High Manganese hot rolled steel and a method of production thereof

[0001]The present invention deals with a hot rolled high manganese steel presenting a microstructure mainly comprising austenite. The steel according to the present invention is particularly well suited for the manufacture of parts or equipment having good wear resistance such as buckets for excavators or earth movers and tipper bins.

[0002]The present invention may also be used as structural steel or for manufacturing industrial machinery, yellow goods, green goods or any other similar industrial applications.

BACKGROUND

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

SUMMARY OF THE INVENTION

[0004]Intense Research and Development endeavors are put in reducing the amount of material utilized by increasing the strength of material. Conversely, an increase in strength of steel decreases elongation and wear resistance, and thus development of materials combining high strength, elongation and toughness with wear resistance is necessitated.

[0005]Earlier Research and Developments in the field of high strength and good toughness steel have resulted in several methods for producing high strength wear resistant steel, some of which are enumerated herein for conclusive appreciation of the present invention:

[0006]EP2971211 discloses an improved steel compositions and methods of making the same are provided. The present disclosure provides advantageous wear resistant steel. More particularly, the present disclosure provides high manganese (Mn) steel having enhanced wear resistance, and methods for fabricating high manganese steel compositions having enhanced wear resistance. The advantageous steel compositions/components of the present 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. In general, the present disclosure provides high manganese steels tailored to resist wear and/or erosion.

[0007]However, the steel of EP2971211 does not demonstrate total elongation or tensile strength.

[0013]The present invention provides a High Manganese hot rolled steel having a composition comprising of the following elements, expressed in percentage by weight:

$0\%\le \mathrm{Niobium}\le 0.03\%;$$0\%\le \mathrm{Titanium}\le 0.2\%;$$0\%\le \mathrm{Chromium}\le 1.5\%;$$0\%\le \mathrm{Molybdenum}\le 0.5\%;$$0\%\le \mathrm{Calcium}\le 0.005\%;$$0.01\%\le \mathrm{Copper}\le 2\%;$$0.01\%\le \mathrm{Nickel}\le 3\%;$$0\%\le \mathrm{Boron}\le 0.01\%;$$0\%\le \mathrm{Magnesium}\le 0.005\%;$

the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel consisting in, in area fraction, 95% or more of Austenite, 0% to 5% Carbides wherein the grain size of the grains of Austenite is 15 microns or more.

[0015]In a preferred embodiment, the steel sheets according to the present invention may also present hardness of greater than or equal to 180 BHN.

[0016]In a preferred embodiment, the steel sheets according to the present invention may also present Charpy-V impact toughness of greater than or equal to 60 J/cm² when tested at −40° C.

[0017]Preferably, such steel can also have a good suitability for cold forming such as bending, roll forming and drawing.

[0018]Another object of the present invention is also to make available a method for the manufacturing of these steels that is compatible with conventional industrial processes while being robust towards manufacturing parameters shifts.

DETAILED DESCRIPTION

[0019]The hot rolled steel sheet of the present invention may optionally be coated with zinc or zinc alloys, to improve the corrosion resistance.

[0020]Other characteristics and advantages of the present invention will become apparent from the following detailed description of the present invention.

[0021]Without willing to be bound by any theory it seems that the hot rolled steel according to the present invention allows for an improvement of the mechanical properties thanks to this specific microstructure.

[0022]Carbon is present in the steel between 0.8% and 1.3%. Carbon is an element necessary for stabilizing the austenite down to room temperature. It also increases the yield strength of the steel. Carbon is an essential contribution to the tensile strength, elongation and wear resistance of the steel through the TWIP effect. But Carbon content less than 0.8% will not be able to impart the yield 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 properties. A preferable content for the present invention may be kept between 0.85% and 1.25%, and more preferably between 0.9% and 1.2%.

[0023]Manganese content is present from 9.5% to 22% by weight. Manganese is an important alloying element in this system, mainly due to the fact that alloying with very high amounts of manganese stabilizes the austenite down to room temperature, which can assist in reaching the target properties such as wear resistance, elongation and tensile strength. If the Manganese is present above 22% it is difficult to elaborate of the liquid steel and above 22% Manganese does not contribute to enhancement of properties significantly. Manganese when present below 9.5% will not stabilize the austenite at room temperature above 95% of volume fraction. The preferred limit for the presence of Manganese is from 10% to 20% and more preferably from 10% to 18% and even more preferably from 10% to 14%.

[0024]Aluminum content is present from 0.01% to 3% by weight. Aluminum is an essential element to suppress the carbide formation and additionally Aluminum contributes to the increase of yield strength. Below 0.01%, the presence of aluminum becomes less beneficial. Above 3%, the aluminum may promote the formation of ferrite which is detrimental for the wear resistance of the steel of the present invention. Moreover the presence of Al above 3% may forms intermetallics such as Fe—Al, Fe₃—Al and other (Fe,Mn)Al intermetallics which will impart brittleness to the product and may also be detrimental for the toughness of the steel. Preferably, the aluminum content will be limited between 0.01% and 2.7% and more preferred limit is from 0.01% to 2.5%.

[0025]Silicon is an element which is effective in suppressing carbide formation and additionally Silicon increases yield and tensile strength of the steel of the present invention. Silicon content is present from 0.01% to 3% by weight. Silicon is limited up to 3% because above that level this element has a tendency to form strongly adhesive oxides that generate surface defects. Further, when Silicon present above 3% it will form ferrite which is detrimental for achieving the targeted properties of the steel. Therefore, the Si content will preferably present between 0.09% and 2.6% and more preferably 0.1% and 2%.

[0026]Sulfur and phosphorus are impurities that embrittle the grain boundaries. Their respective contents must not exceed 0.03% and 0.1% by weight so as to maintain sufficient hot ductility.

[0027]Nitrogen content must be 0.1% or less by weight so as to prevent the precipitation of AlN and the formation of volume defects (blisters) during solidification.

[0028]Niobium may be added as an optional element in an amount up to 0.03% by weight and preferably from 0.01% to 0.03% by weight to the steel of the present invention to provide grain refinement. The grain refinement allows obtaining a good balance between strength and elongation. But, niobium had a tendency to retard the recrystallization during hot rolling hence the limit is kept till 0.03%.

[0029]Titanium may be added as an optional element in an amount of up to 0.2% by weight and preferably from 0.01% to 0.2% by weight to the steel of the present invention for grain refinement, in a similar manner as niobium.

[0030]Copper may be added as an optional element in an amount of 0.01% to 2.0% by weight to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.01% is required to get such effects. However, when its content is above 2.0%, it can degrade the surface aspect.

[0031]Nickel may be added as an optional element in an amount of 0.01 to 3.0% by weight to increase the strength of the steel and to enhance austenite stability and improve its toughness. A minimum of 0.01% is required to get such effects. However, when its content is above 3.0% it is not cost efficient.

[0032]Molybdenum can be added as an optional element that is present from 0% to 0.5% by weight in the steel of the present invention; Molybdenum plays an effective Mo increases the strength and the strain hardening, as such increasing the wear resistance of the steel. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%. The preferable limit for Molybdenum is from 0% to 0.4% and more preferably from 0% to 0.3%.

[0033]Chromium can be added as an optional element of the steel of the present invention, is from 0% to 1.5% by weight. Chromium provides strength to the steel, but when used above 1.5% impairs surface finish of the steel. The preferred limit for chromium is from 0.01% to 1.4.5% and more preferably from 0.01% to 1.2%.

[0034]Other elements such as calcium, cerium, boron, magnesium or zirconium can be added individually 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,

[0035]Additionally some trace elements such as Sb, Sn can come from processing of the steel. The maximum limit up to which these elements are acceptable and are not detrimental for the steel of the present invention is 0.05% by weight cumulatively or alone. It is preferred by the steel of the present invention to have the content of these elements as low as possible and preferably less than 0.03%.

[0036]The microstructure of the steel sheet according to the present invention comprises, optionally of carbides up to 5%, the remainder being made of austenite.

[0037]The austenitic matrix is present as a primary phase of the steel of the present invention which provide the steel of the present invention with TWIP effect thereby providing the steel with high strain hardening leading to high tensile strength, high total elongation and simultaneously excellent wear resistance. Austenite is present in minimum at 95% by area fraction in the steel of the present invention and preferably between 95% and 100% by volume fraction and more preferably between 98% and 100%. The austenite of the present invention has grain size of 15 microns or more. It is preferred that the austenite grain size of the present invention is from 15 microns to 30 microns.

[0038]The present invention intends to avoid the carbide formation, however carbides may be acceptable for the steel of the present invention up to 5% by area fraction. Carbide presence is preferably less than 4%, more preferably less than 3% and more advantageous if present less than 2% in area fraction. Carbides that are not detrimental for the steel according to the present invention are needle-like intergranular carbides and lamellar carbides. Any other form of carbides, such as Widmanstatten Carbides, are not acceptable for the steel of the present invention.

[0039]In addition to the above-mentioned microstructure, the microstructure of the hot rolled steel should be free from microconstituent components such as Pearlite, Ferrite, Martensite and bainite.

[0040]A steel sheet according to the present invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the present invention. The casting can be done either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220 mm for slabs up to several tens of millimeters for thin strip.

[0041]It is however preferred to use the method according to the present invention, which comprises the following steps successively.

[0042]The steel according to the present invention are preferably produced through a method in which the process according to the present invention shortens the production workflow from the liquid phase steel to the final hot rolled steel. The complete production process takes place continuously wherein the liquid steel having the composition described above, is casted in the form of continuous thin slab having thickness range from 10 mm to 100 mm. This casted thin slab can be used directly at a temperature of more than 1000° C. after casting, without intermediate cooling or may 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.

[0043]The continuously casted thin slab then undergoes hot rolling. The hot-rolling finishing temperature must be at least 800° C. and preferably at least 850° C. and more preferably between 850° C. and 950° C. The hot rolling finishing is kept above 800° C. to ensure that hot rolling must be completed in a region where the microstructure is not 100% un-recrystallized.

[0044]The hot rolled strip obtained in this manner is then cooled, such cooling starting immediately after the finishing of hot rolling and the hot rolled strip being cooled from finishing of hot rolling to a cooling stop temperature which is less than 490° C. at a cooling rate CR1 from 1° C./s to 150° C./s. In a preferred embodiment, the cooling rate CR1 is from 2° C./s to 120° C./s and more preferably the cooling rate CR1 is between 3° C./s and 100° C./s.

[0045]Thereafter, the hot rolled steel is coiled at a coiling temperature CT and CT is less than 490° C. Preferably the CT is from 20° C. to 480° C. and more preferably the coiling is performed from 25° C. to 4750° C.

[0046]The coiled hot rolled steel is cooled from coiling temperature to room temperature at a cooling rate CR2 from 0.0001° C./s to 1° C./s. In a preferred embodiment, the cooling rate CR2 is from 0.0001° C./s to 0.8° C./s. to obtain a hot rolled steel.

[0047]The hot rolled steel thus obtained preferably has a thickness between 0.5 mm and 12 mm and more preferably between 0.5 mm and 10 mm and even more preferably between 0.5 mm and 8 mm.

[0048]Thereafter an optional pickling or any other scale removal process may be performed to facilitate further processing of the obtained hot rolled steel.

[0049]To protect the steel according to the preset invention from corrosion, in a preferred embodiment, the steel may be optionally covered by a metallic coating or paint or any other know coating for having adequate corrosion resistance. The metallic coating may be an aluminum-based coating or a zinc-based coating.

[0050]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.

[0051]Advantageously, the zinc-based coating comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

Examples

[0052]The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only, and will display the advantageous features of the present invention.

[0053]Steel sheets made of steels with different compositions are gathered in Table 1, where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties.

[0054]The resulting samples were then analyzed and the corresponding microstructure elements and mechanical properties were respectively gathered in table 3 and 4.

Table 3

[0055]Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as SEM, EPMA, EBSD, XRD or any other microscope for determining microstructural composition of both the inventive steel and reference trials. The area fractions for the carbides is measured on polished samples after etching them in 2% Nital etching solution for 10 seconds and observed by an SEM. Austenite is measured and observed optically by a SEM.

[0056]The results are stipulated herein

Table 4—Properties

[0057]Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are conducted in accordance of JIS Z2241 standards.

[0058]The results of the various mechanical tests conducted in accordance to the standards are gathered

[0059]The examples show that the steel sheets according to the present invention are the only one to show all the targeted properties thanks to their specific composition and microstructures.