Non-Quenched and Tempered Steel Rod Wire with Improved Machinability and Toughness and Method for Manufacturing Same

The present disclosure relates to a non-quenched and tempered steel rod wire with excellent machinability and impact toughness and a method for manufacturing the same, and more particularly, to a non-quenched and tempered steel rod wire suitable for use as a material for automobiles or mechanical parts and a method for manufacturing the same.

Unlike quenched and tempered steels, which obtain certain levels of strength and toughness by quenching and tempering (QT) heat treatment, the QT heat treatment process is omitted in non-quenched and tempered steels. Therefore, non-quenched and tempered steels are not only economically advantageous by reducing heat treatment costs, simplifying processes to shorten delivery time, and improving productivity, but also eco-friendly by reducing COthat is generated by operating a furnace during heat treatment. At the beginning of development, non-quenched and tempered steels were applied only to parts that do not require high toughness due to relatively inferior toughness thereof to that of quenched and tempered steels. However, with a recent increase in the demand for environmental feasibility and cost reduction, demand for improving toughness of non-quenched and tempered steel is increasing. In addition, because a cutting process is often conducted to obtain final shapes of parts, machinability is also required. In general, a large amount of MnS is generated by adding S to improve machinability, thereby causing a problem of reduction in toughness of products.

The present disclosure provides a non-quenched and tempered steel rod wire having impact toughness and providing cutting tools with abrasion resistance by adjusting a microstructure, for example, refining a structure via AlN grain boundary peening and low-temperature rolling or by obtaining a sufficient fraction of ferrite, which is a soft phase, in order to improve impact toughness inferior to that of conventional quenched and tempered steels, and a method for manufacturing the same.

A non-quenched and tempered steel rod wire with improved machinability and impact toughness according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.3% to 0.5% of C, 0.4% to 0.9% of Si, 0.5% to 1.2% of Mn, 0.02% or less of P, 0.01% to 0.05% of S, 0.015% to 0.05% of sol·Al, 0.1% to 0.3% of Cr, 0.007% to 0.020% of N, and the balance being Fe and inevitable impurities, wherein a microstructure includes ferrite and pearlite, and Relational Expression 1 below is satisfied.

A method for manufacturing a non-quenched and tempered steel rod wire with improved machinability and impact toughness according to an embodiment of the present disclosure includes: reheating a steel piece including, in percent by weight (wt %), 0.3% to 0.5% of C, 0.4% to 0.9% of Si, 0.5% to 1.2% of Mn, 0.02% or less of P, 0.01% to 0.05% of S, 0.015% to 0.05% of sol·Al, 0.1% to 0.3% of Cr, 0.007% to 0.020% of N, and the balance being Fe and inevitable impurities at a temperature of 950° C. to 1100° C.; finish rolling the reheated steel piece into a steel rod wire at a temperature of 750° C. to 850° C.; and winding and cooling the steel rod wire, wherein the cooling performed after the winding includes a process of cooling the steel rod wire to 400° C. at an average cooling rate more than 0.1° C./s but not more than 5.0° C./s, and the steel rod wire satisfies the Relational Expression 1.

In the non-quenched and tempered steel rod wire with improved machinability and impact toughness according to an embodiment of the present disclosure, Al combines with N to form an AlN nitride, which inhibits the growth of grain boundaries during heating and refines grains, thereby improving impact toughness. In addition, impact toughness is further improved by adjusting an area fraction of ferrite to 20% to 40% in a region from the center to a quarter or more of the diameter of the steel rod wire from the surface. Deterioration of impact toughness may be minimized and machinability, particularly, abrasion resistance of cutting tools, may be obtained by decreasing the size of MnS, which improves machinability but may deteriorate impact toughness. Therefore, the steel rod wire may be applied to materials for automobiles or mechanical parts that require machinability and impact toughness even after omitting heat treatment.

A non-quenched and tempered steel rod wire with improved machinability and impact toughness according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.3% to 0.5% of C, 0.4% to 0.9% of Si, 0.5% to 1.2% of Mn, 0.02% or less of P, 0.01% to 0.05% of S, 0.015% to 0.05% of sol·Al, 0.1% to 0.3% of Cr, 0.007% to 0.02% of N, and the balance being Fe and inevitable impurities, wherein a microstructure includes ferrite and pearlite, and Relational Expression 1 below is satisfied.

This specification does not describe all elements of the embodiments of the present disclosure and detailed descriptions on what are well known in the art or redundant descriptions on substantially the same configurations may be omitted. In addition, the term “include” an element does not preclude other elements but may further include another element, unless otherwise stated. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. Hereinafter, the present disclosure will be described in detail.

The present inventors have examined a method for providing a steel rod wire with machinability and impact toughness from various angles and have found that machinability and toughness may be obtained by appropriately controlling a composition of alloying elements and a microstructure of the steel rod wire without heat treatment, thereby completing the present disclosure.

A non-quenched and tempered steel rod wire with improved machinability and impact toughness according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.3% to 0.5% of C, 0.4% to 0.9% of Si, 0.5% to 1.2% of Mn, 0.02% or less of P, 0.01% to 0.05% of S, 0.015% to 0.05% of sol·Al, 0.1% to 0.3% of Cr, 0.007% to 0.02% of N, and the balance being Fe and inevitable impurities, wherein a microstructure includes ferrite and pearlite, and Relational Expression 1 below is satisfied.

Hereinafter, reasons for numerical limitations on the contents of alloying elements in the embodiment of the present disclosure will be described. Hereinafter, the unit is wt % unless otherwise stated.

The content of C is 0.3% to 0.5%.

Carbon (C) is an element serving to improve strength of a steel rod wire. To obtain the above-described effect, it is preferable to include C in an amount of 0.3% or more. However, an excessive C content may deteriorate toughness and machinability, and thus the upper limit of the C content may be controlled to 0.5%.

The content of Si is 0.4% to 0.9%.

Silicon (Si), an element effective as a deoxidizer, serves to improve strength. With a Si content less than 0.4%, the above-described effect cannot be obtained. With a Si content exceeding 0.9%, deformation resistance of a steel rapidly increases due to solid solution strengthening resulting in deterioration of cold workability, and therefore, the upper limit of the Si content may be controlled to 0.9%.

The content of Mn is 0.5% to 1.2%.

Manganese (Mn) is an element effective as a deoxidizer and a desulfurizer. With a Mn content less than 0.5%, the above-described effect cannot be obtained. With a Mn content exceeding 1.2%, strength of the steel excessively increases to rapidly increase deformation resistance of the steel, resulting in deterioration of cold workability, and therefore, the upper limit of the Mn content may be controlled to 1.2%.

The content of P is 0.02% or less.

Phosphorus (P), as an impurity inevitably contained in steels, is a major causative element of segregation into grain boundaries resulting in deterioration of toughness and reduction in delayed fracture resistance. Therefore, it is preferable to control the P content as low as possible. Theoretically, it is preferable to control the P content to 0% but P is inevitably included therein during a manufacturing process. Therefore, it is important to control the upper limit, and the upper limit of the P content may be controlled to 0.02% in the present disclosure.

The content of S is 0.01% to 0.05%.

Sulfur(S), as a major causative element of segregation into grain boundaries resulting in significant deterioration in ductility and formation of an emulsion in a steel impairing delayed fracture resistance and stress relaxation, is an impurity inevitably contained in the steel during a manufacturing process. However, as in the present disclosure, S may actively be used to improve machinability. Because S combines with Mn to form MnS that improves machinability, the S content is controlled within a range of 0.01% to 0.05% in the present disclosure in consideration of an S content effective for improvement of machinability without significantly impairing toughness of the steel.

The content of sol·Al is 0.015% to 0.05%.

The sol·Al is an element effective as a deoxidizer. The sol·Al may be contained in an amount of 0.015% to obtain the above-describe effect. However, with an Al content exceeding 0.05%, difficulties may arise during a casting process due to Al oxides. Therefore, the upper limit of the Al content may be controlled to 0.05% in the present disclosure.

The content of Cr is 0.1% to 0.3%.

Chromium (Cr) is an element serving to promote transformation of ferrite and pearlite during hot rolling. In addition, Cr does not increase the strength of the steel more than necessary, reduces an amount of a solid solution of C by precipitating carbides, and contributes to reduction in dynamic deformation aging caused by the solid solution of carbon. With a Cr content less than 0.1%, the above-described effects cannot be obtained, and with a C content exceeding 0.3%, strength of the steel excessively increases to rapidly increase deformation resistance of the steel, resulting in deterioration of cold workability. Therefore, the upper limit of the Cr content may be controlled to 0.3%.

The content of N is 0.007% to 0.02%.

N is an essential element for implementing an effect on improving impact toughness by decreasing grain sizes via formation of a nitride with Al. With a N content less than 0.007%, it is difficult to obtain a sufficient amount of the nitride, resulting in a decrease in production of AlN precipitates, failing to obtain toughness desired in the present disclosure. With a N content exceeding 0.02%, a solid solution of N, not present as a nitride, increases to deteriorate toughness and ductility of the steel rod wire. Therefore, the upper limit of the N content may be controlled to 0.02% in the present disclosure.

The remaining component of the non-quenched and tempered steel rod wire of the present disclosure is iron (Fe). However, the non-quenched and tempered steel rod wire may include other impurities incorporated during common industrial manufacturing processes of steels. The impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.

The non-quenched and tempered steel rod wire according to an embodiment of the present disclosure may satisfy Relational Expressions 1 and 2. In Relational Expressions 1 and 2, [Al], [N], [C], [S], [Mn], and [Si] respectively represent contents (wt %) of the elements.

Relational Expression 1 is an expression related to toughness. According to the present disclosure, AlN is formed by adding high contents of N and Al. Because fine AlN precipitates in a steel inhibit the growth of crystal grains, particles are refined to improve impact toughness of the non-quenched and tempered steel rod wire according to the present disclosure. In order to express the above-described effect, it is preferable to control a [N]-[Al]/1.93 ratio to 0.009 or less. At a [N]-[Al]/1.93 ratio exceeding 0.009, a significant amount of N added in a state where Al is insufficient is present in a steel without combining with Al, and thus impact toughness may deteriorate.

Relational Expression 2 is an expression related to tool wear among machinability. In general, addition of S causes formation of MnS that serves as a stress concentrator during a cutting process to deteriorate cutting resistance and performs lubrication to improve lifespan of tools. However, because tool wear is accelerated in the case where hardness increases due to added alloying elements, they should be considered together. Relational Expression 2 reflects these effects, complexly, and at a value of 0 or more, tool wear is not serious.

The non-quenched and tempered steel rod wire according to an embodiment of the present disclosure includes ferrite and pearlite as microstructures, wherein an area fraction of ferrite, measured in a region from the center to a quarter or more of the diameter of the steel rod wire from the surface, satisfies a range of 20 to 40%.

In the non-quenched and tempered steel rod wire according to an embodiment of the present disclosure, an area fraction of the formed AlN may be 0.03% or more.

In the non-quenched and tempered steel rod wire according to an embodiment of the present disclosure, a size of the formed AlN may be 150 nm or less.

In addition, in the non-quenched and tempered steel rod wire according to an embodiment of the present disclosure, the number of carbonitrides having an average equivalent circular diameter of 100 nm or less per unit area may be 2 ea./μmor more.

In addition, the non-quenched and tempered steel material according to the present disclosure may have a tensile strength of 700 MPa or more.

In addition, the non-quenched and tempered steel material according to the present disclosure may have a yield strength of 350 to 450 MPa.

In addition, the non-quenched and tempered steel material according to the present disclosure may have a yield ratio of 0.45 to 0.65.

In addition, the non-quenched and tempered steel material according to the present disclosure may have an impact toughness of 60 J/cmor more.

In addition, the non-quenched and tempered steel material according to the present disclosure may have a product of tensile strength and impact toughness of 30000 to 60000.

Hereinafter, a method for manufacturing a non-quenched and tempered steel rod wire according to an embodiment of the present disclosure will be described.

The non-quenched and tempered steel rod wire with improved machinability and impact toughness according to the present disclosure may be manufactured in various methods, and the manufacturing method therefor is not particularly limited. However, the steel rod wire may be manufactured according to the following method.

A method for manufacturing a non-quenched and tempered steel rod wire with improved machinability and impact toughness according to an embodiment of the present disclosure includes: reheating a steel piece including, in percent by weight (wt %), 0.3% to 0.5% of C, 0.4% to 0.9% of Si, 0.5% to 1.2% of Mn, 0.02% or less of P, 0.01% to 0.05% of S, 0.015% to 0.05% of sol·Al, 0.1% to 0.3% of Cr, 0.007% to 0.020% of N, and the balance being Fe and inevitable impurities; hot rolling the reheated steel piece into a steel rod wire; and winding and cooling the steel rod wire,

wherein the cooling performed after the winding includes a process of cooling the steel rod wire to 400° C. at an average cooling rate more than 0.1° C./s but not more than 5.0° C./s, and the steel rod wire satisfies Relational Expression 1 below.