CN119278290A - High manganese hot rolled steel and production method thereof - Google Patents

Abstract

A high manganese hot rolled steel having a composition comprising 0.8% or less carbon 1.3%, 9.5% or less manganese 22%, 0.01% or less silicon 3%, 0.01% or less aluminum 3%, 0.03% or less phosphorus 0.1%, 0.03% or less sulfur 0.1%, 0% or less nitrogen 0.01%, 0% or less niobium 0.03%, 0% or less titanium 0.2%, 0% or less chromium 1.5%, 0% or less molybdenum 0.5%, 0% or less calcium 0.005%, 0.01% or less copper 2%, 0.01% or less nickel 3%, 0% or less boron 0.01%, 0% or less magnesium 0.005%, the remainder being composed of iron and unavoidable impurities due to processing, the microstructure of the steel comprising, in terms of area fraction, 95% or more austenite, 0% to 5% carbide, wherein the grain size of austenite is 15 μm or more.

Description

High-manganese hot rolled steel and production method thereof

Technical Field

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

The invention may also be used as structural steel or for manufacturing industrial machines, work machine parts (yellow goods), prototype parts (green goods), or any other similar industrial application.

Background

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, as the wear resistance or abrasion resistance of the steel increases, elongation and toughness generally deteriorate. Therefore, in the development of high-strength steel, an important problem is to increase wear resistance without deteriorating elongation and toughness.

Considerable research and development effort has been devoted to reducing the amount of material utilized by increasing the strength of the material. Conversely, an increase in strength of steel decreases elongation and wear resistance, and therefore materials combining high strength, elongation and toughness with wear resistance must be developed.

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:

EP2971211 discloses an improved steel composition and provides a method for preparing the same. The disclosure provides advantageous wear resistant steel. 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 composition/composition of the disclosure improves one or more of wear resistance, ductility, crack resistance, erosion resistance, fatigue life, surface hardness, stress corrosion resistance, fatigue resistance, and/or environmental crack resistance. In general, the disclosure provides high manganese steels suitable for resistance to wear and/or corrosion. However, the steel of EP2971211 does not show a total elongation or tensile strength.

Disclosure of Invention

The object of the present invention is to solve these problems by making available hot rolled steel having at the same time:

A yield strength of 350MPa or more, and preferably equal to or greater than 375MPa,

A tensile strength of 850MPa or more, and preferably 880MPa or more,

A total elongation of greater than or equal to 25%, and preferably greater than or equal to 30%,

Less than 82G/mm ³ according to the G65 test, and preferably less than 77G/mm ³ according to the G65 test.

In a preferred embodiment, the steel sheet according to the present invention may further exhibit a hardness of 180BHN or more.

In a preferred embodiment, the steel sheet according to the present invention may further exhibit a Charpy-V impact toughness of greater than or equal to 60J/cm ² when tested at-40 ℃.

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

It is also an object of the invention to make available a method for manufacturing these steels compatible with conventional industrial processes while being robust to manufacturing parameter variations.

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

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

Without wishing to be bound by any theory, it appears that the hot rolled steel according to the invention allows to improve the mechanical properties thanks to this particular microstructure.

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 steel by the TWIP effect. Carbon contents less than 0.8% will not impart yield and tensile strength and elongation to the steel of the present invention. On the other hand, at carbon contents 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%.

The manganese content is present in the range of 9.5 wt% to 22 wt%. 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 to achieve target properties such as wear resistance, elongation and tensile strength. If manganese is present at more than 22%, it is difficult to refine the molten steel, and manganese at more than 22% does not significantly contribute to improvement of characteristics. Manganese, when present at less than 9.5%, will not stabilize austenite at room temperature at volume fractions above 95%. The preferable limit of the presence of manganese is 10% to 20%, and more preferably 10% to 18%, and even more preferably 10% to 14%.

The aluminum content is present in the range of 0.01 wt.% to 3 wt.%. Aluminum is an essential element to inhibit carbide formation, and furthermore, aluminum contributes to an increase in yield strength. Below 0.01%, the presence of aluminium becomes less beneficial. Above 3%, aluminum may promote the formation of ferrite detrimental to the wear resistance of the steel of the invention. Furthermore, the presence of more than 3% Al may form intermetallic compounds that will impart brittleness to the product and may also be detrimental to the toughness of the steel, such as Fe-Al, fe ₃ -Al and other (Fe, mn) Al intermetallic compounds. Preferably, the aluminum content will be limited to 0.01% to 2.7%, and more preferably to a limit of 0.01% to 2.5%.

Silicon is an element effective in suppressing carbide formation, and furthermore, silicon improves the yield strength and tensile strength of the steel of the present invention. The silicon content is present in the range of 0.01 wt% to 3 wt%. Silicon is limited to up to 3% because above this level, this element has a tendency to form a strongly adherent oxide that creates surface defects. Furthermore, when silicon is present at more than 3%, it will form ferrite detrimental to achieving the target properties of the steel. Therefore, the Si content will preferably be present at 0.09% to 2.6%, and more preferably 0.1% to 2%.

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

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

Niobium may be added as an optional element to the steel of the 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. Niobium, however, has a tendency to hinder recrystallization during hot rolling, and thus the limit is kept to 0.03%.

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

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

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

Molybdenum may be added as an optional element, which is present in the steel of the invention in an amount of 0 to 0.5 wt.%, molybdenum is effective, mo increasing the strength and strain hardening, thus increasing the wear resistance of the steel. However, excessive addition of molybdenum increases the addition cost of the alloy element, and thus the content thereof is limited to 0.5% for economic reasons. The preferable limit of molybdenum is 0% to 0.4%, and more preferably 0% to 0.3%.

Chromium may be added as an optional element of the steel of the present invention in an amount of 0 to 1.5% by weight. Chromium provides strength to the steel but compromises the surface finish of the steel when used at greater than 1.5%. The preferable limit of chromium is 0.01% to 1.4.5%, and more preferably 0.01% to 1.2%.

Other elements such as calcium, cerium, boron, magnesium or zirconium may be added singly or in combination in proportions of Ce 0.1%, B0.01, ca 0.005, mg 0.005 and Zr 0.005 by weight. Up to the indicated maximum level of content.

In addition, some trace elements such as Sb, sn may come from the processing of steel. These elements are acceptable and harmless to the steel of the invention, either cumulatively to the maximum or individually to 0.05% by weight. The content of these elements of the steel of the invention is preferably as low as possible and preferably less than 0.03%.

The microstructure of the steel sheet according to the invention optionally comprises up to 5% carbides, the remainder being composed of austenite.

The austenitic matrix exists as the main phase of the steel of the present invention, which provides the steel of the present invention with the TWIP effect, thereby providing the steel with a high strain hardening, resulting in a high tensile strength, a high total elongation and at the same time excellent wear resistance. 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 austenite of the present invention has a grain size of 15 microns or more. It is preferred that the austenite grain size of the present invention is 15 microns to 30 microns.

The present invention aims to avoid carbide formation, however for the steel of the present invention up to 5% carbide by area fraction may be acceptable. The carbide is preferably present at less than 4%, more preferably at less than 3%, and even more advantageously if it is present at less than 2% in area fraction. Carbides harmless to the steel according to the present invention are needle-like inter-crystalline carbides and lamellar carbides. Any other form of carbide, such as welsh carbide (WIDMANSTATTEN CARBIDE), is not acceptable for the steel of the present invention.

In addition to the above-described microstructure, the microstructure of the hot rolled steel should be free of microstructure constituent components such as pearlite, ferrite, martensite, and bainite.

The steel sheet according to the present invention may be produced by any suitable method. A preferred method comprises providing a semifinished casting of steel having the chemical composition according to the invention. The castings may be ingot made or continuously manufactured in the form of thin slabs or strips (i.e. thickness ranging from about 220mm for slabs to tens of millimeters for thin strips).

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

The steel according to the invention is preferably produced by a method wherein the production workflow from liquid phase steel to final hot rolled steel is shortened according to the process of the invention. The entire production process is continuously carried out in which molten steel having the above composition is cast in the form of a continuous thin slab having a thickness ranging from 10mm to 100 mm. The cast sheet bar may be used directly after casting at temperatures greater than 1000 ℃ without intermediate cooling, or may be heated to temperatures greater than 1000 ℃, preferably greater than 1050 ℃, and more preferably greater than 1100 ℃ or 1150 ℃ prior to being hot rolled.

The continuously cast sheet bar is then subjected to hot rolling. The hot rolling finishing temperature must be at least 800 ℃, and preferably at least 850 ℃, and more preferably from 850 ℃ to 950 ℃. The hot rolling finish is maintained above 800 ℃ to ensure that the hot rolling must be completed in areas where the microstructure is not 100% unrecrystallized.

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

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

The coiled, hot rolled steel is cooled from coiling temperature to room temperature at a cooling rate CR2 of 0.0001 ℃ per second to 1 ℃ per second. In a preferred embodiment, the cooling rate CR2 is from 0.0001 ℃ per second to 0.8 ℃ per second to obtain a hot rolled steel.

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

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

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

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.

Advantageously, the zinc-based coating comprises 0.01% to 8.0% Al, optionally 0.2% to 8.0% Mg, the remainder being Zn.

Examples

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

Steel sheets made of steels having different compositions are summarized in table 1, wherein the steel sheets were produced according to the process parameters as specified in table 2, respectively. Thereafter, table 3 summarizes the microstructure of the steel sheet obtained during the test, and table 4 summarizes the evaluation results of the obtained characteristics.

TABLE 1 composition

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

TABLE 2 Process parameters

The resulting samples were then analyzed and the corresponding microstructural elements and mechanical properties are summarized in tables 3 and 4, respectively.

TABLE 3 Table 3

Table 3 summarizes the results of tests performed according to the standard on different microscopes, such as SEM, EPMA, EBSD, XRD or any other microscope, for determining the microstructure composition of both the steel of the invention and the reference test. After the polished sample was etched in a 2% nitric acid ethanol (Nital) etching solution for 10 seconds, the area fraction of carbide was measured and observed by SEM. Austenite was measured and optically observed by SEM.

The results are noted herein:

TABLE 4 Properties

Table 4 illustrates the mechanical properties of both the inventive steel and the reference steel. To determine the tensile strength, yield strength and total elongation, a tensile test was performed according to JIS Z2241 standard.

The results of various mechanical tests performed according to the standards are summarized.

I=according to the invention, r=reference, underlined values: not according to the invention.

The examples show that the steel sheet according to the invention is the only steel sheet exhibiting all target properties due to its specific composition and microstructure.

Claims (15)

1. A high manganese hot rolled steel having a composition, expressed in weight percent, comprising:

carbon content of 0.8% to 1.3%

Manganese is more than or equal to 9.5 percent and less than or equal to 22 percent

Silicon is more than or equal to 0.01 percent and less than or equal to 3 percent

Aluminum is more than or equal to 0.01 percent and less than or equal to 3 percent

Phosphorus is more than or equal to 0.03 percent and less than or equal to 0.1 percent

Sulfur is more than or equal to 0.03 percent and less than or equal to 0.1 percent

Nitrogen is more than or equal to 0 percent and less than or equal to 0.01 percent

And can include one or more of the following optional elements:

Niobium is more than or equal to 0 percent and less than or equal to 0.03 percent

Titanium is more than or equal to 0 percent and less than or equal to 0.2 percent

Chromium is more than or equal to 0 percent and less than or equal to 1.5 percent

Molybdenum is more than or equal to 0 percent and less than or equal to 0.5 percent

Calcium content of 0% or more and 0.005% or less

Copper is more than or equal to 0.01 percent and less than or equal to 2 percent

Nickel is more than or equal to 0.01 percent and less than or equal to 3 percent

Boron is more than or equal to 0 percent and less than or equal to 0.01 percent

Magnesium is more than or equal to 0 percent and less than or equal to 0.005 percent

The remainder of the composition is composed of iron and unavoidable impurities resulting from processing, and the microstructure of the steel contains, in area fraction, 95% or more austenite, 0% to 5% carbide, wherein the grain size of the austenite grains is 15 μm or more.

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

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

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

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

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

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

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 steel sheet has a total elongation of 25% or more.

10. A method of producing a high manganese hot rolled steel comprising the sequential steps of:

-providing a steel composition according to any one of claims 1 to 7;

-casting a thin slab having a thickness in the range of 10mm to 100mm at a temperature of more than 1000 ℃ and without intermediate cooling;

-rolling said cast sheet bar in an austenitic range wherein the hot rolling finish temperature should be at least 800 ℃ to obtain a hot rolled steel;

-then cooling the hot rolled strip, wherein the cooling starts immediately after the hot rolling is finished;

-then cooling the hot rolled steel from the end of hot rolling to a cooling stop temperature range of less than 490 ℃ at a cooling rate CR1 of 1 ℃ to 150 ℃ per second;

-thereafter coiling the hot rolled steel in a coiling temperature range of less than 490 ℃;

-then cooling the coiled hot rolled strip to room temperature at a cooling rate CR2 of 0.0001 ℃ to 1 ℃ per second to obtain a high manganese hot rolled steel.

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

12. The method of claim 10 or 11, wherein the hot rolling finish rolling temperature is at least 850 ℃.

13. The method according to any one of claims 10 to 12, wherein the cooling rate CR1 from the end of hot rolling to a cooling stop temperature is 2 ℃ per second to 120 ℃ per second.

14. Use of a steel sheet according to any one of claims 1 to 9 or produced according to the method of 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.