Claims:CLAIMS:

1. A process for production of boron containing hot rolled soft steel through ferritic rolling comprising the steps of:
casting Liquid metal of boron added low carbon unalloyed steel were made in 300 T LD converter, passed through ladle furnace for refining and continuously cast into 200 mm thick slabs having a composition in percentages by weight comprising:

C Mn S P Si Al B* N*
0.03-0.06 0.15-0.25 0.015 max 0.025 max 0.025-0.035 0.02-0.04 15-20 ppm 60-80 ppm
and rest iron and impurities resulting from the smelting.
heating vide thermomechanical simulator said cast steel strip samples, in the austenitic phase, at a rate of 3°C/Sec up to 1200°C, soaked for 120 seconds and then cooled at a rate of 1°C/sec up to 1050°C before subjecting to hot compression tests;
applying rough deformation, to the cast steel strip of 0.25 strain at a strain rate of 5/ sec with first hot rolling operation in one or more steps and subsequently the rough-deformed steel strip were cooled down to 750°C at a cooling rate of 2°C/sec;
applying finish deformation by imparting 0.7 strain at a strain rate of 10/sec, finish deformation were given at a temperature of between 850 - 9500 so as to obtain a hot-rolled sheet having a thickness of less than or equal to 2 mm; crystallizing said hot-rolled sheet having a thickness of less than or equal to 2 mm over its entire thickness at all temperature; and
cooling deformed samples at a lower temperature of around 550 – 650° C in order to simulate the coiling temperatures.

2. A process for production of boron containing hot rolled soft steel through intercriticalrolling as claimed in claim 1, wherein under condition that the finish rolling temperature is maintained at 850°C, and then the increase in coiling temperature did not influence ferrite grain size of 10-12 micron.

3. A process for production of boron containing hot rolled soft steel through ferritic rolling as claimed in claim 1, wherein under condition that the finish rolling is performed at 750°C then the recrystallised ferrite grain size of 30 micron with hardness value of 40 HRB is achieved.

4. A process for production of boron containing hot rolled soft steel through ferritic rolling as claimed in claim 1, wherein the addition of boron in aluminium killed low C-Mn steel led to formation of recrystallised ferrite grains even when austenitising temperature is maintained at 1200oC and hot deformed / rolled subsequently in ferritic region followed by simulated coiling at 650oC.

5. A process for production of boron containing hot rolled soft steel through ferritic rolling as claimed in claim 1, wherein the coarse ferrite grains (Hardness as low as 40 HRB) is produced after austenitising the steel to high temperature (1200°C) followed by ferritic rolling at 750°C.

6. A boron containing hot rolled soft steel obtained through ferritic rolling having a composition in percentages by weight comprising:

C Mn S P Si Al B* N*
0.03-0.06 0.15-0.25 0.015 max 0.025 max 0.025-0.035 0.02-0.04 15-20 ppm 60-80 ppm
and rest iron and impurities resulting from the smelting.

7. A boron containing hot rolled soft steel as claimed in claim 6, wherein the presence of boron in the composition of aluminium killed low C-Mn steel leads to formation of recrystallised ferrite grains even when austenitising temperature was maintained high i.e. 1200oC and hot deformed / rolled subsequently in ferritic region followed by simulated coiling at 650oC.

8. A boron containing hot rolled soft steel as claimed in claim 6, wherein the process of ferritic rolling involves two stage deformation schedules similar to roughing and finishing rolling.

Dated: this 26th day of November, 2016.

(N. K. Gupta)
Patent Agent
Of NICHE
For SAIL

To,
The Controller of Patents,
The Patent Office, Kolkata.

, Description:PROCESS FOR PRODUCTION OF BORON CONTAINING HOT ROLLED SOFT STEEL THROUGH FERRITIC ROLLING AND THE PRODUCT THEREOF.

FIELD OF INVENTION
The present invention relates to a process for production of boron containing hot rolled soft steel through ferritic rolling. More particularly the present invention relates to a method employing optimum conditioning of hot rolled austenite prior to transformation for ensuring the appropriate ferrite microstructure to be formed after transformation.
BACKGROUND ART
Cold rolled thin gauge steel sheet is normally rolled from low carbon hot rolled steel of relatively thin gauge. To keep the roll force within limits even during severe rolling, the hot rolled (HR) coils must have low hardness. For a given composition of steel, properties are dependent on processing conditions and resulting microstructure of the material. Hot rolled strip is generally finish rolled above Ar3 temperature (Austenite to ferrite transformation start temperature) to ensure a uniform transformed ferrite grain structure after cooling. Rolling slightly below Ar3 temperature results in microstructural in homogeneity, which is detrimental to properties of hot strip. If finish rolling is conducted entirely below Ar1 temperature (Austenite to ferrite transformation finish temperature), the microstructure of steel strip is predominantly ferrite during rolling and following recrystallisation of the deformed ferrite, a uniform structure is present in as rolled product. However, ferritic rolling temp is usually kept above Ar1 to optimise finishing mill load in hot strip mill.
Many cold rollers have found that the ferritic rolled hot rolled material is suitable for thin gauge cold rolling. However, the steel chosen for ferritic rolling requires very low carbon, which necessitates special treatment. Furthermore, there are limitations in the roughing mill in terms of mill power, roll wear, roll life and in the electrical and hydraulic system owing to lowering of rolling temperature.
In view of limited information available on the effect of boron addition in low C-Mn steel on ferritic hot deformation/ rolling, hot deformation simulation study was performed keeping finish rolling at 750oC.
SUMMARY OF INVENTION
The object of the present invention is provide a process for fabrication of ferritic steel strip must comply with the specified quality requirements such as the chemical composition, microstructure and texture of the hot-rolled sheet, the percentage of cold reduction (CR) and the recrystallization treatment in the continuous annealing process or batch process.
Another object of the present invention is provide boron containing steel wherein the coarse ferrite grains (Hardness as low as 40 HRB) is produced after austenitising the steel to high temperature (1200°C) followed by ferritic rolling at 750°C.
Another object of the present invention is provide boron containing soft steel with better formability in hot rolled condition and improved cold reducibility for downstream application.
Therefore such as herein described there is provided a process for production of boron containing hot rolled soft steel through ferritic rolling comprising the steps of: casting Liquid metal of boron added low carbon unalloyed steel were made in 300 T LD converter, passed through ladle furnace for refining and continuously cast into 200 mm thick slabs having a composition in percentages by weight comprising:
C Mn S P Si Al B* N*
0.03-0.06 0.15-0.25 0.015 max 0.025 max 0.025-0.035 0.02-0.04 15-20 ppm 60-80 ppm
and rest iron and impurities resulting from the smelting.in thermomechanical simulator, heating vide thermomechanical simulator said cast steel strip samples, in the austenitic phase, at a rate of 3°C/Sec up to 1200°C, soaked for 120 seconds and then cooled at a rate of 1°C/sec up to 1050°C before subjecting to hot compression tests; applying rough deformation, in the ferritic phase, to the cast steel strip of 0.25 strain at a strain rate of 5/ sec with first hot rolling operation in one or more steps and subsequently the rough-deformed steel strip were cooled down to 750°C at a cooling rate of 2°C/sec; applying finish deformation by imparting 0.7 strain at a strain rate of 10/sec,. finish deformation were given at a temperature of between 850 - 9500 C with an overall reduction ratio of at least 50% in the presence of a lubricant, so as to obtain a hot-rolled sheet having a thickness of less than or equal to 2 mm; ecrystallizing said hot-rolled sheet having a thickness of less than or equal to 2 mm over its entire thickness at all temperature of 850°C;andcooling deformed samples at a lower temperature of around 550 – 650° Cel in order to simulate the coiling temperatues.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 illustrates the Ferrite grains as a function of simulated finishing and coiling temperatures in accordance with the present invention;
Fig. 2 illustrates the Microstructures under ferritic rolling conditions (FRT: 750C) in accordance with the present invention;
Fig. 3 illustrates the Microstructures under austenitic rolling conditions in accordance with the present invention;
Fig. 4 illustrates the schematic representation of precipitation in boron free steel in accordance with the present invention;
Fig. 5 illustrates schematic representation of precipitation in boron added steel in accordance with the present invention.
DETAILED DESCRIPTION
The processing routes of a conventional austenitic rolling and a novel ferritic rolling is apparent to a person skilled in the art. The most evident is a reduced reheating temperature in the ferritic rolling practice, which gives also the potential for an increased throughput of the furnace. Lower reheating temperatures for ferritic rolling (between 1050and 1100 °C) result to a reduced AlN dissolution (enhancing ferrite recrystallization kinetics) and a smaller initial austenite grain size. At low reheating temperatures, a portion of previously formed AlN particles (which occurred in coarsed form due to slow cooling of slab) remains un-dissolved. The reduction of alloy solute available for precipitation to take place during processing reduces the volume fraction of fine AlN precipitates which typically favours more rapid and complete recrystallisation than those with high volume fraction of fine precipitates. Higher slab reheating temperature favours more complete dissolution of existing precipitates, thus increasing amount of fine AlN precipitates and thereby results in more sluggish recrystallisation behavior.So, for low C-Mn steel, it has been shown (Figure 4) that when steel specimens were reheated to ~1230 C and hot deformed in ferritic region, they did not fully recrystallise even when coiling temperature was maintained at ~ 675C.Although, this low rolling temperature practice leads also to an improved hot rolled product quality with less surface defects, improved flatness of the hot strips due to reduced internal stresses owing to the fact that steel strips are already transformed prior to cooling on the run-out table , however proper conditioning of hot rolled austenite prior to transformation is necessary to ensure the appropriate microstructure for the desired final ferrite microstructure to be formed after transformation. Higher soaking temperature (>1100C) may also reduce roughing mill load created by smaller and stronger austenite coming out of reheating furnace as a result of lower soaking temperature of 1100C
In accordance with the process according to the invention, heats of boron added low carbon unalloyed steel were made in 300 T LD converter, passed through ladle furnace for refining and continuously cast into 200 mm thick slabs. Continuous cast slabs were further hot rolled in hot strip mill. The chemical composition (wt %) of steel is given below
Table 1: Chemical composition (wt%) of boron alloyed low carbon hot rolled steel
C Mn S P Si Al B* N*
0.03-0.06 0.15-0.25 0.015 max 0.025 max 0.025-0.035 0.02-0.04 15-20 ppm 60-80 ppm

To examine combined effect of phase transformation and hot deformation parameters on the evolution of microstructure, simulations of hot rolling was carried out in thermo-mechanical simulator. Hot rolling simulation involved two stage deformation schedules similar to roughing and finishing rolling. Cylindrical test specimens (15 mm long with 10 mm diameter) were prepared from the crop ends of 32 mm transfer bar plates, collected after rough rolling of slabs.
Specimens were heated at a rate of 3°C/Sec upto 1200°C, soaked for 120 seconds and then cooled at a rate of 1° C/sec upto 1050°C before subjecting to hot compression (rough deformation) of 0.25 strain at a strain rate of 5/ sec. Subsequently, the rough-deformed specimens were cooled down to 750°C at a cooling rate of 2°C/sec and finish deformed by imparting 0.7 strain at a strain rate of 10/sec. To compare the results, finish deformation temperatures was also kept at 850°C, 900°C and 950°C. Further, in order to simulate different coiling temperatures, the deformed specimens were cooled upto 550°C, 600°C and 650°C.
The results obtained were found to be interesting in terms of coarsening of ferrite grains and thereby relevant to commercial cold rolling application. Fig. 1 shows the extent of coarsening of ferrite grains as a function of simulated finishing and coiling temperatures.
Recrystallised ferrite grain size of 30 micron with hardness value of 40 HRB was achieved when finish rolling was performed at 750°C. Microstructures under ferritic rolling conditions (FRT: 750°C) is shown in Fig. 2. In contrast, when finish rolling temperature was maintained at 850°C, the increase in coiling temperature did not influence ferrite grain size and it remained comparatively fine (10-12 micron).
However, ferrite grain size coarsened to 14 and 18 micron corresponding to finish deformation temperature of 900°C and 950°C respectively when simulated coiling temperature was as high as 650°C (Fig. 3). The lowest hardness value of 48HRB was measured against ferrite grain size of 18 micron.
The evolution of coarse ferrite grains was attributed to the fact that the number of newly recrystallised ferrite grains was likely to be fewer than that of freshly transformed grains. The difference in number of newly formed grains may be a consequence of the higher driving force made available for micro-structural transformation in an under cooled strip during typical cooling on the run out table of a hot strip mill in comparison to the lower driving force (dislocation density reduction) for ferrite recrytallsation in a ferritic hot rolling regime.
For low C-Mn steel, it has been shown (Fig. 4) that when steel specimens were reheated to ~1230°C and hot deformed in ferritic region, they did not fully recrystallise even when coiling temperature was maintained at ~ 675°C. However, when reheated at 1100°C and hot deformed in ferritic region, it recrystallised fully after adhering to identical coiling temperature of ~ 675°C. At low reheating temperatures, a portion of previously formed AlN particles (which occurred in coarsed form due to slow cooling of slab) remains un-dissolved. The reduction of alloy solute available for precipitation to take place during processing reduces the volume fraction of fine AlN precipitates which typically favours more rapid and complete recrystallisation than those with high volume fraction of fine precipitates. Higher slab reheating temperature favours more complete dissolution of existing precipitates, thus increasing amount of fine AlN precipitates and thereby results in more sluggish recrystallisation behavior.
Present innovation revealed that addition of boron in aluminium killed low C-Mn steel led to formation of recrystallised ferrite grains even when austenitising temperature was maintained high i.e. 1200oC and hot deformed / rolled subsequently in ferritic region followed by simulated coiling at 650oC. Schematic representation of precipitation in boron added steel is shown in Fig. 5. The role of boron addition in this steel was to consume free nitrogen through formation of boron nitride precipitates, retarding formation of aluminium nitrides, which suppresses recrystallisation and grain growth.

Inventive step
Proper conditioning of hot rolled austenite prior to transformation is necessary to ensure the appropriate microstructure for the desired final ferrite microstructure to be formed after transformation. Present investigation has significant industrial implication because it will permit practicing comparatively high slab reheating temperature of 1200°C, without adversely affecting recrystallisation and grain growth kinetics while rolling in ferritic region.
It may reduce roughing mill load created by stronger austenite coming out of reheating furnace as a result of lower soaking temperature of 1100°C.
The process facilitates formation of coarser and softer ferrite grains in hot rolled coils, which is considered suitable for cold forming and reducing applications. Obtained recrystallized grains at even 650°C CT, which may minimize need for run out table water cooling.
Salient Features of Innovation
Coarse ferrite grains (Hardness as low as 40 HRB) could be produced after austenitising the steel to high temperature (1200°C) followed by ferritic rolling at 750°C. This has been attributed to reduced volume of fine AlN precipitation potential in presence of boron.
Soft steel will facilitate better formability in hot rolled condition and improved cold reducibility for downstream application.
Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.