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 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
  • C21D8/0278—Modifying the physical properties 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 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
• • 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 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
  • C21D8/0247—Modifying the physical properties 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. [0003] Several documents are describing such UHSS products.
• 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 . This extra process step will not only increase the total processing cost, but it will also complicate logistics as well as weldability at the entrance of the continuous annealing or hot dip galvanising line.
• 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 lOmin. 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 650MPa.
• US-A-5470529 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. This enables to use lower Si- levels and thus avoid red scale formation.
• Al-range is set as 0.1-2.0wt% 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%. The following constraints also apply: Ni (wt%) >Cu(wt%) /3 when Cu>0.5wt% and Mn+Ni>0.5wt% . Cr may be added as well to stabilise the retained austenite and to further improve corrosion resistance.
• Nb and V might be added as well.
• Their upper limit is preferably 0.05wt% for Nb and Ti and 0.10wt% for V.
• Si-content in this invention is limited to ⁇ lwt% in order to avoid red scale formation, most of the cold rolled example steels have a Si-content in the range 0.5-l.lwt%. The latter is considered to give rise to hot dip galvanising difficulties (bad wettability by molten Zn) and a deteriorated surface appearance (bare spots) .
• 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%Ti, max. 30ppm B and 100-350ppm 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%.
• Lab-experiments performed by the inventors of the present invention have however shown that Al -additions as low as 0.6wt%, render the obtained mechanical properties very sensitive to process parameter variations such as line speed and overageing temperature. This can lead to a non-compatibility between different galvanising lines (with e.g. different lengths of the levelling zone around 490-460°C) or even to strongly thickness-dependent mechanical properties.
• the carbon content was limited to (in wt%) 0.15% and the manganese content to 1.5%.
• the steels studied was also a (in wt%) 0.25-0.45% Si, 1.5-2.0% Al and 0.05-0.10% P TRIP steel. No mechanical properties were mentioned in the article for the latter composition.
• the proposed chemical composition is insufficiently alloyed in carbon to reach tensile strengths in the range 700-850 MPa .
• the high Al -content requires the use of an adapted very fine casting powder that can give rise to health problems.
• the weldability can be impaired due to the presence of Al -oxides in the welded area, a consequence of the high Al-contents.
• 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 : - C : between 1300ppm and 2600ppm
• the novelty and inventive step of this compostion lies in the specific combination of elements P, Si, Al and C.
• 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 : - UTS (Ultimate Tensile strength) > 590MPa : carbon between 1300ppm and 1900ppm. Two particular embodiments are characterized by a carbon content of 1350ppm to
• 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: - soaking said substrate at a temperature between 760°C and 850°C,
• 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 carbon content between 1300ppm and 1900ppm.
• Said product has a yield strength between 320MPa and 480MPa, a tensile strength above 590MPa, an elongation A80 higher than 26% and a n- alue (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 2300ppm. Said product has a yield strength between 350MPa and 510MPa, a tensile strength above 700MPa, an elongation A80 higher than 24% and a n-value (calculated between 10% and uniform elongation) 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 2000ppm and 2600ppm.
• 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 2600ppm.
• Said product has a yield strength between 450MPa and 700MPa, a tensile strength above 980MPa, 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 40MPa 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 780MPa, or to more stringent requirements on weldability (maximum of C-range, see next paragraph) .
• C between 1300ppm and 2600ppm.
• a first preferred subrange is 1300-1900ppm.
• a second preferred subrange is 1700-2300ppm.
• a third preferred subrange is 2000-2600ppm.
• 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.
• composition A 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) .
• reference compositions B, C and D 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 600MPa.
• Mn between lOOOOppm and 22000ppm, preferably between 13000-22000ppm.
• 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 micrestructures .
• Al between 8000ppm and 15000ppm, preferably between 8000-14000ppm and most preferably between 9000-13000ppm. 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 15000ppm are however known to require the use of an adapted very fine casting powder that can cause health problems.
• Si between 2000ppm and 6000ppm, preferably between 2500-4500ppm. 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.
• 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 lOOOppm, preferably between 600-lOOOppm.
• 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 400ppm do not allow a sufficiently large reduction of C-content.
• S maximum 120ppm. The S-content has to be limited because a too high inclusion level can deteriorate the formability.
• N maximum 200ppm, preferably maximum 150ppm otherwise too much AlN and/or TiN precipitates can form which are detrimental to formability.
• Ti maximum lOOOppm, preferably below 200ppm for products produced according to the present invention having a tensile strength below 980MPa. 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 980MPa, 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 lOOOppm, preferably below lOOppm for products produced according to the present invention having a tensile strength below 980MPa.
• Niobium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening.
• tensile strengths below 980MPa 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 lOOOppm, preferably below lOOppm for products produced according to the present invention having a tensile strength below 980MPa. Vanadium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening.
• B maximum lOppm, preferably maximum 5ppm.
• Stepwise cooling may be used as well, hot rolling mill coiling of said substrate at a coiling temperature CT comprised between 500°C and 680°C, preferably between 600°C and 680°C.
• This temperature range is chosen so as to create a hot band which is as soft as possible in order to facilitate cold rolling, pickling the substrate to remove the oxides, cold rolling to obtain a reduction of thickness.
• the cold rolling reduction is preferably higher than 40%.
• these steps are followed by an annealing treatment in a continuous annealing line, comprising the following steps: soaking said pickled cold rolled substrate in a temperature range between 760 and 850°C in order to create a microstructure that consists of ferrite and austenite. If the soaking temperature is chosen above 850°C, the amount of austenite formed will be too large, which leads to a less stable retained austenite in the end product. Due to the reduced austenite stability, a substantial part of it could also transform to martensite during final cooling to room temperature which deteriorates elongation properties. If, on the other hand, the soaking temperature would be chosen too low, insufficient austenite would be formed during soaking. This could lead to over-stabilisation of the retained austenite, which again deteriorates mechanical properties .
• the holding time in said temperature range is less than 700 seconds.
• the holding temperature would be chosen below 360°C, a substantial part of the retained austenite will transform to martensite, leading to a DP-like behaviour (high initial n-value that decreases as a function of increasing strain) of the final product.
• Holding temperatures above 450°C will on the other hand lead to a decomposition of the retained austenite by carbon precipitation. This will again deteriorate elongation properties .
• 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 substrate can be subjected to a skinpass reduction which is preferably in the range 0.3% to 1.5%.
• the thickness of the steel substrates of the invention after cold rolling can be lower than 1mm 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 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: 8000-14000ppm and most preferably in the range 9000- 13000ppm. Adding less Al will render the retained austenite less stable.
• - Si is furthermore limited in order to avoid the need of hot charging the slabs to prevent crack formation.
• 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. 1 . 1 Cold rolled and continuously annealed product [0055] The processing steps were:
• Table 7 contains the mechanical properties obtained after applying several continuous annealing simulations on steel samples of compositions E and F. Looking at the data in table 5 and 7 (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 600ppm more carbon and no intentionally added phosphor.
• Table 3 Mechanical properties of the cold rolled and continuously annealed P-alloyed Al-Si TRIP steel, composition A, according to the present invention. No temper rolling applied. Thickness 1mm.
• Table 5 Mechanical properties of the cold rolled and continuously annealed reference Al-Si TRIP steels. (RD: rolling direction; TD: transverse direction)
• Table 6 Mechanical properties of the hot dip galvanised reference Al-Si TRIP steels. (RD: rolling direction; TD : transverse direction)
• Table 7 Mechanical properties of the cold rolled and continuously annealed P-alloyed Al-Si TRIP steel, compositions E and F, according to the present invention. No temper rolling applied. Thickness 1mm.

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