KR20140014248A - A steel sheet suitable for enamelling and method for producing such a sheet - Google Patents

Abstract

The present invention relates to a rolled steel sheet suitable for enameling, wherein the sheet is at level C at the bulk of the sheet at a level C _surface at at least one _surface of the sheet. is defined by a slope at the C- levels up to _bulk, has a carbon profiles, C _bulk is higher than the C _surface, and
-C _bulk , greater than 0 and up to 0.08% by weight
0 to 0.015 wt.% C _surface ,
0.012% to 0.07% Al,
0.12% to 0.45% by weight Mn,
O lower than 0.01% by weight,
And optionally:
0.025% to 0.1% by weight Cu,
0.008% to 0.04% by weight S,
0.0005% to 0.005% by weight Ca,
With the remainder being Fe and incidental impurities, the depth when the C-level reaches (C _bulk + C _surface ) / 2 is higher than 75 μm.
The invention equally relates to a method for producing the steel sheet.

Description

A STEEL SHEET SUITABLE FOR ENAMELLING AND METHOD FOR PRODUCING SUCH A SHEET}

The present invention relates to a steel sheet suitable for enameling, and a method for superficial decarburization of the surface of a steel sheet as preparation for enameling the steel.

The carbon level of the steel sheet has a significant effect on the results in terms of the surface properties of the enamel layer applied on the surface of the sheet. High carbon levels at the steel surface may result in CO-gas bubble formation, represented as craters and black spots at the enamel surface. On the other hand, when sufficiently high carbon levels are initially present in the bulk, these carbons form coarse cementite during hot rolling, which splits during cold rolling. do. Such craters are capable of capturing hydrogen entering the steel during the enameling process. If hydrogen is insufficiently captured, the pressure will be increased at the steel / enamel surface, causing a so-called 'fish-scale' deformation of the enamel.

Thus, it is advantageous to decarbonize only the steel in the layer at the surface of the steel, ie to treat superficial decarburization of the surface. Document JP-A-2282421 describes such a method in which continuous cast and annealed non-aging steel sheets for enameling are produced, C 0.0025 To 0.0050% by weight, Si max. 0.03 weight%, Mn 0.1-0.6 weight%, P 0.005-0.03 weight%, S 0.005-0.03 weight%, Al max. 0.01 wt.%, N max. The continuous-cast steel slab containing 0.004% by weight, 0.01 to 0.06% by weight of Cu, 0 0.02 to 0.06% by weight, V 0.01 to 0.06% by weight, remaining Fe and inevitable impurity. Silver is hot-rolled to a finishing temperature of at least 800 ° C., and a coiling temperature of 600 to 800 ° C., and cold rolled to a reduction ratio of at least 60%. -rolled), decarburization annealing at 700 to 900 ° C. for 30 sec to 3 min, and 1 to 20% water vapor, more than the amount of water vapor, to reduce the C-level below 0.002% by weight. It is carried out in a continuous annealing furnace having a decarburizing atmosphere consisting of hydrogen in the gas in the amount of twice, and the remainder which is mainly nitrogen in the gas. In JP-A-6116634 a similar method is described, but the starting material does not have vanadium, the initial C level is up to 0.015% by weight, and B is added instead of V for H-trapping. do.

Both prior art methods result in completely or surface decarburized steel sheets suitable for being enameled by Direct White Enamelling (DWE), wherein one white enamel One white enamel coating is applied on the surface, followed by one firing step. Since the initial carbon level is quite low, fish-scale resistance due to cementite formation is not achieved. To compensate for this, steels with high oxygen levels and limited Al-levels have been used in the prior art with certain alloying elements such as B, V. Because of this limited Al-content, oxides of Si and Mn and nitrides of B and V are formed in the bulk of the steel, which is beneficial against fish-scaling. However, such alloys involve extra costs and lead to some technical difficulties, for example involving casting with V. In addition, the prior art decarbonized sheet has a fairly low formability, as evidenced by the numerical value of Lankford coefficient (r _m ). This value does not exceed 1.8, a concern when deep-drawing is foreseen. In particular, FIG. 1 of JP6116634 showed that an r _m value of 1.6 to 1.8 achieved only a very narrow range of carbon levels before decarburizing annealing. Below 0.0050% C and above 0.0150% C (Below 0.0050% C and above 0.0150% C), the r _m value is deteriorated.

Object of the present invention

The present invention aims to provide a partially decarburized steel sheet suitable for enameling that does not suffer from the problems of the prior art cited above.

SUMMARY OF THE INVENTION

The present invention relates to steel sheets and products and methods of production as indicated in the appended claims. The present invention therefore relates to a rolled steel sheet suitable for enameling, the sheet being in bulk of the sheet at a level C _surface at at least one _surface of the sheet. _{Has a} carbon profile, defined by the slope at the C-level up to the level C _bulk , the C _bulk is higher than the C _surface , and

-C _bulk , greater than 0 and up to 0.08% by weight

0 to 0.015 wt.% C _surface ,

0.012% to 0.07% Al,

0.12% to 0.45% by weight Mn,

O lower than 0.01% by weight,

And optionally:

0.025% to 0.1% by weight Cu,

0.008% to 0.04% by weight S,

0.0005% to 0.005% by weight Ca,

With Fe and the remainder being incidental impurities,

The depth of the C- when the level reaches the (C + C _{bulk surface)} / 2 is greater than 75 ㎛ (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 _bulk in the bulk of the sheet, C _bulk being higher than C _surface , and with:

C _bulk higher than 0 and lower than or equal to 0.08wt%,

C _surface between 0 and 0.015wt%,

Al between 0.012 wt% and 0.07 wt%,

Mn between 0.12 wt% and 0.45 wt%,

O lower than 0.01wt%,

and optionally:

Cu between 0.025wt% and 0.1wt%,

S between 0.008wt% and 0.04wt%,

Ca between 0.0005wt% and 0.005wt%

the balance being Fe and incidental impurities,

and wherein the depth where the C-level reaches (C _bulk + C _surface ) / 2, is higher than 75mm).

According to a preferred embodiment, the steel sheet of the present invention has an r _m value of 1.8 to 2.1.

According to a particular embodiment, C _surface Is 0.005% to 0.015%, or 0 to 0.005% by weight.

According to another specific embodiment, C _bulk Is 0.02% to 0.08%, or 0.025% to 0.08% or 0.025% to 0.06% by weight.

According to another embodiment, the Al-level is from 0.02% to 0.06% by weight.

According to a further embodiment, the depth is from 130 μm to 200 μm.

The invention equally relates to an enamelled steel sheet composed of a steel sheet according to any of the above paragraphs, provided with an enamel layer.

The invention further relates to steel products produced from the sheets according to the invention, and to enameled steel products composed of such products, provided with an enamel layer.

The invention also relates to a method for producing a rolled steel sheet for enameling, comprising the following steps:

In order to obtain a cold-rolled steel sheet, hot rolling the steel slab followed by coiling and cold rolling,

Wherein the slab comprises the following initial composition

0.02% to 0.08% by weight C,

0.012% to 0.07% Al,

0.12% to 0.45% by weight Mn,

O lower than 0.01% by weight,

And optionally:

0.025% to 0.1% by weight Cu,

0.008% to 0.04% by weight S,

0.0005% to 0.005% by weight Ca,

The remainder being Fe and incidental impurities;

The cold-rolled sheet undergoes a continuous annealing step,

The sheet exposes a decarburizing atmosphere comprising water vapour and hydrogen gas during the decarburizing time, wherein the H ₂ content is from 1% to 95% by volume, and Wherein the H ₂ O content is from 0.04% to 33% by volume, the remainder is primarily nitrogen gas and the ratio pH ₂ 0 / pH ₂ is from 0.04 to 0.5.

According to a preferred embodiment, the continuous annealing takes place at an annealing temperature of 760 ° C. to 850 ° C. and for a decarbonation time of 45 s to 300 s.

According to a particular embodiment, the annealing temperature is between 800 ° C and 850 ° C.

According to a particular embodiment of the method of the invention, said initial C-level is between 0.025% and 0.08% or 0.025% and 0.06% by weight.

According to a further embodiment, said initial Al-level is between 0.02% and 0.06% by weight.

According to a specific embodiment, said ratio pH ₂ O / pH ₂ Is 0.04 to 0.25.

The method of the present invention may further comprise an over-ageing step at a temperature of 350 ° C. to 450 ° C. for a timespan of 100 s to 500 s. The method may further comprise a skinpass step with a reduction of 0.3% to 1.5%.

1 shows the carbon profile in a steel sheet according to the invention.
2 shows an example of an annealing step that can be used in the method of the present invention.

The steel sheet of the present invention has a C-profile, which is defined by the slope at the C-level from a lower C _surface at the _surface to a higher C _bulk at the _bulk . The sheet can be obtained by a method comprising a continuous decarburization step, as will be described further in this text. 1 shows the carbon-distribution across the thickness of two sheets according to the invention, with a thickness of 0.7 mm. Curve 10 shows a bulk portion 11 where the C-level C _bulk is substantially unchanged, and each surface portion representing the C-profile, two surface portions 12 (the sheet). One on each side of the sheet]. The surface level is applied to an appropriate measurement technique (e.g. Glow Discharge Optical Emission Spectroscopy (GD-OES), which allows composition measurement as depth analysis). It is shown as the minimum value of the C-profile as measured by. In the steel sheet according to the invention, the C-level at the surface is at most 0.015% by weight, while C _bulk Is greater than 0 and less than or equal to 0.08 weight percent. At the same time, C _bulk The c _surface Respectively. According to an embodiment, the C _surface is 0.005% to 0.015% by weight. According to yet another embodiment, the C _surface is 0 to 0.005 weight percent.

Curve 10 is an example of a sheet in which the decarbonization does not occur on the entire thickness of the sheet. This means that the level C _bulk is identical to the initial C-level applied in the production method (described in further detail). According to an embodiment according to an embodiment of the process of the invention (see further), the C _bulk is from 0.02% to 0.08%, or from 0.025% to 0.08%, or from 0.025% to 0.06% or 0.025% % To 0.05% by weight. Curve 13 shows the case where decarbonization continued to the middle plane of the sheet. In this case, C _bulk Is less 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. In contrast to the prior art reference, the oxygen level is kept below 0.01% by weight. According to a preferred embodiment of the steel sheet of the present invention, oxygen is not intentionally added to the composition, but is possible at the impurity level (oxygen is not added deliberately to the composition, but is allowed only at impurity levels). Fish scaling resistance is ensured by higher initial C-levels so that no oxide formation is required for this purpose. This means that it does not contain any specific alloying elements such as V. Also, N is kept as low as possible

The Al-level in the sheet of the invention is from 0.012% to 0.07% by weight, higher than the alloyed Al-level in the cited prior art reference. In the prior art document cited above, Al needs to be limited to avoid deoxidation to ensure the formation of the oxide which will act against fish-scaling. In the process of the present invention (see additional reference), Al is essential for the binding and deoxidation of free N in order to avoid ageing the mechanical properties. If the Al-level is lower than 0.012% by weight, insufficient deoxidation occurs and the binding of N is required through other means. Adding Al at a level higher than 0.07% by weight means an increase in the cost of the process and deterioration of the enameling quality. With respect to more optimized conditions in terms of deoxidation and / or cost / enamel properties, a more preferred range for the Al-level is from 0.02% to 0.06% by weight.

Mn represents 0.12% to 0.45% by weight. These components are added to avoid the formation of free sulphur and to adjust the strength properties of the steel.

Copper, sulfur and calcium are optionally added on the impurity levels, more precisely in the range of 0.025% to 0.1%, 0.008% to 0.04% and 0.0005% to 0.005% by weight, respectively. May be This component improves the enameling properties.

According to the invention the balance of the composition of the steel sheet consists of Fe and incidental impurities. The following components may be represented as impurities, preferably at levels below the numerical values (in wt%) shown in Table 1:

[Table 1]

In the steel sheet of the present invention, the depth of the C-profile, defined as the depth when the C-level reaches (C _bulk + C _surface ) / 2, ensures good enamelling capability. The depth of the C-profile, being defined as the depth where the C-level reaches (C _bulk + C _surface ) / 2, is higher than 75 μm, to ensure good enamelling capability. According to an embodiment, the depth is from 130 μm to 200 μm.

That is, the steel sheet according to the invention, having a C-level at the surface of 0 to 0.015% by weight, undergoes 2C / 1F enameling, i.e. both coatings undergoing one firing step. , Enameling after applying ground coat enamel, followed by outer enamel coating, and enameling with 1C / 1F enamel, ie one layer of enamel Suitable for rings Steel sheets with low C-levels (ie, 0.005 wt.% Or less) at the surface may also be suitable for Direct White Enamelling (DWE).

According to a preferred embodiment, the r _m value of the steel sheet according to the invention is between 1.8 and 2.1. This means that the steel sheet has better formability than the aforementioned steel sheet of the prior art. In the present description, the 'r' value is a true strain in the width direction when the sheet material is pulled in a uniaxial tension beyond its elastic limit. The plastic strain ratio (also known as the anisotropy factor), which is the ratio of true strain in the thickness direction. The 'r _m ' value is defined as ¼ (r ₉₀ + 2 * r ₄₅ + r ₀ ), r ₉₀ , r ₄₅ and r ₀ And r-values, as indicated above, are measured on samples facing 90 °, 45 ° and 0 ° with respect to the rolling direction, respectively. In the 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 present invention can be produced by steel rolls being hot rolled, coiled and cold rolled together with certain initial steel compositions, and the cold-rolled sheets being subjected to continuous superficial decarburization. The initial composition is mainly characterized by higher C-levels compared to the prior art, and by higher Al-levels and lower oxygen levels. Enameled steel with high fish scale resistance and good enamel surface quality, without any deliberate addition of components such as V, Nb or B It still makes it possible to produce sheets. The initial C-level is from 0.02% to 0.08% by weight, more preferably from 0.025% to 0.08% by weight. This is greater than the initial C-level shown in the prior art reference indicated above. Despite this higher initial C-level, the method of the present invention makes it possible to obtain a steel sheet having improved formability properties compared to the prior art. JP6116634 is at least 0.015% by weight of the initial carbon, which indicates that it is not possible to obtain acceptable decarbonization and good formability, while the starting composition of the present invention does not face this problem. Decarbonization is possible up to an acceptable level, while the formability is excellent. When the initial C-level is lower than 0.02 wt%, insufficient cementite formation occurs which deteriorates fish scale resistance ). Above 0.08% by weight the C-level leads to too high strength levels, thus reducing moldability. Specific ranges for these initial C-levels, related to more optimized properties in terms of fish scale resistance and strength / forming, are from 0.025% to 0.06% by weight and from 0.025% to 0.05% by weight.

The initial steel composition according to the method of the present invention is Al, Mn and possible O, S, Cu and Ca, the incidental impurities shown in Table 1 and Fe remaining in the same range as the decarbonized sheet shown above. It further includes. A more preferred range for the initial Al-level, relating to more optimized conditions in terms of deoxidation and cost / enamelling quality, is from 0.02% to 0.06% by weight. According to a preferred embodiment of the process of the invention, oxygen is not intentionally added to the composition, but 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 this preferred embodiment, the slab is (re) heated at a temperature of at least 1050 ° C, hot rolled with a final temperature of 850 ° C to 950 ° C, and coiled at a coiling temperature of 620 ° C to 770 ° C. According to a still preferred embodiment, cold rolling is carried out by a reduction of at least 50%. The final thickness of the cold rolled sheet is preferably 0.2 to 2 mm.

The decarburization anneal is provided in a annealing furnace for continuous annealing as known in the art, provided a vapor injection device for applying a given annealing atmosphere is possible. Is performed (ie, the cold-rolled sheet is annealed as it moves through the furnace at a fixed speed, the speed being a measure of the time of the annealing time, ie the time of the annealing temperature). ].

Figure 2 shows an example of the lay-out of an annealing furnace usable in the method of the invention, starting with heating phase 1, wherein the temperature is the annealing temperature ( increased to annealing temperature. State 2 represents the actual annealing (soaking) state. State 3 is an overageing step. State 2 may consist of one or more periods with different (constant or average) annealing temperatures and different annealing atmospheres in each period. Practically speaking, different periods of time at different conditions, the annealing line at various points along the (annealing line), the annealing zone (annealing zone) to share the details (subsection), into an atmosphere comprising H ₂ H ₂ O By injecting steam, it can be obtained (see further examples in this detailed description).

According to the invention, the superficial decarburization of the surface is characterized by the fact that the H ₂ content of 1% by volume to 95% by volume, the H ₂ O content of 0.04% by volume to 33% by volume and the pH ₂ O / pH ₂ partial pressure Treatment is carried out under a decarbonation atmosphere comprising water and hydrogen gas, the remainder being essentially nitrogen gas, having a ratio of 0.04 to 0.5, more preferably 0.04 to 0.25. The above composition describes the atmosphere at the start of the decarburizing time. During decarbonization, it is evident that the atmospheric composition will change, mainly because the decarbonation reaction (formation of H ₂ and CO) occurs. In addition, at the beginning of the decarbonation time, small amounts of other gases may already be formed in the atmosphere or may be present as impurities. The total pressure at which the superficial decarburization anneal occurs is at atmospheric pressure, or a pressure other than atmospheric pressure, but may also be a pressure within the generally known boundaries applied to this type of annealing process.

According to one embodiment, the decarbonation atmosphere is a mixture of H ₂ and N ₂ with 1.5 to 5% H ₂ infused with H ₂ O steam such that pH ₂ O / pH ₂ is between 0.04 and 0.5. Can be prepared together. This minimum value of this ratio ensures that sufficient H ₂ O is present to obtain decarburization according to the formula C + H ₂ O-> CO + H ₂ . The maximum of said range ensures that oxidation of Fe and of the furnace is avoided. A more preferred range for pH ₂ O / pH ₂ , which avoids the occurrence of Fe-oxidation and which is more optimized in terms of sufficient decarbonization, is from 0.04 to 0.25.

In the process of the invention, the decarbonation atmosphere is at least one of the above having a different (constant or average) annealing temperature and a different annealing atmosphere, in each period, preferably during the totality of phase 2. Applied during the period. As follows, the 'decarburizing time' represents the time spent under the conditions of the decarbonizing atmosphere.

The decarbonation time and the annealing temperature are selected to obtain a steel sheet according to the invention. It is within the skill of the person skilled in the art to find the appropriate combination of decarbonation time and annealing temperature based on the examples provided further in this description. According to a preferred embodiment, the decarbonation time is between 45 s and 300 s and the annealing temperature is between 760 ° C. and 850 ° C. The ratio pH ₂ O / pH ₂ If it is lower than about 0.1, the decarbonation time is preferably higher than 70 s. The more preferred range of the annealing temperature, corresponding to a combination having any decarbonation time of 45 s to 300 s, is from 800 ° C. to 850 ° C. The temperature is not necessarily constant during the decarbonation time. The change in temperature may occur due to a change in line speed for example. The over-ageing step may be applied at a temperature of 350 ° C. to 450 ° C. for a period of 100 s to 500 s. Skinpass may additionally be applied with a reduction of 0.3% to 1.5%.

Example

The results from industrial trials conducted by the applicant will be described later, as well as some experimental trials. All tested samples were produced from the starting composition according to the present invention. The coiling temperature is 725 ° C. Two industrial attempts were made. The thickness of the cold rolled sheet subjected to decarburization annealing in industrial trial 1 is 0.6 mm; In industrial trial 2 the thickness is 1 mm.

In this industrial approach, a continuous annealing line consists of a heating section, two soaking areas, a cooling and overaging part. The annealing atmosphere consists mainly of a mixture of H ₂ and N ₂ together with H ₂ O vapor injected into the first and / or second immersion zones. In the first attempt, H ₂ O vapor was injected only in the second immersion zone. In a second attempt, H ₂ O was injected in the first and second immersion regions. Overageing was performed in both trials at 400 ° C. The over- mature time is determined by line speed, for example a line speed of 180 m / min, and the over- mature time is 232 s. Table 2 shows the annealing conditions in both trials (numbered trial 1 and trial 2). Table 3 shows the above compositions other than C for some of the samples shown in Table 1.

In the industry attempts 1, wherein the pH ₂ O / pH ₂ ratio is below the range of 0.04 to 0.5 in the first immersion region [due to the fact that any non-H ₂ O injection is not carried out (due to the fact that no H ₂ O injection is done)]. In this approach the decarbonation time is the pH ₂ O / pH ₂ When is in the above range, it is the time spent in the second immersion region. Thus, this is an example of a process in which state 2 as shown in FIG. 1 includes a first period in which the conditions of the present invention are not met, and a second period in which such conditions are met. Such a process is within the scope of the present invention.

In industrial trial 2, H ₂ O-injection is carried out in both immersion zones. The decarbonation time shown herein is the time spent in immersion zones 1 and 2. The annealing temperature is the average of the temperatures in immersion zones 1 and 2. Wherein the pH ₂ O / pH ₂ values shown in Table 1 is an average value of from, soaking zone 1 which is more H ₂ O injection. However, the pH ₂ O / pH ₂ in the crack zone ₂ Is also estimated to be in the range of 0.04 to 0.5. In industrial trial 2, longer decarbonation time could be achieved as compared to the first trial, for similar line velocity leading to stronger decarbonization.

The experimental trial (indicated as 'Trial 3' in Table 2) was performed on a simulated sample of the continuous annealing step under the conditions shown in Table 2. This trial was carried out in an atmosphere of HN _x with 5% H ₂ , with H ₂ O added to obtain pH ₂ O / pH ₂ in the above range [0.04-0.5] (These trials were conducted in an atmosphere of HN _x with 5% H ₂ , with H ₂ O added to obtain pH ₂ O / pH ₂ in the range [0.04-0.5]).

Samples from all three trials are subjected to an enameling process, where a ground coat enamel is deposited, which enamel is specifically designed to determine the roll of C in the enamel properties. (Samples from all three trials were subjected to an enamelling process wherein a ground coat enamel is deposited, this enamel being designed especially to determine the role of C in the enamel characteristics). The adhesion of the enamel layer was found to be good for all tested samples. The enamel side is good for C-levels on surfaces up to 0.015% by weight, and profile depths from 75 μm to 250 μm, as shown in Table 1. There is no reason to draw conclusions from the results, but the enameling properties deteriorate at higher depth values than 250 μm. Thus, these higher depth figures are not excluded from the scope of the present invention. All tested samples exhibit good fish scale resistance.

Table 2 shows the properties of the C-level at the surface (ie, the minimum level of the C-profile, as measured by GD-OES), the depth of the C-profile, and the enamel layer produced at the surface of the sample. In summary, the results after decarbonization are summarized. Samples 25-35 may have insufficient depth of the C-profile, as the C-level is measured by the depth when reaching (C _surface + C _bulk / 2), and / or at the surface. Poor enameling aspects were obtained, which can be described at very high C levels of. The reason for this negative result is that the annealing temperature is too low, the decarbonation time is too short, or the pH ₂ O / pH ₂ ratio is too low, or a combination of these factors may result from the experimental conditions. Can be.

Table 4 shows the mechanical properties of some samples taken from the sheets of industrial trials 1 and 2. Importantly, despite the initial C-level above the prior art, formability is excellent in terms of the r _m value: r _m Is 1.8 to 2.1. These results make it possible for the process of the invention to produce steel sheets suitable for enameling, starting from an initial C-level of higher than 0.02% by weight, resulting in good enameling properties and It enables fish scale resistance and demonstrates having very good formability properties.

[Table 2]

The depth-values provided in the table are numerical values of depth when the C-level reaches (C _surface + C _bulk ) / 2. The depth measurements, denoted by '*', represent the maximum depth that can be measured with the applied equipment. Thus, the real value is higher than this value.

[Table 3]

[Table 4]

Claims (21)

As a rolled steel sheet suitable for enameling,
The sheet has a carbon profile, defined by the slope at the C-level from the bulk of the sheet to the level C _bulk at a level C _surface at at least one _surface of the sheet, wherein C _bulk is Higher than C _surface , and
-C _bulk , greater than 0 and up to 0.08% by weight
0 to 0.015 wt.% C _surface ,
0.012% to 0.07% Al,
0.12% to 0.45% by weight Mn,
O lower than 0.01% by weight,
And optionally:
0.025% to 0.1% by weight Cu,
0.008% to 0.04% by weight S,
0.0005% to 0.005% by weight Ca,
With Fe and the remainder being incidental impurities,
A rolled steel sheet, the depth of when the C-level reaches (C _bulk + C _surface ) / 2 is higher than 75 μm.
The method of claim 1,
Steel sheet having an r _m value of 1.8 to 2.1.
3. The method according to claim 1 or 2,
C _surface is 0.005% to 0.015% by weight of the steel sheet.
3. The method according to claim 1 or 2,
C _surface is 0 to 0.005% by weight of the steel sheet.
5. The method according to any one of claims 1 to 4,
C _bulk is from 0.02% to 0.08% by weight.
6. The method of claim 5,
C _bulk is from 0.025% to 0.08% by weight.
6. The method of claim 5,
C _bulk is from 0.025% to 0.06% by weight.
8. The method according to any one of claims 1 to 7,
The Al-level is from 0.02% to 0.06% by weight.
The method according to any one of claims 1 to 8,
Wherein the depth is from 130 μm to 200 μm.
Provided with an enamel layer Enamelled steel sheet consisting of the steel sheet according to any one of claims 1 to 9.
Steel product produced from the sheet according to claim 1.
An enameled steel product, consisting of the product according to claim 11, provided with an enamel layer.
In order to obtain a cold-rolled steel sheet, hot rolling of steel slabs followed by coiling and cold rolling,
Wherein the slab comprises the following initial composition
0.02% to 0.08% by weight C,
0.012% to 0.07% Al,
0.12% to 0.45% by weight Mn,
O lower than 0.01% by weight,
And optionally:
0.025% to 0.1% by weight Cu,
0.008% to 0.04% by weight S,
0.0005% to 0.005% by weight Ca,
The remainder being Fe and incidental impurities;
The cold-rolled sheet undergoes a continuous annealing step,
The sheet exposes a decarburizing atmosphere comprising water vapour and hydrogen gas during the decarburizing time, wherein the H ₂ content is from 1% to 95% by volume, and The H ₂ O content is 0.04% to 33% by volume, the remainder is primarily nitrogen gas and the ratio pH ₂ 0 / pH ₂ is 0.04 to 0.5;
Including a method for producing a rolled steel sheet for enameling.
14. The method of claim 13,
Wherein the continuous annealing occurs at annealing temperatures of 760 ° C. to 850 ° C. and for a decarburizing time of 45 s to 300 s.
15. The method of claim 14,
Wherein the annealing temperature is from 800 ° C. to 850 ° C.
16. The method according to any one of claims 13 to 15,
Wherein said initial C-level is from 0.025% to 0.08% by weight.
17. The method of claim 16,
Wherein said initial C-level is from 0.025% to 0.06% by weight.
18. The method according to any one of claims 13 to 17,
Wherein said initial Al-level is between 0.02% and 0.06% by weight.
19. The method according to any one of claims 13 to 18,
The ratio pH ₂ 0 / pH ₂ Is 0.04 to 0.25.
20. The method according to any one of claims 13 to 19,
Further comprising an over-ageing step at a temperature of 350 ° C. to 450 ° C. for a time span of 100 s to 500 s.
21. The method of claim 20,
And a skinpass step having a reduction of 0.3% to 1.5%.

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