US20140363694A1 - Low density high strength steel and method for producing said steel - Google Patents
Low density high strength steel and method for producing said steel Download PDFInfo
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- US20140363694A1 US20140363694A1 US14/241,644 US201214241644A US2014363694A1 US 20140363694 A1 US20140363694 A1 US 20140363694A1 US 201214241644 A US201214241644 A US 201214241644A US 2014363694 A1 US2014363694 A1 US 2014363694A1
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- steel sheet
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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Definitions
- the present invention relates to a low density and high strength steel sheet and to a method of producing a low density and high strength steel sheet, for instance for use as inner or outer steel sheets of a structural member for an automobile.
- steel Because of its excellent strength and ductility, and very low cost as compared to aluminium or magnesium, steel has been generally used to make a body of the automobile lighter by developing stronger grades which allow the use of thinner high strength steel sheet. However, in order to overcome a future limitation to the reduction in weight, it may be required to use alternative sources for reducing the weight of steel parts.
- TWIP steels with very high manganese contents of over 20% result in a lighter steel matrix.
- aluminium, as a lightweight element, is sometimes added so as to reduce the density of the steel. Additions of up to 15% aluminium have been used.
- a problem with the known steels is that processability in the existing facilities of the steel industry is problematic due to their proneness for cracking and their high deformation resistance during rolling. Other problems are weldability issues, particularly with the high aluminium content, and the likelihood of formation of undesirable martensite components.
- a low density and high strength steel sheet comprising 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al and 0.01% or less N, the balance being iron and inevitable impurities, wherein 1.7 ⁇ (Mn—Al)+52.7 ⁇ C is at least 3 and at most 4.5 as claimed in claim 1 .
- Preferable embodiments are claimed in the dependent claims.
- composition of the present invention will be described in detail (all compositions in weight %).
- Carbon serves to create cementite (Fe,Mn) 3 C and kappa carbide (Fe,Mn) 3 AlC. Carbon is also an austenite stabilising element, and provides dispersion strengthening by forming precipitates. Particularly, since the columnar dendrite created during continuous casting is rapidly re-crystallized so as to create a coarse structure, the formation of carbides at high temperatures carbide is used to refine the structure. Additionally, the addition of carbon between 0.15 to 0.25% is used to increase the strength. However, if the added amount of carbon increases, the amounts of cementite and kappa carbide increase to contribute to an increase in strength, but greatly decrease ductility of steel.
- the carbon content is at least 0.16%, more preferably at least 0.17%.
- the carbon content is at most 0.23, more preferably at most 0.20%.
- Manganese contributes to the formation of austenite at high temperature, together with carbon. Further, manganese increases the lattice constant of steel thereby decreasing the density of the steel. A minimum amount of 2.5% of manganese was found to result in stable austenite and a significant decrease in density of the steel. However, if the amount of manganese is excessive, then the occurrence of central segregation results in an excessive band structure in a hot rolled sheet, which causes a decrease in ductility. The upper limit of manganese is therefore restricted to 4%. A preferable upper limit of manganese is 3.8%.
- Phosphorus is an element which is added in an amount as small as possible. It segregates on the grain boundary and causes hot shortness and cold shortness, so that workability of steel may be greatly reduced.
- the upper limit of phosphorus is restricted to 0.02%, but preferably the amount is limited to at most 0.01% or even 0.005%.
- sulphur promotes hot shortness. Particularly, it creates coarse MnS, which upon hot rolling and cold rolling, may causes a rolling plate to break, so that it is limited to 0.015% or less. Preferably the sulphur amount is limited to at most 0.01% or even 0.005%.
- Aluminium is an important element in the present invention, together with carbon and manganese.
- the addition of aluminium decreases the density of the steel. Taking into consideration the decrease in specific gravity, it is preferred that a great quantity of Al be added.
- the addition of 6% aluminium or more results in a significant decrease in density of the steel.
- the amount of intermetallic compounds such as kappa carbide, FeAl or Fe 3 Al increases which causes a reduction of the ductility of steel, leading to cracking during cold rolling, so that the upper limit is restricted to 9%.
- aluminium causes an increase of the ductile-brittle transition temperature from sub-zero temperatures to around ambient temperatures. Therefore the upper aluminium limit is restricted to 9%.
- a preferable lower limit of aluminium is 6.2%.
- N Nitrogen causes the formation of AlN-precipitates if a great quantity of aluminium is added as in the present invention. These precipitates are effective in the refinement of columnar dendrite and the improvement in a ratio of equiaxed dendrite and for this reason a small amount of nitrogen in the steel is advantageous. However, large amounts of nitrogen cause large and amounts of potentially coarse AlN-precipitates which is undesirable. Thus, the upper limit of N is restricted to 0.01%. Preferably the nitrogen amount is limited to at most 0.008% or even 0.005%.
- composition of the steel sheet is chosen such that the value of (36.0+Mn)/Al is at least 1.3 and at most 2.0.
- small to intermediate amounts of one or two or more elements of the group consisting of Si, Cr, Mo, Ni, Cu, B, Ti, Zr, Nb, W and Ca may optionally be added.
- silicon Similar to aluminium, silicon also decreases the specific gravity of steel and contributes to the improvement in strength, but if being excessively added, it may create a thick, irregular high temperature oxide film on the surface of steel. Also, silicon causes a stronger increase of the ductile-brittle transition temperature from sub-zero temperatures to around ambient temperatures than aluminium. Therefore the upper silicon limit is restricted to 2%. Thus, it is preferred that the amount of silicon is within the range of 0.1 to 2.0%.
- Chromium is a ferrite-forming element which forms Cr-based carbides which may serve to refine the microstructure, so that the amount can be 0.1% or more. However, if added too much, ductility is reduced, so that the upper limit is restricted to 0.3%.
- molybdenum is a ferrite-forming element which forms fine carbide, and is added by 0.05% or more. However, if excessively added, it decreases the ductility of steel, so that the upper limit thereof is restricted to 0.5%.
- Nickel is an austenite-forming element, which may introduces partial austenite during hot rolling to refine the structure, to thereby greatly improve the ridging resistibility.
- the price of nickel is high and increases the manufacturing cost, so that the limit is restricted to a range of 0.1 to 2.0%.
- Copper acts similar to nickel, but generally the price of copper is lower than that of nickel, so that it can be added in the range of 0.1% or more. However, if excessively added, it exists on a grain boundary in a liquid state to cause intergranular brittleness, owing to fused metal, and causes edge cracking, so that the amount is restricted to a range of 0.1 to 1.0%.
- Boron restricts the recovery and recrystallization of ferrite in the process of hot rolling so as to contribute to the structure refinement thanks to cumulative rolling reduction and increase the strength of steel, so that the amount is 0.0005% or more.
- it may create boron-carbide, decreases the ductility of steel, and deteriorates the wettability of a hot-dipped galvanized coating layer, so that the upper limit is restricted to 0.003%.
- Titanium forms TiN or TiC or the like to thereby improve the grain refinement of the cast structure and contributes to the dispersion of kappa-carbide, so that it is added in the range of 0.01% or more.
- it is expensive and increases the manufacturing cost, and it reduces ductility due to the increase in strength through precipitation, so that the upper limit is restricted to 0.2%.
- Zirconium acts similar to titanium, and forms strong nitride and carbide relative to titanium, so that it is added in the range of 0.005% or more. However, it is expensive, so that the upper limit is restricted to 0.2%.
- Niobium acts similar to titanium, and thus it is added in the range of 0.005% or more. However, unlike titanium, it delays recrystallisation of the steel at high temperature to thereby greatly increase the rolling load of hot rolling. This may make it impossible to manufacture a thin steel sheet, so that the upper limit is restricted to 0.2%.
- Tungsten is a heavy element which increases the specific gravity of steel so the addition, if any is within a range of 0.05 to 1.0%.
- Antimony (Sb) segregates on the grain boundaries restricts the formation of kappa carbide so that antimony, if added, is added in the range of 0.005% or more. However, if excessively added, antimony segregates on a grain boundary to degrade ductility, so that the upper limit thereof is restricted to 0.2%.
- Ca forms sulphides such as CaS, and thereby prevents the formation of MnS, so that it is added in the range of 0.001% or more to improve hot workability of steel.
- the upper limit is restricted to 0.2%.
- the steel sheet of the invention includes a retained austenite structure.
- the retained austenite complements the low strength of a ferrite matrix structure and also contributes to improvement in ductility thereof, so that it is included in the range of 5% or more by area.
- the upper limit thereof is preferably restricted to 20%, more preferably the upper limit is 15% or even 12%.
- a slab i.e. a thin slab ( ⁇ 150 mm), thick slab (150-400 mm) or cast strip ( ⁇ 20 mm)
- a slab is first heated in the temperature range of 1000 to 1250° C. If the reheating temperature exceeds 1250° C., coarse grains are formed in the slab, possibly creating ridging and hot shortness, whereas if it is below 1000° C., the finishing hot-rolling temperature becomes too low to both manufacture a steel sheet and remove an oxide film on a high temperature surface using the spraying of pressurized water, thereby causing surface defects.
- the reheating temperature is restricted to 1000 to 1250° C.
- the reheating temperature is at least 1100° C.
- the finishing rolling is implemented at a temperature of 900° C. or less, preferably at most 850° C. in order to refine crystal grains by dynamic or static recrystallisation during the hot rolling process.
- the material is subjected to the last hot deformation step while it is at least at the aforesaid temperature.
- the temperature is too low, hot deformation resistance increases to make it difficult to manufacture a steel sheet, and kappa carbide is precipitated to provide elongated structures, thereby increasing ridging defects, so that the rolling temperature is in the range of 700° C. or more, preferably 750° C. or more and more preferably 800° C. or more.
- the hot-rolled steel strip is coiled at a temperature of 600° C. or less. This temperature restricts the coarsening of the grain size and the excessive-precipitation of kappa carbide. It also reduces the risk of formation of abnormally coarsened grains caused by secondary recrystallisation of the coarse grains.
- the coiling temperature is below 550° C.
- the coiling temperature should be at least 200° C., and preferably at least 300° C., as quenching the material to ambient temperature proved to cause severe cracking during cold rolling.
- the resulting hot-rolled material can be manufactured into a hot-rolled steel sheet after being treated with pickling, and temper rolling and oiling.
- the steel sheet is a low density steel sheet having the specific gravity of 7400 kg/m 3 or less, preferably of 7300 kg/m 3 or less.
- the hot-rolled steel sheet can be manufactured into a cold rolled steel sheet after being pickled and cold rolled.
- cold rolling reduction is set to 40% or more. This is because, if the cold rolling reduction is set to 40% or more, stored energy by cold working can be secured, and a new recrystallised structure can be obtained.
- the minimum cold rolling reduction is 50%.
- the upper limit thereof is restricted to 90% or less in consideration of production efficiency and economy.
- the material may be subjected to intermediate annealing in between cold rolling reductions or steps.
- the cold rolled steel sheet is treated with continuous annealing or continuous hot-dip galvanizing after cleaning the surface if necessary.
- the annealing rate is preferably selected in the range of 1° C./s to 20° C./s. If the annealing rate is less than 1° C./s, productivity is too low, and the steel sheet is exposed to high temperature condition for a long time to thereby cause the coarsening of crystal grains and reduction in strength, deteriorating the quality of material. On the other hand, if the annealing rate exceeds 20° C./s, because of insufficient re-melting of carbide, the formation of austenite also becomes insufficient and thus retained austenite is reduced to thereby reduce the ductility.
- the cooling rate after annealing is preferably between 10 and 50° C./s, either to ambient temperatures, or to the galvanising bath and/or the overaging treatment. After galvanising or overageing, the cooling rate is preferably between 10 and 50° C./s, more preferably between 10 and 25° C./s,
- Annealing is implemented in the temperature range between the recovery temperature and 900° C. Between the recovery temperature and below the recrystallisation temperature, some ductility is recovered. This may be used to create high strength steels whilst securing adequate ductility by selecting the recovery annealing temperature and time. Above the recrystallisation temperature and below 900° C., the cold deformed structure readily recrystallises. The combination of annealing temperature and annealing time to obtain full recrystallisation of the cold-rolled steel strip can be easily determined. The inventors found a lower austenite content in the final product after annealing if the cold-rolled material was annealed at a higher annealing temperature.
- the lower limit is at least 800° C.
- the annealing is carried out for 10 seconds or more so as to achieve excellent strength and workability. However, if the annealing time exceeds 180 seconds, the productivity is excessively lowered and the properties may be adversely affected by the prolonged annealing process.
- the steel sheet After annealing the steel sheet is cooled to the temperature of the bath and may be coated with Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, Al—Mg—Si, or the like in the thickness of 10 to 200 pm per one side thereof, thereby forming coated steel sheet, by a hot dip coating process.
- These or other metal coating layers may also be applied by an electroplating process.
- the coating thickness on the or each surface is between 10 ⁇ m and 200 ⁇ m.
- the material is subjected to overageing after annealing. If applicable this overageing may precede or follow after the hot dip coating process, depending on the lay-out of the plant or depending on metallurgical preferences.
- the overageing temperature is preferably between 350 and 500° C., and preferably about 400° C.
- the overageing time is preferably at least 30 and/or at most 180 s.
- the annealing temperature is preferably at least 825° C. and/or preferably at most 875° C.
- the steel sheet produced as above carbides and 5% or more of retained austenite is dispersed in a ferrite matrix, so that the tensile strength is high in the level of 600 to 900 MPa, the ductility is excellent, and therefore the combination of strength-ductility is also excellent.
- the steel sheet has a tensile strength of 600 to 900 MPa.
- the steels were then subjected to cold-rolling at a reduction (CRR) of 67% and continuously annealed at annealing temperatures (AT) between 800 and 1050° C. (see Table 3) and with and without overageing at 400° C. (see Table 4).
- the annealing time was 60 seconds and the overageing time was 80 seconds.
- the overageing treatment appears to have a beneficial effect in combination with a higher annealing temperature.
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EP11180669.1 | 2011-09-09 | ||
EP11180669 | 2011-09-09 | ||
PCT/EP2012/052920 WO2013034317A1 (en) | 2011-09-09 | 2012-02-21 | Low density high strength steel and method for producing said steel |
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US14/241,644 Abandoned US20140363694A1 (en) | 2011-09-09 | 2012-02-21 | Low density high strength steel and method for producing said steel |
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US (1) | US20140363694A1 (pt) |
EP (1) | EP2753725B1 (pt) |
CN (1) | CN103827338B (pt) |
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WO (1) | WO2013034317A1 (pt) |
Cited By (2)
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CN113430453A (zh) * | 2021-05-27 | 2021-09-24 | 南京钢铁股份有限公司 | 一种低密度复合钢板的制备方法 |
US11549163B2 (en) * | 2017-12-19 | 2023-01-10 | Arcelormittal | Cold rolled and heat treated steel sheet, method of production thereof and use of such steel to produce vehicle parts |
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CN105164295A (zh) | 2013-05-01 | 2015-12-16 | 新日铁住金株式会社 | 点焊性优异的高强度低比重钢板 |
CA2910439C (en) * | 2013-05-01 | 2018-02-20 | Nippon Steel & Sumitomo Metal Corporation | Galvanized steel sheet and method for producing the same |
KR101560940B1 (ko) | 2013-12-24 | 2015-10-15 | 주식회사 포스코 | 강도와 연성이 우수한 경량강판 및 그 제조방법 |
CN104789888A (zh) * | 2015-03-20 | 2015-07-22 | 苏州科胜仓储物流设备有限公司 | 一种用于中型货架的高强度钢板及其锻造工艺 |
CN104789902A (zh) * | 2015-03-20 | 2015-07-22 | 苏州科胜仓储物流设备有限公司 | 一种用于钢平台的高强度钢板及其锻造工艺 |
CN104789903A (zh) * | 2015-03-20 | 2015-07-22 | 苏州科胜仓储物流设备有限公司 | 一种用于重型横梁式货架的高强度钢板及其热处理工艺 |
CN104789904A (zh) * | 2015-03-20 | 2015-07-22 | 苏州科胜仓储物流设备有限公司 | 一种用于轻型模具货架的高强度钢板及其热处理工艺 |
CN104789901A (zh) * | 2015-03-20 | 2015-07-22 | 苏州科胜仓储物流设备有限公司 | 一种用于重型模具货架的高强度钢板及其热处理工艺 |
CN106011652B (zh) * | 2016-06-28 | 2017-12-26 | 宝山钢铁股份有限公司 | 一种磷化性能优异的冷轧低密度钢板及其制造方法 |
CA3048131C (en) * | 2016-12-22 | 2021-10-19 | Arcelormittal | Cold rolled and heat treated steel sheet, method of production thereof and use of such steel to produce vehicle parts |
CN107326282B (zh) * | 2017-07-13 | 2018-09-14 | 武汉钢铁有限公司 | 600MPa级高屈强比热轧高强轻质钢及其制造方法 |
CN108950392B (zh) * | 2018-07-19 | 2020-10-30 | 首钢集团有限公司 | 一种超高延性低密度钢及其制备方法 |
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- 2012-02-21 IN IN2603CHN2014 patent/IN2014CN02603A/en unknown
- 2012-02-21 CN CN201280043488.5A patent/CN103827338B/zh not_active Expired - Fee Related
- 2012-02-21 US US14/241,644 patent/US20140363694A1/en not_active Abandoned
- 2012-02-21 EP EP12704434.5A patent/EP2753725B1/en active Active
- 2012-02-21 WO PCT/EP2012/052920 patent/WO2013034317A1/en active Application Filing
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CN113430453A (zh) * | 2021-05-27 | 2021-09-24 | 南京钢铁股份有限公司 | 一种低密度复合钢板的制备方法 |
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EP2753725B1 (en) | 2015-09-16 |
IN2014CN02603A (pt) | 2015-08-07 |
WO2013034317A1 (en) | 2013-03-14 |
CN103827338A (zh) | 2014-05-28 |
EP2753725A1 (en) | 2014-07-16 |
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