High-manganese hot rolled steel and production method thereof

Abstract

A high-manganese hot-rolled steel having a composition containing the following elements: 0.8% < = carbon < = 1.3%, 9.5% < = manganese < = 22%, 0.01% < = silicon < = 3%, 0.01% < = aluminum < = 3%, 0.03% < = phosphorus < = 0.1%, 0.03% < = sulfur < = 0.1%, 0% < = nitrogen < = 0.01%, 0% < = niobium < = 0.03%, 0% < = titanium < = 0.2%, 0% < = chromium < = 1.5%, 0% < = molybdenum < = 0.5%, 0% < = calcium < = 0.005%, 0.01% < = copper < = 2%, 0.01% < = nickel < = 3%, 0% < = boron < = 0.01%, 0% < = magnesium < = 0.005%, the remainder is composed of iron and inevitable impurities due to processing, the microstructure of the steel contains, in area fraction, 95% or more of austenite and 0% to 5% of carbide, and the grain size of grains of the austenite is 15 [mu] m or more.

Description

Technical Field

[0001] The present invention relates to hot rolled high manganese steels presenting a microstructure comprising mainly austenite. The steel according to the invention is particularly well suited for manufacturing components or equipment having good wear resistance, such as buckets for excavators or bulldozers and dump truck hoppers.

[0002] The present invention may also be used as structural steel or in the manufacture of industrial machinery, yellow goods, green goods, or any other similar industrial application. Background Art

[0003] In recent years, efforts have been actively made to reduce the weight of equipment and structures by applying high-strength steel for the purpose of improving fuel efficiency and reducing environmental impact. However, when the wear resistance or abrasion resistance of steel increases, generally the elongation and toughness deteriorate. Therefore, in the development of high-strength steel, an important issue is to increase the wear resistance without deteriorating the elongation and toughness.

[0004] A great deal of research and development effort has been put into reducing the amount of material utilized by increasing the strength of the material. Conversely, increasing the strength of steel reduces elongation and wear resistance, so it is necessary to develop materials that combine high strength, elongation and toughness with wear resistance.

[0005] Early research and development in the field of high strength and good toughness steels has resulted in several methods for producing high strength wear resistant steels, some of which are listed herein for a clear understanding of the present invention:

[0006] EP2971211 discloses improved steel compositions and provides methods for preparing them. The disclosure provides advantageous wear-resistant steels. More particularly, the disclosure provides high manganese (Mn) steels with enhanced wear resistance and methods for manufacturing high manganese steel compositions with enhanced wear resistance. The advantageous steel compositions / components of the disclosure improve one or more of the following properties: wear resistance, ductility, crack resistance, erosion resistance, fatigue life, surface hardness, stress corrosion resistance, fatigue resistance and / or environmental cracking resistance. Typically, the disclosure provides high manganese steels suitable for resisting wear and / or erosion. However, the steel of EP2971211 does not show total elongation or tensile strength. Summary of the invention

[0007] The object of the present invention is to solve these problems by making available a hot rolled steel having:

[0008] - a yield strength of 350 MPa or more, and preferably equal to or greater than 375 MPa,

[0009] - a tensile strength of 850 MPa or more, and preferably 880 MPa or more,

[0010] - a total elongation greater than or equal to 25%, and preferably greater than or equal to 30%,

[0011] - Less than 82g / mm2 according to G65 test 3 , and preferably less than 77 g / mm tested according to G65 3 wear loss.

[0012] In a preferred embodiment, the steel sheet according to the present invention may also exhibit a hardness greater than or equal to 180 BHN.

[0013] In a preferred embodiment, the steel sheet according to the present invention may also exhibit a strength greater than or equal to 60 J / cm2 when tested at -40°C. 2 Summer-V impact toughness.

[0014] Preferably, such steel may also have good suitability for cold forming such as bending, roll forming and drawing.

[0015] Another object of the invention is also to make available a method for manufacturing these steels that is compatible with conventional industrial processes while being robust to variations in manufacturing parameters.

[0016] The hot rolled steel sheet of the present invention may be optionally coated with zinc or a zinc alloy to improve corrosion resistance.

[0017] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0018] Without wishing to be bound by any theory, it appears that the hot rolled steel according to the invention allows improved mechanical properties due to this specific microstructure.

[0019] Carbon is present in the steel at 0.8% to 1.3%. Carbon is an element necessary for stabilizing austenite down to room temperature. It also increases the yield strength of the steel. Carbon contributes significantly to the tensile strength, elongation and wear resistance of the steel through the TWIP effect. But a carbon content of less than 0.8% will not impart yield strength and tensile strength and elongation to the steel of the present invention. On the other hand, at a carbon content exceeding 1.3%, the steel exhibits reduced elongation and wear resistance characteristics. The preferred content of the present invention may be maintained at 0.85% to 1.25%, and more preferably 0.9% to 1.2%.

[0020] Manganese content is present in 9.5% to 22% by weight. Manganese is an important alloying element in this system mainly due to the fact that alloying with very large amounts of manganese stabilizes austenite down to room temperature, which can help achieve target properties such as wear resistance, elongation and tensile strength. If manganese is present above 22%, it is difficult to refine the molten steel, and manganese above 22% does not contribute significantly to the improvement of properties. Manganese will not be able to stabilize austenite at room temperature with a volume fraction above 95% when it is present below 9.5%. The preferred limit of the presence of manganese is 10% to 20%, and more preferably 10% to 18%, and even more preferably 10% to 14%.

[0021] The aluminum content is present in a range of 0.01% to 3% by weight. Aluminum is an essential element that inhibits carbide formation and, in addition, contributes to increased yield strength. Below 0.01%, the presence of aluminum becomes less beneficial. Above 3%, aluminum may promote the formation of ferrite that is detrimental to the wear resistance of the steel of the present invention. Furthermore, the presence of Al above 3% may form intermetallic compounds such as Fe-Al, Fe3-Al and other (Fe,Mn)Al intermetallic compounds that will impart brittleness to the product and may also be detrimental to the toughness of the steel. Preferably, the aluminum content will be limited to 0.01% to 2.7%, and a more preferred limit is 0.01% to 2.5%.

[0022] Silicon is an element effective in inhibiting carbide formation, and furthermore, silicon increases the yield strength and tensile strength of the steel of the present invention. The silicon content is present in the range of 0.01% to 3% by weight. Silicon is limited up to 3% because above this level this element has a tendency to form strongly adherent oxides that create surface defects. Furthermore, when silicon is present above 3%, it will form ferrite that is detrimental to achieving the target properties of the steel. Therefore, the Si content will preferably be present in the range of 0.09% to 2.6%, and more preferably 0.1% to 2%.

[0023] Sulfur and phosphorus are impurities that embrittle grain boundaries. Their respective contents must not exceed 0.03 wt. % and 0.1 wt. % in order to maintain sufficient hot ductility.

[0024] The nitrogen content must be 0.1 wt % or less to prevent precipitation of AlN and formation of volume defects (bubbles) during solidification.

[0025] Niobium may be added as an optional element to the steel of the present invention in an amount of up to 0.03 wt. %, and preferably 0.01 wt. % to 0.03 wt. %, to provide grain refinement. Grain refinement allows a good balance between strength and elongation to be obtained. However, niobium has a tendency to hinder recrystallization during hot rolling, so the limit is kept to 0.03%.

[0026] In a similar manner to niobium, titanium may be added as an optional element to the steel of the present invention for grain refinement in an amount of up to 0.2 wt. %, and preferably 0.01 wt. % to 0.2 wt. %.

[0027] Copper can be added as an optional element in an amount of 0.01 wt % to 2.0 wt % to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% is required to obtain such effects. However, when its content is higher than 2.0%, it may deteriorate the surface appearance.

[0028] Nickel can be added as an optional element in an amount of 0.01 wt % to 3.0 wt % to increase the strength of the steel, and enhance austenite stability and improve its toughness. A minimum of 0.01% is required to obtain such an effect. However, when its content is higher than 3.0%, it is not cost effective.

[0029] Molybdenum can be added as an optional element, which is present in the steel of the present invention at 0% to 0.5% by weight; Mo plays an effective role, Mo improves strength and strain hardening, thus increasing the wear resistance of the steel. However, excessive addition of Mo increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.5%. The preferred limit of Mo is 0% to 0.4%, and more preferably 0% to 0.3%.

[0030] Chromium can be added as an optional element to the steel of the present invention at 0 to 1.5 wt%. Chromium provides strength to the steel but compromises the surface finish of the steel when used above 1.5%. The preferred limit of chromium is 0.01 to 1.4.5%, and more preferably 0.01 to 1.2%.

[0031] Other elements such as calcium, cerium, boron, magnesium or zirconium may be added alone or in combination in the following proportions by weight: Ce≦0.1%, B≦0.01, Ca≦0.005, Mg≦0.005 and Zr≦0.005, up to the maximum content levels indicated.

[0032] In addition, some trace elements such as Sb, Sn may come from the processing of the steel. The maximum cumulative or individual limit of these elements that are acceptable and harmless to the steel of the present invention is 0.05% by weight. The content of these elements in the steel of the present invention is preferably as low as possible, and preferably less than 0.03%.

[0033] The microstructure of the steel sheet according to the invention optionally contains up to 5% carbides, the remainder consisting of austenite.

[0034] Austenite matrix exists as the main phase of the steel of the present invention, which provides the steel of the present invention with TWIP effect, thereby providing the steel with high strain hardening, resulting in high tensile strength, high total elongation and excellent wear resistance at the same time. Austenite is present in the steel of the present invention at a minimum of 95% by area fraction, and preferably 95% to 100% by volume fraction, and more preferably 98% to 100%. The grain size of austenite of the present invention is 15 microns or more. It is preferred that the austenite grain size of the present invention is 15 microns to 30 microns.

[0035] The present invention aims to avoid carbide formation, however, up to 5% carbide by area fraction may be acceptable for the steel according to the present invention. Carbides are preferably present in less than 4%, more preferably less than 3%, and even more advantageously if present in less than 2% by area fraction. Carbides that are not harmful to the steel according to the present invention are needle-shaped intergranular carbides and lamellar carbides. Any other form of carbide, such as Widmanstatten carbide, is unacceptable for the steel according to the present invention.

[0036] The microstructure of the hot rolled steel should not contain microstructural constituent components such as pearlite, ferrite, martensite and bainite in addition to the above-mentioned microstructures.

[0037] The steel plate according to the invention can be produced by any suitable method. A preferred method comprises providing a semi-finished casting of a steel having a chemical composition according to the invention. The casting can be made into an ingot or continuously produced in the form of a thin slab or thin strip (i.e., a thickness ranging from about 220 mm for a slab to tens of millimeters for a thin strip).

[0038] However, it is preferred to use a method according to the invention which comprises the following steps in sequence.

[0039] The steel according to the present invention is preferably produced by a method in which the process according to the present invention shortens the production workflow from liquid phase steel to the final hot rolled steel. The entire production process is carried out continuously, wherein the molten steel having the above composition is cast in the form of a continuous thin slab with a thickness ranging from 10 mm to 100 mm. The cast thin slab can be used directly at a temperature greater than 1000°C after casting without intermediate cooling, or can be heated to a temperature above 1000°C, preferably above 1050°C, and more preferably above 1100°C or 1150°C before being hot rolled.

[0040] The continuously cast thin slab is then subjected to hot rolling. The hot rolling final rolling temperature must be at least 800°C, and preferably at least 850°C, and more preferably 850°C to 950°C. The hot rolling final rolling is kept above 800°C to ensure that the hot rolling must be completed in a region where the microstructure is not 100% unrecrystallized.

[0041] The hot rolled strip obtained in this way is then cooled, such cooling starting immediately after the end of hot rolling, and the hot rolled strip is cooled from the end of hot rolling to a cooling stop temperature of less than 490° C. at a cooling rate CR1 of 1° C. / sec to 150° C. / sec. In a preferred embodiment, the cooling rate CR1 is 2° C. / sec to 120° C. / sec, and more preferably, the cooling rate CR1 is 3° C. / sec to 100° C. / sec.

[0042] Thereafter, the hot rolled steel is coiled at a coiling temperature CT, and CT is less than 490° C. Preferably, CT is 20° C. to 480° C., and more preferably coiling is performed at 25° C. to 4750° C.

[0043] The coiled hot rolled steel is cooled from the coiling temperature to room temperature at a cooling rate CR2 of 0.0001°C / sec to 1°C / sec. In a preferred embodiment, the cooling rate CR2 is 0.0001°C / sec to 0.8°C / sec to obtain hot rolled steel.

[0044] The thickness of the hot-rolled steel thus obtained is preferably 0.5 mm to 12 mm, and more preferably 0.5 mm to 10 mm, and even more preferably 0.5 mm to 8 mm.

[0045] Thereafter, optional pickling or any other scale removal process may be performed to facilitate further processing of the obtained hot rolled steel.

[0046] In order to protect the steel according to the invention from corrosion, in a preferred embodiment the steel may optionally be covered with a metallic coating or paint or any other known coating to have sufficient corrosion resistance. The metallic coating may be an aluminum based coating or a zinc based coating.

[0047] Preferably, the aluminum-based coating comprises less than 15% Si, less than 5.0% Fe, optionally 0.1% to 8.0% Mg and optionally 0.1% to 30.0% Zn, the remainder being Al.

[0048] Advantageously, the zinc-based coating comprises from 0.01% to 8.0% of Al, optionally from 0.2% to 8.0% of Mg, the remainder being Zn. Example

[0049] The following tests, examples, graphic illustrations and tables presented herein are non-limiting in nature and must be considered for purposes of illustration only and will demonstrate the advantageous features of the present invention.

[0050] Steel plates made of steels having different compositions are summarized in Table 1, wherein the steel plates were respectively produced according to the process parameters as specified in Table 2. Thereafter, Table 3 summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the evaluation results of the obtained properties.

[0051] Table 1 - Composition

[0052] test C Mn Si Al P S N Cu B Ti I1 1.07 12.12 0.26 0.04 0.005 0.003 0.055 0.028 0.0005 0.003 I2 1.07 12.12 0.26 0.04 0.005 0.003 0.055 0.028 0.0005 0.003 I3 1.11 11.9 0.69 0.011 0.003 0.0018 0.0029 0 0 0 I4 1.10 118 1.51 0.015 0.0044 0.0015 0.003 0 0 0 R1 1.05 10.6 0.186 0.021 0.0028 0.0019 0.0033 0 0 0 R2 1.04 11.7 0.676 0.022 0.003 0.0019 0.0032 0 0 0 R3 1.05 11.7 1.49 0.021 0.0044 0.002 0.0034 0 0 0 R4 1.06 107 0.21 168 0.0044 0.002 0.0027 0 0 0

[0053] Table 2 - Process parameters

[0054]

[0055] The obtained samples were then analyzed, and the corresponding microstructural elements and mechanical properties are summarized in Tables 3 and 4, respectively.

[0056] Table 3

[0057] Table 3 summarizes the results of tests carried out according to standards on different microscopes such as SEM, EPMA, EBSD, XRD or any other microscope to determine the microstructural composition of both the steel according to the invention and the reference test. The area fraction of carbides was measured on the polished samples after etching them for 10 seconds in a 2% nital etching solution and observed by SEM. The austenite was measured and observed optically by SEM.

[0058] The results are documented in this article:

[0059]

[0060] Table 4 - Characteristics

[0061] Table 4 illustrates the mechanical properties of both the inventive steel and the reference steel. In order to determine the tensile strength, yield strength and total elongation, a tensile test was carried out according to the JIS Z2241 standard.

[0062] The results of various mechanical tests performed according to the standard are summarized.

[0063]

[0064] I = according to the invention; R = reference; underlined values: not according to the invention.

[0065] The examples show that the steel sheet according to the invention is the only steel sheet which exhibits all the targeted properties due to its specific composition and microstructure.

Claims

1. A high manganese hot rolled steel having a composition comprising the following elements, expressed in weight percentage: 0.8%≤Carbon≤1.3% 9.5%≤Manganese≤22% 0.01%≤Silicon≤3% 0.01%≤Aluminum≤3% 0.03%≤Phosphorus≤0.1% 0.03%≤Sulfur≤0.1% 0%≤Nitrogen≤0.01% and can include one or more of the following optional elements: 0%≤Niobium≤0.03% 0%≤Titanium≤0.2% 0%≤Cr≤1.5% 0%≤Molybdenum≤0.5% 0%≤Calcium≤0.005% 0.01%≤Copper≤2% 0.01%≤Nickel≤3% 0%≤boron≤0.01% 0%≤Mg≤0.005% The remainder of the composition is composed of iron and inevitable impurities caused by processing, and the microstructure of the steel comprises 95% or more of austenite, 0% to 5% of carbides by area fraction, wherein the grain size of the austenite is 15 micrometers or more.

2. The high manganese hot rolled steel according to claim 1, wherein the composition contains 0.09% to 2.6% silicon.

3. The high manganese hot rolled steel according to claim 1 or 2, wherein the composition contains 0.85% to 1.25% carbon.

4. The high manganese hot rolled steel according to claim 3, wherein the composition contains 10% to 20% manganese.

5. The high manganese hot rolled steel according to any one of claims 1 to 4, wherein the composition contains 10% to 18% manganese.

6. The high manganese hot rolled steel according to any one of claims 1 to 5, wherein the composition contains 0.01% to 2.7% aluminum.

7. The high manganese hot rolled steel according to any one of claims 1 to 6, wherein the composition contains 0.01% to 2.5% aluminum.

8. The high manganese hot rolled steel according to any one of claims 1 to 7, wherein the amount of austenite is 98% to 100%.

9. The high manganese hot-rolled steel according to any one of claims 1 to 8, wherein the total elongation of the steel sheet is 25% or more.

10. A method for producing high manganese hot rolled steel comprising the following steps in sequence: - providing a steel composition according to any one of claims 1 to 7; - Casting of thin slabs with a thickness ranging from 10 mm to 100 mm at a temperature greater than 1000° C. and without intermediate cooling; - rolling the cast thin slab in the austenite range where the hot rolling finishing temperature should be at least 800° C. to obtain hot rolled steel; - the hot-rolled strip is then cooled, wherein the cooling begins immediately after the end of the hot-rolling; - then cooling the hot rolled steel from the end of hot rolling to a cooling stop temperature range of less than 490° C. at a cooling rate CR1 of 1° C. / s to 150° C. / s; - thereafter, coiling the hot rolled steel within a coiling temperature range of less than 490°C; - The coiled hot rolled strip is then cooled to room temperature at a cooling rate CR2 of 0.0001°C / sec to 1°C / sec to obtain a high manganese hot rolled steel.

11. The method according to claim 10, wherein the thickness of the high manganese hot rolled steel is 0.5 mm to 12 mm.

12. The method according to claim 10 or 11, wherein the hot rolling finishing temperature is at least 850°C. 13 . The method according to claim 10 , wherein the cooling rate CR1 from the end of hot rolling to the cooling stop temperature is 2° C. / sec to 120° C. / sec.

14. Use of the steel sheet according to any one of claims 1 to 9 or the steel sheet produced according to any one of claims 10 to 13 for manufacturing parts of industrial machinery, engineering machinery parts or prototype parts.

15. An industrial machine comprising a component obtained according to claim 14.

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