Highly wear-resistant hot-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to a hot-rolled steel sheet, which is made from steel having a composition comprising, in weight percent, C: 0.10-0.25%, Mn: 3.0-5.0%, Si: 0.80-1.60%, Al: 0.10%-0.60%, S≦0.010%, P≦0.020%, N≦0.008%, with the remainder being iron and unavoidable impurities resulting from smelting, and which has a microstructure containing, in a surface fraction, 5-10% retained austenite and the remainder being auto-tempered martensite.

Description

[Technical Field]

[0001] The present invention relates to a hot rolled steel sheet having high wear resistance, high strength and high toughness and to a method for obtaining such a steel sheet. [Background technology]

[0002] In the mineral industry, water-quenched martensitic steels are currently used to manufacture wear parts for applications in mines and quarries, the cement and steel industry, public works, and agricultural machinery. In these fields, wear damage caused by heavy impacts and scratches is a major problem, requiring repeated machine repairs and maintenance costs to replace worn parts such as liners, buckets, and jaw crushers. Considerable effort has been made to extend the life of wear parts, and a wide variety of materials and designs are common. To obtain significant improvements in wear resistance, depending on the application, hardness should be higher than 440HB. Knowing that impact toughness is closely related to wear resistance, this property must be the highest, yet classical water-quenched martensitic steels exhibit low impact toughness.

[0003] Furthermore, from the perspective of protecting the global environment, the steel industry is seeking manufacturing routes that consume less energy. Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, the object of the present invention is to solve the above problems and to provide a steel plate which has a high hardness of 400HB or more, preferably 440HB or more, at the mid-thickness of the plate and 430HB or more, preferably 450HB or more, at the surface of the plate, and a high toughness with a Charpy impact energy at -40°C of 25J or more, and which can be easily processed by conventional process routes.

[0005] Preferably, the hot rolled steel sheet has a yield strength YS of 970 MPa or more, preferably 980 MPa or more.

[0006] Preferably, the hot-rolled steel sheet has a tensile strength TS of 1400 MPa or more. [Means for solving the problem]

[0007] The object of the present invention is achieved by providing a steel sheet as set forth in claim 1. Another object is achieved by providing a method as set forth in claim 2. DETAILED DESCRIPTION OF THE INVENTION

[0008] The invention will now be described in detail and illustrated by examples, without introducing any limitations.

[0009] The composition of the steel of the present invention will now be described, with the contents expressed as weight percent.

[0010] The carbon content is between 0.10% and 0.25%. If the carbon content is too high, the weldability of the steel is insufficient. If the carbon content is less than 0.10%, the austenite fraction is not stabilized to obtain the desired properties. In a preferred embodiment of the invention, the carbon content is between 0.15% and 0.20%.

[0011] The manganese content is between 3.0% and 5.0%. Additions above 5.0% increase the risk of center segregation to the extent that toughness is impaired. Below 3.0%, the final structure contains an insufficient fraction of retained austenite to obtain the desired properties. In a preferred embodiment of the invention, the manganese content is between 3.5% and 4.5%.

[0012] According to the present invention, the silicon content is between 0.80% and 1.60%. The addition of at least 0.80% silicon helps to stabilize a sufficient amount of retained austenite. Above 1.60%, silicon is detrimental to toughness. Furthermore, silicon oxide forms on the surface, impairing the coatability of the steel. In a preferred embodiment of the present invention, the silicon content is between 1.00% and 1.60%.

[0013] The aluminum content is 0.10% to 0.60%. Aluminum is a very effective element for deoxidizing steel in the liquid phase during processing. Aluminum also improves the weldability of steel. The aluminum content is less than 0.60% to avoid the formation of inclusions and avoid oxidation problems.

[0014] Optionally, some elements can be added to the composition of the steel according to the invention.

[0015] The boron content can be 0.0003% to 0.004%. The presence of boron can increase toughness. Boron also improves the weldability of steel. If it exceeds 0.004%, it promotes the formation of boron carbides at the prior austenite grain boundaries, making the steel more brittle. If it is less than 0.0003%, free B does not segregate to the prior austenite grain boundaries in sufficient concentrations to increase the toughness of the steel.

[0016] Up to 0.06% titanium can optionally be added to provide precipitation strengthening. Preferably, a minimum of 0.01% titanium is added in addition to boron to protect the boron from the formation of BN.

[0017] Niobium may be added up to 0.05% to refine the austenite grains during hot rolling and provide precipitation strengthening. Preferably, the minimum amount of niobium added is 0.0010%.

[0018] Molybdenum may be added optionally up to a maximum limit of 0.3%. Molybdenum stabilizes austenite and increases the toughness of the steel. Molybdenum also improves the weldability of the steel. Above 0.3%, the addition of molybdenum becomes costly and ineffective considering the properties required.

[0019] Preferably, the minimum amount of molybdenum is 0.0010%.

[0020] A maximum of 0.80% chromium is permitted. Above that, saturation effects become noticeable and adding chromium becomes unnecessary and expensive. Preferably, the minimum amount of chromium is 0.0010%.

[0021] Copper can be added up to 0.2% to increase the toughness of the steel.

[0022] Nickel may be added up to 0.30% to limit the risk of delayed fracture due to hydrogen embrittlement.

[0023] The remainder of the steel composition consists of iron and impurities resulting from smelting. In this respect, P, S and N are considered to be residual elements, at least unavoidable impurities. Their contents are not more than 0.010% for S, 0.020% for P and 0.008% for N.

[0024] Next, the microstructure of the hot-rolled steel sheet of the present invention will be described.

[0025] Hot-rolled steel sheets have a microstructure consisting of 5% to 10% surface fraction of retained austenite, with the remainder being auto-tempered martensite.

[0026] The steel plate is hot rolled at a final rolling temperature FRT above Ac1.

[0027] In a preferred embodiment of the invention, the FRT is less than Ac3 and the microstructure consists of ferrite and austenite, with the ferrite and part of the austenite transforming to martensite upon subsequent cooling.

[0028] In another preferred embodiment of the present invention, the FRT is equal to or greater than Ac3 and the microstructure is fully austenitic, with some of the austenite transforming to martensite upon subsequent cooling.

[0029] In a preferred embodiment of the present invention, the steel plate has a thickness of 5 mm or more and less than 40 mm, and the martensite is automatically tempered by subsequent cooling at a slow rate, thereby achieving the desired mechanical properties. Preferably, the steel plate has a thickness of more than 25 mm and less than 40 mm.

[0030] In another preferred embodiment of the present invention, in which the steel plate has a thickness of 40 mm to 100 mm, in order to obtain a homogeneous microstructure of austenite in the thickness of the plate, a temperature T H A heating step up to a temperature T below 400 °C is required. Q During the quenching process, some of this austenite is transformed into martensite, which then auto-temper during the subsequent air cooling.

[0031] A portion of the austenite remains in the final microstructure. This retained austenite contributes to obtaining excellent wear resistance and is beneficial to toughness. During wear damage and impact, the retained austenite transforms into martensite and induces a volume expansion from an fcc to a bcc structure, which can fill impact cracks and stop their propagation. The TRIP effect also induces significant work hardening during use, which significantly contributes to improving the wear resistance of the bare material.

[0032] The steel sheet according to the invention can be manufactured by any suitable manufacturing method, which can be defined by a person skilled in the art. However, it is preferable to use a method according to the invention comprising the following steps:

[0033] The semi-finished product, which can be further hot rolled, has the above-mentioned steel composition. The semi-finished product can be heated to a temperature of 1100°C to 1300°C to facilitate hot rolling, with the final hot rolling temperature FRT exceeding Ac1. Preferably, FRT exceeds Ac1 + 100°C, more preferably Ac1 + 150°C. If FRT is lower than Ac1, the microstructure does not contain enough austenite to ensure the hardness and toughness of the plate.

[0034] In a preferred embodiment of the invention, the FRT is less than Ac3.

[0035] In another preferred embodiment of the present invention, FRT is greater than or equal to Ac3.

[0036] The hot-rolled steel plate having a thickness of 5 mm or more and less than 40 mm is then air-cooled, preferably at a cooling rate of less than 5°C / s at the mid-thickness of the plate.

[0037] Steel plates from 40mm thick to a maximum of 100mm thick are heated to a temperature of 850℃~950℃. H and maintained at said temperature for a holding time of not more than 30 minutes to obtain a homogeneous microstructure throughout the thickness of the steel plate. The plate is then cooled at a rate higher than 1°C / s to a temperature T below 400°C. Q After quenching, the steel plate is cooled to room temperature after reaching a temperature of 300 to 400°C at the middle of its thickness.

[0038] Calculations carried out using software such as Forge® make it possible to determine the duration of quenching to obtain a temperature of 300°C to 400°C at the middle thickness of the steel plate at the end of quenching. Preferably, the quenching temperature T Q is 200°C or higher, more preferably 250°C or higher.

[0039] The present invention is illustrated by the following examples, which are not intended to be limiting in any way. [Example]

[0040] Four grades, whose compositions are summarized in Table 1, were cast into semi-finished products and processed into steel plates.

[0041] The compositions tested are summarized in the table below, with elemental contents expressed as weight percent.

[0042] [Table 1]

[0043] Ac1 and Ac3 of Steel A were determined by dilatometry.

[0044] Ac1 and Ac3 of steels B to D are determined by the following formula based on the weight percentage of the corresponding elements. Ac3=925-219√(%C)-7%Mn+39%Si-16%Ni+13%Mo Ac1=742-29%C-14%Mn+13%Si+16%Cr-17%Ni-16%Mo+36%Cu.

[0045] The steel ingot was heated to 1250°C and hot rolled at a final rolling temperature FRT to obtain a hot-rolled steel sheet having a thickness d. The hot-rolled steel sheets of Trials 1 to 3, 6, and 8 were then air-cooled. In Trials 4 and 5, the steel sheets were then cooled to a temperature T H In trial 7, a steel plate having a thickness of 40 mm was heated to a temperature T H Heat to a temperature T above RT Q After quenching, the steel plate is cooled to a temperature of 300-400°C at the middle of the thickness. The following specific conditions were applied:

[0046] [Table 2]

[0047] The hot-rolled sheets were then analyzed, and the corresponding microstructural elements and mechanical properties were summarized in Tables 3 and 4, respectively.

[0048] The percentage of phases in the microstructure of the resulting hot rolled steel sheets was determined at the quarter thickness d / 4 of the steel sheets.

[0049] [Table 3]

[0050] The surface fraction of the phases in the microstructure is determined by the following method: specimens are cut from the hot-rolled sheet, polished and etched with reagents known per se to reveal the microstructure, and the sections are then examined in a scanning electron microscope, for example a field emission electron microscope ("FEG-SEM") in secondary electron mode at a magnification of more than 5000 times.

[0051] The surface fractions of retained austenite and auto-tempered martensite are determined by SEM observation after Nital or Picral / Nital reagent etching and by X-ray diffraction analysis.

[0052] The mechanical properties of the tested samples were measured and are summarized in the table below. Hardness is measured on the surface of the steel plate (up to 2 mm below the top surface) and at mid-thickness (d / 2) of the steel plate. Hardness is measured according to standard ASTM E10. Charpy impact energy at -40°C is measured according to standards ISO 148-1:2006(F) and ISO 148-1:2017(F). YS and TS are measured according to ISO standard ISO 6892-1.

[0053] [Table 4]

[0054] The examples show that the steel sheets according to the invention, namely trials 1-3 and 7-8, are the only ones that exhibit the targeted properties thanks to their specific composition and microstructure.

[0055] The steel components in trials 4-6 have a chemical composition with lower manganese. The quenching step in trial 4 is performed at a slow cooling rate to room temperature. Some of the austenite then transforms to martensite, which is auto-tempered due to the slow cooling, and also transforms to bainite, which is detrimental to hardness. In trial 5, the quenching step is performed at a higher cooling rate to room temperature. Therefore, the austenite transforms completely to martensite. The absence of austenite is detrimental to toughness.

[0056] Also, Trials 2 and 6 have different chemical compositions but undergo the same process. The low level of manganese combined with the absence of boron in Trial 6 results in low toughness.

[0057] The thicker steel plate in trial 7 is reheated and quenched. This quenching step is interrupted at a temperature of 350°C and then air-cooled to room temperature. The austenite formed during the heating step then partially transforms into martensite. This martensite is auto-tempered during the air-cooling step, resulting in high hardness and toughness levels.

Claims

1. A hot rolled steel sheet comprising, in weight percent: C: 0.10-0.25% Mn: 3.0-5.0% Si: 0.80~1.60% Al: 0.10% to 0.60% S≦0.010% P≦0.020% N≦0.008% and optionally the following elements in weight percent: B: 0.0003-0.004% T1≦0.06% Nb≦0.05% Mo≦0.3% Cr≦0.80% Cu≦0.2% Ni≦0.30% the remainder of the composition being iron and unavoidable impurities resulting from smelting; The steel plate has a surface fraction of: - containing 5 to 10% retained austenite, - Remainder is auto-tempered martensite A hot-rolled steel sheet having a microstructure in which

2. 1. A method for producing a hot rolled steel sheet, comprising the following successive steps: - casting the steel to obtain a semi-finished product, said semi-finished product having the composition according to claim 1; - the semi-finished product is heated to a temperature T of 1100 ° C to 1300 ° C. ｒｅｈｅａｔ reheating until - hot rolling said semi-finished product at a finish hot rolling temperature FRT above Ac1, Optionally, the steel plate is heated to a temperature T between 850°C and 950°C. Ｈ and maintaining at that temperature for a holding time of 30 minutes or less; Optionally, the steel sheet is subjected to a T-quench in order to obtain a temperature of 300°C to 400°C at the middle thickness of the steel sheet at the end of the quenching. Ｈ from a temperature T below 400°C Ｑ quenching until - cooling the steel plate to room temperature. A method comprising:

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