A martensitic steel, a steel strip and method for production thereof

Abstract

A martensitic steel comprising, in percent by weight (wt.-%): C 0.40 – 0.55, Si 0.2 – 1.1, Mn 0.20 – 0.70, P equal to or less than 0.03, S equal to or less than 0.03, Cr 5.0 – 9.5, Ni 0.4 – 2.2, N 0.030 to less than 0.090, Mo 1.0 – 2.0, Cu 0.15 – 1.2, V 0.15 – 0.40, optionally Al up to 0.25, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally B up to 0.0050, balance Fe and normally occurring impurities up to 0.8 wt.- % in total.

Description

[0001] A MARTENSITIC STEEL, A STEEL STRIP AND METHOD FOR PRODUCTION THEREOF

[0002] TECHNICAL FIELD

[0003] The present disclosure relates in general to a martensitic steel which is suitable for use in valve components. The present disclosure further relates in general to a steel strip comprising said martensitic steel, and to a method for producing such a strip.

[0004] BACKGROUND

[0005] Compressors that are used e.g., in refrigerators and air conditioners for residential, commercial or vehicle use typically comprise flapper vales made of steel to control the flow of the refrigerant in the system. One of the major challenges to be able to increase compressor efficiency and / or develop new compressor designs is the properties of the steel material. This is because an increased performance of the compressor puts higher demands on performance and reliability of the flapper valves, and thereby also on the steel used to manufacture them. Material manufacturers have therefore made great efforts to enhance the properties of existing materials, for example by various process modifications and / or surface treatments, and to develop new materials to keep pace with the needs of the compressor industry. The increase in tomorrow's compressors' performance requires the steel to have excellent mechanical properties, such as for example high tensile strength and high ductility.

[0006] Common steels used for flapper valves within the compressor industry today belong either to the AISI 1095 type of carbon steels or the AISI 420 type of martensitic stainless steels.

[0007] Chai, G. et al., "Flapper Valve Steels With High Performance", International Compressor Engineering Conference, Paper 1702, 2004, discusses inter alia the fatigue strength of three different steels used for manufacturing flapper valves. They concluded that the combination of high tensile strength and high ductility, together with surface treatment to obtain high compressive residual stresses, is beneficial for the fatigue properties.

[0008] Moreover, Chai, G. et al., "Fatigue Behaviours of a Compressor Valve Stainless Steel with small Amount of retained Austenite", Conference paper AT ICF10, Honolulu, 2001, discloses the influences of retained austenite in a CrMo martensitic stainless steel on the fatigue properties. The martensitic stainless steel strip material used in the tests had a composition comprising 0.37 wt.-% C, 0.4 wt.-% Si, 0.60 wt.-% Mn, 13.5 wt.-% Cr and 1.0 wt.-% Mo. It was inter alia found that increase in the strength of the material was the additive of precipitation strengthening of fine dispersed carbides and the softening due to the increase of retained austenite up to about 10 vol%. It was also found that an increase in retained austenite significantly improved the impact fatigue strength.

[0009] It has recently been found that resistance to wear is an important property to consider. When a flapper valve steel is used in compressor applications, a wear mark from the valve seat may typically be seen after a period of operation. The depth of the wear mark generally increases with increasing operating time. Without being bound by theory, it is believed that the increase in depth of the wear mark leads to an increase in stress concentrations. These stress concentrations may ultimately become too high and thereby lead to failure of the valve. It is therefore believed that a reduced depth of the wear mark may reduce the presence of hazardous stress concentrations and thereby avoid failure. Reducing the depth of the wear mark would require an increase in wear resistance of the steel. However, the increase of the wear resistance would have to be achieved without negatively affecting the fatigue properties.

[0010] Larsson, J., "Wear mark evolution and numerical study of impact stresses in stainless steel flapper valves", Degree project in materials design and engineering, KTH Royal Institute of Technology, 2016, discloses a study of wear mark evolution on the surface of flapper valves that impacts with the seat. The steel used for the study was a martensitic stainless steel having a nominal composition comprising 0.38 wt.-% C, 0.40 wt.-% Si, 0.55 wt.-% Mn, 13.5 wt.-% Cr and 1.0 wt.-% Mo.

[0011] One example of a martensitic stainless steel alloy that may be used in flapper valves is disclosed in WO 2020 / 245285 Al.

[0012] EP 3031942 Al discloses yet another example of a martensitic stainless steel strip for flapper valves in compressors.

[0013] SUMMARY

[0014] The aspect of the present disclosure is a steel which is able to obtain a good balance of properties, making it suitable for use in demanding valve applications.

[0015] In accordance with the present disclosure, a martensitic steel is provided. The martensitic steel comprises, in percent by weight (wt.-%):

[0016] C 0.40 - 0.55,

[0017] Si 0.2 - 1.1,

[0018] Mn 0.20 - 0.70, P equal to or less than 0.03,

[0019] S equal to or less than 0.03,

[0020] Cr 5.0 - 9.5,

[0021] Ni 0.4 - 2.2,

[0022] N 0.030 to less than 0.090,

[0023] Mo 1.0 - 2.0,

[0024] Cu 0.15 - 1.2,

[0025] V 0.15 - 0.40, optionally Al up to 0.25, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally B up to 0.005, balance Fe and normally occurring impurities up to 0.8 wt.-% in total.

[0026] The herein described martensitic steel provides a completely new concept for obtaining a good balance of properties making the material suitable for e.g., demanding valve applications; said concept relying neither on modification of the composition or microstructure of, nor modification of the production process or surface treatment of, conventional carbon steels or martensitic stainless steels commonly used in such applications.

[0027] More specifically, the herein described steel enables obtaining a high tensile strength simultaneously with a relatively high ductility. Moreover, the herein described steel also allows for introducing high surface residual stresses after hardening and tempering. A material having high tensile strength, high ductility, as well as high surface residual stresses will also have good fatigue properties. At the same time, it is possible to obtain a high hardness, and thereby a high wear resistance. Thereby, the herein described martensitic steel enables achieving an excellent combination of properties. This in turn makes the martensitic steel highly suitable for use in very demanding valve applications.

[0028] The present disclosure also provides a steel strip comprising the martensitic steel. Said steel strip may for example be used for producing a valve component, such as a valve component for a compressor, such as a compressor reed valve component. The present disclosure also relates to a method for producing such a steel strip. The method comprises the following steps:

[0029] - casting a melt having a composition comprising, in percent by weight:

[0030] C 0.40 - 0.55,

[0031] Si 0.2 - 1.1,

[0032] Mn 0.20 - 0.70,

[0033] P equal to or less than 0.03,

[0034] S equal to or less than 0.03,

[0035] Cr 5.0 - 9.5,

[0036] Ni 0.4 - 2.2,

[0037] N 0.030 to less than 0.090,

[0038] Mo 1.0 - 2.0,

[0039] Cu 0.15 - 1.2,

[0040] V 0.15 - 0.40, optionally Al up to 0.25, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally B up to 0.005, balance Fe and normally occurring impurities up to 0.8 wt.-% in total,

[0041] - optionally heat treating the as-cast material,

[0042] - followed by hot rolling the cast steel to intermediate thickness,

[0043] - optionally heat treating the hot rolled steel,

[0044] - cold rolling the hot rolled steel to intended final strip thickness,

[0045] - hardening the cold rolled strip at a hardening temperature of 990 - 1080°C followed by quenching, and

[0046] - tempering the hardened strip to temperature of 200-460 °C.

[0047] BRIEF DESCRIPTION OF DRAWINGS

[0048] Fig. 1 illustrates the amount of retained austenite as a function of hardening temperature for experimental alloys,

[0049] Fig. 2 illustrates hardness, in the hardened and tempered condition, as a function of tempering temperature for experimental alloys after having been subjected to a hardening temperature aimed for a similar amount of retained austenite, Fig. 3 shows average surface residual stress after tumbling as a function of carbon content of experimental alloys,

[0050] Fig. 4 illustrates a LOM image of experimental alloy 5 after hardening at 1020 °C, and

[0051] Fig. 5 illustrates a LOM image of experimental alloy 8 after hardening at 1020 °C.

[0052] DETAILED DESCRIPTION

[0053] The invention will be described in more detail below with reference to various exemplifying embodiments. The invention is however not limited to the exemplifying embodiments discussed but may be varied within the scope of the appended claims.

[0054] The term "martensitic steel" is in the present disclosure considered to mean a steel which, after hardening, has a primarily martensitic matrix, but which may also comprise retained austenite and precipitates, such as carbides and / or carbonitrides and / or nitrides.

[0055] The terms "wt.-%" and "%" are used interchangeable throughout the present disclosure.

[0056] The present disclosure is directed to a martensitic steel primarily developed for use in valve components, such as a compressor reed valve component or a wear plate. The martensitic steel is, for example, especially suitable for use in valve components for compressors. In contrast to conventionally used steels for valve applications, the herein described martensitic steel does not belong to the AISI 1095 type of carbon steels or the AISI 420 type of martensitic stainless steels. Instead, the herein described martensitic steel provides a completely new concept for the development of steels for valve applications.

[0057] Although the martensitic steel has primarily been developed for use in valve components as mentioned above, it should be noted that the martensitic steel may also be used in other types of components where a high fatigue strength and / or wear resistance is desired.

[0058] The herein described martensitic steel may be present in any possible form and / or condition without departing from the present disclosure, except where explicitly specified otherwise. Thus, the martensitic steel may be present in e.g., as-cast condition, such as in the form of an ingot, a billet, or a bloom. Alternatively, the martensitic steel may be produced into an intermediate or final product form (such as a bar, a strip, a plate or the like, or a component produced thereof). It is generally accepted in the art that the fatigue strength of a steel component subjected to cyclic bending loads, such as a valve component, typically increases with increasing tensile strength. It is also important for the steel component to have a high ductility in combination with the high tensile strength. The ductility of a martensitic steel may for example be increased by increasing the amount of retained austenite. The present inventors have learned that an increased retained austenite (RA) level may reduce build-up of hazardous stress concentrations, and thereby reduce wear mark sensitivity and increase fatigue properties. However, increasing the amount of retained austenite may lead to a reduction of the tensile strength and may often also negatively impact the hardness. Finding a proper balance between desired properties is thus not an easy task.

[0059] Current knowledge within the art of steels for compressor valve applications indicates that a suitable amount of retained austenite would be about 14-18 vol.-% as long as a high tensile strength is simultaneously achieved. Increased tensile strength may for example be achieved through precipitation strengthening. However, it is important to have a microstructure comprising fine dispersed precipitates in order not to introduce defects which may otherwise introduce unwanted stress concentrations in the material, which in turn may reduce the fatigue strength.

[0060] It is also well accepted in the art that the fatigue strength of a steel component subjected to cyclic bending and impact loads may be improved by introducing residual compressive stresses at its surface. This is common practice used in flapper valve manufacturing, where compressive residual stresses are typically introduced by surface treatment, such as shoot peening or tumbling. The fatigue strength is known to increase with increasing compressive surface residual stress.

[0061] Moreover, it would be desirable to increase the wear resistance of a steel intended to be used in e.g., valve applications in order to be able to reduce the problem associated with wear mark evolution.

[0062] The composition of the hereinabove or hereinafter described martensitic steel enables excellent tensile properties and high ductility to be obtained in combination with a high wear resistance when the steel is in a hardened and tempered condition. This is achieved inter alia through enabling a desired amount of retained austenite together, through control of processing steps such as the hardening temperature, with small and finely distributed precipitates of carbides and possibly also carbonitrides. Furthermore, the composition allows for very high compressive residual stresses to be introduced, after hardening and tempering, through surface treatment, such as by tumbling, shoot peening or the like. A component, such as a compressor reed valve component, comprising the herein described martensitic steel may be produced from a strip of the martensitic steel. Therefore, the present disclosure is also specifically directed to a steel strip comprising, or consisting of, said martensitic steel. In case the steel strip does not entirely consist of the martensitic steel, the steel strip may for example be a coated steel strip.

[0063] The martensitic steel according to the present disclosure has the following composition, in percent by weight (wt.-%):

[0064] C 0.40 - 0.55,

[0065] Si 0.2 - 1.1,

[0066] Mn 0.20 - 0.70,

[0067] P equal to or less than 0.03,

[0068] S equal to or less than 0.03,

[0069] Cr 5.0 - 9.5,

[0070] Ni 0.4 - 2.2,

[0071] N 0.030 to less than 0.090,

[0072] Mo 1.0 - 2.0,

[0073] Cu 0.15 - 1.2,

[0074] V 0.15 - 0.40, optionally Al up to 0.25, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally B up to 0.005, balance Fe and up to 0.8 wt.-% in total of normally occurring impurities.

[0075] When ranges are disclosed in the present disclosure, such ranges include the respective end values of the range, unless explicitly disclosed otherwise. Similarly, when an open range is disclosed, the open range also include the single end value of the open range, unless explicitly disclosed otherwise.

[0076] In the following, the importance of the different alloying elements of the herein described steel will be briefly discussed. All percentages for the chemical composition are given in weight-% (wt.-%), the term is also used, unless explicitly disclosed otherwise. Any herein disclosed upper and / or lower limit of the individual elements of the composition, as specified below, can be freely combined within the broadest limits of the composition of the martensitic steel as set out in the claims, unless explicitly disclosed otherwise. Carbon (C): 0.40 - 0.55 %

[0077] Carbon is an important element for the hardenability of the steel as it contributes to solid solution strengthening. Furthermore, carbon is important for the precipitation of secondary carbides (and / or carbonitrides), such as carbides of M23C6 and M7C3 type. Secondary carbides and carbonitrides contribute to increased strength when being finely dispersed and of small size, i.e. nanosized.

[0078] Therefore, the herein described steel comprises at least 0.40% of C. Increasing the content of carbon leads to an increase of the number of secondary carbides that may be formed. Therefore, carbon may be present in an amount of equal to or higher than 0.45%, or even equal to or higher than 0.46%.

[0079] However, too high contents of carbon may lead to formation of primary carbides during solidification. Such primary carbides may be difficult to dissolve and may tend to grow large during further processing to intended final product form and condition. If carbon is consumed by formation of primary carbides, the hardenability decreases, and a lower amount of the beneficial finely dispersed secondary carbides may be formed. Therefore, the herein described steel comprises equal to or less than 0.55% of C. To further reduce the risk for formation of large sized carbides, the steel may comprise equal to or less than 0.53% of C.

[0080] Silicon (Si): 0.2 - 1.1 %

[0081] Silicon is an element frequently used in steel production as a deoxidation agent. However, more importantly, it contributes to increased strength by solid solution strengthening. Silicon also has the advantage of increasing the carbon activity. Therefore, the herein described steel comprises equal to or more than 0.2% of Si. To further improve the properties of the herein described steel, silicon may be present in an amount of equal to or higher than 0.3%, or even equal or higher than 0.4%, or even equal or higher than 0.5%, even equal or higher than 0.6%, or even equal or higher than 0.7%, or even equal or higher than 0.8%, or even equal to or higher than 0.85%.

[0082] However, if silicon is present in an excessive amount, the risk for formation of unwanted intermetallic phases increases. Such intermetallic phases may deteriorate strength as well as bending fatigue resistance and impact fatigue resistance. Furthermore, silicon is a strong ferrite former, and an excessive amount may therefore lead to increased risk for presence of delta ferrite, which is not desired in the herein described steel. Silicon is therefore present in an amount of equal to or lower than 1.1%. To further reduce the risk of formation of unwanted intermetallic phases, silicon may be present in an amount of equal to or lower than 1.0%, or even equal to or lower than 0.95%. nese n): 0.20 - 0.70%

[0083] Manganese is an austenite stabilizing element and contributes to increased hardenability of the steel. It may also be used as a deoxidation agent. Furthermore, Mn has the advantage of increasing the solubility of nitrogen. Therefore, the herein described steel comprises equal to or more than 0.20% of Mn. To further increase the hardenability, Mn may be present in an amount of equal to or higher than 0.30%, or equal to or higher than 0.35%.

[0084] In view of being an austenite stabilizing element, manganese could possibly be used for increasing the amount of retained austenite. However, manganese may also form manganese sulfide inclusions in combination with sulfur. Thus, the austenite stabilizing effect in the herein described steel is primarily achieved by one or more other elements, in particular Ni and Cu, to avoid too high amounts of Mn. Therefore, the herein described steel comprises equal to or less than 0.70%. The Mn content may alternatively be equal to or less than 0.65% or equal or less than 0.60% or equal or less than 0.55% of Mn. The Mn content may alternatively be equal to or less than 0.50%, or equal to or less than 0.45%. to or less than 0.03%

[0085] Phosphorus is not a purposively added element but may be present as an impurity. It is well known that P, if present in too high amounts, has a negative effect on cold embrittlement, hot workability and resistance to hot cracking. In the herein described alloy, P may therefore be allowed in amounts of equal to or less than 0.03% without substantially affecting the properties negatively. Lower amounts of P are beneficial, and the P content may therefore alternatively be limited to equal to or less than 0.02% or even equal to or less than 0.01%.

[0086] Sulfur (S): equal to or less than 0.03%

[0087] Like phosphorus, sulfur is an impurity element in the herein described steel. Sulfur is an unwanted element that in combination with other elements (in particular Mn) forms sulfides. These sulfides can drastically reduce the toughness of the steel and deteriorate the hot workability if present in too high amounts. S may be allowed in contents of equal to or less than 0.03% without substantially affecting the properties of the alloy negatively. Lower amounts of S are beneficial for reducing the risk of formation of harmful sulfides, and the steel may therefore comprise equal to or less than 0.01% of S.

[0088] Chromium (Cr): 5.0 - 9.5%

[0089] Chromium is an important element of the herein described steel as it contributes to solution strengthening. Cr also increases the solubility of carbon and nitrogen, which in turn contributes to increased strength. Furthermore, Cr forms secondary carbides (such as M23C6 and M7C3) together with carbon (and vanadium) and therefore contributes to the strength also for this reason. If nitrogen is added to the steel, chromium may also contribute to the formation of carbonitrides. The carbides and carbonitrides may also increase wear resistance of the steel. Furthermore, Cr has the advantage of contributing to corrosion resistance as well as to the resistance to softening during tempering. The herein described steel therefore comprises equal to or higher than 5.0% of Cr. To further increase e.g., the strength, chromium may be present in an amount of equal to or higher than 5.5%, or equal to or higher than 6.0%, or equal to or higher than 6.5%, or equal to or higher than 7.0%, or even equal to or higher than 7.5%.

[0090] However, high amounts of chromium may reduce toughness and may also increase the risk for formation of primary carbides. Therefore, the herein described steel comprises equal to or less than 9.5 % of Cr. Alternatively, Cr may be present in an amount of equal to or less than 9.0%, or even equal to or less than 8.5%.

[0091] It should be noted that the chromium content of the herein described steel is considerably lower than previously known martensitic stainless steels used in compressor valve applications (the AISI 420 type of martensitic stainless steels), which typically comprise about 13-14% of Cr. The lower chromium content in combination with the high carbon content and the high vanadium content of the present steel leads to the presence of a different type of structure of precipitates, such as carbides and / or carbonitrides and / or nitrides, compared to the conventional martensitic stainless steels used for compressor reed valve component, which in turn contributes to an increased wear resistance. This structure of the herein described steel also contributes to the ability to introduce high compressive residual stresses by surface treatments, such as shot peening or tumbling or polishing.

[0092] Nickel (Ni): 0.4 - 2.2%

[0093] Nickel is an element added to the herein described steel for the purpose of stabilizing the austenite, to thereby enable a desired amount of retained austenite after hardening. An increase of the amount of retained austenite in the material increases the ductility and thereby also affects the fatigue properties. Therefore, the herein described steel comprises at least 0.4% of Ni. To further improve the ductility and fatigue properties, nickel may be present in an amount of equal to or more than 0.5%, or even equal to or more than 0.6%. However, if present in too high amounts, nickel may lead to a considerable reduction of the Al temperature. This in turn leads to a reduction of amount of dissolved carbon and nitrogen in the matrix, which is not desired. Furthermore, Ni is an expensive alloying element. Therefore, the herein described steel comprises equal to or less than 2.2% of Ni. To further improve the solubility of carbon and nitrogen in the matrix, nickel may be present in an amount of equal to or less than 1.8%, such as less than or equal to 1.5, such as less than or equal to 1.4%. Alternatively, nickel may be present in an amount of equal to or less than 1.0%.

[0094] : 0.030 to less than 0.090%

[0095] Nitrogen is an element which will contribute to increased strength of the herein described steel. Nitrogen may be interstitially solved in the austenite to thereby increase strength through solid solution. N may also, when added in sufficient amount contribute to formation of carbonitrides, such as chromium vanadium carbonitrides. Nitrogen is also an austenite stabilizing element and may therefore be added to facilitate achieving a desired amount of retained austenite after hardening. It has further been found that, when nitrogen is added, higher compressive residual stresses may be obtained when the steel is subjected to surface treatment. Therefore, nitrogen is added in an amount of equal to or more than 0.030%. To facilitate precipitation of chromium vanadium carbides, which inter alia contribute to increased wear resistance, nitrogen may be present in an amount of equal to or more than 0.040%, or even equal to or more than 0.045%.

[0096] However, if present in too high amounts, nitrogen may lead to a reduction of the hot workability. Based on the experimental results given below, it is believed that a nitrogen content of 0.090% constitutes a reasonable upper limit for ensuring a sufficient hot workability. Therefore, the herein described steel comprises less than 0.090% of N. For example, the herein described steel may comprise equal to or less than 0.085% of N. In order to further ensure a desired hot workability of the steel, nitrogen may be present in an amount of equal to or less than 0.080%. urn (Mo): 1.0 - 2.0%

[0097] Molybdenum is an element which contributes to corrosion resistance. Mo is also a strong carbide former and may therefore also contribute to formation of secondary carbides. Mo may also contribute to increased ductility as well as hardenability. Moreover, molybdenum may also contribute to the resistance to softening during tempering. Therefore, the herein described steel comprises equal to or more than 1.0% of Mo. The Mo content may for example be equal to or higher than 1.1%. Alternatively, Mo may be present in an amount of equal to or higher than 1.2%. However, molybdenum is an expensive alloying element. Furthermore, like chromium, molybdenum may lead to a reduction of the toughness if present in too high amounts. Therefore, the herein described steel comprises equal to or less than 2.0% of Mo. Alternatively, Mo may be present in an amount of equal to or less than 1.7 %, or even equal to or less than 1.6%.

[0098] Copper is an element added to the herein described steel for the purpose of stabilizing the austenite, to thereby enable a desired amount of retained austenite after hardening which in turn affects the ductility. Furthermore, copper may contribute to the strength through substitutional solid solution strengthening. Copper is therefore an important element for the fatigue properties. The herein described steel therefore comprises at least 0.15%. To further improve the properties, Cu may be added in an amount of equal to or more than 0.2%, or even equal to or more than 0.5%.

[0099] However, too high contents of copper, in particular in combination with higher amounts of nickel, may lead to an unduly high amount of retained austenite. This may in turn lead to a deterioration of the desired properties, such as a reduction of tensile strength and hardness. Therefore, the herein described steel comprises at most 1.2 % of Cu. According to an example, Cu may be present in an amount of equal to or less than 1.0%. Alternatively, Cu may be present in an amount of equal to or less than 0.9%.

[0100] Vanadium (V): 0.15 - 0.40 %

[0101] Vanadium is a strong carbide former which forms small (nanosized) dispersed carbides and, if nitrogen is added, carbonitrides in the matrix. These secondary carbides provide an increased hardness, and thus also wear resistance, without negatively affecting the toughness as long as the content of vanadium is not too high. Therefore, the herein described steel comprises equal to or more than 0.15% of V. The amount of carbides and carbonitrides increases with increasing amount of vanadium. Thus, vanadium may for example be added in an amount of equal to or more than 0.18%, or equal to or more than 0.20%.

[0102] However, too high contents of vanadium may increase the risk for formation of unwanted primary carbides. Higher contents of vanadium may also lead to a reduction in ductility, which in turn may negatively affect the fatigue strength. Therefore, the herein described steel comprises equal to or less than 0.40% of vanadium. To further reduce the risk for deterioration of the desired properties, V may be present in the herein described steel in an amount of equal to or less than 0.35%. Boron (B): optionally equal to or less than 0.005 %

[0103] The steel does not need to comprise boron, i.e. the steel may be boron-free. However, boron may be added to improve the hot ductility. It can also be added as a grain refiner and thereby increasing the strength of the steel. However, too high contents should be avoided so as to not adversely affect hot workability. If added, boron may be present in amounts equal to or less than 0.005%. According to embodiments, the content of B is equal to or less than 0.003%.

[0104] Aluminum (Al): optionally equal to or less than 0.25 %

[0105] The steel does not need to comprise aluminum, i.e. the content of Al may be zero %, but aluminum may be added as a deoxidation agent during steel production. If added, aluminum may be present in amounts equal to or less than 0.25%, such as amounts equal to or less than 0.125%.

[0106] Calcium (Ca) and / or Magnesium (Mg): optionally equal to or less than 0.15%

[0107] The steel does not need to comprise Ca or Mg, i.e. the content of Ca or Mg may be zero %. Nevertheless, one or both of Ca and Mg may, if desired, be added up to a total amount of 0.15% to improve the hot ductility of the material during the production process. According to embodiments, the calcium content is at most 0.05%, suitably equal to or less than 0.01%. If Mg is added, the content of Mg may then suitably be at most 0.05%.

[0108] Rare Earth Metals (REM): optionally equal to or less than 0.5%

[0109] The steel does not need to comprise REM, thus the content of REM may be 0%. However, REM may be added in an amount of equal to or less than 0.5%, if desired, to improve hot ductility during the production process. Furthermore, even if not purposively added, REM may be present as impurity resulting from the raw material, e.g. in amounts of equal to or less than 0.1%.

[0110] Normally occurring impurities: up to 0.8% in total

[0111] The herein described alloy may, in addition to the elements already specified and discussed above, comprise up to at most 0.8% in total of normally occurring impurities. In the present disclosure, normally occurring impurities are considered to be impurities resulting from the manufacturing process and / or the raw material used. Normally occurring impurities are herein intended to encompass both impurities and trace elements. The amount of normally occurring impurities may according to embodiments suitably be equal to or less than 0.6 % in total or alternatively equal to or less than 0.5% in total. Impurity elements selected from the group consisting of titanium (Ti), niobium (Nb), zirconium (Zr) and tantalum (Ta) may for example be limited to equal to or less than 0.15 % in total, or even equal to or less than 0.10% in total. These are all examples of elements that are strong carbide and / or nitride formers, but may result in e.g. an undesired morphology of the carbides and / or nitrides or primary carbides / nitrides that may be difficult to dissolve. To further reduce the risk of formation carbides and / or nitrides of a non-desired type, the content of Ti, Nb, Zr and Ta may be limited to equal to or less than 0.05% each, or even equal to or less than 0.03% each. According to embodiments, the steel may contain 0% of Ti, Nb. Zr and / or Ta.

[0112] Other examples of impurity elements that may be present in the herein described steel include, but are not limited to, cobalt (Co), tungsten (W), and tin (Sn). The content of Co may for example be equal to or less than 0.3%, or equal to or less than 0.2%. The content of W may for example be equal to or less than 0.3%, or even equal to or less than 0.1%. The content of Sn may for example be equal to or less than 0.1%, or even equal to or less than 0.05%. According to embodiments, the content of Co, W and / or Sn may be 0.

[0113] Method of production

[0114] The herein described martensitic steel may be produced by providing a melt having the above described composition, followed by casting. The melting process may for example be conducted by use of an electric arc furnace (EAF), which may be followed by an AOD process and optionally final adjustments of the composition. Casting may for example be conducted by DC casting.

[0115] The as-cast material, which may for example be in the form of a bloom, may thereafter optionally be subjected to a heat treatment, if desired. Such a heat treatment may be performed at an austenitizing temperature, such as about 1000 - 1350 °C. If desired, the heat treatment may comprise a preheating step at an intermediate temperature before continuing to heat to the selected austenitizing temperature.

[0116] The cast material may thereafter be subjected to hot rolling into an intermediate (strip) thickness. Such a hot rolling may be performed at a starting temperature of about 1100 - 1350 °C. The finishing rolling temperature may suitably be at least 800 °C. Hot rolling may be performed in several passes. If needed in order to maintain a suitable hot rolling temperature, the material may be reheated between rolling passes. The hot rolled strip may thereafter be coiled, if desired. Coiling may suitably be performed at a temperature of about 400 - 800 °C, such as about 600 to 750°C. Thereafter, the hot rolled steel may optionally be heat treated. Such a heat treatment may for example comprise annealing at a temperature of about 650 - 900 °C for at least 1 h, and is partly dependent on the Al temperature.

[0117] The hot rolled, and optionally heat treated, steel strip may thereafter be cold rolled to intended final strip thickness. Depending on the intended use of the strip of the martensitic steel, the final thickness may for example be 0.040 - 3 mm but is not limited thereto. Cold rolling may be performed in several passes, if needed in order to reach the intended final thickness of the strip. Optionally, intermediate annealing may be performed between at least two of the cold rolling passes. Such intermediate annealing may be performed at temperatures of about 700 - 875 °C.

[0118] The obtained strip may thereafter be subjected to hardening. Said hardening comprises heating to a hardening temperature followed by quenching. The hardening temperature may for example be 990 - 1100°C, but is not limited thereto. Hardening temperatures of 1000 - 1080 °C are possible.

[0119] Quenching may suitably be performed at a rate of at least 30°C / s at least down to 460 °C. Thereafter, the hardened strip may be further cooled down to a temperature equal to or below 100 °C.

[0120] After hardening, the hardened strip may be subjected to tempering to a temperature of 200 - 460 °C, such as 220 - 450 °C.

[0121] The martensitic steel may, depending on the intended use thereof, be subjected to further processing steps. For example, in case of intended to be used in a valve component (such as a compressor reed valve), the martensitic steel may be subjected to a processing step for the purpose of introducing surface residual stresses. Such a step may for example be made after punching the valve component from the hardened and tempered steel strip. The introduction of surface residual stress may be made by tumbling although other processes, such a shot peening, are also possible. Alternatively, or additionally, the hardened and tempered steel strip may be coated, if desired.

[0122] Microstructure

[0123] The herein described martensitic steel has a primarily martensitic microstructure comprising finely dispersed carbides and, when nitrogen is added, carbonitrides. The carbides are typically M23C6 and M7C3 carbides. The microstructure further comprises retained austenite. The amount of retained austenite is dependent of the composition as well as processing conditions. More specifically, the amount of retained austenite will vary depending on the hardening temperature used, wherein higher hardening temperatures typically leads to higher amount of retained austenite. By means of the herein described composition and appropriate selection of hardening temperature, it is possible to obtain at least 14 vol.-% of retained austenite. Based on current knowledge, it is believed that an appropriate amount of retained austenite, when the martensitic steel is to be used in high demanding valve applications, should be in the order of about 14-18 vol.-%. In case the amount of retained austenite is too low, there is a risk for a reduction of the fatigue strength. However, in case the amount of retained austenite is too high, there may be a risk for a decrease in hardness and, in some cases, also a coarser grain structure.

[0124] When discussed in the present disclosure, the amount of retained austenite is intended to refer to the amount thereof as determined using a magnetic balance method. More specifically, the amount of retained austenite may be determined according to the following steps:

[0125] (a) determining magnetic dipole moment of a specimen of the martensitic steel in accordance with the withdrawal method of IEC 60404-14;

[0126] (b) determining weight-specific saturation magnetism, os, as the ratio of the determined magnetic dipole moment of the specimen to weight of said specimen;

[0127] (c) calculating the volume percentage of retained austenite, RA, of the specimen according to Equation 1:

[0128] RA (vol. — %) = 100 — 100 * os / k * crm) (Eq. 1) wherein k is a correction factor selected to be 0.966, and omrepresents theoretical magnetic saturation of the specimen and is calculated based on the chemical composition thereof using the Hoselitz formula as given by Equation 2: om= 217.75 - 12 * [wt. -% C] - 2.4 * [wt. -% Si] - 1.9 * [wt. -% Mn] - 3 * [wt. — %Cr] — 0.75 * [wt. — % / Vi] — 1.2 * [wt. — % Mo] — 2.6 * [wt. — % Al] — 3 * [wt. — % P] — 7 * [wt. — % S] — 2.3 * [wt. — % Cu] — 6 * [wt. — % / V] (Eq. 2).

[0129] The properties of the herein described martensitic steel are dependent on processing steps to which it has been subjected, i.e. the condition, and can be tailored in dependence of the intended use of the martensitic steel. The following properties are however given for purpose of illustration.

[0130] In a hardened and non-tempered condition, the herein described martensitic steel may have: a hardness according to SS-EN-ISO 6507 of at least 680 Hv, or even at least 700 Hv, a tensile strength (Rm) according to ISO 6982-1 of at least 2300 MPa, and an elongation (A5) according to ISO 6982-1 of at least 2 %. In a hardened and tempered condition, the herein described martensitic steel may have: a hardness according to SS-EN-ISO 6507 of at least 570 Hv, or even above 600 Hv, a tensile strength (Rm) according to ISO 6982-1 of at least 1900 MPa, a yield strength (Rp0.2) according to ISO 6982-1 of at least 1400 MPa, and an elongation (A5) according to ISO 6982-1 of at least 6 %.

[0131] The herein described martensitic steel may (after hardening and tempering) be subjected to surface treatments, such as shot peening or tumbling, for the purpose of introducing surface residual stresses, and thereby further improve fatigue properties. The result of such surface treatments may vary largely depending on the equipment used, especially in case of tumbling, and the process parameters selected. It has however been found that surface residual stresses of at least about 800 MPa, as determined by XRD, may be achieved for the herein described martensitic steel.

[0132] Furthermore, it has been found that surprisingly high surface residual stresses and high hardness (and thus also wear resistance) may be obtained in the hardened and tempered condition when the martensitic steel has the following composition, in percent by weight:

[0133] C 0.40 - 0.55,

[0134] Si 0.2 - 1.1,

[0135] Mn 0.20 - 0.70,

[0136] P equal to or less than 0.03,

[0137] S equal to or less than 0.03,

[0138] Cr 5.0 - 9.5,

[0139] Ni 0.4 - 1.4,

[0140] N 0.030 - less than 0.090,

[0141] Mo 1.0 - 2.0,

[0142] Cu 0.15 - 1.2,

[0143] V 0.15 - 0.40, optionally Al up to 0.25, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally B up to 0.005, balance Fe and normally occurring impurities up to 0.8 wt.-% in total. According to embodiments, the martensitic steel as defined hereinabove or hereinafter may have the following composition, in percent by weight:

[0144] C 0.45 - 0.53,

[0145] Si 0.85 - 0.95,

[0146] Mn 0.38 - 0.42,

[0147] P equal to or less than 0.01,

[0148] S equal to or less than 0.01,

[0149] Cr 7.9 - 8.1,

[0150] Ni 0.68 - 0.72,

[0151] N 0.050 - less than 0.090,

[0152] Mo 1.40 - 1.55,

[0153] Cu 0.68 - 0.72,

[0154] V 0.22 - 0.28, balance Fe

[0155] Experimental results

[0156] Experimental alloys were produced on a small laboratory scale using vacuum induction melting (VIM) followed by casting into molds. The obtained compositions of the experimental alloys are specified in Table 1 below. In the experimental alloys 1-4, no nitrogen was added, and any nitrogen was therefore present merely as an impurity in these alloys. Alloy 1 has a composition targeted to correspond to a commercially available tool steel known to possess excellent wear resistance, and which is currently used for knives in challenging operations such as garden tools and harvest knives. Alloy 1 was included for comparison in view of the similarities with regard to composition according to the herein described martensitic steel. The composition of Alloy 1 does not comprise any purposively added nickel or copper, and these elements are therefore present in said composition only as impurities.

[0157] After casting, the experimental alloys were each subjected to a heat treatment comprising preheating at about 700 °C for about 30 minutes followed by heating to about 1150 °C for about 30 minutes, and thereafter hot rolled to rods of 7x7mm cross section. Cold rolling was not performed on the present Experimental alloys as they received their desired thickness during hot rolling as they are experimental alloys and therefore may not need to be cold worked in order to obtain their desired thickness. Table 1. Composition of experimental alloys. The numbers are in wt.-%.

[0158] *Comparative, outside claimed scope

[0159] It was found that some cracks were formed in Alloy 9 during hot rolling. Without being bound by theory, it is believed that the reason for the formation of cracks was the high nitrogen content in Alloy 9, which could possibly lead to formation of gas voids during melt production in view of being close to the nitrogen solubility limit of said composition. Crack formation was also seen in Alloy 8, albeit to a lesser extent. It should here be noted that the experimental alloys were produced in a small laboratory scale. It is believed that the possible formation of gas voids due to high nitrogen contents would not present the same problem if these alloys would have been produced in full scale production. For example, the pressure created during DC casting would likely reduce the amount of gas inclusions in the as-cast material. Moreover, the possible formation of gas voids can likely be overcome by adjusting the composition so as to increase the solubility of nitrogen, for example by increasing the chromium content. Still, it is believed that a nitrogen content of 0.090% is a reasonable upper limit to avoid the risk of crack formation also in full scale production.

[0160] Hardening trials and determination of amount of retained austenite

[0161] 3 mm slices of the obtained rods of the different experimental alloys were cut out and subjected to hardening at different temperatures ranging from 990 °C to 1100 °C. The amount of retained austenite after hardening was determined by a magnetic balance method. According to said technique, the amount of magnetic phase is determined by measuring the specific saturation magnetism of a test piece and, via an empirical method, converting this into a volume percentage of magnetic phase.

[0162] More specifically, the amount of retained austenite was determined using a FOERESTER KOERZIMAT 1.097 MS, which is a computer controlled measuring instrument for precise determination of the weight-specific saturation magnetism, os, for ferromagnetic materials and components. For each test piece (i.e. a hardened slice), the weight of the test piece was first determined using a precision balance and the result entered into the controller of the instrument. A pusher of the instrument was thereafter used to move the test piece into the air gap of a saturation magnet (Halbach array) and the magnetic dipole moment was measured using Helmholtz measuring coils and a flux meter when the test piece was pulled out of the magnet. The weight-specific saturation magnetism, os, of the test piece was thereafter calculated by the controller of the instrument from the ratio of the determined magnetic dipole moment to the weight of the test piece. The measurements followed the guidelines in IEC 60404-14.

[0163] For each test piece, the volume percentage of magnetic phase, MP, was thereafter calculated according to Equation 3 using the weight-specific saturation magnetism os, determined as described above.

[0164] MP (vol - %) = 100 * k* crm

[0165] In Equation 3, <JSrepresents the determined weight-specific magnetic saturation, k represents a correction factor, andmrepresents the theoretical magnetic saturation. The correction factor k is used for compensating for expected non-magnetic phases other than retained austenite (such as carbides) and may be selected based on experience from previous measurements on similar materials. Here, a correction factor / c=0.966 was utilized. For each test piece, the theoretical magnetic saturation, <jm, was calculated based on the respective chemical composition using the Hoselitz formula, which is represented by Equation 2 already disclosed above.

[0166] The magnetic phase in the samples represents martensite whereas the retained austenite is nonmagnetic. Therefore, the volume percentage of retained austenite, RA, is given by Equation 4.

[0167] RA (vol - %) = 100 - MP (vol. —%) (Eq. 4) Inserting Equation 3 into Equation 4 results in Equation 1 as specified above.

[0168] The determined amount of retained austenite, after hardening at different hardening temperatures, is presented in Table 2 below and illustrated in Figure 1. Table 2. Retained austenite [vol.-%] as a function of hardening temperature [°C]

[0169] *Comparative, outside claimed scope

[0170] It can be seen from the test results that the amount of retained austenite increases with increasing hardening temperature, which is expected.

[0171] Moreover, it can be seen that the amount of retained austenite in Alloy 1 is lower than for the other alloys at the corresponding hardening temperatures. Alloy 1 would also require hardening at a temperature of more than 1050 °C to ensure at least 12 vol.-% of retained austenite and is not able to reach an amount of retained austenite of 14 vol.-%.

[0172] As already mentioned above, it is believed that the amount of retained austenite should be in the order of about 14 to 18 vol.-%. The results presented above demonstrate that this may be obtained for each of the Alloys 2 to 9 by appropriate selection of hardening temperature.

[0173] As may possibly be expected in view of the relatively low total amount of austenite stabilizing elements, Alloy 2 appears to require the highest hardening temperatures to obtain at least 14 vol.-% retained austenite. However, Alloy 3 surprisingly demonstrates higher amounts of retained austenite compared to Alloy 4 when hardened at the same temperatures, although Alloy 4 has a higher Ni content. For Alloys 5-9, comprising about 0.7 wt.-% Ni and about 0.7 wt.-% Cu, it is possible to obtain a desired amount of retained austenite over a relatively broad range of hardening temperatures despite the fact that some of the nitrogen added will be bound in carbonitrides. Hardness after hardening

[0174] Samples, in the form of 8 mm slices, were cut from the above described rods, obtained from Alloys 1- 9, and subjected to different hardening temperatures as shown in Table 3 below. Thereafter, the hardness in the non-tempered state was determined in accordance with SS-EN-ISO 6507. The result is presented in Table 3 below, in which the values given within paratheses are non-measured values, derived by interpolation based on the results from hardening at the other temperatures.

[0175] The results shown in Table 3 demonstrates that an increase of hardness is achieved when increasing the hardening temperature from 1010 °C to 1020 °C. However, the hardness starts to decrease at a hardening temperature of 1060 °C, which is also a hardening temperature that may lead to about 18 vol.-% of retained austenite for many of the experimental alloys (compare with Table 2).

[0176] It can further be seen from the results that, for hardening temperatures within the temperature interval 1020-1040 °C, the hardness of the non-tempered martensite is about 720-735 HV for Alloys 5-9. It is surprising how equal the hardness is between said alloys after hardening within said temperature interval despite the differences in carbon and nitrogen content between the different compositions.

[0177] Table 3. Hardness [HV] in non-tempered condition as a function of hardening temperature [°C]

[0178] *Comparative, outside claimed scope

[0179] Tempering trial

[0180] Samples, in the form of 8 mm slices, were cut from the above described rods, obtained from Alloys 1- 9, and subjected to different hardening temperatures (HT) as shown in Table 4 below. The hardening temperatures were selected with an aim to obtain about 15 vol.-% retained austenite. However, for practical reasons, the highest hardening temperature was selected to be 1050°C. This means that Alloy 2 was not expected to reach 15 vol.-% RA. Furthermore, it should be noted that it is not possible to obtain an amount of retained austenite above 14 vol.% for Alloy 1. .

[0181] Table 4. Hardening temperatures selected for tempering trial

[0182] *Comparative, outside claimed scope

[0183] The samples were thereafter tempered to various temperatures from 200 °C to 450°C for 2 hours. The hardness after tempering was determined in accordance with SS-EN-ISO 6507. The result is shown in Figure 2.

[0184] The results show that, for Alloys 5-9, the largest reduction in hardness was achieved when tempered to about 300 °C, resulting in a hardness of about 620-630 Hv. For alloys 1-4, the largest reduction in hardness was achieved when tempered to about 350 °C. The lowest hardness after tempering was observed for Alloys 3 and 4, with a minimum hardness in the range 570 - 590 Hv. However, these alloys had also been subjected to a hardening temperature resulting in a lower hardness before tempering compared to the other experimental alloys.

[0185] Tensile testing

[0186] Samples were cut from the above described rods, obtained from Alloys 1-7. The samples were subjected to annealing at 825-875 °C for about 6 hours and thereafter machined to tension test specimens having a diameter of 4 mm and a gauge length of 30 mm. Before tensile testing, the tension test specimens were hardened and tempered. More specifically, the tension test specimens of Alloys 1-7 were hardened at a temperature of 1030 °C. All the tension test specimens were thereafter tempered at 350 °C for 2 hours.

[0187] Tensile testing was performed in accordance with ISO 6892-1:2019. The result of the tensile testing is shown in Table 5.

[0188] It should be noted that tension test specimens were not prepared from the rods of the Alloy 8 and Alloy 9, respectively, in view of the above described formation of cracks therein during hot rolling. Thus, neither Alloy 8 nor Alloy 9 were subjected to tensile testing. Table 5. Tensile properties in hardened and tempered condition (hardening temperature about

[0189] 1030 °C)

[0190] *Comparative, outside claimed scope

[0191] From the results presented in Table 5 above, it can be seen that all alloys present a good combination of tensile strength and elongation, which in turn indicates that it is possible to obtain excellent fatigue properties. The results from the tensile testing may be compared with the results described above with regard to amount of retained austenite after hardening since the amount of retained austenite is substantially unaffected by the above described tempering.

[0192] Alloys 3 and 4, which comprise the highest amounts of nickel, have a somewhat lower yield and tensile strength, but a higher elongation, compared to the other alloys. This indicates that the amount of retained austenite after hardening and tempering is not the sole reason for the differences in results. Furthermore, the benefit of a nitrogen addition can be seen when comparing the tensile and yield strength of Alloys 5-7 with that of Alloys 3 and 4.

[0193] Surface residual stress and hardness after tumbling

[0194] Compressive residual stresses have an important influence on the fatigue life of compressor reed valve materials in compressor applications. Thus, to evaluate surface residual stress after tumbling, 3 mm samples were cut from the obtained rods. The samples were thereafter subjected to hardening and tempering, and thereafter tumbled. The hardening temperatures for the respective samples were selected based on the above described hardening trials to aim for a similar amount of retained austenite for all samples. The amount of retained austenite was determined, prior to tumbling, according to the magnetic balance method as already described above. Table 6 specifies the hardening temperatures used and the resulting amount of retained austenite for the respective samples.

[0195] It is well known that the result of tumbling may vary largely depending on the equipment used as well as the tumbling conditions. Therefore, tumbling of the samples was performed in the same batch to allow comparison of the results. Furthermore, tumbling was performed under conditions aimed to maximize surface residual stress.

[0196] The residual stress after tumbling was determined by means of X-Ray Diffraction (XRD), both in the rolling direction and in a direction transversal to the rolling direction. Moreover, the hardness after tumbling was determined according to SS-EN-ISO 6507. Table 6 specifies the average of residual stress in the different directions.

[0197] Table 6. The average of residual stress in the different directions

[0198] *Comparative, outside claimed scope

[0199] It can be seen from the results that a higher average surface residual stress and higher hardness after tumbling was achieved for Alloys 2 to 8 compared to Alloy 1. Alloy 9 shows a relatively high average surface residual stress, but has a lower hardness after tumbling compared to Alloys 2 to 8. Alloy 2 showed a high hardness after tumbling, but the average surface residual stress was not significantly improved compared to Alloy 1. The highest average surface residual stress, in combination with a high hardness, after tumbling was achieved for Alloys 5 to 8. It also appears from the results that a lower carbon content may have a positive effect on the surface residual stress. This may be seen from Figure 3, showing the average surface residual stress after tumbling as a function of carbon content.

[0200] Microscopy

[0201] The microstructure after hardening was investigated by light optical microscopy (LOM). For this purpose, samples were produced in the same way as described above for the hardness testing of non-tempered martensite. Figure 4 illustrates a LOM image of Alloy 5 after hardening at 1020 °C. Figure 5 illustrates a LOM image of Alloy 8 after hardening at 1020 °C.

[0202] It was found that all samples demonstrated a fine structure with well distributed precipitates such as carbides and / or carbonitrides and / or nitrides. However, as may be expected, there was a tendency towards a somewhat coarser austenite structure for the higher hardening temperatures.

[0203] Thermodynamic calculations

[0204] In order to gain a better understanding of the results, thermodynamic calculations were performed on the compositions for the experimental alloys. The calculations were made in the Thermo-Calc Software using the database TCFE10: Steel / Fe-Alloys, database version 10.1.

[0205] Phase diagrams for Alloys 1-4 demonstrated that the main precipitations therein constitute M23C6 and M7C3 carbides. However, the presence of nitrogen in Alloys 5-9 results in several additional phases, from which the most interesting may be chromium vanadium carbonitrides (hereinafter abbreviated CrVCN) since these are stable at higher temperatures.

[0206] Table 7 discloses the sum of carbon and nitrogen in the austenite at equilibrium at different temperatures, these temperatures selected based on the temperatures at which the hardening trials described above were performed. Furthermore, Table 8 discloses the dissolution temperatures (Tmax) of M7C3 and CrVCN, respectively, as well as the Al temperature.

[0207] The results show that an addition of nitrogen implies an increase of available interstitial dissolved in the matrix, even though CrVCN is present. For temperatures around 1030-1040 °C, the sum of interstitial ly dissolved C and N is about 0.05-0.09% higher for the nitrogen alloyed compositions (compare Alloys 2 and 3 with Alloys 5 and 6). Moreover, it can be seen that the M7C3 carbides dissolve at about 30 °C lower temperature for the nitrogen alloyed compositions. Instead, CrVCN are formed and remains up to temperatures of 1188-1240 °C. Furthermore, the alloys having a higher content of Ni (i.e. Alloys 3 and 4) demonstrates a drop in the Al temperature.

[0208] Table 7. Sum of C+N in austenite at equilibrium *Comparative, outside claimed scope

[0209] Table 8. Dissolution temperatures and Al temperature

[0210] *Comparative, outside claimed scope

Claims

CLAIMS1. A martensitic steel comprising, in percent by weight:C 0.40 - 0.55,Si 0.2 - 1.1,Mn 0.20 - 0.70,P equal to or less than 0.03;S equal to or less than 0.03;Cr 5.0 - 9.5;Ni 0.4 - 2.2,N 0.030 to less than 0.090,Mo 1.0 - 2.0,Cu 0.15 - 1.2,V 0.15 - 0.40, optionally Al up to 0.25; optionally Mg and / or Ca up to 0.15 in total; optionally REM up to 0.5; optionally B up to 0.005; balance Fe and normally occurring impurities up to 0.8 wt.-% in total.

2. The martensitic steel according to any one of claims 1, wherein the composition comprises 0.4 - 1.4 wt.-% Ni.

3. The martensitic steel according to any one of claims 1 or 2, , wherein the composition comprises 0.45 - 0.55 wt.-% of carbon.

4. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 0.3 - 1.1 wt.%, such as 0.4 - 1.1 wt.%, such as 0.5 to 1.1 wt.%, such as 0.6 to 1.1 wt.%, such as 0.7 - 1.0 wt.-% of silicon.

5. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 0.20 - 0.65 wt.-%, such as 0.20 - 0.60 wt.-%, such as 0.30 - 0.55 wt.-%, such as 0.30 - 0.50 wt.-% of manganese.

6. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 5.5 to 9.5 wt.-%, such as 6.0 to 9.5 wt.-%, such as 6.5 to 9.0 wt.-%, such as 7.0 - 9.0 wt.-% of chromium.

7. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 0.030 - 0.085 wt.-% of nitrogen.

8. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 1.1 - 1.7 wt.-% of molybdenum.

9. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 0.2 - 1.0 wt.-% of copper.

10. The martensitic steel according to any one of the preceding claims, wherein the composition comprises 0.18 - 0.35 wt.-% of vanadium.

11. The martensitic steel according to any one of the preceding claims, wherein the martensitic steel, in a hardened and tempered condition, has a microstructure comprising 14-18 vol.-% of retained austenite, as determined according to the following steps:(a) determining magnetic dipole moment of a specimen of the martensitic steel in accordance with the withdrawal method of IEC 60404-14;(b) determining weight-specific saturation magnetism, os, as the ratio of the determined magnetic dipole moment of the specimen to weight of said specimen;(c) calculating the volume percentage of retained austenite, RA, of the specimen according to Equation 1:RA (vol. — %) = 100 — 100 * os / k * crm) (Eq. 1) wherein k is a correction factor selected to be 0.966, and omrepresents theoretical magnetic saturation of the specimen and is calculated based on the chemical composition thereof using the Hoselitz formula as given by Equation 2: om= 217.75 - 12 * [wt. -% C] - 2.4 * [wt. -% Si] - 1.9 * [wt. -% Mn] - 3 * [wt. — %Cr] — 0.75 * [wt. — % / Vi] — 1.2 * [wt. — % Mo] — 2.6 * [wt. — % Al] — 3 * [wt. — % P] — 7 * [wt. — % S] — 2.3 * [wt. — % Cu] — 6 * [wt. — % / V] (Eq. 2).

12. A steel strip comprising the martensitic steel according to any one of the preceding claims.

13. The steel strip according to claim 12, wherein the steel strip is in a hardened and tempered condition.

14. Use of the steel strip according to any one of claims 12 or 13 for producing, or in, a valve component, such as a valve component for a compressor.

15. A method for producing a steel strip, the method comprising:- casting a melt having a composition comprising, in percent by weight:C 0.40 - 0.55,Si 0.2 - 1.1,Mn 0.20 - 0.70,P equal to or less than 0.03,S equal to or less than 0.03,Cr 5.0 - 9.5,Ni 0.4 - 2.2,N 0.030 to less than 0.090,Mo 1.0 - 2.0,Cu 0.15 - 1.2,V 0.15 - 0.40, optionally Al up to 0.25, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally B up to 0.005, balance Fe and normally occurring impurities up to 0.8 wt.-% in total,- optionally heat treating the as-cast material,- followed by hot rolling the cast steel to intermediate thickness,- optionally heat treating the hot rolled steel,- cold rolling the hot rolled steel to intended final strip thickness,- hardening the cold rolled strip at a hardening temperature of 990 -1100 °C followed by quenching, and- tempering the hardened strip to temperature of 200-460 °C.

16. The method according to claim 16, wherein hardening is performed at a hardening temperature of 1000-1080 °C.

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