Classifications

• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0278—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment 
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • 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
• • 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
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing

Definitions

• the present invention is related to a steel composition comprising phosphor, to be used for the production of TRIP steel products.
• the invention is equally related to the process of production of said products, and to the end products themselves.
• Ultra high strength steel (UHSS) sheet products and in particular TRIP steel products showing a remarkable combination of high strength and good formability, can provide the solution for this problem. Additionally, an increased corrosion resistance of these steel sheet products by means of electro or hot dip galvanising, is frequently asked for.
• EP-A-1096029 is related to the production of a tempered martensite TRIP steel, whose chemical composition is silicon-manganese based and contains (in wt %) 0.05-0.20% C, 0.3-1.8% Si and 1.0-3.0% Mn as well as one or more of the following additions (in wt %): 0.05-1% Cr+Mo, ⁇ 0.003% B, 0.01-0.1% Ti+Nb+V and ⁇ 0.01% Ca+REM.
• the cold rolling production process consists of three consecutive annealing steps.
• the sheet is completely austenised during at least 5 seconds and subsequently rapidly cooled (>10° C./s) below the Ms (Martensite start) temperature in order to produce lath martensite.
• the second and third step are combined in a continuous annealing or galvanising line and consist of reheating the sheet in the intercritical region (Acl ⁇ T ⁇ Ac3) during 5 to 120 seconds, cooling (>5° C./s) to 500° C. or lower and than subjecting the sheet to a galvanising or galvannealing treatment.
• the first one being the additional annealing step that is required to produce the lath martensite starting micro-structure.
• EP-A-0922782 also describes the production of a cold rolled Si—Mn based TRIP steel which contains (in wt %) 0.05-0.40% C, 1.0-3.0% Si, 0.6-3.0% Mn, 0.02-1.5% Cr, 0.01-0.20% P and 0.01-0.3% Al.
• this product does not require the use of an additional annealing step.
• Cr is added to the analysis in order to retard the bainite formation and promote acicular ferrite and martensite formation as it is thought by the inventors that bainite is detrimental to the crushing behaviour in Si—Mn based TRIP steels.
• P is added to avoid pearlite formation and to increase the strength of the ferritic phase.
• the maximal P content is limited to 0.2% because of weldability.
• the high Si content in this invention will however again impair hot dip galvanisability resulting in an insufficient surface appearance and a very high risk on bare spots.
• the occurrence of red scale which is difficult to remove, on the hot strip, due to the higher Si content, is also expected to cause processing difficulties.
• EP-A-0796928 describes the production of an Al-based Dual Phase steel which contains (in wt %) 0.05-0.3% C, 0.8-3.0% Mn, 0.4-2.5% Al and 0.01-0.2% Si. Additionally the steel can contain one of the following elements (in wt %) ⁇ 0.05% Ti, ⁇ 0.8% Cr, ⁇ 0.5% Mo, ⁇ 0.5% Ni, ⁇ 0.05% Nb and ⁇ 0.08% P. After cold rolling with a reduction rate higher than 40%, the material is intercritically annealed at temperatures between 740 and 850° C. and subsequently cooled at a cooling rate of 10 to 50 K/s to the Zn-bath temperature.
• EP-A-1170391 describes the production of a low carbon ( ⁇ 0.08wt %), low silicon ( ⁇ 0.5wt %) and low aluminium ( ⁇ 0.3wt %) TRIP steel obtained by adding a nitriding step to the processing (0.03-2wt % N).
• the Al and Si contents have to be kept low in order to avoid nitride precipitation and thus loss of free N.
• the Si content is preferably lower than 0.2wt % because of hot dip galvanisability.
• the carbon content is kept very low because of weldability and because of the fact that the presence of nitrogen in the steel also stabilises the retained austenite.
• This nitrogen is incorporated in the steel sheet either during or immediately after hot finish rolling, during recrystallisation annealing, during intercritical annealing or via a combination of one or more of these processes. All of them require the steel sheet to be held for 2sec to 10 min. in an atmosphere containing not less than 2% ammonia in the temperature range 550-800° C. It is clear that this nitriding step makes processing a lot more difficult and requires complicated technical modifications to existing installations. At the moment this process is internationally not considered to be industrially feasible. Furthermore the very low alloying content of this steel grade, does not allow to reach tensile strength levels above 650 MPa.
• U.S. Pat. No. 5,470,529 deals with the production of cold rolled TRIP steels based upon a wide variety of combined Al—Si analyses.
• the carbon content range is set as 0.05-0.3wt %, but more preferably is 0.1-0.2wt %.
• the Si-content is kept below 1.0wt % in order to avoid red scale formation, but more preferably is in the range 0.2-0.9wt %.
• Manganese is added in 0.005 to 4.0wt %, but more preferably 0.5-2.0wt %.
• part of the Si is replaced by Al for various reasons.
• Al also avoids cementite precipitation during bainitic holding.
• the Al-range is set as 0.1-20.wt % and more preferably as 0.5-1.5wt %.
• Al and Si are both ferrite stabilizers, their sum is limited in order to avoid over-stabilizing the retained austenite.
• the Al+Si content should be in the range 0.5-3.0wt % and more preferably in the range 1.5-2.5wt %.
• P is considered as an incidental impurity that should be limited as much as possible.
• the P-limit is set at 0.1wt % or less and preferably less than 0.02wt %.
• Cu is added to the analysis to facilitate the removal of red scale, to improve the corrosion resistance of the as cold rolled product and to improve the wettability by molten Zn. Therefore the Cu-range is 0.1-2.0wt % and more preferably 0.1-0.6wt %.
• Ni is added as well. For economics its content is limited to 1.0wt % and preferably 0.5wt %.
• Cr may be added as well to stabilise the retained austenite and to further improve corrosion resistance. It is added in the range 0.5-5.0wt % and more preferably in the range 0.6-1.6wt %.
• Nb and V might be added as well. Their upper limit is preferably 0.05wt % for Nb and Ti and 0.10wt % for V.
• the maximum Si-content in this invention is limited to ⁇ 1wt % in order to avoid red scale formation
• most of the cold rolled example steels have a Si-content in the range 0.5-1.1wt %. The latter is considered to give rise to hot dip galvanising difficulties (bad wettability by molten Zn) and a deteriorated surface appearance (bare spots). None of these example steels contained micro-alloying additions as in high Si-TRIP steels, these are known to markedly increase the hot strip hardness, leading to strongly increased cold rolling forces.
• the low Si (0.2-0.4wt %) example steels showed a high yield stress (570-590 MPa) and only moderate ultimate tensile strength ( ⁇ 700 MPa) and total elongation values (A50 ⁇ 30%). In the latter steels no P was added additionally.
• a large disadvantage of these compositions is the necessity of adding Cu and Ni, elements which are considered as impurities in bulk flat carbon steel production. If a steelmaking plant has to cast this, extra logistic problems with scrap recycling occur. Moreover, the use of Ni, Cu and Cr makes the alloying cost much more expensive.
• EP-A-1154028 describes the manufacturing of a P-alloyed low-Al, low-Si TRIP steel, which contains (in wt %) : 0.06-0.17% C, 1.35-1.80% Mn, 0.35-0.50% Si, 0.02-0.12% P, 0.05-0.50% Al, max. 0.07% Nb, max. 0.2% V, max. 0.05%T i, max. 30 ppm B and 100-350 ppm N.
• the carbide forming elements Ti, Nb or V are added, the carbon content is preferably 0.16wt %.
• the amount of residual austenite is limited to a maximum of 10%.
• low-Al steel 0.19% C, 1.5% Mn, 0.26% Si, 0.086% P and 0.52% Al
• high-Al steel 0.17% C, 1.46% Mn, 0.26% Si, 0.097% P and 1.81% Al.
• the low-Al steel will suffer from mechanical properties that are very sensitive to process parameter variations such as line speed and overageing temperature. This can lead to a non-compatibility between different galvanising lines or even to strongly thickness-dependent mechanical properties.
• the high-Al steel on the other hand again requires the use of an adapted casting powder that can give rise to health problems. Furthermore the weldability will be impaired due to the presence of Al-oxides in the welded area.
• the present invention is related to a cold rolled Al—Si P-alloyed TRIP steel composition intended to be used as uncoated, electro-galvanised or hot dip galvanised material.
• Said composition is characterised by the following contents :
• the novelty and inventive step of this composition lies in the specific combination of elements P, Si, Al and C.
• adding P in excess of prior art levels, whilst limiting the maximum Si- and Al-content, allows to decrease the C-content for reaching a certain strength level, in combination with better weldability.
• Three specific embodiments are related to the same chemical composition, but having three different subranges for carbon which are related to the strength level that is aimed at:
• the present invention is equally related to a process for manufacturing a cold rolled TRIP steel product, comprising the steps of:
• the process of the invention further comprises the steps of:
• the process of the invention further comprises an electrolytic zinc coating step.
• the process of the invention further comprises the following processing steps after the cold rolling step:
• the process comprising a hot dip galvanising step may further comprise the step of subjecting said substrate to a skinpass reduction of maximum 1.5%.
• the invention is equally related to a steel product produced according to the process of the invention and having a microstructure comprising 30-75% ferrite, 10-40% bainite, 0-20% retained austenite and possibly 0-10% martensite.
• the invention is equally related to a steel product produced according to the process of the invention and having a carbon content between 1300 ppm and 1900 ppm.
• Said product has a yield strength between 320 MPa and 480 MPa, a tensile strength above 590 MPa, an elongation A80 higher than 26% and a n-value (this is the strain hardening coefficient, calculated between 10% and uniform elongation) higher than 0.2.
• the invention is further related to a steel product produced according to the process of the invention and having a carbon content between 1700 and 2300 ppm.
• Said product has a yield strength between 350 MPa and 510 MPa, a tensile strength above 700 MPa, an elongation A80 higher than 24% and a n-value (calculated between 10% and uniform longation) higher than 0.19.
• the invention is further related to a steel product produced according to the process of the invention and having a carbon content between 2000 ppm and 2600 ppm.
• Said product has a yield strength between 400 MPa and 600 MPa, a tensile strength above 780 MPa, an elongation A80 higher than 22% and a n-value (calculated between 10% and uniform elongation) higher than 0.18.
• the invention is also related to a steel product produced according to the process of the invention and having a carbon content between 2000 and 2600 ppm.
• Said product has a yield strength between 450 MPa and 700 MPa, a tensile strength above 980 MPa, an elongation A80 higher than 18% and a n-value (calculated between 10% and uniform elongation) higher than 0.14.
• a steel product according to the invention may have a bake hardening BH2 higher than 40 MPa in both longitudinal and transversal directions.
• a steel composition is proposed for the production of a P-alloyed Al—Si TRIP steel product.
• Application of the broadest chemical composition ranges which are indicated, will be able, in combination with the right process parameters, to result in products having a desired TRIP microstructure, good weldability as well as excellent mechanical properties, with very high values of the product of tensile strength and total elongation (this value is characteristic for a high energy absorption potential in case of a crash).
• the preferred ranges are related to more narrow ranges of mechanical properties, for example a guaranteed minimum tensile strength of 780 MPa, or to more stringent requirements on weldability (maximum of C-range, see next paragraph).
• a first preferred subrange is 1300-1900ppm.
• a second preferred subrange is 1700-2300 ppm.
• a third preferred subrange is 2000-2600 ppm.
• the minimum carbon content per sub-range is needed in order to ensure the strength level as carbon is the most important element for the hardenability.
• the maximum of the claimed range per sub-range is related to weldability.
• the effect of carbon on mechanical properties is illustrated by exemplary composition A, E and F and reference compositions B, C and D (tables 1, 3-8).
• the effect of carbon content on spot weldability is illustrated by reference compositions B, C and D (table 2).
• Two specific subranges for carbon are characteristic for two specific embodiments: 1350-1900ppm and 1400-1900ppm. These subranges are aimed at ensuring an Ultimate Tensile Strength of at least 600 MPa.
• Mn between 10000 ppm and 22000 ppm, preferably between 13000-22000 ppm.
• Manganese acts as an austenite stabiliser and thus decreases the Ms temperature of the retained austenite. Furthermore Mn suppresses pearlite formation and also contributes to the overall strength level of the steel by solid solution hardening. Adding excess Mn results on the other hand in insufficient ferrite formation upon cooling from the soaking temperature and thus to insufficient carbon concentration in the retained austenite, rendering the latter less stable. Too much Mn will also increase the hardness of the weld and will enhance the formation of detrimental banded microstructures.
• Al between 8000 ppm and 15000 ppm, preferably between 8000-14000 ppm and most preferably between 9000-13000 ppm. Aluminium is added because, to an even stronger degree than Si, it is a ferrite stabiliser and thus enhances the ferrite formation during soaking and during cooling from the soaking temperature, thereby stabilising the retained austenite. The latter is stabilised even more by the fact that Al also suppresses the precipitation of carbon from the retained austenite during the overageing stage. Unlike Si, Al has no detrimental effect on galvanisability. Al-contents above 15000 ppm are however known to require the use of an adapted very fine casting powder that can cause health problems.
• a minimum Al content is however required to allow the material to be processed on different hot dip galvanising lines with different lengths of the levelling zones and to ensure a high process robustness.
• Si between 2000 ppm and 6000 ppm, preferably between 2500-4500 ppm. Silicon has essentially the same function as Al, albeit slightly less pronounced. That is: Si is a ferrite stabiliser and prevents carbide precipitation during the overageing stage, thereby stabilising the retained austenite at room temperature. Besides this, Si also contributes to the overall strength level of the steel by solid solution hardening. The maximum Si-content is however limited as Si is well known to provoke problems as to surface quality because of the presence of Si-oxides which after pickling create a surface with irregular and very high roughness. Moreover, in view of corrosion protection, hot dip galvanising of high Si-containing substrates in general leads to insufficient surface appearance for automotive applications, with moreover a high risk on the presence of bare spots on the surface.
• P between 400ppm and 1000 ppm, preferably between 600-1000 ppm.
• Phosphorous is added primarily to allow the carbon content to be decreased to obtain improved weldability, while maintaining the same tensile strength level.
• P in combination with Si is known to enhance the retained austenite stability by suppressing carbide precipitation during the overageing stage.
• P additions below 400 ppm do not allow a sufficiently large reduction of C-content.
• S maximum 120 ppm.
• the S-content has to be limited because a too high inclusion level can deteriorate the formability.
• N maximum 200 ppm, preferably maximum 150ppm otherwise too much AlN and/or TiN precipitates can form which are detrimental to formability.
• Ti maximum 1000 ppm, preferably below 200 ppm for products produced according to the present invention having a tensile strength below 980 MPa. Titanium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening. However for tensile strengths below 980 MPa, even without adding Ti, using the appropriate processing parameters, will result in the targeted mechanical properties per carbon sub-range and thus avoid an increase in analysis cost or extra processing difficulties (e.g. rolling forces).
• Nb maximum 1000 ppm, preferably below 100 ppm for products produced according to the present invention having a tensile strength below 980 MPa.
• Niobium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening.
• tensile strengths below 980 MPa even without adding Nb, using the appropriate processing parameters, will result in the targeted mechanical properties per carbon sub-range and thus avoid an increase in analysis cost or extra processing difficulties (e.g. rolling forces).
• V maximum 1000 ppm, preferably below 100 ppm for products produced according to the present invention having a tensile strength below 980 MPa. Vanadium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening. However for tensile strengths below 980 MPa, even without adding V, using the appropriate processing parameters, will result in the targeted mechanical properties per carbon sub-range and thus avoid an increase in analysis cost.
• B maximum 10 ppm, preferably maximum 5 ppm. Boron is avoided because of its detrimental influence on ferrite nucleation.
• the present invention is equally related to the process for producing said steel product. This process comprises the steps of:
• these steps are followed by an annealing treatment in a continuous annealing line, comprising the following steps:
• a second preferred embodiment comprises the same processing steps mentioned above, but additionally also comprises an electrolytic zinc coating step.
• the cold rolling step is followed by an annealing treatment in a continuous hot dip galvanising line, comprising the following steps:
• the thickness of the steel substrates of the invention after cold rolling can be lower than 1 mm according to the initial hot rolled sheet thickness and the capability of the cold rolling mill to perform the cold rolling at a sufficiently high level. Thus, thicknesses between 0.3 and 2.5mm are feasible.
• the resulting cold rolled product has a multiphase structure with 30-75% ferrite, 10-40% bainite, 0-20% retained austenite and possibly amounts of martensite (0-10%) present at room temperature.
• the amount of martensite at room temperature should however be limited in order to maintain an n-value behaviour (constant or increasing with strain) and mechanical properties that are characteristic for TRIP-steels. Specific mechanical properties as a function of processing parameter values are given in the examples.
• the cold rolled non-temper rolled product showed in all cases a yield point elongation, which is typical for TRIP-steels and indicates that no or only very small amounts of martensite are present in the microstructure.
• This yield point elongation can be suppressed by temper rolling the final product. Small temper rolling reductions are sufficient to avoid the occurrence of a yield point elongation and temper rolling reductions above 1.5% should be avoided in order to prevent a too large yield strength increase.
• the final cold rolled product furthermore preferably exhibits a constant or increasing n-value with increasing strain. This behaviour implies that the retained austenite is gradually transformed into martensite as the tensile test progresses thereby postponing the occurrence of necking, leading to an excellent combination of tensile strength and total elongation.
• the robustness of TRIP steel products produced according to this invention is ensured by the minimum Al-content specified in the preferred Al-range: 8000-14000 ppm and most preferably in the range 9000-13000 ppm. Adding less Al will render the retained austenite less stable. This will increase the risk of loss of mechanical properties by austenite decomposition through carbon precipitation and on the other hand the less stable retained austenite will more easily transform into martensite during straining, limiting the formability of the material. Adding less Al will also retard the bainite transformation kinetics. As a consequence the mechanical properties will become more dependent on processing conditions such as line speed and overageing temperature as well on the actual line lay-out (short or long overageing section). Using an Al-content within the preferred range, avoids such line dependency and loss of robustness.
• Table 1 shows examples of compositions of laboratory castings of the P-alloyed Al—Si TRIP steel product according to the present invention (codes A, E and F), and of reference compositions (B,C and D) having either a C-content which is higher than the claimed range and/or no intentionally added phosphor.
• Laboratory thermal cycle simulations and tensile tests were performed to obtain the mechanical properties of the test specimens of these example compositions. It is to be noted that in what follows, all mentioned tensile test mechanical properties are measured according to the standard EN10002-1.
• Table 8 contains the mechanical properties obtained after applying several hot dip galvanising simulations on steel samples of compositions E and F. Looking at the data in table 6 and 8 (in particular E compared to B), it is clear that the tensile strength is even higher for the composition of the invention, as compared to the reference composition which has 600 ppm more carbon and no intentionally added phosphor. TABLE 1 Compositions (ppm) of Al—Si TRIP steels. Compositions A, E, F according to the invention, B, C and D are reference compositions.

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