Non-Quenched and Non-Tempered Steel Wire Rod for Hot Forging with Excellent Machinability and Impact Toughness and Method for Manufacturing Same

The present disclosure relates to a non-quenched and non-tempered steel wire rod with improved machinability and impact toughness and a method for manufacturing the same, and more particularly, to a non-quenched and non-tempered steel wire rod 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 non-tempered steels. Therefore, non-quenched and non-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 non-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 non-tempered steels 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 relates to a non-quenched and non-tempered steel wire rod with excellent machinability and impact toughness by improving toughness inferior to that of conventional quenched and tempered steels and by adding high contents of S and N without additional heat treatment and a method for manufacturing the same.

A non-quenched and non-tempered steel wire rod 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, 0.01% to 0.05% of sol.Al, 0.1% to 0.3% of Cr, 0.01% to 0.02% of Ti, 0.0005% to 0.002% of Ca, 0.007% to 0.02% of N, and the balance of Fe and inevitable impurities, and includes ferrite and pearlite as a microstructure, wherein Relational Expression 1 below is satisfied and an area fraction of MnS satisfies a range of 0.10 to 0.60%.

According to an embodiment of the present disclosure, in the non-quenched and non-tempered steel wire rod with improved machinability and impact toughness, a number density of MnS may be 70 ea/mmor more and an aspect ratio of MnS may be 40 or less.

According to an embodiment of the present disclosure, the non-quenched and non-tempered steel wire rod with improved machinability and impact toughness may have a tensile strength of 700 MPa or more, a yield strength of 350 to 500 MPa, and a yield ratio of 0.45 to 0.65.

According to an embodiment of the present disclosure, the non-quenched and non-tempered steel wire rod with improved machinability and impact toughness may have an impact toughness of 60 J/cmor more and a tensile strength×impact toughness value of 45000 MPa·J/cmor more.

A method for manufacturing a non-quenched and non-tempered steel wire rod 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, 0.01% to 0.05% of sol.Al, 0.1% to 0.3% of Cr, 0.01% to 0.02% of Ti, 0.0005% to 0.002% of Ca, 0.007% to 0.02% of N, and the balance of Fe and inevitable impurities, and including ferrite and pearlite as a microstructure in a temperature range of 950 to 1120° C.; finish rolling the reheated steel piece into a steel wire rod at a temperature of 750° C. to 850° C.; and winding and cooling the steel wire rod, wherein the cooling performed after the winding includes: a process of cooling to 400° C. at an average cooling rate of 0.1 to 5.0° C./s, wherein the steel wire rod includes ferrite and pearlite as a microstructure, Relational Expression 1 is satisfied, and an area fraction of MnS is 0.10 to 0.60%.

In the non-quenched and non-tempered steel wire rod with improved machinability and impact toughness according to an embodiment of the present disclosure, Ti and Al combine with N to form nitrides such as TiN and AlN, and such nitrides interfere with the growth of grain boundaries to decrease grain sizes, thereby improving toughness. In addition, a Ca-based oxide resulting from addition of Ca serves as a nucleus of MnS formation and inhibits elongation of MnS during rolling to improve machinability and toughness. Therefore, even if heat treatment is omitted, the steel wire rod may be applied to materials for automobiles or mechanical parts that require both machinability and toughness.

A non-quenched and non-tempered steel wire rod 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, 0.01% to 0.05% of sol.Al, 0.1% to 0.3% of Cr, 0.01% to 0.02% of Ti, 0.0005% to 0.002% of Ca, 0.007% to 0.02% of N, and the balance of Fe and inevitable impurities, and includes ferrite and pearlite as a microstructure, wherein Relational Expression 1 below is satisfied and an area fraction of MnS satisfies a range of 0.10 to 0.60%.

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 wire rod 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 wire rod without an additional heat treatment, thereby completing the present disclosure.

A non-quenched and non-tempered steel wire rod 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, 0.01% to 0.05% of sol.Al, 0.1% to 0.3% of Cr, 0.01% to 0.02% of Ti, 0.0005% to 0.002% of Ca, 0.007% to 0.02% of N, and the balance of Fe and inevitable impurities, and includes ferrite and pearlite as a microstructure, wherein Relational Expression 1 below is satisfied and an area fraction of MnS is 0.10 to 0.60%.

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 wire rod. 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), as 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. 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. Therefore, the upper limit of the Mn content may be controlled to 1.2%.

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 a steel more than necessary, reduces an amount of a solid solution of C by precipitating carbides in a steel, and contributes to reduction in dynamic deformation aging caused by the solid solution of carbon. With a Cr content less than 0.10%, 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 P is 0.02% or less.

Phosphorus (P), as an impurity inevitably contained in steels, is segregated into grain boundaries as a major causative element of 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 significant deterioration in ductility as being segregated into grain boundaries and deterioration in delayed fracture resistance and stress relaxation due to formation of an emulsion in a steel, is an impurity inevitably contained in a 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 0.01% to 0.05%.

The sol.Al is an element effective as a deoxidizer. The sol.Al may be contained in an amount of 0.01% or more to obtain the above-describe effect. However, with an Al content exceeding 0.05%, difficulties may arise during a manufacturing process due to Al oxides produced during casting process. Therefore, the upper limit of the Al content may be controlled to 0.05% in the present disclosure.

The content of Ti is 0.01% to 0.02%.

Titanium (Ti) is an element that plays a major role in improving toughness of a steel by decreasing grain sizes of a final structure by forming TiN precipitates during a solidification process of the steel to inhibit the growth of austenite crystal grains during heating and hot rolling processes of a slab. With a Ti content less than 0.01%, it is difficult to obtain a sufficient amount of TiN precipitates for inhibiting migration of austenite grain boundaries. On the contrary, with a T content exceeding 0.02%, a coarse titanium nitride may be formed rather deteriorating toughness, and thus the upper limit of the Al content may be controlled to 0.02% in the present disclosure.

The content of Ca is 0.0005% to 0.002%.

Ca is an essential element to implement an effect on improving machinability and impact toughness by reducing an aspect ratio of MnS. Addition of Ca causes formation of an oxide, which serves as a nucleus of MnS formation, to inhibit elongation of MnS while rolling the steel wire rod and maintain a low aspect ratio. The low aspect ratio of MnS not only improves machinability but also inhibits deterioration of toughness by reducing anisotropy of a microstructure. However, Ca should be added in an amount of 0.0005% or more to obtain the above-described effects, but a Ca content exceeding 0.002% may cause difficulties in a manufacturing process. Therefore, the upper limit of the Ca content is controlled to 0.002%

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 Ti and 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 precipitates of Al, Ti, and the like, 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 wire rod. 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 non-tempered steel wire rod of the present disclosure is iron (Fe). However, the non-quenched and non-tempered steel wire rod may include other impurities incorporated during common industrial manufacturing processes of steels. Types and contents of 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 non-tempered steel wire rod according to an embodiment of the present disclosure may satisfy Relational Expression 1 below. In Relational Expression 1, [S] and [Mn] respectively represent contents (wt %) of the elements.

Relational Expression 1 is an expression related to machinability. According to the present disclosure, MnS is formed by adding high contents of S and Mn. MnS, as an elongated inclusion, has a shape and an orientation elongated in a rolling direction and significantly improves machinability of the medium-carbon non-quenched and non-tempered steel wire rod of the present disclosure. However, MnS serving as a starting point of cracks and a propagation path thereof in the case of impact applied thereto, thereby deteriorating impact toughness. When the [Mn]/[S] ratio is less than 20, machinability may be satisfied, but impact toughness may deteriorate. When the [Mn]/[S] ratio exceeds 70, machinability may be insufficient. Therefore, the [Mn]/[S] ratio may be controlled to 20 to 70, preferably 30 to 60, in the present disclosure.

In addition, in the non-quenched and non-tempered steel material according to an embodiment of the present disclosure, an area fraction of MnS may be 0.10 to 0.60%, preferably 0.15 to 0.50%, and more preferably 0.15 to 0.45%.

In addition, in the non-quenched and non-tempered steel material according to an embodiment of the present disclosure, a number density of MnS may be 70 ea/mmor more, preferably 80 ea/mmor more, and more preferably 90 ea/mmor more. As the density of MnS increases, MnS serves as a stress concentration source during cutting, to reduce cutting resistance, thereby improving machinability. To this end, the density of MnS needs to be at least 70 ea/mm.

In addition, in the non-quenched and non-tempered steel material according to an embodiment of the present disclosure, the aspect ratio of MnS may be 40 or less, preferably 30 or less, and more preferably 20 or less. In this case, when the aspect ratio of MnS is greater than 40, impact toughness may rapidly decrease.

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

In addition, the non-quenched and non-tempered steel wire rod according to an embodiment of the present disclosure may have a yield strength of 350 to 500 MPa.

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

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

In addition, the non-quenched and non-tempered steel wire rod according to an embodiment of the present disclosure may have a tensile strength of impact toughness value of 45000 MPa·J/cmor more.

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