Field of invention

The present invention relates to metallurgy. More particularly, it relates to manufacturing of steel suitable for automotive application.
Background of invention:
Most of the automotive vehicles utilises steel due to its excellent strength, crash worthiness, recyclability, affordability, etc. Therefore, efforts are ongoing worldwide to make steel with strength as high as possible. This will allow use of thinner section to sustain a given load and, therefore, reduce the weight of automotive vehicle without compromising on the safety. As a result, the greenhouse gas emission can be reduced due to improved fuel efficiency.
However, it is well known that an increase in strength of steel generally leads to a concurrent decrease in the ductility. Therefore, some grades of steel although can achieve high strength, they are limited by a reduced ductility, and the vice-versa is also true. This leads to restriction on the energy absorption capability of any steel, which is the product of tensile strength and total elongation.
The different grades of steel are categorized into various generation based on this energy absorption capability value. For example, various steel grades within 1st generation steels (TRIP, DP, CP, Martensitic, HF, etc.) can achieve energy absorption capability (UTS × TE) of about 15 ± 10 GPa%. As a result, the need to improve the energy absorption capability led to the development of the 2nd generation steels, i.e. TWIP and austenitic stainless steels. These steels with a complete austenitic structure can achieve energy absorption capability of about 60 ± 10 GPa%, which is a 4-fold increase in comparison to the 1st generation steels. This is due to the work-hardening mechanisms of austenite, such as transformation-induced plasticity (TRIP) and/or twinning-induced plasticity (TWIP), which contributes to the enhancement of both strength and ductility. However, the need of significant alloying additions in these steels poses issues such as increased cost and difficulties in steelmaking, processing, fabrication and welding. Therefore, there is a need to develop steel with simultaneous enhancement of both strength and ductility by the use of minimum alloying additions and applying novel processing routes.
Based on the above, the 3rd generation steel, which generally contains a combination of retained austenite and martensite, can achieve the strength-ductility combinations superior to the 1st generation and close to the 2nd generation steels. This is because the martensite gives rise to the desired strength and the ductility is improved the metastable retained austenite. These steel grades are generally produced using processing techniques, such as Quenching & Partitioning (Q&P), Intercritical annealing, etc. The medium manganese steel, which contains about 3–10 wt.% Mn, are first hot and cold rolled for thickness reduction, followed by intercritical annealing. The conventional practise in these steel is to quench to room temperature after annealing or to an intermediate temperature for galvanizing, depending on the requirement.
In this regard, He et al. have also shown to improve the strength of medium manganese steel by warm rolling process; however, a drop in ductility has been reported for the warm-rolled sample. Moreover, the warm rolling process proposed in He et al. required a reheating after every two passes of warm rolling, to maintain a constant temperature. This reheating after every two passes of warm rolling, is quite cumbersome and may not be suitable industrially.
Objects of the invention
An object of the invention is to design a process to produce method for producing superior strength and high ductility steel and product thereof.
Another object of the invention is to design a process adaptable in industrial conditions.
Disclosure of the Invention
The present invention provides a method for producing superior strength and high ductility steel, comprising:
casting a steel with composition
Carbon (C): 0.21-0.23, Manganese (Mn): 4-5.5, Silicon (Si): 0.8-0.95, Aluminium (Al): 0.35-0.45, Sulphur (S): 0.005 to 0.01, Phosphorous (P): 0.003 to 0.05 and Nitrogen (N): 0.004 to 0.01, Rest iron (Fe) and trace elements (all in wt%) at caster;
reheating and soaking the steel to 1150 °C to 1250°C in a reheating furnace;
hot rolling the steel in a Hot rolling mill;
cold rolling the steel in a Cold rolling mill;
inter-critically annealing the steel to 640-660°C in an annealing furnace;
first cooling the steel in a first cooling unit;
treating the steel isothermally at 275-315 deg. C in a heat treatment furnace;
warm rolling the steel at 275-315 deg. C. in a warm rolling mill; and
second cooling the steel at a second cooling unit to produce superior strength and high ductility steel.
In the method, the retained austenite stability has been tailored by the warm rolling process during cooling to room temperature after intercritical annealing. This warm rolling process increases the dislocation density and therefore, the improves the retained austenite stability against deformation. As a result, the warm rolled steel shows improved strength with a comparable ductility, in comparison to the condition of no warm rolling in the same alloy.
In an embodiment, the soaking can be done for 2 to 6 hr.
The rate of reheating can be 5-10 deg. C/sec.
The finish rolling temperature can be 840-860°C
Post hot rolling, the steel can be air /furnace cooled to be cold rolled.
The steel can be deformed 85-95% during hot rolling.
The steel can be deformed at 40-70% during cold rolling.
The thickness of the steel post cold rolling can be 2.0-2.8 mm.
The inter critically annealing can be performed for 1.8-2.3 hrs.
The rate of heating the steel for inter critically annealing can be 1-3°C/s.
The first cooling of the cold rolled steel can be done at 275-315°C for 2-10 mins.
Deformation applied to the steel is 10-40% is during warm rolling.
The first and second cooling of the steel can be done by water.
The soaking can be done for 2 to 6 hr.
In another embodiment, the present invention describes a superior strength and high ductility steel, comprising:
Composition of Carbon (C): 0.21-0.23, Manganese (Mn): 4-5.5, Silicon (Si): 0.8-0.95, Aluminium (Al): 0.35-0.45, Sulphur (S): 0.005 to 0.01, Phosphorous (P): 0.003 to 0.05 and Nitrogen (N): 0.004 to 0.01, Rest iron (Fe) and trace elements (all in wt%) with
Ultimate Tensile Strength is 1227-1255 MPa and Total elongation is 26.5-31.0 %.
The Yield strength of the superior strength and high ductility steel is 969-978 MPa.
Uniform elongation of the superior strength and high ductility steel is 25.4-27.0 %.
Tensile toughness of the superior strength and high ductility steel is 32.6-38.9 GPa%.
The superior strength and high ductility steel comprises 25-30% austenite and rest martensite (by volume).
Brief description of the accompanying drawings
FIG. 1: shows a method for designing a developed steel (defined later) in accordance with an embodiment of the invention.
FIG. 2: shows a schematic illustration of heat treatment schedule adopted in accordance with an embodiment of the present invention
FIG. 3: shows an XRD profiles of samples of the developed steel in accordance with an embodiment of the present invention.
FIG. 4: shows a secondary electron micrograph of the developed steel in accordance with an embodiment of the present invention.
FIG. 5: shows a tensile plot of the developed steel in accordance with an embodiment of the present invention.
Detailed description of a preferred embodiment
In accordance with an embodiment of the invention a method (100) for producing a superior strength and high ductility steel (hereinafter referred as “developed steel”) is described.
The method (100) comprises various steps (104-136) to develop the developed steel. At step (104) a steel is casted at caster. The composition of the steel is
Carbon (C): 0.21-0.23,
Manganese (Mn): 4-5.5,
Silicon (Si): 0.8-0.95,
Aluminium (Al): 0.35-0.45,
Sulphur (S): 0.005 to 0.01,
Phosphorous (P): 0.003 to 0.05 and
Nitrogen (N): 0.004 to 0.01, Rest iron (Fe) and trace elements (all in wt%) at caster.
Carbon (C): Adequate amount of carbon is necessary to ensure that the desired strength levels are reached. Carbon also increases stability of retained austenite which is essential to achieve enhanced ductility. For ensuring both strength and ductility are maximized, carbon content is kept preferably at 0.21-0.23 wt%. Also, at this range of Carbon, the weldability of the steel is good.
Manganese (Mn): Optimum Mn is necessary to stabilize austenite and obtain desired austenite.
Silicon (Si): Silicon is a ferrite stabilizer. It also restricts carbide precipitation during isothermal holding resulting in a larger amount of retained austenite. However, addition of Si leads to surface scale problems during rolling and therefore should be limited to the range mentioned.
Aluminium (Al): Optimum Al is added because, to an even stronger degree than Si, as it is a ferrite stabilizer. Al also suppresses the precipitation of carbon from the retained austenite during the bainitic transformation step, which results in a higher amount of retained austenite. Unlike Si, Al has no detrimental effect on galvanisability.
Sulphur (S): Sulphur is detrimental for steel as it becomes brittle, hence it is kept as low as possible.
Phosphorous (P): high Phosphorous is not desirable for strength and ductility, hence it is kept as low as possible.
Nitrogen (N): high Nitrogen is not desirable for strength and ductility; hence it is kept as low as possible.
The preferrable steel composition is mentioned below in Table 1
Table 1:
Steel C Mn Si Al P S N
1 0.22 5.1 0.85 0.42 0.05 0.005 0.004
2 0.23 5.0 0.84 0.42 0.05 0.005 0.005
3 0.21 5.1 0.85 0.40 0.004 0.005 0.006
At step (108), the steel is reheated to 1150 °C to 1250°C in a reheating furnace. The steel is further soaked for 2-6 hrs at 1150 °C to 1250°C. This is done to ensure the steel gets properly homogenised and austenitized.
The rate of reheating for step (108) is 5-10 deg. C/sec.
At step (112), the steel is hot rolled in a Hot rolling mill. The finished rolling temperature after hot rolling is 840-860 deg. C. The steel is deformed at 85-95% during hot rolling.
Post hot rolling, the steel is cold rolled at cold rolling mill at step (116). The steel is deformed at 40-70% during cold rolling. In an embodiment, the thickness of the steel post cold rolling is 2.0-2.8 mm.
The steel is air /furnace cooled before to be cold rolled. The furnace cooling is sometimes preferred as it reduces chances of crack generation in the steel.
In an embodiment, a softening treatment is also given to the hot rolled sheets at 620-660 °C for 5-6.5 hours prior to cold rolling.
Post cold rolling, at step (120), the steel is inter-critically annealed to 640-660°C deg. C at an annealing furnace.
The inter critically annealing is done for 1.8-2.3 hrs.
The rate of heating the cold rolled steel for inter critically annealing is 1-3°C/s.
At step (124), post inter-critical annealing first cooling is done in a first cooling unit. The first cooling unit in an embodiment can be a heating furnace where the temperature can be set below against the temperature at previous unit. At the instant step (124), the first cooling unit may receive the steel with temperature 640-660 deg. C, at current furnace temperature 275-315 deg. C. This difference in the temperature will eventually lead to cooling of the steel to 275-315 deg. C.
The first cooling of the annealed steel is done at 275-315°C for 2-10 mins.
The purpose of this first cooling step (124) is also to prevent the decomposition of austenite, which has formed during intercritical annealing step (120).
After first cooling, at step (128) the steel is treated isothermally at 275-315 deg. C in a heat treatment furnace. The heat treatment furnace in an embodiment can be the first cooling unit (as mentioned above) depending upon the set temperature. The isothermal treatment of the steel is done for microstructure deformation and dislocation activity.
At step (132), the steel is warm rolled at 275-315 deg. C in a warm rolling mill.
The usage of reheating furnace, the annealing furnace, and the heat treatment furnace are driven by the temperature maintainability. In an embodiment, they can be same furnace if they can maintain the temperature as desired.
At step (136), post warm rolling the steel is second cooled at a second cooling unit to produce superior strength and high ductility steel (developed steel). The second cooling is done till the steel achieves the room temperature.
The deformation of the steel is 10-40% applied during warm rolling.
The first cooling and the second cooling of the steel is done by water.
The mechanical properties of medium manganese steels are greatly influenced by the retained austenite content as well as its characteristics, such as its composition, size, morphology, distribution, dislocation density, etc. This is due to the fact that the retained austenite experiences the transformation-induced plasticity (TRIP) and/or twinning-induced plasticity (TWIP) effect during deformation, leading to improved work-hardening and strength-ductility combination. However, it is important to have a suitable metastability of retained austenite to achieve its anticipated benefits, i.e. it should not transform to martensite over a range of deformation strain rather than a quick or no transformation. Therefore, in the developed steel, the retained austenite stability has been tailored by the warm rolling process (132) during cooling to room temperature after intercritical annealing (120). This warm rolling process increases the dislocation density and therefore, the improves the retained austenite stability against deformation. As a result, the warm rolled steel shows improved strength with a comparable ductility, in comparison to the condition of no warm rolling in the same alloy.
In the conventional practise of annealing, the cold rolled steel is heated into intercritical region followed by cooling to room temperature. In contrast to this, the method (100) includes a warm rolling process (132) during first cooling after annealing, which provides a cumulative reduction of about 15-20% in the temperature range of 275-315°C, followed by second cooling (136) the steel to room temperature. This warm rolling process (132) is non-isothermal in nature, i.e. a continuous drop in temperature during warm rolling, which makes it feasible to apply in industries without need of any additional facilities to maintain a constant temperature during/after warm rolling.
The cold rolled and warm rolled medium manganese steel, i.e. the developed steel has Yield Strength of 969-978 MPa.
The developed steel has 25-30% austenite and rest martensite (by volume).
The developed steel has Ultimate Tensile Strength of 1227-1255 MPa.
The developed steel has Total elongation of 26.5-31.0 %.
The developed steel has Uniform elongation of is 25.4-27.0 %.
The developed steel has Tensile toughness of 32.6-38.9 GPa%.
The developed steel comprises 25-30% austenite and rest martensite (by volume).
Experimental Analysis:
The steel with composition as mentioned in Table 2 below was melted in an air induction furnace of 2 tons capacity and was cast to a 500 mm X 500 mm ingot (steel).
Table 2
Steel Composition C Mn Si Al P S
0.22 5.1 0.85 0.42 0.005 0.005
The heat treatment schedule applied on the steel is schematically illustrated in FIG 2. The steel was austenitized (reheating and soaking) at 1200-1250°C for 3-4 hrs. Thereafter it was hot rolled into slabs and further hot rolled into 350 X 50 mm cross-section. The steel was hot rolled into 5-7 mm thick. The finish rolling temperature was maintained in the austenite in the temperature range 850-860. The hot rolled steel were subsequently furnace cooled to room temperature.
A softening treatment was given to the hot rolled sheets at 650 °C for 6 hours prior to cold rolling. Now the steel is cold rolled to 2.5 mm. The cold rolled steel was heated to 640-650°C (inter-critical annealing) at a heating rate of 1-3°C/s and held there about 2 h. After this, the inter critically annealed steel is first cooled in the annealing furnace to 320-330°C and held there for 3-4 min for temperature homogenization.
Thereafter, the steel is warm rolled in the range of 225-250°C with a cumulative reduction of 15-20% and then second cooled in water to room temperature producing the developed steel.
In order to differentiate the effect of warm rolling and second cooling with no warm rolling, one of the steel plate was first cooled to room temperature after inter-critically annealing at 630-650°C for 2 h (keeping the other parameters same as that of experiment above). The properties obtained are mentioned in the Table 2 below.
Table 2
Process Tensile Strength (MPa) Uniform elongation (%) Total elongation (%) YS (MPa) Toughness
(GPa%)
Without warm rolling 1123-1150 26.9-28.7 28.5-29.8 724-730 32.4-34.2
The X-ray diffraction profiles of the developed steel is shown in Fig 3. The XRD results confirm presence of various phases in the microstructure. The a represents martensite whereas ? represents austenite. Similarly, secondary electron micrographs in Fig 4 also depicts the presence of martensite and austenite.
The amount of retained austenite was calculated to be about 30 vol.% in the warm rolled steel.
The tensile curves and the corresponding mechanical properties of the developed steel is shown in Fig 5 and Table 3, respectively. Tensile curve shows excellent combination of yield and tensile strength with large elongation. Hence, resulted extraordinary toughness.

Table 3: Tensile Properties (range) of the developed steel
Process Tensile Strength (MPa) Uniform elongation (%) Total elongation (%) YS (MPa) Toughness
(GPa%)
Warm rolled 1227-1255 25.4-27.0 26.5-31.0 969-978 32.6-38.9
It is evident that the developed steel shows superior combination of yield and tensile strength while maintaining very good elongation and tensile toughness. Excellent combination of mechanical properties in the steel is attributed to presence of retained austenite.

Claims:

1. A method for producing superior strength and high ductility steel, comprising:
casting a steel with composition
Carbon (C): 0.21-0.23, Manganese (Mn): 4-5.5, Silicon (Si): 0.8-0.95, Aluminium (Al): 0.35-0.45, Sulphur (S): 0.005 to 0.01, Phosphorous (P): 0.003 to 0.05 and Nitrogen (N): 0.004 to 0.01, Rest iron (Fe) and trace elements (all in wt%) at caster;
reheating and soaking the steel to 1150 °C to 1250°C in a reheating furnace;
hot rolling the steel in a Hot rolling mill;
cold rolling the steel in a Cold rolling mill;
inter-critically annealing the steel to 640-660°C in an annealing furnace;
first cooling the steel in a first cooling unit;
treating the steel isothermally at 275-315 deg. C in a heat treatment furnace;
warm rolling the steel at 275-315 deg. C. in a warm rolling mill; and
second cooling the steel at a second cooling unit to produce superior strength and high ductility steel.

2. The method as claimed in claim 1, wherein the soaking is done for 2 to 6 hrs.

3. The method as claimed in claim 1, wherein the rate of reheating is 5-10 deg. C/sec.

4. The method as claimed in claim 1, wherein the finish rolling temperature is 840-860°C

5. The method as claimed in claim 1, wherein post hot rolling the steel is air /furnace cooled to be cold rolled.

6. The method as claimed in claim 1, wherein the steel is deformed 85-95% during hot rolling.

7. The method as claimed in claim 1, wherein the steel is deformed at 40-70% during cold rolling.

8. The method as claimed in claim 1, wherein the thickness of steel post cold rolling is 2.0-2.8 mm.
9. The method as claimed in claim 1, wherein the inter critically annealing is performed for 1.8-2.3 hrs

10. The method as claimed in claim 1, wherein the rate of heating the steel for inter critically annealing is 1-3°C/s

11. The method as claimed in claim 1, wherein the first cooling of the steel post inter critical annealing is done at 275-315°C for 2-10 mins.

12. The method as claimed in claim 1, wherein deformation 10-40% is applied during warm rolling.

13. The method as claimed in claim 1, wherein first and second cooling of the steel is done by water.

14. A superior strength and high ductility steel, comprising:
Composition of Carbon (C): 0.21-0.23, Manganese (Mn): 4-5.5, Silicon (Si): 0.8-0.95, Aluminium (Al): 0.35-0.45, Sulphur (S): 0.005 to 0.01, Phosphorous (P): 0.003 to 0.05 and Nitrogen (N): 0.004 to 0.01, Rest iron (Fe) and trace elements (all in wt%) with
Ultimate Tensile Strength is 1227-1255 MPa and
Total elongation is 26.5-31.0 %.

15. The superior strength and high ductility steel as claimed in claim 14, wherein the Yield Strength is 969-978 MPa.
16. The superior strength and high ductility steel as claimed in claim 14, wherein Uniform elongation is 25.4-27.0 %.

17. The superior strength and high ductility steel as claimed in claim 14, wherein Tensile toughness is 32.6-38.9 GPa%.

18. The superior strength and high ductility steel as claimed in claim 14, wherein the steel comprises 25-30% austenite and rest martensite (by volume).