A martensitic stainless steel

Abstract

The present invention relates to a martensitic stainless steel comprising, in percent by weight: C more than 0.50 to 0.60, Si 0.10 to 0.60, Mn 0.40 to 0.80, Cr 13.0 to 15.0, Ni 0.10 to 1.0, Mo 0.90 to 1.50, N 0.08 to 0.15, Cu 0.2 to 1.0, V 0.15 to less than 0.40, S max 0.03, P max 0.03, optionally Mg and / or Ca up to 0.15 in total, optionally REM up to 0.5, optionally Al up to 0.25, optionally B up to 0.005, balance Fe and normally occurring impurities up to 0.8 wt% in total; and wherein the composition fulfils the following requirement [wt% V] / ([wt% N] + [wt% C]) is greater than or equal to 0.10 (RQ1). The present invention also relates to a steel strip and a compressor valve comprising said martensitic stainless steel and to the manufacturing of them both.

Description

[0001] A martensitic stainless steel

[0002] TECHNICAL FIELD

[0003] The present disclosure relates in general to a martensitic stainless steel which is suitable for use in valve components. The present disclosure further relates in general to a stainless steel strip comprising said martensitic stainless 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 stainless steel to control the flow of the refrigerant in the system. One of the major challenges in order to be able to increase compressor efficiency and / or develop new compressor designs is the properties of the stainless 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 stainless 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 stainless steel to have excellent mechanical properties, such as for example high tensile strength and high ductility.

[0006] The martensitic stainless steels of today have in general high performance and good mechanical properties, such as high strength and high ductility making them suitable to use in different strip applications. For example, EP 3031942 discloses a martensitic stainless steel, which may be used for flapper valves. However, this stainless steel will not be suitable in demanding and high temperature applications as said stainless steel will lose its mechanical strength due to its composition and to the manufacturing process used.

[0007] With the enhanced utilization of the valve material, which follows with the increased demands of coefficient of performance or increased flow rate of cooling media, the compressor manufacturer has to strive towards larger openings and higher impact velocities. This will in turn provide tougher conditions for the surface of the valve and a wear mark will be formed. Often during material qualification work, maximum limitations of wear mark are set as an indication of materials ability to withstand wear. As the wear mark depth increases, the stress concentration grows and with time it is likely that the wear mark will lead to a crack formation followed by a fracture initiation and a failure. There is therefore a need for new materials which will solve or at least reduce the before mentioned problems.

[0008] Thus, one of the aspects of the present disclosure is therefore to provide a new material which will solve or at least reduce these before mentioned problems.

[0009] SUMMARY

[0010] The present disclosure therefore relates to a martensitic stainless steel which is able to obtain a good balance of properties, making it suitable for use in demanding valve applications. Furthermore, the present martensitic stainless steel will provide a product made thereof with superior performance as it will provide for enhanced wear resistance as well as enhanced capability to introduce compressive residual stresses which will enable a high level of fatigue properties.

[0011] The present disclosure therefore provides a martensitic stainless steel having the following composition in percent by weight (wt%):

[0012] C more than 0.50 to 0.60,

[0013] Si 0.10 to 0.60,

[0014] Mn 0.40 to 0.80,

[0015] Cr 13.0 to 15.0,

[0016] Ni 0.10 to 1.0,

[0017] Mo 0.90 to 1.50,

[0018] N 0.08 to 0.15,

[0019] Cu 0.2 to 1.0,

[0020] V 0.15 to less than 0.40,

[0021] S max 0.03,

[0022] P max 0.03,

[0023] optionally Mg and / or Ca up to 0.15 in total,

[0024] optionally REM up to 0.5,

[0025] optionally Al up to 0.25,

[0026] optionally B up to 0.005,

[0027] balance Fe and normally occurring impurities up to 0.8 wt% in total;

[0028] and wherein the composition fulfils the following requirement [wt% V] / ([wt% N] + [wt% C]) is greater than or equal to 0.10 (RQ1).

[0029] The martensitic stainless steel as defined hereinabove or hereinafter will have a combination of improved wear resistance and improved ability to build surface compressive residual stresses, especially after hardening and tempering. A material having high tensile strength and high ductility as well as high surface compressive residual stresses will also have good fatigue properties. Additionally, the herein described martensitic stainless steel will provide for that a product made thereof will obtain a combination of good mechanical properties and thus be highly suitable for use in very demanding valve applications. Furthermore, the present martensitic stainless steel will have a combination of high ductility and high strength.

[0030] The present disclosure also provides a stainless steel strip comprising the martensitic stainless steel. Said stainless 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.

[0031] The present disclosure also relates to a method for producing such a stainless steel strip. The method comprises the following steps:

[0032] - casting a melt having a composition as defined hereinabove or hereinafter,

[0033] - optionally heat treating the as-cast material,

[0034] - followed by hot rolling the cast stainless steel to intermediate thickness,

[0035] - optionally heat treating the hot rolled stainless steel,

[0036] - cold rolling the hot rolled stainless steel, with one or more intermediates annealing steps, to intended final strip thickness,

[0037] - hardening the cold rolled strip at a hardening temperature of 950 to 1100 °C followed by quenching, and

[0038] - tempering the hardened strip to a temperature of 200 to 470 °C.

[0039] DETAILED DESCRIPTION

[0040] 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.

[0041] The term "martensitic stainless steel" is in the present disclosure considered to mean a stainless 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.

[0042] The terms "wt%" and "weight%" are used interchangeable throughout the present disclosure and means percentage by weight.

[0043] The terms "surface compressive residual stresses", "compressive residual stresses" and "residual stresses" are used interchangeable throughout the present disclosure and means the compressive residual stresses which are on the surface of an object composed of a steel.

[0044] Although the martensitic stainless steel as described herein above or hereinafter has primarily been developed for use in valve components as mentioned above, the present martensitic stainless steel may also be used in other types of components where an ability to form high compressive residual stresses and / or wear resistance is desired. Furthermore, the herein described martensitic stainless 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 stainless steel may be present in e.g., as-cast condition or as annealed condition, such as in the form of an ingot, a billet, or a bloom. Alternatively, the martensitic stainless steel may be produced into an intermediate or final product form (such as a bar, a strip, a sheet, a plate or the like, or a component produced thereof).

[0045] A component, such as a compressor reed valve component, comprising the herein described martensitic stainless steel may be produced from a strip of the martensitic stainless steel. Therefore, the present disclosure is also specifically directed to a stainless steel strip comprising or consisting of the herein described martensitic stainless steel. In case the stainless steel strip does not entirely consist of the martensitic stainless steel, the stainless steel strip may for example be a coated stainless steel strip.

[0046] The composition of the hereinabove or hereinafter described martensitic stainless steel enables the stainless steel to be able to achieve high compressive residual stresses and high wear resistance after hardening and tempering. Hence, the martensitic stainless steel according to the present disclosure has the following composition, in percent by weight (wt%):

[0047] C more than 0.50 to 0.60,

[0048] Si 0.10 to 0.60, Mn 0.40 to 0.80,

[0049] Cr 13.0 to 15.0,

[0050] Ni 0.10 to 1.0,

[0051] Mo 0.90 to 1.50,

[0052] N 0.08 to 0.15,

[0053] Cu 0.2 to 1.0,

[0054] V 0.15 to less than 0.40,

[0055] S max 0.03,

[0056] P max 0.03,

[0057] optionally Mg and / or Ca up to 0.15 in total,

[0058] optionally REM up to 0.5,

[0059] optionally Al up to 0.25,

[0060] optionally B up to 0.005,

[0061] balance Fe and normally occurring impurities up to 0.8 wt% in total;

[0062] and wherein the composition fulfils the following requirement [wt% V] / ([wt% N] + [wt% C]) is greater than or equal to 0.10 (RQ1).

[0063] The inventors have found that the present composition will allow for high compressive residual stresses to be introduced especially after hardening and tempering through surface treatment, such as by tumbling, shoot peening or the like.

[0064] Carbon (C) more than 0.50 to 0.60 wt%

[0065] C is an important element for the formation of precipitates, such as M23C6, M7C3 and VCr(NC) and / or carbides of other types, wherein M represents one or more metallic atoms. C is also important for the hardenability. In the present disclosure, the term "VCr(NC)" denotes a precipitate comprising mainly of the elements V and N but also comprises minor portions of Cr and C.

[0066] Hence, a high carbon content is desired as this will have a positive impact on the wear resistance, therefore the carbon content is more than 0.50 wt%, such as equal to or more than 0.510 wt%, such as equal to or more than 0.520 wt%.

[0067] However, a too high content of C may however, in combination with other elements, give rise to unwanted primary carbides formation during the primary production stage and these primary carbides may have a negative impact on both strength and ductility. Additionally, a high content of C makes the martensite more brittle and lowers the temperature at which martensite starts to form (Ms-temperature) and may also increase the amount of retained austenite to too high levels. Thus, the maximum C content of the present composition is equal to or less than 0.60 wt%, such as equal to or less than 0.550 wt%. such as equal to or less than 0.540 wt%.

[0068] Further, according to embodiments, the content of C is from 0.51 to 0.55 wt%, such as 0.510 to 0.550 wt%, such as from 0.51 to 0.54 wt%, such as 0.510 to 0.540 wt%, such as from 0.52 to 0.54 wt%, such as 0.520 to 0.540 wt%.

[0069] Silicon (Si) 0.10 to 0.60 wt%

[0070] Silicon is an element frequently used in stainless 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 stainless steel comprises equal to or more than 0.10 wt% of Si. To further improve the properties of the herein described stainless steel, silicon may be present in an amount of equal to or more than 0.20 wt%, such as equal to or more than 0.25 wt%, such as equal to or more than 0.30 wt%. However, a too high content of Si may result in formation of unwanted inclusions and the maximum content of Si is therefore equal to or less than 0.60 wt%, such as equal to or less than 0.50 wt%.

[0071] Copper (Cu) 0.2 to l.0 wt%

[0072] Cu is an austenite stabilizer and is present on purpose in the present composition for stabilizing the austenite in order to enable a desired amount of retained austenite after hardening. Furthermore, Cu will contribute to the substitutional solid solution strengthening and will also form a type of clusters and / or precipitates which will increase the strength. Copper is therefore an important element for the fatigue properties. Hence, Cu is purposely added. The herein described martensitic stainless steel therefore comprises at least 0.2 wt%, such as equal to or at least 0.3 wt%. To further improve the properties, Cu may be added in an amount equal to or more than 0.5 wt%, such as equal to or more than 0.55 wt%.

[0073] However, too high contents of copper 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 stainless steel comprises at an amount equal to or less than 1.0 wt% of Cu. According to embodiments, Cu may be present in an amount equal to or less than 0.9 wt%, such as equal to or less than 0.85 wt%.

[0074] Manganese (Mn) 0.40 to 0.80 wt%

[0075] Mn is an austenite stabilizer and acts as a deoxidation agent. Mn also increases the solubility of N and improves the hot workability. The Mn content is therefore equal to or more than 0.40 wt%, or equal to or more than 0.50 wt%.

[0076] However, a too high content of Mn can contribute to the formation of MnS inclusions in combination with S, which will have a detrimental impact on the mechanical properties. Therefore, the herein described stainless steel comprises equal to or less than 0.80 wt% Mn. The Mn content may alternatively be equal to or less than 0.75 wt%, such as equal or less than 0.70 wt%.

[0077] Chromium (Cr) 13.0 to 15.0 wt%

[0078] Cr is essential for the corrosion resistance of the stainless steel which is determined by the amount of Cr in the stainless steel matrix. Cr also forms carbides (M23C6, M7C3 and carbonitrides) and increases the solubility of C and N. In order to obtain the desired amount of these carbides, the content of Cr is therefore equal to or more than 13.0 wt%, such as equal to or more than 13.5 wt%.

[0079] Cr is a ferrite stabilizer, and a too high amount can result in the formation of delta ferrite. Furthermore, too much chromium may also reduce the toughness and may increase the risk for formation of primary carbides, therefore the maximum chromium content is equal to or less than 15.0 wt%, such as equal to or less than 14.5 wt%.

[0080] Molybdenum (Mo) 0.90 to 1.50 wt%

[0081] Mo is a ferrite stabilizer and a strong carbide former. Mo has a positive effect on both the corrosion resistance and the hardenability of the stainless steel. Mo also contributes to an improved ductility. In order to achieve these properties, the content of Mo is from 0.90 wt%, such as at more than or equal to 0.95 wt%. Since Mo is an expensive element, the content should not be more than necessary for economic reasons. The amount of Mo is therefore limited to 1.50 wt%, such as less than or equal to 1.30 wt%, such as less than or equal 1.20 wt%.

[0082] Nitrogen (N) 0.08 to 0.15 wt%

[0083] Nitrogen is an element that has been purposely added to the herein described stainless steel in order to increase strength. Nitrogen is an important element in the present martensitic stainless steel as it will contribute to the formation of carbonitrides. Additionally, N is an austenite stabilizer and is added to facilitate achieving a desired amount of retained austenite after hardening as it will provide for solution hardening of the retained austenite. Nitrogen is therefore present in an amount of equal to or more than 0.08 wt%, such as equal to or more than 0.085 wt%, such as equal to or more than 0.09 wt%, such as equal to or more than 0.095 wt%.

[0084] However, if present in too high amounts, the hot workability may be reduced, therefore, the content of N is equal to or less than 0.15 wt%. According to embodiments, the content of N is equal to or less than 0.145 wt%, such as equal to or less than 0.14 wt%, such as equal to or less than 0.13 wt%, such as equal to or less than 0.125 wt%.

[0085] Vanadium (V) 0.15 to less than 0.40 wt%

[0086] Vanadium is a strong carbide former which forms small and dispersed carbides and carbonitrides in the matrix. These carbides provide an increased hardness, and thus also contribute to the wear resistance without negatively affecting the toughness as long as the content of vanadium is not too high. The herein described stainless steel therefore comprises equal to or more than 0.15 wt% V, such as equal to or more than 0.20 wt% V.

[0087] The amount of carbides and carbonitrides increases with increasing amount of vanadium. However, too high contents of vanadium may increase the risk for formation of unwanted primary carbides and higher contents of vanadium may also lead to a reduction in ductility, which in turn may negatively affect the fatigue strength. Furthermore, V may reduce the amount of retained austenite as well as consume all N and C in the stainless steel matrix which will lead to a too soft stainless steel matrix. Therefore, the herein described martensitic stainless steel comprises less than 0.40 wt% of vanadium. To further reduce the risk for deterioration of the desired properties, V may be present in an amount of equal to or less than 0.35 wt%, such as equal to or less than 0.30 wt%.

[0088] Nickel (Ni) 0.10 to 1.0 wt%

[0089] Ni is an austenite stabilizer and decreases the solubility of C and N. Due to this, and as Ni is an expensive element, the content should be kept low. Furthermore, Ni will lower the Al temperature. Ni is therefore normally not purposively added to the present martensitic stainless steel but present due to the scrap used.

[0090] In practice, it is almost impossible to remove all Ni when using metal scrap as raw material therefore nickel is allowed in contents up to max 1.0 wt% as this will not provide any substantial negative effect on the desired properties. It is however desirable to seek to reduce the Ni content further as long as this may be performed without unduly increasing the manufacturing costs. Hence, according to embodiments, Ni content may be equal to or less than 0.50 wt%.

[0091] Hence, according to embodiments, the Ni content is 0.10 to 1.0 wt%, such as 0.10 to 0.50 wt%, such as 0.10 to 0.40 wt%, such as 0.10 to 0.30 wt%.

[0092] Phosphorus (P) equal to or less than 0.03 wt%

[0093] 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 composition, P may therefore be allowed in amounts of equal to or less than 0.03 wt% 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 wt% or even equal to or less than 0.01 wt%.

[0094] Sulfur (S) equal to or less than 0.03 wt%

[0095] Like phosphorus, sulfur is an impurity element in the herein described stainless 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 stainless 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 wt% without substantially affecting the properties negatively. Lower amounts of S are beneficial for reducing the risk of formation of harmful sulfides, and the martensitic stainless steel may therefore comprise equal to or less than 0.01 wt% of S.

[0096] Boron (B) optionally equal to or less than 0.005 wt%

[0097] The martensitic stainless steel does not need to comprise boron, but 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 stainless 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 wt%, preferably equal to or less than 0.003 wt%.

[0098] Aluminum (Al) optionally equal to or less than 0.25 wt%

[0099] The martensitic stainless steel does not need to comprise aluminum, but aluminum may be added as a deoxidation agent during stainless steel production. If added, aluminum may be present in amounts equal to or less than 0.25 wt%, such as equal to or less than 0.20 wt%.

[0100] Calcium (Ca) and / or Magnesium (Mg) optionally equal to or less than 0.15 wt% in total The martensitic stainless steel does not need to comprise Ca or Mg. Nevertheless, one or both of Ca and Mg may, if desired, be added up to a total amount of 0.15 wt% to improve the hot ductility of the material during the production process. Preferably, the calcium content is at most 0.05%, suitably equal to or less than 0.01 wt%. The content of Mg may suitably be at most 0.05 wt%.

[0101] Rare Earth Metals (REM) optionally equal to or less than 0.5 wt%

[0102] The martensitic stainless steel does not need to comprise REM. However, REM may be added in an amount of equal to or less than 0.5 wt%, 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 wt%.

[0103] Normally occurring impurities up to 0.8 wt% in total

[0104] The herein described composition of the present martensitic stainless steel may, in addition to the elements already specified and discussed above, comprise up to at most 0.8 wt% 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 0.7 wt% in total or alternatively equal to or less than 0.5 wt% in total.

[0105] 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 wt% in total, or even equal to or less than 0.10 wt% 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 and / or sizes 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 wt% each, or even equal to or less than 0.03 wt% each.

[0106] Other examples of impurity elements that may be present in the herein described stainless 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 wt%, or equal to or less than 0.2 wt%. The content of W may for example be equal to or less than 0.3 wt%, or even equal to or less than 0.1 wt%. The content of Sn may for example be equal to or less than 0.1 wt%, or even equal to or less than 0.05 wt%.

[0107] Moreover, the martensitic stainless steel or the strip as defined hereinabove or hereinafter may comprise or consist of the elements as defined hereinabove or hereinafter herein, in any of the ranges mentioned herein.

[0108] Microstructure

[0109] The composition of the present martensitic stainless steel as defined hereinabove or hereinafter also fulfills the following requirement:

[0110] [wt% V] / ([wt% N] + [wt% C]) > 0.10 (RQ 1).

[0111] According to embodiments, said composition may fulfill the following requirement:

[0112] 0.10 < [wt% V] / ([wt% N] + [wt% C]) < 0.40; such as 0.15 < [wt% V] / ([wt% N] + [wt% C]) < 0.40.

[0113] This requirement will provide for the formation of essentially only desired precipitates during manufacturing, thus substantially avoiding the formation of unwanted primary carbides. Additionally, the present requirement will also provide for the formation of precipitates having a small grain size during annealing.

[0114] According to embodiments, the inventive martensitic stainless steel may comprise the following carbides and carbonitrides depending on what hardening temperature the steel has been exposed to; VCr(NC) and M23C6 and / or M7C3. The presence of different types of precipitates of carbides and carbonitrides will provide for fine precipitates throughout the steel and thereby enhancing the mechanical properties.

[0115] Hence, according to one embodiment, the herein described martensitic stainless steel may have a primarily martensitic microstructure comprising finely dispersed carbides and carbonitrides of M23C6 and / or M7C3 and VCr(NC). These precipitates will contribute to both increased wear resistance and strength. Furthermore, these precipitates will provide for good precipitation hardening properties.

[0116] The microstructure further comprises retained austenite (RA). The amount of retained austenite is dependent on 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 lead to higher amount of retained austenite.

[0117] By means of the herein described composition and appropriate selection of hardening temperature, it is possible to obtain the desired content of retained austenite in the present martensitic stainless steel. Based on current knowledge, it is believed that an appropriate amount of retained austenite, when the martensitic stainless steel is to be used in high demanding valve applications, should be in the order of at least 14 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. Hence, the maximum content of retained austenite should be about 22 vol% after hardening. By means of the herein described composition and appropriate selection of hardening temperature, it is possible to obtain the desired content of retained austenite in the present martensitic stainless steel. Hence, when hardening a martensitic stainless steel strip as defined hereinabove or hereinafter in a temperature of 1020 °C or 1030°C, the retained austenite may be in a range of from 14 vol% to 22 vol%. There are different methods of determining the retained austenite but herein the amount of retained austenite has been determined according to the following steps:

[0118] (a) determining magnetic dipole moment of a specimen of the martensitic steel in accordance with the withdrawal method of IEC 60404-14;

[0119] (b) determining weight-specific saturation magnetism, os, as the ratio of the determined magnetic dipole moment of the specimen to weight of said specimen;

[0120] (c) calculating the volume percentage of retained austenite, RA, of the specimen according to Equation 1:

[0121] RA (vol. -%) = 100 - 100 * as / (k * CTm) (Eq. 1)

[0122] wherein k is a correction factor selected to be 0.92 and

[0123] <jmrepresents theoretical magnetic saturation of the specimen and is calculated based on the chemical composition thereof using the Hoselitz formula as given by Equation 2:

[0124] am= 217.75 - 12 * [wt. -% C] - 2.4 * [wt. -% Si] - 1.9 * [wt. -% Mn] - 3 *

[0125] [wt. — %Cr] — 0.75 * [wt. — % TVi] — 1.2 * [wt. — % Mo] — 2.6 * [wt. — % Al] — 3 *

[0126] [wt. — % P] — 7 * [wt. — % S] — 2.3 * [wt. — % Cu] — 6 * [wt. — % IV] (Eq. 2).

[0127] Method of production

[0128] The martensitic stainless steel as defined hereinabove or hereinafter may be produced by providing a melt having the composition as defined hereinabove or hereinafter, followed by casting. The melting process may for example be performed by the 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 to a bloom of a desired shape;

[0129] The as-cast material may thereafter optionally be subjected to a heat treatment, such as heating the cast material to a temperature of 1000 - 1350°C.

[0130] The casting is followed by a hot rolling process. The hot rolling may be performed in several passes until the desired thickness is obtained. The starting temperature of the hot rolling process is 1000 to 1300 °C. The hot rolled strip may thereafter be coiled, if desired. The hot rolled material may be heat treated if desired in order for example increase ductility. The hot rolled, and optionally heat treated, stainless steel strip may thereafter be cold rolled to intended final strip thickness. Depending on the intended use of the strip, the final thickness may for example be 0.040 - 3 mm but is not limited thereto. Before cold rolling, the obtained stainless steel strip may be pickled or ground. Cold rolling may be performed in several passes, if needed, in order to reach the intended final thickness of the strip.

[0131] Optionally, annealing may be performed between at least two of the cold rolling passes or after cold rolling. Such annealing may be performed at specific temperatures of 600 to 900 °C.

[0132] 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 950 to 1100 °C. . 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.

[0133] After hardening, the hardened strip may be subjected to tempering to a temperature of 150 to 470 °C.

[0134] The martensitic stainless steel strip 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 stainless 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 or otherwise forming the valve component from the hardened and tempered stainless steel strip. The introduction of surface residual stress may be made by tumbling although other processes, such a shot peening, are also possible.

[0135] Alternatively, or additionally, the hardened and tempered stainless steel strip may be coated, if desired. Alternatively, or additionally, the hardened and tempered stainless steel strip may be further worked if desired, for example it may be polished or ground.

[0136] The properties of the herein described martensitic stainless steel are dependent on processing steps to which it has been subjected, i.e. the condition, and may be tailored in dependence of the intended use. The following properties are however given for purpose of illustration. The herein described martensitic stainless 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. Furthermore, the surface treatment will allow for high compressive residual stresses. Hence, the present martensitic stainless steel as defined hereinabove or hereinafter provides for a capability to build compressive residual stresses.

[0137] The present invention is further described in the following non-limiting examples

[0138] EXAMPLES

[0139] Experimental heats were produced on a small laboratory scale using vacuum induction melting (VIM) followed by casting into molds. The mass of each heat was about 1.5 kg. The compositions of the heats are specified in Table 1 below, he production from ingots to rods was conducted according to Table 2 below. It should be noted that since the ingots were so small, no cold forming step (such as cold rolling) was needed or performed. However, when producing a strip of the herein described martensitic stainless steel in full production, a cold rolling step will be needed to obtain a suitable strip thickness.

[0140] After the production described in Table 2 was performed, samples were taken from the obtained rods by cutting the rods longitudinal in desired lengths. From the longitudinal cut rods, the following two set of samples were prepared:

[0141] Samples I - This set of samples were hardened at different temperatures for 20 min and then tempered at 350 °C for 2 h and then exposed to two tumbling processes. The purpose of the first tumbling was to remove oxide formed during heat treatments and also for determining the ability of forming residual stresses. The purpose of the second tumbling trial was to determine the weight loss and thereby the wear resistance. The hardening temperatures for Samples I are written in each table below.

[0142] Samples II - This set of samples were converted into tensile cylindrical test samples and then hardened for 20 minutes and tempered at 350 °C for 2 h. The hardening temperature is written in the table relating to tensile testing. Table la Compositions of the heats. Heats falling withing the scope of the present invention are marked with a "*". The balance for each heat is Fe and unavoidable impurities. All the numbers given in the table are in wt%

[0143] Element / Heat 1 2 3* 4* 5 6

[0144] Cr 13.90 13.99 13.94 14.18 13.93 14.03

[0145] C 0.530 0.516 0.528 0.539 0.508 0.532

[0146] Si 0.37 0.39 0.40 0.41 0.42 0.37

[0147] Mn 0.65 0.68 0.67 0.68 0.68 0.64

[0148] Mo 1.00 1.00 1.00 1.00 1.00 1.00

[0149] Cu 0.70 0.69 0.70 0.72 0.70 0.71

[0150] N 0.089 0.066 0.100 0.120 0.090 0.128

[0151] V 0.05 0.25 0.23 0.25 0.44 0.40

[0152] Ni 0.19 0.18 0.18 0.19 0.18 0.20

[0153]

[0154] Table lb The result of reguirement 1 (RQ1)

[0155] RQl / Heat 1 2 3* 4* 5 6

[0156] V / (N+C) 0.08 0.42 0.37 0.38 0.74 0.61

[0157]

[0158] Table 2 The manufacturing method of rods

[0159] Process step

[0160] 1 Preheating 700 °C 30 min 1170 °C 40 min Ingot 210 x Preheating 24 mm 2 Hot rolling 1170- 1130 °C 11 passes 7 x7 mm23 Air cooling

[0161] 4 Heat treatment About 800°C

[0162]

[0163] Microstructure

[0164] In order to have a better understanding of the results, thermodynamic calculations were performed on the compositions for the experimental heats. The calculations were made in the Thermo-Calc Software using the database TCFE12: Steel / Fe-Alloys, database version 12.0." Phase diagrams for Alloys 1 to 6 demonstrated that the main precipitations therein constitute M23C6. However, as can be seen below, the presence of vanadium in Alloys 2-6 results in additional phases, from which the most interesting may be chromium M7C3 carbides and VCr(NC) since these are stable at higher temperatures.

[0165] Table 3 discloses the vol% of precipitates in the austenite at equilibrium at different temperatures. The temperatures were selected based on the temperatures at which the hardening trials described below were performed.

[0166] Table 3 The sum of volume fraction of different precipitates at different hardening temperatures

[0167] Heat /

[0168] Hardening

[0169] Temp

[0170] (°C) 1015 1025

[0171] 1 0.046M23C6 0.043 M23C6

[0172] 0.052 M23C6 0.049 M23C6

[0173] 2

[0174] 0.0011 VCr(NC) 0.0009 VCr(NC)

[0175] 0.052 M23C6 0.048 M23C6

[0176] 3*

[0177] 0.0018 VCr(NC) 0.0016 VCr(NC)

[0178] 0.054 M23C6 0.050 M23C6

[0179] 4*

[0180] 0.0024 VCr(NC) 0.0023 VCr(NC)

[0181] 0.052 M23C6 0.049M23C6

[0182] 5

[0183] 0.0036 VCr(NC) 0.0035 VCr(NC)

[0184] 0.053 M23C6 0.050 M23C6

[0185] 6

[0186] 0.0044 VCr(NC) 0.0042 VCr(NC)

[0187]

[0188] Retained Austenite

[0189] Samples I were used for the measurements of amount of retained austenite (RA) and these measurements were performed as described in the description and the results are shown in Table 4.

[0190] Table 4 The amount of retained austenite [vol%]for 8 mm pieces.

[0191] Hardening

[0192] Temp [°C] / Heat 1 2 3* 4* 5 6

[0193] 1020 15.7 14.0 15,7 15.8 13.1 14.4

[0194] 1030 19.0 15.9 19.8 21.1 15.3 18.3

[0195]

[0196] As can be seen from Table 4, the amount of retained austenite increases with increasing hardening temperature, which is expected. Moreover, the data indicated a reduction of retained austenite at the highest vanadium content, i.e. 5 and 6, which is, without being bound to any theory, likely an outcome of formation of a too high content of carbides and carbonitrides. As already mentioned above, it is believed that the amount of retained austenite should be in the order of about 14 to 22 vol%, which is shown form the samples hardened in 1020 as well as 1030°C. The results presented in Table 4 demonstrate that this may be obtained.

[0197] Wear mark

[0198] Samples I were tested in a Pin - Disc setup, Tribometer, with a 6 mm WC - ball and a load of 5 N. The largest depth along the pattern was located and at this position but in the transversal direction the max average depth over a width of 60 pm was measured. The wear marks depth for the different alloys are presented in Table 5. It is clearly observed that the samples I of Heats 3 and 4 perform considerably better in comparison to the other alloys. Without being bound to any theory, it is believed that this is due to the fact that these samples comprise a higher volume fraction of different precipitates and these precipitates will provide a hardened effect. Table 5 Result of wear mark testing

[0199] Heat Max wear mark depth

[0200] [pm]

[0201] 1 2.25

[0202] 2 2.85

[0203] 3* 0.73

[0204] 4* 0.63

[0205] 5 3.13

[0206] 6 2.36

[0207]

[0208] Hence, according to embodiments, the stainless steel strip as defined hereinabove or hereinafter may have a wear mark equal to or less than 2.0, such as equal to or less than 1.5 when measured as described above.

[0209] Surface compressive residual stresses

[0210] The results of the average compressive stresses when Samples I were hardened at 1025 °C are shown in are shown in Table 6.

[0211] XRD analysis was performed using XRD- Bruker axs D8 DISCOVER diffractometer (35kV,50mA). Residual stress of was measured according to the sin2i|jsin2^ -method (1) using Cr-Ka (X = 2.2879 A) radiation. (211) diffraction line of martensite at 20orange of 148°-162° (156.08) was measured at 7 tilt angles in the range of O < i|j < < ^ < 60° and azimuthal angles of 0 and 90°. The step size and time per step were set at 0.03° and 1 sec, respectively. The recorded diffraction lines were fitted using a Sliding Gravity function. For stress calculation, the elastic constants E = 220.3 GPa and v = 0.28 for steels were used. Additionally, the X-ray elastic constants were Sl= -1.271E-6 and 1 / S2= 5.811E-6. ASTM E2860 - 12, Standard Test Method for Residual Stress Measurement by X-Ray Diffraction for Bearing Steels Table 6 Average compressive residual stresses results

[0212] Heat Average compressive

[0213] residual stresses [MPa]

[0214] 1 1060

[0215] 2 840

[0216] 3* 980

[0217] 4* 1080

[0218] 5 630

[0219] 6 790

[0220]

[0221] As can be seen from the above, Samples I of Heats 3 and 4 have the capability to form high compressive residual stresses. It can be concluded that the Sample I of Heat 1 has a high level of residual stresses even though it did not have any good wear resistance. It can also be seen that Sample I of Heat 2 sample 2 with similar vanadium content as Heat 4 exhibit a lower level of residual stresses. Heat 3 also provides a good capability to form compressive residual stresses. Another conclusion is that Heat 5 and 6 with the higher level of vanadium will not be suitable for foring high compressive residual stresses.

[0222] Tensile strength

[0223] Sample II were hardened at 1020 °C for 20 min. The results of the mesurements of tensile strength, yield strength and elongation are presented in Table 7.

[0224] Table 7 The result of the mechanical testing

[0225] Heat Tensile strength Yield strength Rpl.O Elongation A5 [MPa] [MPa] [%]

[0226] 1 2020 1910 7.8

[0227] 2 1935 1835 6.4

[0228] 3* 2000 1900 6.9

[0229] 4* 2000 1900 7.0

[0230] 5 2000 1865 6.8

[0231] 6 1960 1900 6.4

[0232]

[0233] As can be seen from the table above, Samples II of Heats 3 and 4 have as good strength and ductility as for example Sample II of Heat 1. However, this heat does .does not provide good wear resistance compared to the Heats 3 and 4. The same is true for samples II of Heats 2, 5 and 6. Hardness

[0234] The hardness was measured on Samples according to ISO6507 Table 6. As can be seen from Table 8, samples 3 and 4 exhibit the highest hardness.

[0235] Table 8 Hardness for the different alloys and hardening temperatures.

[0236] Hardening

[0237] temp.

[0238] 1 2 3* 4* 5 6 (oC) / Heat

[0239] 1020 603 590 614 607 589 604

[0240] 1030 599 590 605 610 600 604

[0241]

[0242] There is always a risk when improving one mechanical property that another mechanical property will be negatively affected. However, as can be seen from the results above, for the herein defined martensitic stainless steel, the inventors have found a composition of element ranges which will allow for introducing high surface residual stresses after hardening and tempering and which will have great wear resistance as well as have good mechanical properties.

Claims

CLAIMS1. A martensitic stainless steel with the following composition, in percent by weight:C more than 0.50 to 0.60,Si 0.10 to 0.60,Mn 0.40 to 0.80,Cr 13.0 to 15.0,Ni 0.10 to 1.0,Mo 0.90 to 1.50,N 0.08 to 0.15,Cu 0.2 to 1.0,V 0.15 to less than 0.40,S max 0.03,P max 0.03,optionally Mg and / or Ca up to 0.15 in total,optionally REM up to 0.5,optionally Al up to 0.25,optionally B up to 0.005,balance Fe and normally occurring impurities up to 0.8 wt% in total;and wherein the composition fulfils the following requirement [wt% V] / ([wt% N] + [wt% C]) is greater than or equal to 0.10 (RQ1).

2. The martensitic stainless steel according to claim 1, wherein the composition fulfils the following requirement:0.10 < [wt% V] / ([wt% N] + [wt% C]) < 0.40.

3. The martensitic stainless steel according to claim 1 or claim 2, wherein the content of Si is from 0.20 to 0.50 wt%.

4. The martensitic stainless steel according any one of claims 1 to 3, wherein the content of Mo is 0.95 to 1.30 wt%.

5. The martensitic stainless steel according to any one of claims 1 to 4, wherein the content of Mn is from 0.50 to 0.75 wt%.

6. The martensitic stainless steel according to any one of claims 1 to 5, wherein the content of C is greater than or equal to 0.510 wt%, such as .greater than or equal to 0.520 wt%.

7. The martensitic stainless steel according to any of claims 1 to 6, wherein the content of Cu is from 0.3 to 0.9 wt%.

8. The martensitic stainless steel according to any of claims 1 to 7 , wherein the content of Ni is from 0.10 to 0.40 wt%.

9. The martensitic stainless steel according to any one of claims 1 to 8, wherein the content of N is from 0.09 to 0.14 wt%.

10. The martensitic stainless steel according to any one of claims 1 to 9, wherein the content of V is from 0.15 to 0.35 wt%.

11. The martensitic stainless steel according to any one of claims 1 to 10, wherein the content of Cr is from 13.5 to 14.5 wt%.

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

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

14. The stainless steel strip according to claim 12 or claims 13, wherein the content of retained austenite of the stainless steel strip is in the range of 14 to 22 vol% when hardened at 1020 °C or 1030°C and when measured as described in the description page 13.

15. Use of the stainless 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.

16. A method for producing a stainless steel strip according to claims 12 to 14, the method comprising:- casting a melt having a composition as defined in any of claims 1 to 11, - optionally heat treating the as-cast material,- followed by hot rolling the cast stainless steel to intermediate thickness, - optionally heat treating the hot rolled stainless steel,- cold rolling the hot rolled stainless steel to intended final strip thickness, - hardening the cold rolled strip at a hardening temperature of 950 -1100 °C followed by quenching, and- tempering the hardened strip to a temperature of 200-470 °C.

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