The present invention relates to hot rolled steel sheets suitable for use as steel sheet for green goods such as parts or ancillary for agriculture machinery, mining machinery and engineering machinery.

Agricultural Machinery, mining machinery and engineering machinery, such as plough wheels, dozer, shovel loader, excavator, wagon tremie and various mining machinery, grab bucket, stacker-reclaimer, crusher jaw, tractor shoe are mandated to have good wear resistance and steel used to manufacture these equipment depend primarily on the hardness of the steel to achieve good wear resistance and higher hardness can provide good wear resistance. However, increase the hardness, is detrimental for other properties, such as ductility and fatigue. In order to obtain steels having both very good wear-resistance and good suitability for use, therefore, means other than increasing the hardness have been sought.

Additionally, such agriculture and mining equipment loose efficacy rapidly owing to wear and tear, and waste of material that causes material as well as financial loss. Today modem industry demands high speed development and the demand running speed of mechanical means is increasingly high, and market increases the increasing demand of wear resisting steel, therefore; Development of high wear resisting steel is mandated to reduce the loss that causes wearing and tearing.

Therefore, intense Research and development endeavors are put in to increase the hardness of the steel while keeping other properties same to improve the wear resistance of the steel.

Earlier research and developments in the field of high strength and high hardness steel sheets have resulted in several methods for producing steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:

EP2695960 is an abrasion resistant steel plate or steel sheet suitable for use in construction machines, industrial machines, and the like and a method for manufacturing the same. In particular, a steel plate or steel sheet has a composition containing 0.20% to 0.30% C, 0.05% to 1.0% Si, 0.40% to 1.20% Mn, P, S, 0.1% or less Al, 0.01 % or less N, and 0.0003% to 0.0030% B on a mass basis, the composition further containing one or more of Cr, Mo, and W, the composition further containing one or more of Nb, Ti, Cu, Ni, V, an REM, Ca, and Mg as required, the remainder being Fe and inevitable impurities. A semi-finished product having the above steel composition is heated, hot rolling is performed, air cooling is performed, reheating is performed, and accelerated cooling is then performed or accelerated cooling is performed immediately after hot rolling. However the steel of EP2695960 is not able to achieve the hardness of 550Hv or more.

The purpose of the present invention is to solve these problems by making available hot rolled steel sheets that simultaneously have:

- a hardness of greater than or equal to 580Hv and preferably above 600 Hv,

- a wear loss of steel of at most 0.085mm ³ /s in accordance to wear ASTM-G55 standard and preferably at most 0.080mm ³ /s..

In a preferred embodiment, the steel sheets according to the invention may also present a tensile strength 1800 MPa or more

Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and bendability.

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

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

Carbon is present in the steel of present invention is from 0.38% to 0.5%. Carbon is an element necessary for increasing hardness of the Steel of present invention by producing a low-temperature transformation phases such as Tempered Martensite, carbon also impart the steel with strength by precipitate strengthening by forming Iron carbides, Vanadium Carbide or Niobium Carbides. But Carbon content less than 0.36% will not be able to impart the strength as well as hardness to the steel of present invention. On the other hand, at a Carbon content exceeding 0.5%, the steel exhibits poor fatigue properties which limits its application for the agricultural machinery parts. A preferable content for the present invention may be kept from 0.39% to 0.48% and more preferably from 0.39% to 0.45%.

Manganese content of the steel of present invention is from 1 % to 2%. This element is gammagenous and also influence Bs and Ms temperatures therefore plays an important role in controlling the Martensite formation. The purpose of adding Manganese is essentially to impart hardenability to the steel. An amount of at least 1 % by weight of Manganese has been found in order to provide the strength and hardenability to the steel sheet. But when Manganese content is more than 2% it produces adverse effects such as it retards transformation of Austenite during the cooling after hot rolling. In addition, the Manganese content of above 1 .8% it promotes the central segregation hence reduces the formability and also deteriorates the weldability of the present steel. A preferable content for the present invention may be kept from 1 .3% to 1 .8%,

Silicon content of the steel of present invention is from 0.1 % to 0.7%. Silicon is solid solution strengthener. In addition, a higher content of Silicon can retard the precipitation of Cementite. However, disproportionate content of Silicon leads to a problem such as surface defects like tiger strips which adversely effects the steel of present invention. Therefore, the concentration is controlled within an upper limit of 0.7%. A preferable content for the present invention may be kept from 0.2% to 0.6% and more preferably from 0.2% to 0.5%.

Aluminum is an element that is present in the steel of the present invention from 0.01 % to 0.1 %. Aluminum is an alphagenous element and imparts ductility to steel of present invention. Aluminum in the steel has a tendency to bond with nitrogen to form aluminum nitride hence from point of view of the present invention the Aluminum content must be kept as low as possible and preferably from 0.02% to 0.06%.

Chromium of steel of present invention is from 0.3% to 1 %. Chromium is an essential element that provide strength to the steel by solid solution strengthening and a minimum of 0.3% is required to impart the strength but when used above 1 % impairs surface finish of steel. The preferred limit for the presence of Chromium is from 0.3% to 0.9 % and more preferably from 0.3% to 0.8%. Boron is an essential element for the steel of present invention and may be present from 0.002% to 0.05%. Boron forms boro-nitirides and impart additional strength to steel of present invention when added in an amount of at least 0.002%.

Phosphorus constituent of the steel of present invention is from 0.002% to 0.02%. Phosphorus reduces the spot weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02% and preferably lower than 0.015%.

Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention the Sulfur content is preferably as low as possible, but is 0.005% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the steel of present invention, therefore preferred below 0.003%

Nitrogen is limited to 0.01 % in order to avoid ageing of material, nitrogen forms the nitrides which impart strength to the steel of present invention by precipitation strengthening with Vanadium and Niobium but whenever the presence of nitrogen is more than 0.01 % it can form high amount of Aluminum Nitrides which are detrimental for the present invention hence the preferable upper limit for nitrogen is 0.005%.

Molybdenum is an optional element that constitutes 0% to 0.5% of the Steel of present invention: Molybdenum increases the hardenability of the steel of present invention and influences the transformation of austenite during cooling after hot rolling. 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%. Preferable limit for molybdenum is from 0.01 % to 0.3%.

Vanadium is an optional element that constitutes from 0 % to 0.5% of the steel of present invention. Vanadium is effective in enhancing the strength of steel by forming carbides, nitrides or carbo-nitrides and the upper limit is 0.5% due to the economic reasons. These carbides, nitrides or carbo-nitrides are formed during the cooling after hot rolling. Preferable limit for Vanadium is from 0 % to 0.3%.

Titanium is an optional element to the Steel of present invention from 0.001 % to 0.1 %. It forms Titanium-nitrides appearing during solidification of the cast product. The amount of Titanium is so limited to 0.1 % to avoid the formation of coarse Titaniumnitrides detrimental for formability. In case the Titanium content below 0.001 % does not impart any effect on the steel of present invention.

Niobium is an optional element for the present invention. Niobium content may be present in the steel of present invention from 0% to 0.05% and is added in the steel of present invention for forming carbides or carbo-nitrides to impart strength to the steel of present invention by precipitation strengthening.

Nickel may be added as an optional element in an amount of 0% to 1 % to increase the strength of the steel present invention and to improve its toughness. A minimum of 0.01 % is preferred to get such effects. However, when its content is above 1 %, Nickel causes ductility deterioration.

Copper may be added as an optional element in an amount of 0% to 1 % to increase the strength of the of Steel of present invention and to improve its corrosion resistance. A minimum of 0.01 % is preferred to get such effects. However, when its content is above 1 %, it can degrade the surface aspects.

Calcium can be added to the steel of present invention in an among from 0.001 % to 0.01 %%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the Steel by binding the detrimental Sulfur content in globular form thereby retarding the harmful effect of Sulfur.

Other elements such as Mg, Sn , Pb or Sb can be added individually or in combination in the following proportions: Mg < 0.0010%, Sn ^0.1 %, Pb ^0.1 % and Sb ^0.1 %. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.

The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.

The microstructure of the Steel sheet comprises:

Martensite constitutes at least 94% of the microstructure by area fraction and preferably 95 to 99% in area fraction. The martensite of the present invention can comprise both fresh and tempered martensite. However, fresh martensite is an optional microconstituent which is preferably limited in the steel at an amount of from 0% to 4%, preferably from 0 to 2% and even better equal to 0%. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from the martensite which forms during the cooling after annealing and particularly after below Ms temperature and more particularly from Ms-10°C to 20°C.Such martensite is then tempered during the holding at a tempering temperature Temper from 100°C to 300°C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is from 95% to 99% and more preferably from 96% to 99%.

Residual Austenite is an optional constitutes for the steel of present invention and may be present from 0% to 5% by area fraction. When Residual Austenite is in excess of 5 % it lowers the hardness of the steel of present invention below an acceptable level. In a preferred embodiment, residual austenite is from 0% to 4% and more preferably from 0% to 3%.

Carbides of alloying elements might be present in the steel of present invention in a cumulated amount from 0% to 5% by area fraction such as of Chromium, Niobium, Vanadium and Iron. These carbides may increase the strength of the steel of present invention by precipitation strengthening, but whenever the presence of carbides is 5% or more, their precipitation consume partly the amount of Carbon required for the strengthening of tempered martensite. In addition to the above-mentioned microstructure, the microstructure of the hot rolled steel sheet is free from microstructural components, such as Pearlite, ferrite and Bainite but may be found in traces.

A steel sheet according to the 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 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 220mm for slabs up to several tens of millimeters for thin strip.

For example, a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.

The temperature of the slab, which is subjected to hot rolling, is preferably at least 1100° C and must be below 1300°C. In case the temperature of the slab is lower than 1100° C, excessive load is imposed on a rolling mill. Therefore, the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in the in 100% austenitic range. Reheating at temperatures above 1275°C must be avoided because it causes productivity loss and is also industrially expensive. Therefore, the preferred reheating temperature is from 1200°C to 1275°C.

Hot rolling finishing temperature for the present invention is from 850°C to 975°C and preferably from 860°C to 930°C.

The hot rolled strip obtained in this manner is then cooled wherein the cooling starts immediately after the finishing of hot rolling and in the cooling step hot rolled strip is cooled from finishing of hot rolling to a coiling temperature range from550°C to 750°C, preferably at a cooling rate from1 °C/s to 150°C/s. In a preferred embodiment, the cooling rate for the of cooling step is from1 °C/s to 120°C/s. Thereafter the hot rolled strip fromis coiled in the temperature range of 550°C to 750°C and preferably from570°C to 720°C and more preferably from 580°C to 700°C. Then cooling the coiled hot rolled strip to room temperature.

The coiled hot rolled strip may be optionally cut into steel pieces to subjected to at least one mechanical manufacturing operation. Mechanical operation may comprise tapering, cutting, forming, turning, honing or any other suitable mechanical operation or manufacturing procedure that is required to form the part or ancillary for agriculture machinery, mining machinery and engineering machinery.The preferred temperature for all the mechanical operations is from 20° C to Ac3 +300° C and more preferable temperature for all the mechanical operations is from 20° C and Ac3 +250° C. After the completion of the mechanical operations the part is cooled to room temperature to obtain an non-heat treated part or ancillary for agriculture machinery, mining machinery and engineering machinery. The obtained non-heat treated part or ancillary according to the present invention must be heat treated in the identical manner as the hot rolled strip to obtain final microstructure described herein below.

Thereafter, the hot rolled strip is being heat treated which will impart the steel of present invention with requisite mechanical properties and microstructure.

The hot rolled strip is then being heated, to an annealing temperature Tsoak which is from Ac3 to Ac3 + 100°C, preferably from Ac3 +10°C to Ac3 + 100°C, at a heating rate HR1 which is from 1 °C/s to 100°C/s. In a preferred embodiment, the heating rate HR1 is from 1 °C/s to 50°C/s. Ac3 for the steel sheet is calculated by using the following formula:

Ac3 = 910 - 203 [C] ^A (l/2) - 15.2[N i] + 44.7[5i] + 104[V] + 31.5[Mo] + 13.1[W ]

- 30[Mn] - ll[Cr] - 20[Cu] + 700[P] + 400 [Al] + 120[As] + 400[Ti] wherein the elements contents are expressed in weight percentage of the cold rolled steel sheet. The hot rolled strip is held at Tsoak during 10 seconds to 1000 seconds to ensure a complete recrystallization and full transformation to austenite of the strongly work hardened initial structure.

Then the hot rolled strip is cooled from Tsoak at a cooling rate CR1 from 1 °C/s and 150°C/s, to a temperature T1 which is in a range from Ms-75°C and 20°C. In a preferred embodiment, the cooling rate CR1 for such step of cooling is from 15°C/s and 120°C/s. The preferred T1 temperature for such step is from Ms-100°C and 20°C.

Ms for the steel sheet is calculated by using the following formula:

Ms = 545 - 601.2 * (1 - EXP(-0.868[C])) - 34.4[Mn] - 13.7[5i] - 9.2[Cr] - 17.3[N i]

- 15.4[Mo] + 10.8[V] + 4.7[Co] - 1.4[Al] - 16.3[Cu] - 361[N b]

- 2.44[Ti] - 3448[B]

Thereafter the hot rolled strip is reheated to a tempering temperature Ttemper from 100°C to 300°C, preferably with a heating rate of at least 1 °C/s and preferably of at least 2°C/s and more of at least 5°C/s during 10s and 10 hours. The preferred temperature range for tempering is from 150°C to 250°C and the preferred duration for holding at Ttemper is from 200 s to 9hours.

Then, the hot rolled strip is cooled down to room temperature to obtain a the hot rolled steel sheet.

EXAMPLES

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.

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. Table 1

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

Table 2

Table 2 gathers the process parameters implemented on steels of Table 1 .

5

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

Table 3

Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of the steel.

The results are stipulated herein:

Table 4

Table 4 exemplifies the mechanical properties of the steel. In order to determine the tensile strength the tensile tests are conducted in accordance of JIS Z2241 standards and the wear test is conducted in accordance of Wear ASTM-G55 standards.

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

Table 4