A hot rolled steel sheet and a method of manufacturing thereof

Abstract

A hot rolled steel sheet having a composition comprising of the following elements,0.02 % ≤ Carbon ≤ 0.06 %, 1.1 % ≤ Manganese ≤ 1.60%,0.01% ≤ Aluminum ≤ 0.08 %, 0.01% ≤ Niobium ≤ 0.07%, 0.05 % ≤ Titanium ≤ 0.15%, 0.0005% ≤ Calcium ≤ 0.005%, 0% ≤ Phosphorus ≤ 0.03 %, 0 % ≤ Sulfur ≤ 0.015 %, 0 % ≤ Nitrogen ≤ 0.02%,and can contain one or more of the following optional elements 0.001% ≤ Silicon ≤ 0.09%, 0% ≤ Chromium ≤ 0.2%, 0 ≤ Copper ≤ 0.25%, 0% ≤ Nickel ≤ 0.2%,0% ≤ Vanadium ≤ 0.1%, 0 % ≤ Boron ≤ 0.003%, 0 % ≤ Magnesium ≤ 0.010%, 0% ≤ Cerium≤ 0.1%, 0% ≤ Zirconium ≤ 0.010%,the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 35% to 75% Bainite, 25% to 65% ferrite, from 0% to 2% Pearlite and Martensite-Residual Islands from 0% to 2% wherein said hot rolled steel sheet has an inclusion density of 200 inclusions per square micro-meter, the inclusions having size of 2 microns or more being 25% or less of the total number of inclusions.

Description

[0001] A HOT ROLLED STEEL SHEET AND A METHOD OF MANUFACTURING THEREOF

[0002] The present invention relates to hot rolled steel sheets suitable for use as steel sheet for automobiles.

[0003] Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns. Thus, now automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency.

[0004] Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated.

[0005] Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:

[0006] US20140178712 is hot rolled steel sheet which has a maximum tensile strength of 600 MPa or more and has an excellent low temperature impact energy absorption and HAZ softening resistance and a method of production of the same are provided, that is, sheet which contains, by mass %, C: 0.04 to 0.09%, Si: 0.4% or less, Mn: 1 .2 to 2.0%, P: 0.1 % or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02 to 0.09%, Ti: 0.02 to 0.07%, and N: 0.005% or less, where 2.0^Mn+8[% Ti]+12[% Nb]^2.6, has a balance of Fe and unavoidable impurities, has an area percentage of pearlite of 5% or less, has a total area percentage of martensite and retained austenite of 0.5% or less, has a balance of a metal structure of ferrite and / or bainite, has an average grain size of ferrite and bainite of 10 pm or less, has an average particle size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less, and has a yield ratio of 0.85 or more. However, the steel of US20140178712 does not demonstrate a HER of 55% or more.

[0007] The purpose of the present invention is to solve these problems by making available a hot rolled steel sheets that simultaneously have:

[0008] - A TS / YS ratio greater than or equal to 1.10.

[0009] - an ultimate tensile strength greater than or equal to 625MPa to 850MPa and preferably from 650MPa to 825MPa, a yield strength greater than or equal to greater than or equal to 575MPa to 750MPa and preferably from 600MPa to 725MPa,

[0010] - a total elongation greater than or equal to 16% and preferably greater than or equal to 18%.

[0011] - a hole expansion ratio of greater than or equal to 55% and preferably above 65%

[0012] Preferably, such steel can also have a good suitability for forming, for rolling with good weldability and coatability.

[0013] 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.

[0014] The hot rolled steel sheet of the present invention is be coated with zinc or zinc alloys, to improve its corrosion resistance.

[0015] Carbon is present in the steel from 0.02% to 0.06%. Carbon is an element necessary for increasing the strength of the steel sheet by interstitial strengthening as well as via forming micro-alloyed precipitates. If C is lower than 0.02%, it is difficult to achieve the required tensile strength of 625 MPa or more in combination with the required elongation higher than 16%. On the other hand, at a Carbon content exceeding 0.06%, the steel exhibits poor spot weldability which limits its application for the automotive parts. High Carbon content may promote the formation of second phases like pearlite, martensite or cementite during cooling after hot rolling which decreases holeexpansion ratio. The preferred range for carbon for the steel of present invention is therefore 0.025% to 0.055%. Manganese content of the steel of present invention is from 1.1 % to 1.60%. The purpose of adding Manganese is essentially to impart strength to the steel by solid solution strengthening. This element is gammagenous and also influences the Bs and Ms temperatures therefore plays an important role in controlling the bainite and martensite formation. If Mn is lower than 1.1 %, it is difficult to achieve the required strength for the steel according to the present invention in combination with the required elongation higher than 16%. In addition, the Manganese content of above 1 .60% it produces adverse effects such as it retards transformation of Austenite during the cooling after hot rolling thereby reducing the ductility hence the elongation targets may not be achieved. A preferable content for the present invention may be kept from 1.15% to 1.55%

[0016] Aluminum is an essential element and is present in the steel of present invention from 0.01 % to 0.08%. Aluminum promotes ferrite formation which allows the present invention to have ferrite in adequate amount to achieve the desired strength and ductility combination for the steel of present invention. However, when the presence of Aluminum is more than 0.08%, which makes hot rolling finishing temperature in complete Austenitic region. The Aluminum content is preferably limited from 0.015% to 0.07%.

[0017] Niobium is an essential element for the steel of present invention from 0.01 % to 0.07% and suitable for forming carbides and carbonitrides to impart strength of the steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbides and by retarding the recrystallization during hot rolling and also retarding the austenitic grain size. Thus, finer microstructure formed in the final product as a consequence the steel of present invention is able to reach the targeted strength. However, Niobium content above 0.07% is not economically interesting. Additionally, when the content of niobium is 0.07% or more is detrimental for steel because it increase the rolling force in the finishing rolling mill hence increase the difficulties during steel hot rolling. The preferred limit for niobium content is from 0.01 % to 0.06% and more preferably from 0.02% to 0.05%. Titanium is an essential element and may be added to the steel of present invention from 0.05% to 0.15%. As Niobium, it is involved in carbo-nitrides formation so plays a role in hardening of the steel of present invention. In addition, Titanium also forms Titanium-nitrides, Titanium-oxynitrides and Titanium-carbonitrides which appear during solidification of the cast product. The amount of Titanium is so limited to 0.15% to avoid the formation of coarse Titanium-nitrides inclusions that are detrimental for steel formability. The preferred limit for titanium content is from 0.06% to 0.12%.

[0018] Calcium is a mandatory element and added from 0.0005% to 0.005% in the steel of present invention. Calcium is added to steel of present invention especially during the inclusion treatment with a preferred minimum amount of 0.0005%. Calcium contributes towards the refining of steel by arresting the detrimental Sulfur content in globular form, further Calcium also facilitates casting by avoiding the clogging during the casting, thereby, retarding the harmful effects of Sulfur.

[0019] Phosphorus is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the phosphorus content is preferably as low as possible, and below 0.03%. 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 less than 0.03%, preferably less than 0.02%.

[0020] Sulfur is not an essential element but may be contained as an impurity in steel and from the point of view of the present invention the Sulfur content is preferably as low as possible but is 0.015% or less from the viewpoint of desulphurization manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and decreases the positive influence of Manganese on the steel of present invention.

[0021] Nitrogen is limited to 0.02% to avoid ageing of material and to control the precipitation of nitrides inclusion during solidification which are detrimental for the mechanical and formability properties of the steel.

[0022] Silicon is an optional element and may be present from 0.001 % to 0.09%. Silicon adds strength to ferrite through solid solution strengthening. However, when contained in an amount more than 0.09%, silicon concentrates at the steel sheet surface in the form of an oxide during hot rolling. For this reason, the Silicon content is restricted to 0.09% or less. The Silicon content is preferably from 0.005% to 0.08%.

[0023] Copper may be present as an optional element and may be present up to 0.25% to increase the strength of the steel and to improve its corrosion resistance. A minimum of 0.03% of copper is preferable to get such effect. However, when its content is above 0.25%, it can degrade the surface aspects. The most preferred limit is from 0.05% to 0.2%.

[0024] Chromium is an optional element for the present invention. Chromium content may be present in the steel of present invention from 0% to 0.2%. Chromium provides strength and hardening to the steel but when used above 0.2% it impairs surface finish of steel. The preferred limit for Chromium for the present invention is from 0% to 0.15%.

[0025] Nickel may be present as an optional element in an amount up to 0.2% to increase the strength of the steel and to improve its toughness. A minimum of 0.01 % is preferred to produce such effects. However, when its content is above 0.2%, Nickel causes ductility deterioration as well as excessively increases the cost of the addition of alloy elements. Molybdenum is an optional element that constitutes 0% to 0.2% of the Steel of present invention; Molybdenum increases the hardenability of the steel of present invention and influences the transformation of austenite to Ferrite and Bainite 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 0.2%. Preferable limit for molybdenum is from 0% to 0.15%.

[0026] Vanadium is an optional element that may be found in traces in the steel of present invention and is not added voluntarily as an element which is effective in enhancing the strength of steel by forming carbides, nitrides or carbo-nitrides and the upper limit is 0.1 %.

[0027] Other elements such as Cerium, Boron, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Cerium 1=0.1 %, Boron 0.003%, Magnesium 0.010% and Zirconium 0.010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.

[0028] The remainder of the composition of the steel is iron and unavoidable impurities resulting from the smelting process and depending on the process route. In the case of a production route using a blast furnace, the level of unavoidable impurities is very low. In the case of a production route using an Electric Arc Furnace loaded with scraps, the steel sheet can further comprise residual elements coming from such scraps such as Copper, Nickel, Molybdenum, Zinc, Antimony, Arsenic and Lead, up to a cumulated amount of 1 %, in addition to the amounts obtained by a blast furnace route.

[0029] The microstructure of the Steel sheet will now be described.

[0030] Ferrite constitutes from 25% to 65% of microstructure by area fraction for the Steel of present invention. Ferrite cumulatively comprises of polygonal ferrite and acicular ferrite. Ferrite imparts elongation as well as formability to the steel of the present invention. To ensure an elongation of 16% and preferably 18% or more it is necessary to have 25% of Ferrite. Ferrite is formed during the cooling after hot rolling in steel of present invention. But whenever ferrite content is present above 65% in steel of the present invention the tensile strength is not achieved. The preferred limit for presence of the ferrite for the present invention is therefore from 30% to 60% by area fraction.

[0031] Bainite constitutes from 35% to 75% of microstructure by area fraction for the Steel of present invention. Bainite constitutes the primary phase of the steel as a matrix and cumulatively consists of Upper Bainite and Lower Bainite. To ensure tensile strength of 625 MPa and preferably 650 MPa or more it is necessary to have 35% of Bainite. Bainite starts forming during the coiling step and during the cooling after hot rolling especially below the Bs temperature. The preferred limit for presence of the bainite for the present invention is therefore from 40% and 70% by area fraction.

[0032] Pearlite is an optional microstructure of the steel of present invention and present from 0% to 2%. Pearlite may impart strength and toughness to the steel. Pearlite is formed during the cooling after hot rolling temperature and till coiling temperature. Whenever the Pearlite is present more than 2%, the steel of the present invention is not able to achieve the 55% hole expansion ratio.

[0033] Martensite-Austenitic islands and Austenite also may be optionally present from 0% to 2% as trace microstructure.

[0034] Conventionally it is know that the inclusions deteriorates the ductility and flangeability of the steel sheet and also causes the defects such as internal defects. This happens because inclusion forms voids in the steel during deformation of the steel sheet and promotes the ductile fracture to cause the deterioration of the HER. However, the inventors did not bound themselves by this phenomenon and inventors controlled the size and density of the inclusions to reach HER of more than 55% for the steel of present invention. The inclusions of present invention are from one or more selected from the group consisting of inclusions as oxide, sulphides, oxynitrides, oxysulphides, nitrides and / or carbo-nitrides. The inclusions of the present invention are formed during the cooling after the casting processes and are contained in an amount of at least 200 inclusions per square micrometre when measured on any surface of the steel. A preferred amount of inclusions is from 200 inclusions per square micrometre to 680 inclusions per square micrometre. The inclusion presence for the hereinbefore mentioned inclusions having size of 2 microns or more must be controlled to 25% or less of the total number of inclusions present and preferably less than 20% to have of total number of inclusions the steel of present invention is able to achieve a HER of 55%.

[0035] 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 is done continuously or in batches in form of slabs or, i.e. with a thickness ranging from 40mm to 120mm for slabs, wherein a minimum casting speed of 3.5m / min must be maintained during the casting process and preferably greater than or equal to 4m / min. The preferred range for the slab thickness is from 50mm to 70mm.

[0036] 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. 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.

[0037] The slab is reheated to a homogenously reheating temperature from 1075° C to 1 175° C and preferably from 1 100° C to 1 150° C. Thereafter the temperature of the slab, which is subjected to hot rolling, is at least 1075° C and must be below 1 175°C. In case the temperature of the slab is lower than 1075° C, excessive load is imposed on a rolling mill and the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in 100% Austenitic range.

[0038] The final rolling pass has to be performed at a temperature greater than 850°C to finish the hot rolling, because below this temperature the steel sheet exhibits a significant drop in rollability. The preferred hot rolling finishing temperature is from 850°C to 975°C. The hot rolled steel obtained in this manner is then cooled at a cooling rate above 30°C / s to the average coiling temperature which must be from 525°C to 675°C. The preference is to keep the average coiling temperature from 550°C to 650°C is to maximize the precipitation of Niobium and Titanium during cooling after hot rolling and coiling. Preferably, the cooling rate will be less than or equal to 150° C / s.

[0039] The hot rolled steel sheet can optionally be coated with hot dip coating process known industrially.

[0040] If required, skin pass rolling may be optionally performed on the hot rolled steel sheet with a minimum skin pass reduction ranging from 0.3 to 1 .5%.

[0041] Thereafter the hot rolled steel sheet of the present invention is obtained.

[0042] EXAMPLES

[0043] 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.

[0044] 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.

[0045] Table 1

[0046] 5 underlined values: not according to the invention.

[0047] Table 2 io Table 2 gathers the annealing process parameters implemented on steels of Table 1 . The Steel compositions 11 ,12 and R1 , R2 serve for the manufacture of sheets according to the invention.

[0048] Following processing parameters are same for all the steels of Table 1 .

[0049] The table 2 is as follows:

[0050] 15 Table 2

[0051] I = according to the invention; R = reference; underlined values: not according to the invention.

[0052] 20 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 both the inventive and reference steels.

[0053] The results are stipulated herein: I = according to the invention; R = reference; underlined values: not according to the invention.

[0054] The steels according to the invention include more than 350 inclusions per square pm and even more than 400 inclusions per square pm.

[0055] Table 4 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 NBN EN ISO 6892-1 method B. Hole expansion tests are carried out according to ISO 16630

[0056] The results of the various mechanical tests conducted in accordance with the standards are gathered

[0057] Table 4

[0058] I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

CLAIMS1. A hot rolled steel sheet having a composition comprising of the following elements, expressed in percentage by weight:0.02 % < Carbon < 0.06 %1.1 % < Manganese < 1 .60%0.01 % < Aluminum < 0.08 %0.01 % < Niobium < 0.07%0.05 % < Titanium < 0.15%0.0005% < Calcium < 0.005%0% < Phosphorus < 0.03 %0 % < Sulfur < 0.015 %0 % < Nitrogen < 0.02% and can contain one or more of the following optional elements 0.001 % < Silicon < 0.09%0% < Chromium < 0.2%0 < Copper < 0.25%0% < Nickel < 0.2%0% < Vanadium < 0.1%0 % < Boron < 0.003%0 % < Magnesium < 0.010% 0% < Cerium< 0.1 %0% < Zirconium < 0.010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 35% to 75% Bainite, 25% to 65% ferrite, from 0% to 2% Pearlite and Martensite-Residual Islands from 0% to 2% wherein said hot rolled steel sheet has an inclusion density of at least 200 inclusions per square micro-meter, the inclusions having size of 2 microns or more being 25% or less of the total number of inclusions.

2. Hot rolled steel sheet according to claim 1 , wherein the composition includes 0.025% to 0.055% of Carbon.

3. Hot rolled steel sheet according to claim 1 or 2, wherein the composition includes 0.06% to 0.12% of Titanium.

4. Hot rolled steel sheet according to anyone of claim 1 to 3, wherein the composition includes 1.15% to 1 .55% of Manganese.

5. Hot rolled steel sheet according to anyone of claim 1 to 4, wherein the composition includes 0.015% to 0.07% of Aluminum.

6. Hot rolled steel sheet according to anyone of claims 1 to 5, wherein the amount of Bainite is from 40% and 70%7. Hot rolled steel sheet according to anyone of claims 1 to 6, wherein said steel sheet has a tensile strength from 625MPa to 850 MPa, and a hole expansion ratio of 55% or more.

8. Hot rolled steel sheet according to claim 7, wherein said steel sheet has a strength from 650MPa to 825 MPa, and a total elongation of 16% or more.

9. A method of production of a hot rolled steel sheet comprising the following successive steps:- providing a steel composition according to anyone of claims 1 to 5- wherein the said steel composition is provided in form of casted semifinished product with a thickness ranging from 40mm to 120mm for slabs, while a minimum casting speed of 3.5m / min must be maintained during the casting- the said slab provided by casting process may optionally be used directly at a high temperature after the casting or may be first cooled to room temperature and then reheated for hot rolling- reheating said slab to a temperature from 1075°C to 1 175°C;- rolling the said semi-finished product in the 100% austenitic range wherein the hot rolling finishing temperature shall be more than 850°C to obtain a hot rolled steel strip;- then cooling the said hot rolled strip to a temperature range from 525°C to 675°C, with an average cooling rate more than 30°C / s;- thereafter coiling the said hot rolled steel strip at a temperature range from 525°C to 675°C;- cooling the coiled hot rolled steel strip to room temperature to obtain a hot rolled steel sheet .

10. A method according to claim 9, wherein the reheating temperature for semifinished product is from 1100°C to 1 150°C.1 1. A method according to claim 9 or 10, wherein the hot rolling finishing temperature is from 850°C to 975°C.

12. A method according to anyone of claims 9 to 1 1 , wherein the coiling temperature range is from 550°C to 650°C.

13. A method according to anyone of claims 9 to 12, wherein the average cooling rate after hot rolling for cooling is from 30°C / s to 150°C / s.

14. Use of a steel sheet according to anyone of claims 1 to 8 or of a steel sheet produced according to the method of claims 9 to 13, for the manufacture of structural or safety parts of a vehicle.

15. Vehicle comprising a part obtained according to claim 14.

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