Field of Invention
The present invention relates to a hot rolled steel strip with toughness of at least 30 GPa%. This hot rolled steel strip can be employed widely as auto components requiring high strength along with ductility.
Background of Invention
In the mid-20th century, steels with UTS of about 300MPa were deemed sufficient. But, as the days went by, the vehicles became more and more advanced in the fields of performance, output, speed, etc. This meant that the vehicle was more prone to accidents and safety hazards. This, in turn, led to demands for steels with superior mechanical properties. Steels with higher and higher UTS could always be produced. But a new problem arised: as the strength of the steels kept on increasing, there was a significant drop in its total elongation, or formability. This inhibited the manufacturers from choosing such grades, as their commercial production was difficult. This gave rise to one of the most advanced class of steels ever designed: Advanced High Strength Steels (AHSS).
Also for the past several decades, the automobile industry has been focusing
on improving fuel efficiency and passenger safety and making great strides in reducing CO2 emission by adopting advanced high strength steels (AHSS) with both high strength and high elongation for manufacturing various components. The evolution of AHSS has taken place to cater to the ever growing needs for passenger safety and fuel efficiency demands from the automobile sector. AHSS is classified into three broad categories of first generation AHSS, second generation AHSS and third generation AHSS on the basis of strength levels. The variation of strength and ductility of different AHSS is shown in fig 1.

The first generation of AHSS consisted of Dual Phase steel, Transformation Induced Plasticity (TRIP) steel, complex phase steel, martensitic steel, etc. Their Ultimate Tensile Strength (UTS) varied from 500MPa to about 1200MPa. The common feature of the first generation AHSS is that all have a ferritic base in them. During the time of the introduction, these steels had higher strength than the pre-existing steels. But they have become obsolete now as they lacked good combination of strength and ductility. Because of the problems associated with first generation of AHSS, the second generation of AHSS were developed, which mainly comprised of High manganese Transformation induced plasticity steel (TRIP), Twinning induced plasticity steel(TWIP), etc. The third generation AHSS are mainly austenitic steels. Because of high alloying content in these steels (about 20% Mn by Wt.), there is a marked increase in the time and cost of production. Also, because of the high alloying content there are associated problems during the production like castability issues, scale formation. Hence need for development of a better steel arose, which could fulfil the stringent mechanical property requirements from automobile manufacturers. These steel fall in the genre of third generation AHSS, which are in between the first and second generation AHSS in terms of mechanical properties. The third generation of AHSS possess unique combination of strength and ductility; have better crash resistance along with good formability.
There have been recent developments of manufacturing aforementioned steel grade using Quench and partitioning (Q&P) route. The microstructure of Q&P steel consists of Retained Austenite (RA) and bainite/martensite. Presence of martensite attributes to high strength in excess of 1000 MPa in these steel along with good ductility due to the presence of RA. Q&P steels are developed by adopting a two-step heat treatment which involves quenching between Ms and Mf and partitioning above Ms to form a duplex microstructure. Q&P steels are used in manufacturing B-Piller reinforcement, door panels, side members, etc. Research has already been put into development of high strength and high plasticity steels, light weight ferritic steels, wear resistance steels through hot rolling as well as cold rolling routes (CN103103438A, KR20140030969A,

CN104694829A) with Mn content of 4-15 % by Wt.. Steels with tensile strength 800 MPa and elongation more than 20% have already been developed through combined hot rolling and cold rolling routes coupled with series of heat treatment steps. However, the alloying content in these steels is too high to be commercially viable, for example Mn content more than 10% by Wt. (KR20140030969A), Al content in excess of 8% by Wt. (KR20140030969A), Si in excess of 1.5% by Wt. (CN103103438A). Though there is a huge demand from automobile customers for these steels, longer heat treatment duration (CN103103438A) along with highly rich composition makes development of these steels a cumbersome process. Hence, there is a need for a technically viable and economically attractive way of developing these grades without aforementioned limitations.
Objects of the Invention
The main objective of the present invention is to propose a steel within the genre of third generation AHSS with toughness of minimum 30 GPa%.
Another objective of the invention is to propose a steel with toughness of minimum 30 GPa% which has maximum 8.25 % by Wt. alloying content and shorter processing time than aforementioned inventions.
Another objective of the current invention is to develop steel with toughness of minimum 30 GPa% which has maximum 8% by weight of Manganese, preferably less than 6%.
A still another object of the present invention is to propose a hot-rolled steel strip having a minimum toughness of 30 GPa%, which possesses a microstructure consisting of ferrite and second phase (Retained Austenite).
A further object of the present invention is to propose a hot-rolled steel strip having a minimum toughness of 30 GPa%, which is adaptable to automotive industry in particular for manufacturing components where high toughness is required.

SUMMARY OF THE INVENTION
The hot rolled steel strip having a minimum of toughness of 30 GPa%, according to the present invention contains in weight percent 0.15 - 0.25 % of carbon, 4 -8% of manganese - preferably 4 - 6 %, up to 1 % of silicon (excluding zero), up to 0.02 % of titanium, up to 1 % of aluminum (excluding zero), maximum up to 0.01 % of sulphur, maximum up to 0.03 % of phosphorous and up to 0.01 % of nitrogen, the balance being iron and impurities. The hot rolled steel strip according to the present invention has a microstructure comprising 20 to 30 % of second phase (Retained Austenite) and rest ferrite.
The method of manufacturing the hot rolled steel strip with a ferrite + second phase (Retained Austenite) microstructure with a minimum toughness of 30 GPa%, includes a new chemistry design, heat making at laboratory scale, casting and design of parameters which can be up scaled in plant by casting the slab in a horizontal belt caster and then rolling and followed by cooling on run out table (RoT) before coiling followed by a further batch annealing cycle.
BRIEF DESCRIPTION OF THE DRAWINGS:
Fig.1: Variation of strength and elongation for different advanced high strength steel
Fig. 2: Process flow as per the current invention
Fig. 3: Microstructure of steel 1 and steel 2
Table 1. Chemical composition of two alloys developed at laboratory scale. The weight of each heat is approximately 50 kilograms.


Table 2. Mechanical properties and annealing duration of two alloys developed at laboratory scale.

Detailed Description of Invention
The hot rolled steel strip having a minimum toughness of 30 GPa%, according to the present invention contains in weight percent 0.15 - 0.25 % of carbon, 4 – 8 % of manganese - preferably 4 - 6 %, up to 1 % of silicon (excluding zero), up to 0.02 % of titanium, up to 1 % of aluminum (excluding zero), maximum up to 0.01 % of sulphur, maximum up to 0.03 % of phosphorous and up to 0.01 % of nitrogen, the balance being iron and impurities. The hot-rolled steel strip with a minimum toughness of 30 GPa% according to the present invention has a microstructure comprising 70-80% of ferrite and 20-30% of retained austenite wherein the ferrite is in the form of laths and the retained austenite is in between the ferrite laths in the form of martensite/austenite (MA) constituents.
The present invention relates to a hot rolled dual phase steel strip which has a specific alloying composition and is manufactured with a precise control of the rolling and cooling parameters in order to produce the target microstructure, such that a minimum toughness of 30 GPa%, is achieved.

The chemical composition constituting the hot rolled steel strip produced according to the present invention are described below.
1. Alloying additions: The addition of each alloying element and the limitations imposed on each element are essential for achieving the target microstructure and properties.
C: 0.15 - 0.25%: Carbon is one of the most effective and economical strengthening elements. Carbon readily partitions during the intermediate heat treatment in austenite thereby stabilizing it. Carbon is the component which reduces the Ms temperature to the maximum extent. However, in order to avoid weldability issues and excess stabilization of austenite, the carbon content has to be restricted to less than 0.25%.
Mn: 4.0 - 8.0%: Manganese not only imparts solid solution strengthening to the ferrite but it also lowers the austenite to ferrite transformation temperature thereby refining the ferrite grain size. Mn helps in lowering the stacking fault energy of austenite. Mn beyond 6% by Wt. will reduce the austenite to ferrite transformation kinetics and make it sluggish thus affecting the processing time and hence, Mn is preferred to be less than 6%.
Si: up to 1 % (excluding zero): Silicon like Mn is a very efficient solid solution strengthening element. Si restricts the formation of carbides thereby enriching the austenite with carbon. Si helps in enhancing the ferrite to austenite transformation kinetics which otherwise gets sluggish due to addition of manganese. However, additions of Si should be restricted to less than 1 % in order to prevent excessive formation of surface scales.
P: 0.03% maximum: Phosphorus content should be restricted to 0.03% maximum as higher phosphorus levels can lead to reduction in toughness and weldability due to segregation of P into grain boundaries.

S: 0.01% maximum: The Sulphur content has to be limited otherwise it results in a very high inclusion level that deteriorates formability.
N: 0.01% maximum: Reducing nitrogen levels positively affects ageing stability and toughness in the heat-affected zone of the weld seam, as well as resistance to inter-crystalline stress-corrosion cracking.
Al: up to 1 % (excluding zero): Aluminium is used as a deoxidizer and killing of steel. It limits growth of austenite grains. Al along with Si helps in enhancing the ferrite transformation kinetics. Al content should be restricted to 0.5% maximum as higher aluminium levels can lead to issues during coating.
Ti: 0.05% maximum: Titanium improves strength by limiting austenite grain size. Titanium forms carbides and nitrides which are stable at high temperature.
2. Microstructure: In order to achieve the required mechanical properties, the concept of TWIP/TRIP steel is used in conjunction with possible strengthening mechanisms. As stated above the strengthening contribution from solid solution elements is restricted. In view of the above, the only way by which the target strength can be achieved is by modifying the microstructure and hence a microstructure consisting of second phase (Retained austeniite) and ferrite as matrix was targeted in the present invention. The required amount of strength comes from the solid solution strengthened ferrite due to additions of Si and Mn along with grain refinement and the ductility increases due to the presence of retained austenite which transforms to α’-martensite by TRIP or to ε-martensite by TWIP.
According to the invention, a new processing route is described to develop hot rolled ferrite and austenite dual phase microstructure in a steel whose composition is shown in Table 1.
The method of developing the hot-rolled steel strip according to the present invention consists of a casting step followed by a hot rolling step, a controlled cooling step and an inter-critical annealing step using a steel material which satisfies the composition as shown in Table 1. The various processing steps are described in their respective order below:

A hot rolled strip of the composition as shown in Table 1 was used for the carrying out final heat treatments which will ensure the required properties being achieved. The experimental schedules were designed keeping in mind the final objective to develop a microstructure with 20 -30 % second phase (retained austenite) along with 70-80 % ferrite in order to develop a steel which has high toughness (greater than 30 GPa%). All the processing parameters were tuned keeping the target to fulfill such objective.
The process flow involved in the present invention comprises of steps 100 – 105. As per step 100 the steel was produced through induction mention and casting. As mentioned in step 101 the cast ingot was homogenized at 1200 °C for 4 hours to ensure minimization of manganese segregation, subsequently in step 102 the ingot was hot forged to break the cast dendritic structure and then air cooled to room temperature. As in step 103 the forged steel ingot was then again homogenized at 1200 °C for 2 hours prior to rolling. In step 104 the ingot was finish rolled at a temperature of T1 where (A3 + 50 °C) < T1 < (A3 + 70 °C). A3 being the temperature at which the austenite just starts to transform into ferrite. After the finish rolling, maximum 10-15 °C drop in the temperature is allowed before quenching to T2, where 25 °C < T2 < 28 °C. The next step 105, is annealing the hot rolled strips at a temperature T3 such that (A3 – 50 °C) > T3 > (A3 - 150 °C) for 3 – 4 hours after which there will be another cooling up to room temperature. The process flow involved in the present invention is illustrated in Fig. 3.
The first quenching step to T2 after rolling is to ensure a completely martensitic microstructure. Being martensite, the structure has high defect density which ensures a faster partitioning of substitutional alloying elements while annealing in the inter-critcal region. The annealing of the rolled strip at temperature T3 for 3 – 4 hours ensures enrichment of austenite with carbon and manganese, so to get it stabilized. The duration of annealing is the standard time a coil spends in a batch annealing furnace.

Examples
Two heats as per the composition of present invention were cast in the laboratory. Both heats were hot forged and hot-rolled according to the present invention. However the intercritical annealing for both samples were different. For Steel 2, the intercritical annealing was done in accordance with the present invention, whereas for Steel 1 a lower annealing temperature and a lesser duration of holding was used. The duration of annealing for both steels as well as their mechanical properties is listed in Table 2. The microstructures of the two steels are shown in Fig. 2. It is clear from the mechanical properties and the microstructures achieved, that the target properties cannot be achieved when the annealing duration and temperature do not conform to the processing parameters of the invention.

The invention as per the current invention provides a method of manufacturing a high toughness hot-rolled steel strip with a minimum toughness of at least 30 GPa%. The manufactured steel strip comprises of 70-80 % ferrite and 20-30% retained austenite and can be used to manufacture automotive components such as a front bumper beam, wheel house, door impact beam etc., which will weigh less and will have higher crash resistance than the components made from existing grades of steel.

We Claim:
1. A process of manufacturing hot-rolled steel strip, the process
comprising:
Casting of an ingot comprising in weight percent 0.15 -0.25 % of carbon, 4 - 8 % of manganese, up to 1 % of silicon (excluding zero), up to 0.02 % of titanium, up to 1 % of aluminum (excluding zero), maximum up to 0.01 % of sulphur, maximum up to 0.03 % of phosphorous and up to 0.01 % of nitrogen, the balance being iron and impurities.;
Reheating the ingot upto 1200 °C for homogenization;
hot rolling the steel ingot to produce a steel strip such that finish rolling is done at a temperature (T1), wherein T1 varies in the range A3 + 50 (°C) to A3 + 70 (°C), where A3 is the temperature at which the transformation of austenite to ferrite starts at equilibrium;
Quenching the rolled strip to obtain a martensitic microstructure; and
Annealing the rolled strip at T3 where (A3 – 50 °C) > T3 > (A3 - 150 °C) for 3 - 4 hours and cooling to room temperature.
2. The process as claimed in claim 1, wherein microstructure of the high-toughness hot-rolled steel strip comprises of 70-80% ferrite and 20-30% retained austenite.
3. The process as claimed in claim 1 and claim 2, wherein the austenite is stabilized by carbon and manganese enrichment.

4. The process as claimed in claim 3, wherein the austenite has a stacking fault energy of 20 - 30 mJ / m3.
5. The process as claimed in claim 1, wherein Mn is preferably present in the range of 4.0 - 6.0 wt. %.
6. The process as claimed in claim 1, where in the intercritical annealing time is 3 - 4 hours.
7. A high-toughness hot-rolled steel strip with a toughness > 30 GPa% comprising, on weight percent basis 0.15 - 0.25 % of carbon, 4 - 8 % of manganese, up to 1 % of silicon (excluding zero), up to 0.02 % of titanium, up to 1 % of aluminum (excluding zero), maximum up to 0.01 % of sulphur, maximum up to 0.03 % of phosphorous and up to 0.01- % of nitrogen, the balance being iron and impurities; and a microstructure consisting of 70 - 80 % ferrite and 20 - 30 % austenite.

8. The high-toughness hot-rolled steel strip as claimed in claim 7, wherein high-toughness hot-rolled steel strip has UTS of at least 800 MPa.
9. The high-toughness hot-rolled steel strip as claimed in claim 7, wherein high-toughness hot-rolled steel strip has total elongation of at least 35%.
10. The high-toughness hot-rolled steel strip as claimed in claim 7, wherein manganese content preferably varies in the range of 4 - 6 weight %.