TW201816124A - Martensitic stainless steel and method for the manufacture - Google Patents

Abstract

The invention relates to a martensitic stainless steel and its manufacturing method having high strength and enhanced tensile ductility. The steel consists of in weight % equal or less than 0.05% C, equal or less than 1.0% Si, 2.0-8.0% Mn, 10.5-18.0% Cr, 0.4-less than 4.0% Ni, equal or less than 1.5% Mo, equal or less than 1.0% Cu, equal or less than 0.05% N, equal or less than 0.1% Al, optionally at least one of the following group containing equal or less than 0.8% Ti, equal or less than 0.8% Nb, equal or less than 0.8% Zr, equal or less than 0.8% V, the rest being iron and evitable impurities occupying in stainless steels in such conditions, that the Ductility Index (DI)=[gamma]comp-0.03*Ms ≥ 0 where [gamma]comp=54.7-432*(C+N)FCC-6.43*SiFCC-0.603*MnFCC-2.572*CrFCC-0.818*NiFCC-0.157*HJ, HJ=T*(c+log10 t)/1000 and T is temperature in degrees Kelvin and c is a constant, when the Ms temperature is at the range of 50-350 DEG C.

Description

 Ma Tian scattered iron series stainless steel and its manufacturing method  

This invention relates to a cost effective granulated iron-based stainless steel having high tensile strength and improved elongation for improved formability. The invention also relates to a method of making a granulated iron-based stainless steel.

The most critical point in the development of the Matian iron-based stainless steel with improved elongation is how to heat-treat the steel to optimize the stability of the Wolster iron in order to improve the transformation induced plasticity (TRIP). Formability. The TRIP effect refers to a metastable reversal due to the application of stress or strain during plastic deformation and/or a change in the retention of Worth iron.

The stability of the Worthfield iron phase depends on the chemical composition and heat treatment cycle. The elements used to promote the formation and stability of the Worthite iron are carbon (C), nitrogen (N), nickel (Ni), manganese (Mn), copper (Cu) and cobalt (Co). The main elements that promote ferrite iron are chromium (Cr), molybdenum (Mo), strontium (Si), aluminum (Al), titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V). The cheapest way to stabilize Worthite is to use a gap alloying with carbon and/or nitrogen. However, excessive use of carbon and nitrogen in stainless steel causes corrosion problems due to precipitation of chromium carbide and chromium nitride. In addition, excessive hardening of the granulated iron causes embrittlement especially in the welded state.

Manganese can be effectively used in the stabilization process of Worthite iron because manganese has a weaker nickel potential than Worth. The weaker Vostian iron potential broadens the chemical composition and the range of heat treatment windows, making it possible to utilize the TRIP effect for low-gap zetian loose-iron stainless steel. However, the use of manganese to increase ductility requires good impurity control to prevent excessive formation of manganese sulfide.

Low-carbon ramie-distributed iron-based stainless steel (also known as super-maize-distributed iron-based stainless steel) is usually alloyed using a high nickel content of up to 7% by weight. These steels may also contain up to 3% by weight of molybdenum. All super-mashed iron grades utilize the so-called Worstian iron reversal phenomenon, which results in up to 30% of the Vostian iron converted to a tempered Matian bulk iron matrix when tempered at temperatures close to the A _c1 . When properly annealed, the Worthite iron will nucleate and thicken at the strip boundary of the granulated iron and maintain it at room temperature, thereby improving the ductility and toughness of the material.

Unless otherwise mentioned below, the elemental content is referred to in % by weight.

JP Patent Application No. 2008138270 relates to a high-strength steel sheet having in particular the following composition: 0.001-0.03% C, 0.001-0.03% N, 0.05-0.5% Si, 0.05-5% Mn, ≦0.05% P, 0.3- 5% Ni, 0.01-3% Cu, 10-18% Cr and 0.005-0.50% Al, and the balance is iron (Fe) and unavoidable impurities. According to this JP patent application, the hot-rolled steel sheet is heat-treated at a temperature ranging from 900 to 1100 ° C to form a granulated iron and then cold-rolled and heat-treated at a temperature ranging from 700 to 900 ° C to form a ferrite-based iron system + a granulated iron system. A structure that provides at least 500 MPa tensile strength and at least 15% elongation at break. However, although the ductility is improved, the strength value is medium, and this JP patent application does not consider the effect of TRIP on steel in improving ductility.

JP Patent Application No. 2009120954 describes a high strength and excellent toughness 麻田散铁式 stainless steel having a composition comprising: 0.01-0.1% C, 0.05-1% Si, 0.05-1.5% Mn, equal to or less than 0.03% P, equal to or less than 0.01% S, 9-15% Cr, 0.1-2.0% Ni, equal to or less than 0.05% Al, and equal to or less than 0.1% N, and further including one or more Cu, Mo, V , Nb, B, Ca, Mg and the second element of the rare earth metal, and the rest are Fe and impurities. According to this JP patent application, a small amount of Vostian iron is used to improve toughness.

EP Patent Application No. 1006204 A1 describes a hot rolled low carbon 麻田 dispersion iron stainless steel sheet which has excellent formability and corrosion resistance. The chemical composition includes: equal to or less than 0.05% C, equal to or less than 1% Si, equal to or less than 5% Mn, equal to or less than 0.04% P, equal to or less than 0.01% S, 10-15% Cr, 0-3% Mo, 0-0.75% Ti and 1-8% Ni and the rest are Fe and impurities. Further, a steel sheet having a relief strength equal to or less than 758 MPa was produced, and the Worth iron content was adjusted according to the thickness of the sheet.

SUMMARY OF THE INVENTION The object of the present invention is to eliminate the prior art and to obtain a method for producing a granulated stainless steel and a granulated iron-based stainless steel having high tensile strength and good sheet forming properties. The steel is alloyed and heat treated to achieve sufficient Worth iron stability while maintaining high tensile strength and cost efficiency. The essential features of the invention are set forth in the appended claims.

The chemical composition of the granulated iron-based stainless steel according to the present invention is composed of: equal to or less than 0.05% C, equal to or less than 1.0% Si, 2.0-8.0% Mn, 10.5-18.0% Cr, 0.4-less 4.0% Ni, equal to or less than 1.5% Mo, equal to or less than 1.0% Cu, equal to or less than 0.05% N, equal to or less than 0.1% Al, optionally including at least one of the following groups: equal to or Less than 0.8% Ti, equal to or less than 0.8% Nb, equal to or less than 0.8% Zr, equal to or less than 0.8% V, the balance being Fe and impurities which can be avoided in stainless steel.

According to the method of the present invention, steel is produced by hot rolling a slab into a sheet or sheet. The steel of the present invention can be cold rolled. In this case, after cold rolling, at least one heat treatment is performed. This thermomechanical treatment can utilize phase change induced plasticity (TRIP) behavior for the high manganese 麻田 dispersion iron stainless steel of the present invention. The TRIP effect occurs due to the high manganese alloying required to replace nickel. Manganese alloying also changes the phase transition range of Worthite and Matian iron to occur at lower temperatures and further allows for greater control of tempering, Worthite stability and phase.

The role of each alloying element is discussed below: carbon (C) enhances the stability of the Worth iron in the solid solution, but promotes the precipitation of chromium carbide and the hardening of the granulated iron. Preferably, C is removed as much as possible during the steel making process. To prevent precipitation of chromium carbide, the solid solution C can be fixed to a carbide by Ti, Nb, Zr, and V as described below. The C content is limited to be equal to or less than 0.05%, preferably equal to or less than 0.02%, but preferably at least 0.005%.

Strontium (Si) is used in steelmaking to reduce the chrome self-slag back into the melt. Some Si residue in the steel is required to ensure that the reduction proceeds smoothly. However, Si promotes the ferrite iron phase, and therefore, the Si content is equal to or less than 1%, preferably less than 0.7%, but at least 0.05%, most preferably 0.3-0.5%.

Manganese (Mn) moderately stabilizes the hot workability of the Worthfield iron phase and the modified steel. Mn is an economically viable element compared to nickel. However, when combined with sulfur, manganese degrades corrosion resistance by forming manganese sulfide. At low sulfur contents, the Mn content is 8.0% or less, preferably less than 7.5%, but at least 2.0%. A better range is 3-6%.

Chromium (Cr) is the main element that ensures the corrosion resistance of stainless steel, but chromium also promotes the ferrite phase. In order to achieve corrosion resistance comparable to other Ma Tian loose iron stainless steels, the Cr content must be 10.5-18%, preferably 11.0-17.0%, and most preferably 12-16%.

Nickel (Ni) is a Worstian iron stabilizer which advantageously promotes the element of improved toughness. However, since nickel is relatively expensive and subject to price fluctuations, the Ni content is less than 4.0%, preferably less than 3.5%, and most preferably less than 3%, such that the Ni content is at least 0.4%, preferably at least 0.5%.

Molybdenum (Mo) promotes corrosion resistance and tempering resistance, but also promotes the formation of ferrite and second precipitates. The Mo content is equal to or less than 1.5%, preferably less than 0.5%, but at least 0.005%.

Copper (Cu) is a Worstian iron stabilizer and improves corrosion resistance, but a high Cu content can reduce elongation at break by introducing small precipitates. Therefore, the Cu content is equal to or less than 1%, preferably less than 0.5%, but at least 0.05%.

Nitrogen (N) has an effect similar to that of carbon. A nitrogen-based Worstian iron stabilizer, but based on the nitrogen content, precipitation of chromium nitride and excessive hardening of the granulated iron are caused. Therefore, the N content is equal to or less than 0.05%, preferably less than 0.045%, most preferably less than 0.03%, but at least 0.005%.

Aluminum (Al) is used to remove oxygen from the melt. The Al content is equal to or less than 0.1%.

It is not necessary to add titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V) as microalloying elements, but Ti, Nb, Zr and/or V are extremely useful for preventing chromium carbide and nitriding. Chromium precipitation, due to the formation of various carbides and nitrides at very high temperatures due to Ti, Nb, Zr and/or V. Advantageously, at least one of the elements titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V) is added. These elements also improve temper resistance, ie delay excessive softening. Titanium nitride improves grain structure in casting and welding. However, Ti promotes the formation of ferrite. The Ti content is equal to or less than 0.8%, preferably 0.005 to 0.6%. Nb can be used to control the recrystallization behavior in hot rolling. However, the lanthanide precipitate forms the most expensive element in the element. The Nb content is equal to or less than 0.8%, preferably from 0.005 to 0.6%. Zr can be used similarly to Ti. The Zr content is equal to or less than 0.8%, preferably from 0.005 to 0.6%. V forms carbides and nitrides at lower temperatures near chromium. The V content is equal to or less than 0.8%, preferably 0.005 to 0.6%.

The 麻田散铁-based stainless steel of the present invention generally has a tensile strength of 850 to 1000 MPa and a total elongation of 15 to 19%. Steel is manufactured by hot rolling a slab into a sheet or sheet. After the hot rolling, heat treatment is performed at a temperature of 500 ° C or higher, but not higher than 950 ° C (calculated by the formula (1)).

T=950-15.80 * Mn-33.28 * Ni (1)

Among them, Mn and Ni are based on the weight % of steel.

More specifically, according to the present invention, the heat treatment satisfies the conditions described by the "extensibility index" (DI) as calculated by the formula (2).

DI=γ _comp -0.03 * M _s ≧0 (2)

Where γ _comp contains the temperature-specific equilibrium concentration of carbon (C), strontium (Si), manganese (Mn), chromium (Cr), nickel (Ni) and nitrogen (N) in the iron phase of Vostian and the M _s system The starting temperature of the loose iron. γ _comp is calculated by the formula (3).

γ _comp =54.7-432 *(C+N) _FCC -6.43 * Si _FCC -0.603 * Mn _FCC -2.572 * Cr _FCC -0.818 * Ni _FCC -0.157 * HJ (3)

Wherein C, Si, Mn, Cr, Ni and N are based on the weight % of the Wostian iron phase and the HJ system is calculated from the Hollomon-Jaffe parameter of the formula (4).

HJ=T *(c+log ₁₀ t)/1000 (4)

Where T is the temperature in Kelvin, c is dependent on the constant of the material (here 30), and t is the time in minutes.

Where M _s is the temperature at which the transition of the granulated iron in the field is initiated (here 50-350 ° C).

Depending on the desired thickness of the granulated stainless steel, the hot rolled steel is cold rolled to obtain the desired thickness of the granulated stainless steel.

In a specific example of cold rolling the steel of the present invention, the subsequent annealing treatment is always performed at a temperature higher than 580 ° C but at most 950 ° C, preferably 600 to 750 ° C, and then cooled. After cooling, a similar heat treatment as in the specific example of the hot rolled material is performed. Therefore, in the method of the present invention, both hot-rolled granulated stainless steel and cold-rolled granulated stainless steel can be obtained.

The heat treatment of the present invention results in a metastable reversal of the Vostian iron phase into the steel microstructure. In order to improve the ductility by metastable reversal of the Vostian iron, the steel of the present invention is subjected to a second heat treatment at a temperature of at least 500 ° C or higher but not higher than the temperature calculated by the formula (1). If the heat treatment is carried out at less than 500 ° C, the formation of the iron phase in the reverse of the Vostian will be severely limited. If the heat treatment is carried out at a temperature higher than the formula (1), the reverse phase of the Vostian iron phase will be too unstable to be maintained at room temperature. Therefore, improvement in the strength-ductility balance cannot be achieved.

The makita iron-based stainless steel alloy AE listed in Table 1 was prepared to test the 麻田散铁-based stainless steel of the present invention. During the preparation, each alloy was melted, cast and hot rolled to a thickness of 6 mm in the temperature range of 900-1200 °C. The material was further cold rolled into a 3 mm sheet and heat treated according to Table 1. The table further contains the chemical composition of the reference steel XZ. The M _s temperature was measured using a dilatometer.

From Table 1, it can be seen that the alloy AE system utilizes low nickel and accompanying high manganese content melting to ensure sufficient Worthite potential in the steel. Heat D further contains a higher chromium content and a lowered M _s temperature. Reference alloys X and Y are typical low carbon 麻田 dispersion iron stainless steels with high nickel content and low manganese content. The reference alloy Z is a low nickel content variant having a low manganese content. All example alloys were microalloyed with vanadium and molybdenum.

The TRIP effect is the most important property of the granulated iron-based stainless steel of the present invention. This is affected by the chemical composition of the steel and the heat treatment applied to the steel during processing. Table 2 describes the effects of various heat treatments of the example steels and the tensile data obtained from the samples. Temperature-specific equilibrium concentration of carbon (C), cerium (Si), manganese (Mn), chromium (Cr), nickel (Ni) and nitrogen (N) in the iron phase of Vostian after final heat treatment using Thermo-Calc Software (product of Thermo-Calc Software AB (Stockholm, Sweden)) to calculate. The transverse tensile properties of the steel were measured at room temperature according to the EN 10002-1 standard using a tensile tester.

The results presented in Table 2 show that only the steels of the present invention satisfy a tensile strength equal to or greater than 850 MPa and a tensile elongation equal to or greater than 15%. In the steel of the present invention, the tensile strength ranges from 863 to 978 MPa and the elongation at break is within 15 to 19%. When the annealing is insufficient (T ₁ ≦ 580 ° C), the steel of the present invention has a high tensile strength (959-1008 MPa), but the elongation is unsatisfactory (9-11%). Furthermore, if the potential of the Wrestfield iron for the TRIP effect is insufficient (DI<0) during annealing, the tensile strength is 912-1055 MPa and the elongation is moderate (6-11%).

The reference steel has a tensile strength range of 788-1052 MPa and a tensile elongation of 7-17%, but none of the samples simultaneously satisfy a tensile strength equal to or greater than 850 MPa and a tensile elongation equal to or greater than 15%. The chemical composition of Vostian Iron does not produce the desired TRIP effect for improved ductility.

Therefore, the granulated stainless steel of the present invention (sample 1-31) has improved strength compared to the reference steel (samples 44-79) and the inventive steel (samples 32-43) which are heat-treated without considering a given limit standard. - Extensibility balance.

Claims (16)

A granulated iron-based stainless steel having high strength and improved tensile ductility, characterized in that the steel system is composed of the following components (in % by weight): equal to or less than 0.05% C, equal to or less than 1.0 % Si, 2.0 to 8.0% Mn, 10.5 to 18.0% Cr, 0.4 at least 4.0% Ni, equal to or less than 1.5% Mo, equal to or less than 1.0% Cu, equal to or less than 0.05% N, equal to or less 0.1% Al, as the case may include at least one of the following groups: equal to or less than 0.8% Ti, equal to or less than 0.8% Nb, equal to or less than 0.8% Zr, equal to or less than 0.8% V, and the remainder Iron and impurities that can be avoided in stainless steel under the following conditions, ductility index (DI) = γ _comp -0.03 * M _s ≧0 where γ _comp =54.7-432 *(C+N) _FCC -6.43 * Si _FCC - 0.603 * Mn _FCC -2.572 * Cr _FCC -0.818 * Ni _FCC -0.157 * HJ, HJ = T * (c + log ₁₀ t) / 1000 and T is the temperature and c-system constant in terms of Kelvin degree. Wherein the temperature of the M _s is in the range of 50 to 350 °C.  The granulated iron-based stainless steel of claim 1, wherein the carbon content is less than 0.045% by weight, but at least 0.005%.    The granulated iron-based stainless steel of claim 1 or 2, wherein the cerium content is 0.05 to 0.7% by weight.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the manganese content is less than 7.5% by weight, preferably 3.0 to 6.0%.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the nickel content is less than 3.0% by weight, but at least 0.5%.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the molybdenum content is 0.005 to 0.5% by weight.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the copper content is less than 1.0% by weight, but at least 0.2%.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the nitrogen content is less than 0.045% by weight, but at least 0.005%.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the titanium content is 0.005 to 0.60% by weight.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the cerium content is 0.005 to 0.60% by weight.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the zirconium content is from 0.005 to 0.60% by weight.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the vanadium content is 0.005 to 0.60% by weight.    A granulated iron-based stainless steel according to any one of the preceding claims, wherein the chromium content is from 11.0 to 17.0% by weight.    A method of producing a granulated iron-based stainless steel according to any one of the preceding claims, characterized in that after hot rolling, at least one heat treatment is carried out at a temperature ranging from 500 ° C to 950 ° C.    The method of claim 14, wherein after the hot rolling, at least one heat treatment is performed in a temperature range of from 500 ° C to (950 - 15.80 * Mn - 33.28 * Ni) ° C.    The method of claim 14 or 15, wherein the steel is cold rolled and heat treated and cooled at a temperature ranging from 580 ° C to 950 ° C, preferably from 600 to 750 ° C, and further at 500 ° C to (950-15.80) * Mn-33.28 * Ni) Heat treatment in the temperature range of °C.