High-manganese hot-rolled steel and method for producing the same

Abstract

A high-manganese hot-rolled steel containing the following elements, namely, 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%, and having a composition in which the remaining composition is composed of iron and inevitable impurities generated by processing. The microstructure of the steel consists of, in area fraction, 95% or more austenite and 0% to 5% carbide, and the particle size of the austenite particles is 15 microns or more.

Description

Technical Field

[0001] The present invention mainly relates to a hot-rolled high-manganese steel exhibiting a microstructure containing austenite. The steel according to the present invention is particularly well-suited for the manufacture of components or equipment having good wear resistance, such as buckets for excavators or parts of bulldozers and tipper bins.

[0002] The present invention can also be used as a structural steel or for the manufacture of industrial machinery, yellow goods, green goods, or any other similar industrial applications.

Background Art

[0003] In recent years, in order to reduce the impact on the environment and improve fuel efficiency, weight reduction of equipment and structures has been actively carried out by applying high-strength steel. However, when the wear resistance or abrasion resistance of steel is increased, the elongation and toughness generally decrease. Therefore, in the development of high-strength steel, it is an important issue to increase the wear resistance without impairing the elongation and toughness.

[0004] Research and development are actively carried out to reduce the amount of material used by increasing the strength of the material. Conversely, since increasing the strength of steel decreases the elongation and wear resistance, the development of materials combining high strength, high elongation, high toughness, and wear resistance is required.

[0005] Previous research and development in the field of high-strength and good-toughness steel have led to several methods for manufacturing high-strength wear-resistant steel, some of which are listed herein for the ultimate understanding of the present invention.

[0006] EP2971211 discloses an improved steel composition and a method for manufacturing the same. This disclosure provides an advantageous wear-resistant steel. More specifically, this disclosure provides a high manganese (Mn) steel with enhanced wear resistance and a method for manufacturing a high manganese steel composition with enhanced wear resistance. The advantageous steel composition / components of this disclosure improve one or more of the following properties: wear resistance, ductility, crack resistance, corrosion resistance, fatigue life, surface hardness, stress corrosion resistance, fatigue resistance, and / or environmental cracking resistance. Generally, this disclosure provides a high manganese steel adapted to be resistant to wear and / or corrosion. However, the steel of EP2971211 does not illustrate the total elongation or tensile strength.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] The object of the present invention is to solve these problems by making available a hot-rolled steel having the following simultaneously: - A yield strength of 350 MPa or more, preferably 375 MPa or more, - A tensile strength of 850 MPa or more, preferably 880 MPa or more, - A total elongation of 25% or more, preferably 30% or more, - A wear loss of less than 82 g / mm according to the G65 test, 3 preferably less than 77 g / mm according to the G65 test. 3

[0009] In a preferred embodiment, the steel plate according to the present invention can also exhibit a hardness of 180 BHN or more.

[0010] In a preferred embodiment, the steel plate according to the present invention can also have a Charpy impact energy of 60 J / cm when tested at -40°C. 2 ​The above Charpy-V impact toughness can be exhibited.

[0011] Preferably, such steel can also have good compatibility with cold forming such as bending, roll forming, and drawing.

[0012] Another object of the present invention is also to make available a method for manufacturing these steels that is stable against shifts in manufacturing parameters while being compatible with conventional industrial processes.

[0013] The hot-rolled steel sheet of the present invention may be coated with zinc or a zinc alloy in order to improve corrosion resistance.

Means for Solving the Problems

[0014] Other features and advantages of the present invention will become apparent from the following detailed description of the present invention.

[0015] Although not intended to be bound by any theory, the hot-rolled steel according to the present invention enables improvement of mechanical properties thanks to this specific microstructure.

[0016] Carbon is present in the steel between 0.8% and 1.3%. Carbon is an element necessary to stabilize austenite to room temperature. Carbon also increases the yield strength of the steel. Carbon essentially contributes to the tensile strength, elongation, and wear resistance of the steel by the TWIP effect. However, a carbon content of less than 0.8% cannot impart yield strength, tensile strength, and elongation to the steel of the present invention. On the other hand, when the carbon content exceeds 1.3%, the elongation and wear resistance of the steel decrease. The preferred content of the present invention can be maintained between 0.85% and 1.25%, more preferably between 0.9% and 1.2%.

[0017] The manganese component is present in an amount of 9.5 wt% to 22 wt%. Manganese is an important alloying element in this system and is mainly due to the fact that alloying with a very large amount of manganese stabilizes austenite down to room temperature. This helps to achieve the target properties such as wear resistance, elongation and tensile strength. When manganese is present in an amount exceeding 22%, it is difficult to refine the molten steel, and manganese exceeding 22% does not significantly contribute to the improvement of properties. When present in an amount less than 9.5%, manganese does not stabilize austenite with a volume fraction exceeding 95% at room temperature. The preferred limit for the presence of manganese is 10% to 20%, more preferably 10% to 18%, and even more preferably 10% to 14%.

[0018] The aluminum component is present in an amount of 0.01 wt% to 3 wt%. Aluminum is an essential element for suppressing carbide formation, and furthermore, aluminum contributes to an increase in yield strength. When the amount is less than 0.01%, the presence of aluminum becomes less beneficial. When exceeding 3%, aluminum may promote the formation of ferrite, which is harmful to the wear resistance of the steel of the present invention. Furthermore, the presence of Al exceeding 3% may form intermetallic compounds such as Fe-Al, Fe3-Al and other (Fe,Mn)Al intermetallic compounds, which may impart brittleness to the product and may also be harmful to the toughness of the steel. Preferably, the aluminum content is limited to between 0.01% and 2.7%, and a more preferred range is 0.01% to 2.5%.

[0019] Silicon is an element effective for suppressing carbide formation, and furthermore, silicon increases the yield strength and tensile strength of the steel of the present invention. The silicon component is present in an amount of 0.01 wt% to 3 wt%. Silicon is limited to 3% because when its level exceeds this, this element tends to form strongly adhesive oxides that generate surface defects. Furthermore, when silicon is present in an amount exceeding 3%, it forms ferrite that is harmful to achieving the target properties of the steel. Therefore, the Si component is preferably present between 0.09% and 2.6%, more preferably between 0.1% and 2%.

[0020] Sulfur and phosphorus are impurities that embrittle grain boundaries. Their respective contents should not exceed 0.03 wt% and 0.1 wt% in order to maintain sufficient hot ductility.

[0021] To prevent the precipitation of AlN during solidification and the generation of volume defects (blisters), the nitrogen content must be 0.1 wt% or less.

[0022] Niobium can be added as an optional element to the steel of the present invention up to 0.03 wt%, preferably in an amount of 0.01 wt% to 0.03 wt%, to bring about grain refinement. Grain refinement can improve the balance between strength and elongation. However, niobium tends to retard recrystallization during hot rolling, and its limit is maintained up to 0.03%.

[0023] For grain refinement, titanium can be added to the steel of the present invention as an optional element up to 0.2 wt%, preferably 0.01 to 0.2 wt%, in the same manner as niobium.

[0024] By adding copper as an optional element in an amount of 0.01 to 2.0 wt%, the strength of the steel can be increased and its corrosion resistance can be improved. A minimum of 0.01% is required to obtain such effects. However, if its content exceeds 2.0%, copper may deteriorate the surface morphology.

[0025] Nickel can be added as an optional element in an amount of 0.01 to 3.0 wt% to increase the strength of the steel, enhance austenite stability, and improve its toughness. A minimum of 0.01% is required to obtain such effects. However, if its content exceeds 3.0%, the cost efficiency is not high.

[0026] Molybdenum can be added as an optional element present in the steel of the present invention in an amount of 0 wt% to 0.5 wt%. Molybdenum plays an effective role, and Mo promotes strength and strain hardening, thus enhancing the wear resistance of the steel. However, the addition of molybdenum excessively increases the cost of adding alloying elements. For economic reasons, its content is limited to 0.5%. The preferred range of molybdenum is 0% to 0.4%, more preferably 0% to 0.3%.

[0027] Chromium can be added as an optional element of the steel of the present invention and is 0 wt% to 1.5 wt%. Chromium provides strength to the steel, but when used in excess of 1.5%, it impairs the surface finish of the steel. The preferred range of chromium is 0.01% to 1.45%, more preferably 0.01% to 1.2%.

[0028] Other elements such as calcium, cerium, boron, magnesium or zirconium can be added individually or in combination at the following weight ratios, i.e., Ce≤0.1%, B≤0.01, Ca≤0.005, Mg≤0.005, Zr≤0.005. The maximum content levels are indicated up to.

[0029] Furthermore, some trace elements such as Sb and Sn may be derived from the processing of the steel. The maximum range in which these elements are acceptable and not harmful to the steel of the present invention is 0.05 wt% either cumulatively or individually. In the steel of the present invention, it is preferable that the content of these elements is as low as possible, preferably less than 0.03%.

[0030] The microstructure of the steel sheet according to the present invention optionally contains up to 5% carbide, and the remainder consists of austenite.

[0031] The austenite matrix exists as the main phase of the steel of the present invention, bringing about the TWIP effect in the steel of the present invention, thereby resulting in high tensile strength, high total elongation and high strain hardening that leads to excellent wear resistance at the same time in the steel. Austenite exists in the steel of the present invention in an area fraction of at least 95%, preferably between 95% and 100% in volume fraction, more preferably between 98% and 100%. The austenite of the present invention has a grain size of 15 microns or more. The austenite grain size of the present invention is preferably 15 microns to 30 microns.

[0032] Although the present invention intends to avoid the formation of carbides, carbides can be tolerated in the steel of the present invention up to an area fraction of 5%. The presence of carbides is preferably less than 4%, more preferably less than 3%, and when present, less than 2% in area fraction is more advantageous. Carbides that are not harmful to the steel according to the present invention are acicular intergranular carbides and lamellar carbides. Any other form of carbides such as Widmanstätten carbides are not tolerated in the steel of the present invention.

[0033] In addition to the above microstructure, the microstructure of the hot-rolled steel should not contain fine components such as pearlite, ferrite, martensite and bainite.

[0034] The steel sheet according to the present invention can be manufactured by any suitable method. A preferred method is to provide a semi-finished casting of steel having the chemical composition according to the present invention. Casting can be done into ingots or continuously in the form of thin slabs or thin strips, i.e., with a thickness ranging from about 220 mm for slabs to several tens of millimeters for thin strips.

[0035] However, it is preferable to use the method according to the present invention which continuously performs the following steps.

[0036] The steel according to the present invention is preferably manufactured by a method in which the manufacturing workflow from liquid steel to the final hot-rolled steel is shortened by the method according to the present invention. The complete manufacturing process is carried out continuously, and the molten steel having the above-described composition is cast in the form of a continuous thin slab having a thickness in the range of 10 mm to 100 mm. This cast thin slab can be used directly at a temperature higher than 1000°C after casting without intermediate cooling, or can be heated to a temperature higher than 1000°C, preferably higher than 1050°C, more preferably higher than 1100°C or 1150°C before hot rolling.

[0037] Next, the continuously cast thin slab is hot rolled. The hot rolling finishing temperature must be at least 800°C, preferably at least 850°C, more preferably between 850°C and 950°C. The hot rolling finish is maintained above 800°C to ensure that the hot rolling is completed in a region where the microstructure is not 100% recrystallized.

[0038] The hot-rolled strip thus obtained is then cooled, and such cooling is started immediately after the finish of hot rolling. The hot-rolled strip is cooled at a cooling rate CR1 of 1°C / second to 150°C / second from the finish of hot rolling to a cooling stop temperature of less than 490°C. In a preferred embodiment, the cooling rate CR1 is 2°C / second to 120°C / second, and more preferably, the cooling rate CR1 is between 3°C / second and 100°C / second.

[0039] Thereafter, the hot-rolled steel is wound up at a winding temperature CT, and CT is less than 490°C. Preferably, CT is 20°C to 480°C, and more preferably, the winding is carried out at 25°C to 4750°C.

[0040] The wound hot-rolled steel is cooled from the winding temperature to room temperature at a cooling rate CR2 of 0.0001°C / second to 1°C / second. In a preferred embodiment, the cooling rate CR2 is 0.0001°C / second to 0.8°C / second for obtaining the hot-rolled steel.

[0041] The hot-rolled steel thus obtained preferably has a thickness between 0.5 mm and 12 mm, more preferably between 0.5 mm and 10 mm, and even more preferably between 0.5 mm and 8 mm.

[0042] Subsequently, an optional pickling or any other scale removal process can be carried out to facilitate further processing of the obtained hot-rolled steel.

[0043] To protect the steel according to the present invention, in a preferred embodiment, the steel may optionally be coated with a metal film or paint, or any other known film for having appropriate corrosion resistance. The metal film can be an aluminum-based film or a zinc-based film.

[0044] Preferably, the aluminum-based film contains less than 15% Si, less than 5.0% Fe, optionally 0.1% - 8.0% Mg, and optionally 0.1% - 30.0% Zn, with the balance being Al.

[0045] Advantageously, the zinc-based film contains 0.01 - 8.0% Al, optionally 0.2 - 8.0% Mg, with the balance being Zn.

Examples

[0046] The following tests, examples, illustrative examples, and tables presented in this specification are essentially non-limiting and must be considered for illustrative purposes only and show the advantageous features of the present invention.

[0047] Steel plates made of steels with different compositions are summarized in Table 1, and the steel plates are manufactured according to the process parameters defined in Table 2 respectively. Subsequently, the microstructure of the steel plates obtained during the test is summarized in Table 3, and the results of evaluating the obtained properties are summarized in Table 4.

[0048]

Table 1

[0049]

Table 2

[0050] Next, the obtained samples were analyzed, and the corresponding microstructure elements and mechanical properties were summarized in Tables 3 and 4, respectively.

[0051] Table 3 summarizes the results of tests conducted according to standards on different microscopes such as SEM, EPMA, EBSD, XRD, or any other microscope to determine the microstructure composition of both the steel of the present invention and the reference tests. The area fraction of carbide was measured for the polished sample after etching for 10 seconds in a 2% nital etching solution and observed by SEM. Austenite was optically measured and observed by SEM.

[0052] The results are specified in this specification.

[0053]

Table 3

[0054] Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. Tensile tests were conducted according to JIS Z2241 standard to measure the tensile strength, yield strength, and total elongation.

[0055] Summarize the results of various mechanical tests conducted according to standards.

[0056]

Table 4

[0057] The examples show that the steel sheets according to the present invention are the only ones that exhibit all the target properties due to their specific composition and microstructure.

Claims

1. High manganese hot-rolled steel sheet, containing the following elements expressed in weight percentages, namely: 0.8% ≤ Carbon ≤ 1.3% 9.5% ≤ Manganese ≤ 22% 0.01% ≤ Silicon ≤ 3% 0.01% ≤ Aluminum ≤ 0.04% 0.003% ≤ Phosphorus ≤ 0.005% 0.0015% ≤ Sulfur ≤ 0.003% 0% ≤ Nitrogen ≤ 0.01% It includes any of the following elements, namely, 0% ≤ Titanium ≤ 0.003% 0.01%≦Copper≦0.028% 0% ≤ Boron ≤ 0.01% It may contain one or more of the above, and the remaining composition shall consist of iron and unavoidable impurities resulting from the processing, and the microstructure of the steel sheet shall contain 0% to 5% carbides consisting of 95% or more austenite, acicular intergranular carbides and layered carbides in area fraction, and the particle size of the austenite particles shall be 15 to 30 microns. The steel sheet is a high-manganese hot-rolled steel sheet having a tensile strength of 850 MPa or more, a yield strength of 350 MPa or more, and a total elongation of 25% or more, and a wear loss of less than 82 g / mm³ according to the G65 test, with the tensile strength, yield strength and total elongation tests being conducted according to the JIS Z2241 standard.

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

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

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

5. The high-manganese hot-rolled steel sheet according to claim 1, wherein the composition contains 10% to 18% manganese.

6. The high-manganese hot-rolled steel sheet according to claim 1, wherein the amount of austenite is 98% to 100%.

7. The high-manganese hot-rolled steel sheet according to claim 1, wherein the steel sheet has a total elongation of 25% or more.

8. A method for manufacturing a high-manganese hot-rolled steel sheet, comprising the following series of steps. - A step of providing the steel composition described in claim 1, - A step of casting a thin slab having a thickness range of 10 mm to 100 mm at a temperature exceeding 1000°C without intermediate cooling. - A step of rolling the cast thin slab in the austenite range, wherein the hot rolling finish temperature is set to at least 800°C to obtain a hot-rolled strip. - Next, a step of cooling the hot-rolled strip, the step of starting the cooling immediately after the finishing of the hot rolling, - Next, the hot-rolled strip is cooled at a cooling rate CR1 of 1°C / sec to 150°C / sec from the hot-rolling finish down to a cooling stop temperature range of less than 490°C. - Subsequently, the hot-rolled strip is wound up in a winding temperature range of less than 490°C. - Next, the winded hot-rolled strip is cooled to room temperature at a cooling rate CR2 of 0.0001°C / sec to 1°C / sec, thereby obtaining a high-manganese hot-rolled steel sheet.

9. The method according to claim 8, wherein the high-manganese hot-rolled steel sheet has a thickness of 0.5 mm to 12 mm.

10. The method according to claim 8, wherein the hot rolling finishing temperature is at least 850°C.

11. The method according to claim 8, wherein the cooling rate CR1 from hot rolling finish to the cooling stop temperature is 2°C / second to 120°C / second.

12. Use of a steel sheet according to any one of claims 1 to 7, or a steel sheet manufactured according to the method described in claims 8 to 11, for the manufacture of industrial machinery, yellow goods, or green goods components.

13. An industrial machine comprising the component described in claim 12.