Claims:WE CLAIM

1. A process for developing cost effective multiphase steel with increased strength and increased formability in boron added low carbon steel comprising subjecting the said steel to a austentising temperature followed by cooling the same at an increased rate.

2. A process as claimed in claim 1 wherein the said boron added low carbon steel has low manganese content and does not contain any alloying elements like chromium, nickel etc.

3. A process as claimed in claims 1 and/or 2, wherein said boron added steel has the composition in wt% of C(0.04-0.06),Mn(0.20-0.25),Si(0.02-0.04),S(0.015 max.),P(0.02 max.),Al(0.05 max.),B(20-25 ppm) and N(30-50 ppm).

4. A process as claimed in claims 1 to 3, wherein the austentizing temperature is about 850 deg.C, 890 deg.C or 930 deg.C.

5. A process as claimed in claims 1 to 4, wherein the increased cooling rate is achieved under oil or water quenched condition.

6. A process as claimed in claims 1 to 5, wherein the increased cooling rate is achieved under water quenched condition.

7. A process as claimed in claims 1 to 6, wherein the austentising temperature is about 890 deg.C.

8. A process as claimed in claims 1 to 7, wherein the strength value of developed steel increases with increase in cooling rate.

9. A process as claimed in claims 1 to 8, wherein the yield with high YS (425MPa), high UTS (560MPa) and low YS/UTS ratio (0.75) is achieved when the unalloyed

steel with boron(20-25 ppm) in the low carbon(0.04-0.06 wt%) unalloyed steel is austentised at 890 deg.C.

Dated: this 17th day of February, 2016.

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

To,
The Controller of Patents, The Patent Office, Kolkata.
, Description:PROCESS FOR DEVELOPMENT OF LOW CARBON BORON ADDED MULTI PHASE STEEL WITH INCREASED STRENTH AND INCREASED FORMABILITY

FIELD OF INVENTION

Boron in presence of other alloying elements like manganese, chromium, nickel etc influences the hardening behavior of steel significantly and so it has been added extensively in quenched and tempered alloyed steel since decades. The present invention relates to a process for developing multiphase steel with increased strength and increased formability in boron added low carbon low manganese steel. More particularly a process for developing cost effective multiphase steel. Present study was carried out to understand the influence of boron towards increasing the harden ability in low carbon unalloyed steel with lean chemistry.

BACKGROUND ART

Significant progress has been made towards the development of new steel products with improved attributes by addition of micro-alloying elements in the past. Out of these elements boron is a unique one, which can increase or decrease the hardening behaviour of steel depending on presence of titanium or zirconium (Zr), nitrogen (N) content and process conditions i.e. austenitising temperature, cooling rate etc. It retards the nucleation site of ferrite at the austenite grain surfaces, forms bainite / martensite and increases harden ability of steels and is very effectively being used in low alloyed quenched and tempered steel since many years.

Recently the Time Temperature Transformation (TTT) diagram has been modified to reflect the boron effect in low carbon unalloyed steel and it is interesting to note that bainite or martensite are absent even at higher cooling rate, rather ferrite has retained its polygonality and the extent of grain refinement has increased with increase in cooling rate. Boron has the largest effect on the cooling curve at higher temperature representing reconstructive transformation to ferrite or pearlite, leaving the lower cooling curve representing displacive transformation such as Widmanstatten ferrite, bainite and acicular ferrite, hardly changed. Boron, not alone, but in presence of C, Mn, Cr contributes to hardenability. In present work role of

boron as hardener in steel low carbon unalloyed steel with a lean chemistry has been explored and process technology developed for low carbon boron added multi-phase steel.

OBJECT OF THE INVENTION

Primary object of the invention is to develop boron added low carbon multiphase steel with increased strength and increased formability.

Another object of the present invention is to provide a process for developing multiphase steel with increased strength and increased formability in boron added low carbon low manganese steel. More particularly a process for developing cost effective multiphase steel.

Yet another object of the present invention is to see that yield with high YS (425MPa), high UTS (560MPa) and low YS/UTS ratio (0.75) is achieved.

SUMMARY OF THE INVENTION

Response of boron addition towards harden ability vary with alloy chemistry such as amount of carbon or /and other alloying elements and heat treating conditions like soaking temperature, and more significantly cooling rate. This developmental work has been carried out to explore the hardening behavior of boron in low carbon low manganese steel in absence of other alloying elements like chromium, nickel etc. Samples were soaked at different austenitising temperatures and subsequently quenched in air, oil and water. Outcome of the study showed that Irrespective of austenitising temperature, boron added steel resulted in continuous yielding on water quenching. With optimizing soaking (austenitising) temperature and cooling rate, excellent combination of strength and formability could be achieved even in unalloyed steel.

Therefore such as herein described there is provided a process for developing cost effective multiphase steel with increased strength and increased formability in boron

added low carbon steel comprising subjecting the said steel to a austentising temperature followed by cooling the same at an increased rate.

In other embodiment there is provided a process for developing cost effective multiphase steel with increased strength and increased formability in boron added low carbon steel wherein the yield with high YS (425MPa), high UTS (560MPa) and low YS/UTS ratio (0.75) is achieved when the unalloyed steel with boron(20-25 ppm) in the low carbon(0.04-0.06 wt%) unalloyed steel is austentised at 890 deg.C.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 illustrates the Yield strength of samples heated to varying austenitising temperature and subsequently quenched in different cooling media in accordance with the present invention;

Fig. 2 illustrates the ultimate tensile strength of samples heated to varying austenitising temperature and subsequently quenched in different cooling media in accordance with the present invention;

Fig. 3 illustrates the Yield point elongation (%)of samples heated to varying austenitising temperature and subsequently quenched in different cooling media in accordance with the present invention;

Fig. 4 (a & b) llustrates transmission electron Micrographs of low carbon unalloyed steel samples heated to 930 0C and subsequently ( a) air Cooled (b) water quenched in accordance with the present invention;

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention there is provided a process for developing cost effective multiphase steel with increased strength and increased formability in boron

added low carbon low manganese steel which comprises subjecting the said steel to a austenitizing temperature followed by cooling the same at an increased rate.

The boron added steel has broadly a non limiting composition in wt.% : C (0.04-

0.06), Mn (0.20-0.25), Si(0.02- 0.04), S(0.015 max.), P(0.02 max.), Al(0.05 max), B(20-25 ppm), N(30-50 ppm). The said boron added steel does not contain any alloying element like Chromium, Nickel etc. Different austenizing temperatures applied are about 850 deg C, 890 deg C and 930 deg C. Increased cooling rate is achieved under oil or water quenched conditions.

The strength value of the developed steel increases with increase in cooling rate which is maximum under water quenching conditions.

According to a specific embodiment of the invention the yield with YS(425MPa), high UTS(560MPa) and low YS/UTS ratio (0.75) is achieved when the unalloyed steel with boron (20-25 ppm) in the low carbon (0.04-0.06 wt%) unalloyed steel is austenitized at about 890 deg. C followed by water quenching.

SALIENT FEATRURES OF THE INNVENTION

Irrespective of austenitising temperature, boron added steel resulted in continuous yielding on water quenching.

Excellent combination of strength and formability could be achieved in boron added steel subjected to varying austenitising temperatures followed by water quenching.

Process technology for development of cost effective multiphase steel established.

EXAMPLE

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 into 2.8 mm thickness in hot strip mill. Hot rolling parameters for were as follows; slab reheating temperature of
1250 C, finish rolling temperature (FRT) of 880 + C and coiling temperature (CT) of

680 + C. The chemical composition (wt %) of steel is given below.

Table 1: Chemical composition (wt%) of boron added low carbon unalloyed hot

rolled steel

C Mn Si S P Al B N
0.04 –
0.06 0.20 -
0.25 0.02-
0.04 0.015 max 0.02 max 0.05 max 20-25 ppm 30-50 ppm

To understand the hardening behavior of boron, a series of heat treatment experiments were carried out. Thermal cycles followed for heat treatment are as follows; Three different austenitising temperatures were: (a) 850oC (???? range), (b)
890oC (closer to Ac3 temperature i.e. Austenite to ferrite transformation

temperature), and (c) 930oC (well above Ac3 temperature). It may be noted that the calculated Ac3 temperatures for the investigated steel is ~890°C. Air cooling (AC), oil quenching (OQ) and water quenching (WQ) constituted the three cooling conditions. Fig. 1 and 2 shows influence of boron towards increasing yield strength and ultimate tensile strength of low carbon unalloyed steel respectively. Both the strength values are found to increase with increase in cooling rate, i.e. in oil and water quenched conditions.

Interesting feature of the present study is emergence of continuous yielding which is found to occur in water quenched samples subjected to all the austenitising temperatures. Phenomenon of upper and lower yielding behavior in mechanical testing is known as yield point phenomenon or discontinuous yielding. The reason for this behaviour is the presence of interstitial atoms like carbon and nitrogen as the alloying elements. The dislocations which are understood to be the cause of plastic deformation are pinned to these interstitial solutes and hence become immobile. Also with the initiation of the plastic deformation there is an ample increase in the number of dislocations due to the formation of cottrell atmosphere. This leads to high population of dislocations and the movement of the same are restricted and causes the yield point phenomenon. The phenomenon is usually seen in mild steels. With stress getting higher, the pinning is unlocked and thus stress decreases. But then due to high dislocation density, it increases again. The load at which sudden drop occurs is called upper yield point, the constant load is called lower yield point and the elongation that occurs at constant load is called yield point elongation (YPE). The deformation that occurs throughout the YPE is

heterogeneous. Several bands form at different point of stress concentration. These bands are generally 450 to the tensile axis. They are usually called “Luders band or stretcher strains”, and this type of deformation is sometimes referred to as “Piobert effect”.

Fig. 3 shows percentage yield point elongation ((YPE) of samples heated to varying austenitising temperature and quenched in different cooling media. It can be seen that YPE values are found to decrease with increase in cooling rate and led to zero in water quenched condition.

Austenite at 850oC (????? ) is rich is carbon and transforms to martensite and gives rise to continuous yielding with low YS and high UTS values as typically observed in dual phase steels. With increase in austenitising temperature, amount of boron in solution increases and its effectiveness is more with enhanced cooling rate.

In spite of the higher amount of boron in solution at 930 oC, YS and UTS reached its maximum values at 890oC in water quenched condition for boron added steel. It can be explained based on the segregation of boron to grain boundary.

Harden ability is dominated mainly by the grain boundary concentration of boron atoms. When boron segregates to austenite grain boundary, austenite to ferrite transformation is retarded. When it coagulates (or precipitates) on the other hand, its retarding effect is decreased. Segregated boron is effective but the coagulated or precipitated boron is ineffective in suppressing the transformation. Hence it is reasonable to define, the boron which has been segregated at austenite just before
the transformation, as “effective boron”. Austenitising temperature of 890 oC is just

close to austenite to ferrite transformation temperature (Ac3) in the present study and effective boron to contribute towards increase in harden ability is more at this temperature compared to that at 930oC.

The water-cooled specimens exhibited continuous yielding while the air-cooled ones exhibited discontinuous yielding. Internal stresses plays very important role for the continuous yielding of multi phase steels. Figure 4 (a & b) depicts transmission

electron micrographs (TEM) of low carbon unalloyed steel samples heated to 930

0C and subsequently air cooled and water quenched respectively. The dislocations which are understood to be the cause of plastic deformation are pinned and hence become immobile (Figure 4 a), whereas it is mobile in case of water quenched samples. There are many preferentially yielding zones around the harder phases, and initial yielding begins simultaneously from these zones under a very low applied stress compared to the normal yield strength of the ferrite because the internal stress assists the initial yielding. Thus, the steels yield continuously.

Presence of boron has led to an attractive combination of strength and formability even in such low carbon (0.04-0.06) unalloyed cost effective steel. Continuous yielding with high YS (425 MPa), high UTS (560 MPa) and low YS/UTS ratio (0.75), when austenised at 890 C followed by water quenching. Range of multiphase microstructures and thus combination of strength & formability can be obtained in commercial production with optimised cooling rates in hot rolling mill.

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.