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
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • 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
  • C21D8/0257—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 with diffusion of elements, e.g. decarburising, nitriding
• • 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
  • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
  • C21D3/02—Extraction of non-metals
  • C21D3/04—Decarburising
• • 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
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper

Definitions

• the present invention is related to a steel sheet suitable for enamelling, and to a method for the superficial decarburization of a steel sheet, as a preparation for enamelling the steel. State of the art.
• the carbon level of a steel sheet has an important influence on the results in terms of surface quality of an enamel layer applied on the surface of the sheet.
• a high carbon level at the steel surface may give rise to CO-gas bubble formation, which shows up as black spots and craters in the enamel surface.
• this carbon forms coarse cementite during hot rolling which cracks upon cold rolling. These cracks are capable of capturing hydrogen which enters the steel during the enamelling process.
• pressure will rise at the steel/enamel surface which gives rise to the so-called x fish-scale' deformation of the enamel.
• Document JP-A- 2282421 describes such a method, wherein a continuously cast and annealed non-aging steel sheet for enamelling is produced, characterized in that a continuous-cast steel slab containing C between 0.0025 and 0.0050wt ⁇ 6 # Si max . 0.03wt%, Mn between 0.1 and 0.6wt%, P between 0.005 and 0.03wt%, S between 0.005 and 0.03wt%, Al max. 0.01wt%, N max.
• JP-A-6116634 describes a similar method, but wherein the starting material has no vanadium and the initial C level is up to 0.015wt% and B is added instead of V for H-trapping.
• the prior art decarburized sheets have rather low formability as testified by the values of the Lankford coefficient (r m ) . These values do not exceed 1.8 which is a concern when deep-drawing is foreseen.
• figure 1 of JP6116634 shows that r m values between 1.6 and 1.8 are only achieved for a very narrow range of carbon level before decarburizing annealing. Below 0.0050wt%C and above 0.0150wt%C, the r m value is deteriorated. Aims of the invention
• the present invention aims to provide a partially decarburized steel sheet suitable for enamelling which do not suffer from the drawbacks of the above cited prior art.
• the invention is related to steel sheets and products and to a production method as disclosed in the appended claims.
• the invention is thus related to a rolled steel sheet suitable for enamelling, said sheet having a carbon profile, defined by a gradient in the C- level from a level C surface at at least one surface of the sheet, to a level C bu i k in the bulk of the sheet, C bu i k being higher than C surface , and with :
• the steel sheet of the invention has an r m value between 1.8 and 2.1.
• C surface is between 0.005wt% and 0.015wt%, or between 0 and 0.005wt%.
• C bu i k is between 0.02wt% and 0.08wt%, or between 0.025wt% and 0.08wt% or between 0.025wt% and 0.06wt%.
• the Al-level is between 0.02wt% and 0.06wt%.
• said depth is between 130 ⁇ and 200 ⁇ .
• the invention is equally related to an enamelled steel sheet consisting of a steel sheet according to any of the above paragraphs, provided with an enamel layer.
• the invention is further related to a steel product produced from a sheet according to the invention, and to an enamelled steel product consisting of a such a product, provided with an enamel layer.
• the invention is also related to a method for producing a rolled steel sheet for enamelling, comprising the steps of :
• said continuous annealing takes place at an anneal temperature between 760°C and 850°C, and during a decarburizing time between 45s and 300s.
• the anneal temperature is between 800°C and 850°C.
• the initial C-level is between 0.025wt% and 0.08wt% or between 0.025wt% and 0.06wt%.
• the initial Al-level is between 0.02wt% and 0.06wt%.
• the ratio pH 2 0/pH 2 is between 0.04 and 0.25.
• the method of the invention may further comprise an over-ageing step at a temperature between 350 °C and 450°C during a timespan between 100s and 500s.
• the method may further comprise a skinpass step with a reduction of between 0.3% and 1.5%.
• Figure 1 illustrates the carbon profile in a steel sheet according to the invention.
• Figure 2 illustrates an example of an annealing step usable in the method of the invention.
• the steel sheet of the invention has a C- profile, defined by a gradient in the C-level from a lower value C sur f aC e at the surface to a higher value C bu ik in the bulk.
• the sheet is obtainable by a method which includes a continuous decarburi zat ion step, as will be described further in this text.
• Figure 1 illustrates the carbon- distribution across the thickness of two sheets according to the invention, with a thickness of 0.7mm.
• Curve 10 illustrates a sheet which comprises a bulk portion 11, where the C-level Cbuik is substantially constant, and two surface portions 12 (one on each side of the sheet) , each surface portion exhibiting the C-profile.
• the surface level is defined as the minimum value of the C-profile, measured by a suitable measurement technique (e.g. Glow Discharge Optical Emission Spectroscopy (GD-OES) , which allows composition measurement as depth analysis) .
• a suitable measurement technique e.g. Glow Discharge Optical Emission Spectroscopy (GD-OES) , which allows composition measurement as depth analysis.
• the C-level at the surface is maximum 0.015wt%, whereas C uik is higher than zero and lower than or equal to 0.08wt%.
• Cbuik is higher than C sur face-
• C S urface is between 0.005wt% and 0.015wt%.
• Csurface is between 0 and 0.005wt%.
• Curve 10 is an example of a sheet where the de c a rbu r i z a t i on has not taken place over the entire thickness of the sheet.
• the level Cbuik is equal to the initial C-level applied in the production method (described further in more detail) .
• C u i k is then between 0.02wt% and 0.08wt%, or between 0.025wt% and 0.08wt, or between 0.025wt% and 0.06wt% or between 0.025wt% and 0.05wt%.
• Curve 13 illustrates the case where decarburization has continued until the middle plane of the sheet. In this case, C u i k is smaller than the initial C- level of the method, and the C-profile extends over each half-width of the sheet.
• the decarburized sheet according to the invention further comprises Al, Mn and possibly S, Cu and Ca.
• the oxygen level is to be kept lower than 0.01wt%.
• oxygen is not added deliberately to the composition, but is allowed only at impurity levels. Fish scaling resistance is ensured by the higher initial C-level, so no oxide formation is required for this purpose. This means that no special alloying elements such as V are included. Also, N is kept as low as possible.
• the Al-level in the sheet of the invention is between 0.012wt% and 0.07wt%, which is higher than the allowed Al-level in the prior art references cited above.
• Al needs to be limited to avoid deoxidation, so as to ensure the formation of the oxides that will work against fish-scaling.
• Al is mandatory for deoxidation and binding of free N to avoid the ageing of the mechanical properties.
• Adding Al at levels higher than 0.07wt% means an increase in cost of the process, and a deterioration of the enamelling quality.
• a more preferred range for the Al-level, related to more optimized conditions in terms of deoxidation and cost/enamelling quality is between 0.02wt% and 0.06wt%.
• Mn is present between 0.12wt% and 0.45wt%. This element is added to control the strength properties of the steel and to avoid the formation of free sulphur.
• Copper, Sulphur and Calcium may optionally be added above the impurity level, more precisely in the ranges 0.025wt% to 0.1wt%, 0.008wt% to 0.04wt% and 0.0005wt% to 0.005wt% respectively. These elements improve the enamelling quality.
• the balance of the composition of the steel sheet according to the invention consists of Fe and incidental impurities.
• the following elements may be present as impurities at levels which are preferably lower than the values (in wt%) given in table 1 :
• the depth of the C-profile being defined as the depth where the C- level reaches (C bu i k + C surface )/2, is higher than 75 ⁇ , to ensure good enamelling capability. According to an embodiment, said depth is between 130 ⁇ and 200 ⁇ .
• Steel sheets according to the invention i.e. with a C-level at the surface between 0 and 0.015wt% are suitable for 2C/1F enamelling, i.e. enamelling by applying a ground coat enamel, followed by an outer enamel coating, both coatings being subjected to one firing step, and for 1C/1F enamelling, i.e. enamelling by applying one enamel layer subjected to one firing step.
• Steel sheets with low C-levels (i.e. 0.005wt% and less) at the surface may be suitable also for Direct White Enamelling (DWE) .
• DWE Direct White Enamelling
• the r m value of a steel sheet according to the invention is between 1.8 and 2.1. This means that the steel sheet has better formability than the prior art steel sheets referred to above.
• the x r' value refers to the plastic strain ratio (also known as the anisotropy factor) , being the ratio of the true strain in the width direction to the true strain in the thickness direction when a sheet material is pulled in uniaxial tension beyond its elastic limit.
• the ⁇ r m ' value is defined as 1 ⁇ 4(r 90 + 2*r 45 + r 0 ) , with r 90 , r 45 and r 0 the r- values as defined above, measured on samples oriented respectively at 90°, 45° and 0° with respect to the rolling direction.
• fish scaling resistance is ensured by the higher initial C- level applied in the method (see further) .
• the steel sheet of the invention can be produced by subjecting a steel slab with a specific initial steel composition to hot rolling, coiling and cold rolling, and by subjecting the cold-rolled sheet to continuous superficial decarburization .
• the initial composition is mainly characterized by a higher C-level compared to the prior art, and by a higher Al-level and a lower oxygen level. No deliberate addition of elements like V, Nb or B is done, while still allowing to produce enamelled steel sheets with a high fish scale resistance and good enamel surface quality.
• the initial C-level is between 0.02wt% and 0.08wt%, more preferably between 0.025wt% and 0.08wt%. This is higher than the initial C-levels disclosed in the prior art references referred to above.
• the method of the invention allows to obtain steel sheets with improved formability characteristics compared to the prior art.
• JP6116634 indicates that above 0.015wt% of initial carbon, it is not possible to obtain acceptable decarburization and good formability, the starting composition of the invention does not encounter these problems. Decarburization is possible down to an acceptable level, while formability is excellent.
• the initial C-level is lower than 0.02wt%, insufficient cementite formation occurs which deteriorates fish scale resistance.
• C-levels above 0.08wt% lead to too high strength levels and thus reduced formability.
• Specific ranges for the initial C-level, related to more optimized characteristics in terms of fish scale resistance and strength/formability are between 0.025wt% and 0.06wt% and between 0.025wt% and 0.05wt%.
• the initial steel composition according to the method of the invention further comprises Al, Mn and possibly 0, S, Cu and Ca in the same ranges as the decarburized sheet described above, the balance being Fe and the incidental impurities listed in Table 1.
• a more preferred range for the initial Al-level, related to more optimized conditions in terms of deoxidation and cost/enamelling quality is between 0.02wt% and 0.06wt%.
• oxygen is not added deliberately to the composition, but is allowed only at impurity levels.
• the method of the invention comprises standard steps of hot rolling and cold rolling a steel slab of the above composition.
• the slab is (re) heated at a temperature above 1050°C, subjected to hot rolling with a finishing temperature between 850°C and 950°C, and coiling at coiling temperature between 620°C and 770°C.
• cold rolling is performed with a reduction of minimum 50%.
• the final thickness of the cold rolled sheet is preferably between 0.2 and 2mm.
• the decarburi zat ion anneal is done in an annealing furnace for continuous annealing (i.e. annealing while the cold-rolled sheet moves through the furnace at a given speed, said speed determining the anneal time, i.e. the time spent at the annealing temperature) as known in the art, possibly provided with a vapour injection device for applying a given annealing atmosphere.
• Figure 2 shows an example of a lay-out of an annealing furnace usable in the method of the invention, starting with heating phase 1 wherein the temperature rises to the annealing temperature.
• Phase 2 represents the actual annealing (soaking) phase.
• Phase 3 is an overageing step.
• Phase 2 can consist of one or more periods with a different (constant or average) annealing temperature and a different annealing atmosphere in each period. Practically speaking, the different periods at different conditions can be obtained by dividing the annealing zone in subsections and by injecting 3 ⁇ 40 vapour into an atmosphere comprising 3 ⁇ 4, at various points along the annealing line (see example further in this description) .
• the superficial decarburization is done under a decarburizing atmosphere comprising water vapour and hydrogen gas, the remainder being essentially nitrogen gas, with the 3 ⁇ 4 content between lvoll and 95vol%, the 3 ⁇ 40 content between 0.04vol% and 33vol%, the ratio of partial pressures p3 ⁇ 40/pH 2 being between 0.04 and 0.5, more preferably between 0.04 and 0.25.
• a decarburizing atmosphere comprising water vapour and hydrogen gas, the remainder being essentially nitrogen gas, with the 3 ⁇ 4 content between lvoll and 95vol%, the 3 ⁇ 40 content between 0.04vol% and 33vol%, the ratio of partial pressures p3 ⁇ 40/pH 2 being between 0.04 and 0.5, more preferably between 0.04 and 0.25.
• the above composition describes the atmosphere at the start of the decarbur i z ing time. It is clear that during decarburization, the atmosphere composition will change, primarily due to the decarburization reaction taking place (formation of 3 ⁇ 4 and CO) .
• the total pressure under which the superficial decarburization anneal takes place may be atmospheric pressure, or a pressure different from atmospheric but within generally known boundaries applied in this type of annealing process.
• the decarburizing atmosphere can be prepared with a mixture of 3 ⁇ 4 and 2 with between 1,5 and 5% 3 ⁇ 4 in which 3 ⁇ 40 vapour is injected so that pH 2 0/pH 2 is between 0.04 and 0.5.
• the minimum value of this ratio ensures that sufficient 3 ⁇ 40 is present to obtain decarburization according to the formula C + H 2 O CO + 3 ⁇ 4 .
• the maximum of said range ensures that oxidation of Fe and of the furnace is avoided.
• a more preferred range for p3 ⁇ 40/pH 2 related to more optimized conditions in terms of sufficient decarburization and avoiding the occurrence of Fe-oxidation is between 0.04 and 0.25.
• the decarburizing atmosphere is applied during at least one of said periods with a different (constant or average) annealing temperature and a different annealing atmosphere in each period, preferably during the totality of phase 2.
• the ecarburi zing time' refers to the time spent under the conditions of the decarburizing atmosphere .
• the decarburizing time and the anneal temperature are chosen so as to obtain a steel sheet according to the invention. It is within the skilled person' s knowledge to find suitable combinations of decarburizing time and anneal temperature based on the examples given further in this description. According to a preferred embodiment, the decarburizing time is between 45s and 300s and the anneal temperature between 760°C and 850°C. When the ratio p3 ⁇ 40/pH 2 is lower than about 0.1, the decarburizing time is preferably higher than 70s. A more preferred range of the anneal temperature, applicable in combination with any decarburizing time between 45s and 300s is between 800°C and 850°C. The temperature is not necessarily constant during the decarburizing time.
• Fluctuations of the temperature may occur due to variations in the line speed for example.
• An over-ageing step may be applied at a temperature between 350°C and 450°C during a timespan between 100s and 500s.
• a skinpass may further be applied with a reduction of between 0.3% and 1.5%.
• Results from industrial trials performed by the applicant will be described hereafter, as well as a number of laboratory trials. All tested samples were produced from starting compositions according to the invention.
• the coiling temperature was 725 ° C .
• Two industrial trials were conducted.
• the thickness of the cold rolled sheet subjected to decarburization annealing in industrial trial 1 was 0.6mm; in the second industrial trial the thickness was 1mm.
• the continuous annealing line in which the industrial trials were conducted consists of a heating section, two soaking areas, a cooling and an overaging part.
• the annealing atmosphere consisted mainly of a mixture of 3 ⁇ 4 and 2 , with 3 ⁇ 40 vapour being injected in the first and/or the second soaking area.
• phase 2 as shown in figure 1 comprises a first period wherein the conditions of the present invention are not met, and a second period wherein these conditions are met. Such a process falls within the scope of the present invention .
• Table 2 summarizes the results after decarburization in terms of the C-level at the surface (i.e. minimum level of the C-profile, measured by GD-OES), the depth of the C-profile, and the quality of an enamel layer produced on the surface of the samples.
• Samples 25 to 35 yielded a bad enamelling aspect, which can be ascribed to either an insufficient depth of the C-profile (as determined by the depth where the C-level reaches (Csurface+Cbuik/2 ) , and/or a C level at the surface which is too high.
• the reason for these negative results can be ascribed to the test conditions, either the anneal temperature which is too low, the decarburizing time too short, or the p3 ⁇ 40/pH 2 ratio too low, or a combination of these factors.
• Table 4 shows the mechanical properties of a number of samples taken from the sheets of the industrial trials 1 and 2. Importantly, the formability in terms of the r m value is excellent, despite the initial C-level which is higher than in the prior art : r m is between 1.8 and 2.1. These results prove that the method of the invention allows to produce steel sheets suitable for enamelling, starting from an initial C-level higher than 0.02wt%, the resulting sheets allowing good enamelling quality and fish scale resistance, and having very good formability characteristics.
• the depth-values given in table 1 are values of the depth where the C-level reaches ( C sur f ace +Cbuik) /2.
• the depth measurements indicated with ⁇ *' show the maximum depth which could be measured with the applied equipment. The real value is thus higher than this value.
• Table 3 composition of samples (C-level in table 1, remaining elements are beneath impurity level, the remainder is Fe)

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