The present invention relates to a cost-efficient martensitic stainless steel having high tensile strength and enhanced elongation for improved formability. The invention also relates to a method for manufacturing the martensitic stainless steel.

The most critical point in developing martensitic stainless steel with enhanced elongation is how to heat treat the steel to optimize austenite stability in order to utilize transformation induced plasticity (TRIP) for improving the formability. The TRIP effect refers to the transformation of metastable reversed and/or retained austenite during plastic deformation as a result of imposed stress or strain. The stability of the austenite phase is dependent upon the chemical composition and the heat treatment cycle. The elements that are used for promoting austenite formation and its stability are carbon (C), nitrogen (N), nickel (Ni), manganese (Mn), copper (Cu) and cobalt (Co). The elements that predominantly promote ferrite are chromium (Cr), molybdenum (Mo), silicon (Si), aluminium (Al), titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V). The most affordable way to stabilize austenite is to use interstitial alloying with carbon and/or nitrogen. However, excessive use of carbon and nitrogen in stainless steels leads to corrosion problems due to chromium carbide and nitride precipitation. In addition, an excessive hardening of martensite leads to brittleness especially in as-welded condition.

Manganese can be effectively used in the stabilization process of austenite as manganese has weaker austenite potential than that of nickel. Weaker austenite potential broadens the range of chemical composition and heat treatment window, making the utilization of TRIP effect feasible for low interstitial martensitic stainless steel. However, the utilization of manganese for enhanced ductility requires good impurity control to prevent excessive manganese sulphide formation.

Low carbon martensitic stainless steels, also known as supermartensitic stainless steel, are usually alloyed with high nickel content up to 7 weight %. These steels may also contain molybdenum up to 3 weight %. All supermartensitic grades utilize so-called austenite reversion phenomena, which results in up to 30 % austenite to the tempered martensite matrix, when tempering is carried out near the A _c1 temperature. When annealing is done properly, austenite will nucleate and enrichen to martensite lath boundaries and retain at room temperature, improving ductility and toughness of the material.

Hereafter, referring to the element contents are in weight %, if not anything else mentioned.

The JP patent application 20081 38270 relates to a high-strength steel sheet having amongst others the 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, 1 0 - 18 % Cr and 0.005 - 0.50 % Al, and the balance iron (Fe) with inevitable impurities. According to this JP patent application a hot rolled steel sheet is heat-treated at the temperature range of 900 - 1 1 00 °C to form martensite and subsequently cold rolled and heat-treated at the temperature range of 700 - 900 °C to form ferritic+martensitic structure, which provides at least 500 MPa tensile strength and at least 1 5% fracture elongation. However, despite having improvements in ductility, the strength level is moderate and this JP patent application does not consider the effect of TRI P in the steel for enhanced ductility.

The JP patent application 20091 20954 describes a high strength and excellent toughness martensitic stainless steel with a composition comprising 0.01 - 0.1 % C, 0.05 - 1 % Si, 0.05 - 1 .5 % Mn, equal or less than 0.03 % P, equal or less than 0.01 % S, 9 - 1 5 % Cr, 0.1 - 2.0 % Ni, equal or less than 0.05 % Al and equal or less than 0.1 % N, and further comprising one or more secondary elements of Cu, Mo, V, Nb, B, Ca, Mg and rare earth metals, and the balance of Fe with impurities. In accordance with this JP patent application the minor amount of austenite is utilized to improve the toughness.

The EP patent application 1006204 A1 describes a hot rolled low carbon martensitic stainless steel plate, which has excellent formability and corrosion resistance. The chemical composition is comprising equal or less than 0.05 % C, equal or less than 1 % Si, equal or less than 5 % Mn, equal or less than 0.04 % P, equal or less than 0.01 % S, 10 - 15 % Cr, 0 - 3 % Mo, 0 - 0.75 % Ti and 1 - 8 % Ni with a balance of Fe and impurities. In addition, the steel plate is manufactured to have yield strength of equal or less than 758 MPa and austenite content is adjusted depending on the plate thickness.

The object of the present invention is to eliminate drawbacks of the prior art and to achieve a martensitic stainless steel and a manufacturing method for a martensitic stainless steel having high tensile strength and good sheet forming properties. The steel is alloyed and heat treated to achieve sufficient stability of austenite, while maintaining high tensile strength and cost efficiency. The essential features of the present invention are enlisted in the appended claims.

The chemical composition of the martensitic stainless steel according to the invention consists of 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 Fe and evitable impurities occupying in stainless steels. According to the method of the invention the steel is manufactured by hot rolling a slab into a plate or a sheet. It is possible that cold rolling is carried out to the steel of the invention. In that case, after cold rolling, at least one heat treatment is carried out. This thermomechanical treatment achieves to utilize the transformation induced plasticity (TRIP) behaviour for the high manganese martensitic stainless steel of the invention. The TRIP effect occurs due to the high manganese alloying required to replace nickel. Manganese alloying also alters the phase transformation range of austenite and martensite to occur at lower temperatures and further allows greater control of tempering, austenite stability and the amount of the phases.

The effects of each alloying element are discussed in the following:

Carbon (C) increases austenite stability in solid solution but promotes precipitation of chromium carbides and hardening of martensite. Preferably, C is removed as much as possible during the steelmaking process. To prevent chromium carbide precipitation, the solid-solution C can be fixed as carbides by Ti, Nb, Zr and V as described below. The C content is limited equal or less than 0.05 %, preferably equal or less than 0.02 %, but preferably at least 0.005 %.

Silicon (Si) is used in steelmaking to reduce chromium from slag back to melt. Some Si remainders in steel are necessary to make sure that reduction is done well. However, Si promotes ferrite phase and, therefore, the Si content is equal or less than 1 %, preferably less than 0.7 %, but at least 0.05 %, most preferably 0.3 - 0.5 %.

Manganese (Mn) stabilizes the austenite phase moderately and improves hot workability of the steel. Mn is one of economically feasible elements compared to nickel. However, when combined with sulphur manganese can degrade corrosion resistance by forming manganese sulphides. With low sulphur content the Mn content is equal or less than 8.0 %, preferably less than 7.5 %, but at least 2.0 %. The more preferable range is 3 - 6 %. Chromium (Cr) is the main element to secure stainless steel's corrosion resistance, but chromium also promotes ferrite phase. In order to achieve corrosion resistance comparable to other martensitic stainless steels, the Cr content must be 10.5 - 18 %, preferably 1 1 .0 - 17.0 %, most preferably 12 - 16 %.

Nickel (Ni) is an austenite stabilizer, element favourably contributing to the improvement of toughness. However, because nickel is very expensive and subjected to price fluctuation, the Ni content is less than 4.0 %, preferably less than 3.5 %, most preferably less than 3 % so that the Ni content is at least 0.4 %, preferably at least 0.5 %.

Molybdenum (Mo) enhances corrosion resistance and brings about tempering resistance but promotes ferrite and formation of secondary precipitates. The Mo content is equal or less than 1 .5 %, preferably less than 0.5 %, but at least 0.005 %.

Copper (Cu) is an austenite stabilizer and improves corrosion resistance, but high Cu content can reduce elongation to fracture by introduction of small precipitates. The Cu content is thus equal or less than 1 %, preferably less than 0.5 %, but at least 0.05 %.

Nitrogen (N) has effects similar to the effects of carbon. Nitrogen is an effective austenite stabilizer, but based on the nitrogen content precipitation of chromium nitrides and excessive hardening of the martensite may occur. Thus, the N content is equal or less than 0.05 %, preferably less than 0.045 %, most preferably less than 0.03 %, but at least 0.005 %. Aluminium (Al) is used to remove oxygen from the melt. The Al content is equal or less than 0.1 %. Titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V) as microalloying elements are not necessarily added but Ti, Nb, Zr and/or V can be very useful to prevent chromium carbide and chromium nitride precipitation because Ti, Nb, Zr and/or V form various carbides and nitrides at very high temperatures. Advantageously at least one of the elements titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V) is added. These elements also improve the tempering resistance, i.e. retard excessive softening. Titanium nitrides refine the grain structure in casting and welding. However, Ti promotes ferrite formation. The Ti content is equal or less than 0.8 %, preferably 0.005 - 0.6 %. Nb can be used to control the recrystallization behaviour in hot rolling. However, niobium is the most expensive element of precipitate forming elements. The Nb content is equal or less than 0.8 %, preferably 0.005 - 0.6 %. Zr can be used similar to that of Ti. The Zr content is equal or less than 0.8 %, preferably 0.005 - 0.6 %. V forms carbides and nitrides at lower temperatures close to that of chromium. The V content is equal or less than 0.8 %, preferably 0.005 - 0.6 %.

The martensitic stainless steel of the present invention has typically a tensile strength of 850 - 1000 MPa and total elongation 15 - 19 %. The steel is manufactured by hot rolling a slab into a plate or a sheet. After hot rolling heat treatment is carried out at 500 °C or above, but not higher than 950 °C temperature, calculated by the formula (1 ).

T = 950 - 15.80 * Mn - 33.28 * Ni (1 ) where Mn and Ni are in weight % in the steel.

More specifically, according to the invention the heat treatment satisfies conditions described by the "Ductility Index" (Dl) as calculated by the formula (2).

Dl = Ycomp - 0.03 * Ms > 0 (2) where γ _comp contains a temperature specific equilibrium concentration of carbon (C), silicon (Si), manganese (Mn), chromium (Cr), nickel (Ni) and nitrogen (N) in the austenite phase and Ms is the martensite start temperature. The Vcomp is calculated by the formula (3).

Ycomp = 54.7 - 432 * (C+N)FCC - 6.43 * SIFCC - 0.603 * MnFcc - 2.572 * CrFcc - 0.818 * Ni _F cc - 0.157 * HJ (3) where C, Si, Mn, Cr, Ni and N are in weight % in the austenite phase and HJ is a Hollomon-Jaffe parameter, calculated by the formula (4).

HJ = T * (c + logio t) / 1000 (4) where T is temperature in degrees Kelvin, c is a constant depending on the material (here 30), and t is time in minutes. where Ms is the temperature where martensite transformation begins (here 50 - 350 °C). Dependent on the desired thickness of the martensitic stainless steel, the hot rolled steel is cold rolled in order to have the desired thickness for the martensitic stainless steel.

In an embodiment where cold rolling is carried out to the steel of the invention, a subsequent annealing treatment is always carried out at a temperature above 580 °C but up to 950 °C, preferably 600 - 750 °C and then cooled. After cooling a similar heat treatment as in the embodiment for the hot rolled material is carried out. Thus in the method of the invention both a hot rolled martensitic stainless steel and a cold rolled martensitic stainless steel can be produced.

The heat treatment of the invention brings about meta-stable reversed austenite phase into the steel microstructure. In order to utilize meta-stable reversed austenite for enhanced ductility, the steel of the invention is subjected to a secondary heat treatment at a temperature of at least 500 °C or above but not higher than the temperature calculated by the formula (1 ). If heat treatment is carried out below 500 °C the formation of reversed austenite phase will be severely limited. If heat treatment is carried out above the formula (1 ), the reversed austenite phase will be too unstable to retain at room temperature. Consequently, the improvements in strength-ductility balance are not achieved.

The martensitic stainless steel alloys A - E listed in Table 1 were prepared for testing the martensitic stainless steel of the invention. During the preparation every alloy was melted, cast and hot-rolled to a thickness of 6 mm at the temperature range of 900 - 1200 °C. The materials were further cold rolled into 3 mm sheets and heat treated according to Table 1 . The table further contains the chemical compositions of the reference steels X - Z. The Ms temperatures were determined by using a dilatometer.

From Table 1 it is seen that the alloys A - E are melts with low nickel with accompanied high manganese content to secure enough austenite potential in a steel. The heat D further contains higher chromium content and a decrease in the Ms temperature. The reference alloys X and Y are typical low carbon martensitic stainless steels with high nickel content and low manganese content. The reference alloy Z is a low nickel content variant with low manganese content. All example alloys were microalloyed with vanadium and molybdenum. The TRIP effect is the most important property of the martensitic stainless steel of the invention. This is affected by the chemical composition of the steel and the heat treatments applied to the steel during processing. Table 2 describes the effects of various heat treatments of the example steels and tensile data obtained from the specimens. The temperature specific equilibrium concentration of carbon (C), silicon (Si), manganese (Mn), chromium (Cr), nickel (Ni) and nitrogen (N) in the austenite phase after the last heat treatment was calculated using Thermo-Calc software, product of Thermo-Calc Software AB, Stockholm, Sweden. The transversal tensile properties of the steels were measured by using a tension testing machine at room temperature according to standard EN 10002-1 .

Note: T ₁ is the annealing temperature and t ₁ is the duration of the treatment carried out after cold rolling, T ₂ is the secondary temperature and t ₂ is the duration of the treatment. The UTS is the ultimate tensile strength and A50 is the elongation to fracture obtained from tension test, x = failed to meet the criterion, o = meets the criterion of the invention.

The results presented in Table 2 show that only those of the invention steels satisfy equal or greater than 850 MPa tensile strength with equal or greater than 15 % tensile elongation. The range of tensile strength varied 863 - 978 MPa and elongation to fracture 15 - 19 % in steels of the invention. When the annealing was insufficient (T ₁ ≤ 580°C), the steels of the invention had high tensile strength 959 - 1008 MPa, but the elongation was unsatisfactory 9 - 1 1 %. Furthermore, if the austenite potential for TRIP effect at annealing was insufficient (Dl < 0), the tensile strength was 912 - 1055 MPa and the elongation was modest 6 - 1 1 %.

The comparative reference steels had 788 - 1052 MPa range of tensile strength with 7 - 17 % tensile elongation, but none of the samples satisfied the equal or greater than 850 MPa tensile strength with equal or greater than 15 % tensile elongation simultaneously. The chemical composition of the austenite failed to produce the needed TRIP effect for echanced ductility.

Thus, the martensitic stainless steel of the invention (samples 1 -31 ) has enhanced strength-ductility balance compared to reference steels (samples 44- 79) and invention steels heat treated without considering the limiting criteria given (samples 32-43).