Patent Description:
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.

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.

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:
<CIT> is a patent application that claims for a hot-dip galvanized steel sheet having a microstructure, by volume fraction, equal to or more than <NUM>% and equal to or less than <NUM>% in total of one or two of martensite and bainite, a residual structure contains one or two of ferrite, residual austenite of less than <NUM>% by volume fraction, and pearlite of equal to or less than <NUM>% by volume fraction. Further <CIT> reaches to a tensile strength of <NUM> MPa but unable to reaches the elongation of <NUM>%.

<CIT> claims for high strength galvanized steel sheet has a Tensile Strength of <NUM> MPa or more and excellent processability. The component composition contains, by mass %, C: <NUM>% to <NUM>%, Si: <NUM>% to <NUM>%, Mn: <NUM>% to <NUM>%, P: <NUM>% or lower, S: <NUM>% or lower, Al: <NUM>% or lower, and N: <NUM>% or lower, and the balance: Fe or inevitable impurities. The microstructure contains, in terms of area ratio, ferrite phases: <NUM>% to <NUM>%, bainite phases: <NUM>% to <NUM>%, and martensite phases: <NUM>% to <NUM>%, in which, among the martensite phases, martensite phases having an aspect ratio of <NUM> or more are present in a proportion of <NUM>% or more.

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

In a preferred embodiment, the steel sheets according to the invention may also present a yield strength <NUM> MPa or more
In a preferred embodiment, the steel sheets according to the invention may also present a yield strength to tensile strength ratio of <NUM> or more
Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and coatability.

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.

The cold rolled and heat treated steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or with aluminium or aluminium alloys to improve its corrosion resistance.

Carbon is present in the steel between <NUM>% and <NUM>%. Carbon is an element necessary for increasing the strength of the steel sheet by producing low-temperature transformation phases such as bainite, further Carbon also plays a pivotal role in Austenite stabilization hence a necessary element for securing Residual Austenite. Therefore, Carbon plays two pivotal roles one in increasing the strength and another in retaining austenite to impart ductility. But Carbon content less than <NUM>% will not be able to stabilize Austenite in an adequate amount required by the steel of present invention. On the other hand, at a Carbon content exceeding <NUM>%, the steel exhibits poor spot weldability which limits its application for the automotive parts.

Manganese content of the steel of present invention is between <NUM> % and <NUM>%. This element is gammagenous. The purpose of adding Manganese is essentially to obtain a structure that contains Austenite and impart strength to the steel. An amount of at least <NUM>% by weight of Manganese has been found in order to provide the strength and hardenability of the steel sheet as well as to stabilize Austenite. But when Manganese content is more than <NUM>% it produces adverse effects such as it retards transformation of Austenite to Bainite during the over-aging holding for Bainite transformation. In addition the Manganese content of above <NUM>% also reduces the ductility and also deteriorates the weldability of the present steel hence the elongation targets may not be achieved. A preferable content for the present invention may be kept between <NUM>% and <NUM>%, further more preferably <NUM>% and <NUM>%.

Silicon content of the steel of present invention is between <NUM>% and <NUM>%. Silicon is a constituent that can retard the precipitation of carbides during overageing, therefore, due to the presence of Silicon, carbon rich Austenite is stabilized at room temperature. Further, due to poor solubility of Silicon in carbide it effectively inhibits or retards the formation of carbides, hence also promotes the formation of Bainitic structure which is sought as per the present invention to impart steel with its essential features. However, disproportionate content of Silicon does not produce the mentioned effect and leads to a problem such as temper embrittlement. Therefore, the concentration is controlled within an upper limit of <NUM>%. A preferable content for the present invention may be kept between <NUM>% and <NUM>%.

Phosphorus constituent of the steel of present invention is between <NUM>% and <NUM>%. 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 <NUM> % and preferably lower than <NUM>%.

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 <NUM>% 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.

Aluminum is not an essential element but may be contained as a processing impurity in steel due to the fact that aluminum is added in the molten state of the steel to clean steel of present invention by removing oxygen existing in molten steel to prevent oxygen from forming a gas phase hence may be present up to <NUM>% as a residual element. But from point of view of the present invention the Aluminum content is preferably as low as possible.

Nitrogen is limited to <NUM>% in order to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel.

Chromium is an optional element for the present invention. Chromium content may be present in the steel of present invention is between <NUM>% and <NUM>%. Chromium is an essential element that provides strength and hardening to the steel but when used above <NUM>% it impairs surface finish of steel. Further Chromium contents under <NUM>% coarsen the dispersion pattern of carbide in Bainitic structures, hence; keep the density of carbides low in Bainite.

Nickel may be added as an optional element in an amount of <NUM> to <NUM>% to increase the strength of the steel and to improve its toughness. A minimum of <NUM>% is required to produce such effects. However, when its content is above <NUM>%, Nickel causes ductility deterioration.

Niobium is an optional element for the present invention. Niobium content may be present in the steel of present invention between <NUM> and <NUM>% and is added in the Steel of present invention for forming carbo-nitrides 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 carbo-nitrides and by retarding the recrystallization during heating process. Thus finer microstructure formed at the end of the holding temperature and as a consequence after the completion of annealing that will lead to the hardening of the Steel of present invention. However, Niobium content above <NUM>% is not economically interesting as a saturation effect of its influence is observed this means that additional amount of Niobium does not result in any strength improvement of the product.

Titanium is an optional element and may be added to the Steel of present invention between <NUM>% and <NUM>%. 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 which appear during solidification of the cast product. The amount of Titanium is so limited to <NUM>% to avoid formation of coarse Titanium-nitrides detrimental for formability. In case the Titanium content is below <NUM>% it does not impart any effect on the steel of present invention.

Calcium content in the steel of present invention is between <NUM>% and <NUM>%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of Steel by arresting the detrimental Sulfur content in globular form, thereby, retarding the harmful effects of Sulfur.

Copper may be added as an optional element in an amount of <NUM>% to <NUM>% to increase the strength of the steel and to improve its corrosion resistance. A minimum of <NUM>% of Copper is required to get such effect. However, when its content is above <NUM>%, it can degrade the surface aspects.

Molybdenum is an optional element that constitutes <NUM>% to <NUM>% of the Steel of present invention; Molybdenum plays an effective role in determining hardenability and hardness, delays the appearance of Bainite and avoids carbides precipitation in Bainite. 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 <NUM>%.

Vanadium is effective in enhancing the strength of steel by forming carbides or carbo-nitrides and the upper limit is <NUM>% due to the economic reasons. Other elements such as Cerium, Boron, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Cerium ≦<NUM>%, Boron ≦ <NUM>%, Magnesium ≦ <NUM>% and Zirconium ≦ <NUM>%. 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 microstructure of the Steel sheet comprises:
Ferrite constitutes from <NUM>% to <NUM>% of microstructure by area fraction for the Steel of present invention. Ferrite constitutes the primary phase of the steel as a matrix. In the present invention, Ferrite cumulatively comprises of Polygonal ferrite and acicular ferrite Ferrite imparts high strength as well as elongation to the steel of present invention. To ensure an elongation of <NUM>% and preferably <NUM>% or more it is necessary to have <NUM>% of Ferrite. Ferrite is formed during the cooling after annealing in steel of present invention. But whenever ferrite content is present above <NUM>% in steel of present invention the strength is not achieved.

Bainite constitutes from <NUM>% to <NUM>% of microstructure by area fraction for the Steel of present invention. In the present invention, Bainite cumulatively consists of Lath Bainite and Granular Bainite, To ensure tensile strength of <NUM> MPa and preferably <NUM> MPa or more it is necessary to have <NUM>% of Bainite. Bainite is formed during over-aging holding.

Residual Austenite constitutes from <NUM>% to <NUM>% by area fraction of the Steel. Residual Austenite is known to have a higher solubility of Carbon than Bainite and, hence, acts as effective Carbon trap, therefore, retarding the formation of carbides in Bainite. Carbon percentage inside the Residual Austenite of present invention is preferably higher than <NUM>% and preferably lower than <NUM>%. Residual Austenite of the Steel according to the invention imparts an enhanced ductility.

Martensite constitutes between <NUM>% and <NUM> % of microstructure by area fraction and found in traces. Martensite for present invention includes both fresh martensite and tempered martensite. Present invention form martensite due to the cooling after annealing and get tempered during overaging holding. Fresh Martensite also form during cooling after the coating of cold rolled steel sheet. Martensite imparts ductility and strength to the Steel of present invention when it is below <NUM>%. When Martensite is in excess of <NUM> % it imparts excess strength but diminishes the elongation beyond acceptable limit.

In addition to the above-mentioned microstructure, the microstructure of the cold rolled and heat treated steel sheet is free from microstructural components, such as pearlite and cementite without impairing the mechanical properties of the steel sheets.

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 <NUM> 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 <NUM>. 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 at least <NUM>° C and must be up to <NUM>. In case the temperature of the slab is lower than <NUM>° C, excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the slab is preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac3 to Ac3+<NUM> and final rolling temperature remains above Ac3. Reheating at temperatures above <NUM> must be avoided because they are industrially expensive.

The final rolling temperature range is between Ac3 to Ac3+<NUM> to have a structure that is favorable to recrystallization and rolling. It is necessary to have final rolling pass to be performed at a temperature greater than Ac3, because below this temperature the steel sheet exhibits a significant drop in rollability. The sheet obtained in this manner is then cooled at a cooling rate above <NUM>/s to the coiling temperature which must be below <NUM>. Preferably, the cooling rate will be less than or equal to <NUM>° C/s.

The hot rolled steel sheet is then coiled at a coiling temperature below <NUM> to avoid ovalization and preferably below <NUM> to avoid scale formation. The preferred range for such coiling temperature is between <NUM>° C and <NUM>° C. The coiled hot rolled steel sheet is cooled down to room temperature before subjecting it to optional hot band annealing.

The hot rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing. The hot rolled sheet may then subjected to an optional Hot Band Annealing at temperatures between <NUM> and <NUM> for at least <NUM> hours and not more than <NUM> hours, the temperature remaining below <NUM> to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel sheet may performed through, for example, pickling of such sheet. This hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between <NUM> to <NUM>%. The cold rolled steel sheet obtained from cold rolling process is then subjected to annealing to impart the steel of present invention with microstructure and mechanical properties.

In the annealing, the cold rolled steel sheet subjected to two steps of heating to reach the soaking temperature between Ac1+<NUM> and Ac3 wherein Ac1 and Ac3 for the present steel is calculated by using the following formula : <MAT> <MAT> wherein the elements contents are expressed in weight percent.

In step one cold rolled steel sheet is heated at a heating rate between <NUM>/s and <NUM>/s to a temperature range between <NUM> and <NUM>. Thereafter in subsequent second step of heating the cold rolled steel sheet is heated at a heating rate between <NUM>/s and <NUM>/s to the soaking temperature of annealing.

Then the cold rolled steel sheet is held at the soaking temperature during <NUM> to <NUM> seconds to ensure at least <NUM>% transformation to Austenite microstructure of the strongly work-hardened initial structure. Then the cold rolled steel sheet is then cooled in two step cooling to an over-aging holding temperature. In step one of cooling the cold rolled steel sheet is cooled at cooling rate less than <NUM>/s to a temperature range between <NUM> and <NUM>. During this step one of cooling ferrite matrix of the present invention is formed. Thereafter in a subsequent second cooling step the cold rolled steel sheet is cooled to an overaging temperature range between <NUM> and <NUM> at a cooling rate between <NUM>/s and <NUM>/s. Then hold the cold rolled steel sheet in the over-aging temperature range during <NUM> to <NUM> seconds. Then bring the cold rolled steel sheet to the temperature to a coating bath temperature range of <NUM> and <NUM> to facilitate coating of the cold rolled steel sheet. Then the cold rolled steel sheet is coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, Hot dip(GI/GA) etc..

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 <NUM>, where the steel sheets are produced according to process parameters as stipulated in Table <NUM>, respectively. Thereafter Table <NUM> gathers the microstructures of the steel sheets obtained during the trials and table <NUM> gathers the result of evaluations of obtained properties.

Table <NUM> gathers the annealing process parameters implemented on steels of Table <NUM>. The Steel compositions I1 to I4 serve for the manufacture of sheets according to the invention. This table also specifies the reference steel which are designated in table from R1 to R4. Table <NUM> also shows tabulation of Ac1 and Ac3. These Ac1 and Ac3 are defined for the inventive steels and reference steels as follows: <MAT> <MAT> wherein the elements contents are expressed in weight percent.

All sheets were cooled at a cooling rate of <NUM>/s after hot rolling and were finally brought at a temperature of <NUM> before coating.

Table <NUM> 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.

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

Claim 1:
A cold rolled and heat treated steel sheet having a composition comprising of the
following elements, expressed in percentage by weight: <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> and can contain one or more of the following optional elements <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet consisting of in area fraction, <NUM> to <NUM>% Ferrite, <NUM> to <NUM>% Bainite, <NUM> to <NUM>% Residual Austenite, and <NUM>% to <NUM>% Martensite, wherein the cumulated amounts of Bainite and Ferrite less than <NUM>% and an ultimate tensile strength of <NUM> MPa or more, and a total elongation of <NUM>% or more measured according to JIS Z2241 standard.