Hot rolled and steel sheet and a method of manufacturing thereof

Abstract

A hot rolled steel sheet having a composition comprising of the following elements, 0.04% ≤ Carbon ≤ 0.1 %,1 % ≤ Manganese ≤ 1.50%,0.5% ≤ Silicon ≤ 1.3%,0 02% ≤ Aluminum ≤ 0.08 %, 0.1% ≤ Chromium ≤ 0.6%,0.02% ≤ Niobium ≤ 0.08%, 0.02% ≤ Phosphorus ≤ 0.1 %, 0 % ≤ Sulfur ≤ 0.005 %,0 % ≤ Nitrogen ≤ 0.012%, 0% ≤ Molybdenum ≤ 0.3%, 0% ≤ Vanadium ≤ 0.2, 0.0001% ≤ Calcium ≤ 0.005%, 0 % ≤ Nickel ≤ 0. 2%,0 % ≤ Magnesium ≤ 0.0010%, 0 % ≤ Copper ≤ 0.2% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 68% to 90% Ferrite, 5% to 25% Bainite, 2% to 10% Pearlite, Martensite and / or Austenite islands from 0% to 2% wherein the said hot rolled steel sheet has an inclusion density from 10 inclusions per square micro-meter to 100 inclusions per square micro-meters and a the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equal to or more than 35% of the nominal niobium content present in the steel in percentage by weight.

Description

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

[0002]

[0001] The present invention relates to hot rolled steel sheets suitable for use as steel sheet for automobiles.

[0003]

[0002] 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]

[0003] 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]

[0004] 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]

[0005] W02004 / 1 13581 discloses a steel material having a structure consisting mainly of a hard phase and excellent in fatigue crack growth characteristics and a method for producing the same are provided. The structure is mainly composed of ferrite I bainite, and the half width of the X- ray diffraction intensity from the (1 10) plane is 0.13 degrees or more. Specifically, the steel composition is C: 0.01 to 0.10%, Si: 0.03 to 0.6%, Mn: 0.5 to 2.0%, sol. Al: 0.005-0.10%, N: 0.0005-0.008%, and Ft (3Mn + Cu + 1.5Cr + 1.8Ni + 1.5Mo): 4.0-6.0%, or B It may contain 0.0030% or less, Ft: 3.5 to 5.5%, and may further contain one or more of Cu, Ni, Cr, and Mo. Manufactured by quenching after hot rolling. However, W02004 / 1 13581 is not able to reach the hole expansion ratio of 50% or more.

[0007]

[0006] The purpose of the present invention is to solve these problems by making available cold-rolled steel sheets that simultaneously have:

[0008]

[0007] an ultimate tensile strength of 640MPa or more in both rolling direction as well as transversal direction and preferably more than 660 MPa in both rolling direction as well as transversal direction

[0009]

[0008] a yield strength of 525 MPa or more in both rolling direction as well as transversal direction and preferably more than 535 MPa in both rolling direction as well as transversal direction

[0010]

[0009] a uniform elongation of more than 10.5% in both rolling direction as well as transversal direction and preferably a uniform elongation of more than 1 1 % in both rolling direction as well as transversal direction

[0011]

[0010] a hole expansion ratio of more than 50% and preferably above 55%

[0012]

[0011] In a preferred embodiment, the steel sheets according to the invention may also present a yield strength to tensile strength ratio of 0.5 or more

[0013]

[0012] In a preferred embodiment, the steel according to the invention may also have a total elongation of more than 14% in both rolling direction as well as transversal direction and preferably a total elongation of more than 15% in both rolling direction as well as transversal direction.

[0014]

[0013] Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and coat-ability.

[0015]

[0014] 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.

[0016]

[0015] The hot rolled steel sheet of the present invention may optionally be coated with zinc or zinc alloys, to improve its corrosion resistance.

[0017]

[0016] Carbon is present in the steel from 0.04% to 0.1 %. Carbon is an element necessary for increasing the strength of the steel sheet by controlling the ferrite formation and carbon also impart the steel with strength by precipitate strengthening by forming Carbides or carbo nitrides, therefore, Carbon plays a pivotal role in increasing the strength. But Carbon content less than 0.04% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.1 %, the steel exhibits poor spot weldability which limits its application for the automotive parts. A preferable content for the present invention may be kept from 0.05% to 0.09%.

[0018]

[0017] Manganese content of the steel of present invention is from 1 % and 1.50%. This element is gammagenous and influence Bs and Ms temperatures therefore plays an important role in controlling the Ferrite 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 1 .50% 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 .5% it promotes the central segregation hence reduces the formability and also deteriorates the weldability of the present steel. A preferable content of Manganese for the present invention may be kept from 1 % to 1 .4%.

[0019]

[0018] Silicon content of the steel of present invention is from 0.5% to 1 .3%. Silicon is solid solution strengthener especially for microstructures Ferrite and Bainite. In addition, a higher content of Silicon can retard the precipitation of Cementite. However, disproportionate content of Silicon may lead to surface defects which adversely effects the coat-ability of the steel of present invention. Therefore, the concentration is controlled within an upper limit of 1 .3%. A preferable content for Silicon for the present invention may be kept from 0.6% to 1 .2% and more preferably 0.6% to 1.1 %.

[0020]

[0019] Aluminum is an element that is present in the steel of the present invention from 0.02% to 0.08%. 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%.

[0021]

[0020] Chromium is an essential element for the present invention. Chromium content is present in the steel of present invention from 0.1 % and 0.6%. Chromium is an element that provides hardenability to the steel but higher content of Chromium higher than 0.6% leads to central co-segregation similar to Manganese. A preferable content for Chromium for the present invention may be kept from 0.1% to 0.5%.

[0022]

[0021] Niobium is an essential element for the present invention. Niobium content is present in the steel of present invention from 0.02% to 0.08% and is added in the steel of present invention for forming carbides or carbonitrides to impart strength to the steel of present invention by precipitation strengthening. A preferable content for Niobium for the present invention may be kept from 0.02% to 0.07% and more preferably from 0.03% to 0.07%.

[0023]

[0022] Phosphorus essentially constituent the steel of present invention from 0.02% to 0.1%. Phosphorus reduces the weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or cosegregate with manganese. For these reasons, its content is limited from 0.02% to 0.07% and more preferably from 0.02% to 0.05%.

[0024]

[0023] Sulphur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention the Sulphur content is preferably as low as possible, preferably less than 0.005% or less. Further if higher Sulphur is present in steel it combines to form Sulphides especially with Manganese and reduces its beneficial impact on the steel of present invention.

[0025]

[0024] Nitrogen is limited to 0.012% in order to avoid ageing of the material, nitrogen forms the nitrides and carbo-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 0.012% it can form high amount of Aluminum Nitrides which are detrimental for the present invention hence the preferable upper limit for nitrogen is 0.01 %.

[0026]

[0025] Molybdenum is an optional element that constitutes 0% to 0.3% 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 0.3%. Preferable limit for molybdenum is from 0% to 0.2%.

[0027]

[0026] Vanadium is an optional element that constitutes from 0% to 0.2% 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 0.2% due to the economic reasons. These carbides, nitrides or carbo-nitrides are formed during the second and third step of cooling. Preferable limit for Vanadium is from 0% to 0.1%.

[0028]

[0027] Calcium content in the steel of present invention is from 0.0001 % to 0.005%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment, thereby, retarding the harmful effects of Sulphides.

[0029]

[0028] Nickel may be added as an optional element in an amount of 0% to 0.2% to increase the strength of the steel and to improve its toughness.

[0030]

[0029] Copper may be added as an optional element in an amount up to 0.2% to increase the strength of the steel and to improve its corrosion resistance. However, when its content is above 0.2%, it can degrade the surface aspects.

[0031]

[0030] Other elements such as, Magnesium can be added individually or in combination in the following proportions by weight: Magnesium 0.0010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The steel of the present invention is completely free from Titanium and Boron.

[0032]

[0031] The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.

[0033]

[0032] The microstructure of the Steel sheet comprises:

[0034]

[0033] Ferrite constitutes from 68% to 90% of microstructure by area fraction for the Steel of present invention. Ferrite constitutes the primary phase of the steel as a matrix and cumulatively comprises of Polygonal ferrite and acicular ferrite. Ferrite imparts elongation as well as formability to the steel of the present invention. To ensure a hole expansion ratio of 50% and preferably 55% or more it is necessary to have 68% of Ferrite. Ferrite is formed during the cooling after hot rolling in steel of present invention. But whenever ferrite content is present above 90% in steel of the present invention the tensile strength is not achieved. The preferred presence for the ferrite for the present invention is from 70% to 88%.

[0035]

[0034] Bainite constitutes from 5% to 25% of microstructure by area fraction for the Steel of present invention. Bainite of the present invention comprises of Upper Bainite and Lower Bainite. To ensure tensile strength of 640 MPa and preferably 660 MPa or more it is necessary to have at least 5% of Bainite. Bainite starts forming during the cooling step and forms till the coiling. The preferred limit for presence of the bainite for the present invention is therefore from 6% and 22% by area fraction.

[0036]

[0035] Pearlite is an essential microstructure of the steel of present invention and present from 2% to 10%. Pearlite impart strength and toughness to the steel. Pearlite of the present has a lamellar structure. Pearlite is formed during the cooling after hot rolling temperature and till coiling temperature. Whenever the Pearlite is present more than 10% the steel of the present invention is not able to achieve the 50% hole expansion ratio. The preferred limit for presence of the pearlite for the present invention is therefore from 2% to 8% by area fraction.

[0037]

[0036] Conventionally it is known 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 50% 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, oxysulphides 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 from 10 inclusions per square micrometer to 100 inclusions per square micrometer when measured on any surface of the steel. A preferred number of inclusions is from 20 inclusions per square micrometer to 80 inclusions per square micrometer. The inclusion of the present invention preferably has an average size from 1 .5 microns to 2.5microns.

[0037] Further, to obtain the targeted mechanical properties especially yield strength and tensile strength, the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equal to or more than 35% of the nominal niobium content present in the steel by percentage in weight and preferably 40% or more.

[0038]

[0038] Martensite and / or Residual Austenite islands optionally may be present from 0% to 2% cumulatively by area fraction and are found in traces. In a preferred embodiment, the cumulated amounts of Martensite and / or Residual Austenite islands is from 0% to 1%.

[0039]

[0039] In addition to the above-mentioned microstructure, the microstructure of the hot rolled steel sheet is free from microstructural components, such as Cementite but may be found in traces.

[0040]

[0040] 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 50mm to 300mm.

[0041]

[0041] 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.

[0042]

[0042] The temperature of the slab, which is subjected to hot rolling, is preferably at least 1150° C and must be below 1325°C. In case the temperature of the slab is lower than 1 150° C, excessive load is imposed on the 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 1325°C must be avoided because it causes productivity loss and is also industrially expensive. Therefore, the preferred reheating temperature is from 1200°C to 1300°C.

[0043]

[0043] Hot rolling finishing temperature for the present invention is from 825°C to 925°C and preferably from 840°C to 920°C.

[0044] The hot rolled strip obtained in this manner is then cooled in two step cooling process wherein the step one of cooling starts after the finishing of hot rolling and in the cooling step one the hot rolled strip is cooled from finishing of hot rolling to a temperature range from 690°C to 790°C and more

[0044] 5 preferably from 700°C to 780°C. Thereafter the step two of cooling starts from temperature range 690°C to 790°C to the coiling temperature range which is from 450°C to 550°C. The first cooling step may comprise an optional slow cooling sub-step with a cooling rate of 10°C / s or less.

[0045]

[0045] The average cooling rate for both steps is from 10°C / s to 50°C / s in

[0046] 10 accordance with the present invention. During the first cooling step, the ferrite microstructure is formed and the micro-alloying elements such as Niobium forms carbides and carbo-nitrides to impart strength to the steel. In the second step of cooling, the pearlite transformation starts followed by the bainite transformation. This bainite transformation kept on going till the hot rolled strip crosses the Ms temperature while cooling and thereafter the bainite transformation stops.

[0047]

[0046] Thereafter the hot rolled strip is coiled from the temperature range 450°C to 550°C and preferably from 460°C to 540°C. Then cooling the coiled hot rolled strip to room temperature to obtain a hot rolled steel sheet.

[0048] 20

[0047] EXAMPLES

[0049]

[0048] 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.

[0050] 25

[0049] 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.

[0051]

[0052]

[0050] Table 1

[0053]

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

[0054]

[0052] Table 2

[0055] 5

[0053] Table 2 gathers the process parameters implemented on steels of

[0056] Table 1 .

[0057]

[0054] I = according to the invention; R = reference; underlined values: not according to the invention.

[0058]

[0055] Table 3

[0059] 10

[0056] 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. Ferrite, Martensite and Bainite are measured by EBSD method on a FEG-SEM at a magnification of 1500X using Aztec Software. Pearlite is measured by an image analysis on W-SEM at a magnification of 5 3000x using Metalia software. Percentage of Niobium present as carbides, nitrides and / or carbo-nitrides of the nominal niobium content of steel is measured as the report published as Report number EUR 20938 EN Technical steel research of European Commission entitled as “Development of methods for the characterization and modelling of precipitation in steels”

[0060] 10

[0057] The results are stipulated herein:

[0061]

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

[0062]

[0059] Table 4

[0063] 15

[0060] Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and uniform elongation tests are conducted in accordance of NF EN ISO 6892-1 standards. The hole expansion test is conduct in accordance of ISO TS 16630-201 standard. The results of the various mechanical tests

[0064] 20 conducted in accordance to the standards are gathered.

[0065]

[0061] Table 4

[0066]

[0067]

[0062] 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:0.04% < Carbon < 0.1 %1 % < Manganese < 1 .50% 0.5% < Silicon < 1.3%0 02% < Aluminum < 0.08 %0.1 % < Chromium < 0.6%0.02% < Niobium < 0.08%0.02% < Phosphorus < 0.1 %0 % < Sulfur < 0.005 %0 % < Nitrogen < 0.012% and can contain one or more of the following optional elements0% < Molybdenum < 0.3%0% < Vanadium < 0.20.0001 % < Calcium < 0.005%0 % < Nickel < 0. 2%0 % < Magnesium < 0.0010%0 % < Copper < 0.2% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 68% to 90% Ferrite, 5% to 25% Bainite, 2% to 10% Pearlite, Martensite and / or Austenite islands from 0% to 2% wherein the said hot rolled steel sheet has an inclusion density from 10 inclusions per square micro-meter to 100 inclusions per square micro-meters and a the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equal to or more than 35% of the nominal niobium content present in the steel in percentage by weight .

2. Hot rolled steel sheet according to claim 1 , wherein the composition includes 0.6% to 1 .2% of Silicon.

3. Hot rolled steel sheet according to claim 1 or 2, wherein the composition includes 0.05% to 0.09% of Carbon.

4. Hot rolled steel sheet according to claim 3, wherein the composition includes 0.1 % to 0.5% of Chromium5. Hot rolled steel sheet according to anyone of claim 1 to 4, wherein the composition includes 1 % to 1 .4% of Manganese.

6. Hot rolled steel sheet according to anyone of claim 1 to 5, wherein the composition includes 0.03% to 0.07% of Niobium.

7. Hot rolled steel sheet according to anyone of claim 1 to 6, wherein the composition includes 0.02% to 0.06% of Aluminum.

8. Hot rolled steel sheet according to anyone of claims 1 to 7, wherein the amount of ferrite is from 70% to 88%9. Hot rolled steel sheet according to anyone of claims 1 to 8, wherein said steel sheet has a hole expansion ratio of 50% or more.

10. A method of production of a cold rolled heat treated steel sheet comprising the following successive steps:- providing a steel composition according to anyone of claims 1 to 7;- reheating said semi-finished product to a temperature froml 150°C to 1325°C;- rolling the said semi-finished product in the austenitic range wherein the hot rolling finishing temperature shall be from 825°C to 925°C to obtain a hot rolled steel strip;- then cooling the said hot rolled strip in a two-steps cooling wherein: o the step one of cooling the hot rolled steel sheet starts from a temperature range from 825°C to 925°C to a temperature range from 690°C to 790°C, with an average cooling rate from 10°C / s to 50°C / s;o the step two of cooling the hot rolled steel sheet starts from a temperature range from 690°C to 790°C to a temperature range from 450°C to 550°C, with an average cooling rate from 10°C / s to 50°C / s- thereafter coiling the said hot rolled steel strip at a temperature range from 450°C to 550°C;- cooling the coiled hot rolled steel strip to room temperature.1 1 . A method according to claim 10, wherein the reheating temperature for semifinished product is from 1200°C to 1300°C.

12. A method according to claim 10 or 1 1 , wherein the hot rolling finishing temperature is from 840°C to 920°C.

13. A method according to anyone of claims 10 to 12, wherein the coiling temperature range is from 460°C to 540°C.

14. Use of a steel sheet according to anyone of claims 1 to 9 or of a steel sheet produced according to the method of claims 10 to 13, for the manufacture of structural or safety parts of a vehicle.

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