EP4363628A1

EP4363628A1 - A super bainite steel, an object comprising said steel and a method for manufacturing said object

Info

Publication number: EP4363628A1
Application number: EP22833755.6A
Authority: EP
Inventors: Lars NYLÖF; Tomas Antonsson
Original assignee: Alleima Emea AB
Current assignee: Alleima Emea AB
Priority date: 2021-06-29
Filing date: 2022-05-25
Publication date: 2024-05-08
Also published as: SE2130179A1; SE544951C2; WO2023277754A1

Abstract

The 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; 5 Al 0.10 to 0.80; Mn ≤ 0.90; P ≤ 0.03; S ≤ 0.03; Cr 0.40 to 1.50; 0 Ni 0.05 to 1.50; Mo 0.40 to 1.10; Co < 3.20; V < 0.50; Ti < 0.10; 5 Cu < 0.50; Si+Al 2.1 to 3.1; Balance is Fe and unavoidable impurities.

Description

A SUPER BAINITE STEEL, AN OBJECT COMPRISING SAID STEEL AND A METHOD FOR

MANUFACTURING SAID OBJECT

Technical field

The present disclosure relates to a super bainite steel and to a method for manufacturing said super bainite steel and an object comprising said super bainite steel.

Background

Super bainite steels are characterized in that they have both high strength and high hardness. These steels are therefore especially suited for e.g. wear and armor applications. Examples of such steels and processes for manufacturing such steels are described in GB 2352726 A.

However, one major problem relating to these steels is that it will take too long time to obtain the desired super bainitic microstructure in the finished product as the decisive heat treatment time to achieve the desired properties exceeds 24 hours and often significantly beyond that. Thus, these products are therefore not commercially competitive in a full-scale manufacturing process.

An aspect of the present disclosure is therefore to provide a solution to solve or at least to reduce this problem. The present disclosure therefore relates to a new super bainite steel having a composition of alloying elements in ranges which will provide for a more efficient manufacturing process but still provide the desired properties, such as high strength and high hardness.

Summary of the disclosure

The present disclosure therefore provides a super bainite steel comprising in weight-% (wt-%) the following elements:

C 0.60 to 0.90;

Si 1.60 to 3.00;

A1 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 Fe and unavoidable impurities.

The selection and ranges of alloying elements of the present super bainite steel will provide for a faster bainite transformation meaning that the heat treatment time for obtaining the desired super bainitic microstructure will be greatly reduced compared to the heat treatment times for the super bainite steels known today.

The present disclosure also provides an object comprising a super bainite steel, which super bainite steel comprising the following elements in weight-% (wt-%);

C 0.60 to 0.90;

Si 1.60 to 3.00;

A1 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 Fe and unavoidable impurities; and wherein the object has a microstructure with a pearlite content less than 2.0 % at room temperature after continuous cooling from the austenitizing temperature to room temperature at a constant cooling rate of 1 °C/s. According to embodiments, the desired super baimtic microstructure has almost no presence of pearlite.

According to embodiments, the object as defined hereinabove or hereinafter has a Hardness (HV1) > 630 HV1 (SS-EN-ISO 6507) in room temperature (RT) after austenitizing and isothermal transformation of the super bainitic microstructure at 250°C during 16 h. The present object will also possess high strength as shown in Table 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 microstructure, the following definition of the term ‘^'super bainite steel” applies herein: a super bainite steel is a steel containing a super bainitic microstructure. The super bainitic microstruciure is formed during a thermal heat treatment above the Ms- temperature but below 350° C. Further, before this thermal heat treatment, when the super bainite forms, an austenitizing heat treatment step has to be performed.

The super bainite steel as defined hereinabove or hereinafter will essentially comprise a super bainitic microstructure. This super bainitic microstructure is characterized in that it contains thin ferrite laths and retained austenite and that there will be essentially no carbide- precipitation in the ferrite laths. The laths may also appear as discs or plates in a three- dimensional view, and the laths have an average thickness below 100 nm and typically below this.

However, primary carbides, which have already been precipitated during the hot-working processes performed above the temperatures in range for thermal precipitation and growth of bainitic microstructures, may be present in the microstructure. These primary carbides are intended for austenite grain size refinement during the austenitizing heat treatment.

Brief Description of the Figures

Figure 1 shows a non -limiting schematic CCT-diagram (continuous cooling transformations) of a super bainite steel with a hypoeutectoid carbon content, and explains how different alloying elements effect the transformation of austenite phase into different microstructures at various temperatures, during cooling and isothermal heat treatment; Figure 2 shows a schematic dilatometer curve during isothermal heat treatment, “a” denotes the start of transformation, “b” denotes inflection point of transformation, “c” denotes the end of transformation. The increase in length dilatation vs time corresponds to the precipitation and growth of the super bainitic microstructure during the isothermal heat treatment.

Detailed 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 0150-200 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 1, 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-%” and “wt-%” 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 governing 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 major 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 microstructure.

Additionally, carbon will suppress the Ms -temperature, which is the temperature from where the martensitic microstructure starts to form on cooling. The suppression of the Ms temperature 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 microstructure. 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 microstructure and may also reduce the ductility.

A sort of carbides, called primary carbides, may be present. These are precipitated during steelmaking and hot working processes, which processes are performed prior to the heat treatment steps used for precipitation and growth of the super bainitic microstructure and which are performed at much higher temperature ranges compared to the temperature ranges where bainite transformation of the austenite phase will 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-%, such as 0.65 to 0.85 wt-%.

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 microstructure, 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-%, such as 2.00 to 2.60 wt-%.

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 thermal 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 thermal 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 microstructure 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-%, such as 0.10 to 0.50 wt-%.

Silicon + Aluminium (Si + Al): 2.1 to 3.1 wt-%

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 thermal 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 well-balanced combination of the total content of Si and Al 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-%, such as 2.3 to 2.8 wt-%.

Manganese (Mn): < 0.90 wt-%

Mn is an austenite stabilizing alloying element and may optionally be added. If added, it will be beneficial for preventing hot cracking during welding and hot forming. Additionally, Mn prevents the formation of proeutectoid ferrite and normally also pearlite and reduces the Ms- temperature, thus increasing the amount of retained austenite in the microstructure. However, Mn has also an impact on the kinetics of the bainite transformation and if too much Mn is added, the precipitation and growth of bainite during the thermal heat treatment process, used for super bainite transformation, will be slowed down. Hence, if added, the Mn content should be as low as possible and is therefore < 0.90 wt-%, such as < 0.80 wt-%, such as <0.60 wt-%.

Phosphorous (P): < 0.03 wt-%

P is an optional element and is considered to be an impurity as it is normally regarded as a harmful element due to its embrittling effect. Therefore, it is desirable to have < 0.03 wt-% P.

Sulphur (S): < 0.03 wt-%

S is also regarded as an impurity as S may form grain boundary segregations and inclusions and will therefore restrict the hot working properties as well as the mechanical properties. Hence, the content of S should he < 0.03 wt-%.

Chromium (Cr): 0.40 to 1.50 wt-%

Cr will contribute to the solid solution strengthening of the super bainitic microstructure and is thus an important element for improving the mechanical properties. In addition, chromium decreases the transformation kinetics of both proeutectoid ferrite phase and pearlite . The effect of Cr on pearlite transformation of the austenite phase is significant and Cr is therefore added to avoid precipitation and growth of pearlite during cooling to the thermal heat treatment temperature for bainite transformation. Cr will also reduce the bainite transformation kinetics but to a lower degree than Mn. Therefore, Cr is preferable used instead of Mn for retarding the formation of proeutectoid ferrite phase and pearlite.

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-%, such as 0.60 to 1.30 wt-%.

Nickel (Ni): 0.05 to 1.50 wt-%

Nickel is a strong austenite forming element and has also a strong toughening effect. The strong toughening effect will increase the impact strength, especially at low service temperatures.

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-%. According to embodiments, the content ofNi is 0.20 to 1.10 wt-%.

Molybdenum (Mo): 0.40 to 1.10 wt-%

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 1. 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-%.

To ensure that Mo will have the positive effects mentioned above, the amount shall be at least 0.40 wt-%. 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-%.

Cobalt (Co): < 3.20 wt-%

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-%.

According to embodiments, the present super bainite steel contains Co in the range of 2.00 to 3.10 wt-%.

Vanadium (V): < 0.50 wt-%

Vanadium is an optional element and if added it will form precipitates together with carbon and/or nitrogen, V may therefore be added in order to generate grain refinement, specifically by controlling recrystallization and grain growth during hot working and during an austenitizing heat treatment. These grain refinements will facilitate the later precipitation and growth of ferrite laths during bainite transformation, which will improve the mechanical 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 wt-%. If V is added, the lowest content is 0.05 wt-%. In embodiments, the content of V is 0.05 to 0.30 wt-%.

Titanium (Ti): < 0.10 wt-%

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, A1 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-%, such as < 0.05 wt-%. Copper (Cu): < 0.50 wt-%

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-%, such as < 0.30 wt-%.

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-%.

When the terms “max” or “<” are used, the skilled person knows that the lower limit of the range is 0 wt-%, 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 normally 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-%.

According to embodiments, the present super bainite steel may also fulfil the condition of having: an inflection point time of the isothermal 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 isothermally 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.

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 1100 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 forming 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 1100 °C and will depend on the heating time and the size and thickness of the object; quenching the object to the thermal heat treatment temperature for super bainite transformation by for example using a salt bath or a hot oil bath;

- thermally 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; 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 disclosure, the melt may be processed in vacuum.

According to embodiments, the thermal heat treatment is an isothermal 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 (Bs) and growth.

Optionally, the process used for melting may be vacuum induction melting (VIM). Optionally, the process used for melting may also be a vacuum arc remelting (VAR). VAR is used for removing hydrogen and other gaseous impurities, thereby avoiding possible reduction of the ductility.

Additionally, VOD or AOD combined with degassing may also be used for refining the present super bainite steel.

The present invention is illustrated by the following non-limiting examples: Examples

The alloys were manufactured accordingly:

Melting and casting an ingot

- Heating the ingot to hot working temperature

- Hot working the ingot to a billet

Air cooling the billet to room temperature

- Heating and austenitizing at 1000 to 1050 °C Quenching to the isothermal heat treatment temperature

Isothermal heat treatment at 225 to 275 °C until the desired microstructure is obtained; Cooling to room temperature

The chemical compositions of the manufactured alloys (heats) are shown in Table 1. The results of the different tests performed are shown in Table 2 and 3.

The isothermal heat treatment was performed according to the following:

1) 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 Bahr 805A quench dilatometer.

The dilatometer cycle included heating to an austenitizing temperature of 1050°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 (04 mm, L10 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 aLVDT- 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 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 former austenitic microstructure.

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

2) By quenching and isothermal heat treatment in a salt bath

Samples from all alloys were also austenitized, quenched in a salt bath and isothermally heat treated in the same salt hath in order to obtain samples for performing mechanical testing. The salt bath heat treatment was performed at a temperature of about 250°C during 16 h.

3) The samples from the dilatometry testing were then evaluated regarding:

• 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 (1.50 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.

• The hardness of the samples at room temperature, RT, after the isothermal transformation of the super bainitic microstructure at 250°C/ during 16 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 > 630 HV1.

• The pearlite content was determined at room temperature after continuous cooling from the austenitizing temperature (1050 °C) to room temperature at a constant cooling rate of 1 °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 xlOO, using a light optical microscope. 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. Table 1 The Heats of the Example. The balance is Fe and unavoidable impurities. All values are given in wt-% unless stated otherwise.

Heats marked with “*”are within the present invention.

Table 1 cent.

Table 2 Result of testing

In the Tables , the term “free cooling” used during the dilatometer tests refers to the free cooling performed in the dilatometer from a temperature above the austenite transition temperature (i.e. the austenitizing temperature), where the super bainite steel essentially consists of austenite phase (i.e. having an austenitic microstructure), with a free cooling rate high enough for avoiding pearlite precipitation and growth. The hardness values are an average of five indents. Further, in the tables is the term “< 0.5” used for a pearlite content which is too low to be measured. Alloy 23 had too high boron content and therefore cracked during hot working and was scrapped.

Table 2 cont.

Table 3 Mechanical properties for some of the Heats- SS-EN ISO 6892-1, Round Lo=15mm. The values are an average value of three tests

Claims

1. A super bainite steel comprising the following elements in weight-%:

C 0.60 to 0.90;

Si 1.60 to 3.00;

A1 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.

2. The super bainite steel according to claim 1, wherein the content of Mn is <0.80 wt-%, such as < 0.60 wt-%.

3. The super bainite steel according to claim 1 or claim 2, wherein the content of Ni is 0.20 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 claims 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 claims 1 to 8, wherein the content of A1 is 0.10 to 0.50 wt-%.

10. The super bainite steel according to any one of claims 1 to 9, wherein the content of Si is 2.00 to 2.60 wt-%.

11. The super bainite steel according to any one of claims 1 to 10, wherein the content of Si+Al is within the range of 2.3 to 2.8 wt-%.

12. The super bainite steel according to any one of claims 1 to 11, wherein the content of Co is 2.00 to 3.10 wt-%.

13. The super bainite steel according to any one of claims 1 to 12, wherein the content of V is 0.05 to 0.30 wt-%.

14. The super bainite steel according to any one of claims 1 to 13, wherein the super bainite steel has an inflection point time of the isothermal bainite transformation which is less than 90 minutes, measured as the dilatation in a dilatometer test, when the present super bainite steel is austenitized and then rapidly cooled to and directly isothermally heat treated at 250°C.

15. A method for manufacturing an object comprising the super bainite steel according to any one of claims 1 to 14, the method comprises the steps of:

- melting raw material, alloying elements and/or scrap material whereby a molten steel having the element ranges as in claims 1 to 13 is obtained;

- casting the molten steel into a casting;

- hot working the casting to an object having a desired shape and/or dimension in a temperature range of 1100 tol300 °C;

- cooling the object to room temperature;

- heating and austenitizing the object in a temperature range of 850 to 1100 °C;

- quenching the object to the thermal heat treatment temperature for super bainite transformation; - thermally heat-treating the object in a temperature range of higher than the Ms temperature but below 350 °C until the desired super bainitic microstructure has been obtained; and

- cooling the object to room temperature.

16. An object comprising the super hainite steel according to any one of claims 1 to 14 or manufactured according to the process according to the method of claim 15, wherein said object has a microstructure with a pearlite content less than 2.0 % at room temperature after continuous cooling from the austenitizing temperature to room temperature at a constant cooling rate of 1 °C/s.

17. The object according to claim 16, wherein said object has a Hardness

(HV1) > 630 HV1 (SS-EN-ISO 6507) in room temperature after the isothermal transformation of the super bainitic microstructure at 250°C during 16 h.