Title of Invention

A METHOD FOR THE PRODUCTION OF HIGH FORMABLE STEEL

Abstract

A method for the production of high formable steel which will exhibit lower Yield Strength /U tensile Strength (YS/UTS) ratio and lower strain ageing index (SAI).The method involves hot rolling a steel slab having the alloying elements composition comprising C - 0.07 - 0.09, Mn = 0.3 - 0.4, P = 0.02max., S = 0.02 max., Si = 0.04 max., Al- 0.025 - 0.035, N = 50-70 ppm. and selectively incorporating Ti = 0.015 - 0.02 as micro alloying elements for fixing of nitrogen which would enable maintaining the finishing rolling temperature of about 860°C +/- 10°C and lower coiling temperature of around 590°C +/- 10°C in order to achieve uniform mechanical property and reduced scale thickness. The method is particularly suitable for producing steel suited for LPG cylinder.

Full Text

Introduction to the Field of Invention This invention relates to a new method for the production of high formable hot rolled steel, in particular, a method for the production of high formable steel which will exhibit lower Yield Strength /U tensile Strength (YS/UTS) ratio higher elongation and lower strain ageing index (5AI). The method is particularly suitable for producing steel suited for LPG cylinder. BASIC OF INVENTION Highly formable hot rolled steel is required for preparing several end products'of which one important Application is for preparing liquefied petroleum gas cylinder (LPG Cylinders). The chemistry of steel and its process parameter determine the formabihty of steel. Further, the micro-structure of steel is another important factor, which is intimately related to the steel processing conditions. Hot rolled steel with good cold formability are characterized by low yield stress, low YS/UTS ratio combined with high total elongation (Uniform and post necking i elongation). Prior Art and Drawbacks The several combination of properties mentioned above have been achieved in steels with soft ferrite structure, free from second phase. The production of such type of steel would be too expensive and may not fulfill the strength requirements, hence a small amount of pearl ite or cementite and a certain level of ferrite strengthening elements are usually added in low carbon steel (S 0.08%). Except in tne case of hot rolling in the ferrite region, it has been thought to be difficult to improve properties of hot rolled steel by controlling texture (r value being theoretically limited to 1.0). In such conditions, hot rolled steels must exhibit the following characteristics. • Low YS and YS/UTS ratio, high ductility, high n values • Low strain + aging index ( • Maximum lankford co-effoent/high hole expansion ratio (X). Background of the invention It has been found during the several investigation studies conducted by us that the cold formability of mild, unalloyed hot rolled strip is seriously impaired by the presence of dissolved 2 nitrogen in the ferrite. It can be counteracted to great extent in the case of aluminium killed steel by optimized temperature management in the hot strip mill, e.g. high coiling temperature. However, with the high coiling temperature necessary for nitrogen fixation, a limited degree of uniformity of the mechanical properties over the strip length and unfavourable picking behaviour must be expected. Fixing of the nitrogen by Micro Alloying Elements (MAE) such as Ti or B can provide a remedy here. Objects of the Invention: It is therefore a primary object of the invention to propose a method for the production of high formable steel. The other object to propose such a method, which will produce steel suited for LPG cylinder. It is a further object to produce such a high formable steel which will exhibit higher elongation and lower hardness. In addition it is a further object of the invention to propose such a high formable steel which will exhibit lower Yield Strength/U Tensile Strength (YS/UTS) ratio and lower strain aging index (SAI). These and other objects of the invention will be apparent from the following paragraphs: Brief Statement of the Invention: According to this invention, there is provided a method for the production of high formable steel which comprises hot rolling a steel slab with following alloying elements in weight % c Mn P S Si Al Ti N 0.07-0.09 0.3-0.4 0.02 max 0.02 max 0.04 max 0.025-0.035 0.015-0.02 50-70 ppm maintaining the finishing rolling temperature of about 860±10 C and coiling temperature of around 590±10° C in order to get desired property and reduced scale thickness. The manganese content is preferably 0.35%. The steel with Ti bearing heat with reduced Mn content was processed through BOF-LF-CC route to produce the slab before hot rolling. 3 The final product produced has around 39% elongation, higher n (0.23), YS/UTS ratio 0.70, lower hardness of 57 HRB and lower strain aging index (SAI) of 8%. Detailed Description with Reference to the Following Example: A steel was taken to which Ti was added in 0.01 to 0.02% by weight to give a starting material having the following composition: - c Mn P S Si Al Ti N 0.07-0.09 0.3-0.4 0.02 max 0.02 max 0.04 max 0.025-0.035 0.015-0.02 50-70 ppm Ti bearing heat with reduced Mn content (0.35%) was processed through BOF-LF-CC route at BSL. The slabs were hot rolled by maintaining finishing temperature at 860±10° C and coiling temperature at 590±10° C. Contrary to this, processing of non-Ti bearing LPG steel demands higher coiling temperature (690±10° C) to facilitate fixing of free nitrogen to a greater extent by way of precipitation of AIN. However, it implicates on non-uniformity in the mechanical properties alongwith the strip length and formation of thicker oxide scale which is found difficult to remove completely by pickling. Therefore, the newly developed steel with Ti addition when processed in as controlled manner has proved to be a right candidate for substituting the traditional C-Mn LPG steels. We have studied the mechanical properties and the test results of developed steel show that the formability has improved significantly compared to conventional grade (non-Ti) LPG steel. This has been quantified in terms of higher elongation (39%-Ti, 30% non-Ti), higher n (0.23-Ti, 0.21 non-Ti), lower hardness (57 HRB-Ti, 62 HRB-non-Ti) and lower strain aging index (SAI) (8%-Ti, 17% non-Ti). The beneficial effect of lowering in SAI (from 17% to 8%) resulted in improved formability of steel even after aging. Salient Features of Innovations: -High degree of formability properties with following characteristics: - • Reduced level of SAI (even with high nitrogen level of 50-70 ppm) • Higher ductility • Lower hardness 4 A combination of these attributes lead to defect free easy fabrication of LPG cylinders with lower forming load. -Uniformity in tensile property along the coil length and lower amount of scale formation due to adherence to low coiling temperature (590±10°C). 5 We daim: 1. A method for the production of high formable steel comprising: hot rolling a steel slab having the alloying elements composition comprising C - 0.07 - 0.09, Mn = 0.3 - 0.4t P = 0.02max.f S = 0.02 max.. Si = 0.04 max., Al= 0.025 - 0.035, N = 50-70 ppm. and selectively incorporating Ti = 0.015 - 0.02 as micro alloying elements for fixing of nitrogen; maintaining the finishing rolling temperature of about 860°C +/- 10°C and coiling temperature of around 590PC +/- 10°C in order to achieve uniform mechanical property and reduced scale thickness. 2. A method as daimed in Claim 1 wherein the manganese content is preferably 0.35%. 3. A method as daimed in any one of daims 1 and 2 wherein the steel with Ti bearing heat with reduced Mn content is processed through BOF- LF-CC route to produce the slab before hot rolling. 4. A method as daimed in any one of daims l to 3 wherein the final product produced has around 39% elongation, higher n (0.23), low YS/UTS ratio of 0.70, lower hardness of 57 HRB and lower strain ageing index (SAI) of 8%. 5. A method for the preparation of high formable steel substantially as herein described with reference to the accompanying examples. A method for the production of high formable steel which will exhibit lower Yield Strength /U tensile Strength (YS/UTS) ratio and lower strain ageing index (SAI).The method involves hot rolling a steel slab having the alloying elements composition comprising C - 0.07 - 0.09, Mn = 0.3 - 0.4, P = 0.02max., S = 0.02 max., Si = 0.04 max., Al- 0.025 - 0.035, N = 50-70 ppm. and selectively incorporating Ti = 0.015 - 0.02 as micro alloying elements for fixing of nitrogen which would enable maintaining the finishing rolling temperature of about 860°C +/- 10°C and lower coiling temperature of around 590°C +/- 10°C in order to achieve uniform mechanical property and reduced scale thickness. The method is particularly suitable for producing steel suited for LPG cylinder.

Documents:

00460-cal-2001-abstract.pdf

00460-cal-2001-claims.pdf

00460-cal-2001-correspondence.pdf

00460-cal-2001-description(complete).pdf

00460-cal-2001-form-1.pdf

00460-cal-2001-form-13.pdf

00460-cal-2001-form-18.pdf

00460-cal-2001-form-2.pdf

00460-cal-2001-form-3.pdf

00460-cal-2001-g.p.a.pdf

00460-cal-2001-letters patent.pdf

00460-cal-2001-p.a.pdf

460-CAL-2001-CORRESPONDENCE 1.1.pdf

460-CAL-2001-CORRESPONDENCE 1.2.pdf

460-CAL-2001-FORM 27 1.1.pdf

460-CAL-2001-FORM 27.pdf

460-cal-2001-granted-abstract.pdf

460-cal-2001-granted-claims.pdf

460-cal-2001-granted-description (complete).pdf

460-cal-2001-granted-form 2.pdf

460-cal-2001-granted-specification.pdf