as an M1.6 pan head screw, is performed in a state without temper treatment such as quenching and tempering. As for the index of ductility, cracks occurred at area reduction RA of 60.2%. However, when the torsion torque test was carried out, 3.38 kgf cm was obtained which satisfies the breaking torque value of 3.0 kgf cm or more, which is preferable as the M1.6 pan head machine screw, and it was confirmed that it had high torsional strength.

[II] <1> -2)-(b) [Amplification test of cold rolling method and obtained steel wire in Example 2]

The above-mentioned 6.0 mmΦ steel wire material (steel wire material which was processed to 18 mmΦ by warm rolling and then cut into 6.0 mmΦ as mentioned above) at normal temperature is shown in Table 6 at the 1st process-3rd process. By cold rolling with each of the combination rolls, the test was rolled to 1.3 mm phi to manufacture a steel wire.

Cold rolling of Example 2 Rolling process No. 1st process 2nd process 3rd process Number of passes 8 pass 10 pass 5 pass Steel wire diameter after drawing (mmΦ) 5.7 *) 3.3Φ 3.1 to 1.8Φ 1.6 to 1.3Φ Diameter of Test Material (mmΦ) 3.3Φ 1.8Φ 1.3Φ Total rolling reduction rate (%) of test sample collection 69.8 91.3 95.3 Industrial variation e (%) of test sample collection 1.20 2.41 3.06

Note *) However, the starting diameter of the rolled material is 6.0 mm Φ

That is, rolling from 6.0 mm Φ to 3.3 mm Φ in 8 passes of the first process, rolling from 3.3 mm Φ to 1.8 mm Φ in 10 passes of the second process, and rolling from 1.8 mm Φ to 1.3 mm Φ in 5 passes of the third process, Steel wire was prepared. The material temperature during rolling was less than 200 degreeC. In all these rolling processes, it was easy to cold-roll from 6.0 mm phi to 1.3 mm phi, without performing any softening process other than spheroidization annealing. In the meantime, as a test material for expansion, in the three stages of 3.3 mm (total reduction ratio: 69.8%), 1.8 mm (total reduction ratio: 91.0%) and 1.3 mm (total reduction ratio: 95.3%), for cold expansion as it is Test materials were taken. The adequacy test method was performed according to the above.

1) Measurement test of tensile strength (TS) and area plasticity (RA) by tensile test

2) Hardness test by Vickers hardness tester

3) Measurement test of ferrite particle size (d) by microscope test

4) Recess molding test of machine screw

5) Torsional Torque Test of Machine Screw

The test results are shown in Table 7.

exam C (mass%) Test piece wire diameter (mmΦ) Total cold rolling reduction (%) Variant e Tensile Strength TS (Mpa) Area reduction RA (%) Vickers Hardness Hv (-) Average particle diameter d (nm) of ferrite in C direction cross section Recess Formability of Machine Screws Torsional Breaking Torque (kg × cm) Example 2 0.0245 3.3Φ 69.8% 1.20 922 - - - - - 1.8Φ 91.0% 2.41 1147 - - - - - 1.3Φ 95.3% 3.06 - - 328 - crack 2.92

From the test results, the following is confirmed. In addition, the manufacturing conditions of Example 2 differ from that of Example 1 by processing by cold rolling instead of cold drawing. All other conditions are the same. In Example 2, although the linear diameter of the sampling test material differs from Example 1, the tensile strength TS is 922MPa and the linear diameter 1.8mmΦ (total reduction ratio: 91.0%) at 3.3mmΦ (total reduction rate: 69.8%). At 1147MPa is the highest level. In addition, Vickers hardness HV reached a very high level of 328 at a line diameter of 1.3 mm Φ (total reduction ratio: 95.3%).

Comparing Example 2 and Example 1 with Vickers hardness HV at a line diameter of 1.3 mm Φ, Example 2 (cold rolling method) is 328, Example 1 (cold drawing method) is 355, and other conditions may be the same. In the case, it is confirmed that the increase in hardness is slightly larger in the case of cold drawing than in the case of cold rolling. In this way, the cold working method for the raw material (steel wire) is either cold drawn or cold rolling, and the chemical composition of the raw material (steel wire) immediately before cold working, the state of the crystal structure, especially in the C-direction cross section. It is confirmed that the same high-strength steel wire is obtained when the average ferrite grain size in the structure has the same ferrite columnar structure and the tensile strength TS and the area reduction RA are the same. The torsional breaking torque of the M1.6 pan head machine screw is 2.92 kgf · cm, and high torsional strength close to 3.0 kgf · cm of the preferred level is exhibited even in the cold rolling without spheroidizing annealing. .

[II] <2> Example 3

As Example 3 within the scope of the invention of the present application, the test was carried out as follows. Among the cold-rolled carbon steel wires specified in JIS G 3507, commercially available 13mmΦ steel wires having the chemical composition shown in Table 8 belonging to SWRCH5A and manufactured by hot rolling were used. The component of this steel wire is 0.03 mass% of carbon C, and is similar to the component composition of the steel provided in Example 1 and Example 2 above. However, Si content of the sample provision steel of this Example 3 is 0.03 mass% different from Si = 0.30 mass% of Examples 1 and 2, and satisfy | fills Si content specification (Si <= 10.10 mass%) of SWRCH5A. .

Sample source Ingredient equivalent of JIS Chemical composition (mass%) C Si Mn P S sol.Al Example 3 SWRCH5A 0.03 0.30 0.20 0.016 0.010 0.035

The 13 mm Φ hot rolled steel wire was prepared in a 6.0 mm Φ steel wire by multi-direction, multi-pass warm rolling with a caliber roll in the range of a rolling temperature of 450 ° C. to 530 ° C. The warm rolling method was followed by caliber roll rolling in which diamond, square, and ellipse were appropriately combined with the method for preparing the steel wire for sample provision in Examples 1 and 2. Thus, the test material for amplification was extract | collected from the said 6 mm (Φ) steel wire obtained by the warm rolling, and the following item was tested. In addition, the 6.0 mm Φ steel wire material after this test material for amplification was provided for the test of Example 3 (as above-mentioned).

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test

2) Measurement test of ferrite particle size (d) by microscope test

The test results are shown in Table 9.

Sample provider C (mass%) Manufacture process Test piece wire diameter (mmΦ) Tensile Strength TS (Mpa) Area reduction RA (%) Average particle diameter d of the ferrite in section C (μm) Example 3 0.03 Warm rolling 6.0 817 75.0 0.8

The following are confirmed from the test result of Table 9. The microstructure of the metal crystal of the steel wire of Example 3 has ferrite as a main phase, and the ferrite particle diameter is C direction as shown in the microstructure photograph in the L direction cross section by SEM (scanning electron microscope) of FIG. The average ferrite particle diameter in the cross section is made into fine particles of 0.8 μm. Therefore, although the C content is low carbon steel of 0.03% by mass, the high strength of the tensile strength TS of 817 MPa is ensured and the high level of characteristics of the area reduction RA of 75.0% is obtained. It is confirmed that it is a material of the balance excellent in the property. This is because it is a material which satisfies the preparation conditions (manufacturing conditions) of the raw material (steel wire material) for manufacturing the high strength steel wire or steel bar which is excellent in cold workability of this invention, and is prepared by warm rolling.

Next, in Example 3, the test which manufactures steel wire by cold rolling was performed using the 6.0 mm (phi) steel wire material prepared by the warm process as a raw material as mentioned above as a raw material. The cold rolling method was cold-worked to 1.3 mm (phi) in accordance with the 1st-3rd process of cold rolling in Example 2 shown in Table 6, and produced the steel wire. As the test specimen for expansion during this period, cold rolled steel wire test specimens of 2.1 mm Φ (87.8%), 1.8 mm Φ (total reduction ratio: 91.0%) and 1.3 mm Φ (total reduction ratio: 95.3%) were collected.

About the said test material, the following test was done suitably as mentioned above.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test

2) Hardness test by Vickers hardness tester

3) Measurement test of ferrite particle size (d) by microscope test

The test results are shown in Table 10.

The following are confirmed from the said test result.

In Example 3, the tensile strength TS of the steel wire is high at 1140 MPa at a wire diameter of 1.8 mm Φ (total reduction ratio: 91.0%) and 1202 MPa at a wire diameter of 1.3 mm Φ (total reduction ratio: 95.3%). The area reduction RA at this time is at a high level of 72.3% and 70.2%, respectively. In addition, Vickers hardness HV reached a very high level of 310 at a diameter of 1.3 mm Φ (total reduction ratio: 95.3%). In this way, the average ferrite grain size in the C-direction cross section is reduced to 186 m. By cold rolling on the warm rolling material, not only the tensile strength TS is further improved, but the area reduction RA is also maintained at a high level, and it is confirmed that the balance between the two is excellent. As in Example 1, the 1.3 mmΦ steel wire of Example 3 has a ferrite columnar shape in which the average ferrite grain size in the cross section in the C direction is 186 nm and is elongated in a bamboo structure shape in the direction of cold drawing. Because. Here, the result of Example 3 and Example 1 is compared. The comparison between the tensile strength TS and the area reduction RA at 1.3 mm Φ (total reduction ratio 95.3%) of the same linear diameter is shown in Example 3, which has a low Si content of 0.03 mass%, which is 0.30 mass%. Although the tensile strength TS is lower than Example 1 which is high (Example 3: 1202 MPa, Example 1: 1567 MPa), the area reduction RA is reversed by both, and Example 3 is clearly higher (Example 3: 70.2%, Example 1: 60.2%). In addition, regarding C content, it is considered that there is no significant difference between them (Example 3: 0.03 mass%, Example 1: 0.0245 mass%).

[III] <Comparative Example 1 to Comparative Example 3>

Next, the following test was done as a 1st group of the comparative example outside the scope of the invention of this application.

A cold-rolled carbon steel wire stipulated in JIS G 3507, which is a 6.0 mm Φ steel wire having respective component compositions corresponding to SWRCH5A, SWRCH10A, and SWRCH18 of Comparative Examples 1-3 of Sample Providers shown in Table 11. The test sample for amplification was sampled from the commercially available steel wire material which finished processing more than A3 and transformation point which is hot rolling conditions, and the steel wire after test material collection was continued for the test of Comparative Examples 1-3. About the said test material for amplification, the above test of the following item was done.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test

2) Measurement test of ferrite particle size (d) by microscope test

These test results are shown in Table 12.

Sample provider JIS equivalent of ingredient Chemical composition (mass%) C Si Mn P S sol.Al Comparative Example 1 SWCH5A 0.04 0.04 0.33 0.003 0.007 0.028 Comparative Example 2 SWCH10A 0.09 0.01 0.30 0.011 0.025 0.030 Comparative Example 3 SWCH18A 0.18 0.01 0.79 0.017 0.005 0.040

Sample provider JIS equivalent of ingredient C content (mass%) Manufacture process Test material diameter (nmΦ) Tensile Strength TS (MPa) Area reduction RA (%) Average ferrite diameter d (μm) in cross section C direction Comparative Example 1 SWCH5 0.04 Hot rolling 6.0 350 80.0 20 Comparative Example 2 SWCH10 0.09 430 75.0 17 Comparative Example 3 SWCH18 0.18 520 72.0 16

The following are confirmed from the said test result. In addition, this test material for amplification is a general hot rolling material, ie, the steel wire which finished rolling process more than A3 transformation point. This is the manufacturing conditions of the steel wire material outside the range of the manufacturing method of the invention of this application. Therefore, the average particle diameter in the C direction cross section of the ferrite which is columnar structure of a metal crystal is not fine-grained structured about 16-20 micrometers. Therefore, the area reduction RA is 80.1 to 89.9%, which is excellent at a high level, but the tensile strength TS is 350 to 550 MPa, and the C content provided in Examples 1 to 3 is 0.0245 to 0.03% by mass, which is produced by warm rolling. Compared with 817 MPa (refer Table 9) of the steel wire which was made, it is confirmed that it is very low.

Subsequently, using the 6.0 mmΦ hot-rolled steel wire material after taking the said test material for amplification, as a steel wire manufacturing test in the following comparative examples 1-3, it cold-processed to 1.3 mmΦ by cold drawing or cold rolling, and steel wire Was prepared.

First, (i) about the hot-rolled steel wire material corresponding to SWCH5A of the comparative example 1, cold drawing was performed and the steel wire was manufactured. Cold drawing was performed on the conditions similar to Example 1 (refer Table 4. Drawing temperature is less than 200 degreeC). In this cold drawing process, the steel wire test material as it is cold drawn as 2.1 mm Φ (new total reduction rate: 87.8%), 1.8 mm Φ (new total reduction rate: 91.0%) and 1.3 mm Φ (new total reduction rate: 95.3%) for expansion purposes. Was taken. On the other hand, about the hot-rolled steel wire material of (ii) SWCH10A correspondence of the comparative example 2, and SWCH18A correspondence of the comparative example 3, it cold-rolled and manufactured the steel wire. Cold rolling conditions were the same as in the said Example 2 (refer Table 6. The rolling temperature is less than 200 degreeC). In this cold rolling step, steel wires of 3.3 mm Φ (drawn total reduction rate: 69.8%), 2.3 mm Φ (drawn total reduction rate: 85.3%), and 1.3 mm Φ (drawn total reduction rate: 95.3%) as they are Test materials were taken.

The following test was done about the said test material.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test

2) Recess molding test of machine screw: This is also the same as described above. However, about the comparative example 2, about the steel wire of 1.3 mm (phi) diameter, the steel wire test material which cold-rolled as it was spheroidized annealing process was prepared, and the test material which improved cold workability was prepared also about this, The recess molding test of the screw was done.

3) Torsional Torque Test of Machine Screws: As described above, a torsional torque test was conducted for the formation of the M1.6 pan head machine screw by cold rolling and rolling from a steel wire of 1.3 mm Φ.

The test results are shown in Table 13.

exam C (mass%) Test piece wire diameter (mmΦ) Total cold working area (%) Industrial transformation Nodular annealing Tensile Strength TS (Mpa) Area reduction RA (%) Recess Formability of Machine Screws Torsional Breaking Torque (kgf × cm) Comparative example 0.04 2.1 Freshness: 87.8 2.10 none 814 64.0 - - 1.8 Freshness: 91.0 2.41 none 857 64.7 - - 1.3 Freshness: 95.3 3.06 none 962 64.9 Good 2.35 Comparative Example 2 0.09 3.3 Rolled: 69.8 1.20 none 783 - - - 2.3 Rolled: 85.3 1.92 none 828 64.8 - - 1.3 Rolled: 95.3 3.06 none 1025 62.5 Sometimes crack 2.43 has exist - - Good 2.24 Comparative Example 3 0.18 3.3 Rolled: 69.8 1.20 none 868 - - - 2.3 Rolled: 85.3 1.92 none 934 58.0 - - 1.3 Rolled: 95.3 3.06 none 1176 58.9 crack - has exist - - crack -

The cold working total area ratio represents the total area ratio by cold drawing or cold rolling.

The following are confirmed from the said test result. That is, these test materials are steel wire test materials obtained by the test procedures of Comparative Examples 1 to 3 outside the scope of the invention of the present application, and the C content is 0.04 to 0.18%. For the steel wire prepared by hot rolling, cold drawing or cold rolling is performed, and as the total reduction ratio increases, the tensile strength TS increases, and the area decrease RA decreases (FIGS. 4 and 5 described later). ). It is confirmed that the total reduction ratio for the tensile strength TS exceeding 1000 MPa is almost 95.3% corresponding to the wire diameter of 1.3 mm Φ in Comparative Examples 2 and 3. However, the behavior of area reduction RA when the tensile strength TS exceeds 1000 MPa is 62.5 to 64.4% from 85.9 to 83.0% (see Table 12, Comparative Examples 2 and 3) before cold rolling, which is approximately 20%. It is causing a deterioration. On the other hand, in Comparative Example 1, since the tensile strength TS was as low as 350 MPa for cold drawn steel wire, the tensile strength TS stayed at 962 MPa even at a 1.3 mm Φ wire diameter of 95.3%. Nevertheless, the area reduction RA has declined significantly from 80.1% to 64.9%.

[IV] Comparison and Review of Test Results of Examples 1-3 and Comparative Examples 1-3

[IV] (1) Tensile strength TS and area reduction RA

Also in Examples 1-3 and Comparative Examples 1-3, tensile strength TS rises and area decrease RA falls with the increase of the total reduction ratio by cold work. The total reduction rate R is expressed as a value obtained by converting the above-described industrial strain e (by the above formula (3)), and this is the x-axis, and the change in tensile strength TS or area decrease RA with respect to the change in industrial strain e The state of is shown to FIG. 4 and FIG. 5, respectively.

First, as can be seen from FIG. 4 and FIG. 5, in Examples 1 to 3, the tensile strength TS is 700 to 800 steel in the raw material (6.0 mmΦ steel wire: warm rolled material, e = 0). From the level of MPa, the linearly significant rise is shown from the diameter of 1.3 mm Φ steel wire (total reduction rate R = 95.3%, e = 3.06) to the level of 1200 to 1570 MPa.

Thus, according to the approximate increase of the tensile strength TS, which is very large: 500 to 770 MPa, the area reduction RA is about 75 to 80% in the material to 60 to 75% in the wire diameter of 1.3 mm Φ steel wire. The amount of degradation remains at about 10%. In contrast, in Comparative Examples 1 to 3, the tensile strength TS is 1.3mmΦ steel wire diameter (total reduction ratio R = 95.3) from 350 to 550 MPa level in the raw material (6.0 mmΦ steel wire: warm rolled material, e = 0). %, e = 3.06), and it has risen substantially linearly to the level of 1000 steel-1150 steel MPa. Approximate amount of increase: 600 to 650 MPa, area reduction RA ranges from 80 to 85% in the material, 65 to 70% in the diameter of 1.3mm Φ steel wire, and the approximate amount of reduction is about 20%. As compared with Examples 1 to 3, it is larger.

6, the relationship between tensile strength TS and area reduction RA was also shown about Examples 1-3 and Comparative Examples 1-3. Thereby, the advantageous property of the intensity | strength balance in an Example is clear. That is, in Examples, the tensile strength TS of the material is already significantly higher than that of the comparative example, and is significantly increased by cold working. Thus, a high strength exceeding 1500 MPa is also obtained, but in the comparative example, the tensile strength of the material is obtained. Since TS remained at the conventional level, even by the increase of the tensile strength TS by cold working, it was just below 1200 MPa level. In addition, in the Example, the amount of reduction of the area reduction RA due to the high strength is significantly smaller than that of the comparative example, and the level of the area reduction RA after the reduction is also greater than the level in the comparative example. The sex was confirmed. In this way, in the steel wire which concerns on this invention of this application, while being high intensity | strength, ductility is maintained at a very high level, and the thing excellent in the strength-ductility balance is obtained.

[IV] <2> M1.6 Formability of Cross-Head Recessed Panhead Machine Screws

On the other hand, according to the recess formability test, in Comparative Examples 2 and 3 in which the tensile strength TS exceeded 1000 MPa, Comparative Example 2 in which the test material was subjected to spheroidizing annealing in advance, was good without generating a recess crack. In Example 3, cracks are generated even when the spheroidizing annealing treatment is performed. When spheroidizing annealing is not performed while cold working, recess cracks generate | occur | produce in both the comparative examples 2 and 3. However, in Comparative Example 1 (which is 962 MPa at a line diameter of 1.3 mm Φ of 95.3% of total reduction factor), the tensile strength TS is less than 1000 MPa, and the recess crack is good.

The torsional breaking torque is about 2.3 kgf × cm and does not reach the desired level of 3.0 kgf × cm even when the spherical annealing treatment in Comparative Example 1 or Comparative Example 2 in which the recess crack does not occur is performed.

As described above, in the comparative examples outside the scope of the invention of the present application, if the total reduction ratio of cold drawing or cold rolling to the raw material increases and the tensile strength rises above a certain value, unless appropriate softening treatment such as spheroidization annealing is performed, In the recess forming of the M1.6 pan head machine screw, which requires very severe cold pressing, cracks are generated. On the other hand, in an Example, it is confirmed that a crack does not generate | occur | produce even in the recess shaping | molding which such a severe cold rolling property is required even if it is cold drawing or cold rolling which does not perform spheroidization annealing. From the viewpoints of cold workability other than the special cold pressing composition, even when the level of area reduction RA is used as an index, it is confirmed that Examples 1-3 are superior to Comparative Examples 1-3.

Next, the comparison between Examples 1 to 3 and Comparative Examples 1 to 3 is referred to as the difference between the steel materials, and according to the method for producing high strength steel according to the invention of the present application, the C content is almost 0.03 mass%. Using steel as a material, a steel wire having excellent cold-tensionability, which can maintain a high tensile strength TS of 1000 MPa or more, and a significantly reduced area RA, for example, 65% or more, can be formed by spheroidizing annealing. It is confirmed that it can be obtained in the state of cold working as it is without.

In FIG. 7, the level of tensile strength TS with respect to C content of steel wire is shown, and the level of area | region reduction RA with respect to C content of steel wire is shown layered according to an Example and a comparative example. Here, it shows about the case where a wire diameter is 1.3 mm (industrial strain is 3.06) as an example at the time of making a cold work rate into fixed conditions. According to this, even if C content is comparatively lower than a comparative example in an Example, it is confirmed that tensile strength TS is high and area reduction RA is more than the equivalent level.

[IV] <3> Comparative Example

As the second group of the comparative example, the raw screw and the carburizing-quenching screw made from commercially available SWCH16A steel wire manufactured by the prior art were referred to as Comparative Example 4. This screw is a M1.6 panhead machine screw, the chemical composition of which is shown in Table 14.

Sample provider JIS equivalent of ingredient Chemical composition (mass%) C Si Mn P S sol.Al Comparative Example 4 SWCH15A 0.16 0.04 0.74 0.005 0.008 0.30

The manufacturing method of this M1.6 pan head machine screw is according to the prior art, a steel wire is manufactured by hot rolling, and then cold drawn by the prior art to produce a 1.3 mm Φ steel wire, which is subjected to a spheroidizing annealing treatment. After the cold rolling has been improved, the cold forming and rolling have been carried out by forming an M1.6 pan head machine screw (unprocessed screw), and the untreated screw is subjected to carburizing quenching and tempering treatment, thereby providing a predetermined strength. Two types of M1.6 panhead machine screws (carburizing quenching screws) are given.

As the amplification test of Comparative Example 4, a torsional torque test (the test method is as described above) was performed using the unprocessed screw and the carburized quenched screw as test materials. The test results are shown in Table 15.

exam JIS equivalent of ingredient C content (mass%) Screw type Temper treatment Vickers Hardness Hv (-) Torsional Breaking Torque (kgf × cm) Comparative Example 4 SWCH16A 0.16 M1.6 Panhead Machine Screw Not processed - 1.82 Carburizing Quenching & Tempering 330 2.96

The following are confirmed from the said test result. That is, in the comparative example 4 manufactured by the manufacturing method of the range foil of the invention of this application, the torsional breaking torque of the M1.6 pan head machine screw was a low value of 1.82 kgf * cm about the unprocessed thread test material. In the carburized quenching screw, a high torsional strength of 2.96 kgf · cm is obtained and has a preferable torsional strength.

In the torsional torque tests conducted in Comparative Examples 1 and 2 described above, the levels were 2.25 to 2.43 kgf · cm, but in Examples 1 and 2 described above, they were 3.38 kgf · cm and 2.923.38 kgf · cm, respectively. The level of the torsional breaking torque of these Examples is the same level as the level of the comparative example 4 which is a commercial item, and almost all satisfy 3.0 kgf * cm of the preferable torsional breaking torque level.

From the above test, the industrial usefulness of the high strength steel wire or steel bar excellent in the cold workability of this invention of this application, and high strength molded article, and the manufacturing method for manufacturing these were confirmed.

[V] Examples <Carbon steels to low alloy steels having a main phase of the metal crystal structure substantially free of cementite-containing C content at a point Ae1 or less than the solid solution carbon of the ferrite phase, or C content of 0.01O% by mass or less>

[V]-<1> Test tips common to Examples 1 to 5 and Examples 6 to 9

Examples 1 to 9 within the scope of the invention of this application were tested as follows.

Each steel which has the chemical composition of components No. 1-5 shown in Table 16 was melted using the vacuum melting furnace, and casted into steel ingot. The component characteristics here are the ultra low carbon steels which changed carbon C within the range of C content which makes carbon C 0.001 to 0.01109 mass%, and the component No. 4 is Si: 0.01 mass% high compared with others. And component No. 5 to N = 0.0080 mass%, which is a little higher than others.

Ingredient No. Sample provider Chemical composition (mass%) C Si Mn P S N sol.Al One Example 1,6 0.0014 0.30 0.20 0.009 0.001 0.0031 0.0028 2 Example 2,7 0.0047 0.30 0.20 0.009 0.001 0.0024 0.026 3 Example 3,8 0.0098 0.30 0.20 0.009 0.001 0.0025 0.028 4 Example 4,9 0.0109 1.01 0.19 0.010 0.001 0.0020 0.033 5 Example 5 0.0095 0.30 0.19 0.009 0.001 0.0080 0.029

The obtained ingot was shape | molded by the steel bar which is 80 mm square by hot forging. The metal structure of these steel bars consisted of ferrite, and the average particle diameter of the ferrite in C direction cross section was about 20 micrometers or less. The raw material for rolling was extract | collected from each said 80 mm square steel bar, and it shape | molded it to 18 mm four directions by multipass caliber rolling in warm direction, and it cooled by water and prepared the steel bar. This warm rolling prepares the raw material for steel wires or steel bars which concerns on invention of this application, on condition that the average grain size in the cross section perpendicular | vertical to the longitudinal direction of the material obtained by the said warm rolling will be 3 micrometers or less. It was done.

As mentioned above, it performed on the following conditions as a method of the warm caliber rolling that an average crystal grain diameter becomes 3 micrometers or less. After heating the rolling material of 80 mm square formed by the hot forging at 550 ° C., the diamond-like caliber roll was first shown in Table 2 in the range of rolling temperature of 450 to 530 ° C. ), Warm rolling of l9 passes with a reduction rate of about 1% for each pass was performed to form a 24 mm square. Subsequently, warm rolling was performed by an elliptical caliber roll having a maximum short axis length of 11 mm and a long axis length of 52 mm (a, b, respectively, R = 64 mm in FIG. 2 and the lower figure), and finally a warm roll of one pass with a square caliber roll. And all directions were molded into 18 mm in 21 passes. The total reduction ratio from the material for warm rolling (80 mm square) to the material 18 mm square is 95%. Table 2 shows the outline of the pass schedule.

In the one-pass warm rolling with the elliptical caliber roll, since the rod is rolled with the elliptical caliber roll with 24 mm in all directions, the material after the rolling with respect to the stool length of the cross section in the C direction of the pre-rolled material is 24 mm. The ratio of the maximum shortening length of 11 mm in the C-direction cross section is considerably small, (11 mm? / 24 mm?) X 100 = 46%, and the reduction ratio calculated from the hole-shaped dimension at this time is quite large (38%). Therefore, the warm rolling of 1 pass | pass by this elliptical caliber roll is a condition which accelerates | miniaturizes the refinement | miniaturization of the ferrite particle diameter in the steel bar of 18 mm square after completion | finish of warm rolling. Moreover, in the rolling process by the diamond-type caliber roll up to the 19th pass, in order to make the cross-sectional shape of a material as close to a square as possible, rolling which passes two consecutive passes through the same caliber roll (so-called "double pass" double pass) ”), and each double pass was counted as two passes, respectively.

Further, the material was rotated around the longitudinal axis in each pass of the rolling to change the pressing direction, and multipass rolling was performed in multiple directions. In addition, processing heat is also applied, and in the relatively low temperature side region even in the rolling temperature region of the warm rolling, the amount of heat dissipation is relatively small, and there is no need for intermediate heating due to the temperature drop of the material during the rolling. Next, the steel bar of 18 mm square four directions prepared by the warm rolling method mentioned above was reduced by cutting, and was processed into the wire rod of diameter 6.0 mm phi.

Here, the reason for reducing the diameter by cutting from 18 mm Φ to 6.0 mm Φ in all directions is, as will be described later, in this embodiment, M1.6 pan head machine screw specified in JIS B 1111 as the use of steel wire (effective section of the screw portion). Since the diameter of 1.27 mm (phi) was selected, it is for making into a raw material which diameter 1.3 mm (phi) is obtained by the cold drawing process of 95% of target drawing ratio, or the cold rolling process of 95% of target total reduction ratio. The M1.6 panhead machine screw is selected for the M1.6 panhead, which will be described later because very cold cold forming properties are required in order to pressure-form the recesses (torques that give torque to the driver) in the head. This is for evaluating whether or not the machine screw has a particularly good cold rolling capability by the cross-shaped "recess molding test".

In addition, in the above, the particle diameter in the C direction cross section of the bar steel which is 18 mm (phi) square prepared by the warm rolling was equalized over the whole surface.

These 6.0 mm diameter amplification test materials (hereinafter referred to as "A0 group test materials") are sampled and composed of five kinds corresponding to the components No. 1 to 5 in Table 16, and the following items are tested. It was.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test: In this test, a high level balance in strength and cold workability which is particularly excellent in strength as well as in cold workability. It is an object to obtain basic data for evaluating whether or not the material having a.

2) Hardness measurement test by Vickers hardness tester: As one of the strength characteristics, it is effective when the tensile test piece is difficult to collect in order to confirm correlation with tensile strength. It carried out based on the method prescribed | regulated to JISZ2244.

3) Measurement test of ferrite particle size (d) by microscope test: Prepare a suitable speculum test piece from each test material, and measure the average particle diameter of the ferrite constituting the columnar phase with the microstructure of the metal crystal in the longitudinal direction of the test material (the above is 18mm in all directions). The average ferrite particle size of the cross section (cross section in the C direction) perpendicular to the longitudinal direction of the steel bar is measured. At that time, the microstructure in the L direction cross section was actually observed, and the average ferrite particle diameter of the C direction cross section was calculated | required. Hereinafter, it is the same in this specification.

Table 17 shows the test results for the warm rolled material.

Examination Ingredient No. Sample provider C (mass%) Manufacture process Test piece diameter (mm) Tensile Strength TS (Mpa) Area reduction RA (%) Vickers Hardness Hv (-) Ferrite Particle Size d (μm) AO Group (Warm Rolled Material) One Example 1, Example 6 0.0014 Warm rolling 6.0Φ 635 81.9 221 0.9 2 Example 2, Example 7 0.0047 665 80.0 226 0.8 3 Example 3, Example 8 0.0098 795 78.1 234 0.7 4 Example 4, Example 9 0.0109 760 80.7 233 0.7 5 Example 5 0.0095 710 80.1 210 0.7

From the test result of Table 17, the following matters are confirmed. An AO group test material is an amplification test material of the raw material provided for the cold working performed in Examples 1-9. The AO group test material is a material prepared by warm rolling that satisfies the preparation conditions (manufacturing conditions) of the raw material (steel wire) in the structural requirements of the method of manufacturing a high strength steel wire or steel bar having excellent cold workability according to the invention of the present application. In addition, the chemical component composition of the raw material has a cementite-free carbon steel component. Therefore, the microstructure of a metal crystal is cementite-free, and the fine particle whose average ferrite particle diameter is 0.7-0.9 micrometer is obtained. Therefore, it is confirmed that the high strength of the tensile strength TS of 635 MPa or more is ensured, and the extremely high level of characteristic of area reduction RA of 78% or more is obtained, and the balance material is excellent in strength and formability. What is obtained by the manufacturing method of the high strength steel wire or steel bar excellent in cold workability which concerns on this invention of this application, and what is obtained by this can be made by cold-processing the raw material which combines such material characteristics. In particular, it is also confirmed that even in the ultra low carbon steel having a C content of 0.0014 to 0.0109 mass% or less, it has a high level of 600 MPa or more in tensile strength.

[V] <2> Tests of Examples 1 to 5 and Examples 6 to 9

Next, each 6.0 mmΦ steel wire rod after using the A0 group test specimen was used, and in Examples 1 to 5 by cold drawing, and in Examples 6 to 9 by cold rolling, all from 6.0 mm Φ to 1.3 mm Φ The test which cold-processed and manufactures a steel wire was done.

[V] <2> -1) Examples 1 to 5 (Manufacturing Test of Steel Wire by Cold Drawn)

The test which manufactures steel wire by making it the raw material of five 6.0 mmΦ steel wires of the components No. 1-5 (refer Table 16) prepared by the above-mentioned warm rolling, and drawing to 1.3 mmΦ by cold drawing (hereinafter, respectively) Was referred to as "Examples 1 to 5". The conditions of cold drawing in these Examples are as follows. That is, as shown in Table 18, the 6.0 mmΦ steel wire material (steel wire material which was processed to 18 mmΦ by warm rolling, and then cut into 6.0 mmΦ) as normal temperature is shown to the die No.1-No. The wires were drawn in sequence by 17 wire dies to produce steel wires of 1.3 mm Φ. The material temperature in the fresh wire was less than 200 ° C.

Cold Drawn of Examples 1-5 Die No. 1 to 11 12 to 13 14-17 Steel wire diameter after drawing (mmΦ) 5.6 ^{*) to} 2.1Φ 1.9 to 1.8Φ 1.6 to 1.3Φ Linear diameter of test material (mmΦ) 2.1Φ 1.8Φ 1.3Φ Fresh gross reduction rate (%) at the test sample collection stage 87.8 91.0 95.3 Industrial variation e (%) of test sample collection 2.10 2.41 3.06

Note *) However, the starting diameter of the drawing material is 6.0 mm Φ

In the drawing process of all these examples, it was easy to wire from 6.0 mm phi to 1.3 mm phi, without performing any spheroidization annealing or other softening process. In the meantime, in each step of 2.1 mm Φ (drawn total reduction rate: 87.8%), 1.8 mm Φ (drawn total reduction rate: 91.0%) and 1.3 mm Φ (drawn total reduction rate: 95.3%), the test material for expansion as it is (as follows) , "A1 group test piece"). In addition, the A1 group test specimens each had five levels of linear diameters for each of the five types of Examples 1 to 5, and consisted of 5 types x 3 = 15 types in total. In addition, among these, the test piece which cold-formed the M1.6 pan head machine screw was done about the 1.3 mm (PHI) test material. About the test material "A1 group test material" of Examples 1-5, the following items were tested.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test (as described above)

2) Hardness test by Vickers hardness tester (as described above)

3) Measurement test of average ferrite particle size (d) by microscope test (as described above)

4) Recess molding test of machine screw: This was done only for steel wire of 1.3mmΦ wire diameter. As described above, the steel wire having a wire diameter of 1.3 mm Φ is preformed by header processing in the manufacturing process of the M1.6 pan head machine screw specified in JIS B 1111, and then a predetermined cross-shaped recess (on the driver By this, grooves such as cross shapes for fastening the screws are formed by cold pressing. It is a test which observes the situation which a crack generate | occur | produces in this recess at the time of this shaping | molding with a magnifying glass of 10 times. In general, the occurrence of recess cracks varies greatly depending on the recess shape of the machine screw, but the cross-shaped recess forming of the M1.6 panhead machine screw is a very harsh pressure forming and is a practical test, and is particularly excellent. It was set as the evaluation test of cold rolling. "Good" and micro cracks were found that no cracks were found, but "good" and "good" cracks were found to be "good cracks" and "good cracks".

5) Torsional Torque Test of Machine Screws: From a steel wire with a wire diameter of 1.3 mm Φ, M1.6 panhead machine screws are prepared by forming a screw portion by cold rolling on the intermediate screw recessed as described above. Subsequently, follow the method specified in 5.4 "Torsion Test" of JIS B 1060 "Mechanical Properties and Performance of Carburizing Quenching Tempering" and apply torque until the screw is broken by an appropriate torque measuring device. Increase The torque value (breaking torque (kgf · cm)) required to cause breakage was measured. The purpose of this test is to evaluate "torsional strength" which is one of the characteristics of mechanical properties for fasteners such as screws and bolts. Hereinafter, the same in this specification. M1.6 In the case of panhead machine screws, it is desirable that the breaking torque be more than 3.Okgf · cm

6) Torsional Delay Fracture Test of Machine Screw: M1.6 panhead machine screw prepared from steel wire of 1.3mmΦ wire diameter is 70% of breaking torque value obtained by breaking torque test as shown in the picture of Figure 9 The test piece was closed and set in a bent state, and the delayed fracture resistance was evaluated by whether or not screw break occurred within 72 hours. The number of sets of torsional specimens is ten. In addition, this torsional delayed breaking test was carried out only for Example 2. The test results are shown in Table 19 and Table 20.

Examination exam Ingredient No. C (mass%) Test piece wire diameter (mm) Fresh gross reduction rate (%) transform Tensile Strength TS (Mpa) Area reduction RA (%) A1 group Example 1 One 0.0014 2.1Φ 87.8 2.10 1070 81.2 Example 2 2 0.0047 1125 78.7 Example 3 3 0.0098 1214 73.1 Example 4 4 0.0109 1252 73.1 Example 5 5 0.0095 1220 74.5 Examination exam Ingredient No. C (mass%) Test piece wire diameter (mm) Fresh gross reduction rate (%) transform Tensile Strength TS (Mpa) Area reduction RA (%) A1 group Example 1 One 0.0014 1.8Φ 91.0 2.41 1142 76.6 Example 2 2 0.0047 1199 76.1 Example 3 3 0.0098 1247 73.9 Example 4 4 0.0109 1322 69.8 Example 5 5 0.0095 1272 72.1

From the test result of Table 19 and Table 20, the following is confirmed. That is, first, all the A1 group test materials are the test materials collected from the steel wire obtained by the Example which falls within the scope of the invention of this application. More specifically, the test group A1 has a very low C content (C: 0.0014-0.0109%), and as described above, cementite-free TS with fine cementite (average particle diameter d ≦ 0.9 μm) containing no cementite. Cold drawing is performed by the total reduction ratio of 88% or more of the material (steel wire) which has a high level of area reduction RA and excellent balance. Therefore, also in the obtained steel wire, in any of Examples 1-5, the tensile strength TS rises remarkably with the increase of the total reduction rate by cold drawing. Nevertheless, the amount of reduction in area reduction RA is very small. Although this state is shown in FIG. 10 and FIG. 11 (these figures also show the result of the comparative example 1-the comparative example 3 mentioned later), It is clear when these two figures are referred together collectively. Here, in FIG. 10 and FIG. 11, the total reduction ratio R by cold drawing on the horizontal axis was described by the value (based on said Formula (3)) which was converted into the above industrial deformation e. In addition, these two tables together describe the industrial variant e. This also applies to the following.

As can be seen from this, the tensile strength TS is from the level of 635 to 795 MPa of the material, from 87.8% to 1070 to 1252 MPa, from 81.0% to 1142 to 1322 MPa, In this 95.3%, it is remarkably raised to the level of 1370-1568 MPa. Despite the remarkable increase in tensile strength TS, the decrease in area reduction RA is very small. In other words, the level of fresh material was 78.1 ~ 81.9%, the fresh gross reduction rate was 87.8% to 73.1 ~ 81.2% level, the fresh gross reduction rate was 91.0% to 69.8 ~ 76.6% level, and the fresh gross reduction rate was 95.3%. Although the decrease to 62.1 ~ 71.8% level, the decrease is very small. In addition, since it contains no cementite, the softening process, such as spheroidization annealing, in the process between them is not performed at all.

In addition, the relationship between the industrial strain ε and the tensile strength from these two figures shows that the tensile strength TS is already high at the level of 635 to 795 MPa in the material, and the tensile strength TS is further increased by slight deformation. do. For example, according to Example 3, it can be confirmed that even by cold working of industrial strain ε = 0.17, a high strength such as C content exceeding 800 MPa is obtained at 0.0095 mass%. Since the total wire reduction rate R when epsilon = 0.17 is calculated by 17%, the wire diameter of the steel wire at this time is 5.5 mmΦ. In this embodiment, since the diameter (corresponding to the diameter of the steel) of the raw material immediately before cold drawing was set to 6.0 mmΦ, by setting this larger, steel wires of more than 800 MPa can be manufactured even in a thick wire diameter of 5.5 mmΦ or more. The area reduction is then secured in excess of 75%. 12 shows the relationship between the tensile strength TS and the area reduction RA. According to the same figure, (1) TS ≥ 1000 MPa, RA ≥ 70% is secured, (2) TS ≥ 1200 MPa, RA ≥ 65%, or (3) TS ≥ 1500 MPa, RA ≥ 60% It is confirmed that the production of high strength steel wires or bars excellent in the balance between strength and ductility is possible.

As described above, it is confirmed that the high-strength steel excellent in the cold workability according to the invention of the present application described above is in a state of being cold drawn and that the above-described material characteristics are obtained in a steel wire which is not subjected to temper treatment such as quenching and tempering. . In addition, the crystal structure of the steel wire having such excellent material properties is cementite-free ferrite showing a form extending in the form of a bamboo structure in the direction of cold drawing, and the average ferrite in the C-direction cross section of the steel wire having a diameter of 1.3 mm Φ. The particle diameter consists of ultrafine particles of 138 to 175 nm (see Table 20). 13 shows a tissue photograph by TEM (transmission electron microscope) in Example 2. FIG. This average ferrite particle diameter is 150 nm. Here, from the idea that the ferrite particle size in the C-direction cross section after the cold working is controlled by the amount of processing deformation, the measured value of the particle size before and after cold working is examined. For example, in Example 2, the average ferrite particle diameter in the C direction cross section in the steel wire material (steel wire just before cold work start) prepared by the warm rolling was 0.8 micrometer (refer Table 17).

Therefore, the average ferrite grain size (referred to = d2) of the C-direction cross section of the steel wire having the chemical component composition and the production history of the steel wire rod according to the present embodiment,

Equation 5 below:

d2 = (1-R / 100) 1/2 x d1 (5)

(However, R: total section reduction rate (%) by cold working

    d1: It estimates by the ferrite particle diameter of C cross section immediately before cold work start. Here, R is calculated by the total reduction ratio from the wire diameter of 6.0 mm Φ to the wire diameter of 1.3 mm Φ of the steel wire, and is R = 95.3%. d1 is calculated from d2 to 173 nm from 0.8. This calculated value of 173 nm is in good agreement with the measured value of 150 nm.

Therefore, in the manufacturing method of the steel wire or the steel bar which concerns on this invention, when manufacturing a steel wire by cold working the steel wire material of a warm rolling material, it is a control means of the ferrite particle diameter in the C direction cross section of the said steel wire material. It is effective to use Formula (5) above. Next, the steel wire according to the invention of the present application manufactured as described above is formed in a recess, which is a molding process in which a very severe cold press is performed, such as an M1.6 pan head machine screw, in a state without temper treatment such as quenching and tempering. Also, Example 1 and Example 2 were all favorable, and Example 3 also reached the level which is almost no problem. And the M1.6 pan head machine screw formed by the cold working method called cold rolling and cold rolling from the steel wire which has such excellent cold rolling property has a high torsional strength of nearly 3.0 kgf * cm as the torsional breaking torque. It is confirmed.

[V] <2> -2) Examples 6-9 (Manufacturing test of steel wire by cold rolling)

Similarly, the test which manufactures a steel wire by making it into the raw material of four types of 6.0 mm (3) steel wires of components No. 1-4 (refer Table 16) prepared by the above-mentioned warm rolling, and drawing to 1.3 mm (Φ) by cold rolling ( Hereinafter, each was called "Example 6-9." In Examples 1 to 5, the warm-rolled steel wire material was cold-fresh, whereas in these Examples 6 to 9, the method of manufacturing the steel wire differs in that the warm-rolled steel wire material was cold rolled. The conditions of this cold rolling are as follows. 6.0 mmΦ steel wire material (steel wire material processed to 18 mmΦ by warm rolling, and then cut into 6.0 mmΦ as mentioned above) of normal temperature as shown in Table 21, each in the 1st process-the 3rd process It cold-rolled with the combine roll.

Cold rolling of Examples 6-9 Rolling process No. 1st process 2nd process 3rd process Number of passes 8 pass 10 pass 5 pass Steel wire diameter after rolling (mmΦ) 5.7 *) to 3.3Φ 3.1 to 1.8Φ 1.6 to 1.3Φ Linear diameter of test material (mmΦ) 3.3Φ 2.3Φ, 1.8Φ 1.3Φ Total draw reduction rate (%) of test sample collection 69.8 85.3, 91.3 95.3 Industrial variation e (%) of test sample collection 1.20 1.92, 2.41 3.06

Note *) However, the starting diameter of the rolled material is 6.0 mm Φ

That is, rolling from 6.0 mm Φ to 3.3 mm Φ in 8 passes of the first step, rolling from 3.3 mm Φ to 1.8 mm Φ in 10 passes of the second step, and rolling from 1.8 mm Φ to 1.3 mm Φ in 5 passes of the third step. To prepare a steel wire. The material temperature during rolling was less than 200 degreeC. In the rolling process of all these examples, it could cold-roll from 6.0 mm phi to 1.3 mm phi without performing softening process other than integral spherical annealing. In the meantime, as test materials for expansion, 4 of 3.3 mm Φ (total reduction ratio: 69.8%), 2.3 mm Φ (total reduction ratio: 85.3%), 1.8 mm Φ (total reduction ratio: 91.0%), and 1.3 mm Φ (total reduction ratio: 95.3%) In the step, the steel wire test material (hereinafter referred to as "A2 group test material") as it was rolled out was collected. In addition, the A2 group test specimens have a linear diameter of four levels for each of the four types of Examples 6 to 9, and are composed of four kinds in total and four types of four kinds. In addition, among these, the test piece which cold-formed the M1.6 pan head machine screw was done about the 1.3 mm (PHI) test material.

About the test material (A2 group test material) of Examples 6-9, the test of the following item was done.

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test (as described above)

2) Hardness test by Vickers hardness tester (as described above)

3) Recess forming test for machine screws: Only steel wire with a diameter of 1.3 mm Φ is applied (as described above).

4) Torsional Torque Test of Machine Screws: Only M1.6 panhead machine screws (as described above) are shown in Table 22 and Table 23.

Examination exam Ingredient No. C (mass%) Test piece wire diameter (mm) Total rolling reduction rate (%) transform Tensile Strength TS (Mpa) Area reduction RA (%) A2 group Example 6 One 0.0014 3.3Φ 69.8% 1.20 773 - Example 7 2 0.0047 836 - Example 8 3 0.0098 895 - Example 9 4 0.0109 999 - Examination exam Ingredient No. C (mass%) Test piece wire diameter (mm) Total rolling reduction rate (%) transform Tensile Strength TS (Mpa) Area reduction RA (%) A2 group Example 6 One 0.0014 2.3Φ 85.3% 1.92 875 83.2 Example 7 2 0.0047 968 82.7 Example 8 3 0.0098 1001 76.9 Example 9 4 0.0109 1094 78.5

From the test results of Table 22 and Table 23, the followings were confirmed. That is, first, all the A2 group test materials are the test materials collected from the steel wire obtained by the Example which falls within the scope of the invention of this application. The steel wires, which are the raw materials of the steel wires, are the same as those of Examples 1 to 5, and the C content produced by appropriate warm rolling is very low (C: 0.0014 to 0.0109 mass%), and the crystals are cementite-free fine ferrite particles (average). It is a material having a particle diameter d = 0.7 to 0.9 mu m), a high level of tensile strength TS and area reduction RA, and excellent balance. About such a material, the cold rolling by the rolling total reduction rate of 69.8% (when it is 6.0 mm? 3.3 mm) is performed.

As described above, the manufacturing conditions of Examples 6 to 9 differ from those of Examples 1 to 5 by cold rolling instead of cold drawing. The material characteristics of the steel wire obtained in this way were written together to the said FIG. 10, FIG. 11, and FIG. As mentioned above, the tensile strength TS of the obtained steel wire rises remarkably also with the increase of the total reduction rate by cold rolling here.

In addition, although the tensile strength TS is remarkably increased, the amount of decrease in area reduction RA is very small. The change of this material characteristic is the same also in any of Example 6-9, and the result of Example 1-5 is similar. It is also confirmed that the tensile strength TS and the area reduction RA of the cold rolled steel wire maintain a high level, and both have a good balance.

It is confirmed that the superiority of such material characteristics is obtained without performing temper treatment such as quenching and tempering as the state as it is cold rolling. Further, even in cold rolling without performing spheroidizing annealing, in Example 6, Example 7 and Example 8 having a low C content, recess molding of the M1.6 pan head machine screw can be performed, and very cold workability is achieved. It was also confirmed that it is excellent. This material characteristic is also based on Examples 1-3.

Further, in Examples 7 and 8 having such a material characteristic level, excellent torsional breaking torque of almost 3.0 kgf · cm was obtained even in the absence of temper treatment such as quenching and tempering after molding with an M1.6 panhead machine screw. Strength is being exerted. In addition, as described above, in comparison with the results of Examples 1 to 4 and the results of Examples 6 to 9, in the manufacturing method of high strength steel excellent in cold workability according to the invention of the present application, the cold-treated steel wire material It is confirmed that any of a cold drawing method and a cold rolling method may be sufficient as a processing method.

[V] <3> Comparative Example

Next, the following test was done as a comparative example outside the scope of the invention of this application. The comparative example was divided into a first group and a second group.

[V] <3> Comparative Example 1 (Comparative Example 1 to Comparative Example 3)

As a first group of the comparative example, it is a cold-rolled carbon steel wire material specified in JIS G 3507, and is a 6.0 mm Φ steel wire material having respective component compositions equivalent to SWRCH5A, SWRCH10A and SWRCH18 shown in the components No. 6 to 8 of Table 24, A test specimen for expansion (hereinafter referred to as "B0 group test specimen") was sampled from a commercially available steel wire rod which finished processing above the A3 transformation point which is a general hot rolling condition of the prior art, and the following items were tested. .

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test (as described above)

2) Measurement test of ferrite particle size (d) by microscope test (as described above)

These test results are shown in Table 24 and Table 25.

Ingredient No. Sample provider JIS equivalent of ingredient Chemical composition (mass%) C Si Mn P S sol.Al 6 Comparative Example 1 SWCH5A 0.04 0.04 0.33 0.003 0.007 0.028 7 Comparative Example 2 SWCH10A 0.09 0.01 0.30 0.011 0.025 0.030 8 Comparative Example 3 SWCH18A 0.18 0.01 0.79 0.017 0.005 0.040

Examination Ingredient No. Sample provider JIS equivalent of ingredient C content (mass%) Manufacture process Test piece diameter (mm) Tensile Strength TS (MPa) Area reduction RA (%) Average Ferrite Particle Size d (µm) CO Group (Hot Rolled Material) 6 Comparative Example 1 SWCH5 0.04 Hot rolling 6.0Φ 350 80.0 20 7 Comparative Example 2 SWCH10 0.09 430 75.0 17 8 Comparative Example 3 SWCH18 0.18 520 72.0 16

From the test results of Table 24 and Table 25, the followings were confirmed. That is, the B0 group test material is a test material for amplification of the raw material used for the cold working performed by the comparative examples 1-3. This B0 group test material is a material (steel wire material) manufactured by hot rolling which is a preparation condition of the raw material in the manufacturing method of steel out of the scope of the invention of this application. Therefore, the average particle diameter in the C direction cross section of ferrite which is columnar structure of a metal crystal is 16-20 micrometers. This is confirmed to be very large compared with the average ferrite particle diameter (0.7-0.9 micrometer) of the material used as steel wire in Examples 1-9.

Therefore, although C content is remarkably high compared with Example 1-Example 9, area reduction RA is 80.1 to 89.9%, and it is excellent in high level. However, despite the high C content, the tensile strength TS is 350 to 550 MPa, and it is confirmed that the tensile strength TS is significantly lower than the tensile strength TS: 635 to 795 MPa of the steel wire used in Examples 1 to 9. . On the other hand, the steel wire cold-processed to 1.3 mm (phi) by cold drawing or cold rolling was prepared using the said 6.0 mm (phi) hot-rolled steel wire after collecting the said B0 group test material.

(1) First, about the hot-rolled steel wire material of component No. 6 (equivalent to SWCH5A), cold drawing was performed and the steel wire was manufactured. Cold drawing is performed on the same conditions as Example 1-5 (refer Table 18. Drawing temperature is less than 200 degreeC). This is called "Comparative Example 1." In this cold drawing process, the steel wire test material as it is cold drawn as 2.1 mm Φ (new total reduction rate: 87.8%), 1.8 mm Φ (new total reduction rate: 91.0%) and 1.3 mm Φ (new total reduction rate: 95.3%) for expansion purposes. Was taken.

(2) On the other hand, about the hot-rolled steel wire material of component No. 7 (equivalent to SWCH10A) and component No. 8 (equivalent to SWCH18A), it cold-rolled and manufactured the steel wire. Cold rolling conditions are the same as in Examples 6-9 (refer Table 21. The rolling temperature is less than 200 degreeC). In this cold rolling process, 3.3 mm (the total reduction of freshness: 69.8%) for amplification, Cold rolled steel wire test specimens of 2.3 mm Φ (drawn total reduction rate: 85.3%) and 1.3 mm Φ (drawn total reduction rate: 95.3%) were collected. This test is called "Comparative Example 2" and "Comparative Example 3", respectively. In the above, the test materials of Comparative Examples 1-3 were put together, and the following test was done about these test materials called "B1 group test materials."

1) Measurement test of tensile strength (TS) and area reduction (RA) by tensile test (as described above)

2) Recess molding test of machine screws (as described above): For steel wires having a wire diameter of 1.3 mm Φ, a recess forming test of M1.6 pan head machine screws was performed on a test piece as it was cold rolled. . Moreover, about the steel wire of 1.3 mm (phi) diameter of the comparative examples 2 and 3, in addition to the steel wire test material as it is cold-rolled, the test material which carried out the spheroidizing annealing process and improved cold workability was prepared, and this also relates to M1.6. A recess forming test of a panhead machine screw was performed.

3) Torsional Torque Test of Machine Screws (as described above): A torsion torque test was conducted for the formation of M1.6 panhead machine screws by cold rolling and rolling from a steel wire of 1.3 mm Φ.

The test results are shown in Table 26.

Examination exam Ingredient No. C (mass%) Test piece wire diameter (mmΦ) Cold work reduction rate (%) transform Nodular annealing Tensile Strength TS (Mpa) Area reduction RA (%) M1.6 Recess Formability of Machine Screw Torsional Breaking Torque (kgf × cm) C1 group Comparative Example 1 6 0.04 2.1 Freshness: 87.8 2.10 none 814 64.0 - - 1.8 Freshness: 91.0 2.41 none 857 64.7 - - 1.3 Freshness: 95.3 3.06 none 962 64.9 Good 2.35 Comparative Example 2 7 0.09 3.3 Rolled: 69.8 1.20 none 783 - - - 2.3 Rolled: 85.3 1.92 none 828 64.8 - - 1.3 Rolled: 95.3 3.06 none 1025 62.5 Sometimes crack 2.43 has exist - - Good 2.24 Comparative Example 3 8 0.18 3.3 Rolled: 69.8 1.20 none 868 - - - 2.3 Rolled: 85.3 1.92 none 934 58.0 - - 1.3 Rolled: 95.3 3.06 none 1176 58.9 crack - has exist - - crack -

From the test results of Table 26 (Comparative Examples 1 to 3), the following matters were confirmed. B1 group test material is a steel wire test material obtained by the test process of Comparative Examples 1-3 outside the scope of the invention of this application, and C content is the level of 0.04-0.18 mass%. When cold drawing or cold rolling is performed on the raw material (steel wire) prepared by hot rolling, the tensile strength TS increases as the total reduction ratio increases, and the area reduction RA decreases. The total reduction rate for tensile strength TS exceeding 1000 MPa is achieved in 95.3% corresponding to the wire diameter of 1.3 mm Φ in Comparative Examples 2 and 3. However, the area reduction RA at this time is falling to 64.4 to 66.2%. The fall situation from the material of this area reduction RA is 85.9-83.0% → 64.4-62.5%, falling about 20%, and the fall amount is remarkably large. In addition, the level of the area reduction RA value after lowering is also considerably lower than that of the area reduction RA when the tensile strength TS exceeds 1000 MPa in Examples 1 to 9: about 70 to 75% (see Fig. 12). Has become.

Thus, the tendency of the change of the material characteristic that tensile strength rises with the increase of the total reduction rate in cold working with respect to a raw material, and the area reduction RA falls is compared with Example 1-3 also in Comparative Examples 1-3. Same as the case of 9. However, quantitatively, the decrease of the area reduction RA at that time was remarkably small in the case of Examples 1 to 9 ((6 mmΦ warm rolled material: 78.1 to 89.1%) → (1.3 mmΦ cold drawn material: 62.1) -71.8%) or → (1.3mmΦ cold rolled material: 64.0-80.1%) is comparatively large ((6mmΦ hot rolled material: 80.1%) → (1.3mmΦ cold drawn material: 64.9%) in Comparative Examples 1-3. ), (6 mmΦ hot rolled material: 83.0 to 85.9%) → (1.3 mmΦ cold rolled material: 62.5 to 64.4%)).

The change of the said material characteristic was written together in FIGS. 10-12.

In each of these figures, the above matters are further clarified by comparing the examples with the comparative examples. On the other hand, according to the recess formability test of the mechanical screw, even in the case where the tensile strength TS exceeds 1000 MPa, in Comparative Examples 2 and 3, the recess crack does not occur when the test material is subjected to the spheroidizing annealing in advance. Although there exist favorable cases (comparative example 2) (however, a crack generate | occur | produces in the comparative example 3), when the spheroidizing annealing was not performed as it was cold working, recess cracks generate | occur | produce in both the comparative examples 2 and 3. However, in Comparative Example 1 (which is 962 MPa at a line diameter of 1.3 mm Φ of 95.3% of total reduction factor), the tensile strength TS is less than 1000 MPa, and the recess crack is good.

As described above, in the comparative examples outside the scope of the invention of the present application, when the total reduction ratio of cold drawing or cold rolling to the raw material increases and the tensile strength rises above a certain value, an appropriate softening treatment such as spheroidization annealing is not performed. However, cracks occur in the recess forming of M1.6 panhead machine screws, which require very severe cold pressing. On the other hand, in the Example, even if it is cold drawing or cold rolling which does not perform spheroidization annealing, it is confirmed that a crack does not generate | occur | produce in such a strict recess test, unless tensile strength TS exceeds 1500 Mpa fully. In addition, it is confirmed that Examples 1 to 9 are superior to Comparative Examples 1 to 3 even when the level of area reduction RA is used as an index from the viewpoint of cold workability other than the particularly strict cold rolling. Next, in comparison with Examples 1 to 9 and Comparative Examples 1 to 3, the components of the steel are different, according to the method for producing high strength steel according to the invention of the present application, the C content is 0.0014 to 0.0109 mass% The low-carbon steel is used as a material, and the tensile strength TS is, for example, 1000 to 1400 MPa in the high level range, and the area reduction RA can be maintained at 65% or more of the considerably high level. It is confirmed that it can be obtained in the state of cold working as it is (see FIG. 12).

In Fig. 14, the level of tensile strength TS with respect to the C content of the steel wire is shown in the case of the wire diameter of 1.3 mmΦ, and the area reduction of the C content of the steel wire with respect to the C content of the steel wire is shown for the case of the wire diameter of 1.3 mmΦ as in Fig. 1. The graph which compares a level with Examples 1-9 and Comparative Examples 1-3 is shown. In addition, the industrial deformation corresponds to 3.06 under a certain condition of a cold working rate with a diameter of 1.3 mm Φ.

[V] <3> (b) Comparative Group 2 (Comparative Example 4)

As the second group of the comparative example, the raw screw and the carburizing-quenching screw manufactured from commercially available SWCH16A equivalent steel wire manufactured by the prior art were referred to as Comparative Example 4.

This screw is a M1.6 pan head machine screw, the chemical composition of which is shown in component No. 9 of Table 27.

Ingredient No. Sample provider JIS equivalent of ingredient Chemical composition (mass%) C Si Mn P S sol.Al 9 Comparative Example 4 SWCH15A 0.16 0.04 0.74 0.005 0.008 0.030

The production method is a prior art, a steel wire material is manufactured by hot rolling, and then cold drawn by the prior art to produce a steel wire of 1.3 mm Φ, which is subjected to a spheroidizing annealing treatment to improve cold rolling, and then cold M1.6 panhead machine screws that have been formed by M1.6 panhead machine screws by pressing and rolling (unprocessed screws), and that the raw threads are carburized and tempered to impart a predetermined strength. Carburizing and quenching screws). As the amplification test of Comparative Example 4, the torsional torque test (as described above) was performed on the unprocessed screw and the carburized quenched screw as a test material (called "B2 group test material"). The test results are shown in Table 28.

Examination exam Ingredient No. JIS equivalent of ingredient C content (mass%) Screw type Temper treatment Vickers Hardness Hv (-) Torsional Breaking Torque (kgf × cm) C2 Group Comparative Example 4 9 SWCH16A 0.16 M1.6 Panhead Machine Screw Not processed - 1.82 Carburizing Quenching & Tempering 330 2.96

From the said test result, the following is confirmed. In Comparative Example 4 manufactured by a manufacturing method outside the scope of the invention of the present application, for the unprocessed screw test member, the torsional breaking torque of the M1.6 pan head machine screw was a low value of 1.82 kgf · cm, but it was hardened In a screw, a high torsional strength of 2.96 kgf · cm is obtained and has a preferable torsional strength. In the torsional torque test performed in the above-described example, it was 2.63 kgf · cm in Example 6, but in the test conducted in the other examples, all exceeded 2.9 kgf · cm, and it was confirmed that it had sufficiently torsional strength. From the above tests, the industrial usefulness of the high strength steel wire or the steel bar excellent in the cold workability according to the invention of the present application, and the high strength molded article, and the manufacturing method for producing them, the steel wire or the steel bar according to the present invention, and the manufacturing method of the high strength molded article Industrial usefulness was confirmed.

Claims (63)

A high-strength steel wire or steel bar having excellent cold workability, which has a cementite-free ferrite structure with an average particle diameter of 500 nm or less in a vertical section of the steel wire or steel bar in the longitudinal direction. A high-strength steel wire having excellent cold workability, characterized by having a ferrite structure having a C content of not more than the solid-solution limit of the ferritic carbon at the Ae1 point, and having an average particle diameter of 500 nm or less in the vertical section with respect to the longitudinal direction of the steel wire or steel bar or Bar. A high strength steel wire or steel bar having excellent cold workability, wherein the C content is 0.010% by mass or less and has a ferrite structure having an average particle diameter of 500 nm or less in the vertical section in the longitudinal direction of the steel wire or steel bar. The high strength steel wire or steel bar of any one of Claims 1-3 WHEREIN: Tensile strength TS becomes 900 Mpa or more, The high strength steel wire or steel bar excellent in cold workability characterized by the above-mentioned. The high-strength steel wire or steel bar excellent in cold workability as described in claim 4, wherein the tensile strength TS is 1000 MPa or more and the area reduction RA is 70% or more. The high strength steel wire or steel bar excellent in the cold workability as described in Claim 4 in which the tensile strength TS is 1200 Mpa or more, and area reduction RA is 65% or more. The high strength steel wire or steel bar of the high strength steel wire or steel bar of Claim 4 which is added that tensile strength TS is 1500 Mpa or more and area reduction RA is 60% or more. The high strength steel wire or steel bar in any one of Claims 1-3 WHEREIN: Hardness is 285 or more in Vickers hardness Hv, The high strength steel wire or steel bar excellent in cold workability characterized by the above-mentioned. The high-strength steel wire or bar excellent in cold workability according to any one of claims 1 to 8, wherein the average grain diameter of the ferrite structure is 200 nm or less. The high-strength steel wire or the steel bar according to any one of claims 1 to 9, wherein any of the hardenability promoting elements other than Cr and Mo, and the solid solution strengthening elements other than Cu and Ni are unavoidable contents or more. High strength steel wire or steel bar excellent in cold workability, characterized in that it is not contained in. The high strength steel wire or the steel bar according to any one of claims 1 to 10, wherein the Si content is 1.0% by mass or less, and the Mn content is 2.0% by mass or less. Or bar. A high-strength molded article characterized by having a cementite-free ferrite structure with an average particle diameter of at least one cross section in an arbitrary cross section.  A high-strength molded article having a C content of not more than the solid solution limit of carbon in the ferrite phase at the Ae1 point, and having a ferrite structure having an average particle diameter of 500 nm or less in at least one cross-section of an arbitrary cross section. A high-strength molded article having a C content of 0.01% by mass or less and having a ferrite structure having an average particle diameter of 500 nm or less in at least one cross section of an arbitrary cross section. The high strength molded article according to any one of claims 12 to 14, wherein a tensile strength TS of 1000 MPa or more is added. The high strength molded article according to claim 15, wherein a tensile strength TS of 1500 MPa or more is added. The high strength molded article according to claim 15, wherein the hardness is added to Vickers hardness Hv of 285 or more. The high strength molded article according to claim 15, wherein the hardness is added to Vickers hardness HV of 300 or more. The high-strength molded article according to any one of claims 12 to 18, wherein an average particle diameter of the ferrite structure is 200 nm or less. 20. The method according to any one of claims 12 to 19, wherein any one of Cr, Mo, other hardenable promoting elements, and Cu, Ni, and other solid solution strengthening elements is not contained at an unavoidable content or higher. High strength molded products. The high-strength molded article according to any one of claims 12 to 20, wherein the Si content is 1.0% by mass or less, and the Mn content is 2.0% by mass or less. The high strength molded article according to any one of claims 1 to 11, which is made from a high strength steel wire or a bar having excellent cold workability. 23. The high strength molded article according to claim 22, wherein the molded article is manufactured by cold rolling, cold forging and / or cutting. The high strength molded article according to any one of claims 12 to 23, wherein the molded article is not subjected to temper treatment. C content of 0.01% to 0.45% by mass, ferrite structure having an average particle diameter of 500 nm or less in the vertical section in the longitudinal direction of the steel wire or steel bar, with a tensile strength of 700 MPa or more and an area reduction of 65% or more. High strength steel wire or bar with excellent cold workability, characterized by having mechanical properties. The high strength steel wire or the steel bar excellent in cold workability according to claim 25, wherein the area reduction is 70% or more. 27. The high strength steel wire or bar having excellent cold workability according to claim 26, wherein the tensile strength is 1000 MPa or more. C content is over 0.01 to 0.45% by mass, the ferrite structure having a mean particle diameter of 500 nm or less in the vertical section in the longitudinal direction of the steel wire or steel bar as a main phase, tensile strength of 1500 MPa or more, and area reduction of 60% High strength steel wire or steel bar excellent in cold workability, characterized by having the above mechanical properties. Cold workability characterized by having a C content of more than 0.01 to 0.45% by mass, a ferrite structure having a mean particle diameter of 500 nm or less in a vertical section in the longitudinal direction of the steel wire or steel bar, and a hardness of 285 or more in Vickers hardness HV. This excellent high strength steel wire or bar. 30. The high-strength steel wire or steel bar according to any one of claims 25 to 29, wherein the C content exceeds the solid solution limit of the ferritic carbon at the Ae1 point and is 0.45 mass% or less. The steel wire or steel bar excellent in cold workability as described in any one of Claims 25-30 whose average particle diameter of a ferrite structure is 200 nm or less. 32. The cold according to any one of claims 25 to 31, wherein all of Cr and Mo and other hardenable promoting elements, and Cu and Ni and other solid solution strengthening elements are not contained at an unavoidable content or higher. Steel wire or bar with excellent workability. A steel wire or steel bar excellent in cold workability, wherein the Si content is 1.0% by mass or less and the Mn content is 2.0% by mass or less. The C content is more than 0.01 to 0.45% by mass, and the ferrite structure having an average particle diameter of 500 nm or less in at least one cross section of an arbitrary cross section is a main phase, and the hardness in at least one cross section of an arbitrary cross section is Vickers hardness HV. High strength molded article, characterized in that 285 or more. A high-strength molded article having a C content of more than 0.01 to 0.45 mass%, a ferrite structure having an average particle diameter of 500 nm or less in at least one cross section of an arbitrary cross section, and a tensile strength TS of 900 MPa or more. The high-strength molded article according to claim 34 or 35, wherein the average particle diameter is 200 nm or less. 34. The high strength molded article according to any one of claims 25 to 33, which is made from a steel wire or a bar having excellent cold workability. 38. The high strength molded article according to any one of claims 34 to 37, which is produced by cold rolling, cold forging and / or cutting. The high-strength molded article according to any one of claims 34 to 38, wherein the tempering treatment is not performed. The steel ingot, cast piece, steel piece or steel semi-finished product having a cementite-free ferrite structure is subjected to warm processing, and a material having an average grain size of 3 µm or less in a cross section perpendicular to the longitudinal direction is prepared, followed by cold working. And forming a ferrite structure having an average crystal grain diameter of 500 nm or less in a cross section perpendicular to the longitudinal direction. Ingots, castings, slabs or steel semi-finished products having a C content of not more than the solubility limit of ferrite carbon at Ae1 point are warm-processed to prepare a material having an average grain size of 3 µm or less in a cross section perpendicular to the longitudinal direction. And then performing cold working to form a ferrite structure having an average crystal grain diameter of 500 nm or less in a cross section perpendicular to the longitudinal direction. 42. The method for producing a high strength steel wire or steel bar having excellent cold workability according to claim 41, wherein the C content is 0.010 mass% or less. 43. The method for producing a high strength steel wire or steel bar according to any one of claims 40 to 42, wherein the average grain size of the ferrite structure after the cold working is 200 nm or less. 44. The plastic working according to any one of claims 40 to 43, wherein the warm working has a processing temperature in the range of 350 to 800 ° C, and the plastic deformation introduced and remaining in the material by rolling and / or forging is three-dimensional finite. A method for producing a high strength steel wire or steel bar having excellent cold workability, which is a process of forming 0.7 or more by an average plastic deformation in the material calculated by the urea method. The said warm work is a total reduction ratio R represented by following formula (1) by rolling and / or forging, in the said warm processing, in the range of 350-800 degreeC. A method for producing a high strength steel wire or bar having excellent cold workability, characterized in that the processing is 50% or more. R = {(S0-S) / S0} × 100 (1) (Where R is the total reduction rate (%) of the cast piece or steel piece)     S0: C-section cross-sectional area of the casting piece or steel piece immediately before the start of warm working     S: cross-sectional area in the C direction of the material obtained after completion of warm processing) The method for producing a high strength steel wire or steel bar according to any one of claims 40 to 45, wherein the warm work is performed in a plurality of passes and in a plurality of directions. 47. The cold working according to any one of claims 40 to 46, wherein the cold working is carried out in the material by rolling and / or drawing at a processing temperature of less than 350 deg. A method for producing a high strength steel wire or steel bar having excellent cold workability, which is a process of forming 0.05 or more by an average plastic deformation in a material. The said cold working is 5% of the total reduction ratio R 'represented by following formula (2) by rolling and / or drawing, when the said cold working is less than 350 degreeC. The manufacturing method of the high strength steel wire or steel bar excellent in cold workability characterized by the above-mentioned process. R '= {(S0'-S') / S0 '} × 100 (2) (R ': total reduction rate (%) performed on the warm processed material)    S0 ': C-direction cross-sectional area of the material immediately before the start of cold working    S ': C-direction cross-sectional area of the material obtained after the end of cold working) The high strength excellent in cold workability as described in any one of Claims 40-48 which does not contain a spheroidization annealing process and / or a quenching and tempering process in any one of the said warm process and the said cold process. Method of manufacturing steel wire or bar. A high strength steel wire or a steel bar having excellent cold workability manufactured by the manufacturing method according to any one of claims 40 to 49, which is produced by cold rolling, cold forging and / or cutting. Manufacturing method of the molded article. 51. The method for producing a high strength molded article according to claim 50, wherein the molded article is not tempered. 52. The method for producing a high strength molded article according to claim 50 or 51, wherein the molded article is not subjected to stress relief annealing treatment and / or baking treatment. C content: The steel ingot, the casting piece, the steel piece, or the steel semi-finished product which exceeds 0.01 to 0.45 mass% is warm-processed, the material whose average grain size is 3 micrometers or less in the cross section perpendicular | vertical to the longitudinal direction is prepared, and then cold A process for producing a high strength steel wire or steel bar having excellent cold workability, comprising: forming a ferrite columnar structure having an average grain size of 500 nm or less in a cross section perpendicular to the longitudinal direction. 54. The method for producing a high strength steel wire or steel bar having excellent cold workability according to claim 53, wherein said average crystal grain size of the ferrite structure after cold working is 200 nm or less. 55. The method according to claim 53 or 54, wherein the warm working is performed in which the plastic strain flowing into the material by rolling and / or forging and remaining in the processing temperature is in the range of 350 to 800 ° C is calculated by the three-dimensional finite element method. A process for producing a high strength steel wire or steel bar having excellent cold workability, comprising: performing a process of 0.7 or more due to an average plastic deformation in a material for the cast piece or steel piece. 55. The warm working is performed at a working temperature of 350 to 800 ° C, and by rolling and / or forging, the total reduction ratio R represented by the following formula (1) is 50% or more. Process for producing a high strength steel wire or steel bar excellent in cold workability, characterized in that the machining is performed on the cast or steel pieces. R = {(S0-S) / S0} × 100 (1) (Where R is the total reduction rate (%) of the cast piece or steel piece)     S0: C-section cross-sectional area of the casting piece or steel piece immediately before the start of warm working     S: cross-sectional area in the C direction of the material obtained after completion of warm processing) 57. The method for producing a high strength steel wire or steel bar according to any one of claims 53 to 56, wherein the warm working is performed in a plurality of passes and in a plurality of directions. 58. The cold working according to any one of claims 53 to 57, wherein, in the cold working, the plastic deformation that flows into the material by rolling and / or drawing and remains at a processing temperature of less than 350 ° C is calculated by the three-dimensional finite element method. A method for producing a high strength steel wire or a steel bar having excellent cold workability, wherein a processing of 0.05 or more is performed on the warm processed material by an average plastic deformation in the material to be used. The cold working is performed at a processing temperature of less than 350 ° C, and by rolling and / or drawing, the total reduction ratio R 'represented by the following formula (2) is 5% or more. Process for producing a high-strength steel wire or bar excellent in cold workability, characterized in that for performing the warm working material. R '= {(S0'-S') / S0 '} × 100 (2) (R ': total reduction rate (%) performed on the warm processed material)     S0 ': C-direction cross-sectional area of the material immediately before the start of cold working     S ': C-direction cross-sectional area of the material obtained after the end of cold working) 60. The method for producing a high strength steel according to any one of claims 53 to 59, wherein the method for producing high strength steel does not include spheroidizing annealing treatment and / or quenching and tempering treatment in any one of the above-mentioned warm working and cold working. Method for producing high strength steel wire or steel bar having excellent cold workability. A high strength steel wire or a steel bar excellent in cold workability manufactured by the manufacturing method according to any one of claims 53 to 60, which is produced by cold rolling, cold forging and / or cutting. Manufacturing method of the molded article. 64. The method for producing a high strength molded article according to claim 61, wherein the molded article is not tempered. 63. The method for producing a high strength molded article according to claim 61 or 62, wherein the molded article is not subjected to stress relief annealing treatment and / or baking treatment.

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