WO2018182480A1

WO2018182480A1 - Hot work tool steel

Info

Publication number: WO2018182480A1
Application number: PCT/SE2018/050213
Authority: WO
Inventors: Sebastian Ejnermark
Original assignee: Uddeholms Ab
Priority date: 2017-03-29
Filing date: 2018-03-07
Publication date: 2018-10-04
Also published as: TW201840866A

Abstract

The invention relates hot work tool steel. The steel comprises the following main components (in wt. %): C 0.27 -0.40, Si 0.10 -0.35, Mn 0.2 -1.0, Cr 4.0 -6.0, Mo 1.0 -3, V 0.4 -0.9, balance optional elements, iron and impurities, wherein the steel in the annealed condition has a microstructure comprising: a) at least 75 vol. % packets of tempered martensite and/or bainite and b) 1-20 vol. % of hard phase including carbides, nitrides and carbo-nitrides.

Description

HOT WORK TOOL STEEL

TECHNICAL FIELD

The invention relates to a hot work tool steel.

BACKGROUND OF THE INVENTION

Vanadium alloyed matrix tool steels have been on market for decades and attained a considerable interest because of the fact that they combine a high wear resistance with an excellent dimensional stability and because they have a good toughness. These steels have a wide range of applications such as for die casting and forging. The steels are generally produced by conventional metallurgy followed by Electro Slag Remelting (ESR). Uddeholm DIEVAR"⁹ is a high performance chromium-molybdenum-vanadium steel, containing balanced carbon and vanadium contents as described in WO 99/50468 Al. It is a modified H13 premium hot work tool steel, which is machined in the soft annealed delivery condition. The recommended soft annealing is heating to 850 °C in protecting atmosphere for 4 hours to obtain a uniform temperature followed by cooling at a rate of 10 °C/h to 600 °C and then freely in air. This results in a hardness of approximately 160 HB.

Although the known steel has superior properties than other conventionally produced tool steels with respect to heat checking, gross cracking, hot wear and plastic deformation, there is a need for further improvements of its property profile. In particular, it would be beneficial to improve the machinability of the steel in the unhardened condition. DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a hot work tool steel having an improved property profile, i.e. the inventive steel should be superior over the modified H13 tool steel known in the art in at least one respect.

Another object of the present invention is to improve the machinability of the steel in the unhardened condition. In addition, it would be advantageous if the distortion during the hardening and tempering of the tool steel could be reduced in order to further reduce the machining allowances. The foregoing objects, as well as additional advantages are achieved to a significant measure by providing a hot work tool steel having a composition and a microstructure as set out in the claims.

The invention is defined in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the structure of the inventive steel 800 °C/6h according to the Example. The length of the bar in the lower right corner is 20 μιη.

Fig. 2 shows the structure of the comparative subjected to conventional soft annealing according to the Example. The length of the bar in the lower right corner is 20 μιη. Fig. 3 shows the structure of the inventive steel 800 °C/6h according to the Example.

Fig. 4 shows the structure of the comparative subjected to conventional soft annealing according to the Example.

DETAILED DESCRIPTION OF THE INVENTION

The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. The amount of hard phases is given in volume % (vol. %). Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims. Carbon (0.27 - 0.40 %)

is to be present in a minimum content of 0.27 %, preferably at least 0.28, 0.29, 0.30, 0.31, 0.32, 0.33 or 0.34 %. The upper limit for carbon is 0.40% and may be set to 0.39, 0.38, 0.37, 0.36 or 0.35%. Preferred ranges are 0.30 - 0.38 % and 0.33 - 0. 37 %. In any case, the amount of carbon should be controlled such that the amount of primary carbides of the type M23C6, M7C3 and Me in the steel is limited, preferably the steel is free from such primary carbides.

Silicon (0.10 - 1.0 %)

Silicon is commonly used for deoxidation. Si is present in the steel in a dissolved form and may positively influence the machinability. However, Si is a strong ferrite former and increases the carbon activity and therefore the risk for the formation of undesired carbides, which negatively affect the impact strength. Silicon is also prone to interfacial segregation, which may result in decreased toughness and thermal fatigue resistance. Si is therefore limited to 1.0 %. The upper limit may be 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.35 0.34, 0.32, 0.30, 0.28, 0.26, 0.24 or 0.22%. The lower limit may be 0.12, 0.14, 0.16, 0.18 and 0.20%. Preferred ranges are 0.10 - 0.25 % and 0.15 - 0.24%.

Manganese (0.2 - 1.0 %)

Manganese contributes to improving the hardenability of the steel and together with sulphur manganese contributes to improving the machinability by forming manganese sulphides. Manganese shall therefore be present in a minimum content of 0.2 %, preferably at least 0.3, 0.35, 0.4, 0.45 or 0.5 %. At higher sulphur contents manganese prevents red brittleness in the steel. The steel shall contain maximum 1.0 %, preferably maximum 0.8, 0.7, 0.6, 0.55 or 0.5 %.

Chromium (2.0 - 6.0 %)

Chromium is to be present in a content of at least 2.0 % in order to provide a good

hardenability in larger cross sections during heat treatment. If the chromium content is too high, this may lead to the formation of high-temperature ferrite, which reduces the hot- workability. The lower limit may be 2.0, 2.5, 3.0, 3.5, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8 or 4.9%. The upper limit may be 6.0, 5.8, 5.6, 5.5, 5.4, 5.2 or 5.1 %.

Molybdenum (1.0 - 3.5 %)

Mo is known to have a very favourable effect on the hardenability. Molybdenum is essential for attaining a good secondary hardening response. The minimum content is 1.0 %, and may be set to 1,2, 1.4, 1.6, 1.8, 2.0, 2.1, 2.2, 2.25 or 2.3 %. Molybdenum is a strong carbide forming element and also a strong ferrite former. The maximum content of molybdenum is therefore 3.5 %. Mo may be limited to 3.4, 3.3, 3.2, 3.1, 3.0, 2.9, 2.8, 2.7, 2.6, 2.5, 2.4 or 2.35 %. Vanadium (0.4 - 0.9 %)

Vanadium forms evenly distributed primary precipitated carbides and carbonitrides of the type V(N,C) in the matrix of the steel. This hard phase may also be denoted MX, wherein M is mainly V but Cr and Mo may be present and X is one or more of C, N and B. Vanadium shall therefore be present in an amount of 0.4 - 0.9%. The upper limit may be set to 0.9, 0.8, 0.7, 0.6, 0.58, 0.56 or 0.55 %. The lower limit may be 0.42, 0.44, 0.46, 0.48, 0.50 or 0.52%. Aluminium (0.001 - 0.06 %)

Aluminium is an optional element and may be used for deoxidation in combination with Si and Mn. The lower limit may be set to 0.001, 0.003, 0.005 or 0.007% in order to ensure a good deoxidation. The upper limit is restricted to 0.06% for avoiding precipitation of undesired phases such as AIN. The upper limit may be 0.05, 0.04, 0.03, 0.02 or 0.015%.

Nitrogen (< 0.08 %)

Nitrogen is restricted to 0.08 % in order to obtain the desired type and amount of hard phases, in particular V(C,N). When the nitrogen content is properly balanced against the vanadium content, vanadium rich carbonitrides V(C,N) will form. These will be partly dissolved during the austenitizing step and then precipitated during the tempering step as particles of nanometer size. The thermal stability of vanadium carbonitrides is considered to be better than that of vanadium carbides, hence the tempering resistance of the tool steel may be improved and the resistance against grain growth at high austenitizing temperatures is enhanced. The lower limit may be 0.001, 0.004, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016 or 0.017%. The upper limit may be 0.07, 0.06, 0.05, 0.04, 0.03 or 0.01 %.

Hydrogen (< 0.0004 %)

Hydrogen is known to have a deleterious effect on the properties of the steel and to cause problems during processing. In order to avoid problems related to hydrogen the molten steel is subjected to vacuum degassing. The upper limit can be set to 0.0004% (4 ppm) and it may be limited to 3, 2.5, 2, 1.5 or 1 ppm.

Nickel (<1.5 %)

Nickel may be present in an amount of <1.5 %. It gives the steel a good hardenability and toughness. However, because of the expense, the nickel content of the steel should be limited. The upper limit may therefore be set to 1.0, 0.8, 0.5 or 0.3%. The lower limit may be set to 0.05, 0.10, 0.15 or 0.20 %.

Copper (< 2.0%)

Cu is an optional element, which may contribute to increasing the hardness and the corrosion resistance of the steel. If used, the preferred range is 0.02 - 1%. The lower limit may beset to 0.05, 0.1 or 0.15 %. The upper limit may be set to 0.6, 0.4 0.3 or 0.2 %. However, it is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally not deliberately added.

Cobalt (< 8 %)

Co is an optional element. Co causes the solidus temperature to increase and therefore provides an opportunity to raises the hardening temperature, which may be 15 - 30 °C higher than without Co. During austenitization it is therefore possible to dissolve larger fraction of carbides and thereby enhance the hardenability. Co also increases the M_s temperature.

However, large amount of Co may result in a decreased toughness and wear resistance. The maximum amount is 8 % and, if added, an effective amount may be 2 - 6 %, in particular 4 to 5 %. However, for practical reasons, such as scrap handling, deliberate additions of Co is not made. The maximum impurity content may then be set to 1 % or 0.3 %.

Tungsten (< 1 %)

In principle, molybdenum may be replaced by twice as much with tungsten because of their chemical similarities. However, tungsten is expensive and it also complicates the handling of scrap metal. The maximum amount is therefore limited to 1 %, preferably 0.5 %, more preferably 0.3 % and most preferably no deliberate additions are made.

Niobium (< 0.5%)

Niobium is similar to vanadium in that it forms carbonitrides of the type M(N,C) and may in principle be used to replace part of the vanadium but that requires the double amount of niobium as compared to vanadium. However, Nb results in a more angular shape of the

M(N,C). The maximum amount is therefore 0.5%, preferably 0.05 % and most preferably no deliberate additions are made.

Ti, Zr and Ta

These elements are carbide formers and may be present in the alloy in the claimed ranges for altering the composition of the hard phases. However, normally none of these elements are added.

Boron (< 0.01%)

B may be used in order to further increase the hardness of the steel. The amount is limited to 0.01%, preferably < 0.005%. A preferred range for the addition of B is 0.001 - 0.004 %. Selenium (< 0.05%)

Selenium may be added to the steel in an amount of < 0.05% in order to further improve the machinability of the steel.

Yttrium (< 1 %)

Y may optionally be added in an amount of up to 1 % in order to improve adherence of the oxide scale and thereby improving the wear resistance of the steel. The addition is particularly effective when evenly distributed in the steel matrix. Y is therefore preferably added to steels produced by powder metallurgy. The lower limit may be set to 0.2, 0.3 or 0.4 %. The upper limit may be set to 0.9, 0.8, 0.7 or 0.6 %. Ca, Mg and REM (Rare Earth Metals)

These elements may be added to the steel in the claimed amounts in order to further improve the machinability, hot workability and/or weldability. REM is here defined to consist of the elements 57 to 71 in the periodic table.

Impurity elements

P, S and O are the main impurities, which have a negative effect on the mechanical properties of the steel. P may therefore be limited to 0.03%, preferably to 0.01%. S may be limited to 0.03, 0.01, 0.00 5, 0.003, 0.001, 0.0008, 0.0005 or even 0.0001%. O may be limited to 0.0015, 0.0012, 0.0010, 0.0008, 0.0006 or 0.0005%.

The invention provides a steel for hot working having a composition consisting of in weight % (wt.%):

c 0.27 - 0.40

Si 0.10 - 1.0

Mn 0.2 - 1.0

Cr 2.0 - 6.0

Mo 1.0 - 3.5

V 0.4 - 0.9

optionally one or more of

Al 0.001 - 0.06

N < 0.08

Ni < 1.5 Cu <2

Co <8

W < 1

Nb <0.5

Ti <0.05

Zr <0.05

Ta <0.05

B <0.01

Se <0.05

Ca 0.0001 - 0.009

Mg <0.01

Y < 1

REM <0.2

balance Fe apart from impurities,

wherein the steel is in an annealed condition, has a hardness of not more than 360 HB and a microstructure comprising:

a) at least 75 vol. % packets of tempered martensite and/or bainite, and b) 1-20 vol. % of hard phase including carbides, nitrides and carbo-nitrides.

According to another aspect the steel should fulfil at least one of the following requirements c 0.30-0.38

Si 0.15-0.35

Mn 0.4-0.8

Cr 3.5-5.5

Mo 1.8-2.8

V 0.5-0.6

N 0.001-0.03

Cu 0.02-1

According to another aspect the steel should fulfil at least one of the following requirements

C 0.30-0.38

Si 0.15-0.30

Mn 0.4-0.6 Mo 2.1-2.8

V 0.5-0.6

N 0.001-0.056

C 0.33-0.37

Si 0.16-0.26

Mn 0.45-0.55

Cr 4.8-5.2

Mo 2.2-2.6

V 0.51-0.58

N 0.004 - 0.02

C 0.30-0.38

Si 0.15-0.30

Mn 0.4-0.6

Cr 4.5-5.5

Mo 2.1-2.8

V 0.5-0.6

N <0.01

According to another aspect the steel should fulfil the following requirements:

C 0.33-0.37

Si 0.16-0.26

Mn 0.45-0.55

Cr 4.8-5.2

Mo 2.2-2.6

V 0.51-0.58

According to another aspect the packets of the tempered martensite and/or bainite should have a maximum size of 40 μιη, preferably 20 μιη, more preferably 15 μιη. According to another aspect the hard phase comprises Me , M7C3, M23C6 and MC, wherein M is one or more metals of Cr, Mo, V and Nb and wherein the hard phase, apart from C, may comprise N and B.

Preferably, the amount of hard phase is 3-8 vol. % and/or the maximum size of the hard phase Ϊ5 3 μιη.

According to another aspect the steel in the annealed condition has a hardness of not more than 300 HB, preferably not more than 250 HB. A preferred hardness range is 180-250 HB, preferably 190-230 HB.

According to another aspect the steel fulfils the machinability value V1000 (HSS) > 35 m/min. A method of producing a steel comprising the steps of:

a) providing a steel having a composition as defined in any of claims 1-6, b) subjecting said steel to hot deformation,

c) subjecting the hot deformed steel to a heat treatment at a temperature in the range of 970 °C to 1050 °C for 2-8 hours in order to austenitize the steel,

d) cooling the steel to a temperature of 70-220 °C in order to obtain a martensitic and/or bainitic structure, and

e) reheating the steel to a temperature of Aci-100 °C to A_ci for 2-10 hours.

Preferably the method of producing a steel fulfils at least one of the following requirements: the temperature in step c) is 980-1030 °C, the reheating temperature in step e) is A_ci-50 °C to Aci-10 °C and the holding time is 4-8 hours.

The method of producing a steel may further comprise the step of:

f) hardening the steel from a temperature of 980-1080 °C, and

optionally

g) tempering the steel. A tool steel having the claimed chemical composition can be produced by conventional metallurgy including melting in an Electric Arc Furnace (EAF) and further refining in a ladle and vacuum treatment. Optionally the steel may be subjected to Electro Slag Remelting (ESR) in order to further improve the cleanliness and the microstructural homogeneity. Normally the steel is subjected to hardening and tempering before being used e.g. in a mould for die casting. Austenitizing may then be performed at an austenitizing temperature (TA) in the range of 1000-1070 °C, preferably about 1000-1030 °C. A typical T_A is 1025 °C with a holding time of 30 minutes followed by rapid quenching. The tempering temperature is chosen according to the hardness requirement and is performed at least twice at 550 - 650 °C for 2 hours (2x2h) followed by cooling in air. This results in a hardness of 44 - 54 HRC.

However, the steel block used to make a mould or die is machined in the unhardened condition. Although, the prior art steel Uddeholm DIEVAFf has a low hardness of

approximately 160 HB in the soft annealed condition and easily meet NADAC 207-97 requirements of a maximum hardness of 230 HB for premium hot work tool steels, it has a moderate machinability. The hardness test used for all values in this application was

HBW10/3000, i.e. a 10 mm diameter tungsten carbide ball indenter and a load of 3000 kgf. Accordingly, in this application HB is the same as HBW10/3000.

The inventors of the present invention has surprisingly found that if the conventional soft annealing is replaced by an alternate high temperature annealing slightly below the Aci temperature it is possible to improve the machinability of the steel by providing the steel with a completely different microstructure.

The annealing of a martensitic and/or bainitic structure slightly below the Aci temperature results in a redistribution of the carbon and precipitation of carbides. The structure of the lath martensite and/or bainite remains because the material was not subjected to any

transformation into austenite. The carbides are mainly found at the grain and sub-grain boundaries. The precipitation of carbides results in a decrease in tetragonality of the martensite and finally, the packets of lath martensite and/or bainite are transferred into packets of ferrite. Accordingly, term "packets of tempered martensite and/or bainite" refers to a structure of carbon depleted packets of tempered martensite and/or bainite with carbides distributed in the grain- and sub-grain boundaries as well as to a packet structure of ferrite having most of the carbides distributed in the grain- and sub-grain boundaries.

EXAMPLE

In this example a steel was produced by EAF-melting, ladle refining, and vacuum degassing followed by uphill casting. The ingots were subjected to forging to the dimension 799x379 mm. Test pieces of were taken for examination.

The steel had the following composition in weight % (wt.%): C: 0.35, Si: 0.18, Mn: 0.45, Cr: 5.0, Mo 2.3, V: 0.6 and Al:0.01. The Aci for this composition is 820 °C. The samples were subjected to austenitizing at 990 °C for 4 hours and cooling time of 100 second between 800 and 500 °C (t₈/₅ = 100s) and thereafter freely in air to 180 °C. The steel sample according to the invention was subjected to annealing at 800 °C, which is slightly below the Aci temperature. This resulted in a hardness value of 213 HB. The hardness of the sample subjected to conventional soft annealing above the Aci temperature was 164 HB. The results are shown in Table 1.

Table 1. Results of the heat treatments

Fig. 1 discloses the microstructure of the inventive steel, Light Optical Microscope (LOM). Fig. 2 discloses the microstructure of the comparative steel, which was subjected to conventional soft annealing (LOM). The length of the bar in the lower right corner in Figures 1 and 2 is 20 μιη. It can be seen that the inventive steel has a much finer and more homogeneous structure as well as a finer and more uniform carbide distribution than the soft annealed steel. These differences can be more clearly recognized in the pictures taken in the Scanning Electron Microscope (SEM). Fig. 3 discloses a SEM picture of the inventive steel consisting of packets of carbon depleted tempered martensite and carbides mainly distributed in the grain- and sub- grain boundaries. Fig. 4 discloses the structure of the comparative steel having coarse carbides uniformly distributed in a matrix of polygonal ferrite.

Machinability was examined by drilling, since this is one of the toughest operations in tool body manufacture. The tests were carried out on a MODIG 7000 machining center. The steels were subjected to V1000 drilling test. This test gives the cutting speed for a cutting length of 1000 mm. The drills used were HSS Wedevag Double-X 05 mm.

Table 2 discloses the result according to the invention and Table 3 for the comparative steel.

Table 2. Result of the drilling test for the inventive steel. V1000=40 m/min.

Table 3. Result of the drilling test for the comparative steel. V1000=30m/min.

The results of the drilling tests were used to derive the VIOOO-values for the steels examined. The inventive steel had a V1000 value of 40 m/min and the comparative steel a V1000 value of 30 m/min. Accordingly, the inventive steel revealed a remarkable improved machinability in this test. A result of this microstructure is that the hardness of the annealed material increases from about 160 HB to e.g. about 210 HB. In addition thereto, it has surprisingly been found that the new structure give rise to a very much improved machinability.

The reason for the improved machinability is as present not fully understood and the inventors do not want to be bound by any theory. However, one reason for the improved machinability might be that the fine grained structure resulting from the small packet size influences positively on the machinability and offsets the negative effect of an increased hardness. Possibly, the fine grained structure results in smaller chip segments and thereby in lower cutting forces. INDUSTRIAL APPLICABILITY

The tool steel of the present invention is particular useful in large dies requiring a good hardenability and a good machinability in the unhardened condition.

Claims

1. A steel for hot working having a composition consisting of in weight % (wt.%):

C 0.27 - 0.40

Si 0.10-1.0

Mn 0.2-1.0

Cr 2.0-6.0

Mo 1.0-3.5

V 0.4-0.9

optionally one or more of

Al 0.001-0.06

N <0.08

Ni < 1.5

Cu <2

Co <8

W < 1

Nb <0.5

Ti <0.05

Zr <0.05

Ta <0.05

B <0.01

Se <0.05

Ca 0.0001 - 0.009

Mg <0.01

Y < 1

REM <0.

2

balance Fe apart from impurities,

a) at least 75 vol. % packets of tempered martensite and/or bainite and b) 1-20 vol. % of hard phase including carbides, nitrides and carbo-nitrides. A steel according to claim 1 fulfilling at least one of the following requirements: c 0.30-0.38

Si 0.15-0.35

Mn 0.4-0.8

Cr 3.5-5.5

Mo 1.8-2.8

V 0.5-0.6

N 0.001-0.03

H < 0.0003

Cu 0.02-1

Co < 1

W <0.

3

Nb <0.05

Ti <0.01

Zr <0.01

Ta <0.01

B < 0.005

Se <0.03

Mg < 0.001

Y 0.2-1.0

and wherein the impurity contents of P, S and O fulfils the following requirements

P < 0.03

S < 0.03

O < 0.0015

A steel according to claim 1 or 2 fulfilling at least one of the following requirements:

C 0.30-0.38

Si 0.15-0.30

Mn 0.

4-0.6

Cr 4.5-5.5

Mo 2.1-2.8 V 0.

5-0.

6

N 0.001-0.056

H < 0.0002

Cu 0.02-0.5

Co < 1

W <0.3

Nb <0.5

Ti <0.01

Zr <0.01

Ta <0.01

B < 0.005

Se <0.03

Mg < 0.001

Y 0.3-0.

7

P <0.01

S < 0.0015

O < 0.0010 A steel according to any of the preceding claims fulfilling at least one of the following requirements:

c 0.33-0.37

Si 0.16-0.26

Mn 0.45 - 0.55

Cr 4.8-5.2

Mo 2.2-2.6

V 0.51-0.58

N 0.004 - 0.02

H < 0.0003

Cu 0.02-0.3

Co <0.3 W <0.1

Nb <0.05

Y 0.4-0.6

P <0.01

S < 0.0005

O < 0.0008

A steel according to any of the preceding claims fulfilling the following requirements:

c 0.30-0.38

Si 0.15-0.30

Mn 0.4-0.6

Cr 4.5-5.5

Mo 2.1-2.8

V 0.5-0.6

N <0.01

C 0.33-0.37

Si 0.16-0.26

Mn 0.45-0.55

Cr 4.

8-5.2

Mo 2.2-2.6

V 0.51-0.58

A steel according to any of the preceding claims, wherein the packets of the tempered martensite and/or bainite have a maximum size of 40 μιη, preferably 20 μιη, more preferably 15 μιη.

A steel according to any of the preceding claims, wherein the hard phase comprises MeC, M₇C₃, M23C6 and MC, wherein M is one or more metals of Cr, Mo, V and Nb and wherein the hard phases, apart from C, may comprise N and B.

9. A steel according to any of the preceding claims, wherein the amount of hard phase is 3-8 vol. % and/or the maximum size of the hard phase is 3 μιη.

10. A steel according to any of the preceding claims, wherein the steel in the annealed

condition has a hardness of not more than 300 HB, preferably not more than 250 HB.

11. A steel according to any of the preceding claims, wherein the steel in the annealed

condition has a hardness of 180-250 HB, preferably, 190-230 HB.

12. A steel according to claim 10, wherein the steel fulfils the following machinability value V1000 (HSS) > 35 m/min.

13. A method of producing a steel comprising the steps of:

d) cooling the steel to a temperature of 70-220 °C in order to obtain a martensitic and/or bainitic structure and

e) reheating the steel to a temperature of Aci-100 °C to A_ci for 2-10 hours.

14. A method of producing a steel according to claim 13 fulfilling at least one of the following requirements: the temperature in step c) is 980-1030 °C, the reheating temperature in step e) is A_ci-50 °C to A_ci-10 °C and the holding time is 4-8 hours.

15. A method of producing a steel according to claim 13 or 14 further comprising the step of:

f) hardening the steel from a temperature of 980-1080 °C and

optionally

g) tempering the steel.