Patent Application
20190112684
The present invention is related to a steel sheet suitable for enameling, and to a method for the superficial decarburization of a steel sheet, as a preparation for enameling the steel.
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. On the other hand, when a sufficiently high carbon level is present initially in the bulk, 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 enameling process. When hydrogen is insufficiently captured, pressure will rise at the steel/enamel surface which gives rise to the so-called ‘fish-scale’ deformation of the enamel.
It is therefore advantageous to decarburize the steel only in a layer at the surface of the steel, i.e. to conduct a superficial decarburization. Document JP-A-2282421 describes such a method, wherein a continuously cast and annealed non-aging steel sheet for enameling is produced, characterized in that a continuous-cast steel slab containing C between 0.0025 and 0.0050 wt %, Si max. 0.03 wt %, Mn between 0.1 and 0.6 wt %, P between 0.005 and 0.03 wt %, S between 0.005 and 0.03 wt %, Al max. 0.01 wt %, N max. 0.004 wt %, Cu between 0.01 and 0.06 wt %, 0 between 0.02 and 0.06 wt %, V between 0.01 and 0.06 wt %, the balance Fe and inevitable impurities, is hot-rolled with a finishing temperature higher than or equal to 800° C., and a coiling temperature of 600-800° C., cold-rolled with a reduction ratio higher than or equal to 60% and subjected to a decarburization annealing at 700-900° C. for 30 sec-3 min, carried out in a continuous annealing furnace having a decarburizing atmosphere composed of 1-20% water vapour, gaseous hydrogen in an amount higher than or equal to twice the water vapour amount, and the balance being mainly gaseous nitrogen, so as to reduce the C-level to be less than or equal to 0.002 wt %. JP-A-6116634 describes a similar method, but wherein the starting material has no vanadium and the initial C level is up to 0.015 wt % and B is added instead of V for H-trapping.
Both prior art methods result in fully or superficially decarburized steel sheets which are suitable for enameling by Direct White Enameling (DWE), wherein one white enamel coating is applied on the surface, followed by one firing step. As the initial carbon level is quite low, fish-scale resistance due to cementite formation is not achieved. To compensate for this, a steel with a high oxygen level and a limited Al-level is used in the prior art together with specific alloying elements such as B, V. Thanks to the limited Al-content, oxides of Si and Mn and nitrides of B and V are formed within the bulk of the steel which are beneficial against fish-scaling. However, this alloying involves extra costs and leads to some technical difficulties, e.g. involving casting with V. Furthermore, the prior art decarburized sheets have rather low formability as testified by the values of the Lankford coefficient (rm). These values do not exceed 1.8 which is a concern when deep-drawing is foreseen. In particular, FIG. 1 of JP6116634 shows that rm values between 1.6 and 1.8 are only achieved for a very narrow range of carbon level before decarburizing annealing. Below 0.0050 wt % C and above 0.0150 wt % C, the rm value is deteriorated.
The present invention aims to provide a partially decarburized steel sheet suitable for enameling 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 enameling, said sheet having a carbon profile, defined by a gradient in the carbon level (C-level) from a level Csurface at at least one surface of the sheet, to a level Cbulk in the bulk of the sheet, Cbulk being higher than Csurface, and with:
and optionally:
According to a preferred embodiment, the steel sheet of the invention has an rm value between 1.8 and 2.1.
According to specific embodiments, Csurface is between 0.005 wt % and 0.015 wt %, or between 0 and 0.005 wt %.
According to other specific embodiments, Cbulk is between 0.02 wt % and 0.08 wt %, or between 0.025 wt % and 0.08 wt % or between 0.025 wt % and 0.06 wt %.
According to another embodiment, the Al-level is between 0.02 wt % and 0.06 wt %.
According to a further embodiment, said depth is between 130 mm and 200 mm.
The invention is equally related to an enameled 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 enameled steel product consisting of a such a product, 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 enameled 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 enameling, comprising the steps of:
According to a preferred embodiment, said continuous annealing takes place at an anneal temperature between 760° C. and 850° C., and during a decarburizing time between 45 s and 300 s.
According to a specific embodiment, the anneal temperature is between 800° C. and 850° C.
According to specific embodiments of the method of the invention, the initial C-level is between 0.025 wt % and 0.08 wt % or between 0.025 wt % and 0.06 wt %.
According to a further embodiment, the initial Al-level is between 0.02 wt % and 0.06 wt %.
According to a specific embodiment, the ratio pH2O/pH2 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 100 s and 500 s. The method may further comprise a skinpass step with a reduction of between 0.3% and 1.5%.
The steel sheet of the invention has a C-profile, defined by a gradient in the C-level from a lower value Csurface at the surface to a higher value Cbulk in the bulk. The sheet is obtainable by a method which includes a continuous decarburization step, as will be described further in this text.
Curve 10 is an example of a sheet where the decarburization has not taken place over the entire thickness of the sheet. This means that the level Cbulk is equal to the initial C-level applied in the production method (described further in more detail). According to embodiments which correspond to embodiments of the method of the invention (see further), Cbulk is then between 0.02 wt % and 0.08 wt %, or between 0.025 wt % and 0.08 wt, or between 0.025 wt % and 0.06 wt % or between 0.025 wt % and 0.05 wt %. Curve 13 illustrates the case where decarburization has continued until the middle plane of the sheet. In this case, Cbulk 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. Contrary to the prior art references, the oxygen level is to be kept lower than 0.01 wt %. According to a preferred embodiment of the steel sheet of the invention, 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 0.012 wt % and 0.07 wt %, which is higher than the allowed Al-level in the prior art references cited above. In the cited prior art documents Al needs to be limited to avoid deoxidation, so as to ensure the formation of the oxides that will work against fish-scaling. In the method of the invention (see further), Al is mandatory for deoxidation and binding of free N to avoid the ageing of the mechanical properties. When the Al-level is lower than 0.012 wt %, insufficient deoxidation takes place, and binding of N is required through other means. Adding Al at levels higher than 0.07 wt % means an increase in cost of the process, and a deterioration of the enameling quality. A more preferred range for the Al-level, related to more optimized conditions in terms of deoxidation and cost/enameling quality is between 0.02 wt % and 0.06 wt %.
Mn is present between 0.12 wt % and 0.45 wt %. 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 0.025 wt % to 0.1 wt %, 0.008 wt % to 0.04 wt % and 0.0005 wt % to 0.005 wt % respectively. These elements improve the enameling 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:
In a steel sheet of the invention, the depth of the C-profile, being defined as the depth where the C-level reaches (Cbulk+Csurface)/2, is higher than 75 mm, to ensure good enameling capability. According to an embodiment, said depth is between 130 mm and 200 mm.
Steel sheets according to the invention, i.e. with a C-level at the surface between 0 and 0.015 wt % are suitable for 2C/1F enameling, i.e. enameling by applying a ground coat enamel, followed by an outer enamel coating, both coatings being subjected to one firing step, and for 1C/1F enameling, i.e. enameling by applying one enamel layer subjected to one firing step. Steel sheets with low C-levels (i.e. 0.005 wt % and less) at the surface may be suitable also for Direct White Enameling (DWE).
According to a preferred embodiment, the rm 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. In the present description, the ‘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 ‘rm’ value is defined as ¼(r90+2*r45+r0), with r90, r45 and r0 the r-values as defined above, measured on samples oriented respectively at 90°, 45° and 0° with respect to the rolling direction. In a steel sheet according to the invention, 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 enameled steel sheets with a high fish scale resistance and good enamel surface quality. The initial C-level is between 0.02 wt % and 0.08 wt %, more preferably between 0.025 wt % and 0.08 wt %. This is higher than the initial C-levels disclosed in the prior art references referred to above. Despite such higher initial C-levels, the method of the invention allows to obtain steel sheets with improved formability characteristics compared to the prior art. Whereas JP6116634 indicates that above 0.015 wt % 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. When the initial C-level is lower than 0.02 wt %, insufficient cementite formation occurs which deteriorates fish scale resistance. C-levels above 0.08 wt % 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 0.025 wt % and 0.06 wt % and between 0.025 wt % and 0.05 wt %.
The initial steel composition according to the method of the invention further comprises Al, Mn and possibly O, 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/enameling quality is between 0.02 wt % and 0.06 wt %. According to a preferred embodiment of the method of the invention, 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. According to the preferred embodiment, the slab is heated or reheated 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. Still according to the preferred embodiment, cold rolling is performed with a reduction of minimum 50%. The final thickness of the cold rolled sheet is preferably between 0.2 and 2 mm.
The decarburization 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.
According to one embodiment, the decarburizing atmosphere can be prepared with a mixture of H2 and N2 with between 1.5 and 5% H2 in which H2O vapour is injected so that pH2O/pH2 is between 0.04 and 0.5. The minimum value of this ratio ensures that sufficient H2O is present to obtain decarburization according to the formula C+H2O→CO+H2. The maximum of said range ensures that oxidation of Fe and of the furnace is avoided. A more preferred range for pH2O/pH2, related to more optimized conditions in terms of sufficient decarburization and avoiding the occurrence of Fe-oxidation is between 0.04 and 0.25.
In the method of the invention, 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. In the following, the ‘decarburizing 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 45 s and 300 s and the anneal temperature between 760° C. and 850° C. When the ratio pH2O/pH2 is lower than about 0.1, the decarburizing time is preferably higher than 70 s. A more preferred range of the anneal temperature, applicable in combination with any decarburizing time between 45 s and 300 s 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 100 s and 500 s. 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 0.6 mm; in the second industrial trial the thickness was 1 mm.
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 H2 and N2, with H2O vapour being injected in the first and/or the second soaking area. In the first trial, H2O vapour was injected only in the second soaking area. In the second trial, H2O was injected in the first and the second soaking area. Overageing was performed in both trials at 400° C. The overageing time depended on the line speed, e.g. at 180 m/min line speed, the overageing time was 232 s. Table 2 shows the annealing conditions for both trials (numbered trial 1 and trial 2). Table 3 shows the composition besides C, for a number of the samples shown in table 1.
In industrial trial 1, the pH2O/pH2 ratio is below the range of 0.04-0.5 in the first soaking area (due to the fact that no H2O injection is done). The decarburizing time in this trial is the time spent in the second soaking area, where the pH2O/pH2 is within said range. This is an example therefore of a process wherein phase 2 as shown in
In industrial trial 2, H2O-injection was performed in both soaking areas. The decarburizing time indicated here is the time spent in soaking areas 1 and 2. The anneal temperature is the average of the temperatures in soaking areas 1 and 2. The pH2O/pH2 values indicated in table 1 are the average values in soaking area 1 in which more H2O was injected. However, the pH2O/pH2 in soaking area 2 is estimated to be also within the range of 0.04-0.5. In industrial trial 2, longer decarburizing times could be achieved as compared to the first trial, for similar line velocities, leading to stronger decarburization.
The laboratory trials (marked ‘trial 3’ in table 2), were conducted on samples which were subjected to a simulation of the continuous annealing step, at the conditions shown in table 2. These trials were conducted in an atmosphere of HNx with 5% H2, with H2O added to obtain pH2O/pH2 in the range between 0.04 and 0.5.
Samples from all three trials were subjected to an enameling process wherein a ground coat enamel is deposited, this enamel being designed especially to determine the role of C in the enamel characteristics. It was found that the adhesion of the enameling layer was good for all tested samples. The enameling aspect was good for C-levels at the surface of maximum 0.015 wt %, and for profile depths of 75 mm up to 250 mm, as shown in table 1. There is no reason to conclude from the results however that the enameling quality deteriorates at higher depth values than 250 mm. Such higher depth values are therefore not excluded from the scope of the invention. All tested samples showed a good fish scale resistance.
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 enameling 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+Cbulk/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 pH2O/pH2 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 rm value is excellent, despite the initial C-level which is higher than in the prior art: rm is between 1.8 and 2.1. These results prove that the method of the invention allows to produce steel sheets suitable for enameling, starting from an initial C-level higher than 0.02 wt %, the resulting sheets allowing good enameling quality and fish scale resistance, and having very good formability characteristics.
11 wt % = 104 ppm
The depth-values given in table 1 are values of the depth where the C-level reaches (Csurface+Cbulk)/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.
This patent application is a divisional of co-pending U.S. patent application Ser. No. 13/502,566, filed Nov. 29, 2012, which is a National Stage Entry of PCT/EP11/55477, filed Apr. 8, 2011, the entire teachings and disclosure of which are incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13502566 | Nov 2012 | US |
| Child | 16230485 | US |
