CLIAMS:WE CLAIM:
1. A process for producing low-carbon fine-grained bainitic steel plates
comprising the steps of:
(i) heating the steel to a temperature 1100°C for transforming its
structure into fully austenitic structure, whereby the temperature 1100°C and the
holding time 3 hours at the temperature 1100°C are constrained for hindering
the grain growth of the austenite,
(ii) performing hot rolling at temperatures at 1000 – 900°C where the
structure of the steel is essentially austenitic.
(iii) performing air cooling upto 300°C at a cooling rate of 10°C/s to
5°C/s to produce fully bainitic structure.
2. A process for producing low-carbon fine-grained bainitic steel plates as
claimed in claim 1, wherein the steel composition comprise of 0.025 to 0.027 wt%
of carbon, 0.27 to 0.29 wt% of silicon, 1.59 to 1.69 wt% of manganese, 0.01-0.27
wt% chromium, 0.88-0.89 wt% nickel, 0.28 wt% molybdenum,0.041-0.042 wt% of
niobium, 0.008-0.025 wt% titanium, 0.039-0.040 wt% of vanadium, 0.47 wt%
copper and 0.001 wt% boron and balance essentially iron.
3. A process for producing low-carbon fine-grained bainitic steel plates as
claimed in claim 1, where in rolling of the steel is continued after soaking at a
temperature of 1100oC for 3 hours and rolled in rolling mill into 20 mm thick
plates.
4. A process for producing low-carbon fine-grained bainitic steel plates as
claimed in claim 1, where in the steel plates are further rolled into 6 mm plates in
3 passes with finish rolling temperature of 900C and subsequently cooled in air.
5. A process for producing low-carbon fine-grained bainitic steel plates as
claimed in claim 1, where in the hot compression studies of the cylindrical steel is
made using a dynamic thermomechanical simulator at a strain of 0.4.
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6. .A process for producing low-carbon fine-grained bainitic steel plates as
claimed in claim 1, where in strain rate is 5s-1 and finish rolling temperature of
900C followed by air cooling at the rates of 5 -10C/s.

Dated: this 13th day of October, 2014. ,TagSPECI:A PROCESS FOR PRODUCING LOW-CARBON FINE-GRAINED BAINITIC STEELS WITH IMPROVED STRENGTH AND TOUGHNESS

FIELD OF INVENTION

This invention relates to a process for producing low-carbon fine-grained bainitic steel plates with micro alloy additions by multi-pass air-cooled rolling route.

PRIOR ART

Bainite is an important microstructural constituent, either wholly or in part, of a wide range of commercial steels. The last decade has seen a particularly strong resurgence of interest in the bainitic transformation. It is also known that bainitic structures can confer certain other beneficial properties to steels. For example, amongst the most widely recognized is that bainitic structures give better creep resistance to steels than ferrite/pearlite structures. Perhaps less widely appreciated is that bainite can be more resistant to certain environmentally controlled conditions than some other steel microstructures, for example, in situations involving hydrogen embrittlement, corrosion fatigue and in some circumstances, rolling contact fatigue. High-strength bainitic steel plates can be a good candidate for use in machineries, buildings and heavy equipment for construction which requires high strength as well as high toughness. It can be further applied for line pipe applications and offshore constructions requiring excellent low temperature toughness.

The strengthening of steel via a reduction in grain size is a very attractive option because a smaller grain size leads also to an improvement in toughness. This simple fact has led to the development of impressive technology, designed to impart thermo-mechanical treatments capable of refining the austenite grain structure prior to its transformation to bainite. A fine austenite grains size leads to a correspondingly refined bainitic structure. The growth of these grains during the hot rolling process is hindered by the use of microalloying additions such as niobium or titanium. These elements have a low solubility in austenite and are added in small concentrations to form stable carbides or carbonitrides which impede austenite grain growth during hot deformation and subsequent cooling.

Controlled rolling has been used successfully over the past 30 years or so, for steels containing allotriomorphic ferrite and a small amount of pearlite, but has only recently been adapted for bainitic alloys. There are two ways in which a bainitic microstructure can be obtained; the first, involves an increase in the cooling rate in order to allow the austenite to supercool into the bainite transformation range. The second alternative, discussed here, is to modify the steel hardenability without substantially changing the processing conditions. Alloying elements such as manganese are boosted in order to retard the formation of allotriomorphic ferrite relative to the bainite reaction. The hardenability of the steels is such that in spite of their small grain size, they transform to a “uniform and fine” bainitic structure on further cooling.

In steels with low carbon contents (0.1 wt %) where the carbide particle size in bainite is small it is found that the main structural feature affecting toughness is the colony size. Cleavage cracks are deflected at the colony boundaries and the measured cleavage facet size on the fracture surface can therefore be related to the colony size.

In order to increase further the strength levels of structural steels whilst maintaining toughness and improving weldability by reducing the carbon equivalent value (CEV) particularly for pipeline applications, it has been necessary to evaluate lath and dislocation strengthening from low temperature reaction products as an alternative to other strengthening methods which are either exhausted or are incompatible with the stated objectives. Consequently, over the years interest has focused on steels with very low carbon contents which transform to structures where the displacive nature of the reaction mechanism introduces a significant density of transformation dislocations within a lath or irregular a polygonal ferrite grain. The first bainitic steels available commercially were the so-called Fama steels containing up to 4 wt % Mn, although other combinations studies include Mn-Cr, Ni-Mo and Ni-Cr-Mo. Good high strength and toughness combinations have been achieved from this family of steels.

In spite the early optimism about the potential properties of bainitic steels, major commercial exploitation took many years to become established. The steels were not in general found to be better than quenched and tempered martensitic steels, partly because of the relatively course cementite particles associated with bainite and partly because the continuous cooling heat treatments which were popular in industry, could not in practice produce fully bainitic steels. The use of lean alloys gave mixed microstructures whereas heavy alloying led to a considerable quantity of martensite in the final microstructure. Therefore, martensitic steels dominated the high strength steel market, with their better overall mechanical properties and well understood physical metallurgy principles.

It is natural to reduce the carbon concentration even further to produce better bainitic steels, which acquire their strength and toughness via the submicron size grain structure of bainite. However, technology was not in those days sufficiently advanced to cope with the necessarily higher cooling rates required to produce bainite in very low carbon steels, as the steel left the hot-rolling mill. The technology of accelerated cooling designed to produce partially or wholly bainitic microstructures in very low carbon, micro alloyed steels has been perfect within past few years or so, and has resulted in the production of a new class of steels which are the cause of much excitement. However, these variety of steels mostly need a tempering treatment to relieve the quenching stresses which is not cost effective, Therefore, in recent times attempts are being made to produce low-carbon fine-grained bainitic steels with proper alloy combination with relatively slower cooling after hot-rolling i.e. through air-cooling route and thus, the tempering step can be eliminated from the production process.

Earlier, the high strength bainitic steels could be developed through faster cooling and tempering route. The present invention proposes a novel method to achieve high strength as well as high toughness bainitic steel through a slow cooling (air cooling) route without the necessity of going for further tempering of the plates after air cooling.

SUMMARY OF THE INVENTION

In view of the foregoing disadvantages inherent to the known types of the processes for achieving fine-grained bainitic structures in steel plates, the present invention relates to an improved and easier technique for producing fine- grained bainitic structure in micro alloyed low-carbon steel plates.

Therefore such as herein described there is provided a process for producing low-carbon fine-grained bainitic steel plates comprising the steps of - heating the steel to a temperature 1100°C for transforming its structure into fully austenitic structure, whereby the temperature 1100°C and the holding time 3 hours at the temperature 1100°C are constrained for hindering the grain growth of the austenite, performing hot rolling at temperatures at 1000 – 900°C where the structure of the steel is essentially austenitic and performing air cooling up to 300°C at a cooling rate of 10°C/s to 5°C/s to produce fully bainitic structure.

Therefore the principal objective of the present invention is to provide an improved method for producing fine-grained bainitic structure in low-carbon micro alloyed steel plates.

The second objective of the present invention is to provide an improved method for producing low-carbon fine-grained bainitic steel plates through multi-pass hot rolling and subsequent air cooling.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig 1 illustrates a table showing the chemical composition of the steel in accordance with the present invention;

Fig 2 illustrates the stress-strain plots of steel 1 in accordance with the present invention;
Fig 3 illustrates the stress-strain plots of steel 2 in accordance with the present invention;

Fig 4 illustrates the stress-strain plots of steel 3 in accordance with the present
invention;

Fig 5 illustrates the light optical and scanning electron micrographs of the as-rolled steel plates and air-cooled; (a) steel 1, (b) steel 2 and (c) steel 3 in accordance with the present invention;

Fig 6 illustrates the transmission electron micrographs of air-cooled (a) steel 1, (b) steel 2 and (c) steel 3 showing fine grained closely spaced bainitic laths in accordance with the present invention;

Fig 7 illustrates Hardness and tensile properties of the (a) steel 1, (b) steel 2 and (c) steel 3 in accordance with the present invention

Fig 8 illustrates Charpy V-notch impact test results of the (a) steel 1, (b) steel 2 and (c) steel 3 in accordance with the present invention

Fig 9 illustrates SEM images of the fracture surface of steel 1 after Charpy impact test at room temperature at (a) 1,000x and (b) 2,000x magnifications showing ductile fracture with fine dimples in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In consideration of that state of the art the inventors set themselves the aim of improving the known methods, avoiding the disadvantages noted, and dealing with the task, which is considerably different in comparison with the conventional process of producing low-carbon fine-grained bainitic steel plates.

The first operation is making three experimental heats of 25 kg each were made in Inductotherm make laboratory air induction furnace with varying alloying combinations The cast cylindrical ingots were 100 mm in diameter. The defective portion comprising of about 25% from top and 10% from bottom of the ingots was discarded. The remaining portion of the ingots were then hot rolled into 20 mm thick plates in a HILLE, UK make 2-Hi/ 4-Hi experimental hot/cold rolling mill. Prior to rolling all the ingots were heated to 1100 C and soaked for 3 hours. Samples for hot compression studies were prepared from the as-rolled plates. After determining the optimum process parameters for obtaining desired microstructural morphology of the steels, the plates were further rolled into 6 mm plates in three passes with the final rolling temperature (FRT) of 900 C followed by air cooling (AC).

The second operation is hot compression simulation studies were carried out in a ‘Gleeble 3500C model dynamic thermo mechanical simulator with designed parameters (strain: 0.4, strain rate 5s-1 and finish rolling temperature 900C) followed by air cooling at the rates of 5/10 C/s. cylindrical samples of 10 mm diameter and 15 mm length were used for hot-compression studies.

The third operation is chemical analysis of the as-rolled plates was done on an Applied Research Laboratories (ARL), Switzerland make Optical Emission Spectrometer (OES) [Model 3460] using BAS standard. The chemistry of the steels is given in Fig.1.

The fourth operation is the microstructural examination of the rolled as well as of the samples pertaining to the hot compression studies under light metallurgical microscope, scanning electron microscope and transmission electron microscope. The L-T section of all the hot rolled samples and transverse sections of the hot compression studied samples are ground and mechanically polished following conventional metallographic procedures. The polished samples are etched in 2% nital solution for microstructural examination.

The fifth operation is the property evaluation of the hot rolled steel samples through hardness, tensile testing and Charpy impact testing. Hardness measurements were carried out by Rockwell indentation method using a standard hardness testing machine at 100 kg load. Tensile tests are carried out in a universal testing machine using flat specimens of 25 mm gauge length in accordance with ASTM A 370 standard. Charpy impact tests are carried out at 25C, 0C and -20C using sub-sized specimens as per ASTM E23 specification. For sub-zero temperature testing (0C and -20C), methanol and liquid nitrogen mixture bath is used and each sample is soaked for 25 minutes in the bath before testing.

The sixth operation is fractographic examination of the freshly broken selected Charpy impact tested specimen surfaces under a Scanning electron microscope at various magnifications.

The steel composition comprises of the components in weight percentage as under 0.025 to 0.027 wt% of carbon, 0.27 to 0.29 wt% of silicon, 1.59 to 1.69 wt% of manganese, 0.01-0.27 wt% chromium, 0.88-0.89 wt% nickel, 0.28 wt% molybdenum, 0.041-0.042 wt% of niobium, 0.008-0.025 wt% titanium, 0.039-0.040 wt% of vanadium, 0.47 wt% copper and 0.001 wt% boron and balance essentially iron.

Hot deformation simulation studies were carried out to investigate the influence of hot deformation as well as cooling rate on the flow stress behavior and microstructural evolution. In all, 3 different steels with Ti-Cu-B alloying were studied (Steel 1 alloyed with 0.27 wt% Cr, 0.89 wt% Ni ,0.28 wt% Mo, 0.042 wt% Nb, 0.025 wt %Ti ,0.040 wt% V, Steel 2 additionally alloyed with 0.47 wt% Cu and Steel 3 additionally alloyed with 0.47 wt% Cu and 0.001 wt% B). Three hot deformation passes were imparted at 1000C, 950C and 900C assuming that the first pass will be in recrystallization zone, the second pass close to Tnr and the third one below Tnr. Fig 2 to Fig 4 illustrates the stress-strain plots of a steel sample (hereinafter referred as steel 1) wherein the heating rate is carried out at 5°C /s with holding temp./ time of 1100°C / 120 s. The cooling rate was carried out up to deformation temp. of 1000, 950, 900°C at 10°C/s. The strain 0.4 and strain rate 5 s-1 was done at all deformation temp with soaking time at all deformation temperature of 10 s. The air cooling rate was carried out up to 300°C:(a) 5°C/s and (b) 10°C/s. Further fig 3 illustrates the stress-strain plots of another steel sample (hereinafter referred as steel 2) and fig 4 illustrates stress-strain plots of yet another sample (hereinafter referred as steel 3).

It can be seen from Fig. 2 (a-b) to Fig. 4(a-b) illustrates that the peak flow stress at the end of each deformation of 0.4 strain increased continuously showing continuous work hardening without any recovery in steel 1 containing 0.025 Ti. The peak stress values for the three passes for steel 1 varied in the range of about 160 – 185 MPa. Steel 2 with 0.014 Ti and 0.47 Cu also showed similar trend like steel 1 and the peak stress values were also almost in the similar range which suggests that the addition of Cu had no influence on the stress-strain behavior of the experimental steel. Addition of 0.001 B, in addition to 0.008 Ti and 0.47 Cu, changed the behavior and dynamic recovery was observed immediately after the first hot deformation pass. This resulted in lowering and narrowing the peak stress ranging between 150 – 160 MPa. During the third deformation pass, a drop in stress after a peak stress was observed reflecting occurrence of dynamic recrystallization.

Light and scanning electron micrographs of the steels 1, 2 and 3 are depicted in Fig. 5 (a-c) respectively at various magnifications. All of the micrographs essentially showed granular bainitic microstructures. The transmission electron micrographs taken on steels 2 and 3 are depicted in Fig. 6, which showed fine grained closely spaced bainitic laths (~0.25 µm), very fine Cu and NbC precipitates.

The hardness of all the three steels ranged between 205-240 VHN. Tensile properties of the experimental steels are given in Fig. 7. The yield strength (YS) varied between 507-575 MPa. The ultimate tensile strength (UTS) varied between 668-705 MPa. The YS and UTS of all the three steels were very high, typical of steel with bainitic microstructure. The % El was also found to be better (16.94 to 22.96%). Similarly, the % reduction in area was in general better (60.62 to 63.55 %).

Charpy impact toughness of the experimental steels at various testing temperatures are given in Fig. 8. The Charpy impact toughness of the steels 1, 2 and 3 varied between 131 to 173 J, 121 to 171 J and 114 to 170 J respectively at Room temperature, 0 and -20o C test temperatures. The Charpy impact toughness of all the three steels were found to be excellent and did not deteriorate much even at -20o C particularly. The low-carbon bainitic steels are known to have excellent impact toughness. The impact toughness of the present steels was therefore in conformation with the observations made on bainitic steels by earlier workers. Fractography of the impact tested samples were carried out. Typical fractographs of the Charpy impact tested steel 1 (tested at room temperature) are shown in Fig. 9 (a) and (b) at 1000 and 2000X magnifications respectively. Both micrographs revealed fine dimpled structures typical of a steel with very high ductility.

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.