B. K. Jha

Microstructural evolution during tempering of a multiphase steel containing retained austenite

Abstract Double cycle heat treatment has been imparted to a low carbon low alloy steel with a view to maximize volume fraction of retained austenite. e -Carbide has been found to occur in the austenite phase. Its unusual presence has been explained as a result of enrichment by interstitials during isothermal holding. The steel has been subjected to low temperature ageing treatment to assess the stability of retained austenite against thermal severity. Retained austenite phase remained stable up to a temperature of 100°C. Decomposition of the austenitic phase having predominantly chunky morphology commenced at and above 200°C. Tempering at a temperature of 300°C led to the rearrangement of dislocations into parallel arrays. The mismatch between parent austenite and product ferrite phase resulted in the formation of interfacial dislocations. Upon tempering at 400°C, e -carbide gave way to the formation of e ′( η )-carbide. The observation is extraordinary in view of low carbon content of the steel. Tempering at 500°C eventually led to the formation of stable cementite. Thus a sequence of events leading to the precipitation of cementite ( θ ) from parent austenite phase can be traced as: γ R →α+e→α+e′ η →α+θ

Assessment of Formability of Hot-Rolled Steel through Determination of Hole-Expansion Ratio

Hole expansion ratio has been designed in recent past to characterize formability/stretch flangeability of hot-rolled high-strength steel. Various factors influencing the determination of hole-expansion ratio has been discussed. Parameters such as yield strength, ultimate tensile strength, elongation, carbon equivalent have been correlated with the hole-expansion ratio of various grades of hot-rolled steel. It is found that the ultimate tensile strength of steel is the most important criterion that correlates with hole expansion ratio provided the strengthening mechanism in the steel is the same.

Development of API X-70 Grade Plates and Strips in SAIL

API X-70 grade plates and hot strips microalloyed with Nb, V, and Ti have been developed simultaneously from two units of SAIL at Bhilai and Bokaro Steel Plants, respectively. Suitably designed steel (C: 0.1% max., Mn: 1.7% max., Nb: 0.06% max., V: 0.04% max., and Ti: 0.02% max.) was processed through vacuum arc degassing (VAD), and cast into 240 mm thick slabs and hot rolled into 20 mm thick plates, deploying an appropriate thermomechanical controlled processing (TMCP) technology. The tensile properties (YS: 473–531 MPa, YS/UTS: 0.74–0.88, %El: 32–48%) and Charpy impact toughness properties (CIE: 113–159 J at 0°C) were found to be quite attractive, owing to their fine microstructure of ferrite (∼10 µm) and low pearlite (volume fraction of ∼12%). In case of hot strips, the chemistry was suitably modified with leaner alloy chemistry (C: 0.08% max., Mn: 1.5% max., Nb: 0.06% max., V: 0.05% max., and Ti: 0.02% max.) and slabs were control rolled into 10/12 mm thick strips followed by accelerated cooling prior to coiling. The mechanical properties of hot strips varied within a narrow range of YS: 535–557 MPa, YS/UTS: 0.86–0.88, %El: 33–50%, and Charpy impact toughness property: 150–180 J at 0°C showing appearance of finer ferrite grains: ∼8 µm. Examination of precipitates in both plates and hot strips revealed presence of fine NbV(CN) and complex M4C3 type coarse carbides. The mechanical properties of Electrical Resistance Welding (ERW) pipes manufactured using hot strips satisfied all specified requirements of American Petroleum Institute (API) norms. Both the products are now ready for their commercial exploitation.

Strain Hardening Behavior and Cold Reducibility of Boron-added Low-carbon Steel

Addition of boron in low carbon (0.06% max) hot rolled steel has improved its formability. A unique combination of properties with low strain hardening exponent (n) and high total elongation has resulted into higher percentage of cold reducibility of hot rolled coils.

Effect of B/N ratio on plastic anisotropy behaviour in low carbon aluminium killed steel

It is well known that dissolved nitrogen in ferrite seriously impairs the formability of hot rolled unalloyed steel. Boron being a strong nitride former, combines aggressively with dissolved nitrogen in steel, and thereby improves the forming properties. Further, atomic ratio of boron to nitrogen (B/N) plays an important role in influencing the microstructure and properties of low carbon steel. Whenever excess boron is present in solution in austenite, it segregates to the c grain boundary, thus inhibiting the transformation of austenite to ferrite, and resulting in increase in hardenability of steel. Although plenty of works have been carried out on the effect of boron on properties of hot rolled steels, limited literature is available on its effect in cold rolled formable grades particularly when carbon is in the range 0?03–0?06 wt-%. The present paper discusses the effect of B/N atomic ratio on the forming properties in general and plastic anisotropy ratio rm in particular, in low carbon aluminium killed batch annealed steel. The present study has been carried out on the industrially produced low carbon (0?04–0?06 wt-%) steel with varying B/N atomic ratio. The chemical composition of steels used for the present study is shown in Table 1. Steel A is the typical chemistry used for producing extra deep drawing steel. All the steels were continuously cast to 210 mm thick slabs and were hot rolled to 2?8 mm thickness. The hot rolled bands were finish rolled at 880i10uC and coiled at 620i10uC. As lower coiling temperature (,600uC) results in higher rm values in batch annealed aluminium killed steel, some coils were coiled at 540uC also. Hot rolled coils were cold reduced to 1 mm thickness. The cold rolled coils were annealed with shorter and longer annealing cycles as schematically shown in Fig. 1. Conventionally shorter annealing cycle is practiced for normal cold rolled steel whereas longer annealing cycle is used for extra deep drawing grade. Table 2 shows the mechanical properties of steels with varying B/N ratio processed under different annealing cycles. Properties of boron added steel has shown a significant improvement compared to steel without boron in terms of lower yield strength and higher elongation. In spite of being subjected to similar hot rolling conditions and annealing cycle parameters, lower YS (242 MPa), lower UTS (360 MPa) and higher elongation has been obtained in boron added steel B1 as compared to boron free steel A. It can be attributed to the reduced solute nitrogen and carbon contents in Steel. As expected, increasing the annealing time has led to lowering the strength values and increasing elongation further. As the tensile properties alone does not depict the forming behaviour of cold rolled steel completely, plastic anisotropy ratio r, which is a good measure of deep drawability of steel, has been assessed. A mean value rm is defined as rm5(r0z2r45zr90)/4, where subscripts refer to the angles of tensile tests to the rolling direction. Figure 2 shows the effect of B/N ratio on rm for the steels (with and without boron) annealed with longer cycle. Steel with B/N ratio of 0?8 (steel C) shows lower value of rm (1?12) as compared to rm value of 1?66 in steel with B/N ratio of 0?3 (steel B2) processed under identical conditions of hot/cold rolling and annealing. The rm value of steel A, subjected to coiling temperature of 540uC and longer annealing cycle, has also been compared to steel B3 to assess the effect of boron. rm for both the steel were found to be nearly same (Fig. 2) with value of 1?76 for steel A and value of 1?74 for steel B3. The results show that lower value of B/N ratio does not affect rm adversely. It can be explained in terms of availability of nitrogen for AlN precipitation in steel during batch annealing. Depending on the Al, B and N concentration in steel, the range of temperature at which AlN and BN precipitate coincide in general. However, Ohmari and Yamanaka have reported that BN will form first compared to AlN due to higher diffusivity of boron. In the present study also, it appears that most of nitrogen has been combined by boron before precipitation of AlN in the hot rolled stage, which in turn has resulted in lower availability of nitrogen in solution depending on B/N atomic ratio for combining with aluminium during batch annealing. It is well known that there is strong influence of aluminium nitride during batch annealing of aluminium killed steel. High rm values are produced by textures containing a high proportion of grains with (111) planes and low proportion of (100) planes parallel to the sheet surface. The aluminium nitrides lead to enhancement of the (111) texture components and a concurrent reduction of the (100) components. While developing the desirable texture, aluminium nitrides also help at the same time in formation of a pancake grain structure which results in better rm value in steel. This emphasises the Research and Development Centre for Iron and Steel, Steel authority of India Limited, Ranchi, 834002, India

Development and characterization of steel containing very fine ferrite grains

Ferrite grain sizes of the order of 1 to 2 µm were obtained by optimizing the strain, strain rate, the stage of cooling, as well as the cooling rate during hot rolling of 0.15C-0.92Mn-0.01Si-0.036S-0.04P-0.013Nb steel. It was found that in single-pass rolling of a 10 mm plate to a thickness of 3.5 mm with an entry temperature of 800 °C, and early-stage water cooling, very fine grains of ferrite (1–2 µm) were formed at the surface and in subsurface regions. It was also found that the threshold level of reduction during rolling, which is required for the refinement of ferrite grains, is >50%. The 3.5 mm thermomechanically processed plate was found to possess very attractive mechanical properties in terms of the yield strength (485 MPa), the ultimate tensile strength (UTS) (763 MPa), and particularly the yield strength to ultimate tensile strength (YS/UTS) ratio (0.63). This combination of properties can be explained on the basis of the composite microstructure consisting of ferrite and bainite that was obtained as a result of the thermomechanical processing.

Optimization of Annealing Parameters for Improvement in Formability of Extra Deep Drawing Quality Steel

A number of annealing cycles were investigated in an attempt to find the optimum cycle that results in an attractive combination of mechanical and formability properties of an extra deep drawing (EDD) quality steel. It was found that the cycle that involved an intermediate anneal at 600 °C followed by further soaking at 700 °C resulted in the best combination of mechanical and formability properties. It was also found that the rate of heating up to 600 °C can be kept at 50 °C/h while the heating has to be done at a rate of 30 °C/h from 600 °C to the final annealing temperature of 700 °C. The desirable combination of mechanical and formability properties has been correlated with the microstructure that shows pancaking of the annealed grains accompanied by precipitation of carbides. Precipitates of carbides are more in number and smaller in size in the case of samples annealed by the cycle mentioned above compared to the ones annealed by other cycles. They are spherical in shape, which is desirable for forming applications.

Development of High Performance Steel Products in Sail Through R&D Initiatives

AbstractSteel Authority of India Limited (SAIL), through its Corporate R&D i.e. RDCIS, has been working incessantly for the development of new steel products with superior quality attributes to meet the requirements of Indian market. The product development activities have always been supported by in-house/collaborative research work which has led to deeper understanding on the process know-how, resulting in launching of cost-effective products in Indian market. Some of the products developed by SAIL in the flat product category include high strength thin gauge HR coil for automobile sectors, SAILMA 550 HI/SAILMA 600 grade plates for penstocks and earthmovers, SAIL HITEN 690 AR plates for automated teller machines chest, high strength plates with guaranteed through thickness ductility for railways and Defence sectors, boiler quality plates (ASTM 387 Gr. 12) for power sector, API X 70 grade HR coils and plates for Linepipe application. In the long products category, EQR TMT rebar, Roof/Rock Bolt (Fe—600) TMT rebars for mines and tunnels, corrosion resistant TMT rebars for construction sector, Al-killed low carbon CC blooms for cold reducers, Al-killed medium carbon CC blooms for forging have been developed recently through R&D initiatives in SAIL. This paper describes some of the products which were recently developed in SAIL.

Optimization of Hot Rolling and Annealing Process of Low Ni Stainless Steel Using Simulation Studies

Processing of low Ni Stainless Steel poses a serious problem during rolling both in roughing and finishing stands owing to high load. This is primarily attributed to high work hardening rate due to low Ni and high Mn in such steels. An increase in temperature to bring down the rolling load deteriorates the surface quality. In addition to this, high hardness in the cold rolled and annealed product posses problem during further forming operation. To overcome these problems, simulation studies were carried out using Gleeble Thermo-mechanical Simulator (i) to determine flow stress under different hot rolling conditions and (ii) to simulate continuous strip annealing process to optimize annealing parameters to bring down the hardness. It was observed that the flow stress increases gradually with a decrease in temperature till 1050C and thereafter, at a significantly high rate. It was further observed that continuous dynamic recovery occurred at 1050 – 1150C which gradually diminished and resulted in work hardening at lower temperatures. In view of this, rolling schedule was modified which led to smooth rolling without any over-load problem. It was also found based on simulation studies that the hardness was minimum in the temperature range of 950-975 C, however, no/ little recrystallisation occurred at these temperatures. A fully recrystallised grain with significant drop in hardness was observed at 1050C. Annealing cycle was accordingly modified for regular commercial production which has given good result.

Characterization of a precrept modified 9Cr-1Mo steel through room temperature tensile and strain rate change tests

Abstract A Nb–V containing 9Cr–1Mo ferritic steel was allowed to age under creep conditions in the temperature range of 873–973 K. Its microstructure in normalized and tempered condition contained tempered martensite with well recognized laths. However, under high stress creep conditions at 973 K, cell walls and dislocation clusters are formed. A cellular structure develops instead with cell walls being formed at carbides. These microstructural features are reflected in their subsequent room temperature deformation behavior, which was studied through an analysis of strain rate change (SRC) tests and stress–strain curves. In the new analysis, Cottrell–Stokes (C–S) ratio and modified Haasen plots are derived to study the nature of thermal obstacles and component internal stresses. SRC results are used also to estimate long-range internal stresses, which are interpreted to be associated with cell walls, dislocation clusters etc. The nature of variation of activation area, long range stress, and work hardening rate as determined from room temperature tensile tests carry sufficient information to suggest a methodology for evaluating the damage state of the parent service exposed material.