SE2130179A1 - A new super bainite steel, method for manufacturing an object of said steel and an object manufactured by the method - Google Patents

Abstract

Ab tractThe present invention relates to a super bainite steel comprising the following elements in weight- % (wt-%)C 0.60 to 0.90;Si 1.60 to 3.00;Al 0.10 to 0.80;Mn < 0.90;P < 0.03;S < 0.03;Cr 0.40 to 1.50;Ni 0.05 to 1.50;Mo 0.40 to 1.10;Co < 3.20;V < 0.50;Ti < 0.10;Cu < 0.50;Si+Al 2.1 to 3.1;Balance is Fe and unavoidable impurities.

Description

The present tiiscitvsu e. :reizites to a, super iiairiite steei and to a, inethriti for iiianufaetiiriiig said super haiiiite steel and an tihject coinprisiiig said super ihainite steei.

Baekgroiitid Super haiiiite steeis are charaeterized in that they iiawfe hoth iiigii strengtii and iiigii iiardness, Ti rise steeis are tiiereiftire espettiziiiy' stiiteti trir e. g. tifear aiid ariritir appiiezttitviis. Exztriipies of stieh steeis and processes for manufacturing such steeis are described in GB 23š27lti A i-ioiraetfer, one niajoi' probieni ieiatiiig to these steeis is that it twiii take too iong tiine to ohtaiii the desired super bainitic iriicrostructiire in the finished. product as the decisive iieat treatment time to achieve the tiesireti properties exeeeds 124 hours and often si gniticantiy beyond that.

Tiins, these products are tiiiëreftire not tfoirirnerciaiiy' troinpetitixfe iii a fuihstfaie rharnifaitturiiig ivrorttas s .

An aspect of ti' e present tiisttitisiirië is theiefoiï: to pJroVititë a sohititiii to stiixfe or at least to reduce this prohieni. The present disciostire tiierefore reiates to a iietv siiper ihainite steei iiaving a coniposition of aiioyiiig eieiiierits in raiiges *which vviii iaroviiii: for a rnore eificieiit nianutaeturing process hut stiii provifie the desired properties, such as high strength and high ir "tiiiiëss.

Summary of the diseiosnre Ti e prtësent diseiostirtë ttieriëtrire provides a super hztinite steei tttiiriprisiiig in *weight-WG (ttft-*ï/tií) the ftiiiotviiig eieinents: C 0.60 to 0.90; Si 1.60 to 3.00; A1 0.10 to 0.80; Mn S 0.90; P :I 0.03; S 0.03; Cr 0.40 to 1.50; Ni 0.05 to 1.50; M0 0.40 to 1.10; Co < 3.20; V <1 0.50; Ti < 0.10; Cu < 0.50; Si+A1 2.1 to 3.1; Balance Fe and unavoidable impurities.

The, selection and irangtfs of zttioyirtg eierïitërits of the preserit super iiaiiiittë steei vviii provide, for a faster hainite transforrnation rneaniitg that the heat treatment time for ohtainiitg the desiirecí super' tßztinitic rnicrtästructtrre xviii he, grexzttiy' reduced cmrtipztretí to the, heztt treatment times for the super hainite steeEs kitowri todayz The gïæresent disclosure aEso provides an object cornprising, a super 'oainite steel, tvhiciä strper baiiiite steel troinprising the foiitwwing eienitëiits in Weight-Wo (Wt-°/0); C 0.60 to 0.90; Si 1.60 to 3.00; A1 0.10 to 0.80; Mn S 0.90; P 0.03; S 0.03; Cr 0.40 to 1.50; Ni 0.05 to 1.50; Mo 0.40 to 1.10; Co < 3.20; V <1 0.50; Ti < 0.10; Cu < 0.50; Si+A1 2.1 to 3.1; Balance Fe and unavoidable impurities; and Wherein the object has a microstructure With a pearlite content 1ess than 2.0 % at room temperature after continuous Cooling from the austenitizing temperature to room temperature at a constant coo1ing rate of 1 °C/s.

According to embodiments, the desired super hztiiiitit: rnicrostructure has almost no presence of pearlite.

According to enihodiinents, the ffibject as defined iiereinahotvfe or iiereinaftei' has a Hardness (HV1) 2 630 HV1 (SS-EN-ISO 6507) in room temperature (RT) after austenitizing and isotherrnal transformation of the super bainitic microstructure at 250°C during 16 h. The priëseiit ohjeet ufiii aistw possess iiigii strength as shoxvii in Tahie 2.

Further, according to embodiments, the desired super bainitic microstructure Will have almost no presence or no presence of proeutectoid ferrite phase.

With regard to the niiertvstiuttture, the ftiiitnviiig definition of the term “super hainite steel” appiies iierein: a seuper hainite steel is a steei containing a super hainitie inicrffistructtire. Tiie siipei' bainitic iriicrostrutfitiire is formed during a thermai iieat treatment above the Ms- ternperattire hut ibeiovif 3500 (f. Further, ibefore this thermai heat treatment, vvhen the super hainite forins, an austenitizing heat treatment step has to be performed. 'the super hainite steei as defined iiereinahove or iiereihafter tviii essentiaiiy coniprise a super hainitie niierostrtittture. Tiiis siipei' bainitic iiiicrostnictiire is charattterizetí in that it contains thin ferrite iaths and retained austenite and that there vviii he essentiaiiy' no earhide~ precipitatitiii in the ferrite iaths. Tiie iaths rhay aiso appear as distfis or piates in a tiiree- fiinieiisifonai view, and tiie iaths iiavfe an average thieitiiess heiovvf 100 hm and tyfpicaiiy tieiow this .

Hovvevfer, primary carhicies. Which have already been precipitated ciuriiig the iiot--ifvorkiii processes perfortrieoi aiitive, the teniivtfiifattires ih iangt: tfor tiieifiiiai pireeipitatiriti and grovt/th of bainitic iiiicrostructures, may he present in the iiiicrostructure. These priniary earhides are ihttfiitieti for attstenite grain size refinenieiit during the austenitizihg iieat treatment.

Brief Deseriptieii ef the Figures Figure i shoiafs a iioii~iiitiitiiig sciieitizttie CCïïriiagraiii (continuous coriiiiig transformations) of a super ihainite steei tvith a iiyptietitecttiici carhon eonteiit, and expiaiiis iiotv' tiiiÉfeiffeiiit aihiyictig eieriitëiits tiffect the trztristiirtiiatiriii of austenite phase into different niicrostrncttires at tfarious ternperatnres, during ttoralin g amd istäthtërirnal iteat trfeatrritfrtt; Figure 2 sltotxfs a schentzttitt dilatrnrieter tturve during isotlterrrrai lteat trtäatrrient. “a” tleitfotes the start of transfonnation, “b” ttenotes infiectiort point of transformation, “c” denotes the end of transformation. The increase in lruigtii dilatatirrti vs time correstvotitls to the garecipitation and grovtftih of the super bainitic rnicrostmcttrre during the istatlitëirnal lteat treatment. Üetaišed description The inventors have found an inventive composition which Will provide the desired super bainitic microstructure within shorter thermal heat treatment times compared to other known super bainite steels. Additionally, the present inventive composition will, even though the use of shorter thermal heat treatment times, still be both hard and strong. Further, the present inventive composition will provide for the use of conventional production processes as essentially no pearlite will form during cooling to the thermal heat treatment temperature for the super bainite transformation, even in objects being large in size and weight, such as bars up to øl50-200 mm. Without being bound to any theory, it is believed that the shorter thermal heat treatment times for transformation of austenite to a super bainitic microstructure are due to increased transformation kinetics, The inventors have, as can be seen from Figure l, investigated and carefully selected the necessary elements having the herein specified ranges so that non-desired phase transformations will be avoided during the different process steps.

The alloying elements of the super bainite steel and of the object comprising the super bainite steel will now be described. The terms “weight-°/0” and “wt-°/0” are used interchangeably. Also, the list of properties or contributions mentioned for a specific element should not be considered exhaustive.

Carbon (C): 0.60 to 0.90 wt-% Carbon is included to increase both strength and hardness but also for goveming precipitation and growth of the desired super bainitic microstructure during the thermal heat treatment process for super bainite transformation. The thermal heat treatment may be performed in e. g. a salt bath or in a hot oil bath.

Carbon is a very efficient austenite stabilizer and will thus influence the transformation kinetics and the amount of retained austenite in the super bainitic microstructure. The carbon content should preferably be high enough to inhibit precipitation and growth of the proeutectoid ferrite phase as well as pearlite during cooling. These phases would otherwise prevent or reduce the bainite transformation at lower temperatures. Carbon is also a maj or component in providing improved mechanical properties of the super bainitic microstructure due to an extended interstitial solid solution strengthening. However, an increased C-content will delay the kinetics of the austenite to bainite transformation, which means that extended thermal heat treatment process times will be required to achieve the super bainitic rnicrostructure.

Additionally, carbon will suppress the Ms-temperature, vvhicih is the temperature from vxfhere the niartensitirt rnirrrrtstrrtctttrtë starts to form on rtoolirig. The suppressictri of thte Ms ternptëraturïf. means that lower thermal heat treatment temperatures may be used for the bainite transformation, thereby facilitating the precipitation and growth of an even finer super bainitic rnicrostructure. This will in turn enhance the mechanical properties even more. A too low content of carbon will both result in inferior mechanical properties and have an impact on the type of microstructure being formed during the manufacturing process. A too high carbon content will on the other hand increase the risk of eXcessive carbide precipitation in the rnicrostructure and may also reduce the ductility.

A sort of carbides, called primary carbides may be present, these are prerfipitated, dtiririg steehnaking and hot vvorking processes, tvhich processes are performed prior to the heat treatment steps tised for precipitation arrd groytfth of the super hairiitirt rnirtrrtstructttrtf and. xvhielt are performed at ntucih higher tennperatttre ran ges compared to the ternperature ranges where bainite transformation of the attstenite phase »vill occur. The primary carbides will also prevent grain growth during austenitizing heat treatment. These carbides will thus, enable grain refinement and will thus facilitate nucleation of ferrite laths at the grain boundaries during the bainite transformation. The content of carbon is therefore between 0.60 to 0.90 wt-°/0, such as 0.65 to 0.85 wt-°/0.

Silicon (Si): 1.60 to 3.00 wt-% Silicon is an essential alloying element as it will promote the precipitation and growth of thin ferrite laths in the austenite matrix and at the same time retard carbide formation. Silicon has also a positive effect on the strength as being an effective solution strengthening element.

However, Si will increase the transformation kinetics of both ferrite and pearlite in the rnicrostructure, see Figure 1. The precipitation and growth of pearlite is especially enhanced by Si and this effect must therefore be counteracted by carefully balancing the content of other alloying elements. The amount of silicon is therefore limited to 1.60 to 3.00 wt-°/0, such as 2.00 to 2.60 wt-°/0.

Aluminium (Al): 0.10 to 0.80 wt-% Aluminium is a ferrite forming element and enhances the precipitation and growth of an essentially carbide-free bainitic microstructure during the therrnal heat treatment process for bainite transformation.

Al will have an impact on the precipitation and growth of proeutectoid ferrite phase and it will also have a small impact on the precipitation and growth of pearlite. Al will increase Ac3, Ac1 and Ms-temperatures (Figure 1). In combination with Si, Al will inhibit the precipitation of secondary carbides during prolonged therrnal bainite transformation times. It has been found that the content of Al and Si should thus be balanced in order to achieve a super bainitic rnicrostructure and to achieve optimal properties.

A too high content of Al may however reduce the mechanical properties by decreasing the ductility. A high Al content will also restrict the available temperature interval for the bainite transformation due to an elevated Ms-temperature. The content of Al is therefore 0.10 to 0.80 wt-°/0, such as 0.10 to 0.50 wt-°/0.

Silicon + .fälurïiiriiurn (Si + fälä: 2.1 to 3.1 wt-°/0 As mentioned above, Si and Al will act together to inhibit secondary carbide precipitation during cooling from the austenitizing temperature to the thermal heat treatment temperature for bainite transformation and during the therrnal heat treatment. However, a difference between Si and Al is that Si will lower the Ms temperature while Al may increase the Ms temperature. Al will also have less impact on the formation of pearlite compared to Si. A we11-ba1anced combination of the total content of Si and A1 is therefore essential in order to achieve the desired super bainitic microstructure and properties. The content of Si + A1 is therefore in the range of 2.1 to 3.1 wt-°/0, such as 2.3 to 2.8 wt-°/0.

Manganese (Mn): S 0.90 wt-°/0 Mn is an austenite stabi1izing a11oying e1ement and may optiona11y be added. If added, it wi11 be beneficia1 for preventing hot cracking during we1ding and hot forrning. Additiona11y, Mn prevents the formation of proeutectoid ferrite and norrna11y a1so pear1ite and reduces the Ms- temperature, thus increasing the amount of retained austenite in the microstructure. However, Mn has a1so an impact on the kinetics of the bainite transformation and if too much Mn is added, the precipitation and growth of bainite during the therrna1 heat treatment process, used for super bainite trans formation. wi11 be s1owed down. Hence, if added, the Mn content shou1d be as 1ow as possib1e and is therefore 5 0.90 wt-°/0, such as S 0.80 wt-°/0, such as 50.60 Wt-O/o.

Fhosphorous (im: Q' 0.03 ifat--Q/š» P is an optioriai e1ernent and is tfonsiritëreti to be an irnpurity' as it is riorrnaiiy regarded. as a harinfui e1ernent due to its einbrittiing effect. 'I"hereibre, it is tiesiraiiie to iiatxfe (103 wi11/á; P.

Suipliui' (5): (103 »vt-WG S is a1so regarded as an impiirity' S itnay form grain bounciary and inciusioiis and *vi/iii tiieretïire rïfstrict tiie hot *avoritiii g pJropiëJrties wveii 'as the nientiiaiiicai properfti es.

Hence, the content of S shou1d be :T 0.03 wt--Ü/ß.

Chromium (Cr): 0.40 to 1.50 wt-°/0 Cr wi11 contribute to the so1id so1ution strengthening of the super bainitic microstructure and is thus an important e1ement for improving the mechanica1 properties. In addition, chromium decreases the transformation kinetics of both proeutectoid ferrite phase and pear1ite . The effect of Cr on pear1ite transformation of the austenite phase is significant and Cr is therefore added to avoid precipitation and growth of pear1ite during coo1ing to the therma1 heat treatment temperature for bainite transformation. Cr wi11 a1so reduce the bainite transformation kinetics but to a 1ower degree than Mn. Therefore, Cr is preferab1e used instead of Mn for retarding the formation of proeutectoid ferrite phase and pear1ite.

However, chromium Will decrease the Ms temperature and a too high Cr content Will increase the risk for precipitation of grain boundary carbides during cooling as well as during the thermal heat treatment. These precipitates will have a negative impact on the ductility and create an undesirable microstructure. Further a too low Cr content will result in too low mechanical strength and an inferior microstructure, including too large amounts of pearlite.

The Cr content is therefore set to 0.40 to 1.50 wt-°/0, such as 0.60 to 1.30 wt-°/0.

Nickel (Ni): 0.05 to 1.50 wt-% Nickel is a strong austenite forrning element and has also a strong toughening effect. The strong toughening effect will increase the impact strength, especially at low service temp eratures.

A too high content of Ni will lead to a considerable stabilization of the austenite phase and thereby to a too high content of retained austenite in the microstructure and this will provide unacceptable low hardness and strength. Ni also reduces the transformation kinetics of the proeutectoid ferrite phase as well as the pearlite and bainitic microstructures. An eXcessive Ni content will thus slow down the bainite transformation during the thermal heat treatment process for bainite transformation too much. Ni is further considered as an expensive alloying element.

The Ni content should therefore be limited to 0.05 to 1.50 wt-°/0. According to embodiments, the content of Ni is 0.20 to l.l0 wt-°/0.

Molybdenum (Mo): 0.40 to l.l0 wt-°/0 Molybdenum will improve the strength of the super bainitic microstructure by solid solution strengthening. Mo is also very efficient in retarding the precipitation and growth of pearlite during cooling. Mo is thus an important alloying element as it will decrease the transformation kinetics of both the proeutectoid ferrite phase and pearlite while not having a significant impact on the bainite transformation kinetics. Thus, the addition of Mo will be very beneficial to achieve the desired manufacturing process properties, such as slow proeutectoid ferrite and pearlite kinetics combined with a fast bainite transformation at the thermal heat treatment temperature, see Figure l. Further, Mo will decrease Ms temperature, which will provide for the use of lower thermal heat treatment temperatures.

HoWever, Mo is also a strong carbide former and too high amount Will result in undesired carbide precipitation. The upper limit for molybdenum is therefore set to 1.10 Wt-°/0.

To ensure that Mo Will have the positive effects mentioned above, the amount shall be at least 0.40 Wt-°/0. As Mo is considered as an expensive alloying element, its content should be kept as low as possible but still in a range Where it Will have significant impact on the properties.

According to embodiments, the content of molybdenum is from 0.65 to 0.95 Wt-°/0.

Cobalt (Co): < 3.2.0 *fvt-fï/-Éi Co may be added and if added it Will have a strong impact on the ferrite forming and on the strengthening of ferrite. The inventors have found that Co Will increase the kinetics of the thermal bainite transformation and also increase hardness. HoWever, Co Will also raise the Ms temperature, Which in turn Will restrict the available temperature range for the thermal bainite transformation. Co might also have a negative impact on the hot Working properties and the ductility.

As being a ferrite former, Co must thus be balanced against the other alloying elements in order to achieve the desired properties needed for the manufacturing process and during service. The content of Co is therefore limited to less than 3.20 Wt-°/0.

According to embodiments, the present super bainite steel contains Co in the range of 2.00 to 3.10 Wt-°/0.

Vanadiuin (VÜ: <2 0.50 vift--O/o Variadiiirii. is :inr iiptioiiai eienniëiit anti if added it ttvill ioriri precipitates tiigetiirër' vvitii Ltarlioii anal/oi' nitrogen, V rnay' therefore be added in order to generate grain refinenient, specificailjv by controllirig refzryfstzillizatiriri anti grain grovt/iii: riuring liot wvrirfiriiig 'and riiiririg an austenitizing heat treatment. 'These grain refinenients “fill facilitate the later precipitatioii and grovvtii of ferrite. laths during bainite transformatioii, yifliich vvili irriprovfe the rnechariirral properties. The hardness of the formed V carbonitrides may also improve Wear resistance and retard softening at elevated service temperatures. A too high content of V may however impair the mechanical properties by reduction of the ductility. The content of V is therefore < 0.50 *fvt--lï/Éi. If V is added, the loWest content is 0.05 Wt-°/0. In embodiments, the content of V is 0.05 to 0.30 Wt-°/0.

Titanium (Ti): < 0.10 wt-°/0 Titanium is a highly reactive element which easily reacts with C, O, N and S. Ti is an optional element and if added it may be used to control these elements by preventing them from binding to other alloying elements and thereby reducing any optional harmful effects. In the present disclosure, Al is an important alloying element and Ti may thus be added to reduce the risk for formation of aluminium nitride precipitates, which otherwise could have a negative impact on the mechanical properties, especially ductility. A too high content of Ti may on the other hand impair the mechanical properties by a reduction of the ductility. The content of Ti is therefore < 0.10 wt-°/0, such as < 0.05 wt-°/0.

Copper (Cu): < 0.50 wt-°/0 Cu may be included due to the scrap or raw materials used but if present, the element should be present in as low amounts as possible as Cu will have a negative impact on the transformation kinetics of bainite, thereby causing a delayed bainite transformation during the thermal heat treatment. It should however be noted that small amounts of Cu may be allowed.

Thus, the content of Cu is < 0.50 wt-°/0, such as < 0.30 wt-°/0.

Optionally small amounts of other alloying elements may be added as defined hereinabove or hereinafter in order to improve for example but not limited to the machinability or the hot working properties, such as the hot ductility. Example, but not limiting, of such elements are Ca, Mg, B, and Ce. The amounts of one or more of these elements are of max. 0.05 wt-°/0.

When the terms “max” or “§” are used, the skilled person knows that the lower limit of the range is 0 wt-°/0, unless stated otherwise.

The present super bainite steel or object comprising the present super bainite steel may contain traces of Tungsten (W), Niobium (Nb), Tantalum (Ta), Tin (Sn), Nitrogen (N) and OXygen (O) as these elements may be included in the scrap metal, the raw material or during the steelmaking process. These elements are to be considered as impurities, meaning that they are allowed to be present but only in such amounts that the properties are not negatively affected. Thus, impurities are elements and compounds which have not been added on purpose but cannot be fully avoided as they norrnally occur in e. g. the scrap metal or the raw material.

The balance of elements is Iron (Fe) and unavoidable occurring impurities as discussed above.

The present disclosure also relates to the following embodiments. The ranges are mentioned as weight-°/0.

Element Broad Interrnediate Narrow 1 Narrow 2 C 0.60 to 0.90 0.60 to 0.90 0.65 to 0.85 0.65 to 0.85 Si 1.60 to 3.00 1.60 to 3.00 2.00 to 2.60 2.00 to 2.60 Mn S 0.90 S 0.80 S 0.60 S 0.60 P S 0.03 S 0.03 S 0.03 S 0.03 S S 0.03 S 0.03 S 003 S 0.03 Cr 0.40 to 1.50 0.40 to 1.50 0.60 to 1.30 0.60 to 1.30 Ni 0.05 to 1.50 0.05 to 1.50 0.20 to 1.10 0.20 to 1.10 Mo 0.40 to 1.10 0.40 to 1.10 0.65 to 0.95 0.65 to 0.95 Al 0.10 to 0.80 0.10 to 0.80 0.10 to 0.50 0.10 to 0.50 Co < 3.20 < 3.20 < 3.20 2.00 to 3.10 V < 0.50 < 0.50 0.05 to 0.30 0.05 to 0.30 Ti < 0.10 < 0.10 < 0.05 < 0.05 Cu < 0.50 < 0.50 < 0.30 < 0.30 Si+Al 2.1to 3.1 2.1to 3.1 2.3 to 2.8 2.3 to 2.8 Fe + Balance Balance Balance Balance unavoidable impurities .According to ernbcvdiinents, the ífireseiit super bainite steel niay also fulfii the condition of having: an inflection point time of the isotherrnal bainite transformation which is less than 90 minutes (1.50 h), measured as the dilatation in a dilatometer test, when the present super bainite steel is austenitized and then rapidly cooled to and directly isotherrnally heat treated at 250°C (See Figure 2) According to embodiments, the present disclosure relates to a super bainite steel comprising or consisting of the alloying elements as defined hereinabove or hereinafter. 11 According to embodiments, the present disclosure relates to an object comprising or consisting of the super bainite steel and fulfilling at least one of the conditions as defined hereinabove or hereinafter. According to embodiments, the object may be used in wear and armor applications. According to embodiments, the object may be selected from an armor, a safety vest, a military application, a rock tool, a rock drill bit, ball bearings, an eXcavator bucket or a machete and a sword.

The present disclosure also relates to a method for manufacturing an object comprising or consisting of the super bainite steel as defined hereinabove or hereinafter, the method comprising the following steps: melting raw material and alloying elements and/or scrap, whereby a molten steel having the composition as defined hereinabove or hereinafter is obtained; casting said molten steel into a casting; The casting may be a slab, a bloom, a billet, or an ingot; hot working the casting to an object having a desired shape and/or dimension; The hot working is performed at a temperature between ll00 to 1300 °C depending on the steel composition and process used; cooling the object to room temperature; The cooling rate should be slow enough to avoid cracking as well as eXcessive martensite formation; optionally machining and/or cold forrning the object; heating the object; The object is heated to a temperature above the austenitizing temperature and kept at this temperature until a desired austenitic microstructure is obtained; The heating temperature is in the range of 850 to ll00 °C and will depend on the heating time and the size and thickness of the object; quenching the object to the therrnal heat treatment temperature for super bainite transformation by for example using a salt bath or a hot oil bath; therrnally heat-treating the object until the desired super bainitic microstructure has been obtained; The time of heat treatment will depend on the composition, the shape of the object, the dimension of the object and the heat treatment temperature. The present super bainitic microstructure is formed during this thermal heat treatment step. The heat treatment is performed at a temperature above the Ms temperature but below 350 °C; cooling the object to room temperature; 12 - optionally performing additional steps such as cleaning, machining, grinding and/ or surface hardening and/or surface treatment (e. g. laser hardening, PVD, shoot peening, induction hardening). Depending on the surface hardening method used, the surface of an object comprising the super bainite steel as defined hereinabove or hereinafter may have a martensitic microstructure containing martensite phase. However, this martensitic microstructure will have a composition within the present ranges.

The thermal heat treatment for super bainite transformation may be an isothermal heat treatment.

According to the present disclosuire, the melt may be processed in vacuum.

According to embodiments, the thermal heat treatment is an isotherrnal heat treatment and is performed by quenching the object by using for example a salt bath or an oil bath, at for example a temperature range between 200 to 350 °C, such as a temperature range between 220 to 260° C. However, the temperature must be above the Ms temperature and the Ms temperature will depend on the steel composition. A salt bath or an oil bath has a high energy capacity and a high energy absorption which will ensure that the outer and/or the inner surface layers are cooled fast enough to avoid pearlite prior to the isothermal super bainite transformation and that the core of the object is cooled in a way so that essentially no pearlite will be formed in the microstructure. It is very important that the cooling of the present super bainite steel is fast enough to avoid pearlite in the super bainitic microstructure. Thus, the present super bainite steel is cooled from a temperature above its austenite transition temperature to a temperature above its martensite start temperature (Ms) but well below the start temperature for bainite precipitation (B s) and growth. (Éêptionallyg the process trsed for meltin rhay be vacuum induction melting (ÄJIM). (Åšptiortallyf, the process trsed for rneltiri g rnay also be a vacuun: arc remeätirig (XIzXR). 't/ÜAR. is used for renrovirig lrytirogen arid other gaseous impurities, therebjf avoidirtg possible reduction of the fiuctilityf.

Additionztlly, VOD or AGD tttärnlvitrirëfí vvith tiegassirrg rriay also lie used for retirtihg the present super bainite steel. 'llhe present iriverition is illustrated by the follovving non--liniiting examples: 13 Examples The ztlloyfs *were iriatiufacttiriëti. aeeriraiiiiglyf: ivieltiiig and Casting an ingot Heating the inget to hot *working ternperattire H ot *wrirki rig tiie iiigiät to a “oiile Air" cooling the ibillet to room teinperature Heating and austenitizing at 1000 to 1050 °C Quenching to the isothermal heat treatment temperature lsothernial heat trtfatnitfiit at 225 to 275 OC tiiitil the tiesireti rnicrostructiire is twbtairiiëti; (Üoriliiig to frootii ternpeiature 'The clieinical cornprisitioris of the rnanufacttireti alloys (heats) are shoiwn in Table i. 'The results of the differciit tests perfornicti are shovvii in Table 2 and S.

The isotiitërrnal lieat treatment *was perfornicti aecordirig to tlic follovairig: l) By dilatometry testing A dilatometer test was used for simulating the heat treatment cycle to Capture the actual behavior of the bainite transformation and for being able to determine any differences in transformation time depending on alloy composition of the different heats. By this method, the precipitation, growth, and completion of the isothermally formed super bainitic microstructure was continuously measured and the heats were exposed to a predetermined thermal cycle by using a Bähr 805A quench dilatometer.

The dilatometer cycle included heating to an austenitizing temperature of l050°C with 5 min holding time followed by a free, fast cooling to 250°C and a subsequent isothermal hold at this temperature for 16 h, before a final free cooling to room temperature. All samples (ø4 mm, Ll0 mm) were inductively heat treated in a Vacuum atmosphere, with thermoelements spot-welded onto the sample surface for temperature control. The sample dilatation was measured continuously by a LVDT- measuring unit. An example of a dilatometer curve is shown in Figure 2.

The isothermal transformation of the metastable austenite structure into the super bainitic microstructure was captured as a continuous length expansion in the dilatation 14 2) 3) curve with time, due to the change in specific volume between the phases, as the growing super bainitic microstructure has a higher specific volume compared to the forrner austenitic microstructure.

An example of a schematic and typical dilatometry curve measured during the isotherrnal heat treatment is presented in Figure 2, which also shows the used definitions of start time and inflection point time of the bainite transformation.

By quenching and isothermal heat treatment in a salt bath Sarnples from all alloys were also austenitized, quenched in a salt bath and isotherrnally heat treated in the same salt lbath in order to obtain samples for prärtorriiirig rnefzliariirtztl testing. Thie salt liath lieat trcëatriïiërit was perfoirrïiaicí at' a teniperature of about ZSÛÜC during 16 h.

The samples from the dilatometry testing were then evaluated regarding: 0 The isothermal transformation time to reach the inflection point of the measured dilatation curve, i.e. the point where the second derivative of the dilatation curve changes from positive to negative. The time needed to reach this point had been set to < 90 min (l.50 h) at 250°C in order to achieve a sufficiently transformed super bainitic microstructure in a fast enough time frame for a continuous full- scale production process. 0 The hardness of the samples at room temperature, RT, after the isothermal transformation of the super bainitic microstructure at 250°C/ during l6 h was measured according to the SS-EN-ISO 6507 standard and in order to comply with the requirements of the final product, it was set to be 2 630 HVl. 0 The pearlite content was deterrnined at room temperature after continuous cooling from the austenitizing temperature (1050 °C) to room temperature at a constant cooling rate of l °C/ s. The amount of pearlite was measured with an image analysis software (Zeiss Axiovision) in cross-sectional sample surfaces after grinding, polishing and etching at a magnification of Xl00, using a light optical rnicroscope. The amount of pearlite in the microstructure of the continuously cooled samples was set to be < 2.0 %, to assure a sufficient safety margin against undesirable microstructures formed during cooling of the super bainite steel in a full-scale production process. In the tables and throughout the applications Will “RT” refer to room temperature unless stated otherwise.

Claims (17)

1. A super bainite steel comprising the following elements in Weight-°/0: C 0.60 to 0.90; Si 1.60 to 3.00; A1 0.10 to 0.80; Mn S 0.90; P íï 0.03; S 0.03; Cr 0.40 to 1.50; Ni 0.05 to 1.50; Mo 0.40 to 1.10; Co < 3.20; V ~< 0.50; Ti < 0.10; Cu < 0.50; Si+A1 2.1 to 3.1; Balance is Fe and unavoidable impurities.

2. The super bainite steel according to claim 1, Wherein the content of Mn is 50.80 Wt-°/0, such as S 0.60 Wt-°/

3. The super bainite steel according to claim 1 or claim 2, Wherein the content of Ni is 0.to 1.10 Wt-°/

4. The super bainite steel according to any one of claims 1 to 3, Wherein the content of Cu is < 0.30 Wt-°/

5. The super bainite steel according to any one of claims 1 to 4, Wherein the content of Ti is < 0.05 Wt-°/

6. The super bainite steel according to any one of claims 1 to 5, Wherein the content of C is 0.65 to 0.85 Wt-°/

7. The super bainite steel according to any one of claims 1 to 6, Wherein the content of Cr is 0.60 to 1.30 Wt-°/

8. The super bainite steel according to any one of c1aims 1 to 7, Wherein the content of Mo is 0.65 to 0.95 Wt-°/

9. The super bainite steel according to any one of c1aims 1 to 8, Wherein the content of Ais 0.10 to 0.50 Wt-°/

10. The super bainite stee1 according to any one of c1aims 1 to 9, Wherein the content of Si is 2.00 to 2.60 Wt-°/

11. The super bainite stee1 according to any one of c1aims 1 to 10, Wherein the content of Si+A1 is Within the range of 2.3 to 2.8 Wt-°/

12. The super bainite stee1 according to any one of c1aims 1 to 11, Wherein the content of Co is 2.00 to 3.10 Wt-°/

13. The super bainite stee1 according to any one of c1aims 1 to 12, Wherein the content of V is 0.05 to 0.30 Wt-°/

14. The super bainite stee1 according to any one of c1aims 1 to 13, Wherein the super bainite stee1 has an inflection point time of the isotherma1bainite transformation Which is 1ess than 90 rninutes, measured as the di1atation in a di1atometer test, When the present super bainite steeis austenitized and then rapid1y coo1ed to and direct1y isotherma11y heat treated at 250°C.

15. A method for manufacturing an object comprising the super bainite stee1 according to any one of c1aims 1 to 14, the method comprises the steps of: - me1ting raw materia1, a11oying e1ements and/or scrap materia1 Whereby a mo1ten stee1 having the e1ement ranges as in c1aims 1 to 13 is obtained; - casting the mo1ten stee1 into a casting; - hot Working the casting to an object having a desired shape and/or dimension in a temperature range of 1100 to1300 °C; - coo1ing the object to room temperature; - heating and austenitizing the object in a temperature range of 850 to 1100 °C; - quenching the object to the therrna1 heat treatment temperature for super bainite transformation; - therrna11y heat-treating the object in a temperature range of higher than the Ms temperature but be1oW 350 °C unti1 the desired super bainitic microstructure has been obtained; and- Cooling the object to room temperature.

16. .An object contprising the super' bainite steel according to any one of ciairns 1 to 14 or rnariufactured attcordirig to the process actrording to the. rnethiati o f ciairn 15, wfherein said oißjeßt has a microstructure With a pear1ite content 1ess than 2.0 % at room temperature after continuous cooling from the austenitizing temperature to room temperature at a constant coo1ing rate of 1 °C/s.

17. The object according to c1aim 16, Wherein said object has a Hardness (HV1) 2 630 HV1 (SS-EN-ISO 6507) in room temperature after the isothermatransformation of the super bainitic microstructure at 250°C during 16 h.