DESC:FIELD OF INVENTION
The present invention relates to low Carbon Steel sheet having improved ageing resistance, and a method for manufacturing the same through continuous annealing route. More particularly, the present invention is directed to provide low carbon steel sheets having composition of C weight percentage containing about 0.02% to 0.05%,Mn weight percentage containing about 0.08% to 0.2%,V weight percentage containing about 0.01 to 0.03, Cr weight percentage containing about 0.02 to 0.04, Al weight percentage containing about 0.025 to 0.05, Nitrogen about 0.005 weight percent or less, wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel slabs and precipitation size of cementite in ferrite grain boundary is greater than 1µmand is suitable for automotive formable parts with ageing guarantee for 9 months at room temperature processed through continuous annealing. In order to achieve good ageing property, cold rolled steel is continuously annealed with soaking temperature of 820°C or more which provides complete recrystallization and forms uniform grains. To fix the free carbon and nitrogen desired amount of chromium, Vanadium is added and overaged at 380°C or more with residence time of 130Sec to 400Sec. To suppress the yield point elongation material is skin-passed with 1.2% elongation or more. The above parameters are set to achieve desired properties of yield strength 240Mpa or less, UTS 360Mpa or less with ageing guarantee of 9 months or more at room temperature.
BACKGROUND OF THE INVENTION
In press forming of automobile body components from sheet metal, yield point elongation is an important factor to be considered. It is known that the deformation occurring throughout the yield-point elongation is heterogeneous. At upper yield point a discrete band of deformed metal at a stress concentration such as a fillet, and coincident with the formation of the band the load drops to the lower yield point. The band then propagates along the length of the specimen, causing the yield-point elongation. This can be associated with small amounts of Interstitial or Substitutional impurities of which carbon and nitrogen are more effective. Suppression of yield point elongation is thus required to have components free of stretcher strain marks.

Strain aging phenomenon is known to be reappearance of yield point in which strength of metal increases and ductility decreases and a low value of strain rate sensitivity on heating at a relatively low temperature after cold-working. This is due to the diffusion of C and N atoms to the dislocations during the aging to form new solute atmospheres anchoring the dislocations. Nitrogen plays a more important role in the strain-aging of iron than carbon because it has a higher solubility and diffusion coefficient and produces less complete precipitation during slow cooling.

It is thus important to eliminate strain aging in deep drawing steels because the reappearance of yield point can lead to difficulties with surface markings or stretcher strains due to localized heterogeneous deformation.

US4374682 disclosed a process for producing a cold rolled steel strip having excellent workability by a short-time continuous annealing, which process is constituted by hot rolling a low carbon steel slab of steel having a carbon content of from about 0.003 to 0.08% into a steel strip; cold rolling the hot rolled steel strip; and subjecting the steel strip to said short time annealing and then cooled steel strip to an over-ageing treatment, with selected parameters. Cold rolled steel strip is widely used for cold forming articles such as press-formed automobile parts, and as such the strip is required to have an excellent press-forming property. Thus this prior art targets a shorter annealing time to be more effective in preventing dissolving of carbides formed in the hot rolled steel strip, thus shortening the subsequent over-ageing treatment. Low carbon having C: 0.003 to 0.04% is manufactured without alloying elements wherein little solute carbon will be left to cause yield point elongation after ageing.
EP0857794A1 discloses process to obtain the excellent deep drawability and excellent anti aging properties by eliminating over ageing section and with 0.015% -0.15%of Carbon, 1% or less silicon, 0.1% to 1.0% of Manganese, with combination of Ti and Nb 0.001% to 0.03% and by adding Boron less than 50ppm, Chromium in range of 0.05 to 1.00% with annealing at 800°C and temper rolling with 0.8% to achieving ageing index less than 40Mpa.
Ageing index with 40Mpa or less will cause the problem at the time of final component making using press forming , high chance of material to crack if yield strength is 210,tested after production , after period of time yield strength will be 250 Mpa which will cause a problem during press forming and also out of the desirable property.
Problem to be solved by the Invention
The Present invention aims to solve the problem of the prior arts by providing the good ageing resistance for 9 months and also change in yield strength after 9 months within 20Mpa with alloying of vanadium and Chromium with temper rolling of 1.2% or more with process consisting of over ageing section where the residence time of the steel strip is around 130Sec to 400Sec to suppress the yield point elongation ensuring ageing guarantee for 9 months at 30°C. The Ratio of (Al+V)/N to be minimum 14 to ensure the ageing guarantee and also providing difference in yield strength after processed condition and after 6 hrs of heated at 100 °C and tested condition is around 10 to 20 Mpa.
Batch annealing has been so far an ordinary procedure for annealing a cold rolled steel sheet for working which has very long production cycle time, but recently the cold rolled steel sheets are often produced by continuous annealing owing to remarkable quality and very less production cycle.
But the continuous annealing has such a serious disadvantage that no satisfactory ageing resistance can be given to Al-killed steel, and thus the remarkable quality and less production cycle time of the continuous anneal¬ing have not been fully obtained in the case of the Al-¬killed steel up to now because of less AlN precipitation which is required to fix N.

OBJECTS OF THE INVENTION

The basic object of the present invention is thus directed to provide low Carbon Steel sheet having improved ageing resistance, and a method for manufacturing the same through continuous annealing route.

A further object of the present invention is directed to provide low carbon steel sheet having selective composition along with processing through hot rolling, cold rolling, continuous annealing followed by temper rolling with selective parameters at each stage to ensure desired strength properties and substantially suppress/eliminate yield point elongation of resulting sheet metal.

A still further object of the present invention is directed to provide low carbon steel sheet with aging resistance wherein to achieve good ageing property, cold rolled steel is continuously annealed with selected soaking temperature to ensure complete recrystallization and form uniform grains.

Yet another object of the present invention is directed to provide low carbon steel sheet with aging resistance and a method of producing the same wherein to fix the free carbon and nitrogen desired amount of chromium, Vanadium is added and overaged at 380°C or more with residence time of 130 to 400Sec.

A still further object of the present invention is directed to provide low carbon steel sheet with aging resistance wherein to ensure desired properties of steel sheet, a composition is selected to maintain the ratio of (Al+V)/N in between 14 to 30.
A still further object of the present invention is directed to provide low carbon steel sheet with aging resistance wherein to ensure desired properties of steel sheet, a composition is selected to maintain precipitation size of cementite in ferrite grain boundary is greater than 1µm
A still further object of present invention is directed to provide low carbon steel sheet with aging resistance where in to ensure the surface quality, hot rolled is done with slab reheating temperature of below 1220°C to attain roughing mill delivery temperature below 1060°C, to prevent the surface defects like rolled in scale.
A still further object of the present invention is directed to provide low carbon steel sheet with ageing resistance where in to ensure the surface quality having reduction 33% or less at 1st and 2nd stand, 27% or less at 3rd stand and 19 to 27 % at 4th stand with emulsion concentration 3.6 % or more to avoid snaky lines generated because excessive heat.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to ageing resistant continuous annealed low carbon steel composition comprising:-
(In wt %) (In wt %)
C: 0.02-0.05 Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.005 or less more preferably 0.004 or less
V: 0.01-0.03 Cr: 0.02-0.04

wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel slabs, have precipitation size of cementite in ferrite grain boundary is greater than 1µmand with having ageing guarantee for atleast 9 months at room temperature.
A further aspect of the present invention is directed to ageing resistant continuous annealed low carbon steel composition for said steel slabs having yield strength 240Mpa or less, UTS 360Mpa or less.
A still further aspect of the present invention is directed to ageing resistant low carbon steel having ageing guarantee for atleast 9 months at room temperature which is obtained involving
(a) ageing resistant continuous annealed low carbon steel composition comprising:-
(In wt %) (In wt %)
C: 0.02-0.05 Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.005 or less more preferably 0.004 or less
V: 0.01-0.03 Cr: 0.02-0.04

wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel slabs, have precipitation size of cementite in ferrite grain boundary is greater than 1µmand with having ageing guarantee for atleast 9 months at room temperature; and
(b) steel manufacturing including subjecting the cold rolled steel to step of continuous annealing including (i) soaking temperature in the range of 820 to 850 preferably 8200C or more , (ii) fixing of free carbon and nitrogen involving said chromium and vanadium and overaging at 380 to 410 preferably 3800C or more with residence time of 139Sec to 400Sec and (iii) suppression of yield point elongation by skin passing with 1.2 to 1.7 % preferably 1.2% or more .
A still further aspect of the present invention is directed to ageing resistant low carbon steel having yield strength 240Mpa or less, UTS 360Mpa or less.
Another aspect of the present invention is directed to a process for manufacture of ageing resistant low carbon steel having ageing guarantee for atleast 9 months at room temperature comprising:
(a) providing selective ageing resistant continuous anneal able low carbon steel composition comprising:-
(In wt %) (In wt %)
C: 0.02-0.05 Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.005 or less more preferably 0.004 or less
V: 0.01-0.03 Cr: 0.02-0.04

wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel slabs, have precipitation size of cementite in ferrite grain boundary is greater than 1µmand with having ageing guarantee for atleast 9 months at room temperature and carrying out
(b) steel manufacturing including subjecting the cold rolled steel to step of controlled continuous annealing including (i) soaking temperature in the range of 8200C to 8500C preferably 8200C or more, (ii) fixing of free carbon and nitrogen involving said chromium and vanadium and overaging at 3800C to 4100C preferably 3800C or more with residence time of 139Sec to 400Sec and (iii) suppression of yield point elongation by skin passing with 1.2 to 1.7 % preferably 1.2% or more .
Yet another aspect of the present invention is directed to a process for manufacture of ageing resistant low carbon steel comprising said steel manufacture including steel slab manufacture involving said selective ageing resistant continuous anneal able low carbon steel composition in hot strip mill followed by pickling to obtain said cold rolled steel being subjected to said controlled continuous annealing.
A further aspect of the present invention is directed to said process for manufacture of ageing resistant low carbon steel comprising manufacture in hot strip mill to produce slab including hot rolling with HSM Finishing Temperature 8900C to 925°C and HSM Coiling temperature 6500C to 680°C to precipitate AlN and then cold rolling with 72% or more reduction.
A still further aspect of the present invention is directed to said process for manufacture of ageing resistant low carbon steel comprising
hot rolling with slab reheating temperature of below 1220°C to attain roughing mill delivery temperature below 1060°C, to prevent the surface defects, finishing temperature of 890°C to 925°C and then coiling at 650°C to 680°C and processing through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduction of 72% or more .
A still further aspect of the present invention is directed to said process for manufacture of ageing resistant low carbon steel wherein said step of cold reduction comprise five stands reduction distributed with first stand having less than or equal to 33% , second strand with 34%, third strand with 27.85% and Fourth strand with 19.61% to desired final thickness followed by fifth strand for roughness for reduced heat generation and cooling free of heat band with the combination of emulsion concentration more than 3.6.
A still further aspect of the present invention is directed to said process for manufacture of ageing resistant low carbon steel comprising
after pickling and cold rolled steel strip is processed through said continuous annealing line, where electrolytic cleaning removes emulsion present on the surface, cleaned surface passes through the preheating section where the strip is heated and then passes through soaking section where it is heated above 820°C or more to achieve complete recrystallization tend make the steel softer to get desired properties of UTS 360Mpa or less;
steel strip soaked at 820°C or more, passed through slow cooling section with 690°C and above to get the yield strength less than 240Mpa or less, rapid cooling to 480°C and overaged at 380°C or more to reduce the solute carbon. Followed by processing through skin-pass mill with 1.2% or more elongation to eliminate the yield point for desired yield strength increasing.
According to yet another aspect of the present advancement there is provided for a process for manufacture of ageing resistant low carbon steel as above wherein the steel composition comprises element selected from the group consisting of Cu, Sn, Ni, Ca, Mg, Co, As, Mo, Sb, W, Pb, Ta, Ti, REM, Zr, B and Hf of total content 0.5% or less.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings and examples.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: shows graphically the effect of Vanadium content in steel composition on Yield Strength.
Figure 2: shows graphically the effect of Skin pass mill (SPM) Elongation on Yield Strength.
Figure 3: shows graphically the effect of (V+Al)/N on Yield Point Elongation after 6hrs at 100°C.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO DRAWINGS AND PREFERRED EMBODIMENT
The present invention relates to low carbon steel sheet having good ageing resistance processed through continuous annealing. More particularly, the steel composition comprises low Carbon 0.02-0.05 weight %, Manganese 0.08- 0.2% ,Cr -0.02 to 0.03%, Al -0.025 to 0.05%, N- 0 to 0.005%, Vanadium 0.01-0.03,wherein the ratio (Al+V)/N should be in between 14 to 30. Slab produced with above chemistry is hot rolled with HSM Finishing Temperature 890 to 925°C and HSM Coiling temperature 650 to 680°C to precipitate AlN and then cold rolled with 72% or more reduction. To get good ageing property cold rolled steel is continuously annealed with soaking temperature of 820°C or more with residence time of 45 to 180 Sec which provides complete recrystallization and forms uniform grains. To fix the free carbon and nitrogen desired amount of chromium, Vanadium and overaged at 380°C or more with residence time of 130 to 400Sec. To suppress the yield point elongation material is skin-passed with 1.2% or more elongation. The above parameters are set to achieve desired properties of yield strength 240Mpa or less, UTS 360Mpa or less with ageing guarantee of 9 months or more at 30°C.

Ageing Test is testing that uses aggravated conditions of heat, oxygen, sunlight, vibration, etc. to speed up the normal aging processes of material to determine the shelf life. Processed steel strip is made to dumbbell shape and heated using an oil bath at 100°C for 3 hours and tested again. While testing, if yield point elongation is observed to be 0.3 or less, that is equivalent to 4 month shelf life of the product at 30°C as per Hundy’s equation. A limit of 0.2% yield point elongation for no stretcher strain has been considered as mentioned in patent no 1055-KOL-2007.For calculating of 9 months aging 6 hours heating is considered.

Following abbreviations have been used in the description of the invention:
SS- Soaking Section
SCS – Slow Cooling Section
RCS-Rapid Cooling Section
OAS Over-ageing section
YPE-Yield Point Elongation
HSM-Hot Strip Mill
UTS-Ultimate Tensile Strength
YS-Yield Strength
El – Elongation
SPM-Skin Pass Mill Elongation

Thus the present invention is directed to provide a steel grade comprising carbon 0.02-0.05 wt%, manganese 0.08- 0.2 wt%, nitrogen up to 0.005 wt%, Chromium 0.02-0.04 wt%, aluminum 0.025-0.05wt%, and V- 0.01-0.03 and wherein the ratio (Al+V)/N should be in between 14 to 30 with the balance being Fe and other unavoidable impurities. The chromium and Vanadium is added as carbide and nitride forming agent, which reduces the solute carbon and nitrogen and improves the ageing property. Following are the justification for keeping the claimed compositions.

CARBON ( 0.02-0.05 wt.%) – While carbon increases the steel strength, it reduces the cold workability, r value and the deep drawability of the cold rolled steel sheet remarkably and thus the higher limit of the cold rolled sheet is set at 0.1 wt%. For improving the r-value i.e. drawability it desirable to reduce the C level less than 0.05 wt%. Lowering the carbon content below 0.02 wt% results in poor ageing property since below 0.02 wt% steel is in complete a-ferrite region in Iron-Cementite phase diagram resulting no cementite formation. Consequently more free carbon will be available in steel matrix which deteriorates the ageing property. Cementite should thus desirably be formed in grain boundaries and its precipitate size should be greater than 1µm which helps in reducing solute carbon and suppress the YPE. Keeping low carbon <0.02 % also requires vacuum degassing of the molten steel which add more cost of production. To avoid YPE and to have yield strength within the desired limit, the level of Carbon is maintained between 0.02-0.05 wt percent.
MANGANESE (0.08-0.2) wt% - The lower manganese is better for providing a softer material, but the lower limit will affect surface property and hot brittleness. More than 0.2%Mn makes the yield strength more than 240Mpa or more and drawability of the steel becomes low.

ALUMINIUM (0.025-0.05) wt% -If sol. Al were less than 0.020%, AlN would be delayed in precipitation, and growth of ferrite grain would be unsatisfactory. If precipitation occurred, ferrite grain size becomes fine. On the other hand, greater than 0.05% sol. Al invites surface related problems and more inclusions.

NITROGEN (less than 0.005 wt %) – The lower nitrogen is better, higher N lead to free N which causes poor ageing resistance so maximum limit of N is 0.005 and more preferably 0.004 %.

CHROMIUM (0.02-0.04 wt %) - Chromium is strong carbide former so it helps in scavenging off Carbon and stabilizes it by forming chromium carbide (Cr23C6). Addition of Cr more than 0.04 adds more costs and makes the steel harder, Cr less than 0.02 leaves the solute carbon which affects the ageing property of steel, 0.02% to 0.04% of chromium suppress YPE by fixing solute C completely, so chromium is restricted to 0.02 to 0.04%,

VANADIUM (0.010 to 0.03%): Vanadium is strong carbide former, reduces solute carbon in the mild steel and forms Vanadium Carbide, as it forms coarser carbide particles it restricts the yield strength within 240Mpa and promotes the ageing resistance. As shown in Figure 1 to achieve stretcher strain free material minimum vanadium to be added is 0.01 and to restrict yield strength maximum vanadium is to be 0.03. To achieve the desired properties vanadium is restricted to 0.01 to 0.03.
The effect of Al and V have been explained before, but it is desirable that the ratio of (Al+V) and N shall be 14 to 30 to fix the N completely and get the desired ageing guarantee of min 9 Months as well as yield strength less than 240 Mpa As the total amount of Al+V increases i.e more than 30 the ferrite grains become finer because of increase in free Al and or V which increases the yield strength of material as shown in Fig 3. On the other hand if the ratio of (Al+V) and N ratio is less than 14 then it results free N which deteriorate in the ageing property.
Method of Manufacturing
Slabs are casted with the chemistry said above and hot rolled with slab reheating temperature of below 1220°C to attain roughing mill delivery temperature below 1060°C, to prevent the surface defects like rolled in scale, finishing temperature of 890°C to 925°C and then coiled at 650°C to 680°C and processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduction of 72% or more with the five stands reduction percentage is distributed with first stand having less than or equal to 33% and second strand with 34% and third strand with 27.85% and Fourth strand with 19.61% required final thickness, fifth strand for roughness, if this pattern is followed heat generated will be less and the cooling will also take place eliminating generation of heat band, in combination with emulsion concentration more than 3.6.
After pickled and cold rolled steel strip is processed through continuous annealing line, where electrolytic cleaning removes emulsion present on the surface. Cleaned surface passes through the preheating section where the strip is heated and then passes through soaking section where it is heated above 820°C or more where complete recrystallization takes and makes the steel softer to get desired properties of UTS 360Mpa or less, where the soaking temperature increases from 800 to 850°C yield strength decreases, to achieve yield strength less than 240Mpa soaking temperature is set to 820°C or more. Microstructure of various soaking temperatures from 720°C to 840°C, at 720°C it shows the incomplete recrystallization where yield strength is very high grain size observed is 11.5 and at 840°C it shows complete recrystallization and the yield strength observed is within the desired limit of 240MPa max, grain size observed is 9.5. Steel strip soaked at 820°C or more, sheet passes through slow cooling section with 690°C and above to get the yield Strength less than 240Mpa or less, as slow cooling temperature increases yield strength decreases and rapidly cooled to 480°C and overaged at 380°C or more to reduce the solute carbon. Increase in overaging temperature decreases the yield strength, and then processed through skin-pass mill with 1.2% or more elongation to eliminate the yield point elongation; skin pass elongation of 0.8% to 1.2% increases yield strength decreases from 226Mpa to 210Mpa, when skin passed at 1.2% or more, dislocation starts and yield strength starts increasing; Figure 2 show at SPM elongation of 1.2% or more, yield point elongation is suppressed, yield strength obtained at 1.2% is 210Mpa. To achieve yield strength of 240Mpa or less and with yield point elongation of 0, skin pass elongation to be maintained 1.2% or more. Additionally Cold rolled sheet processed can be coated using galvanizing process and used as Galvannealed and Galvanized steel for similar applications.

The low carbon steel sheet with improved aging resistance according to the present invention are illustrated with help of following examples and the experimental data as presented in tables I-III.
Table I- Compositions of the invented steel sheets along with some comparative examples.
Table II- Hot rolling, cold rolling, annealing parameters.
Table III – Mechanical properties and Ageing properties of respective steels.
Example 1- from table I, II and III it is shown sample number 1 and 5 with carbon wt of 0.05% and 0.022% with cold rolling reduction of 74% and 75% annealed respectively at 820°C and 830°C shows higher UTS of 360 Mpa with Carbon of 0.05%, lower the carbon of 0.022% shows the lower UTS of 332 Mpa. Hence percentage of carbon is restricted to 0.02% to 0.05%. Sample No. 23, having C wt % 0.017 suffer from ageing problem. So, C wt% is restricted to more than 0.02 wt %.
Example 2- Sample number 3 with same chemical composition and having same hot rolling parameters and cold rolling reduction with different annealing temperature of 800°C and 820°C, steel sample annealed with 800 shows having UTS of 362 and sample annealed with 820°C shows UTS of 334 within the desired range. Sample annealed at 800°C shows YPE 0.80 and at 820°C it shows 0.09 ensures ageing guarantees of 9 months, to achieve the desired properties annealing temperature is set to 820°C or more.
Example 3- Sample number 8 having same chemical composition with different coiling temperature of 638°C annealed at same temperature of 820°C and overaged at 380°C and 360°C respectively, sample overaged at 380°C show good ageing guarantee of 9 months, but sample overaged at 360°C has YPE of 0.88 so overaging of steel is set to 380°C or more.

Example 4-Sample number 1 having slow cooling section temperature of 670°C with Yield strength of 238 Mpa and UTS of 360Mpa, where sample number 2 and 3 processed at 690°C and 699°C respectively has yield strength of 192Mpa and 212Mpa.
Example 5- Sample number 4 having chromium with 0.02 with soaking temperature of 830°C have r90 of 1.5 where else sample number 7 having chromium of 0.045 with 830°C have reduced r90 of 1.3 less than desired property , chromium is restricted to 0.04 or less.
Example 6- Sample 15 with vanadium 0.016 wt % and sample no 17 with vanadium 0.03 wt%, steel sheet processed with Vanadium which is also qualifying the ageing test and yield strength less than 240 Mpa. The element V is sufficient to fix the ageing problem in low carbon steel processed through continuous annealing.

Example 7- Sample number 17 having vanadium 0.03 with soaking temperature of 850°C shows yield strength of 195Mpa and YPE after 9 months 0.06, so to achieve yield strength of 240 Mpa or less with aging guarantee of 9 months Vanadium preferably to be 0.03 or less.

Example 8- Sample number 12 having only chromium of 0.01 and without addition of vanadium , aging after 9 months Yield point elongation observed is 3.5; in inventive steels sample no 15 and where vanadium and chromium is added YPE observed is very negligible and passes aging test of 9 months.
Table I-

I- Inventive
C-Comparative
Table

Table II-

Table III-
S. No. YS YPE UTS EL R90 YPE after heating 6 hours at 100°C (Equivalent to 9 Months) Cementite in ferrite grain (Precipitate Size)µm Remarks
1 238 360 42 1.3 0.10 2 I
2 192 319 47.2 1.7 0.13 1.5 I
3 212 334 46 1.55 0.09 1.7 I
3 245 362 42 1.28 0.80 C
4 224 344 43.3 1.5 0.08 1.8 I
5 208 332 43.3 1.5 0.08 1.7 I
6 239 0.6 352 42.1 1.35 1.20 C
7 247 330 44.4 1.3 0.11 C
8 235 350 44.3 1.7 0.07 2.2 I
8 250 0.8 350 44.3 1.7 0.88 C
9 270 0.7 346 43 1.5 1.2 C
10 252 0.9 374 39 1.3 1.5 C
11 254 0 356 43.2 1.4 1.4 C
12 290 0 308 40 1.4 3.5 C
13 212 351 35.9 1.5 0.14 1.8 I
14 238 357 29.5 1.38 0.123 2 I
15 205 338 39 1.55 0.09 1.7 I
16 200 335 42 1.56 0.04 1.8 I
17 196 332 40.5 1.7 0.06 2.2 I
18 213 324 41.2 1.52 0.12 2.3 I

I- Inventive
C-Comparative

It is thus possible by way of the present invention to provide low carbon steel sheets having good ageing resistance processed through continuous annealing, having selective composition wherein the ratio (Al+V)/N should be in between 14 to 30. Slab having such composition is hot rolled with HSM Finishing Temperature 890 to 925°C and HSM Coiling temperature 650 to 680°C to precipitate AlN and then cold rolled with 72% or more reduction. To get good ageing property cold rolled steel is continuously annealed with soaking temperature of 820°C or more which provides complete recrystallization and forms uniform grains. To fix the free carbon and nitrogen desired amount of chromium, vanadium is added and overaged at 380°C or more. To suppress the yield point elongation material is skin-passed with 1.2% or more elongation. The above parameters are set to achieve desired properties of yield strength 240Mpa or less, UTS 360Mpa or less with ageing guarantee of 9 months or more at 30°C, to ensure advantageous application of such sheets in automobile body components.

,CLAIMS:We Claim:
1. Ageing resistant continuous annealed low carbon steel having YPE less than 0.15% after accelerated aging and composition comprising:-
(In wt %) (In wt %)
C: 0.02-0.05 Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.005 or less more preferably 0.004 or less
V: 0.01-0.03 Cr: 0.02-0.04
wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel having precipitation size of cementite in ferrite grain boundary is greater than 1µm and with having ageing guarantee for atleast 9 months at room temperature.
2. Ageing resistant continuous annealed low carbon steel composition as claimed in claim 1 for said steel slabs having yield strength 240Mpa or less, UTS 360Mpa or less .
3. Ageing resistant low carbon steel having ageing guarantee for atleast 9 months at room temperature which is obtained involving (a) ageing resistant continuous annealed low carbon steel composition comprising:-
(In wt %) (In wt %)
C: 0.02-0.05 Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.005 or less more preferably 0.004 or less
V: 0.01-0.03 Cr: 0.02-0.04
wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel having precipitation size of cementite in ferrite grain boundary is greater than 1µmand with having ageing guarantee for atleast 9 months at room temperature and
(b) steel manufacturing including subjecting the cold rolled steel to step of continuous annealing including (i) soaking temperature in the range of 8200C to 8500C preferably 8200C or more, (ii) fixing of free carbon and nitrogen involving said chromium and vanadium and overaging at 3800C to 4100C preferably 3800C or more with residence time of 139Sec to 400Sec and (iii) suppression of yield point elongation by skin passing with 1.2 to1.7 % preferably 1.2% or more.
4. A process for manufacture of ageing resistant low carbon steel having ageing guarantee for atleast 9 months at room temperature comprising:
(a) providing selective ageing resistant continuous anneal able low carbon steel composition comprising:-
(In wt %) (In wt %)
C: 0.02-0.05 Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.005 or less more preferably 0.004 or less
V: 0.01-0.03 Cr: 0.02-0.04

wherein the ratio (Al+V)/N is between 14 to 30 and the balance being Fe and other unavoidable impurities suitable for steel having precipitation size of cementite in ferrite grain boundary is greater than 1µm and with having ageing guarantee for atleast 9 months at room temperature; and carrying out
(b) steel manufacturing including subjecting the cold rolled steel to step of controlled continuous annealing including (i) soaking temperature in the range of 8200C to 8500C preferably 8200C or more, (ii) fixing of free carbon and nitrogen involving said chromium and vanadium and overaging at 3800C to 410 preferably 3800C or more with residence time of 139Sec to 400Sec and (iii) suppression of yield point elongation by skin passing with 1.2 to1.7 % preferably 1.2% or more.
5. A process for manufacture of ageing resistant low carbon steel as claimed in anyone of claims 4 comprising
hot rolling with slab reheating temperature of below 1220°C to attain roughing mill delivery temperature below 1060°C, to prevent the surface defects, finishing temperature of 890°C to 925°C and then coiling at 650°C to 680°C and processing through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduction of 72% or more.
6. A process for manufacture of ageing resistant low carbon steel as claimed in claim 5 wherein said step of cold reduction comprise five stands reduction distributed with first stand having less than or equal to 33%, second strand with 34%, third strand with 27.85% and Fourth strand with 19.61% to desired final thickness followed by fifth strand for roughness for reduced heat generation and cooling free of heat band with the combination of emulsion concentration more than 3.6.
7. A process for manufacture of ageing resistant low carbon steel as claimed in anyone of claims 4 to 6 comprising
after pickling and cold rolled steel strip is processed through said continuous annealing line, where electrolytic cleaning removes emulsion present on the surface, cleaned surface passes through the preheating section where the strip is heated and then passes through soaking section where it is heated above 820°C or more to achieve complete recrystallization tend make the steel softer to get desired properties of UTS 360Mpa or less;
steel strip soaked at 820°C or more, passed through slow cooling section with 690°C and above to get the yield strength less than 240Mpa or less, rapid cooling to 480°C and overaged at 380°C or more to reduce the solute carbon. Followed by processing through skin-pass mill with 1.2% or more elongation to eliminate the yield point for desired yield strength increasing.

8. A process for manufacture of ageing resistant low carbon steel as claimed in anyone of claims 4 to 7 wherein the steel composition comprises element selected from the group consisting of Cu, Sn, Ni, Ca, Mg, Co, As, Mo, Sb, W, Pb, Ta, Ti, REM, Zr, B and Hf of total content 0.5% or less.

Dated this the 2nd day of August, 2016 Anjan Sen
Of Anjan Sen and Associates
(Applicants Agent)