Powder-metallurgical chromium- and niobium-containing steel and method of manufacturing the same

Abstract

A powder-metallurgical steel consists of, in % in weight: C: 0.35% to 0.6%, N: 0.01% to 0.3%, Cr: 12% to 15%, Mo: 3% to 5.5%, Co: 1.6% to 5%, Nb: 1.2% to 1.75%, and optionally one or more of Si: less than 1.5%, Mn: less than 1.5%, W: less than 0.5%, Ni: less than 0.5%, Al: less than 0.3%, V: less than 1.0%, Ti: less than 1.0%, Ta: less than 0.5%, the balance being Fe and incidental impurities.

Description

POWDER-METALLURGICAL CHROMIUM- AND NIOBIUM-CONTAINING STEEL AND METHOD OF MANUFACTURING THE SAMETechnical Field

[0001] This disclosure relates to a powder-metallurgical steel , and in particular to a powder-metallurgical chromium- and niobium-containing steel .Background

[0002] In speci fic technical areas , steels are required which exhibit a combination of high corrosion resistance and high hardness , while still providing good toughness for avoiding cracks or chipping during operation . For example, applications in plastics processing, e . g . inj ection molding or other plastic forming processes , and applications such as industrial blade materials or hand knives , particularly in the food industry, require highly corrosion-resistant steels with high hardness and wear resistance combined with good toughness and polishability .

[0003] An NbC-containing powder-metallurgical martensitic steel has been described by Hahn, Isabel , et al . , "Novel Development of an NbC-Containing Powder-Metallurgical Martensitic Steel with Outstanding Tribocorrosion Resistance", steel research int . , 2023 , 94 , 2200474 , pages 1-7 . This steel consists of a martensitic matrix with evenly dispersed Nb-rich monocarbides having a fraction in volume of 2 . 5 vol% . The fine dispersion of monocarbides increases the wear resistance, whereas high contents of chromium, molybdenum and nitrogen dissolved in the metal matrix exert a positive effect on the corrosion properties .

[0004] Powder-metallurgical production of Nb- and C- containing steel s i s di f ficult , because primary carbides , which form in an Nb- and C-containing melt during atomi zation, tend to agglomerate in the noz zl e and clog it . There fore , to avoid clogging, a conventional manuf acturing route for producing powder-metallurgical steel s wi th hi gh Nb-content i s to atomi ze an Nb-containing melt without carbon . The carbon i s subs equently added af ter atomi z ation in the form of graphite-powder . The formation of NbC and the carboni zation of the metal matrix are achi eved during hot i sostatic pres sing ( HI P ) by di f fusion . However , thi s proces s i s complex and not applicable for large- scale industrial implementation .Summary

[0005] According to an aspect o f the di sclosure a powder-metallurgical steel consi sts of a s teel composition, in % in wei ght : C : 0 . 35 % to 0 . 6 % , N : 0 . 01 % to 0 . 3 % , Cr : 12 % to 15 % , Mo : 3 % to 5 . 5 % , Co : 1 . 6% to 5 % , Nb : 1 . 2 % to 1 . 75 % , and optional ly one or more of Si : le s s than 1 . 5 % , Mn : l es s than 1 . 5 % , W : les s than 0 . 5 % , Ni : les s than 0 . 5 % , Al : l es s than 0 . 3 % , V : les s than 1 . 0 % , Ti : l es s than 1 . 0 % , Ta : l es s than 0 . 5 % , the bal ance being Fe and incidental impurities .

[0006] According to another aspect of the di s closure , a hardened powder-metallurgical steel compri s es the powder-metallurgical steel recited above after having been subj ected to a hardening proces s and a tempering proces s , the hardening proces s compri sing heating the powder-metallurgical steel to a temperature between 1000 ° C and 1150 ° C followed by rapid cooling, and the tempering proces s compri sing heating the cool ed powder-metallurgical steelto a tempering temperature in a range between 200 ° C and 600 ° C, wherein the tempering i s repeated at least once .

[0007] According to another aspect of the di sclosure , a method of manufacturing a powder-metallurgical steel compri s es mel ting a steel composi tion cons i sting of , in % in weight : C : 0 . 35 % to 0 . 6 % , N : 0 . 01 % to 0 . 3 % , Cr : 12 % to 15 % , Mo : 3 % to 5 . 5 % , Co : 1 . 6% to 5 % , Nb : 1 . 2 % to 1 . 75 % , and optionally one or more of Si : les s than 1 . 5 % , Mn : les s than 1 . 5 % , W : les s than 0 . 5 % , Ni : les s than 0 . 5 % , Al : l es s than 0 . 3 % , V : le s s than 1 . 0 % , Ti : les s than 1 . 0 % , Ta : les s than 0 . 5 % , the bal ance being Fe and incidental impurities . The melted steel composi tion i s then atomi zed to obtain a metal powder . The metal powder i s hot i sostatic pres sed to obtain a powder-metallurgical steel product . The powder-metallurgical steel product i s formed .

[0008] According to another aspect of the di s closure , a method of manufacturing a hardened powder-metallurgical steel further compri se s hardening the powder-metallurgical steel product recited above by heating the powder-metallurgical steel product to a temperature between 1000 ° C and 1150 ° C fol lowed by rapid cooling, and tempering the hardened powder-metallurgical s teel product by heating the cool ed powder-metallurgical steel product to an tempering temperature in a range between 200 ° C and 600 ° C , wherein the tempering i s repeated at least once .Brief description of the drawings

[0009] The features of the various illustrated exampl es can be combined unl es s they exclude each other and / or can be s electivel y omi tted i f not described to be neces sarily required . Examples aredepicted in the drawings and are exemplarily detailed in the description which follows .

[0010] Figure 1 is an illustration of an exemplary manufacturing route of a powder-metallurgical steel according to the disclosure .

[0011] Figure 2 is a schematic illustration of exemplary processes of stress relief heating, hardening and tempering a powder-metallurgical steel according to the disclosure .

[0012] Figure 3 is a diagram illustrating a size distribution of NbC and NbCN primary carbides in the microstructure of an example of a powder-metallurgical steel according to the disclosure after hardening and tempering .

[0013] Figure 4 is a diagram showing a tempering curve of a hardened powder-metallurgical steel according to the disclosure as a function of the tempering temperature .

[0014] Figure 5 is a Schaeffler diagram illustrating the microstructure of the metal matrix of steel dependent on the Cr-equivalent and the Ni-equivalent of alloying elements , and further showing the microstructure of an example of a powder-metallurgical steel according to the disclosure after hardening and before tempering .

[0015] Figure 6 is a diagram showing the hardness and the impact toughness of an example of a powder-metallurgical steel according to the disclosure and an example of a commercially available powder-metallurgical steel after hardening and low temperature tempering and after hardening and high temperature tempering .

[0016] Figure 7 is a diagram showing the corrosion resistance of the examples of powder-metallurgical steels of Figure 6.Detailed description

[0017] In the powder-metallurgical steel according to the disclosure, Cr-rich carbides have been substituted with primary carbides such as NbC and NbCN ( in the following written as "NbC (N) " ) . As the solubility of Cr in these monocarbides is low, Cr remains dissolved in the metal matrix and thus increases the corrosions resistance of the steel .

[0018] In other words , Nb causes the formation of primary carbides NbC (N) during atomization . The primary carbides consume C, resulting in very few Cr-rich carbides ( e . g . CraCa and CraaCe) being formed during atomization and hot isostatic pressing . As a result, most of Cr remains in the metal matrix . The high Cr content in the metal matrix, together with a high Mo content, results in the desired corrosion resistance .

[0019] Further, it has been found that the addition of Co in a certain ratio to the Nb content is helpful to alleviate the problem of nozzle clogging and to improve the hardness of the powder-metallurgical steel .

[0020] Table 1 provides the chemical composition in % in weight of the powder-metallurgical steel according to the disclosure .Table 1: Chemical composition (in wt%) of steel melt according to the present disclosureTable 1: (continuation)Effects of the alloying elements

[0021] The effects of the alloying elements in relation to their content are described below. Percentages given below are % in weight (wt% ) . The balance of the chemical composition are Fe and incidental impurities .Carbon [C]

[0022] Carbon is essentially used to set the desired hardness . At least 0 . 35% of C is needed . However, the carbon content should not be too high, as this can lead to an excessively high proportion of precipitates in the form of carbides , which couldhave a negative e f fect on toughnes s and fatigue strength .There fore , the upper l imit i s set to 0 . 6% .Nitrogen [N]

[0023] Next to Carbon , Nitrogen i s the second most signi ficant element contributing to the high achievable hardne s s level when di s solved interstitial ly . Additionall y, it forms ni tride s and carbonitrides with elements such as niobium or vanadium . To achieve these e f fects , a lower nitrogen limit o f 0 . 01 % i s required . An upper limi t of 0 . 3 % i s s et because exce s sive nitrogen content stabi li zes aus tenite , potenti ally resulting in high amounts of retained austeni te .Sil icon [ Si ]

[0024] Si i s a solid solution strengthener and not a carbide-forming element , but influences carbide precipitation kineti cs in steel . I t shi fts the formation of secondary hardening carbides to higher tempering temperatures , at whi ch the non Cr-rich secondary hardening carbide s M7C3 and M23C6 pre ferential ly form . S i serves as a deoxidi zing agent and i s there fore present in low concentrations in almost al l steel s due to the manufacturing proces s . I t increases the re si stance to scaling , the yield strength and the tensi le strength wi thout signi ficantly reducing the elongation . However , an exces sively high Si content can signi ficantly reduce toughne s s . There fore , the upper limit i s set to 1 . 5 % . I f Si i s used as an alloying element , lower limits may, e . g . , be set to 0 . 05 % , 0 . 10 % , 0 . 15 % , 0 . 20 % , 0 . 25 % , 0 . 30 % , 0 . 35 % , 0 . 40 % , 0 . 45 % , 0 . 50 % , 0 . 55 % , or 0 . 60 % .Mangane se [Mn]

[0025] Mn can inf luence the hardnes s of the steel . In addition, Mn acts as a solid solution strengthener similar to S i . Mnincrease s the hardnes s , but too high a proportion can l ead to a reduction in toughnes s . There fore , the upper limit i s set to 1 . 5 % . I f Mn i s us ed as an alloying el ement , lower l imits may, e . g . , be set to 0 . 05 % , 0 . 10 % , 0 . 15 % , 0 . 20 % , 0 . 25 % , 0 . 30 % , 0 . 35 % , 0 . 40 % , 0 . 45 % , 0 . 50 % , 0 . 55 % , or 0 . 60 % .Chromium [ Cr ]

[0026] Cr leads to corrosion res i stance in the metal matrix at a content greater than 12 % . There fore , the lower l imit i s set to 12 % . I t al so leads to the desired solid solution strengthening . In general , Cr reduces the critical cooling rate and thus increase s hardenability . Furthermore , increased Cr contents can lead to carbide precipitation and thus increas e the hardnes s . Exces sively high Cr contents can there fore al so lead to negative e f f ects in terms o f toughnes s . In addi tion , exces sive Cr content can have a negative e f fect on the retained austenite content after hardening, which may become too high . There fore , the upper limit i s set to 15 % .Mol ybdenum [Mo ]

[0027] Mo forms special carbides and, on the other hand, mixed carbides with iron . The se are of the type M2C and MeC . The addition of Mo increase s the activation energy for C di f fusion in the austenite and thus lowers the di f fusion coe f fi cient for C and thus C di f fus ion . On the other hand, the addition o f Mo leads to a re finement of the microstructure , i . e . a fine microstructure prevail s regardle s s of the cool ing rate ( 0 . 1 ° C / s to 60 ° C / s ) during hardening . Grain coarsening remains low due to the low di s solution rate and the hi gh di s solution temperature o f the carbides ( carbide s counteract grain coars ening ) . Thi s means that aus teni ti zing ( solution tempering ) at higher hardening temperatures can achi eve improved tempering res i stance , as morecarbide- forming elements can be precipitated and thus more carbides are formed . The hard carbides also increase the hot yield strength and wear-resistance . At high operation temperatures , Mo improves the scale resistance of the steel . Too high a content impairs machinability and, if Mo remains dissolved in the metal matrix, thermal conductivity . It could also happen that embrittlement occurs during tempering due to the formation of carbide precipitates at the former austenite grain boundaries . Therefore, the upper limit according to the disclosure is 5 . 5% by weight, preferably 5% by weight, particularly preferably 4 .2% by weight . For the above-mentioned reasons , the lower limit according to the invention is 3% by weight, preferably 3 . 5% by weight, particularly preferably 3 . 8% by weight .Nickel [Ni ]

[0028] Ni is an austenite-stabilizing element . Therefore, the upper limit is set to 0 . 5% .Cobalt [ Co]

[0029] Co is an austenite-stabili zing element . It does not form carbides , but remains dissolved in the metal matrix and thus influences carbon diffusion . This leads to an increase in hot hardness (hardness at temperatures higher than room temperature to about 600 °C) and improved toughness . Further, Co promotes the formation of secondary hardening carbides , as Co acts as nucleation site during tempering . This may increase the hardness caused by secondary hardening carbides ( to an extent depending on the tempering temperature ) . An upper limit is determined due to saturation of carbide precipitation .Niobium [Nb]

[0030] Due to i ts high a f fini ty to C , Nb i s one of the stronge st carbide- forming elements . I t forms fine and evenly di stributed precipi tates of the monocarbide (MC ) type , namely NbC (N) . Thes e are pre f erred due to their higher thermal resi s tance and hi gher hardne s s compared to other carbide types . Thi s results in an increase in high-temperature strength, an increase in yi eld strength , wear res i stance and an improvement in tempering res i stance . However , at higher concentrations , a higher hardening temperature i s required to di s solve the thermally stabl e , primary carbides NbC (N) . High amounts of NbC (N) reduce toughnes s as well as poli shabili ty . There fore , a lower limit of 1 . 2 % and an upper limit of 1 . 75 % are set .Vanadium [V]

[0031] V i s a MC- forming element similar as Nb . However , an advantage of Nb over V i s its low solubil ity in the metal matrix , whi ch means that the NbC (N) content in the steel can be adj usted very preci s ely ( and more preci sely than the VC ( N) content ) .There fore , the upper l imit i s s et to 1 . 0 % , pre ferably to 0 . 7 % especial ly pre ferably to 0 . 5 % .Ti tanium [ Ti ]

[0032] Ti i s a MC- forming element similar as Nb or V . However , in N-alloyed steel s it caus es the formation of coarse TiN, which i s detrimental to toughnes s . There fore , the upper limit i s set to 1 . 0 % pre ferably to 0 . 5 % , especial ly pre ferably to 0 . 2 % .Al umi num [ Al ]

[0033] Al acts as a deoxidi zing agent and may optionally be added up to 0 . 3 % .Tungsten [W]

[0034] W is a f errite-stabilizing element . It is a strong carbide- former and is used to increase wear resistance . The addition of W improves the hot hardness and tempering resistance, which is why it is used as an additive in high-speed steel and hot-work tool steel . However, W is also a cost-intensive alloying element . It may optionally be added up to 0 . 5% .Nb / Co ratio

[0035] It was found that the Nb / Co ratio affects the prevention of nozzle clogging during atomization and can improve the hot hardness and other properties of the powder-metallurgical steel . Further, since there is an optimum amount of primary carbides NbC (N) to obtain high hardness ( if there are too few primary carbides NbC (N) , wear resistance and hardness deteriorate, but if there are too many primary carbides NbC (N) , the metal matrix is depleted of C, causing the hardness of the metal matrix and also the overall hardness of the powder-metallurgy steel to decrease ) , carbon diffusion, which depends on the Co content, and the ability of Co to form nucleation sites for secondary hardening carbide formation, are important to achieve the optimum amount of primary carbides NbC (N) formation and secondary hardening carbides formation . Therefore, the Nb / Co ratio is preferably set to a range from 0 . 4 to 1 . 0 and more preferably to a range from 0 . 50 to 0 . 85 .Manufacturing process

[0036] Figure 1 illustrates exemplary stages of a process of manufacturing a powder-metallurgical steel according to the disclosure .

[0037] At S I , a s teel composition consi sting of the elements of Table 1 , the balance being Fe and incidental impurities , i s mel ted by heating .

[0038] At S2 , the Nb- and C-containing melt i s atomi zed . For example , nitrogen may be used as an atomi z ing gas . Atomi z ation of the mel t produce s steel particles (powder ) of a si ze equal to or smaller than 350 pm, for example . During atomi zation, the particl es solidi fy behind the no z zle . A cooling rate equal to or more than 1000 ° C / s can be achieved .

[0039] When Nb- and C- containing melts are atomi z ed, primary carbides NbC (N) are formed . The se primary carbides tend to accumulate in the noz zle and clog it . However , as mentioned above , given the chemical compos ition of the melt , i t was pos sible to atomi ze the mel t wi thout clogging the noz zl e i f the formation of the primary carbides NbC (N) was l es s than 3 vol % of the microstructure of the steel s .

[0040] At S 3 , the metal powder i s fil led into a capsule which i s then evacuated and sealed by welding, for example .

[0041] At S 4 , the metal powder i s hot i sostatic pre s sed to obtain a powder-metallurgical steel product . Hot i sostatic pres sing ( HI P ) may be carried out at a temperature between 1000 ° C and 1200 ° C and under a pres sure of , e . g . , 800 bar to 1200 bar . The HI P proce s s may be carri ed out for 5 to 10 hours , for example . That way, the metal powder i s converted into the powder-metal lurgi cal steel product . In contrast to , e . g . , a s intering proces s , in which only the surf aces of the particl es are bonded, the powder-metal lurgi cal s teel product i s homogeneous and free of any residual parti cle s tructure .

[0042] Di f ferently put , the HI P proces s allows to produce a powder-metallurgical steel product which has a highly homogenous microstructure wi th very f inely di s tributed primary carbides NbC (N) in the metal matrix . As a result , a high proportion of finely di stributed primary carbide can be achieved in the powder-metal lurgical steel product , which would not be pos sible without powder-metal lurgi cal steel production .

[0043] At S 5 , the powder-metallurgi cal steel product may be formed by, e . g . , forging or rol ling . The forming temperature may be in a range between 1050 ° C and 1150 ° C . The degree o f forming may, e . g . , be equal to or more than 3 or 4 . Thus , a change of the cros s- sectional shape of a f actor more than 3 or 4 may be obtained . Thi s may mean that a main surface of the powder-metallurgical steel product of si ze A i s reduced to a main sur face of si ze A / 3 or A / 4 , respectively .

[0044] At S 6 , the powder-metallurgical steel product may, e . g . , be subj ected to a heat treatment . The the heat treatment i s used to soften the steel product . Af ter heat treatment , the steel product i s slowly cool ed to prevent martensite formation ( hardening ) .

[0045] At S 7 , the steel product can optional ly be machined ( e . g . by milling, grinding, poli shing or another metalworking proce s s ) to give it a desired shape and / or sur face fini sh .

[0046] The powder-metallurgical steel ( al so re ferred to as steel product ) may then be sold to a customer , for example . The following production stages relate to subsequent proces sing of the powder-metal lurgi cal steel as produced a fter some or al l ofthe proces s stage s S I to S 7 . Thi s subsequent proces s ing may ( optionally) be carried out by the customer and not by the s teel manufacturer , for example . In other example s , the subsequent proces s ing i s carri ed out by the steel manufacturer .

[0047] Re f erring to Fi gure 2 , be fore hardening at S 9 , the powder-metallurgical steel may, e . g . , be subj ected to a stres s rel ie f heat treatment at S 8 . For example , the stres s rel ie f heat treatment may be conducted at a temperature above 700 ° C , for example at a temperature of about 750 ° C . The s tres s rel ie f heat treatment S 8 reduces stre s s in the metal matrix .

[0048] The hardening treatment at S 9 includes heating the powder-metallurgical steel to a temperature in the austenite phase region between 1000 ° C and 1150 ° C followed by rapid cooling . Hardening may compri se a certain holding time at the maximum temperature ( hardening temperature ) . For example , the holding time may, e . g . , be between 0 to 2 hours , in particul ar between 10 min and 1 hour .

[0049] Fol lowing the heat treatment , hardening further includes rapid cooling . Rapid cooling causes the transition o f the aus teni tic matrix into fresh martensite and retained austenite . As indicated in Fi gure 2 , by using a cooling medium such as , e . g . , oi l , gas , water or a polymer solution , the cooling rate may be substanti ally higher than under ambient atmosphere ( dash-dotted line ) .

[0050] As already mentioned, the primary carbides , among them NbC (N) , were formed during atomi zation . During hardening, the concentration of primary carbides i s s et . The primary carbides are partiall y di s solved depending on the hardening temperature .The higher the hardening temperature , the more primary carbides , among them NbC (N) , are di s solved, i . e . they di sappear .

[0051] The partial di s solution of the primary carbides during hardening re sults in the steel matrix being al loyed by the corresponding metal el ements . In particul ar , the di s solution of Cr- rich primary carbides (which, however , have already largely been substituted by the primary carbides NbC (N) ) increases the corrosion res i stance of the steel .

[0052] Aft er hardening, the primary carbides NbC (N) may have a fraction in volume in a range between 2 vol % and 5 vol % , in particul ar 2 vol % and 3 . 5 vol % o f the hardened powder-metallurgical steel .

[0053] Figure 3 i s a diagram showing a si ze di stribution o f the primary carbides NbC (N) contained in the metal matrix o f a powder-metallurgical steel a fter hardening and tempering . Figure 3 shows the si ze di stribution o f NbC (N) in terms o f the surf ace area coverage fraction in % versus ECD ( Equivalent Circular Diameter ) for two measurements (Measurement 1 and Measurement 2 ) at di f ferent locations of the s ame s teel sampl e . Obviously, the mean si z e (mean ECD ) o f the NbC (N) primary carbides i s small er than 0 . 5 pm, 0 . 45 pm, 0 . 4 pm, 0 . 35 pm or 0 . 3 pm . A maximum s i ze of les s than 0 . 70 pm was observed in all ECD di stributions . Due to the small si ze of the primary carbides , the steel has a high pol i shability, i . e . can be poli shed very well .

[0054] The steel product had a martens itic microstructure with about 3 vol % primary carbides NbC (N) .

[0055] The (maximum) temperature of the hardening treatment s ets the grain si z e and the amount or fraction in volume of the primary carbides NbC (N) in the steel . In other words , the ECD di s tribution of the primary carbides NbC (N) ( see Figure 3 ) may be modi fied by the hardening proces s . Increas ing the hardening temperature shi fts the maximum o f the ECD di stribution towards smaller grain si z es ( smaller ECD ) .

[0056] Further , the (maximum) temperature of hardening may be used to af fect the PREN ( Pitting Resi stance Equivalent Number ) of the powder-metallurgi cal steel af ter hardening .

[0057] Optional ly, a fter rapid cooling to room temperature , cool ing may be continued to temperatures below 0 ° C , in particular below - 50 ° C or - 80 ° C or - 196 ° C ( not shown ) . Deep cool ing may compri se a certain holding time at the minimum temperature . For example , the holding time may, e . g . , be between 0 to 4 hours , in particular 1 to 3 hours . Deep cooling to temperatures below 0 ° C i s e f fective in reducing retained austenite in the metal matrix . Thi s increas es the hardnes s of the hardened powder-metallurgical steel .

[0058] After hardening , the powder-metallurgical steel product i s tempered at S 10 . Tempering includes heating the powder-metallurgical steel product to a tempering temperature in a range between 200 ° C and 600 ° C . In particular , a pre ferred temperature range for tempering i s between 300 ° C and 600 ° C , pre ferably between 400 ° C and 600 ° C and in parti cular between 450 ° C and 580 ° C . Tempering temperature s in a range between 480 ° C to 560 ° C may be particularl y pre ferred .

[0059] As shown by way of example in Figure 2 , the tempering process at S 10 is repeated at least once . For example, three tempering cycles are used . Each tempering cycle may reach any of the temperatures mentioned above .

[0060] Further, each tempering cycle may comprise a certain holding time at a maximum temperature, e . g . at the tempering temperature . For example, the holding time may, e . g . , be between 0 to 4 hours , in particular 1 to 3 hours .

[0061] During tempering, secondary hardening carbides are formed if the tempering temperature exceeds approximately 300 °C . The formation of secondary hardening carbides M7C3, MeC, M23C6 and M3C, among them the chromium carbides Cr?C3 and Cr23Ce, changes the properties of the hardened powder-metallurgical steel , in particular its hardness , toughness and corrosion resistance ( the latter is determined by the corrosion resistance of the metal matrix, which is mainly based on the Cr and Mo content of the metal matrix) .

[0062] Further, tempered martensite is formed by tempering . In this disclosure, martensite means fresh martensite and tempered martensite . Fresh martensite refers to martensite which is not tempered and which is formed during hardening . Tempered martensite is tempered during a tempering cycle .

[0063] In other words , fresh martensite means matensite after hardening . Martensite means fresh + tempered martensite .Low Temperature Tempering (LTT )

[0064] According to one embodiment, the tempering temperature is in a range between 200 °C and 350 °C, in particular between 200 °Cand 300 ° C or 200 °C and 250 °C . The stress in the martensite is reduced with the effect that the toughness of the steel is increased . Very few secondary hardening carbides M7C3, MeC, M23C6 and M3C to improve the hardness are formed during LTT .Consequently, more Cr and Mo remain in the metal matrix .Therefore, the hardened and low temperature tempered powder-metallurgical steel is highly corrosion resistant .

[0065] Since very few or no secondary hardening carbides are formed, the low temperature tempered steel has not such a high hardness as the high temperature tempered steel ( see below) . That is , during LTT, for example , the fraction in volume of secondary hardening carbides M7C3, MeC, M23C6 and M3C in the hardened powder-metallurgical steel may, e . g . , be equal to or less than 1 vol% .High Temperature Tempering (HTT )

[0066] High temperature tempered steel is defined herein as a steel which has been tempered at temperatures above 450 °C, for example at temperatures in a range between 450 °C and 600 °C or between 480 ° C and 560 °C . At these temperatures , secondary hardening carbides M7C3, MeC, M23C6 and M3C, among them the chromium carbides Cr7C3 and Cr23Ce, are formed . Further, temperatures above 450 °C are needed to reduce the fraction in volume of retained austenite in the metal matrix .

[0067] The upper temperature limit of 600 °C is given since above this temperature, carbides coarsen and hardness decreases . Due to the formation of secondary hardening carbides , in particular Cr-containing carbides , the metal matrix becomes increasingly depleted of Cr . Therefore, the corrosion resistance is lower than for low temperature tempered steels . However, tempering in the HTT temperature range may provide the best combination betweenhardness , toughness and still sufficient corrosion resistance, and may therefore be preferred for many applications .

[0068] The effect of tempering temperature on hardness is shown in Figure 4 . The curve shows a typical behavior for steels that form secondary hardening carbides . In general , hardness decreases with increasing tempering temperature without the formation of secondary hardening carbides ( see the low temperature section of the curve ) . At tempering temperatures above 300 °C, the formation of the secondary hardening carbides begins . As can be seen from Figure 4 , the hardness of the hardened powder-metallurgical steel product then increases with increasing tempering temperature .

[0069] After HTT, the fraction in volume of carbides ( e . g . primary carbides such as NbC (N) and secondary hardening carbides , among them Cr?C3, C^aCe) may be equal to or less than 15 vol% . For example, a fraction of primary carbides NbC (N) may be in a range between 1 and 5 vol% . The hardened powder-metallurgical steel after HTT may comprises a fraction in volume of less than 12 . 0 vol% , in particular less than 10 . 0 vol% of secondary hardening carbides M7C3, MeC, M23C6 and M3C . Specifically, a fraction in volume of Cr?C3 and C23C6 may be equal to or less than 5 vol% .Metal matrix microstructure

[0070] The metal matrix microstructure of the hardened powder-metallurgical steel for an example Ex_l ( compare Table 2 for the chemical composition) of the disclosure is shown in Figure 5 . Figure 5 shows a Schaef fler diagram in which the Cr-equivalent and the Ni-equivalent represent the X-axis and the Y-axis , respectively . The Schaeffler diagram is divided into different areas that represent the microstructure present .

[0071] The Cr-equivalent represents the ef fectiveness of the ferrite- forming elements and is given by their mass fractions as Cr-equivalent = %Cr+%Mo+l . 5%Si+0 . 5%Nb+2%Ti . The Ni-equivalent is calculated from the mass fractions of the alloying elements that cause iron to contain austenite in its structure, and is calculated as Ni-equivalent = %Ni+30%C+0 . 5%Mn+30 ( %N-0 . 02 ) .

[0072] As apparent from Figure 5 , the example Ex_l is in the microstructure area "Austenite + Martensite" . No ferrite fraction is present . The Cr-equivalent is 15 and the Ni-equivalent is 10 .

[0073] It is to be noted that the Cr-equivalent and the Ni-equivalent exclusively rely on the elements of the metal matrix . Precipitations , such as , e . g . , carbide precipitations , are not part of the metal matrix . Therefore, the Cr-equivalent of example Ex_l would be 18 if computed based on the chemical composition of example Ex_l ( see Table 2 ) . Analogously, the Ni-equivalent would be 19, i f computed based on the chemical composition of example Ex_l . These calculations were done using the thermodynamic simulation software "Thermocalc 2024a with database TCFE7" .

[0074] Regarding strength and wear resistance, an upper limit of the C and / or N content is given by the austenite stability (no martensite forms in the "Austenite" area) . The C and / or N content is limited downwards by low martensite hardness and an insufficient fraction in volume of carbides or carbonitrides .

[0075] Regarding corrosion resistance, an upper limit of the Cr-content is given due to the stability range of ferrite and austenite . A lower limit is given by about 11 % free Cr in the metal matrix to ensure the required minimum corrosion resistance .

[0076] In general , a fter hardening and tempering, the fraction in volume of martensite ( fresh + tempered martensite ) in the metal matrix may be equal to or greater than 65 vol % or 70 vol % or 75 vol % or , more pre ferably, 80 vol % or 90 vol % . The f raction in volume of retained aus teni te after hardening and tempering may be equal to or les s than 15 vol % , more pre ferably equal to or les s than 10 vol % .ExamplesTable 2 shows the chemical composition o f example Ex_l of a powder-metallurgical steel in accordance wi th the di s closure . Examples Ex_2 ( re f . ) , Ex_3 ( re f . ) and Ex_4 ( re f . ) are re f erence example s .Table 2: Chemical compositions (in wt%) of a steel melt example Ex_l according to the present disclosure and reference examples Ex_2(ref.), Ex_3(ref.), Ex_4(ref.) < < <Table 2: (continuation)< < < < < <<

[0077] In Table 2 , the compos ition values that l ie outside the ranges o f Table 1 are printed in bold . In re ference example Ex_2 ( re f . ) , too much Nb i s present . There i s too little Nb in re f erence example Ex_3 ( re f . ) . Re f erence example Ex_4 ( re f . ) i s notbas ed on the concept underl ying the di sclosure , inter alia s ince too much Cr and too little Nb i s used .

[0078] Examples Ex_l , Ex_2 ( re f . ) and Ex_3 ( re f . ) were produced by a proces s according to Figure 1 ( stages S I to S 7 ) . Subsequentl y, thes e powder-metallurgical steel s were hardened and tempered ( s ee Figure 2 , S 9 and S 10 ) .Hardening

[0079] Ex_l , Ex_2 ( re f . ) and Ex_3 ( re f . ) were hardened at 1130 ° C in a chamber furnace . The holding time was 0 . 5 hours . Rapidcooling was carri ed out in oil down to room temperature .Alternatively, water , a pol ymer solution or pres suri zed N2 could be used as a cool ing medium . In addi tion, the example s were cooled down to - 80 ° C , wi th a holding time o f 2 hours .

[0080] Aft er hardening and be fore tempering , the steel of Ex_l had a mainly martensitic mi cros tructure with retained aus teni te and about 3 vol % NbC ( N) .

[0081] Example Ex_4 ( re f . ) was hardened at 1150 ° C in a vacuum furnace with a holding time of 0 . 5 hours . Rapid cooling down was carried out in N2 at a pre s sure of 6 bar . Thi s so-cal led vacuum tempering prevents the sur face form scal ing .Tempering

[0082] Two di f ferent tempering proces se s were applied, namely low temperature tempering ( LTT ) and hi gh temperature tempering ( HTT ) .

[0083] Examples Ex_l , Ex_2 ( re f . ) and Ex_3 ( re f . ) were tempered as fol lows :- LTT : two tempering cycles were carried out , each at 300 ° C and at a holding time of 2 hours .- HTT : two tempering cycl es were carried out , each at 520 ° C and at a holding time of 2 hours .

[0084] Aft er hardening and tempering, the fraction in volume o f the martensi te ( fresh martens ite and tempered martensi te ) was equal to or greater than 79 vol % . The fraction in volume o f NbC (N) and secondary hardening carbide s was equal to or l es s than 13 vol % of the microstructure . The volume fraction of retained aus teni te was <10 vol . % .

[0085] For example Ex_4 ( re f . ) , high temperature tempering ( HTT ) and low temperature tempering ( LTT ) were carried out as follows : - LTT : two tempering cycles were carried out , each at 200 ° C and at a holding time of 2 hours .- HTT : two tempering cycles were carried out , each at 540 ° C and at a holding time of 2 hours .

[0086] Further , it i s to be noted that the samples of Ex_4 ( re f . ) for LTT were cool ed down to -80 ° C , wi th a holding time o f 2h , at the end of the hardening proces s . In contrast , the s ample s for HTT were not cooled below room temperature be fore tempering .Measurement result

[0087] Table 3 shows the mechanical propertie s and corrosion res i stance properti es of the examples as obtained by measurements .

[0088] The hardnes s was determined in units of HRc according to ASTM E 18 -24 , Rev . 04 24 . Toughnes s was determined as impact toughne s s in units of J according to SEP 1314 , Rev . 04 90 .

[0089] Corrosion tes t : Exposure te st in 20 % boiling acetic acid for 24 hours . Mas s los s of the s ample s ( in g ) was determined and rel ated to time ( in h ) and surface area of the s ampl es ( in m2) . Sample geometry was 40mm x 15mm x 5mm, the surf ace was ground with 120 grit .Table 3: Mechanical and corrosion resistance properties of the examples

[0090] Impact toughnes s and hardne s s are il lustrated in Figure 6 . The columns indicate impact toughnes s , while the diamonds indicate hardnes s . Corrosion re si stance in terms o f mas s los s i s illustrated in Fi gure 7 .

[0091] In example Ex_2 ( re f . ) , the s teel i s Nb-ri ch andNb / Co = 1 . 45 . More coars e carbide s are formed and there fore , a lower toughne s s i s obtained . There i s les s C in the metal matrix , and there fore , a lower hardnes s i s reached . However , Ex_2 ( re f . ) yields a good corrosion res i stance as more Cr remains in the metal matrix . Co i s no longer as e f fective because too much C i s already bound from the metal matrix in the form of NbC (N) .

[0092] In example Ex_3 ( re f . ) , the s teel has a substantial ly lower content of Nb , and Nb / Co = 0 . 37 . As a resul t , the hardne s s i s lower because le s s primary carbides , in particul ar NbC ( N) , are formed . Even in the cas e of HTT , thi s cannot be compensated by secondary hardening carbides ( al though there i s more C in the metal matrix , which in turn forms more Cr-rich secondaryhardening carbides ) . Thi s results in a worse corros ion res i stance .

[0093] In example Ex_l according to the di sclosure , the steel has a ratio Nb / Co = 0 . 74 . About thi s ratio has been found to be an optimum between primary carbide NbC (N) formation and remaining C in the metal matrix for formation of secondary hardening carbides , wherein the formation of secondary hardening carbides ( nucleation s ites ) i s promoted by Co .

[0094] As shown in Table 3 , Ex_l ( LTT ) provides the best combination of mechani cal properties ( impact toughnes s , hardne s s ) at highe st corros ion resi stance (mas s los s = 0 g / m2h ) . Ex_l ( HTT ) provides the best combination of mechanical properti es ( impact toughnes s , hardne s s ) at sti ll good corros ion resi s tance (mas s los s = 36 . 7 g / m2h ) . The combination of hi gh res i stance to wear ( hi gh hardne s s ) and corrosion opens-up the pos sibility of applications in areas such as steel s for pl asti c proces sing ( e . g . , mold plate s , tool inserts , hot runners , gating s ystems for , e . g . , inj ection molding or other plastic manuf acturing tool s , etc . ) , indus trial cutting tool s especial ly in the food industry and tool s in the medical sector .

[0095] Generally, the LTT hardened powder-metallurgical s teel can compri se a hardne s s of equal to or more than 57HRc and an impact toughnes s of equal to or more than 90J .

[0096] Generally, the HTT hardened powder-metallurgical s teel can compri se a hardne s s o f equal to or more than 60HRc and an impact toughnes s of equal to or more than 45J .

[0097] Although speci fic examples have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations may be substituted for the specific examples shown and described without departing from the scope of the present invention . This application is intended to cover any adaptations or variations of the specific examples discussed herein . Therefore , it is intended that this invention be limited only by the claims and the equivalents thereof .

Claims

Claims1 . A powder-metallurgical steel consi sting of a steel composition, in % in weight :C : 0 . 35 % to 0 . 6% ,N : 0 . 01 % to 0 . 3 % ,Cr : 12 % to 15 % ,Mo : 3 % to 5 . 5 % ,Co : 1 . 6% to 5 % ,Nb : 1 . 2 % to 1 . 75 % ,and optional ly one or more ofSi : les s than 1 . 5 % ,Mn : les s than 1 . 5 % ,W : l es s than 0 . 5 % ,Ni : les s than 0 . 5 % ,Al : les s than 0 . 3 % ,V : les s than 1 . 0 % ,Ti : les s than 1 . 0 % ,Ta : l es s than 0 . 5 % , the balance being Fe and incidental impurities .2 . The powder-metallurgical steel of claim 1 , wherein Nb : 1 . 3 % to 1 . 7 % or 1 . 4 % to 1 . 65 % .3 . The powder-metallurgical steel o f claim 1 or 2 , wherein Co : 1 . 8 % to 4 % or 1 . 95 % to 3 % .4 . The powder-metallurgical steel o f any of the preceding claims , wherein 0 . 4 < Nb / Co < 1 . 0 or 0 . 5 < Nb / Co < 0 . 85 .5 . The powder-metallurgical steel of any of the preceding claims , wherein Cr : 12 . 5 % to 14 % or 12 . 8 % to 13 . 8 % .276 . The powder-metallurgical steel of any of the preceding claims , wherein Mo : 3 . 5 % to 5 . 0 % or 3 . 8 % to 4 . 2 % .7 . The powder-metallurgical steel of any of the preceding claims , wherein Si : more than 0 . 3 % or 0 . 4 % .8 . A hardened powder-metallurgical steel , compri sing the powder-metallurgical steel of any of the preceding claims after having been subj ected to a hardening proces s and a tempering proces s , the hardening proce s s compri ses heating the powder-metallurgical steel to a temperature between 1000 ° C and 1150 ° C fol lowed by rapid cooling, and the tempering proces s compri ses heating the cooled powder-metallurgical steel to a tempering temperature in a range between 200 ° C and 600 ° C , wherein the tempering i s repeated at least once .9 . The hardened powder-metallurgical steel of claim 8 , compri sing a metal matrix and primary carbides NbC and NbCN di s tributed in the metal matrix , whereina maximum of the equivalent circular diameter , ECD, di s tribution of the primary carbides NbC and NbCN i s in a range between 0 . 05 pm and 0 . 4 pm, in parti cular between 0 . 1 pm and 0 . 35 pm and / orthe primary carbides NbC and NbCN have a total volume in a range between 2 vol % and 5 vol % , in particular 2 vol % and 3 . 5 vol % of the hardened powder-metallurgical steel .10 . The hardened powder-metallurgical steel of claim 8 or 9 , wherein the tempering temperature i s in a range between 200 ° C and 350 ° C , and the hardened powder-metallurgical steel compri se s a hardnes s of equal to or more than 57HRc and an impact toughnes sof equal to or more than 90J, wherein hardnes s i s determined according to ASTM E 18 - 24 , Rev . 04 24 , and impact toughne s s i s determined according to SEP 1314 , Rev . 04 90 .11 . The hardened powder-metallurgical steel of claim 8 or 9 , wherein the tempering temperature i s in a range between 450 ° C and 600 ° C , in particular 480 ° C and 560 ° C , and the hardened powder-metallurgical steel compri s es a hardnes s of equal to or more than 60HRc and an impact toughnes s of equal to or more than 45 J, wherein hardnes s i s determined according to ASTM E 18 -24 , Rev . 04 24 , and impact toughne s s i s determined according to SEP 1314 , Rev . 04 90 .12 . A method of manufacturing a powder-metallurgical steel , the method compri sing :mel ting a steel composition consi sting o f , in % in weight : C : 0 . 35 % to 0 . 6% ,N : 0 . 01 % to 0 . 3 % ,Cr : 12 % to 15 % ,Mo : 3 % to 5 . 5 % ,Co : 1 . 6% to 5 % ,Nb : 1 . 2 % to 1 . 75 % ,and optional ly one or more ofSi : l es s than 1 . 5 % ,Mn : les s than 1 . 5 % ,W : l es s than 0 . 5 % ,Ni : les s than 0 . 5 % ,Al : les s than 0 . 3 % ,V : les s than 1 . 0 % ,Ti : les s than 1 . 0 % ,Ta : l es s than 0 . 5 % , the balance being Fe and incidental impurities ;atomizing the melted steel composition to obtain a metal powder ;hot isostatic pressing the metal powder to obtain a powder-metallurgical steel product; andforming the powder-metallurgical steel product .13 . The method of claim 12 , further comprising :hardening the powder-metallurgical steel product by heating the powder-metallurgical steel product to a temperature between 1000 °C and 1150 °C followed by rapid cooling, andtempering the hardened powder-metallurgical steel product by heating the cooled powder-metallurgical steel product to a tempering temperature in a range between 200 °C and 600 ° C, wherein the tempering is repeated at least once .14 . The method of claim 13 , wherein the tempering temperature is in a range between 200 °C and 350 °C, and the hardened powder-metallurgical steel after tempering comprises a fraction in volume of less than 1 . 0 vol% of secondary hardening carbides M7C3, MeC, M23C6 and M3C, wherein M denotes a metal element of the steel composition .15 . The method of claim 13 , wherein the tempering temperature is in a range between 450 °C and 600 °C, in particular 480 °C and 560 °C, and the hardened powder-metallurgical steel after tempering comprises a fraction in volume of less than 12 . 0 vol% , in particular less than 10 . 0 vol% of secondary hardening carbides M7C3, MeC, M23C6 and M3C, wherein M denotes a metal element of the steel composition .

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