FIELD OF THE INVENTION

This invention relates to a process technology for developing low carbon (0.4 - 0.6 wt %) cold rolled continuous annealed steel with high plastic anisotropy ( rm > 1.5). Most frequently used anisotropy parameter is the normal anisotropy ratio or the r value, which represents strains ratio in sheet metal plane and along its thickness. Due to their crystallographic structure and the characteristic processing, annealed sheet metals generally exhibit a significant anisotropy of mechanical properties.

BACKGROUND OF THE INVENTION AND PRIOR ART

Stresses generated through deformation at room temperature during the cold rolling process get relieved either through batch or continuous annealing. Traditionally, low carbon extra deep drawing cold rolled steels (plastic anisotropy rm>1.6) have always been produced by batch or box annealing. However, the long cycles involved in batch annealing cycles, typically two three days, are expensive in terms of energy consumption and capital costs, for both plant and materials.

Therefore, in recent years, there has been a significant development towards the introduction of continuous annealing lines on which the total processing time is reduced to only a few minutes. Continuous annealed steel sheets also exhibit more uniform mechanical properties, better flatness & cleaner surface as compared to that processed through batch annealing route. However, inferior formability as compared to that of batch annealed steel is the limitation of continuous annealed steel. Good formability of sheet steel requires high elongation & pronounced anisotropy (rm), characterized by width strain to thickness strain.

Therefore, research works are going on globally to produce continuous annealed steel sheets with improved rm value. In general, these steels possess rm value in the range of 1.1 to 1.3 with carbon level of 0.04 wt%. Addition of boron has been found beneficial to improve its forming properties, however not much favourable influence has been noticed on rm value yet.

A few prior art references are given below-
EP1805339A1 of 2004 discloses high strength cold rolled steel sheet having excellent shape freezability……...
US3853636A of 1968 discloses a method for manufacturing cold rolled steel excellent in press-formability.
DE19918581A1 of 1998 discloses a process for casting thin carbon steel strips.
EP0659891A2 of 1993 discloses a method for manufacturing a thin cold rolled mild steel strip for deep drawing.
Quan Li et al. discloses a study on the plastic anisotropy of advanced high strength steel sheet: Experiments and microstructure-based crystal plasticity modeling and reported in International Journal of Mechanical Sciences, Vol. 176, 15 June, 2020, 105569.
Pingguang Xu et al. reported "Plastic Anisotropy of Strip-Cast Low-Carbon steels" in Materials Transactions, Vol. 45, No. 2 (2004) pp. 447 to 456.
"Effect of carbon content on annealing texture, plastic anisotropy, and mechanical properties of 0.7% phosphorus sheet steels" are reported by Hsun Hu in Texture of Crystalline Solids, 1979,Vol. 3, pp- 215-230.

OBJECT OF THE INVENTION

The main object of the invention is to develop a process technology for producing low carbon cold rolled continuous annealed steel with high plastic anisotropy (rm>1.5).

The other object is to achieve very good drawing properties (Total Elongation >30% and Uniform Elongation >20%) in the steel with adequate strength (YS ~ 300 MPa) in the cold rolled continuous annealed steel.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

FIGURE: 1 represents the orientation distribution function (ODF) section at f2= 00 and f2=450 on the RD-TD plane of the continuous annealed samples with B/N ratio of 0.185.

SUMMARY OF THE INVENTION

According to the invention there is provided a process for producing low carbon cold rolled continuous annealed steel with high plastic anisotropy having the composition in wt.% of C - 0.04-0.06, Mn - 0.15 - 0.25, S - 0.010 max., P - 0.025 max., Si - 0.05 max., Al - 0.03 - 0.06, N - 50 - 70, Boron / Nitrogen ratio - 0.15 - 0.20, the balance being Fe and unavoidable impurities comprising of the following steps with required processing parameters :
(a) Hot Rolling : Soaking Temperature : 1200 + 10 Deg. C, Finish rolling Temperature : 870 + 10 Deg. C, Coiling Temperature : 570 + 10 Deg. C ;
(b) Cold Reduction > 70 % ; and
(c) Continuous Annealing Cycle : Heating to 740 + 10 Deg. C with heating rate of 20 Deg. C/sec, soaking for one minute, Cooling to 450 + 10 Deg. C with cooling rate of 30 Deg. C/sec, soaking for two minutes and finally cooled to room temperature.
The resultant steel has the following combination of strength and drawing properties - YS min.- 300 MPa, UTS min. - 380 MPa, % Total El. min. - 30, % Uniform El. min. - 20, YS/UTS max. 0.85, and rm min. - 1.5.

DESCRIPTION OF THE INVENTION

According to the invention there is provided a process for producing low carbon cold rolled continuous annealed steel with high plastic anisotropy having the composition in wt.% of C - 0.04-0.06, Mn - 0.15 - 0.25, S - 0.010 max., P - 0.025 max., Si - 0.05 max., Al - 0.03 - 0.06, N - 50 - 70, Boron / Nitrogen ratio - 0.15 - 0.20, the balance being Fe and unavoidable impurities comprising the following steps with required processing parameters :
(a) Hot Rolling:
(b) Cold Reduction and
(c) Continuous Annealing Cycle.

The hot rolling step has the following process parameters: Soaking Temperature: 1200 + 10 Deg. C, Finish rolling Temperature: 870 + 10 Deg. C, Coiling Temperature: 570 + 10 Deg. C.
In the step (b) the Cold Reduction is made > 70%.
The Continuous Annealing Cycle has the following process parameters -
Heating to 740 + 10 Deg. C with heating rate of 20 Deg. C/sec, soaking for one minute, cooling to 450 + 10 Deg. C with cooling rate of 30 Deg. C/sec, soaking for two minutes and finally cooled to room temperature.
The resultant steel has the following combination of strength and drawing properties:

YS
min UTS
min % Total-El,
min % Uniform –El,
min YS/UTS
max rm
min
300 MPa 380 MPa 30 20 0.85 1.5

The governing factor for achieving good formability in steel is complete recrystallisation and sufficient grain growth in a short time continuous annealing. Approach includes utilization of scavenging effects of carbide, nitride, oxide & sulphide forming elements on impurities such as carbon, nitrogen, oxygen and sulphur.

Carbon could be kept in three range of options; (i) <0.005% (ii) 0.015-0.025 % and (iii) 0.04-0.06%. However, to save the cost of lowering carbon through secondary refining process and the associated difficulties, conventional and convenient range 0.04-0.06% is opted. Concept of boron to nitrogen ratio is introduced in place of absolute boron in Al-killed steel.

Low carbon boron (20-50 ppm) added aluminium killed heats were made industrially through Basic Oxygen Furnace (BOF) - Continuous Casting (CC) – Ladle Furnace (LF) – Hot Strip Mill (HSM) route.

Table 1 shows the chemical composition of the steel. One heat was also made without boron (i.e. B/N = 0), to compare the properties with varying boron to nitrogen ratio in the steel.

Table 1: Chemical composition (wt %) of the experimental steels
C Mn S P Si Al N ppm B/N
0.04-0.06 0.15-0.25 0.01 max 0.025 max 0.05 max 0.03-0.06 50-70 0-1

Table 2 shows the processing temperatures of the experimental steels in terms of soaking, finishing and coiling temperatures during industrial hot rolling which were carefully designed and controlled to keep aluminium and nitrogen in solution. Hot rolled steels were cold reduced to 1mm thickness in laboratory experimental cold rolling mill with 72 % cold reduction.

Table 2: Processing temperatures of the experimental steels
Soaking Temperature Finishing Temperature Coiling Temperature
1200 + 10 C 870 + 10 C 570 + 10 C

Continuous annealing cycles were specially designed which would be suitable for over aging /galvanizing of steel sheet and tensile samples prepared from cold rolled sheets were subjected to these cycles in prototype simulator. Samples were heated to 740 + 10 Deg. C with heating rate of 20 Deg. C/sec and soaked for one minute. Subsequently, samples were cooled to 450 + 10 Deg. C with cooling rate of 30 Deg. C/sec, soaked for two minutes and then kept out of simulator.

All the samples, after conducting continuous annealing experiment, were subjected to detailed characterization with respect to mechanical properties evaluation and texture examination using electron back scattered diffraction (EBSD).

Table 3 depicts the tensile and drawing properties of the developed steel. It is surprisingly noticed that the combination of strength and drawing properties are found to be excellent, which may be attributed to the presence of optimum amount of boron nitride as well as aluminium nitride in the steel. Unlike the fine aluminium nitride particles, coarser boron nitride do not inhibit growth of the recystallized ferrite grain during rapid anneal.

Table 3: Tensile and Drawing properties of the experimental steels
YS, MPa UTS, MPa YS/UTS Total El (%) Uniform El (%) rm
293-313 336-383 0.81-0.87 32-35 20-21 1.23-1.55

Ferrite grain growth following recrystallisation results into development of {111) leading into improved rm value.

Accompanying Figure 1 represents the orientation distribution function (ODF) section at f2= 00 and f2=450 on the RD-TD plane of the continuous annealed samples with B/N ratio of 0.185. Analysis was carried out in EBSD.

Presence of optimum amount of boron nitride (BN) and aluminium nitride (AlN) has resulted into excellent combination of strength and drawability. Lower sulphur in steel necessitated lower amount of manganese to tie up, which has contributed in improved ODF for favourable texture. Evidence of gamma fibre can be clearly seen, which may have contributed in achieving high rm value of 1.55, reported for the first time with 0.04-0.06 % carbon processed through continuous annealing route.

This innovative study has provided product development opportunity and can be utilized for development of deep drawing cold rolled steel with such an excellent combination of product attributes through continuous annealing/ galvanising route.

While there has been shown and described herein some preferred embodiments of the invention disclosed, many minor variations and changes apparent to those skilled in the art may be made without departing from the spirit and scope of the invention which is defined in the appended claims.

WE CLAIM:

1. A process for producing low carbon cold rolled continuous annealed steel with high plastic anisotropy having the composition in wt. % of C - 0.04 - 0.06, Mn - 0.15 - 0.25, S - 0.010 max., P - 0.025 max., Si - 0.05 max., Al - 0.03 - 0.06, N - 50 - 70, Boron / Nitrogen ratio - 0.15 - 0.20 and the balance being Fe with unavoidable impurities comprising the following steps :
(a) Hot Rolling ;
(b) Cold Reduction > 70 % and
(c) Continuous Annealing Cycle.

2. The process as claimed in claim 1, wherein the hot rolling step has the following process parameters: Soaking Temperature: 1200 + 10 Deg. C, Finish rolling Temperature: 870 + 10 Deg. C, Coiling Temperature: 570 + 10 Deg. C.

3. The process as claimed in claim 1, wherein the Cold Reduction is > 70%.

4. The process as claimed in claim 1, wherein the Continuous Annealing Cycle has the following process parameters -
Heating to 740 + 10 Deg. C with heating rate of 20 Deg. C/sec, soaking for one minute, cooling to 450 + 10 Deg. C with cooling rate of 30 Deg. C/sec, soaking for two minutes and finally cooled to room temperature.

5. The process as claimed in claim 1, wherein the processing temperatures of the steel in terms of soaking, finishing and coiling temperatures during industrial hot rolling are carefully designed and controlled to keep aluminium and nitrogen in solution.

6. The process as claimed in claim 1, wherein the hot rolled steels are cold reduced to 1 mm thickness with more than 70 % cold reduction.

7. The process as claimed in claim 1, wherein the resultant steel has the following combination of strength and drawing properties:
YS min.- 300 MPa, UTS min. - 380 MPa, % Total El. min. - 30, % Uniform El. min. - 20, YS/UTS max. - 0.85, and rm min. - 1.5.

8. The process as claimed in claims 1 to 7, wherein the resultant steel has a carbon content of 0.04 -0.06 wt% and the rm min: 1.5.