LOW TEMPERATURE BAINITIC STEEL WITH ENHANCED TRANSFORMATION KINETICS AND A PROCESS FOR PRODUCING THE SAME

FIELD OF THE INVENTION:

This invention relates to a low temperature bainitic steel with enhanced transformation kinetics. This invention particularly relates to a cost effective medium carbon low temperature bainitic steel with mechanical properties comparable to the conventional bainitic low temperature steel where costly alloying elements are avoided. This invention also relates to a process for producing low temperature bainitic steel with enhanced transformation kinetics.

BACKGROUND OF THE INVENTION AND PRIOR ART:
Low temperature bainitic steels are processed by austenitisation followed by austempering at temperatures just above Ms temperature resulting in a microstructure comprising alternate layers of nano-scale bainitic ferrite as well as retained austenite. Nano-structured bainitic structure so obtained is free of the carbide formation due to significant amount of Si (> 1.5%) present in the steel. The bainitic lath size varies from 40 to 100 nm. This kind of nano-bainitic microstructure is achieved in steels having carbon: ~1.0% and Si >1.5% through isothermal annealing by holding for a period ranging from days to weeks. The mechanical properties achieved in these steels are quite remarkable with tensile strength varying from 1.6-2.5 GPa, hardness in the range of 650-700 HV and fracture toughness: 30-50 MPa/m2, depending on the steel chemistry and isothermal transformation temperatures employed.

Though, the strength properties of nano-structured bainitic steels are excellent but the austempering time varies from days to week which makes their production more complicated. Further, since the carbon content of these nano-bainitic steels is quite high, so the weldability of these steels is an issue. Hence, efforts are required to achieve the comparable level of mechanical properties in steel having low/medium carbon content. Efforts made on developing low cabon nano-structured bainitic steel have not resulted in satisfactory results due to various reasons like merger of Bs and Ms temperature, coalescence of bainite etc. However, efforts may be made in developing medium carbon (0.4-0.6%) low temperature bainitic steels with mechanical properties comparable to conventional low temperature bainitic steel. Another issue which needs be addressed is the slow transformation kinetics of these steel. This must be done without the addition of costly alloying elements, only through modification in heat treatment techniques. Some studies have been recently reported where transformation kinetics has been found to get enhanced through formation of prior martensite in the structure.

OBJECTS OF THE INVENTION:

The object of the invention is to provide for a medium carbon low temperature bainitic steel with enhanced transformation kinetics.

Another object of the invention is to provide for a medium carbon bainitic steel with enhanced transformation kinetics where high cost alloying metals are avoided thereby achieving economy in its production.

The other object of the invention is to provide for a medium carbon bainitic steel with enhanced transformation kinetics where further improving the cost effectiveness by innovative treatment cycle of substantially reducing the bainitising time.

A further object of the invention is to provide for a process for producing the said steel.

The following description of the invention fulfills the above objects.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWING:

The invention is described with reference to the accompanying drawings in which

Fig. 1: gives schematic representation of heat treatments applied

Fig. 2: shows dilation as function of time

Fig. 3: shows scanning electron micrographs of the specimens of steel isothermally
annealed for eight hours at temperatures below Ms (a) 180o C (b) 210o C and (c) 240o C, just at Ms (d) 270o C, and above Ms (e) 300o C, (f) 330o C and (g) 360o C

Fig. 4: shows XRD of isothermally annealed steel samples

Fig. 5: shows transformed phase fractions during heat treatment

Fig. 6: shows hardness variation with isothermal annealing temperature

DETAILED DESCRIPTION OF THE INVENTION:

According to the invention there is provided a medium carbon low temperature bainitic steel with enhanced transformation kinetics having a composition comprising in percent by weight, C-0.45 to 0.55, Mn-1.40 to 1.50, Si -1.70 to 1.80, Cr-1.10 to 1.20, and Mo-0.15 to 0.20, the balance being Fe and having been treated with heat treatment cycle by austenitising at 850 deg. C for 15 minutes, quenching at below Ms temperature to 210 -240 deg. C followed by holding it at this temperature for 8 minutes and quenching it at room temperature.

The invented steel has enhanced low temperature bainite transformation kinetics through formation of prior martensite.

The low temperature bainitic steel of the invention has unique microstructure of tempered martenisite & lower bainite with carbides aligned in ferrite platelets.

The invented low temperature bainitic steel has the unique combination of properties i.e. YS: 1070-1152 MPa, UTS: 1895-2202 Mpa, %El: 7-11%, Hardness (HV30): 490-500, Impact toughness at -40o C: 12-14 J.

The invention includes a process for producing the low temperature bainitic steel which comprises in sequence-

i) austenitising the steel at 850 deg. C for 15 minutes,

ii) quenching it below Ms temperature to 210 - 240 deg. C ,
iii) holding it at this temperature for 8 hrs. and
iv) quenching it to room temperature.

It has been found that the formation of prior a thermal martensite causes an acceleration at the start of the subsequent isothermal transformations below Ms temperature compared to those above Ms temperature where prior martensite does not form, which is reflected in high incubation time for bainitic transformation in such cases.

To examine the influence of isothermal annealing treatment both below and above Ms temperature on the transformation kinetics and microstructure evolution in a medium carbon low temperature bainitic steel, dilatometry studies were conducted in a Thermo-mechanical simulator (Gleeble-3500 C) in a steel of compostion C: 0.45-0.55%, Mn: 1.40-1.50%, Si: 1.70-1.80%, Cr: 1.10-1.120% & Mo: 0.15-0.20% having Ac3 and Ms temperature ~834 & ~270oC respectively. The heat treatment cycle (Fig.1) comprised of heating the samples at 5o C/s to austenitizing temperature of 850o C, soaking at 850o C for 180 seconds, cooling at 20oC/s up to the isothermal annealing temperature varying from 180o C to 360o C, holding at these temperatures for 8 hrs. followed by natural cooling up to room temperature.

Transformation kinetics was studied through dilation plot (Fig. 2) generated through dilatometry studies in Gleeble. It can be observed from Fig. 2 that there is no incubation time required for bainitic formation at isothermal annealing temperatures below Ms temperature (180 to 240o C). As the kinetics of diffusion less bainite formation is controlled by nucleation, the presence of prior martensite in the structure leads to more nucleation sites before the isothermal annealing treatment and these results in enhanced transformation kinetics. The final deviation in dilatation plot, subsequent to isothermal holding is because of formation of some fresh martensite (FM) which occurs due to transformation of some fraction of retained austenite (RA).

However, the time required for initiation (incubation time) of bainitic transformation is ~500 s for isothermal annealing temperature of 270o C, while it ~400 s for isothermal annealing temperature of 300o C. Incubation time further decrease to ~200 s for temperature of 330o c, while for 360o C, there is no incubation time needed, the transformation starts immediately to bainite.

Microstructure examination (Fig. 3) of dilatometry samples was carried out through Scanning Electron Microscope. It can be observed that at above Ms temperature microstructure comprises lower bainite (accompanied by carbide precipitation) along with retained austenite (RA) and martensite-austenite (MA) blocks, while below Ms temperature, besides tempered martensite, features of typical lower bainite were observed along with carbides aligned in ferrite platelets. The formation of RA and MA occurs as some fraction of untransformed austenite remains after isothermal holding is over and out of this fraction of retained austenite gets transformed (depending on RA stability) to fresh martensite (FM) during final stage of cooling and some fraction of RA still found in the final microstructure. The blocky type phase of FM was observed. The carbide precipitation was clearly observed in the specimens which were isothermally annealed above Ms temperature (particularly at 330 and 360o C). Presence of RA at interlath/blocky positions was observed particularly in specimens which were isothermally annealed above Ms temperature (300-360o C). Also, some fraction of FM was also observed in specimens which were isothermally annealed at 330o C and 360o C, however, their presence was absent in specimens which were annealed at temperatures below Ms temperature (very small deviation observed in dilatation).

To determine experimentally the phase fractions of bainitic ferrite, retained austenite, and fresh martensite /tempered martensite in the isothermally annealed specimens, X-ray diffraction analysis (Fig.4) and dilatation plots were analysed. It can be seen in the graph that major phase formed in all the samples is bainitic ferrite, while peaks of austenite are also visible. In fact, small peaks of martensite at 2? angle of 43.9 and 82.2 can also be clearly seen in isothermally samples of above Ms temperatures. However, these peaks have got merged with the adjacent peaks of austenite and ferrite respectively. Fraction of phases formed at different isothermal annealing temperatures are shown in Fig. 5. RA was measured through X-ray diffraction analysis. As can be seen in Fig. 5 that the RA fraction varied in the range of 7-12%. The maximum RA content of ~12% was observed in specimen, which was isothermally annealed at 360o C. The fraction of initial martenisite (IM) in terms of TM fractions, as well as FM fractions were determined through dilation plot, while. Phase fraction of bainitic ferrite was estimated through balance of TM, RA and FM phase fractions formed in the samples.

Hardness of the isothermally annealed samples with annealing temperature is shown in Fig. 6. With increase in isothermal annealing temperature, the hardness was found to decrease. This is due to the presence of higher amount of prior matensite formation (which subsequently gets tempered during isothermal holding) in the samples which were isothermally annealed below Ms temperature. It can be observed that harness of samples annealed below Ms temperature varied in the range of 490-500 HV30.

Based on these results, salt bath bainitising treatment of samples (which were to be evaluated for mechanical properties) was carried out by following conventional as well as innovative cycle, as shown in Table I. Mechanical properties achieved in samples, after bainitising treatment are shown in Table II. It can be seen that mechanical properties achieved in samples, as per innovative cycle developed in this work, are comparable or better in comparison to that achieved through conventional cycle.

Table I: Heat treatment parameters for Salt bath bainitising treatment

Austenitisation Temp., oC Austenitisation Time, min. Bainitising Temp., oC Bainitising
Time., hrs/days
Innovative Cycle 850 15 210-240 8 hrs.
Conventional Cycle 850 30 285 7 days

Table II : Properties achieved in plates after Salt bath bainitising treatment

Property Innovative cycle Conventional cycle
YS, MPa 1070 - 1151 1315 - 1471
UTS, MPa 1895 - 2202 1719 - 1919
%El 7 - 11% 10 - 14
Impact Toughness, J (at -40oC) 12 - 14 10 - 12
Hardness, HV30 490 - 500 500 – 520

Usefulness of the invention:

? Through proper design of steel chemistry and innovative heat treatment cycle, substantial reduction in bainitising time could be achieved for production of low temperature bainitic structure
? By achieving unique combination of microstructure i.e. lower bainite along-with tempered martensite, attractive combination of properties in terms of high YS (>1000 MPa), UTS (>2000 MPa) and low YS/UTS ratio (~0.5) could be achieved
? The invention can be easily implemented in steel plants where facilities exist for production of high strength quenched & tempered plates

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

WE CLAIM:

1. Medium carbon low temperature bainitic steel with enhanced transformation kinetics having a composition comprising in percent by weight C-0.45 to 0.55, Mn-1.40 to 1.50, Si -1.70 to 1.80, Cr-1.10 to 1.20, Mo - 0.15 to 0.20, the balance being Fe and having been treated with heat treatment cycle by austenitising at 850 deg. C for 15 minutes, quenching at below Ms temperature to 210 -240 deg. C followed by holding it at this temperature for 8 minutes and quenching it at room temperature.

2. Low temperature bainitic steel as claimed in claim 1, wherein the steel has the composition comprising in percent by weight C - 0.49, Mn - 1.46, Si - 1.78, Cr - 1.12, Mo - 0.17, the balance being Fe.

3. Low temperature bainitic steel as claimed in claim 1 and 2, wherein the steel has the enhanced low temperature bainite transformation kinetics through formation of prior martensite.

4. Low temperature bainitic steel as claimed in claims 1 to 3, wherein the steel has the unique microstructure of tempered martenisite & lower bainite with carbides aligned in ferrite platelets.

5. Low temperature bainitic steel as claimed in claims 1 to 4, has the unique combination of properties i.e YS: 1070-1152 MPa, UTS: 1895-2202 Mpa, %El: 7-11%, Hardness (HV30): 490-500, Impact toughness at -40oC: 12-14 J.

6. A process for producing low temperature bainitic steel as claimed in claims 1 to 5, comprising in sequence-

i) austenitising the steel at 850 deg. C for 15 minutes,

ii) quenching it below Ms temperature to 210 - 240 deg. C ,

iii) holding it at this temperature for 8 hrs. and

iv) quenching it to room temperature.

7. The process as claimed in claim 6, wherein the steel has the composition comprising in percent by weight C - 0.45 to 0.55, Mn - 1.40 to 1.50, Si - 1.70 to 1.80, Cr - 1.10 to 1.20, Mo - 0.15 to 0.20 and the balance being Fe.

8. The process as claimed in claim 6, wherein the steel has the composition comprising in percent by weight C- 0.49, Mn - 1.46, Si - 1.78, Cr - 1.12 , Mo - 0.17 and the balance being Fe.