Bainitic steel of high strength and high elongation and method to manufacture said bainitic steel

Abstract

The invention relates to a bainite steel consisting of the following elements in weight %: C: 0.25-0.55 Si: 0.5-1.8 Mn: 0.8-3.8 Cr: 0.2-2.0 Ti: 0.0-0.1 Cu: 0.0-1.2 V: 0.0-0.5 Nb: 0.0-0.06 Al: 0.0-2.75 N: <0.004 P: <0.025 S: <0.025 and a method for manufacturing a bainite steel strip that comprises the step of cooling the coiled strip of such composition to ambient temperature, during which the bainite transformation takes place.

Claims

1. A bainite steel comprising the following elements in weight %: C: 0.30-0.50 Si: 1.0-1.8 Mn: 1.0-2.5 Cr: 0.7-1.5 Ti: 0.0-0.08 Cu: 0.0-1.2 V: 0.0-0.5 Nb: 0.0-0.06 Al: 0.0-1.50 N: <0.004 P: <0.025 S: <0.025 the balance being iron and unavoidable impurities, wherein the bainite steel comprises no added alloying Ni or Mo, wherein the bainite has a microstructure with bainite plates with a thickness of less than 100 nm, and wherein the steel has an ultimate tensile strength of at least 1350 MPa, wherein the bainite steel is formed by: casting a slab from a liquid steel of the composition; cooling the slab; reheating the cooled slab to 1250° C.; hot rolling a cast slab into a strip at a temperature of 850° C.; and coiling the strip of steel at a temperature above the bainite start temperature in the range of 350° C.-500° C., and continuously cooling the coiled strip by natural cooling, and wherein no further heat is applied during the step of continuously cooling the coiled strip such that the slab transforms to bainite steel during the cooling the coiled strip.

2. The bainite steel according to claim 1, wherein one or more of the following elements are present in weight %: C: 0.30-0.40 Si: 1.2-1.7 Mn: 1.6-2.1 Cr: 0.9-1.2 Ti: 0.0-0.07 Al: 0.0-0.2.

3. The bainite steel according to claim 1, wherein the steel has a hardness of at least 415 VHN.

4. The bainite steel according to claim 1, wherein the steel has at least a total elongation of 20%.

5. The bainite steel according to claim 1, wherein the steel has a microstructure with 15-30% of retained austenite.

6. The bainite steel according to claim 1, wherein the bainite steel is 70-85% carbide-free bainite.

7. The bainite steel according to claim 1, wherein the bainite steel comprises 70-80% nano-structured bainite.

8. The bainite steel according to claim 1, wherein the Ti and N react to form TiN which in turn forms fine TiCN precipitates.

9. The bainite steel according to claim 1, wherein the steel has a dislocation density in the range of 4-6×10.sup.6.

10. The bainite steel according to claim 1, wherein N is present in a weight % of 0.001-0.004.

11. The bainite steel according to claim 1, further comprising, prior to coiling the strip, cooling the strip formed from the slab to a temperature above the bainite start temperature in the range of 400-550° C.

12. The bainite steel according to claim 1, wherein Cu is present in a weight % of 0.1-1.2.

13. The bainite steel according to claim 1, wherein V is present in a weight % of 0.1-0.5.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 Calculated TTT diagram for the designed steel

(2) FIG. 2 Calculated T.sub.0 curve for the designed steel composition

(3) FIG. 3a Calculated amount of retained austenite as a function of isothermal transformation temperature

(4) FIG. 3b Calculated ratio of film type to blocky type austenite as a function of isothermal transformation temperature

(5) FIG. 4 Calculated strength of the designed steel

(6) FIG. 5 Schematic diagram of the hot rolling operation

(7) FIG. 6 Microstructure of the bainitic steel (a) Optical and (b) SEM

(8) FIG. 7 TEM photograph of the microstructure showing nanoscale bainite with high dislocation density

(9) FIG. 8 XRD profile (experimental along with simulated) of the continuously cooled sample

(10) FIG. 9 Tensile test results of three samples exposed to continuous cooling transformation after hot rolling.

DESCRIPTION OF THE INVENTION

(11) In FIG. 1 a TTT diagram is shown for a sample with a composition within the ranges given in Table 1 below.

(12) TABLE-US-00001 TABLE 1 Range of compositions C Si Mn Cr Ti Cu V Nb Al N Min 0.25 0.5 0.8 0.2 0.0 0.0 0.0 0.0 0.01 0.001 Max 0.55 1.8 3.8 2.0 0.1 1.2 0.5 0.06 2.75 0.004

(13) In the diagram B.sub.s and M.sub.s stand for respectively bainite start temperature and martensite start temperature. It can be seen from this figure that a minimal cooling rate 20° C. sec.sup.−1, which is typical of any hot rolling mill, is capable enough to avoid the diffusional bay and in turn avoid the chance of formation of high temperature products like ferrite. The difference between B.sub.s and M.sub.s temperatures provides a reasonably wide processing window to carry out the method for producing bainite.

(14) The M.sub.s will further be suppressed by the formation of bainite where due to the rejection of C from bainitic ferrite, adjacent austenite gets enriched with C, as denoted by the T.sub.0 curve presented in FIG. 2.

(15) From FIG. 2, it can be seen that the lower the transformation temperature, the higher is the enrichment of C in austenite. Consequently all the austenite is expected to be retained till the cessation of bainitic transformation. A sufficiently lower B.sub.s also provides the chance to produce lower bainite which is finer in nature and can contribute for higher strengthening.

(16) During the progress of bainitic transformation, the whole of the austenite grain does not transform instantaneously to bainite. It is a gradual process; when the first plate of bainite forms, it rejects its excess carbon which it can not accommodate into the adjacent austenite. Further advancement of transformation therefore is associated with a lowering of free energy due to the higher carbon content of austenite from which bainite forms. Finally a time is reached when the free energies of both residual austenite and bainitic ferrite of the same composition becomes identical and therefore any further transformation becomes thermodynamically impossible. T.sub.0 represents the locus of all the points, on a temperature versus carbon concentration plot, where the stress-free austenite and ferrite of identical composition have the same free energy. The bainitic transformation can progress by successive nucleation of subunits of bainitic ferrite till the carbon concentration in the remaining austenite reaches to its limit which is defined by the T.sub.0 curve. The maximum amount of bainite which can be produced at any given transformation temperature is restricted by the retained austenite carbon concentration which can not exceed the limit given by the T.sub.0 curve.

(17) In this approach, bainitic transformation is made to occur at such a temperature where the diffusion of any elements except carbon is extremely negligible. Hence it can be considered that during bainitic transformation no other diffusional reaction interacts with it and the temperature is high enough for restricting other diffusionless transformation product. The carbon enrichment in austenite from adjacent bainitic-ferrite plates makes it thermally stable at room temperature and it will only transform to martensite during deformation exhibiting a TRansformation Induced Plasticity (TRIP) effect.

(18) FIG. 3a represents a theoretical calculation of the amount of retained austenite after bainitic transformation at different isothermal temperatures whereas FIG. 3b shows the calculated ratio between the blocky and film type austenite. In the FIG. 3b, volume fraction of blocky and film type austenite are represented by V.sub.γ-b and V.sub.γ-f, respectively. From FIG. 3a and FIG. 3b it is evident that the lower the transformation temperature is, the lower will be the amount of austenite which is detrimental for the expected TRIP effect and final elongation value. On the other hand, lower the transformation temperature, higher the ratio between films to blocky austenite which is required for the good ductility behavior. During TRIP effect, austenite transforms to martensite and the material gets work hardened. As a consequence, it is essential to have a certain amount of austenite remain untransformed at ambient temperature so that TRIP effect can occur.

(19) It can also be found from FIG. 3 that at temperature 350° C., the calculated amount of retained austenite is approximately 24% and the ratio between the thin to blocky austenite is 0.9. At further lower temperature, the kinetics of the transformation becomes very sluggish and further reduction in the amount of retained austenite is not very much expected.

(20) TABLE-US-00002 TABLE 2 Composition in wt % for the 4 casts Heat number C Mn Si Cr S P Al Ti Ni Mo Co 1 0.37 1.84 1.65 0.92 0.01 0.03 0.054 0.068 0.014 0.024 0.005 2 0.345 1.97 1.29 1.03 0.007 0.015 0.036 0.017 0.01 0.01 0.001 3 0.355 2.01 1.46 1.04 0.007 0.016 0.038 0.017 0.01 <0.005 0.001 4 0.32 1.94 1.55 1.01 0.01 0.03 0.01 0.04 0.01 0.01 0.001

(21) FIG. 4 represents the strength of the alloy which shows that the calculated total strength of the designed steel could exceed 1500 MPa. The major source of strengthening is coming from the ultra fine bainite plates. Another major source of strengthening is from the dislocation density which was calculated to be in the range of 4-6×10.sup.6. Since there are some approximations and assumptions, the actual strength will be below the calculated strength. As there is very little knowledge available for bainitic transformation during continuous cooling, all the calculations were carried out at many different temperatures considering isothermal nature of transformation and then extrapolated to the continuous cooling situation.

(22) Four 40 kg heats were made in vacuum induction furnace. The chemical compositions of these four casts are given in Table 2 below.

(23) Subsequently, the cast steels were forged to 40 mm thickness and homogenized at 1100° C. for 48 hours after which the steels were cooled along with the furnace. All the experiments were carried out with this homogenized steel.

(24) Small pieces of samples (150 mm×100 mm×20 mm) were cut for hot rolling in an experimental rolling mill. The soaking was done at 1200° for 3 hours. The rolling operation was completed within 6-7 passes, keeping the final rolling temperature at about 850-900° C. Throughout the experiments, temperature was monitored with laser radiation pyrometer. After the hot rolling, the samples were kept on run-out table where water jet cooling was applied till a temperature of 400-550° C. is reached and finally the samples were kept inside a programmable furnace where very slow cooling rate was applied to simulate the actual coil cooling situation. The cooling rate of a coil after coiling in downcoiler in hot strip mill was first measured with radiation pyrometer over a long period of time and similar cooling rate was simulated in furnace for the simulation purpose. The temperature of the furnace for coiling simulation was kept within 350-500° C. Schematic diagram of the entire hot rolling process is shown in FIG. 5. The hot rolled thickness was about 3.0 mm.

(25) Samples for metallographic observation were cut from the rolling plane of one end of the heat treated samples. The samples were polished using standard procedure, etched with nital and the microstructures are reproduced here in FIG. 6 where FIG. 6a is the optical microstructure and FIG. 6b is the SEM photograph. Image analysis of the optical microstructures was carried out with the help of Axio-Vision Software version 4 equipped with Zeiss 80 DX microscope and shows the presence of significant amount of bainite (˜75%) along with some retained (˜25%) austenite. The products of diffusional transformation, e.g. ferrite, cementite were not seen and the bainite thus produced is a carbide-free bainite. The bainite plate thicknesses, as can be observed from the TEM photograph presented in FIG. 7, are less than 100 nm and the structure is highly dislocated.

(26) The volume fraction and the lattice parameter of retained austenite were calculated from the X-ray data by using commercial software, X'Pert High Score Plus. The X-Ray Diffraction analysis results are shown in Table 3 below.

(27) TABLE-US-00003 TABLE 3 Volume fraction of different phases along with C in austenite C in austenite C in austenite Austenite/ (from XRD)/ (from T.sub.0)/ Ferrite/ wt % wt % wt % wt % 22 ± 1.4 1.07 ± 0.06 0.99 79 ± 2.1

(28) FIG. 8 represents the calculated and experimentally obtained XRD profiles along with the differences between these two. During the XRD analysis, it was assumed that whatever ferrite is present is only bainitic ferrite as the diffusional bay and its products were bypassed. From the Table 3, it is apparent that the C content of retained austenite is higher than that predicted from calculated T.sub.0 curve shown in FIG. 2. It should be kept in mind that the T.sub.0 curve was calculated at isothermal condition and the actual experiments were carried out in continuous cooling form producing different austenites with different C concentration. These different austenites are not separable by XRD and XRD indicates average C concentration only.

(29) After continuous cooling to room temperature, hardness measurement was to carried out in Vicker's Hardness tester using 30 kg load. The hardness value turned out to be 425±9 VHN which is an averaged out value of 100 readings from four different hot rolled and continuously cooled samples. See Table 4 below for all the mechanical properties (hardness, YS, UTS, uniform elongation, total elongation). The ultimate tensile strength is even more than 1350 MPa.

(30) TABLE-US-00004 TABLE 4 Mechanical properties of the 4 casts Hardness/ YS/ UTS/ YS/ Uniform Total VHN MPa MPa UTS elongation/% elongation/% 425 ± 9 864 ± 28 1366 ± 4 0.63 13.6 ± 0.5 22 ± 0.7

(31) Standard tensile samples were prepared from the steel following the ASTM procedure [ASTM E8] for standard samples of 50 mm gauge length and tested in Instron tensile testing machine (Model number: 5582). FIG. 9 shows the results of the first three samples. From this figure, it is evident that the bainite steel according to the invention has an outstanding combination of tensile strength (>1300 MPa) with more than 20% elongation.