Hot-rolled steel plate and method of manufacturing the same

Abstract

This invention relates to a steel plate having a composition consisting of, in terms of weight percentages, wt. %: C: 0.3 - 0.5; Si: 0.5 - 1.5; Mn: 0.5 - 1.5; Cr: 1 - 3; Ni: 0.7 - 1.8; Mo: 0.5 - 2.5; V: 0.1 - 0.6; Ti: 0.0005 - 0.05; Cu: less than or equal to 0.4; Nb: 0.01 - 0.09; P: ≤ 0.02; S: ≤ 0.004; remainder Fe and inevitable impurities. The steel plate has impact toughness of at least 5 J at room temperature, and the surface hardness of the steel plate is at least 480 HV.

Description

TECHNICAL FIELD

[0001] The present invention relates to a hot-rolled steel plate that can be used for example in cutting applications such as knives and blades, wear pieces for tooling applications, and cold stamping dies.

[0002] The present invention further relates to a method for manufacturing a hot-rolled steel plate.BACKGROUND OF THE INVENTION

[0003] The present invention relates to a hot-rolled steel plate having an alloying concept especially suitable for manufacturing steel products for various mechanical engineering applications such as blades, cold forming dies and wear piece applications that require high hardness combined with good toughness and machinability.

[0004] High hardness is essential in e.g. cutting applications in order to reduce the need for sharpening and to increase tool lifetime. Currently, the pre-hardened steels for these types of applications typically exhibit hardness levels of 45 HRC. At present, the tooling steel is typically provided in an annealed state to the customer, who then machines and heat treats the final product. This process, however, has some drawbacks, as the extra heat treatments required may cause cracking and distortions on the final product. However, the rapid development in machining technology enables the use of higher hardness steels already in the pre-hardened condition. In the past, such steel have not been available. When the steel is provided in a pre-hardened condition instead of an annealed condition, extra heat treatments can be excluded. This reduces the risk of cracking and distortion during cooling. It also reduces the amount of energy consumed, and thereby contributes to a lower environmental impact. In addition to a high hardness level, tooling steels need to maintain good machinability and exhibit low internal stresses, and such combination has not been available.SUMMARY OF THE INVENTION

[0005] In view of the state of the art, it is a primary object of the invention to provide a hot-rolled steel plate, which is in at least some aspects improved with respect to known such steel plates of that type. In particular, it is an object of the present invention to solve the problem of providing a hot-rolled, pre-hardened steel plate that has increased hardness combined with good impact toughness and at the same time maintaining good machinability.

[0006] The combination of high hardness and good toughness is achieved by a carbon content of 0.3 - 0.5 wt.% combined with a suitable alloying of Mn, Si, Cr, Ni, Mo and V. The carefully tailored chemical composition is then processed by hot-rolling and quenching and tempering. The composition design for this product is aimed to obtain high hardness, achieved via precipitation hardening.

[0007] According to a first aspect of the invention, at least the primary object is achieved by a hot-rolled steel plate according to claim 1. The hot-rolled steel plate has a composition consisting of, in terms of weight percentages, wt. %: C0.3-0.5Si0.5- 1.5Mn0.5- 1.5Cr1-3Ni0.7- 1.8Mo0.5-2.5V0.1 -0.6Ti0.0005 - 0.05Culess than or equal to 0.4Nb0.01 -0.09P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, wherein the impact toughness at room temperature is at least 5 J, measured according to the description, and the surface hardness is at least 480 HV, measured according to the description.

[0008] The design of the present steel is a medium carbon steel with lower alloying concept compared to conventional tooling steels. Such a chemical composition is beneficial because high hardness is achieved via precipitation hardening from V, Nb, Cr, Mo alloying and at the same time the steel maintains a good level of machinability.

[0009] Preferably, the steel plate has a composition consisting of, in terms of wt. %: C0.34 - 0.42Si0.9 - 1.2Mn0.7 - 1Cr1.5 - 2.2Ni1.1 - 1.7Mo1 -2V0.14 - 0.4Ti0.0005 - 0.015Culess than or equal to 0.4Nb0.017 - 0.05P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, wherein the impact toughness at room temperature is at least 5 J, measured according to the description, and the surface hardness is at least 480 HV, measured according to the description.

[0010] Preferably, the steel plate has a surface hardness ranging from 480 HV to 700 HV, preferably ranging from 480 HV to 670 HV, and more preferably 510 HV to 650 HV. For tooling steels, high hardness levels are preferred as they enable longer service life.

[0011] Preferably, the steel plate has a microstructure comprising mainly of tempered martensite, preferably more than 90 % of tempered martensite and more preferably more than 95 % of tempered martensite. Tempered martensite is a beneficial microstructure for a tooling steel as it exhibits high hardness combined with better toughness and lower internal stresses, compared to fresh martensite.

[0012] Preferably, the steel plate has a grain size in the range of 15-150 µm. A sufficiently small grain size helps to improve both strength and toughness.

[0013] Preferably, the steel plate has a thickness in the range of 10-130 mm.

[0014] According to a third aspect of the invention, the hot-rolled steel plate is used in manufacturing of knives, wear pieces, dies or blades. This illustrates that the present steel can be used to manufacture various types of goods.

[0015] According to a third aspect of the invention, the hot-rolled steel plate according to the invention is manufactured using a method comprising the following steps of - providing a steel slab having the following composition C0.3 -0.5Si0.5- 1.5Mn0.5- 1.5Cr1-3Ni0.7- 1.8Mo0.5-2.5V0.1 -0.6Ti0.0005 - 0.05Culess than or equal to 0.4Nb0.01 -0.09P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, and - preheating the slab to a temperature in the range of 950 °C - 1200 °C; - subjecting the preheated steel slab to a plurality of hot rolling passes, above, at or below recrystallization temperature in order to form a steel plate wherein the final rolling temperature is 850 - 1150 °C, and - air cooling the steel plate to room temperature, and - heating the steel plate to a temperature range of 850 - 1000 °C, preferably 875 - 975 °C, for a time period of 15 - 250 min, and - water quenching the heated steel plate continuously to room temperature with a cooling rate of at least 5 °C / s, - tempering the water quenched steel plate from the previous step at a temperature range of 350 - 700 °C for a time period of 20 - 200 min, and - air cooling the steel plate from previous step to room temperature.

[0016] With this type of processing of the present steel, a pre-hardened steel plate, with at least 480 HV hardness, can be produced.

[0017] Preferably, the manufacturing method comprises a step of reheating the steel plate, after the last air cooling step, to a temperature range of 350 - 700 °C for a time period of 20 - 200 min, followed by air cooling to room temperature. This processing step is sometimes referred to as double-tempering and it may be used to adjust the final properties of the steel plate.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 shows a box diagram of a manufacturing method for the steel plate.DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0019] The term "steel" is defined as an iron alloy containing carbon (C).

[0020] The term "ultimate tensile strength" (UTS, Rm) refers to the limit, at which the steel fractures under tension, thus the maximum tensile stress.

[0021] The term "yield strength" (YS, Rp0.2) refers to 0.2 % offset yield strength defined as the amount of stress that will result in a plastic strain of 0.2 %.

[0022] The term "Q&T" (quench and tempered) is a heat treatment wherein a steel is first fully austenitized, followed by quenching down to room temperature, and then heated to a tempering temperature, after which the steel is typically cooled down to room temperature in air. The quenching step may be conducted using several quenching media (e.g. air, water, water-polymer solution, oil).

[0023] The term "CR" (cooling rate) in this application refers to a cooling rate during the quenching step of a Q&T heat treatment.

[0024] The alloying content of steel together with the processing parameters determines the microstructure, which in turn determines the mechanical properties of the steel.

[0025] The alloying strategy needs to be lean enough to reduce unnecessary costs and at the same time carefully tailored to provide sufficient hardness, mainly through precipitation strengthening. Hence, alloy design is one of the first issues to be considered when developing a steel product with targeted mechanical properties.

[0026] Next, the chemical composition according to the present invention is described in more detail, wherein % of each element refers to weight percentage.Carbon C is used in the range of 0.3 - 0.5%

[0027] C is a critical element for achieving sufficient strength and hardness level in steel as it contributes to various strengthening mechanisms like phase transformation products, solid solution strengthening and precipitation strengthening. Insufficient C alloying would lead to lack of hardness and hardenability, especially in thick gauges.

[0028] On the other hand, excessive C alloying reduces the impact toughness and increases the brittleness and crack susceptibility of steel by promoting transformation products rich in brittle microconstituents.

[0029] Preferably, C is used in the range of 0.34 - 0.42%.Silicon Si is used in the range of 0.5 - 1.5%

[0030] 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 improves machinability. Si contributes to better surface quality and Si alloyed steels are easier to polish. Without sufficient Si alloying these positive effects are not achieved.

[0031] However, an excess Si alloying can stabilize undesired ferrite phases and also prevent carbide formation (i.e. cementite).

[0032] Preferably, Si is used in the range of 0.9 - 1.2 %.Manganese Mn is used in the range of 0.5 - 1.5 %

[0033] Mn is an essential alloying element for improving both strength and hardenability. There is a rough relation between higher Mn content and higher strength level of the steel. Mn alloying enhances strength by solid solution strengthening and increases hardenability by prolonging phase transformation start times. Therefore, Mn is alloyed in an amount of at least 0.5 %.

[0034] However, elevated Mn alloying may potentially result in flame cutting problems. In addition, an excess of Mn could lead to segregation issues. Furthermore, it is unlikely that higher Mn alloying would help to improve the properties of the present steel.

[0035] Preferably, Mn is used in the range of 0.7 - 1 %.Chromium Cr is used in the range of 1 - 3 %

[0036] Cr alloying improves the strength by solid solution strengthening, increases the hardenability and decreases the martensite transformation start (Ms) temperature. Cr is a carbide forming element which results in precipitation hardening as well. Alloying with a lower amount of Cr would lead to insufficient amount of precipitation hardening.

[0037] However, if Cr is alloyed in excess, carbide size could grow too large thus leading to a decrease in hardness.

[0038] Preferably, Cr is used in the range of 1.5 - 2.2 %.Nickel Ni is used in the range of 0.7 - 1.8 %

[0039] Ni is an alloying element that improves strength, hardenability and low temperature toughness.

[0040] If the steel has high amount of Cu, then Ni alloying is needed in order to prevent surface defects from arising during hot rolling, especially when high temperatures (usually above 1100 °C) are used at the start of hot-rolling. As a general rule, a Ni content of at least 30 % of the Cu content is needed to prevent the defects, and preferably even more. Ni alloying may be needed when the Cu content is more than 0.2 %.

[0041] However, Ni contents of above 1.8 % would increase alloying costs too much without significant technical improvement. An excess of Ni may also produce high viscosity iron oxide scales, which deteriorate surface quality of the steel product.

[0042] Ni is preferably used in the range of 1.1 - 1.6 %.Molybdenum Mo is used in the range of 0.5 - 2.5 %

[0043] Mo improves hardenability and contributes to the strength of the steel via solid solution strengthening and precipitation hardening. Mo alloying may also provide tempering resistance. Insufficient Mo alloying may not provide the required level of precipitation hardening thus at least 0.5 % of Mo alloying is needed.

[0044] Excess Mo alloying may, however, increase strength unnecessarily. Mo also has a tendency for segregation, and excess Mo alloying may result in increased Mo segregation. Furthermore, Mo is an expensive alloying element, and to reduce cost excess alloying is not desired. In addition, more than 2.5 % of Mo alloying may also increase crack susceptibility.

[0045] Preferably, Mo is used in the range of 1 - 2 %.Vanadium V is used in the range of 0.1 - 0.6 %

[0046] V contributes to increased strength through precipitation strengthening during tempering. V is a strong carbide and nitride former, and V(C,N) (vanadium carbonitrides) may also form and the solubility of V in austenite is higher than that of Nb or Ti. Thus, V alloying has potential for precipitation strengthening, because large quantities of V are dissolved during austenitization and available for precipitation during tempering. The resulting V precipitates are very small in size, often referred to as nanoprecipitation, which aids reaching the target hardness for this grade.

[0047] V levels of more than 0.1 % is required to provide sufficient precipitation strengthening for increased tempering resistance. However, an addition exceeding 0.6 % V may have substantial negative effects on, e.g. toughness.

[0048] Preferably, V is used in the range of 0.14 - 0.4 %.Titanium Ti is used in a range of 0.0005 - 0.05 %

[0049] Ti may be added to bind free N that is harmful to toughness via formation of stable titanium nitrides (TiN). Furthermore, the TiN together with niobium carbides (NbC) can efficiently prevent austenite grain growth in the reheating stage at high temperatures.

[0050] However, if Ti content is too high, coarsening of TiN and precipitation hardening due to TiC formation may occur and thereby lead to deteriorated toughness properties. Therefore, it is necessary to restrict Ti to an amount of ≤ 0.05 %, preferably to an amount of ≤ 0.015 %.

[0051] However, Ti alloying is not as important as other alloying elements, such as Nb and V, when it comes to precipitation hardening.

[0052] Preferably, if Ti is alloyed in low amounts within the specified range, Nb alloying may be increased. In such a case, Nb alloying should be at least 0.02 % to compensate for the absence of Ti.Copper Cu is used in an amount of less than or equal to 0.4 %

[0053] Cu is not intentionally alloyed to the composition. However, when using scrap-based metallurgy, Cu may appear in the composition.

[0054] If Cu is present in excessive amounts, it may cause problems in the continuous casting and, for example, cause selective oxidation.

[0055] Therefore, the upper limit for Cu is set to 0.4 %.Niobium Nb is used in the range of 0.01 - 0.09 %

[0056] Nb forms niobium carbides (NbC) and niobium carbonitrides (Nb(C,N)). Nb is considered to be a major austenite grain refining element during re-austenitizing heat treatment due to the pinning effect of Nb(C,N) during the reheating and soaking stage at high temperatures by introducing fine Nb(C,N) precipitates. Nb may also contribute to increased strength through precipitation strengthening during tempering. Nb may also contribute to solid solution strengthening when it is dissolved in the matrix.

[0057] Yet, preferably, Nb addition should be limited to 0.09 % since further increase in Nb content does not have a pronounced effect on further increasing the strength and toughness.

[0058] Preferably, Nb is used in the range of 0.017 - 0.05 %, and more preferably in the range of 0.015 - 0.05 %.

[0059] The remainder of the composition consists of iron (Fe) and unavoidable impurities.

[0060] The unavoidable impurities may comprise phosphor P, sulfur S, and nitrogen N. Their contents in terms of weight percentages are preferably defined as follows: P ≤ 0.035 %, preferably ≤ 0.015 %, more preferably ≤ 0.01 %, S ≤ 0.025 %, preferably ≤ 0.01 %, more preferably ≤ 0.003 %, N < 0.02 %, preferably N < 0.01 %, more preferably N < 0.006 %.

[0061] The contents of the unavoidable impurities are limited in order to ensure excellent mechanical properties, such as impact toughness.

[0062] The total content of unavoidable impurities may advantageously be controlled to be equal to or less than 0.2 %. Preferably, unavoidable impurities may be present in an amount of equal to or less than 0.1 % in total. It should here be noted that the content of copper (which is an element often considered to constitute an impurity) is not considered to be included in the total content of unavoidable impurities.

[0063] The steel cording to the invention is designed as a medium carbon steel in the lower carbon range with a relatively low alloy content. It is a pre-hardened, heat treated steel plate which can be used in knives, wear pieces, dies or blade applications. The benefits of the "ready to use" steel plate are several compared to machining soft steels and performing extra heat treatment(s). The benefits include shorter lead time, lower cost, and removed need for further heat treatment. This will also contribute to decreased environmental impact.Manufacturing of a hot rolled steel plate

[0064] Clean steelmaking practice may be applied to minimize unavoidable impurities that may appear as non-metallic inclusions. Clean steelmaking practices commonly include e.g. ladle treatments, such as vacuum degassing, and careful control of the continuous casting process. Non-metallic inclusions disrupt the homogeneity of structure, so their influence on the mechanical and other properties can be considerable.

[0065] The inventive hot-rolled steel plate has a typical thickness of 10 to 130 mm, preferably 8 to 65 mm and more preferably 10 to 55 mm.

[0066] A method for manufacturing the hot-rolled steel product disclosed herein will now be described with reference to Figure 1.

[0067] Figure 1 shows a box diagram of a manufacturing method for the steel plate. In a first step of the method, denoted as 101 in Figure 1, a steel slab with the above defined chemical composition is preheated to a target temperature in the range of 950 °C - 1250 °C, preferably 1000 °C - 1200 °C and more preferably 1100 °C - 1200 °C, for a period of 30 min to 10 hours, preferably 2 hours to 6 hours.

[0068] In a second step, denoted as 102 of Figure 1, the heated steel slab is hot rolled in a plurality of hot rolling passes in order to form a steel plate.

[0069] The plurality of hot-rolling passes are carried out at a temperature above, at, or below the austenite non-recrystallization temperature (Tnr).

[0070] After the plurality of hot-rolling passes, the hot rolled steel plate is cooled in a cooling step, denoted as 103 in Figure 1, preferably in air, down to room temperature such that the required microstructure is achieved. In another embodiment the hot rolled steel plate from step 102 is subjected to accelerated continuous cooling down to room temperature.

[0071] In the fourth and fifth step, denoted as 104 and 105 in Figure 1, the hot-rolled and cooled steel plate is austenitized, quenched and tempered, and cooled to room temperature.

[0072] In the heating stage of step 101 the slabs are heated to a discharging temperature in the range of 950 °C - 1250 °C, preferably 1000 °C - 1200 °C and more preferably 1050 °C - 1200 °C, for a period of 30 min to 10 hours, preferably 2 hours to 6 hours. Higher temperatures enable better Nb dissolution into the austenite.

[0073] In the hot rolling stage of step 102, the slab is hot rolled with a typical pass schedule of 6 - 20 hot rolling passes, for example 10 - 18 passes, depending on the thickness of the slab and the final product in order to form a steel plate. Preferably, the amount of rolling passes is kept as low as possible to ensure high reduction of a single rolling pass.

[0074] The hot rolling steps may be carried out above the austenite non-recrystallization temperature, at the Tnr temperature, or below the Tnr temperature. Typically, at the start of rolling, the rolling temperature is above Tnr and at the end of hot-rolling, the rolling temperature may be below Tnr. The rolling temperature at the end of hot-rolling may, however, also be above Tnr. Typically, the passes start at a temperature above the non-recrystallization temperature and depending on the thickness of the plate it can fall below the non-recrystalization temperature. This means that the final rolling temperature can be between 850 and 1150 C.

[0075] The hot-rolled steel plate in the cooling step 103 is cooled to room temperature. Preferably cooling is conducted as air cooling.

[0076] The fourth step 104 of the process is hardening wherein the cooled hot-rolled steel plate is austenitized at a temperature range 850 - 1000 °C, preferably 875 - 975 °C, for a time period of 15 - 250 min in order to create a fully austenitic microstructure. This is followed by water quenching with a cooling rate of at least 5 °C / s to room temperature for obtaining a martensitic microstructure.

[0077] The fifth step 105 of the process is tempering, which is performed to reduce the internal stresses and brittleness of the steel. Preferably, tempering is performed at a temperature in the range of 350 °C - 700 °C for 20 - 200 min. However, the tempering treatment parameters may be different as well.

[0078] Optionally, a sixth step, denoted as 106 in Figure 1, of heat treatment is double tempering, which in some cases is performed to adjust the final properties, such as impact toughness. Preferably, tempering is performed at a temperature in the range of 350 °C - 700 °C for 20 - 200 min. However, the double-tempering treatment parameters may be different as well.

[0079] Tempered martensite is the target microstructure of the inventive steel plate with distribution of precipitates such as Mo, Cr, V and Nb-precipitates. Such a microstructure in the present steel enables high hardness while maintaining good machinability and toughness.

[0080] The following examples further describe and demonstrate embodiments within the scope of the present invention. The examples are given solely for the purpose of illustration and are not to be construed as limitations of the present invention, as many variations thereof are possible without departing from the scope of the invention.Examples

[0081] Examples from the inventive steels A and B were prepared with chemical compositions according to Table 1. During the preparation, both alloys were melted in lab-scale, cast and hot-rolled to 14 mm thickness. Both example steels were then subsequently Q&T processed. The specific processing parameters are shown in Tables 2 and 3. The resulting mechanical properties are shown in Tables 4 and 5.

[0082] The chemical compositions of the inventive examples differ in carbon, nickel, vanadium and niobium content. Alloy 2 has increased carbon, vanadium and niobium contents and lower nickel content compared to Alloy 1. This is mainly because vanadium and niobium combined with increased carbon can provide more precipitation hardening. Increased carbon content also increases martensite hardness.Processing and mechanical properties of the Q&T heat treated steel

[0083] Table 2 lists hot-rolling and Q&T heat treatment parameters for the inventive steel examples. Table 3 shows an example of a hot-rolling pass schedule for the inventive samples, and Tables 4 and 5 show the resulting mechanical properties and the effects of various Q&T heat treatments on the mechanical properties of the inventive examples. All the samples were processed similarly with the biggest difference being in tempering temperature and time. All the specimens were made in lab-scale. Austenitization and tempering heat treatments were carried out in air laboratory furnaces.

[0084] The hardness tests of the Q&T heat treated specimens were measured with a Q10 Vickers microhardness indenter (HV1kg) using a loading time of 7 seconds. All tests were carried out at room temperature according to ISO 6507-1:2018, standard for Vickers hardness testing, and the average of five hardness tests and the average of three impact toughness tests is reported in Tables 4 and 5.

[0085] The Charpy V impact toughness values for the QT condition were measured according to ASTM E23 standard. The results show an average value from three test pieces with 10x10x55mm dimensions, which were cut perpendicular to the rolling direction.

[0086] Table 4 shows the hardness and impact toughness values for the inventive examples after single tempering at 600 °C for 30 min and the hardness after double tempering (tempering twice at 600 °C for 30 min, cooling in air). Table 4 shows that for both steels, the impact toughness values are on a good level, especially when combined with such a high hardness level. Alloy 2 exhibited better impact toughness values compared to Alloy 1 while maintaining the same hardness level. After the second tempering, the hardness of the studied steels decreased roughly 30 - 40 HV1 to 520 HV1. This is still a good hardness level for certain applications.

[0087] Table 5 shows hardness and impact toughness values for the inventive examples after single tempering at 550 °C for 60 min. The test results show that, in this way, hardness may be increased while maintaining a good level of impact toughness.

[0088] It is to be understood that the present invention is not limited to the embodiments described above; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. Table 1SteelCSiMnPSCrNiMoVTiNbRemarkA0.371.010.74--1.951.661.430.130.010.018Inv.B0.401.070.77--1.941.161.430.180.010.031Inv. Table 2 SteelHeating T (°C)FRT (°C)Cooling method to RTAustenitization T(°C)Austenitization t (min)Cooling method to RTTempering T (°C)Tempering t (min)Cooling method to RTRemarkA1200>950air92013WQ55060airInv.6003030+30B1200>950air92013WQ55060airInv.6003030+30 Table 3 Pass, no.Temperature (°C)Height of the slab (mm)Change in height of the slab (mm)Reduction per pass120060150100.17236140.28326100.2841880.315>950°C1440.22 Table 4 SteelEnergy (J)Energy (J / cm 2< )Hardness (HV1)Hardness, double tempered (HV1)A13.416.8548520B15.519.4559520 Table 5 SteelEnergy (J)Energy (J / cm 2< )Hardness (HV1)A1215595B9.211.5611

Examples

examples

Examples

[0081]Examples from the inventive steels A and B were prepared with chemical compositions according to Table 1. During the preparation, both alloys were melted in lab-scale, cast and hot-rolled to 14 mm thickness. Both example steels were then subsequently Q&T processed. The specific processing parameters are shown in Tables 2 and 3. The resulting mechanical properties are shown in Tables 4 and 5.

[0082]The chemical compositions of the inventive examples differ in carbon, nickel, vanadium and niobium content. Alloy 2 has increased carbon, vanadium and niobium contents and lower nickel content compared to Alloy 1. This is mainly because vanadium and niobium combined with increased carbon can provide more precipitation hardening. Increased carbon content also increases martensite hardness.

Processing and mechanical properties of the Q&T heat treated steel

[0083]Table 2 lists hot-rolling and Q&T heat treatment parameters for the inventive steel examples. Table 3 shows an example o...

TECHNICAL FIELD

[0001] The present invention relates to a hot-rolled steel plate that can be used for example in cutting applications such as knives and blades, wear pieces for tooling applications, and cold stamping dies.

[0002] The present invention further relates to a method for manufacturing a hot-rolled steel plate.BACKGROUND OF THE INVENTION

[0003] The present invention relates to a hot-rolled steel plate having an alloying concept especially suitable for manufacturing steel products for various mechanical engineering applications such as blades, cold forming dies and wear piece applications that require high hardness combined with good toughness and machinability.

[0004] High hardness is essential in e.g. cutting applications in order to reduce the need for sharpening and to increase tool lifetime. Currently, the pre-hardened steels for these types of applications typically exhibit hardness levels of 45 HRC. At present, the tooling steel is typically provided in an annealed state to the customer, who then machines and heat treats the final product. This process, however, has some drawbacks, as the extra heat treatments required may cause cracking and distortions on the final product. However, the rapid development in machining technology enables the use of higher hardness steels already in the pre-hardened condition. In the past, such steel have not been available. When the steel is provided in a pre-hardened condition instead of an annealed condition, extra heat treatments can be excluded. This reduces the risk of cracking and distortion during cooling. It also reduces the amount of energy consumed, and thereby contributes to a lower environmental impact. In addition to a high hardness level, tooling steels need to maintain good machinability and exhibit low internal stresses, and such combination has not been available.SUMMARY OF THE INVENTION

[0005] In view of the state of the art, it is a primary object of the invention to provide a hot-rolled steel plate, which is in at least some aspects improved with respect to known such steel plates of that type. In particular, it is an object of the present invention to solve the problem of providing a hot-rolled, pre-hardened steel plate that has increased hardness combined with good impact toughness and at the same time maintaining good machinability.

[0006] The combination of high hardness and good toughness is achieved by a carbon content of 0.3 - 0.5 wt.% combined with a suitable alloying of Mn, Si, Cr, Ni, Mo and V. The carefully tailored chemical composition is then processed by hot-rolling and quenching and tempering. The composition design for this product is aimed to obtain high hardness, achieved via precipitation hardening.

[0007] According to a first aspect of the invention, at least the primary object is achieved by a hot-rolled steel plate according to claim 1. The hot-rolled steel plate has a composition consisting of, in terms of weight percentages, wt. %: C0.3-0.5Si0.5- 1.5Mn0.5- 1.5Cr1-3Ni0.7- 1.8Mo0.5-2.5V0.1 -0.6Ti0.0005 - 0.05Culess than or equal to 0.4Nb0.01 -0.09P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, wherein the impact toughness at room temperature is at least 5 J, measured according to the description, and the surface hardness is at least 480 HV, measured according to the description.

[0008] The design of the present steel is a medium carbon steel with lower alloying concept compared to conventional tooling steels. Such a chemical composition is beneficial because high hardness is achieved via precipitation hardening from V, Nb, Cr, Mo alloying and at the same time the steel maintains a good level of machinability.

[0009] Preferably, the steel plate has a composition consisting of, in terms of wt. %: C0.34 - 0.42Si0.9 - 1.2Mn0.7 - 1Cr1.5 - 2.2Ni1.1 - 1.7Mo1 -2V0.14 - 0.4Ti0.0005 - 0.015Culess than or equal to 0.4Nb0.017 - 0.05P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, wherein the impact toughness at room temperature is at least 5 J, measured according to the description, and the surface hardness is at least 480 HV, measured according to the description.

[0010] Preferably, the steel plate has a surface hardness ranging from 480 HV to 700 HV, preferably ranging from 480 HV to 670 HV, and more preferably 510 HV to 650 HV. For tooling steels, high hardness levels are preferred as they enable longer service life.

[0011] Preferably, the steel plate has a microstructure comprising mainly of tempered martensite, preferably more than 90 % of tempered martensite and more preferably more than 95 % of tempered martensite. Tempered martensite is a beneficial microstructure for a tooling steel as it exhibits high hardness combined with better toughness and lower internal stresses, compared to fresh martensite.

[0012] Preferably, the steel plate has a grain size in the range of 15-150 µm. A sufficiently small grain size helps to improve both strength and toughness.

[0013] Preferably, the steel plate has a thickness in the range of 10-130 mm.

[0014] According to a third aspect of the invention, the hot-rolled steel plate is used in manufacturing of knives, wear pieces, dies or blades. This illustrates that the present steel can be used to manufacture various types of goods.

[0015] According to a third aspect of the invention, the hot-rolled steel plate according to the invention is manufactured using a method comprising the following steps of - providing a steel slab having the following composition C0.3 -0.5Si0.5- 1.5Mn0.5- 1.5Cr1-3Ni0.7- 1.8Mo0.5-2.5V0.1 -0.6Ti0.0005 - 0.05Culess than or equal to 0.4Nb0.01 -0.09P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, and - preheating the slab to a temperature in the range of 950 °C - 1200 °C; - subjecting the preheated steel slab to a plurality of hot rolling passes, above, at or below recrystallization temperature in order to form a steel plate wherein the final rolling temperature is 850 - 1150 °C, and - air cooling the steel plate to room temperature, and - heating the steel plate to a temperature range of 850 - 1000 °C, preferably 875 - 975 °C, for a time period of 15 - 250 min, and - water quenching the heated steel plate continuously to room temperature with a cooling rate of at least 5 °C / s, - tempering the water quenched steel plate from the previous step at a temperature range of 350 - 700 °C for a time period of 20 - 200 min, and - air cooling the steel plate from previous step to room temperature.

[0016] With this type of processing of the present steel, a pre-hardened steel plate, with at least 480 HV hardness, can be produced.

[0017] Preferably, the manufacturing method comprises a step of reheating the steel plate, after the last air cooling step, to a temperature range of 350 - 700 °C for a time period of 20 - 200 min, followed by air cooling to room temperature. This processing step is sometimes referred to as double-tempering and it may be used to adjust the final properties of the steel plate.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 shows a box diagram of a manufacturing method for the steel plate.DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0019] The term "steel" is defined as an iron alloy containing carbon (C).

[0020] The term "ultimate tensile strength" (UTS, Rm) refers to the limit, at which the steel fractures under tension, thus the maximum tensile stress.

[0021] The term "yield strength" (YS, Rp0.2) refers to 0.2 % offset yield strength defined as the amount of stress that will result in a plastic strain of 0.2 %.

[0022] The term "Q&T" (quench and tempered) is a heat treatment wherein a steel is first fully austenitized, followed by quenching down to room temperature, and then heated to a tempering temperature, after which the steel is typically cooled down to room temperature in air. The quenching step may be conducted using several quenching media (e.g. air, water, water-polymer solution, oil).

[0023] The term "CR" (cooling rate) in this application refers to a cooling rate during the quenching step of a Q&T heat treatment.

[0024] The alloying content of steel together with the processing parameters determines the microstructure, which in turn determines the mechanical properties of the steel.

[0025] The alloying strategy needs to be lean enough to reduce unnecessary costs and at the same time carefully tailored to provide sufficient hardness, mainly through precipitation strengthening. Hence, alloy design is one of the first issues to be considered when developing a steel product with targeted mechanical properties.

[0026] Next, the chemical composition according to the present invention is described in more detail, wherein % of each element refers to weight percentage.Carbon C is used in the range of 0.3 - 0.5%

[0027] C is a critical element for achieving sufficient strength and hardness level in steel as it contributes to various strengthening mechanisms like phase transformation products, solid solution strengthening and precipitation strengthening. Insufficient C alloying would lead to lack of hardness and hardenability, especially in thick gauges.

[0028] On the other hand, excessive C alloying reduces the impact toughness and increases the brittleness and crack susceptibility of steel by promoting transformation products rich in brittle microconstituents.

[0029] Preferably, C is used in the range of 0.34 - 0.42%.Silicon Si is used in the range of 0.5 - 1.5%

[0030] 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 improves machinability. Si contributes to better surface quality and Si alloyed steels are easier to polish. Without sufficient Si alloying these positive effects are not achieved.

[0031] However, an excess Si alloying can stabilize undesired ferrite phases and also prevent carbide formation (i.e. cementite).

[0032] Preferably, Si is used in the range of 0.9 - 1.2 %.Manganese Mn is used in the range of 0.5 - 1.5 %

[0033] Mn is an essential alloying element for improving both strength and hardenability. There is a rough relation between higher Mn content and higher strength level of the steel. Mn alloying enhances strength by solid solution strengthening and increases hardenability by prolonging phase transformation start times. Therefore, Mn is alloyed in an amount of at least 0.5 %.

[0034] However, elevated Mn alloying may potentially result in flame cutting problems. In addition, an excess of Mn could lead to segregation issues. Furthermore, it is unlikely that higher Mn alloying would help to improve the properties of the present steel.

[0035] Preferably, Mn is used in the range of 0.7 - 1 %.Chromium Cr is used in the range of 1 - 3 %

[0036] Cr alloying improves the strength by solid solution strengthening, increases the hardenability and decreases the martensite transformation start (Ms) temperature. Cr is a carbide forming element which results in precipitation hardening as well. Alloying with a lower amount of Cr would lead to insufficient amount of precipitation hardening.

[0037] However, if Cr is alloyed in excess, carbide size could grow too large thus leading to a decrease in hardness.

[0038] Preferably, Cr is used in the range of 1.5 - 2.2 %.Nickel Ni is used in the range of 0.7 - 1.8 %

[0039] Ni is an alloying element that improves strength, hardenability and low temperature toughness.

[0040] If the steel has high amount of Cu, then Ni alloying is needed in order to prevent surface defects from arising during hot rolling, especially when high temperatures (usually above 1100 °C) are used at the start of hot-rolling. As a general rule, a Ni content of at least 30 % of the Cu content is needed to prevent the defects, and preferably even more. Ni alloying may be needed when the Cu content is more than 0.2 %.

[0041] However, Ni contents of above 1.8 % would increase alloying costs too much without significant technical improvement. An excess of Ni may also produce high viscosity iron oxide scales, which deteriorate surface quality of the steel product.

[0042] Ni is preferably used in the range of 1.1 - 1.6 %.Molybdenum Mo is used in the range of 0.5 - 2.5 %

[0043] Mo improves hardenability and contributes to the strength of the steel via solid solution strengthening and precipitation hardening. Mo alloying may also provide tempering resistance. Insufficient Mo alloying may not provide the required level of precipitation hardening thus at least 0.5 % of Mo alloying is needed.

[0044] Excess Mo alloying may, however, increase strength unnecessarily. Mo also has a tendency for segregation, and excess Mo alloying may result in increased Mo segregation. Furthermore, Mo is an expensive alloying element, and to reduce cost excess alloying is not desired. In addition, more than 2.5 % of Mo alloying may also increase crack susceptibility.

[0045] Preferably, Mo is used in the range of 1 - 2 %.Vanadium V is used in the range of 0.1 - 0.6 %

[0046] V contributes to increased strength through precipitation strengthening during tempering. V is a strong carbide and nitride former, and V(C,N) (vanadium carbonitrides) may also form and the solubility of V in austenite is higher than that of Nb or Ti. Thus, V alloying has potential for precipitation strengthening, because large quantities of V are dissolved during austenitization and available for precipitation during tempering. The resulting V precipitates are very small in size, often referred to as nanoprecipitation, which aids reaching the target hardness for this grade.

[0047] V levels of more than 0.1 % is required to provide sufficient precipitation strengthening for increased tempering resistance. However, an addition exceeding 0.6 % V may have substantial negative effects on, e.g. toughness.

[0048] Preferably, V is used in the range of 0.14 - 0.4 %.Titanium Ti is used in a range of 0.0005 - 0.05 %

[0049] Ti may be added to bind free N that is harmful to toughness via formation of stable titanium nitrides (TiN). Furthermore, the TiN together with niobium carbides (NbC) can efficiently prevent austenite grain growth in the reheating stage at high temperatures.

[0050] However, if Ti content is too high, coarsening of TiN and precipitation hardening due to TiC formation may occur and thereby lead to deteriorated toughness properties. Therefore, it is necessary to restrict Ti to an amount of ≤ 0.05 %, preferably to an amount of ≤ 0.015 %.

[0051] However, Ti alloying is not as important as other alloying elements, such as Nb and V, when it comes to precipitation hardening.

[0052] Preferably, if Ti is alloyed in low amounts within the specified range, Nb alloying may be increased. In such a case, Nb alloying should be at least 0.02 % to compensate for the absence of Ti.Copper Cu is used in an amount of less than or equal to 0.4 %

[0053] Cu is not intentionally alloyed to the composition. However, when using scrap-based metallurgy, Cu may appear in the composition.

[0054] If Cu is present in excessive amounts, it may cause problems in the continuous casting and, for example, cause selective oxidation.

[0055] Therefore, the upper limit for Cu is set to 0.4 %.Niobium Nb is used in the range of 0.01 - 0.09 %

[0056] Nb forms niobium carbides (NbC) and niobium carbonitrides (Nb(C,N)). Nb is considered to be a major austenite grain refining element during re-austenitizing heat treatment due to the pinning effect of Nb(C,N) during the reheating and soaking stage at high temperatures by introducing fine Nb(C,N) precipitates. Nb may also contribute to increased strength through precipitation strengthening during tempering. Nb may also contribute to solid solution strengthening when it is dissolved in the matrix.

[0057] Yet, preferably, Nb addition should be limited to 0.09 % since further increase in Nb content does not have a pronounced effect on further increasing the strength and toughness.

[0058] Preferably, Nb is used in the range of 0.017 - 0.05 %, and more preferably in the range of 0.015 - 0.05 %.

[0059] The remainder of the composition consists of iron (Fe) and unavoidable impurities.

[0060] The unavoidable impurities may comprise phosphor P, sulfur S, and nitrogen N. Their contents in terms of weight percentages are preferably defined as follows: P ≤ 0.035 %, preferably ≤ 0.015 %, more preferably ≤ 0.01 %, S ≤ 0.025 %, preferably ≤ 0.01 %, more preferably ≤ 0.003 %, N < 0.02 %, preferably N < 0.01 %, more preferably N < 0.006 %.

[0061] The contents of the unavoidable impurities are limited in order to ensure excellent mechanical properties, such as impact toughness.

[0062] The total content of unavoidable impurities may advantageously be controlled to be equal to or less than 0.2 %. Preferably, unavoidable impurities may be present in an amount of equal to or less than 0.1 % in total. It should here be noted that the content of copper (which is an element often considered to constitute an impurity) is not considered to be included in the total content of unavoidable impurities.

[0063] The steel cording to the invention is designed as a medium carbon steel in the lower carbon range with a relatively low alloy content. It is a pre-hardened, heat treated steel plate which can be used in knives, wear pieces, dies or blade applications. The benefits of the "ready to use" steel plate are several compared to machining soft steels and performing extra heat treatment(s). The benefits include shorter lead time, lower cost, and removed need for further heat treatment. This will also contribute to decreased environmental impact.Manufacturing of a hot rolled steel plate

[0064] Clean steelmaking practice may be applied to minimize unavoidable impurities that may appear as non-metallic inclusions. Clean steelmaking practices commonly include e.g. ladle treatments, such as vacuum degassing, and careful control of the continuous casting process. Non-metallic inclusions disrupt the homogeneity of structure, so their influence on the mechanical and other properties can be considerable.

[0065] The inventive hot-rolled steel plate has a typical thickness of 10 to 130 mm, preferably 8 to 65 mm and more preferably 10 to 55 mm.

[0066] A method for manufacturing the hot-rolled steel product disclosed herein will now be described with reference to Figure 1.

[0067] Figure 1 shows a box diagram of a manufacturing method for the steel plate. In a first step of the method, denoted as 101 in Figure 1, a steel slab with the above defined chemical composition is preheated to a target temperature in the range of 950 °C - 1250 °C, preferably 1000 °C - 1200 °C and more preferably 1100 °C - 1200 °C, for a period of 30 min to 10 hours, preferably 2 hours to 6 hours.

[0068] In a second step, denoted as 102 of Figure 1, the heated steel slab is hot rolled in a plurality of hot rolling passes in order to form a steel plate.

[0069] The plurality of hot-rolling passes are carried out at a temperature above, at, or below the austenite non-recrystallization temperature (Tnr).

[0070] After the plurality of hot-rolling passes, the hot rolled steel plate is cooled in a cooling step, denoted as 103 in Figure 1, preferably in air, down to room temperature such that the required microstructure is achieved. In another embodiment the hot rolled steel plate from step 102 is subjected to accelerated continuous cooling down to room temperature.

[0071] In the fourth and fifth step, denoted as 104 and 105 in Figure 1, the hot-rolled and cooled steel plate is austenitized, quenched and tempered, and cooled to room temperature.

[0072] In the heating stage of step 101 the slabs are heated to a discharging temperature in the range of 950 °C - 1250 °C, preferably 1000 °C - 1200 °C and more preferably 1050 °C - 1200 °C, for a period of 30 min to 10 hours, preferably 2 hours to 6 hours. Higher temperatures enable better Nb dissolution into the austenite.

[0073] In the hot rolling stage of step 102, the slab is hot rolled with a typical pass schedule of 6 - 20 hot rolling passes, for example 10 - 18 passes, depending on the thickness of the slab and the final product in order to form a steel plate. Preferably, the amount of rolling passes is kept as low as possible to ensure high reduction of a single rolling pass.

[0074] The hot rolling steps may be carried out above the austenite non-recrystallization temperature, at the Tnr temperature, or below the Tnr temperature. Typically, at the start of rolling, the rolling temperature is above Tnr and at the end of hot-rolling, the rolling temperature may be below Tnr. The rolling temperature at the end of hot-rolling may, however, also be above Tnr. Typically, the passes start at a temperature above the non-recrystallization temperature and depending on the thickness of the plate it can fall below the non-recrystalization temperature. This means that the final rolling temperature can be between 850 and 1150 C.

[0075] The hot-rolled steel plate in the cooling step 103 is cooled to room temperature. Preferably cooling is conducted as air cooling.

[0076] The fourth step 104 of the process is hardening wherein the cooled hot-rolled steel plate is austenitized at a temperature range 850 - 1000 °C, preferably 875 - 975 °C, for a time period of 15 - 250 min in order to create a fully austenitic microstructure. This is followed by water quenching with a cooling rate of at least 5 °C / s to room temperature for obtaining a martensitic microstructure.

[0077] The fifth step 105 of the process is tempering, which is performed to reduce the internal stresses and brittleness of the steel. Preferably, tempering is performed at a temperature in the range of 350 °C - 700 °C for 20 - 200 min. However, the tempering treatment parameters may be different as well.

[0078] Optionally, a sixth step, denoted as 106 in Figure 1, of heat treatment is double tempering, which in some cases is performed to adjust the final properties, such as impact toughness. Preferably, tempering is performed at a temperature in the range of 350 °C - 700 °C for 20 - 200 min. However, the double-tempering treatment parameters may be different as well.

[0079] Tempered martensite is the target microstructure of the inventive steel plate with distribution of precipitates such as Mo, Cr, V and Nb-precipitates. Such a microstructure in the present steel enables high hardness while maintaining good machinability and toughness.

[0080] The following examples further describe and demonstrate embodiments within the scope of the present invention. The examples are given solely for the purpose of illustration and are not to be construed as limitations of the present invention, as many variations thereof are possible without departing from the scope of the invention.Examples

[0081] Examples from the inventive steels A and B were prepared with chemical compositions according to Table 1. During the preparation, both alloys were melted in lab-scale, cast and hot-rolled to 14 mm thickness. Both example steels were then subsequently Q&T processed. The specific processing parameters are shown in Tables 2 and 3. The resulting mechanical properties are shown in Tables 4 and 5.

[0082] The chemical compositions of the inventive examples differ in carbon, nickel, vanadium and niobium content. Alloy 2 has increased carbon, vanadium and niobium contents and lower nickel content compared to Alloy 1. This is mainly because vanadium and niobium combined with increased carbon can provide more precipitation hardening. Increased carbon content also increases martensite hardness.Processing and mechanical properties of the Q&T heat treated steel

[0083] Table 2 lists hot-rolling and Q&T heat treatment parameters for the inventive steel examples. Table 3 shows an example of a hot-rolling pass schedule for the inventive samples, and Tables 4 and 5 show the resulting mechanical properties and the effects of various Q&T heat treatments on the mechanical properties of the inventive examples. All the samples were processed similarly with the biggest difference being in tempering temperature and time. All the specimens were made in lab-scale. Austenitization and tempering heat treatments were carried out in air laboratory furnaces.

[0084] The hardness tests of the Q&T heat treated specimens were measured with a Q10 Vickers microhardness indenter (HV1kg) using a loading time of 7 seconds. All tests were carried out at room temperature according to ISO 6507-1:2018, standard for Vickers hardness testing, and the average of five hardness tests and the average of three impact toughness tests is reported in Tables 4 and 5.

[0085] The Charpy V impact toughness values for the QT condition were measured according to ASTM E23 standard. The results show an average value from three test pieces with 10x10x55mm dimensions, which were cut perpendicular to the rolling direction.

[0086] Table 4 shows the hardness and impact toughness values for the inventive examples after single tempering at 600 °C for 30 min and the hardness after double tempering (tempering twice at 600 °C for 30 min, cooling in air). Table 4 shows that for both steels, the impact toughness values are on a good level, especially when combined with such a high hardness level. Alloy 2 exhibited better impact toughness values compared to Alloy 1 while maintaining the same hardness level. After the second tempering, the hardness of the studied steels decreased roughly 30 - 40 HV1 to 520 HV1. This is still a good hardness level for certain applications.

[0087] Table 5 shows hardness and impact toughness values for the inventive examples after single tempering at 550 °C for 60 min. The test results show that, in this way, hardness may be increased while maintaining a good level of impact toughness.

[0088] It is to be understood that the present invention is not limited to the embodiments described above; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. Table 1SteelCSiMnPSCrNiMoVTiNbRemarkA0.371.010.74--1.951.661.430.130.010.018Inv.B0.401.070.77--1.941.161.430.180.010.031Inv. Table 2 SteelHeating T (°C)FRT (°C)Cooling method to RTAustenitization T(°C)Austenitization t (min)Cooling method to RTTempering T (°C)Tempering t (min)Cooling method to RTRemarkA1200>950air92013WQ55060airInv.6003030+30B1200>950air92013WQ55060airInv.6003030+30 Table 3 Pass, no.Temperature (°C)Height of the slab (mm)Change in height of the slab (mm)Reduction per pass120060150100.17236140.28326100.2841880.315>950°C1440.22 Table 4 SteelEnergy (J)Energy (J / cm 2< )Hardness (HV1)Hardness, double tempered (HV1)A13.416.8548520B15.519.4559520 Table 5 SteelEnergy (J)Energy (J / cm 2< )Hardness (HV1)A1215595B9.211.5611

Examples

examples

Examples

[0081]Examples from the inventive steels A and B were prepared with chemical compositions according to Table 1. During the preparation, both alloys were melted in lab-scale, cast and hot-rolled to 14 mm thickness. Both example steels were then subsequently Q&T processed. The specific processing parameters are shown in Tables 2 and 3. The resulting mechanical properties are shown in Tables 4 and 5.

[0082]The chemical compositions of the inventive examples differ in carbon, nickel, vanadium and niobium content. Alloy 2 has increased carbon, vanadium and niobium contents and lower nickel content compared to Alloy 1. This is mainly because vanadium and niobium combined with increased carbon can provide more precipitation hardening. Increased carbon content also increases martensite hardness.

Processing and mechanical properties of the Q&T heat treated steel

[0083]Table 2 lists hot-rolling and Q&T heat treatment parameters for the inventive steel examples. Table 3 shows an example o...

Claims

1. A steel plate having a composition consisting of, in terms of weight percentages, wt. %: C0.3-0.5Si0.5-1.5Mn0.5-1.5Cr1-3Ni0.7-1.8Mo0.5-2.5V0.1-0.6Ti0.0005 - 0.05Culess than or equal to 0.4Nb0.01 -0.09P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, wherein the impact toughness at room temperature is at least 5 J, and the surface hardness is at least 480 HV.

2. The steel plate of claim 1, wherein the composition consists of, in terms of wt. %: C0.34 - 0.42Si0.9-1.2Mn0.7-1Cr1.5 - 2.2Ni1.1 - 1.7Mo1 -2V0.14-0.4Ti0.0005 - 0.015Culess than or equal to 0.4Nb0.017 - 0.05P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, wherein the impact toughness at room temperature is at least 5 J, and the surface hardness is at least 480 HV.

3. The steel plate according any one of claims 1-2, wherein the steel product has a surface hardness ranging from 480 HV to 700 HV.

4. The steel plate according to any one of claims 1-3 having a microstructure comprising mainly of tempered martensite.

5. The steel plate according any one of the preceding claims, having a grain size in the range of 15 - 150 µm.

6. The steel plate according to any one of the preceding claims, wherein the steel plate is a hot rolled steel plate having a thickness in the range of 10 - 130 mm.

7. Use of a steel plate according to any one of the preceding claims in knives, wear pieces, dies or blades.

8. A method for manufacturing a steel plate comprising the following steps of - providing a steel slab having the following composition: C0.3 -0.5Si0.5- 1.5Mn0.5- 1.5Cr1-3Ni0.7- 1.8Mo0.5-2.5V0.1 -0.6Ti0.0005 - 0.05Culess than or equal to 0.4Nb0.01 -0.09P≤ 0.02S≤ 0.004 remainder Fe and inevitable impurities, and - preheating the steel slab to a temperature in the range of 950 °C - 1200 °C; - subjecting the preheated steel slab to a plurality of hot rolling passes, above, at or below recrystallization temperature in order to form a steel plate wherein the final rolling temperature is 850 - 1150 C, and - air cooling the steel plate to room temperature, and - heating the steel plate to a temperature range of 850 - 1000 °C, for a time period of 15 - 250 min, and - water quenching the heated steel plate continuously to room temperature with a cooling rate of 5 °C / s, - tempering the water quenched steel plate from previous step at a temperature range of 350 - 700 °C for a time period of 20 - 200 min, and - air cooling the steel plate from previous step to room temperature.

9. The method according to claim 8, comprising the additional step of - reheating the steel plate after the last air cooling step to a temperature range of 350 - 700 °C for a time period of 20 - 200 min.

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