WO2006057470A1

WO2006057470A1 - Steel wire for cold forging

Info

Publication number: WO2006057470A1
Application number: PCT/KR2004/003106
Authority: WO
Inventors: Soon-Tae Ahn
Original assignee: Samhwa Steel Co., Ltd
Priority date: 2004-11-29
Filing date: 2004-11-29
Publication date: 2006-06-01
Also published as: CA2589006A1

Abstract

Disclosed is a steel wire or bar which is used as a material for bolts and shafts applied to parts for machine structures having high strength. Having excellent cold forging properties, the steel wire consists of 0.10 - 0.40 wt% C, 1.0 wt % or less Si, 0.30 - 2.0 wt% Mn, 0.03 wt% or less P, 0.03 wt % or less S, and the balance of Fe and impurities, and is quenched and tempered so as to have a tensile strength of 70 - 130 Kgf/mm2. A ratio (δ/TS) of area reduction (δ, %) to tensile strength (TS) is 0.4 - 1.2. In the present invention, it is unnecessary to conduct spheroidizing for a long time. Furthermore, it is possible to produce a heat-treated steel wire, which has forging processabilitythat is the same as or superior to spheroidized steel wire, through brief quenching and tempering processes using a high frequency induction heating method, thereby improving productivity.

Description

[DESCRIPTION]

[invention Title]

STEEL WIRE FOR COLD FORGING

[Technical Field] The present invention relates to a steel wire or bar which is used as a material for bolts and shafts applied to parts for machine structures having relatively high strength. More particularly, the present invention is directed to a quenched and tempered steel wire with superior cold forging properties, characterized in that additional quenching and tempering process after a cold forging is not needed by maintaining a new material parameter within a specific range which is affecting on cold forging properties of the steel wire.

[Background Art]

Conventionally, with respect to the production of parts for machine structures, such as hexagonal bolts, ball studs, or shafts, which are produced by cold forging a steel wire or bar (hereinafter, referred to as "steel wire") and which have a relatively high tensile strength of 70 - 130 kgf/mm², it is necessary to heat the steel wire to about 700^°C for a few tens of hours to spheroidize the metal structure thereof so as to improve cold forging processability as shown in the following process. Furthermore, it is necessary to conduct additional heat treatment, such as quenching and tempering, after the cold forging so as to increase strength and toughness, and a production process is complicated, thus it is necessary to conduct a plurality of processes.

[Conventional cold forging process] steel wire or bar —*^■ spheroidizing annealing for a long time —> cold forging → heating to a high temperature (850^°C or more) → quenching (water or oil) —» tempering ^→ product

Accordingly, conventional technologies have the following problems, thus there is a need to reduce energy consumption and improve productivity and the operating environment. 1) Since a steel wire is spheroidized for a long time, much heat energy is lost and productivity is low.

2) In a company manufacturing parts for machine structures, quenching and tempering must be additionally conducted after cold forging so as to assure strength and toughness of the parts after forming. Thus, it takes a long time to conduct the above-mentioned processes, and, if heat treatment is directly carried out by a company producing a final product, the operating environment unavoidably becomes poor. If heat treatment is conducted elsewhere, a heat treatment cost is incurred and surplus man-hours are necessary to meet a due date, thus management of the overall process is complicated. 3) If heat treatment, such as quenching and tempering, is conducted after cold forging, deformation inevitably occurs as a result of the heat treatment, and particularly, in the case of a long product, an additional correction operation must be carried out.

4) Productivity is reduced during the heat treatment process due to the above-mentioned facts 1), 2), and 3), thus improvement thereof is urgently required.

As described above, there is an urgent need to develop novel technology capable of avoiding problems, such as poor productivity, increased production cost, complicated treatment processes, and poor operation environment, which are caused by the ^rheat treatmentJ conducted before or after the cold forging process. The present invention has been made keeping in mind problems occurring in the prior art, and an object of the present invention is to provide a quenched and tempered steel wire having high strength and excellent cold forging properties, which is produced through a very simple process in which the steel wire is subjected to quenching and tempering processes, conventionally conducted after a cold forging process, before undergoing the forging process without being spheroidized and is then cold forged.

[Description of Drawings] FIG. 1 is a graph showing a critical compressibility (Hcrit) as a function of a ratio of area reduction to tensile strength for a quenched and tempered specimen;

FIG. 2 illustrates a compression test specimen, in which FIG. 2a is a perspective view of the specimen and FIG. 2b illustrates a notch part; and FIG. 3 is a front view of a hexagonal flange bolt.

[Best Mode]

As known in the art, since a quenched and tempered steel wire has high strength, it is difficult to realize the present invention simply by cold forging the quenched and tempered steel wire.

Based on the present invention, the present inventor has conducted studies of cold forging properties of a highly strong steel wire with respect to various complicated shapes, resulting in the following finding. When the steel wire, which is quenched and tempered so as to have tensile strength of 70 - 130 kgf/mm², has a ratio of area reduction to tensile strength of 0.4 - 1.2, the cold forging properties are excellent. In other words, if the steel wire, which is quenched and tempered before the cold forging process so as to have tensile strength of 70 - 130 kgf/mm², has a ratio of area reduction to tensile strength of 0.4 - 1.2, the cold forging properties are excellent even though the steel wire has high strength, thus a typical cold forging process is nicely achieved even though spheroidizing is not conducted. Furthermore, since it is possible to assure the relatively high strength required in parts for machine structures after the cold forging process, it is unnecessary to conduct additional quenching and tempering to increase strength after the cold forging process.

It is preferable that a raw material of the quenched and tempered steel wire according to the present invention be a C- Si-Mn-based alloy steel wire including 0.10 - 0.40 wt% C, 1.0 wt% or less Si, 0.30 - 2.0 wt% Mn, and the balance of Fe and impurities. If necessary, the steel wire may further comprise one or more components of 0.05 - 2.0 wt% Cr, 0.05 - 1.5 wt% Mo, and 0.0003 - 0.0050 wt% B.

Properties of the components constituting the quenched and tempered steel wire of the present invention and the reason why ranges of the components are limited will be described below. C: 0.10 - 0.40 wt%

C is the most essential element for improving strength during a quenching process. As known in the art, if the content of C is less than 0.10 wt%, it is difficult to assure a hardening effect by the quenching, and, if the content is more than 0.40 wt%, toughness is reduced and deformation resistance is increased due to precipitation of a lot of carbides, causing cracks during a cold forging process and reduction of a lifespan of a tool. Si: 1.0 wt% or less Si is an element used to achieve deoxidation of the steel, and causes solid-solution hardening to improve strength. If the content of Si is more than 1.0 wt%, it leads to the reduction in toughness, and deformation resistance is enhanced upon a cold forging process, resulting in generating cracks and shortening a service life of tools. The reason is that Si is solid-solved in carbide precipitate to hinder movement of carbon, resulting in interruption of spheroidizing of carbide.

Mn: 0.30 - 2.0 wt%

Mn is an element for solid-solution hardening, and is used to prevent an increase in deformation resistance caused by use of an excessive amount of C and Si and to supplement reduction in strength of steel having low C and Si content.

To achieve this, it is necessary to use it in an amount of at least 0.30 wt%. However, if Mn is used in an excessive amount, toughness and deformation resistance increase. Therefore, the Mn content must not exceed 2.0 wt%.

Cr: 0.05 - 2.0 wt%

Cr is an element used to improve strength, quenching ability, and toughness. When the content of Cr is less than 0.05 wt%, improvement of the above physical properties is insignificant. When the content is more than 2.0 wt%, economic efficiency is reduced because Cr is relatively expensive. Accordingly, the lower and upper limits of the Cr content are set to 0.05 wt% and 2.0 wt%, respectively.

Mo: 0.05 - 1.5 wt% The effect caused by use of Mo is almost the same as that of Cr. When the content of Mo is less than 0.05 wt%, insufficient results are realized. When the content is more than 1.5 wt%, since deformation resistance increases, the content is set to 1.5 wt% or less. B: 0.0003 - 0.0050 wt%

B is an element for improving quenching ability. If the content of B is less than 0.0003 wt%, the effect of B is insignificant. If the content is more than 0.0050 wt%, quenching ability is reduced. Meanwhile, B reacts with N in the steel structure to produce BN, which affects to embrittle the grain boundaries. Thus, Ti having higher affinity to N than B is added in the amount of 0.01 - 0.05wt%, so as to increase the effect of addition of B. Additionally, it is preferable to add one or more of Zr, Nb, and Al that act equally with Ti.

P and S are unavoidable impurities, act to reduce a degree of deformation upon cold working process. If their respective contents are more than 0.030 wt%, the frequency of crack occurrence increases during the cold working process, thus it is necessary to limit the content of each of them to 0.030 wt% or less. The reason why the final tensile strength of the heat treated steel wire according to the present invention is limited to 70 - 130 kgf/mm² is as follows. If the tensile strength is less than 70 kgf/mm², the ductility is high. Thus, only when being cold forged in small quantities, a hot rolled wire material (structure: ferrite + pearlite) may be subjected to a cold forging process. Further, the steel wire having tensile strength of the above value is unsuitable for use in machine parts. On the other hand, at tensile strength exceeding 130 kgf/mm², hardness of the wire material is high, thus reducing a service life of tools. In addition, it is difficult to make machine parts having complicated shapes. Further, the reason why the ratio of area reduction to tensile strength is limited to 0.4 - 1.2 so that the quenched and tempered steel wire has excellent cold forging characteristics is as follows.

Referring to FIG.l in a study conducted by the present inventor, 16mm across wire materials, including JIS G 4105 SCM420, JIS G 4051 S30C, and JIS G 4106 SMn420, are drawn to have a diameter of 15.0mm, and heated at Ac3 point or higher and then cooled with water or oil. Each wire material is tempered under various heating temperatures and heating times and then tensile strength test performed.

From the test results, as shown in FIG. 1, it was evident that the ratio of area reduction to tensile strength can be varied depending on the compositions and the heat treatment conditions of the material even though their tensile strengths are the same, and that the cold forging properties depended on the ratio of area reduction to tensile strength. Particularly, when the ratio was 0.4 or more, a critical compressibility (Hcrit) used as an index denoting possibility of cold forging was 40 % or more, which meant excellent cold forging properties. In the present invention, the upper limit of the ratio of area reduction is set to 1.2. This is based on the lots of test results conducted by the present inventor. The present inventor has repeatedly conducted tests, resulting in the finding that it is difficult to assure area reduction of 85 % or more. Furthermore, the limit is set to be based on the fact that the lower limit of tensile strength of the object steel wire according to the present invention is 70 kgf/mm².

The ratio of area reduction of the steel wire according to the present invention can be controlled so as to be within the range of 0.4 - 1.2 by suitably adjusting the composition of the steel wire, the heating temperature, the heating time, the heating speed, and the cooling speed during the quenching and tempering processes within range of the desired tensile strength.

The production of a specimen and measurement of physical properties thereof are conducted as described below in order to calculate the ratio of area reduction (RAR) to tensile strength and the critical compressibility as shown in FIG. 1.

First, the ratio of area reduction (RAR) is calculated using the following Equation 1, and, at this time, any one of a standard specimen and a bar-shaped specimen may be used.

Equation 1 RAR = δ / TS

(wherein, δ: area reduction (%), and TS: tensile strength (kgf/mm²) )

In addition, the measurement of critical compressibility is carried out by subjecting a compression test piece as shown in FIG.2 to a V-notch process and then a compressing process at various heights, whereby a bottom surface of the V- notched part is observed 10 magnifications by a magnifying glass. The critical compressibility (Hcrit) when 1 mm long cracks appear is calculated according to the following Equation 2.

Equation 2 Hcrit = (H₀ - H₁)ZH₀ X 100 (%) wherein,

Ho: an original height of the specimen (mm) , Hi: a height of test piece when lmm cracks are generated on a bottom surface of V-notch (mm) The V-notch compression test is used to evaluate whether cold forging characteristics are superior. The present inventor has practically performed cold forging processing for a plurality of steel wire test pieces having different values of critical compressibility. Thereby, it can be confirmed that cold forging characteristics are superior when the critical compressibility is 40% or more. Thus, the above value is used as an index denoting possibility of cold forging.

Through the above test results, it could be seen that the cold forging processability of the quenched and tempered steel wire is significantly affected by the ratio of area reduction to tensile strength of the quenched and tempered material, and that, when the ratio of area reduction to tensile strength is 0.4 - 1.2, it is possible to produce the quenched and tempered steel wire having the best cold forging processability. Accordingly, it became apparent that the ratio of area reduction to tensile strength is a very important factor in the production of the quenched and tempered steel wire having excellent cold forging processability.

A detailed description will be given of the object, constitution, and effects according to the present invention in the following examples.

To clarify above results, the quenched and tempered steel wire was subjected to a tensile strength test to measure the ratio of area reduction (RAR) to tensile strength, and a V-notch compressed specimen was prepared using the steel wire to measure a critical compressibility (Hcrit) . Furthermore, a hexagonal flange bolt shown in FIG. 3 was subjected to a cold forging process, and the occurrence of cracks was checked, which was used as an index for evaluating cold forging properties. Hot-rolled wire rods, which included compositions shown in the following Table 1 and each had a diameter of 16 mm, were drawn so as to have a diameter of 15.0 mm, and then quenched and tempered using a high frequency induction heating device through series processes while the heating temperature, heating time, heating speed, and cooling rate were changed so that tensile strength was 70 - 130 kgf/mm², thereby the steel wire specimens were produced. TABLE 1. Chemical composition of specimens

The specimens having the compositions shown in Table 1 were subjected to a tensile strength test to measure tensile strengths (TS) , and the specimens for compression were produced as shown in FIG. 2 and subjected to a compression test to measure critical compressibilities (Hcrit) . Further, a hexagonal flange bolts shown in FIG. 3 was subjected to cold forging, whereby whether any cracks appeared at the weakest portions, indicated by arrows, was examined. The results are shown in the following Table 2.

TABLE 2. Cold forging properties of a specimen (quenched and tempered steel wire)

From Table 2, it can be seen that the test pieces of the present invention having the ratio of area reduction of 0.4 - 1.2 represent the critical compressibility of 40% or more, regardless of kinds of steel. Further, since practically forged parts have no cracks, It will signify the assurance that the steel wire of the present invention exhibits superior cold forging characteristics.

Additionally, the following Table 3 shows characteristics of steel wire drawn to 20% after the specimens of Table 1 were subjected to quenching and tempering treatment.

From the results, it can be seen that the quenched and tempered steel wires of the present invention are useful even though they are drawn.

TABLE 3. Cold forging properties of a specimen (a steel wire drawn to a reduction ratio of 20 % after quenching and tempering)

[industrial Applicability]

As described above, the steel wire of the present invention has the following effects and advantages.

1) In a company manufacturing the steel wire, there is required no spheroidizing annealing process requiring a long period, and thus it is possible to manufacture quenched and tempered steel wire having cold forging characteristics equal or superior to those of spheroidizing annealed steel wires, thus increasing productivity.

2) In a company manufacturing machine parts, quenching and tempering processes are not additionally performed for the enhancement of strengths after cold forging process, thereby achieving energy saving and improvement of working conditions. Further, by performing only a forging process, it is possible to manufacture machine parts having strength and toughness equal or superior to those of conventional parts. Thus management of product quality and process are simplified, resulting in improvement productivity. Furthermore, since it is possible to remove the quenching and tempering processes conventionally conducted after the cold forging process, a correction process conducted because of heat treatment deformation is unnecessary, thus it is possible to reduce production cost.

Claims

[CLAIMS]

[Claim l]

A steel wire for cold forging, comprising: 0.10 - 0.40 wt% C; 1.0 wt% or less Si; 0.30 - 2.0 wt% Mn; 0.03 wt% or less P; 0.03 wt% or less S; and a balance of Fe and unavoidable impurities, the steel wire being quenched and tempered so as to have tensile strength of 70 - 130 kgf/mm² and having a ratio (δ/TS) of area reduction (δ, %) to the tensile strength (TS) of 0.4 - 1.2.

[Claim 2] The steel wire as set forth in claim 1, further comprising one or more components of 0.05 - 2.0 wt% Cr, 0.05 - 1.5 wt% Mo, and 0.0003 - 0.0050 wt% B.

[Claim 3]

The steel wire as set forth in claim 1 or 2, wherein the steel wire is drawn.

[Claim 4]

A product cold forged using the steel wire according to any one of claims 1 to 3.