Austenite Stainless Steel and Manufacturing Method Therefor

The present disclosure relates to an austenite stainless steel and a manufacturing method therefor, and more particularly, to an austenite stainless steel having high strength, high ductility, and improved corrosion resistance by realizing ultrafine grain characteristics and a manufacturing method therefor.

304 steels, as common austenite stainless steels, having a yield strength of 200 MPa to 350 MPa are limited in application as structural members. 304 steels are subject to an additional skin pass rolling process to obtain a higher yield strength, but this process causes an increase in costs and rapid decreases in elongation and formability. In addition, 304 steels including large amounts high-priced alloying elements have a problem of low price competitiveness.

Patent Document 0001 discloses an austenite stainless steel and a manufacturing method therefor. Although an austenite stainless steel having a tensile strength of 600 MPa or more is disclosed, price competitiveness decreases due to a high Ni content.

Meanwhile, grain refinement technology has drawn attention as a technology to improve both strength and ductility. Particularly, as a method for manufacturing ultrafine-grained steels for structural members, severe plastic deformation (SPD) has drawn attention. Severe plastic deformation is a method for realizing fine grains by generating new grain boundaries in existing grain boundaries by applying a strong sheer stress to a material. However, severe plastic deformation have problems such as a decrease in productivity and a limitation in product size.

Patent Document 0002 discloses a method including heat treatment performed at a temperature of 600 to 700° C. for a long time over 48 hours to obtain an average grain size of 10 μm or less. However, according to method disclosed in Patent Document 0002, there are problems of a decrease in productivity and an increase in manufacturing costs.

The present disclosure has been proposed to solve the above-described problems, and provided is an austenite stainless steel having high strength, high ductility, and high corrosion resistance by realizing ultrafine grain characteristics as well as price competitiveness.

An austenite stainless steel according to an embodiment of the present disclosure includes, in weight %, at least 0.05% but not more than 0.1% of C, at least 0.2% but not more than 0.7% of Si, at least 2.0% but not more than 4.0% of Mn, more than 0% but less than 0.1% of P, more than 0% but less than 0.01% of S, at least 17% but not more than 19% of Cr, at least 2.0% but not more than 4.0% of Ni, at least 1.0% but not more than 2.5% of Cu, at least 0.15% but not more than 0.25% of N, and the balance being iron (Fe) and inevitable impurities and is 5 μm or less in average grain diameter of the thickness center.

In addition, the austenite stainless steel according to an embodiment of the present disclosure may have an austenite stability parameter (ASP), represented by Expression (1) below, of −30 to 30.

In Expression (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent weight percentages (wt %) of respective elements.

In addition, the austenite stainless steel according to an embodiment of the present disclosure may have a strength stability parameter (SSP), represented by Expression (2) below, of 0 or more.

In Expression (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent weight percentages (wt %) of respective elements.

In addition, the austenite stainless steel according to an embodiment of the present disclosure, may have a pitting resistance equivalent number (PREN), represented by Expression (3) below, of 17 or more.

In Expression (3), [Cr], [Mn], and [N] represent weight percentages (wt %) of respective elements.

In addition, the austenite stainless steel according to an embodiment of the present disclosure may have a yield strength of 600 MPa or more.

In addition, the austenite stainless steel according to an embodiment of the present disclosure may have an elongation of 30% or more.

In addition, the austenite stainless steel according to an embodiment of the present disclosure may have a pitting potential value of 200 mV or more.

In addition, the austenite stainless steel according to an embodiment of the present disclosure may have a thickness of 0.4 to 2.0 mm.

In addition, a method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure may include: manufacturing an ingot including, in weight %, at least 0.05% but not more than 0.1% of C, at least 0.2% but not more than 0.7% of Si, at least 2.0% but not more than 4.0% of Mn, more than 0% but less than 0.1% of P, more than 0% but less than 0.01% of S, at least 17% but not more than 19% of Cr, at least 2.0% but not more than 4.0% of Ni, at least 1.0% but not more than 2.5% of Cu, at least 0.15% but not more than 0.25% of N, and the balance being iron (Fe) and inevitable impurities; hot rolling the ingot into a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and final annealing the cold-rolled steel sheet.

In addition, in the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure, the ingot may have an austenite stability parameter (ASP), represented by Expression (1) below, of −30 to 30.

In Expression (1), [C], [N], [Si], [Mn], [Cr], [Ni], and [Cu] represent weight percentages (wt %) of respective elements.

In addition, in the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure, the ingot may have a strength stability parameter (SSP), represented by Expression (2) below, of 0 or more.

In Expression (2), [C], [Si], [Mn], [Cr], [Ni], [Cu], and [N] represent weight percentages (wt %) of respective elements.

In addition, in the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure, the ingot may have a pitting resistance equivalent number (PREN), represented by Expression (3) below, of 17 or more.

In Expression (3), [Cr], [Mn], and [N] represent weight percentages (wt %) of respective elements.

In addition, the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure may further include intermediate annealing the hot-rolled steel sheet before the cold rolling.

In addition, in the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure, the intermediate annealing may be performed at a temperature of 1050 to 1150° C.

In addition, in the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure, the final annealing may be performed at a temperature of 800 to 850° C.

In addition, in the method for manufacturing an austenite stainless steel according to an embodiment of the present disclosure, the cold rolling may be performed at room temperature such that a thickness reduction ratio of the hot-rolled steel sheet is 50% or more.

According to an embodiment of the present disclosure, provided are an austenite stainless steel having high strength, high ductility, and high corrosion resistance by realizing ultrafine grain characteristics as well as price competitiveness and a manufacturing method therefor.

An austenite stainless steel according to an embodiment of the present disclosure includes, in weight %, at least 0.05% but not more than 0.1% of C, at least 0.2% but not more than 0.7% of Si, at least 2.0% but not more than 4.0% of Mn, more than 0% but less than 0.10% of P, more than 0% but less than 0.01% of S, at least 17% but not more than 19% of Cr, at least 2.0% but not more than 4.0% of Ni, at least 1.0% but not more than 2.5% of Cu, at least 0.15% but not more than 0.25% of N, and the balance being iron (Fe) and inevitable impurities and is 5 μm or less in average grain diameter of the thickness center.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. These embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. The embodiments of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. In the drawings, parts unrelated to the descriptions are omitted for clear description of the disclosure and sizes of elements may be exaggerated for clarity.

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

An austenite stainless steel according to an embodiment of the present disclosure includes, in weight %, at least 0.05% but not more than 0.1% of C, at least 0.2% but not more than 0.7% of Si, at least 2.0% but not more than 4.0% of Mn, more than 0% but less than 0.1% of P, more than 0% but less than 0.01% of S, at least 17% but not more than 19% of Cr, at least 2.0% but not more than 4.0% of Ni, at least 1.0% but not more than 2.5% of Cu, at least 0.15% but not more than 0.25% of N, and the balance being iron (Fe) and inevitable impurities.

The content of carbon (C) may be at least 0.05% but not more than 0.1%.

Carbon (C), as an element effective on stabilizing an austenite phase, needs to be added in an appropriate amount to obtain a yield strength of a steel. In consideration thereof, C may be added in an amount of 0.05% or more. However, an excess of C may deteriorate cold workability due to the solid solution strengthening effect. In addition, an excess of C may cause grain boundary precipitation of a Cr carbide during a low-temperature annealing process, resulting in adverse effects on ductility and corrosion resistance. In consideration thereof, the upper limit of the C content may be controlled to 0.1%.

The content of silicon (Si) may be at least 0.2% but not more than 0.7%.

Si may be added for deoxidation of a steel and is an element effective on improving corrosion resistance. In consideration thereof, Si may be added in an amount of 0.2% or more. However, an excess of Si may promote formation of delta ferrite in a cast material due to the ferrite phase stabilization effect. Therefore, an excess of Si may deteriorate hot workability and adversely affect ductility and impact properties. In consideration thereof, the upper limit of the Si content may be controlled to 0.7%. Preferably, Si may be added in an amount of at least 0.3% but not less than 0.4%.

The content of manganese (Mn) may be at least 2.0% but not more than 4.0%.

In the present disclosure, Mn is an austenite phase-stabilizing element added instead of Ni. In consideration thereof, Mn may be added in an amount of 2.0% or more. However, an excess of Mn may cause excessive formation of S-based inclusions (MnS) resulting in deterioration of ductility and corrosion resistance. In consideration thereof, the upper limit of the Mn content may be controlled to 4.0%. Preferably, Mn may be added in an amount of at least 3.6% but not more than 3.9%.

The content of phosphorus (P) may be more than 0% but less than 0.1%.

P, as an inevitable impurity contained in steels, is an element causing intergranular corrosion and deteriorating hot workability. Therefore, it is preferable to control the P content as low as possible. In consideration thereof, the upper limit of the P content may be controlled to less than 0.1%.