Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium

Definitions

• the present invention relates to steel sheets for deep drawing mainly used for interior or exterior plates of automobile bodies, and the like. More particularly, the present invention relates to steel sheets for deep drawing, which have a tensile strength of 28 ⁇ 50 f/ while exhibiting excellent secondary work embrittlement resistance, fatigue properties of welded joints, and plating properties as well as excellent formability, and to a method for manufacturing the same.
• steel sheets for the automobile body have been required to have further enhanced formability.
• the steel sheets for the automobile body also have been required to have excellent secondary work embrittlement and fatigue properties of welded joints in terms of using conditions of the automobiles, and to have an appealing plated surface.
• steel sheets having enhanced formability and strength are produced in such a way of adding formability enhancing elements, that is, carbide and nitride formation elements such as Ti, Nb and the like, and strength enhancing elements, that is, solid solution strengthening elements such as Mn, P, Si and the like to a highly pure steel which is minimized in contents of impurities in the steel. Due to inherent restrictions in properties of the steel, however, it is difficult to enhance the formability and the strength at the same time.
• formability enhancing elements that is, carbide and nitride formation elements such as Ti, Nb and the like
• strength enhancing elements that is, solid solution strengthening elements such as Mn, P, Si and the like
• the steel sheet for extra deep drawing is produced using the highly pure steel, it commonly suffers from embrittlement of grain boundaries, which results in significant deterioration of secondary work embrittlement resistance and fatigue properties of welded joints.
• the steel sheets for deep drawing are produced using, so called, ultra-low carbon interstitial free (IF) steel, which is produced by adding the carbide and nitride formation elements such as Ti, Nb, and the like as a single component or a combination thereof to ultra-low carbon steel while lowering an amount of interstitial solid solution elements such as C or N to 50 ppm or less during a steel making process in order to ensure good formability.
• IF ultra-low carbon interstitial free
• the carbide and nitride formation elements such as Ti, Nb, and the like are added in an amount of 0.01 ⁇ 0.07% to the ultra-low carbon steel in order to ensure workability
• the steel lacks in the interstitial solid solution strengthening elements which serve to strengthen the grain boundaries, causing the secondary work embrittlement while deteriorating the fatigue properties at the spot welded joints.
• inventors of the present invention invented a high strength steel sheet for extra deep drawing useful for automobiles and the like, and a method for manufacturing the same disclosed in Korean Patent Laid-open Publication No. 2004-0002768, which comprises, by weight %, C: 0.010% or less, Si: 0.02% or less, Mn: 1.5% or less, P: 0.03 ⁇ 0.15%, S: 0.02% or less, Sol. Al: 0.03 ⁇ 0.40%, N: 0.004% or less, Ti: 0.005 ⁇ 0.040%, Nb: 0.002 ⁇ 0.020%, and at least one of B: 0.0001 ⁇ 0.0020% and Mo: 0.005 ⁇ 0.02%, thereby enhancing the workability of Ti—Nb added steel.
• this method can enhance the workability by controlling Ti and Nb in combination, it fails to ensure the secondary work embrittlement and the fatigue properties which have been required for the steel plate of the automobile in recent years.
• the present invention has been made to solve the above problems, and it is an object of the present invention to provide a high strength steel sheet for deep drawing, which is controlled in contents of Ti, Al, B and N, and in contents of Nb, Al and C combinationally, while increasing the content of Al, which is advantageous in terms of formability and plating properties, and reducing the content of Ti, which is disadvantageous in terms of the plating properties, and the like, thereby providing excellent properties in terms of secondary work embrittlement resistance, and fatigue properties of welded joints as well as formability while exhibiting an appealing surface quality.
• a high strength steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties, comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06 ⁇ 1.5%, P: 0.15% or less, S: 0.020% or less, Sol.
• Al 0.10 ⁇ 0.40%
• N 0.010% or less
• Ti 0.003 ⁇ 0.010%
• Nb 0.003 ⁇ 0.040%
• B 0.0002 ⁇ 0.0020%
• Fe and other unavoidable impurities wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0 ⁇ (Ti[%]+Al[%]/16+6B[%])/3.43N[%] ⁇ 4.1, and wherein the composition of Nb, Al, and C satisfies the relationship: 0.7 ⁇ (Nb[%]+Al[%]/20)/7.75C[%] ⁇ 3.5.
• a method for manufacturing a high strength steel sheet for deep drawing having excellent secondary work embrittlement resistance, fatigue properties and plating properties comprises: reheating a steel slab at a temperature of 1,100 ⁇ 1,250° C., the steel slab comprising, by weight %: C: 0.010% or less, Si: 0.02% or less, Mn: 0.06 ⁇ 1.5%, P: 0.15% or less, S: 0.020% or less, Sol.
• Al 0.10 ⁇ 0.40%, N: 0.010% or less, Ti: 0.003 ⁇ 0.010%, Nb: 0.003 ⁇ 0.040%, B: 0.0002 ⁇ 0.0020%, and the balance of Fe and other unavoidable impurities, wherein the composition of Ti, Al, B, and N satisfies the relationship: 1.0 ⁇ (Ti[%]+Al[%]/16+6B[%])/3.43N[%] ⁇ 4.1, and wherein the composition of Nb, Al, and C satisfies the relationship: 0.7 ⁇ (Nb[%]+Al[%]/20)/7.75C[%] ⁇ 3.5; rough rolling the reheated steel slab; finish rolling the rough rolled steel slab at a finish rolling temperature of 880° C. or more, followed by coiling the hot rolled steel sheet; cold rolling the coiled steel sheet at a reduction ratio of 65% or more; and continuously annealing the cold rolled steel sheet at a temperature of 780 ⁇ 860°
• the steel sheets for deep drawing according to the present invention exhibit excellent secondary work embrittlement, fatigue properties of welded joints, and an appealing plated surface as well as excellent formability compared with the conventional high strength steel sheets for deep drawing.
• a high strength steel sheet according to the present invention has characteristics in that it is controlled in contents of Ti, Al, B and N, and in contents of Nb, Al and C combinationally, while increasing the content of Al, which is advantageous in terms of formability and plating properties, and reducing the content of Ti, which is disadvantageous in terms of the plating properties, and the like, thereby exhibiting excellent properties in terms of secondary work embrittlement resistance, fatigue properties of welded joints and plating properties as well as formability.
• the steel sheet according to the present invention will be described in terms of composition and manufacturing method hereinafter.
• C acts as an interstitial solid solution element in steel, and obstructs formation of ⁇ 111 ⁇ texture, which is advantageous in terms of workability in the course of forming the texture in a steel sheet upon cold rolling and annealing.
• carbon content exceeds 0.010%, it is necessary to increase the contents of Ti and Nb which are carbide and nitride formation elements, causing a disadvantage in terms of manufacturing costs.
• the carbon content is preferably 0.010% or less.
• Si is an element which causes a defect of surface scale. If silicon content exceeds 0.02%, there arise problems such as temper color upon annealing, and non-plated parts upon plating. Thus, the silicon content is preferably 0.02% or less.
• Mn is a substitutional solid solution strengthening element for ensuring strength. If Mn content is less than 0.06%, the steel suffers from embrittlement due to S in the steel, whereas, if the Mn content exceeds 1.5%, an r-value of the steel is rapidly deteriorated along with elongation. Thus, the Mn content is preferably in the range of 0.06 ⁇ 1.5%.
• P is also a representative solid solution strengthening element which is added to the steel along with Mn for increasing the strength.
• P is added to Ti—Nb added steel as in the steel of the present invention, it results in growth of the ⁇ 111 ⁇ texture, advantageous in terms of the r-value, through grain refinement, grain boundary segregation, and the like.
• P content exceeds 0.15%, the steel suffers from rapid reduction in elongation along with significant increase in brittleness.
• the P content is preferably in the range of 0.15% or less.
• S content in the steel is generally restricted to a low degree of 0.005% or less. According to the present invention, however, since the steel contains Mn, all amounts of S in the steel are precipitated as MnS, thereby enabling deterioration of formability due to solid solution S to be avoided.
• S content is preferably 0.020% or less, which deviates from a region causing edge cracks during rolling.
• Sol. Al content of the steel is generally controlled to be in the range of 0.02 ⁇ 0.07% while dissolved oxygen in the steel is maintained in a sufficiently low state in consideration of manufacturing costs.
• Sol. Al serves to allow deep drawability to be stably secured at a lower annealing temperature.
• Sol. Al diffuses to the surface of the steel along the grain boundaries, and makes a plated layer dense, thereby enhancing powdering resistance of the steel.
• the Sol. Al content is 0.10% or more in the steel, it coarsens the precipitates in the steel, remarkably obstructs effect of suppressing recrystallization by P, thereby activating the recrystallization, and aids in development of the ⁇ 111 ⁇ texture and enhancement of the powdering resistance. If the Sol. Al content exceeds 0.40%, it causes an increase of the costs, and deterioration in efficiency of continuous casting operation.
• the Sol. Al content is preferably in the range of 0.10 ⁇ 0.40%.
• the Sol. Al content influences formation of Ti or Nb-based precipitates as the carbide and nitride such that the precipitates become coarsened, it serves as a critical component, which provides further enhanced formability of the steel with small added amounts of Ti and Nb in comparison to the conventional IF steel.
• N generally exists in a solid solution state, and deteriorates the formability of the steel. If N content exceeds 0.010%, it is necessary to increase added amounts of Ti and Nb for fixing N as precipitates. Thus, the N content is preferably 0.010% or less.
• Ti is a very important element in terms of the formability. In order to provide effect of enhancing the formability (in particular, r-value), Ti must be added to the steel in an amount of 0.003% or more. However, if Ti content exceeds 0.010%, it is disadvantageous in terms of manufacturing costs and plating properties in galvannealing. Thus, the Ti content is preferably in the range of 0.003 ⁇ 0.010%.
• Nb is also a very important element in terms of the formability like Ti.
• Nb In order to provide the effect of enhancing the formability (in particular, r-value), Nb must be added to the steel in an amount of 0.003% or more.
• Nb content exceeds 0.040%, it is disadvantageous in terms of the manufacturing costs and the plating properties.
• the Nb content is preferably in the range of 0.003 ⁇ 0.040%.
• B is a grain boundary strengthening element, and effective to enhance fatigue properties of spot welded joints while preventing grain boundary embrittlement by P. If B content is less than 0.0002%, the steel fails to achieve the effect described above, whereas, if the B content exceeds 0.0020%, there arise problems of rapid reduction in the formability, and deterioration in surface properties of plated steel sheet. Thus, the B content is preferably 0.0002 ⁇ 0.0020%.
• the steel sheet comprises the balance of Fe and other unavoidable impurities in addition to the above components.
• the steel sheet of the present invention may further comprise Mo in order to further enhance the secondary work embrittlement resistance and the plating properties.
• Mo content is preferably 0.05% or less. The reason is that, if the Mo content exceeds 0.05%, the effect of enhancing the secondary work embrittlement resistance and the plating properties by the Mo content is significantly reduced, and it is disadvantageous in terms of the manufacturing costs.
• the present invention in order to simultaneously secure the formability, plating properties, secondary work embrittlement resistance and fatigue properties of the steel which has the composition with a low Ti content and a high Al content as described above, it is necessary to control the contents of Al, B and N in combination to addition of Ti as in the following Expression 1.
• the present invention since the Ti content is low in comparison to the conventional steel sheet, there is a high possibility of deterioration of formability.
• the present invention suggests the following Expression 1:
• a calculated value of the expression is less than 1.0, the steel suffers from not only the aging phenomenon and the deterioration in drawability, but also failure in ensuring the secondary work embrittlement resistance and the fatigue properties.
• the calculated value exceeds 4.1, the steel suffers from deterioration in the plating properties and the stretching properties. Accordingly, it is preferably to control the content of Ti, Al, B and N so as to satisfy the relationship of 1.0 ⁇ (Ti[%]+Al[%]/16+6B[%])/3.43N[%] ⁇ 4.1
• the present invention in order to ensure the deep drawability and the stretching properties more stably, it is necessary to control the contents of the components so as to satisfy the following Expression 2. Specifically, due to the low Ti content of the steel according to the present invention, it is necessary to further ensure the deep drawability and the stretching properties. To this end, the present invention controls the contents of Nb, Al and C in combination according to the following Expression 2:
• Nb—Ti—Al—N—C based composite precipitates are formed.
• an average size of Nb—Ti—Al—N—C based composite precipitates is controlled to be 40 ⁇ or more, it is more preferable since it can further enhance the formability of the steel sheet.
• the formability and the plating properties can be further enhanced by restricting a fraction of Ti 4 C 2 S 2 to be 50% or more and a fraction of TiC to be below 5% among the Nb—Ti—Al—N—C based precipitates.
• Ti 4 C 2 S 2 is a precipitate advantageous in terms of the formability and the plating properties desired to be obtained by the present invention, if the fraction of Ti 4 C 2 S 2 is controlled to be 50% or more, it is possible to secure further enhanced formability and plating properties.
• TiC is a precipitates disadvantageous in terms of the plating properties, if the fraction of TiC is restricted to be below 5%, it is possible to secure further enhanced plating properties.
• the control of the composite precipitates as described above is closely related to a ratio of a reduction amount of rough rolling to a reduction amount of finish rolling (also hereinafter referred to as a reduction amount ratio) in hot rolling when manufacturing of the steel sheet according to the present invention, which will be described below.
• the steel sheet can be produced to have a desired tensile strength by controlling the components to satisfy the above composition and the following Expression 3:
• the present invention it is possible to control the contents of the components such that a calculated value of 27.6+4.81Mn[%]+90.7P[%]+132Nb[%]+30Mo[%]+180B[%] is in the range of 28 ⁇ 50.
• This expression is a regression expression of tensile strength according to the present invention, which expresses an influential degree of each component to the tensile strength as a coefficient based on experience.
• a steel slab having the composition as described above is reheated to a temperature of 1,100 ⁇ 1,250° C. If the reheating temperature is less than 1,100° C., it is difficult to perform hot rolling, whereas, if the reheating temperature exceeds 1,250° C., surface defects can be created.
• a finish rolling temperature is preferably controlled to be 880° C. or more. The reason is that, if the finish rolling temperature is less than 880° C., mixed grains are created, causing negative properties of products.
• a ratio of a reduction amount of rough rolling to a reduction amount of finish rolling that is, a reduction amount ratio is suitably controlled during the hot rolling.
• the reduction amount ratio is preferably controlled in the range of 1.0 ⁇ 3.5.
• the reason is that, if the reduction amount ratio is less than 1.0, the reduction amount of the finish rolling is significantly increased, causing an increase of load while making it difficult to control the fraction of Ti 4 C 2 S 2 among the precipitates to be 50% or more and to control the fraction of TiC to be below 5%.
• the reduction amount ratio exceeds 3.5, the effect of improving the r-value is negligible. Controlling of the reduction amount ratio will be described in detail hereinafter.
• Ti, Nb and the like react with impurity solid solution elements, and form precipitates, size and distribution of which significantly influence the formability of the final cold rolled products.
• the precipitates mainly having a size of several hundreds of or more are uniformly distributed instead of ultra fine precipitates having a size of several dozens of or less in a state wherein all the impurity elements such as C, N, S and the like in the hot rolled steel sheet are fixed as the precipitates, the r-value of the cold rolled steel sheet as the final product is remarkably improved.
• the size and distribution of the precipitates in the ultra-low carbon steel significantly depend on a hot rolling temperature and the reduction amount. Since formation of the precipitates is promoted via dynamic precipitation during the rolling process, an increase of the reduction amount in the temperature region of most actively enabling the precipitation results in easy formation of the precipitates.
• the reduction amount of the finish rolling is increased, it is advantageous in formation of the precipitates.
• the precipitates mainly have the size of several hundreds of or more so that an average size of Nb—Ti—Al—N—C based composite precipitates in the steel becomes 40 or more.
• the increase in reduction amount of the finish rolling can cause an increase in fraction of Ti 4 C 2 S 2 which is advantageous in terms of the formability and the plating properties, and a decrease in fraction of TiC, which is disadvantageous in terms of the plating properties.
• the reduction amount ratio is restricted due to the following reasons: increasing the reduction amount of the finish rolling serves not only to allow the precipitates mainly having the size of several hundreds of or more to be distributed in the steel sheet without forming the solid solution elements therein, but also to increase the fraction of the precipitate, which is advantageous in terms of the formability and the plating properties, while decreasing the fraction of the precipitate, which is disadvantageous in terms of the plating properties, thereby improving the r-value and the plating properties of the final product.
• the coil hot-rolled steel sheet is subjected to cold rolling and continuous annealing.
• a reduction ratio of the cold rolling is preferably restricted to be 65% or more since the reduction ratio below 65% makes it difficult to obtain a high r-value of 1.9 or more.
• the continuous annealing is preferably performed at a temperature of 780 ⁇ 860° C.
• an annealing temperature less than 780° C. makes it difficult to obtain a high r-value of 1.9 or more, and an annealing temperature above 860° C. provides a high possibility of causing problems to threading of strips during the operation due to high temperature annealing. Since the continuous annealing temperature of the present invention is significantly lower than a temperature region (880 ⁇ 930° C.) used by the conventional method for manufacturing the steel sheet for deep drawing, it is advantageous in manufacturing cost, and provides excellent producibility.
• the cold rolled steel sheet produced as above can be subjected to a typical plating process, if necessary.
• the plating process may be, for example, galvanizing, galvannealing, and the like.
• the fatigue properties were evaluated under a condition wherein, when applying a load repetitiously a total of ten million times to point welded samples with a cycle of 60 Hz, the samples did not fail.
• the powdering resistance was evaluated according to a detached ratio of a plated layer due to cupping, which was calculated in terms of a weight ratio.
• Inventive steels 1 ⁇ 16 satisfying the conditions of the present invention exhibit excellent properties in terms of secondary work embrittlement resistance, fatigue properties, and plating properties (powdering resistance) as well as formability.
• Comparative steels 1 ⁇ 12 not satisfying the conditions of the present invention in terms of composition and relations between the components exhibit deteriorated properties in terms of secondary work embrittlement resistance, fatigue properties, and plating properties (powdering resistance) as well as formability compared with the inventive steels.
• Comparative steels 1, 4, 7 and 10 satisfying the composition according to the present invention while not satisfying the relations between the components elongation, r-value, secondary work embrittlement resistance and fatigue properties are lower than the inventive steels.

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• 2005
  • 2005-07-08 KR KR1020050061691A patent/KR100685030B1/ko active IP Right Grant
• 2006
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