KR100957960B1 - Cold rolled steel sheet having good formability and surface quality and process for producing the same - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet used as a material for automobiles, home appliances, etc., using Mn and Cu as an emulsion-based precipitate to provide a cold rolled steel sheet without surface defects having improved workability and improved workability, and a method of manufacturing the same. Its purpose is to.

In the present invention, by weight%, C: 0.005% or less, Mn: 0.35 to 0.8%, Cu: 0.01-0.2%, S: 0.005-0.02%, Al: 0.1% or less, N: 0.002% or less, P: 0.2 % Or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, remaining Fe and other inevitable impurities, Mn, Cu, S is 0.27 * (Mn + Cu) / S: 5-25, the Nb And C satisfy (Nb / 93) / (C / 12): 0.5 to 3.0, and (Mn, Cu) S precipitates are formed, and the average size of the precipitates is 0.2 µm or less. The cold rolled steel sheet excellent in surface quality and its manufacturing method are made into the summary.

According to the present invention, there is provided a cold rolled steel sheet having no surface defects with improved yield strength and improved workability.

Cold rolled steel, non-aging, high strength, secondary processing brittleness, in-plane anisotropy index, precipitate

Description

COLD ROLLED STEEL SHEET HAVING GOOD FORMABILITY AND SURFACE QUALITY AND PROCESS FOR PRODUCING THE SAME}

The present invention relates to a cold rolled steel sheet used as a material for automobiles, home appliances, and the like, and more particularly, cold rolled steel sheet having improved yield strength and workability by (Mn, Cu) S precipitates in IF steel (Interstitial Free Steel). It relates to a manufacturing method.

Cold rolled steel sheets used in automobiles and home appliances require strength and formability as well as non-aging characteristics. Aging is a kind of strain aging that causes hardening as the solid elements (C, N) adhere to dislocations, leading to a defect called stretcher strain.

The non-aging property of the cold rolled steel sheet can be secured by the annealing of the aluminum-kilted steel, but the annealing has the disadvantage that the annealing time is long and the productivity is low and the material deviation is severe for each part.

Therefore, IF steel which adds strong carbon and nitride forming elements such as Ti and Nb and performs continuous annealing is mainly used. IF steels have a non-aging effect by removing some or all of the carbon and nitrogen employed.

The high-strength method of IF steel is the employment strengthening technology by P. Japanese Laid-Open Patent Publication No. 57-0413349 secures strength by adding 0.04 to 0.12% of P in Ti-added IF steel. However, in the IF steel of the ultra low carbon component system, P causes the problem of secondary processing brittleness due to grain boundary segregation.

Korean Laid-Open Patent Publication No. 2003-0052248 improves processability and secondary processing brittleness by adding 0.5-2.0% Mn, Al, and B instead of P addition.

However, Mn used in ultra low carbon steel is of high quality, and when Mn is added up to a level of 2.0%, manufacturing cost increases, and in particular, plating properties are not good.

In JP-A-6-057336, 0.5-2.5% of Cu is added to IF steel to form an ε-Cu precipitated phase to secure strength. The high strength is ensured by the precipitated phase of? -Cu, but the processing characteristics are not good.

In contrast, techniques for improving the processability or surface defects by carbides using Cu as precipitation nuclei of carbides are disclosed in Japanese Patent Laid-Open Nos. 9-227951 and 10-265900.

Japanese Patent Application Laid-Open No. 9-227951 discloses a slab of IF steel containing 0.01-0.05% of Cu after rough rolling at 1100-950 ° C. for at least 70 seconds to precipitate CuS and hot-roll to precipitate Cu-Ti-CS precipitates. The hot rolled plate is obtained and cold rolled to secure workability. Cold rolling of a hot rolled sheet having a precipitate of Cu-Ti-CS increases the nuclei of crystal grains which form {111} parallel to the plate surface around these precipitates during recrystallization, resulting in improved workability as a result. .

Japanese Patent Laid-Open No. 10-265900 discloses surface defects by reducing the solid solution C in the steel sheet by precipitating CuS in IF steel to which 0.005-0.1% Cu is added and making the CuS precipitate effectively function as precipitation nuclei of carbides. It is a technique to improve.

In Japanese Patent Laid-Open Publication Nos. 9-227951 and 10-265900, the coarse CuS precipitates are used in the intermediate process, and finally, the cold rolled steel sheet secures carbides. It is adding more than S amount by ratio. That is, since the amount of Ti added is larger than the amount of S in atomic ratio so that S reacts most with Ti, the precipitate of CuS alone is not utilized.

In Korean Patent Laid-Open Publication No. 2006-0115319, in order to refine the MnS precipitate, the amount of Mn added is small, so that FeS is crystallized at a high temperature, resulting in surface defects.

An object of the present invention is to provide a cold-rolled steel sheet and a method of manufacturing the same, using Mn and Cu as an emulsion-based precipitate and improving the yield strength while improving workability.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight%, C: 0.005% or less, Mn: 0.35 to 0.8%, Cu: 0.01-0.2%, S: 0.005-0.02%, Al: 0.1% or less, N: 0.002% or less, P: 0.2 % Or less, B: 0.0001-0.002%, Nb: 0.002-0.04%, remaining Fe and other inevitable impurities, Mn, Cu, S is 0.27 * (Mn + Cu) / S: 5-25, the Nb And C satisfy (Nb / 93) / (C / 12): 0.5 to 3.0, and (Mn, Cu) S precipitates are formed, and the average size of the precipitates is 0.2 µm or less. The present invention relates to a cold rolled steel sheet having excellent surface quality.

In the present invention, it has non-aging characteristics and baking hardening characteristics according to the conditions of Nb and C. In order to secure the non-aging characteristics, C must be precipitated as NbC, and the conditions are that Nb and C are (Nb / 93) / (C / 12): 0.5 to 3.0, more preferably (Nb / 93) / ( C / 12): 1-1.80.

In the present invention, the content of C is preferably 0.0005-0.003%, and Cu is preferably 0.03-2.0%.

The cold rolled steel sheet of the present invention has the characteristics of a soft cold rolled steel sheet of 280MPa grade and high strength cold rolled steel sheet of 340MPa or more according to the component design.

If you want to improve the workability in the cold rolled steel sheet of the present invention may further comprise Mo 0.01 ~ 0.2%.

In addition, the present invention is re-heated steel slab that satisfies the component system of the present invention at a temperature of 1100 ℃ or more and hot-rolled with a finish rolling temperature of more than Ar ₃ transformation point and temperature below 700 ℃ at a rate of 300 ℃ / min or more The present invention relates to a method for producing a cold rolled steel sheet having excellent workability and surface quality, which is cooled by, and wound at a temperature of 700 ° C. or lower, followed by cold rolling.

As described above, according to the present invention, unlike the technique of adding a large amount of Mn as a solid solution element in IF steel or using Cu as a precipitated phase of ε-Cu, (Mn, Cu) S precipitates of nanosized By distributing the to the cold rolled steel sheet to provide a cold-rolled steel sheet without surface defects to refine the grains, thereby improving workability and yield strength enhanced by precipitation strengthening.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, the (Mn, Cu) S precipitate is an expression including Mn alone emulsion, Cu alone emulsion, and a complex emulsion of Mn and Cu.

The present invention is completed on the basis of the research results that if the (Mn, Cu) S precipitate is secured in the cold rolled steel sheet is fine grains yield yield strength and workability is improved.

In order to secure fine (Mn, Cu) S precipitates on cold rolled steel sheets, S must be secured as an emulsion of Mn and Cu.

S is preferentially precipitated with Ti and Zr, and the remaining amount of S is precipitated as (Mn, Cu) S.

Since the IF steel of the present invention is an Nb-only IF steel to which no sulfide forming elements such as Ti and Zr are added, S is precipitated as (Mn, Cu) S according to the addition conditions of Cu and S.

According to the present invention, when the grains are refined by the fine (Mn, Cu) S precipitates, the solid solution carbon is more present in the grain boundary than in the grains, thereby ensuring room temperature non-aging characteristics.

The dissolved carbon remaining in the grain is relatively free to move, so it does not affect the room temperature aging characteristics in combination with the operating potential.

In addition, the finely distributed CuS precipitates according to the present invention have a positive effect on the increase in yield strength and the improvement of the strength-ductility balance characteristics, and the in-plane anisotropy and plastic anisotropy. For this purpose, (Mn, Cu) S precipitates should be finely distributed, and the distribution of Mn, Cu and S contents, their component ratios, and the cooling rate after hot rolling are affected.

Cold rolled steel sheet of the present invention has a high yield strength can reduce the thickness of the steel sheet has a light weight effect. In addition, the in-plane anisotropy has the advantage of less wrinkles generated during processing, and less ear generation after processing. The cold rolled steel sheet of the present invention and a manufacturing method thereof will be described in detail below.

First, C, Mn, Cu, S, Al, P, N, B, and Nb as basic components will be described.

The content of carbon (C) is preferably 0.005% or less.

If the content of carbon (C) exceeds 0.005%, a large amount of expensive Nb should be added to remove the solid solution carbon which greatly degrades aging and plastic anisotropy. In this case, the manufacturing cost rises and the recrystallization temperature increases. Therefore, the annealing temperature should be increased, otherwise the crystal grains of the annealing plate become fine and the ductility is greatly lowered, and the plating property is also lowered during plating. More preferable content of carbon (C) is 0.003% or less.

Preferably, the lower limit of the content of carbon (C) is made 0.0005%. When the content of carbon (C) is less than 0.0005%, the grains of the hot rolled sheet may be coarse to reduce strength and increase in-plane anisotropy.

The content of manganese (Mn) is preferably 0.3 to 0.8%.

Manganese is known to prevent hot shortness caused by solid solution sulfur by precipitating sulfur in solid solution state with MnS. From this technical point of view, it is common to add a high content of manganese. However, in the present invention, MnS is very finely precipitated when the sulfur content is properly controlled while lowering the content of manganese, thereby improving the properties of plastic anisotropy and in-plane anisotropy by grain refinement and improving the yield strength by precipitation strengthening. Based on the research results, the manganese content is limited to 0.8% or less. In order to secure these characteristics and prevent red brittleness due to FeS crystallization, the content of manganese should be 0.3% or more. If the content is less than 0.3%, red brittleness may occur due to the large amount of sulfur remaining in solid solution. For example, when the content of manganese exceeds 0.8%, the content of manganese is high, making coarse MnS precipitates, making it difficult to secure strength.

The content of copper (Cu) is preferably 0.01 to 0.2%.

Cu forms fine CuS precipitates to make grains fine to improve in-plane anisotropy and plastic anisotropy, and to increase yield strength by strengthening precipitation. For this purpose, CuS precipitates can be precipitated finely when the Cu content is 0.01% or more, and when the CuS precipitates exceed 0.2%, the CuS precipitates are coarsened. Preferable Cu content is 0.03-0.2%.

The content of sulfur (S) is preferably 0.005-0.02%.

Sulfur (S) reacts with Mn and Cu to form fine (Mn, Cu) S precipitates. When the content of S is less than 0.005%, not only the amount of precipitates precipitated is small but also the number of precipitates precipitated is very small. When the content of sulfur exceeds 0.02%, the content of solute is high, so the ductility and moldability are greatly lowered, and there is a fear of red brittleness.

The content of aluminum (Al) is preferably 0.1% or less.

Aluminum is an element added as a deoxidizer, but is added to precipitate nitrogen in the steel to completely prevent aging by solid nitrogen. If the aluminum content is more than 0.1%, the amount of aluminum present in the solid solution state is high, thereby reducing the ductility. Preferred Al content is 0.01-0.1%.

The content of nitrogen (N) is preferably 0.004% or less.

Nitrogen is an element inevitably contained in steelmaking and is preferably controlled at 0.004% or less.

The content of phosphorus (P) is preferably 0.2% or less.

Phosphorus is an element having a high solid solution strengthening effect and a small decrease in r value, and guarantees high strength in steels for controlling precipitates according to the present invention. In steel grades requiring strength of 280 Mpa, the content of P should be less than 0.015%. For high strength steel of 340Mpa or higher, it is recommended to set it as 0.016 ~ 0.2%. If the content of P is more than 0.2%, the ductility is lowered, so the upper limit is preferably limited to 0.2%. In the present invention, when Si and Cr are added, various strength designs are possible while the content of P is in the range of 0.2% or less.

The content of boron (B) is preferably 0.0001 to 0.002%.

Boron is added to prevent secondary processing brittleness. For this purpose, the boron content is preferably 0.0001% or more. If the boron content exceeds 0.002%, deep drawing may be greatly degraded.

The content of niobium (Nb) is preferably 0.002 to 0.1%.

Nb is added for the purpose of ensuring inaging and improving moldability. Nb is a strong carbide-generating element, added to steel to precipitate NbC precipitates to precipitate carbon in solid solution, thereby securing inaging properties. In addition, the NbC precipitates have an effect of greatly improving the processing ability of the soil by developing the aggregate structure during annealing.

If the amount of Nb added is less than 0.002%, the amount of precipitation of NbC precipitates is so small that there is little development of the aggregate structure, so that there is little effect of improving the processability. When the amount of Nb exceeds 0.1%, the amount of NbC precipitates is too large, resulting in low rimability and low elongation, which may significantly reduce moldability.

In the present invention, (Mn, Cu) S precipitates are affected by the component ratios of Mn, Cu, and S. That is, the value of 0.27 * (Mn + Cu) / S (Mn, Cu, S is weight%) is 5-25, Preferably it is set to 5-20.

Effective precipitates are obtained when 0.27 * (Mn + Cu) / S is 5 or more, and in the case of more than 25, the precipitates are coarse to increase the aging index, and the characteristics of plastic anisotropy index and in-plane anisotropy index are not good.

In the present invention, the average size of the precipitate in the range of 5 to 25 in the ratio of 0.27 * (Mn + Cu) / S is smaller than 0.2 μm or less. In the present invention, the greater the distribution number of precipitates, the better the aging characteristics, the in-plane anisotropy, the secondary processing brittleness, and the like.

In the present invention, Nb and C can be a component design in terms of non-aging and baking properties. In order to secure the non-aging characteristics, it is preferable that 0.13 * Nb / C (where Nb and C are by weight%) satisfies 0.5 to 3.0. More preferably, 1-1.8 is satisfied.

Nb combines with C to precipitate NbC precipitates to remove C in the solid solution state in the steel, thereby securing ineffectiveness and developing aggregated structure during annealing to improve Omrim processability. When 0.13 * Nb / C value is less than 0.5, it is ineffective It is difficult to secure the quality, and the processability of soiling is also lowered. If the value of 0.13 * Nb / C exceeds 3.0, the amount of Nb remaining in solid solution in the steel is large, so the ductility is greatly reduced.

The finer the precipitates in the component system of the present invention, the more advantageous it is. Preferably, the average size of the (Mn, Cu) S precipitates is 0.2 μm or less. According to the results of the present invention, especially when the average size of the precipitate is more than 0.2㎛, the strength is low, and workability is not good.

In general, when the plastic anisotropy index increases, the in-plane anisotropy index rises and there is a limit to increasing the plastic anisotropy index in terms of processability. .

In the cold rolled steel sheet of the present invention, molybdenum (Mo) may be further added.

The content of molybdenum (Mo) is preferably 0.01 to 0.2%.

Mo is added as an element to increase the plastic anisotropy index, the content of the plastic anisotropy index is increased when the content is more than 0.01%, if the content exceeds 0.2%, the plastic anisotropy index is no longer increased and there is a risk of causing hot brittleness.

In the cold rolled steel sheet of the present invention, antimony (Sb) may be further added.

The content of antimony (Sb) is preferably 0.01 to 0.1%.

Sb is an element added to prevent local plating of the steel sheet during annealing to prevent unplating by the formation of oxides of Mn or Si, and to prevent surface defects. In addition, even if it exceeds 0.1%, since an effect does not become large, the upper limit shall be 0.1%.

[Manufacturing method of cold rolled steel sheet]

The present invention is characterized by forming a fine (Mn, Cu) S precipitate on a cold rolled sheet through hot rolling and cold rolling of the steel slab that satisfies the above steel composition, and to have an average size of 0.2 µm or less. The average size of (Mn, Cu) S precipitates in the cold rolled plate is affected by the conditions of addition, reheating temperature, winding temperature, etc., but it is directly affected by the cooling rate after hot rolling.

[Hot Rolling Condition]

In the present invention, the steel slab that satisfies the above steel composition is reheated and hot rolled. The reheating temperature is preferably 1100 ° C or more. This is because when the reheating temperature is lower than 1100 ° C., the coarse precipitates generated during continuous casting remain completely insoluble due to the low reheating temperature, so that many coarse precipitates remain even after hot rolling.

Hot rolling is preferably performed at a finish rolling temperature above Ar ₃ transformation temperature. This is because when the finish rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is degraded due to the formation of the rolled grain but also the strength is lowered.

It is preferable that the cooling rate before winding after hot rolling shall be 300 degreeC / min or more. Even if the component ratio is controlled to obtain a fine precipitate according to the present invention, if the cooling rate is less than 300 ° C / min, the average size of the precipitate may exceed 0.2 ㎛. In other words, as the cooling rate increases, a large number of nuclei are generated and the precipitate becomes fine. The faster the cooling rate, the finer the precipitate is, so it is not necessary to limit the upper limit of the cooling rate.However, even if the cooling rate is faster than 1000 ° C / min, the finer effect of the precipitate is no longer increased, so the cooling rate is 300 to 1000 ° C / min. This is more preferable.

[Coiling condition]

Winding is performed after hot rolling as above, but the winding temperature is preferably 700 ° C or lower. If the coiling temperature exceeds 700 ℃, precipitates grow too coarse, making it difficult to secure strength.

[Cold rolling condition]

Cold rolling is preferably performed at a reduction ratio of 50 to 90%. If the cold reduction rate is less than 50%, the amount of nucleation of the annealing recrystallization is small, so that grains grow too large during annealing, resulting in a decrease in strength and formability due to coarsening of the annealing recrystallization grains. If the cold reduction ratio is more than 90%, the moldability is improved, but the nucleation amount is too high, so the annealing recrystallized grain is too fine to decrease the ductility.

[Continuous Annealing]

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 700-900 degreeC. If the continuous annealing temperature is less than 700 ° C., recrystallization is not completed and the target ductility value cannot be secured. If the annealing temperature is more than 900 ° C., the strength decreases due to coarsening of the recrystallized grains. The continuous annealing time keeps the recrystallization complete. If it is about 10 seconds or more, the recrystallization is completed. Preferably the continuous annealing time is in the range of 10 seconds to 30 minutes,

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

The steel slabs of Table 1 were reheated, hot rolled to finish, cooled to 400 ° C./min, wound up at 650 ° C., and then cold rolled and continuously annealed at a reduction ratio of 75%. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 830 ℃ to 10 ℃ / second.

The obtained annealing plate was processed into a standard specimen according to ASTM E-8 standard to investigate the mechanical properties. The specimen was measured using a tensile tester (INSTRON, Model 6025) to measure yield strength, tensile strength, elongation, plastic anisotropy index (r _m value) and secondary brittle brittleness index DBTT (Ductile Brittle Transition Temperature). It is shown in Table 2 below.

The DBTT was measured at a temperature at which brittle fracture occurs by impacting the workpiece after processing with drawing ratio 2.0.

The plastic anisotropy index was r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, and the occurrence of surface defects was visually observed for surface defects around the edge after hot rolling.

The size of the precipitate is an electron microscope, and the size of the grains is an average value analyzed by an optical microscope and an image analyzer.

Sample Number Chemical composition (% by weight) 0.27 * (Mn + Cu) / S 0.13 * Nb / C Remarks C Mn P S Cu Al N Nb Etc B Scope of invention ≤0.02 0.3- 0.8 0.005- 0.2 0.003-0.02 0.005-0.2 0.01- 0.08 ≤0.004 0.005 ~ 0.1 0.0001 ~ 0.005 5-25 0.5-3.0 One 0.0008 0.35 0.007 0.009 0.05 0.027 0.0018 0.018  - 0.0005 9.00 2.93 Invention steel 2 0.0018 0.35 0.062 0.012 0.11 0.038 0.0016 0.032 - 0.0005 10.35 2.31 3 0.0026 0.47 0.085 0.007 0.08 0.053 0.0028 0.038 Sb: 0.02 0.001 21.21 1.90 4 0.0068 0.58 0.093 0.016 0.12 0.027 0.002 0.078 - 0.0013 11.81 1.49 5 0.0062 0.49 0.072 0.009 0.09 0.031 0.0019 0.069 Mo: 0.052 0.0012 17.40 1.45 6 0.0016 0.08 0.069 0.008 0.07 0.039 0.0016 0.022 - 0.0008 5.06 1.79 Comparative steel 7 0.026 0.18 0.058 0.012 0.12 0.03 0.0022 0 - 0.0009 8.78 0.00 8 0.0022 0.58 0.07 0.01 0 0.04 0.0016 0 Ti: 0.054 0.0005 15.66 0.00 Conventional Steel

Sample number Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Plastic Anisotropy Index (rm) Secondary work brittleness (DBTT- ℃) Surface Defects Average size of precipitate (μm) Grain size (ASTM No.) Remarks One 138 295 52 2.24 -40 Does not occur 0.06 8.3 Invention steel 2 201 355 44 2.11 -50 " 0.07 9.5 Invention steel 3 245 405 40 1.98 -40 " 0.09 10.3 Invention steel 4 294 459 34 1.94 -40 " 0.09 12.2 Invention steel 5 291 463 32 1.92 -40 " 0.11 11.8 Invention steel 6 205 352 42 2.13 -40 Occur 0.06 9.5 Comparative steel 7 259 406 32 1.42 -50 Occur 0.08 12.8 Comparative steel 8 202 345 40 1.93 -20 Does not occur 0.38 8.2 Conventional Steel

As shown in Table 2, Sample Nos. 1 to 5, which are inventive steels, exhibit high characteristics of elongation and plastic anisotropy as well as no surface defects for each strength, while Comparative Nos. 6 and 7 have Mn contents. Low surface defects occurred. Especially, sample No. 7 has high carbon content and low plastic anisotropy.

On the other hand, Sample No. 8, which is a conventional steel, has a disadvantage in that the Cu is not added, so the grain size is large, so that the anisotropy index is low and the strength is secured only by strengthening the solid solution.

Claims (4)

By weight%, C: 0.005% or less, Mn: 0.35 to 0.8%, Cu: 0.01-0.2%, S: 0.005-0.02%, Al: 0.1% or less, N: 0.002% or less, P: 0.2% or less, B : 0.0001-0.002%, Nb: 0.002 ~ 0.04%, remaining Fe and other inevitable impurities, Mn, Cu, S is 0.27 * (Mn + Cu) / S: 5-25, Nb, C is ( Nb / 93) / (C / 12): satisfies 0.5 to 3.0, and (Mn, Cu) S precipitates are formed, and the average size of the precipitates is 0.2 µm or less, and the surface properties are excellent. Cold rolled steel sheet The cold rolled steel sheet having excellent workability and surface quality according to claim 1, wherein the cold rolled steel sheet further comprises 0.01 to 0.2% of Mo. The cold rolled steel sheet according to claim 1 or 2, wherein the cold rolled steel sheet contains Sb additionally 0.01 to 0.1%. By weight%, C: 0.005% or less, Mn: 0.35 to 0.8%, Cu: 0.01-0.2%, S: 0.005-0.02%, Al: 0.1% or less, N: 0.002% or less, P: 0.2% or less, B : 0.0001-0.002%, Nb: 0.002 ~ 0.04%, remaining Fe and other inevitable impurities, Mn, Cu, S is 0.27 * (Mn + Cu) / S: 5-25, Nb, C is ( Nb / 93) / (C / 12): after re-heating a steel slab satisfying the 0.5 to 3.0 with a temperature above 1100 ℃ by a finish rolling temperature above the Ar ₃ transformation point, hot-rolled and 700 ℃ in over 300 ℃ / min speed Process for producing cold rolled steel sheet having excellent workability and surface quality, which is cooled to the following temperature, wound up to a temperature of 700 ° C. or lower, followed by cold rolling and continuous annealing.