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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
  • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high-hardness steel strip product exhibiting a good balance of high hardness and excellent mechanical properties such as impact strength and formability/bendability.
• the present invention further relates to a method of manufacturing the high-hardness steel strip product.
• High hardness has a direct effect on wear resistance of a steel product, the higher hardness the better wear resistance.
• high hardness it is meant that the Brinell hardness is at least 450 HBW and especially in the range of 500 HBW to 650 HBW.
• Wear resistant steels are also known as abrasion resistant steels. They are used in applications in which high resistance against abrasive and shock wear is required. Such applications can be found in e.g. mining and earth moving industry, and waste transportation. Wear resistant steels are used for instance in gravel truck's bodies and excavator buckets, whereby longer service time of the vehicle components is achieved due to the high hardness provided by the wear resistant steels.
• Wear resistant steels can also function as structural steels for making construction components if the wear resistant steels have sufficient mechanical properties such as formability, weldability and fatigue resistance that comply with national standards.
• the advantage of using wear resistant steels in the structural part for construction purposes is that less welding is needed and the weight can be lowered.
• Such high hardness in a steel product is typically obtained by martensitic microstructure produced by quench hardening steel alloy having high content of carbon (0.41-0.50 wt. %) after austenitization in the furnace.
• steel plates are first hot-rolled, slowly cooled to room temperature from the hot-rolling heat, reheated to austenitization temperature, equalized and finally quench hardened.
• This process is hereinafter referred to as the reheating and quenching (RHQ) process.
• RHQ reheating and quenching
• steels produced in this way are wear resistant steels disclosed in CN102199737 or some commercial wear resistant steels. Due to the relatively high content of carbon, which is required to achieve the desired hardness, the resulting martensite reaction causes significant internal residual stresses to the steel.
• CN102392186 and CN103820717 relate to RHQ steel plates having relatively low carbon content (0.25-0.30 wt. % in CN102392186; 0.22-0.29 wt. % in CN103820717) and also relatively low manganese content.
• a tempering step after quench hardening is required for making such RHQ steel plates, which inevitably increases the processing efforts and costs.
• EP2695960 relates to an abrasion-resistant steel product exhibiting excellent resistance to stress corrosion cracking, which steel sheet can be made in a process where direct quenching (DQ) may be performed immediately after hot rolling, without the reheating treatment after hot rolling as in the RHQ process.
• the steel sheet of EP2695960 has a relatively low carbon content (0.20-0.30 wt. %) and a relatively high manganese content (0.40-1.20 wt. %).
• the base phase or main phase of the microstructure of the steel product of EP2695960 must be made of tempered martensite.
• the area fraction of untempered martensite is restricted to 10% or less because the resistance to stress corrosion cracking is reduced in the presence of untempered martensite.
• the steel product of EP2695960 has a surface hardness of 520 HBW or less.
• the present invention extends the utilization of the cost-effective thermomechanically controlled processing (TMCP) in conjunction with direct quenching (DQ) and possibly also tempering to produce a high-hardness steel strip product exhibiting excellent formability/bendability and impact strength values.
• TMCP thermomechanically controlled processing
• DQ direct quenching
• the object of the present invention is to solve the problem of providing a high-hardness steel strip product exhibiting excellent formability/bendability and impact strength values.
• the problem is solved by the combination of specific alloy designs with cost-efficient TMCP procedures which produces a metallographic microstructure comprising mainly martensite.
• the present invention provides a hot-rolled steel strip product comprising a composition consisting of, in terms of weight percentages (wt. %):
• C 0.14-0.35 preferably 0.17-0.31, more preferably 0.20-0.28 Si 0-0.5, preferably 0.01-0.50, more preferably 0.03-0.25 Mn 0.05-0.40, preferably 0.05-0.30 Al 0-0.1, preferably 0-0.08 Cu 0.1-0.4, preferably 0.10-0.35 Ni 0.2-0.9, preferably 0.3-0.8, more preferably 0.3-0.7 Cr 0.2-0.9, preferably 0.3-0.8, more preferably 0.3-0.7 Mo 0-0.2, preferably 0-0.1 Nb 0-0.005 Ti 0-0.035 V 0-0.05 B 0.0005-0.0050, preferably 0.0008-0.0040 P 0-0.025, preferably 0-0.020 S 0-0.008, preferably 0-0.005 N 0-0.01, preferably 0-0.005 Ca 0-0.01, preferably 0-0.005, more preferably 0-0.003
• the steel product has a low content of Mn, which is important for improving impact toughness and bendability.
• the levels of Cr and Ni are set to improve hardenability.
• the level of Ni is further set to improve impact toughness and formability.
• the level of Nb should be restricted to the lowest possible to increase formability or bendability of the steel product. Elements such as Nb may be present as residual contents that are not purposefully added.
• residual contents are controlled quantities of alloying elements, which are not considered to be impurities.
• a residual content as normally controlled by an industrial process does not have an essential effect upon the alloy.
• the present invention provides a method for manufacturing hot-rolled steel strip product comprising the following steps of
• C 0.14-0.35 preferably 0.17-0.31, more preferably 0.20-0.28 Si 0-0.5, preferably 0.01-0.50, more preferably 0.03-0.25 Mn 0.05-0.40, preferably 0.05-0.30 Al 0-0.1, preferably 0-0.08 Cu 0.1-0.4, preferably 0.10-0.35 Ni 0.2-0.9, preferably 0.3-0.8, more preferably 0.3-0.7 Cr 0.2-0.9, preferably 0.3-0.8, more preferably 0.3-0.7 Mo 0-0.2, preferably 0-0.1 Nb 0-0.005 Ti 0-0.035 V 0-0.05 B 0.0005-0.0050, preferably 0.0008-0.0040 P 0-0.025, preferably 0-0.020 S 0-0.008, preferably 0-0.005 N 0-0.01, preferably 0-0.005 Ca 0-0.01, preferably 0-0.005, more preferably 0-0.003
• a step of temper annealing is performed on the direct quenched product at a temperature in the range of 150° C.-250° C.
• the step of temper annealing is not required according to the present invention.
• the steel product is a steel strip having a thickness of 10 mm or less, preferably 8 mm or less.
• the obtained steel product has a microstructure comprising, in terms of volume percentages (vol. %), at least 90 vol. % martensite, preferably at least 95 vol. % martensite, and more preferably at least 98 vol. % martensite, measured from 1 ⁇ 4 thickness of the steel strip product.
• the martensitic structure may be untempered, autotempered and/or tempered.
• the microstructure also comprises retained austenite, bainite, ferrite and/or cementite.
• the obtained steel product has a prior austenite grain size of 50 ⁇ m or less, preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less, measured from 1 ⁇ 4 thickness of the steel strip product.
• the aspect ratio of a prior austenite grain structure is one of the factors affecting a steel product's impact toughness and bendability.
• the prior austenite grain structure should have an aspect ratio of at least 1.5, preferably at least 2, and more preferably at least 3.
• the prior austenite grain structure should have an aspect ratio of 7 or less, preferably 5 or less, and more preferably 1.5 or less.
• the obtained steel product according to the present invention has a prior austenite grain structure with an aspect ratio in the range of 1.5-7, preferably 1.5-5, and more preferably 2-5, which ensures that a good balance of excellent impact toughness and excellent bendability can be achieved.
• the steel product has a good balance of high hardness and excellent mechanical properties such as impact strength and formability/bendability.
• the steel product has at least one of the following mechanical properties: a Brinell hardness in the range of 420-580 HBW, preferably 450-550 HBW, more preferably 460-530 HBW, and even more preferably 470-530 HBW; a Charpy-V impact toughness of at least 50 J/cm 2 at a temperature of ⁇ 40° C.
• the steel product exhibits excellent bendability or formability.
• the steel product has a minimum bending radius of 3.2 t or less in a measurement direction longitudinal to the rolling direction wherein the bending axis is longitudinal to rolling direction; a minimum bending radius of 2.5 t or less in a measurement direction transversal to the rolling direction wherein the bending axis is transversal to rolling direction; and wherein t is the thickness of the steel strip product.
• steel is defined as an iron alloy containing carbon (C).
• Brinell hardness is a designation of hardness of steel.
• the Brinell hardness test is performed by pressing a spherical tungsten carbide ball with a diameter of 10 mm against a clean prepared surface of a metal sheet using a 3000 kilogram force, producing an impression, measured and given a special numerical value.
• a spherical tungsten carbide ball with a diameter of 5 mm and a load of 750 kilogram force are applied to test samples with thinner gauges, e.g. 3 mm in thickness.
• gauge refers generally to a measure of the thickness of a metal sheet.
• yield strength (YS, R p 0.2) refers to 0.2% offset yield strength defined as the amount of stress that will result in a plastic strain of 0.2%.
• total elongation refers to the percentage by which the material can be stretched before it breaks; a rough indicator of formability, usually expressed as a percentage over a fixed gauge length of the measuring extensometer. Two common gauge lengths are 50 mm (A50) and 80 mm (A80).
• minimum bending radius (R i ) is used to refer to the minimum radius of bending that can be applied to a test sheet without occurrence of cracks.
• HAZ heat-affected zone
• the alloying content of steel together with the processing parameters determines the microstructure which in turn determines the mechanical properties of the steel.
• Alloy design is one of the first issues to be considered when developing a steel product with targeted mechanical properties.
• chemical composition according to the present invention is described in more details, wherein % of each component refers to weight percentage.
• Carbon C is used in the range of 0.14% to 0.35%.
• C alloying increases strength of steel by solid solution strengthening, and hence C content determines the strength level.
• C is used in the range of 0.14% to 0.35% depending on targeted hardness. If the carbon content is less than 0.14%, it is difficult to achieve a Brinell hardness of more than 420 HBW. C is also an austenite stabilizing element. However, C has detrimental effects on weldability, impact toughness, formability or bendability, and resistance to stress corrosion cracking. Therefore, C content is set to not more than 0.35%.
• C is used in the range of 0.17% to 0.31%, and more preferably 0.20% to 0.28%.
• Silicon Si is used in an amount of 0.5% or less.
• Si is added to the composition to facilitate formation of a protective oxide layer under corrosive climate conditions, which provides good resistance against climatic corrosion and increases the durability of a paint layer that is easily damaged or removed from machines surfaces due to wear.
• Si is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process.
• Si alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability. Also the presence of Si can stabilize retained austenite.
• silicon content of higher than 0.5% may unnecessarily increase carbon equivalent (CE) value thereby weakening the weldability.
• CE carbon equivalent
• Si is used in the range of 0.01% to 0.50%, and more preferably 0.03% to 0.25%.
• Manganese Mn is used in the range of 0.05% to 0.40%.
• Mn alloying lowers martensite start temperature (M s ) and martensite finish temperature (M f ), which can suppress autotempering of martensite during quenching. Reduced autotempering of martensite leads to higher internal stresses that may enhance the risk for quench-induced cracking or distortion of shape. Although a lower degree of autotempered martensitic microstructures is beneficial to higher hardness, its negative effects on impact strength should not be underestimated.
• Mn alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability. However, if the Mn content is too high, hardenability of the steel will increase at the expense of impact toughness. Excessive Mn alloying may also lead to C—Mn segregation and formation of MnS, which could induce formation of initiation sites for pitting corrosion and stress corrosion cracking.
• Mn is used in an amount of at least 0.05% to ensure hardenability, but not more than 0.40% to avoid the harmful effects as described above and to ensure excellent mechanical properties such as impact strength and bendability.
• a low level of Mn is used in the range of 0.05% to 0.30% to further improve the bendability.
• Aluminum Al is used in the range of 0.1% or less.
• Al is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process.
• Al removes N by forming stable AlN particles and provides grain refinement, which is beneficial to high toughness.
• Al stabilizes retained austenite.
• an excess of Al may increase non-metallic inclusions thereby deteriorating cleanliness.
• Al is used in the range of 0.08% or less.
• Copper Cu is used in the range of 0.1% to 0.4%.
• Cu is added to the composition to facilitate formation of a protective oxide layer under corrosive climate conditions, which provides good resistance against climatic corrosion and increases the durability of a paint layer that is easily damaged or removed from machines surfaces due to wear.
• Cu may promote formation of low carbon bainitic structures, cause solid solution strengthening and contribute to precipitation strengthening.
• Cu may also have beneficial effects of inhibiting stress corrosion cracking. When added in excessive amounts, Cu deteriorates field weldability and the heat affected zone (HAZ) toughness. Therefore, the upper limit of Cu is set to 0.4%.
• Cu is used in the range of 0.10% to 0.35%.
• Nickel Ni is used in the range of 0.2% to 0.9%.
• Ni is used to avoid quench induced cracking and also to improve toughness and formability.
• Ni is an alloying element that improves austenite hardenability thereby increasing strength with no or marginal loss of impact toughness and/or heat-affected zone (HAZ) toughness.
• HZ heat-affected zone
• Ni also improves surface quality thereby preventing pitting corrosion, i.e. initiation site for stress corrosion cracking.
• Ni is added to the composition to facilitate formation of a protective oxide layer under corrosive climate conditions, which provides good resistance against climatic corrosion and increases the durability of a paint layer that is easily damaged or removed from machines surfaces due to wear.
• nickel contents of above 0.9% would increase alloying costs too much without significant technical improvement.
• An excess of Ni may produce high viscosity iron oxide scales which deteriorate surface quality of the steel product. Higher Ni contents also have negative impacts on weldability due to increased CE value and cracking sensitivity coefficient.
• Ni is preferably used in the range of 0.3% to 0.8%, and more preferably 0.3% to 0.7%.
• Chromium Cr is used in the range of 0.2% to 0.9%.
• Cr is added to the composition to facilitate formation of a protective oxide layer under corrosive climate conditions, which provides good resistance against climatic corrosion and increases the durability of a paint layer that is easily damaged or removed from machines surfaces due to wear.
• Cr alloying provides better resistance against pitting corrosion thereby preventing stress corrosion cracking at an early stage.
• mid-strength carbide forming element Cr increases the strength of both the base steel and weld with marginal expense of impact toughness.
• Cr alloying also enhances strength and hardness by increasing austenite hardenability.
• HZ heat-affected zone
• Cr is used in the range of 0.3% to 0.8%, and more preferably 0.3% to 0.7%.
• Molybdenum Mo is used in the range of 0.2% or less.
• Mo alloying improves impact strength, low-temperature toughness and tempering resistance.
• the presence of Mo enhances strength and hardness by increasing austenite hardenability.
• Mo can be added to the composition to provide hardenability in place of Mn.
• B alloying Mo is usually required to ensure the effectiveness of B.
• Mo is not an economically acceptable alloying element. If Mo is used in an amount of above 0.2% toughness may be deteriorated thereby increasing the risk of brittleness. An excessive amount of Mo may also reduce the effect of B.
• the inventors have noticed that Mo alloying retards recrystallization of austenite thereby increasing the aspect ratio of a prior austenite grain structure. Therefore, the level of Mo content should be carefully controlled to prevent excessive elongation of the prior austenite grains which may deteriorate bendability of the steel product.
• Mo is used in the range of 0.1% or less.
• Niobium Nb is used in an amount of 0.005% or less.
• Nb forms carbides NbC and carbonitrides Nb(C,N).
• Nb is considered to be the major grain refining element.
• Nb contributes to strengthening and toughening of steels.
• Nb addition should be limited to 0.005% since an excess of Nb deteriorates bendability, in particular when direct quenching is applied and/or when Mo is present in the composition.
• Nb can be harmful for heat-affected zone (HAZ) toughness since Nb may promote the formation of coarse upper bainite structure by forming relatively unstable TiNbN or TiNb(C,N) precipitates.
• the level of Nb should be restricted to the lowest possible to increase formability or bendability of the steel product.
• Titanium Ti is used in an amount of 0.035% or less.
• TiC precipitates are able to deeply trap a significant amount of hydrogen H, which decreases the H diffusivity in the materials and removes some of the detrimental H from the microstructure to prevent stress corrosion cracking.
• Ti is also added to bind free N that is harmful to toughness by forming stable TiN that together with NbC can efficiently prevent austenite grain growth in the reheating stage at high temperatures.
• TiN precipitates can further prevent grain coarsening in the heat-affected zone (HAZ) during welding thereby improving toughness.
• HZ heat-affected zone
• TiN formation suppresses BN precipitation, thereby leaving B free to make its contribution to hardenability.
• Ti content is too high, coarsening of TiN and precipitation hardening due to TiC develop and toughness may be deteriorated. Therefore, it is necessary to restrict Ti so that it does not exceed 0.035%.
• Vanadium V is used in an amount of 0.05% or less.
• V has substantially the same but smaller effects as Nb.
• V4C3 precipitates are able to deeply trap a significant amount of hydrogen H, which decreases the H diffusivity in the materials and removes some of the detrimental H from the microstructure to prevent hydrogen induced cracking (HIC).
• H hydrogen induced cracking
• V is a strong carbide and nitride former, but V(C,N) can also form and its solubility in austenite is higher than that of Nb or Ti.
• V alloying has potential for dispersion and precipitation strengthening, because large quantities of V are dissolved and available for precipitation in ferrite.
• an addition of more than 0.05% V has negative effects on weldability, hardenability and alloying cost.
• Boron B is used in the range of 0.0005% to 0.0050%.
• B is a well-established microalloying element to increase hardenability. Boron can be added to retard phosphorus segregation to grain boundaries thereby reducing embrittlement during welding in the heat-affected zone (HAZ). Effective B alloying requires the presence of Ti to prevent formation of BN. In the presence of B, Ti content can be lowered to be less than 0.02%, which is beneficial for toughness. However, hardenability deteriorates if the B content exceeds 0.005%.
• B is used in the range of 0.0008% to 0.0040%.
• Calcium Ca is used in an amount of 0.01% or less.
• Ca addition during a steelmaking process is for refining, deoxidation, desulphurization, and control of shape, size and distribution of oxide and sulphide inclusions.
• Ca is usually added to improve subsequent coating.
• an excessive amount of Ca should be avoided to achieve clean steel thereby preventing the formation of calcium sulfide (CaS) or calcium oxide (CaO) or mixture of these (CaOS) that may deteriorate the mechanical properties such as bendability and stress corrosion cracking (SCC) resistance.
• CaS calcium sulfide
• CaO calcium oxide
• SCC stress corrosion cracking
• Ca is used in an amount of 0.005% or less, and more preferably 0.003% or less to ensure excellent mechanical properties such as impact strength and bendability.
• Unavoidable impurities can be phosphor P, sulfur S and nitrogen N. Their content in terms of weight percentages (wt. %) is preferably defined as follows:
• Austenite to martensite transformation in steels depends largely on the following factors: chemical composition and some processing parameters, mainly reheating temperature, cooling rate and cooling temperature. With regard to chemical composition, some alloying elements have a greater impact than others while others have a negligible impact. Equations describing austenite hardenability may be used to assess the impact of different alloying elements on martensite formation during cooling. One such equation is presented below. From this equation we can see that carbon has the biggest impact, Mn, Mo and Cr have an intermediate impact while Si and Ni have a lesser impact.
• the equation shows that any single element is not crucial for martensite formation and that the absence of one element may be compensated with the amount of other alloying elements and processing parameters, such as e.g. cooling rate.
• the steel product with the targeted mechanical properties is produced in a process that determines a specific microstructure which in turn dictates the mechanical properties of the steel product.
• the first step is to provide a steel slab by means of, for instance a process of continuous casting, also known as strand casting.
• the steel slab is heated to the austenitizing temperature of 1150-1300° C., and thereafter subjected to a temperature equalizing step that may take 30 to 150 minutes.
• the reheating and equalizing steps are important for controlling the austenite grain growth. An increase in the heating temperature can cause dissolution and coarsening of alloy precipitates, which may result in abnormal grain growth.
• the final steel product has a prior austenite grain size of 50 ⁇ m or less, preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less, measured from 1 ⁇ 4 thickness of the steel strip product.
• the slab is hot rolled to the desired thickness at a temperature in the range of Ar 3 to 1250° C., wherein the finish rolling temperature (FRT) is in the range of 800° C. to 960° C., preferably 870° C.-940° C., more preferably 880° C.-930° C.
• FRT finish rolling temperature
• the aspect ratio of a prior austenite grain structure is one of the factors affecting a steel product's impact toughness and bendability.
• the prior austenite grain structure should have an aspect ratio of at least 1.5, preferably at least 2, and more preferably at least 3.
• the prior austenite grain structure should have an aspect ratio of 7 or less, preferably 5 or less, and more preferably 1.5 or less.
• a desired aspect ratio of prior austenite grains can be achieved by adjusting a number of parameters such as finish rolling temperature, strain/deformation, strain rate, and/or alloying with the elements such as Mo that retard recrystallization of austenite.
• the obtained steel product according to the present invention has a prior austenite grain structure with an aspect ratio in the range of 1.5-7, preferably 1.5-5, and more preferably 2-5, which ensures that a good balance of excellent impact toughness and excellent bendability can be achieved.
• the obtained steel strip product has a thickness of 10 mm or less, preferably 8 mm or less.
• the hot-rolled steel strip product is direct quenched to a cooling end and coiling temperature of 450° C. or less, preferably 250° C. or less, more preferably 150° C. or less, and even more preferably 100° C. or less.
• the cooling rate is at least 30° C./s.
• the direct quenched steel strip product is coiled at temperature of 450° C. or less, preferably 250° C. or less, more preferably 150° C. or less, and even more preferably 100° C. or less.
• the obtained steel strip product has a microstructure comprising, in terms of volume percentages (vol. %), at least 90 vol. % martensite, preferably at least 95 vol. % martensite, and more preferably at least 98 vol. % martensite, measured from 1 ⁇ 4 thickness of the steel strip product.
• the martensitic structure may be untempered, autotempered and/or tempered.
• the microstructure comprises 1 vol. % or less retained austenite, and more preferably 0.5 vol. % or less retained austenite.
• the microstructure also comprises bainite, ferrite, pearlite and/or cementite.
• an extra step of temper annealing is performed at a temperature in the range of 150° C.-250° C.
• the steel strip product has a good balance of hardness and other mechanical properties such as excellent impact strength and excellent formability/bendability.
• the steel strip product has a high Brinell hardness in the range of 420-580 HBW, preferably 450-550 HBW, more preferably 460-530 HBW, and even more preferably 470-530 HBW.
• the steel strip product with high hardness has a Charpy-V impact toughness of at least 50 J/cm 2 at a temperature of ⁇ 40° C. thereby fulfilling the conventional impact strength requirements.
• the steel strip product exhibits excellent bendability or formability.
• the steel product has a minimum bending radius (R i ) of 3.2 t or less in a measurement direction longitudinal to the rolling direction wherein the bending axis is longitudinal to rolling direction; a minimum bending radius (R i ) of 2.5 t or less in a measurement direction transversal to the rolling direction wherein the bending axis is transversal to rolling direction; and wherein t is the thickness of the steel strip product.
• Steel types A-C are the inventive compositions according to the present disclosure.
• Steel types D and E are comparative compositions which comprise a relatively high Mn content of 1.20 wt. % and 1.19 wt. % respectively (Table 1).
• Microstructure can be characterized from SEM micrographs and the volume fraction can be determined using point counting or image analysis method.
• the microstructures of the tested inventive examples no. 1-3 all have a main phase of martensite in an amount of at least 90 vol. %.
• the Brinell hardness test is performed by pressing a spherical tungsten carbide ball with a diameter of 10 mm against a clean prepared surface of the steel strip samples with a thickness of 6 mm using a 3000 kilogram force, producing an impression, measured and given a special numerical value.
• a spherical tungsten carbide ball with a diameter of 5 mm and a load of 750 kilogram force are applied for the strip samples with a thickness of 3 mm.
• the measurement is done perpendicular to the upper surface of the steel sheet at 10-15% depth from the steel surface.
• each one of the inventive examples no. 1-3 exhibits a Brinell harness in the range of 467-489 HBW.
• the comparative examples no. 4 exhibits a Brinell harness of 485 HBW while the comparative example no. 5 exhibits a Brinell harness of 502 HBW.
• the impact toughness values at ⁇ 40° C. are obtained by Charpy V-notch tests according to the ISO 148 standard.
• Each one of the inventive examples no. 1-3 has a Charpy-V impact toughness in the range of 78-118 J/cm 2 at a temperature of ⁇ 40° C. if the measurement direction is longitudinal to the rolling direction.
• Each one of the inventive examples no. 1-3 has a Charpy-V impact toughness in the range of 65-90 J/cm 2 at a temperature of ⁇ 40° C. if the measurement direction is transversal to the rolling direction.
• the impact toughness of the inventive examples no. 1-3 is improved compared to the comparative examples no. 4 and 5.
• Elongation was determined according ISO 6892 standard using longitudinal specimens.
• the mean value of total elongation (A80) of the inventive examples no. 1, 2 and 3 is 4.5, 7.6 and 7.7 respectively (Table 3).
• the comparative examples no. 4 and 5 have better elongation values than the inventive examples no. 1-3 at the expense of Charpy-V impact toughness and bendability.
• the bend test consists of subjecting a test piece to plastic deformation by three-point bending, with one single stroke, until a specified angle 90° of the bend is reached after unloading.
• the inspection and assessment of the bends is a continuous process during the whole test series. This is to be able to decide if the punch radius (R) should be increased, maintained or decreased.
• the limit of bendability (R/t) for a material can be identified in a test series if a minimum of 3 m bending length, without any defects, is fulfilled with the same punch radius (R) both longitudinally and transversally. Cracks, surface necking marks and flat bends (significant necking) are registered as defects.
• each one of the inventive examples no. 1-3 has a minimum bending radius (R i ) of 2.8 t or less in a measurement direction longitudinal to the rolling direction; a minimum bending radius (R i ) of 2.0 t or less in a measurement direction transversal to the rolling direction; and wherein t is the thickness of the steel strip product (Table 3).
• the comparative examples no. 4 and 5 exhibit a minimum bending radius (R i ) of 3.7 t and 3.3 t respectively in a measurement direction longitudinal to the rolling direction, and a minimum bending radius (R i ) of 3.0 t and 2.7 t respectively in a measurement direction transversal to the rolling direction (Table 3).
• Yield strength was determined according ISO 6892 standard using longitudinal specimens.
• Each one of the inventive examples no. 1-3 has a mean value of yield strength (R p 0.2) in the range of 1310 MPa to 1413 MPa measured in the longitudinal direction (Table 3).
• the comparative examples no. 4 and 5 have a mean value of yield strength (R p 0.2) of 1375 MPa and 1397 MPa respectively, measured in the longitudinal direction (Table 3).
• Ultimate tensile strength (R m ) was determined according ISO 6892 standard using longitudinal specimens.
• Each one of the inventive examples no. 1-3 has a mean value of ultimate tensile strength (R m ) in the range of 1511 MPa to 1609 MPa, measured in the longitudinal direction (Table 3).
• the comparative examples no. 4 and 5 have a mean value of ultimate tensile strength (R m ) of 1617 MPa and 1654 MPa respectively, measured in the longitudinal direction (Table 3).

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