TWI789985B - Enameled steel sheet and method of forming the same - Google Patents

Abstract

A enameled steel sheet and a method of forming the same are provided. A hot rolling operation, a coiling operation, a cold rolling operation, a temper rolling operation and thermal treatment are performed to steel embryo with certain components, and a coiling temperature and temper rolling ratio are controlled to obtain the enameled steel sheet with resistance to thermal process softening and great hydrogen diffusion coefficient.

Description

Enameled steel sheet and manufacturing method thereof

The invention relates to an enamelled steel sheet and a manufacturing method thereof, in particular to an enamelled steel sheet with high temperature softening resistance and a manufacturing method thereof.

The known cold-rolled enamelled steel sheet is generally made of low-carbon steel, and the formability is improved by reducing the carbon content. In addition, by adding elemental additives such as titanium, nitrogen, and sulfur, the ability to absorb hydrogen gas inside the steel is improved, thereby avoiding the phenomenon of iron jumping (or fishscale phenomenon). Among them, the phenomenon of iron jumping is due to the accumulation of hydrogen atoms at the interface between steel and enamel to form hydrogen gas during the cooling process. A large amount of hydrogen gas makes the interface pressure too high, which leads to the phenomenon of local peeling of the enamel layer. Therefore, adding the above-mentioned element additives can make the enamel layer and the steel closely bonded and not easy to fall off, and after subsequent hot rolling, cold rolling and continuous annealing processes, a cold-rolled finished product with low strength and good deep drawability can be obtained.

However, conventional enamel steel sheets are mostly deep-drawing grade materials, so the product has low strength and high elongation, and is suitable for use as utensils with complex shapes such as pots and kitchen utensils. If it is used for appliances that need high strength to resist external shocks or to withstand internal pressure (such as water heater inner tanks), the strength of conventional enamelled steel sheets cannot bear the load. Generally speaking, high-temperature heat treatment is required in the enamelling process to facilitate the dissolution of the glaze and adhere to the steel. However, it is known that the strength of the enamelled steel sheet is greatly reduced after being subjected to high temperature, so that its impact resistance and pressure resistance are deteriorated. Therefore, Not resistant to high temperature softening properties.

In view of this, there is an urgent need to provide an enamelled steel sheet and a manufacturing method thereof, so as to manufacture an enamelled steel sheet with high temperature softening resistance and good hydrogen diffusion coefficient.

One aspect of the present invention is to provide a manufacturing method of enamelled steel sheet, which is to sequentially perform hot rolling operation, coiling operation, cold rolling operation, quenching and tempering rolling operation and heat treatment operation on a steel billet with specific composition To obtain enamelled steel sheets with high temperature softening properties and good hydrogen diffusion coefficient.

Another aspect of the present invention is to provide an enamelled steel sheet, which is produced by the manufacturing method of the above aspect, and has high strength and good hydrogen diffusion coefficient.

According to an aspect of the present invention, a method for manufacturing an enamelled steel sheet is provided, which includes providing a steel billet. Based on 100 wt% of the billet, the billet includes 0.04 wt% to 0.16 wt% of carbon, 0.05 wt% to 1.50 wt% of manganese, 0.01 wt% to 0.30 wt% of silicon, 0.01 wt% to 0.20 wt% of phosphorus , 0.01 to 0.30 wt% copper, 0.01 to 0.30 wt% chromium, 0.01 to 0.10 wt% titanium, 0.01 to 0.12 wt% aluminum, 0.01 to 0.02 wt% nitrogen , not more than 0.03 wt% of sulfur, not more than 0.10 wt% of nickel, and the rest are iron and unavoidable impurities. The steel billet is subjected to a hot rolling operation to obtain finished rolled steel.

Next, coiling is performed on the finished rolled steel to obtain hot-rolled steel coils. The coiling temperature for the coiling operation is 550°C to 650°C. Then, the hot-rolled steel coil is subjected to a cold-rolling operation to obtain a cold-rolled steel strip. The reduction rate of the cold rolling operation is 40% to 80%. The cold-rolled steel strip is tempered and rolled to obtain a substrate. The rolling ratio of temper rolling is 0.5% to 3.0%. Next, a heat treatment operation is performed on the substrate to obtain an enamelled steel sheet.

According to an embodiment of the present invention, the above-mentioned hot rolling operation includes a step of reheating the steel billet to raise the temperature to 1150°C to 1300°C. Next, after the reheating step, the billet is subjected to a hot rolling step to obtain a finished rolled steel product. The finishing temperature of the hot rolling step is not lower than the Ar3 temperature.

According to an embodiment of the present invention, the above method further includes performing an annealing operation on the cold-rolled steel strip before the temper rolling operation.

According to an embodiment of the present invention, the above-mentioned annealing operation includes a step of heating the cold-rolled steel strip to reach 650°C to 850°C. After the heating step, the cold-rolled steel strip is subjected to a temperature holding step at 650° C. to 850° C. for 80 seconds to 240 seconds. Next, after the temperature holding step, the cold-rolled steel strip is subjected to a temperature-lowering step to drop to room temperature.

According to an embodiment of the present invention, the reduction ratio of the above-mentioned cold rolling operation is 40% to 80%.

According to an embodiment of the present invention, the above heat treatment operation includes performing at a temperature of 850° C. to 950° C. for 10 minutes to 20 minutes.

According to an embodiment of the present invention, the difference between the yield strength of the enamelled steel sheet and the yield strength of the substrate is not greater than 20 MPa.

According to another aspect of the present invention, an enamelled steel sheet is provided, which is produced by the manufacturing method described in the above aspect.

According to an embodiment of the present invention, the enamelled steel sheet has a yield strength not less than 300 MPa and a tensile strength not less than 400 MPa.

According to an embodiment of the present invention, the above-mentioned enamelled steel sheet has a hydrogen diffusion coefficient of 6.0×10 ^-7 to 9.0×10 ^-7 cm ² /s.

Applying the enamelled steel sheet and its manufacturing method of the present invention, by sequentially performing hot rolling operation, coiling operation, cold rolling operation, quenching and tempering rolling operation and heat treatment operation on the steel billet with specific composition, and controlling the coiling temperature And quenching and tempering rolling rate to obtain enameled steel sheet with high temperature softening resistance and good hydrogen diffusion coefficient.

As used in this disclosure, "around", "about", "approximately" or "substantially" generally means within 20 percent of the value or range stated , or within 10 percent, or within 5 percent.

Based on the above, the present invention provides an enamelled steel sheet and its manufacturing method, by sequentially performing hot rolling operation, coiling operation, cold rolling operation, quenching and tempering rolling operation and heat treatment operation on the steel billet with specific composition, And control the coil temperature and quenching and tempering rolling rate to obtain enameled steel sheet with high temperature softening resistance and good hydrogen diffusion coefficient.

Please refer to FIG. 1 , which is a flowchart illustrating a method 100 for manufacturing an enamelled steel sheet according to some embodiments of the present invention. First, operation 110 is performed to provide a steel billet. In some embodiments, based on 100 wt% of the steel billet, the steel billet comprises 0.04 wt% to 0.16 wt% carbon, 0.04 wt% to 0.16 wt% carbon, 0.01 wt% to 0.30 wt% silicon, 0.01 wt% to 0.20 wt% phosphorus, 0.01 to 0.30 wt% copper, 0.01 to 0.30 wt% chromium, 0.01 to 0.10 wt% titanium, 0.01 to 0.12 wt% aluminum, 0.01 wt% To 0.02 wt% nitrogen, not more than 0.03 wt% sulfur, not more than 0.10 wt% nickel, the rest is iron and unavoidable impurities.

Generally speaking, the content of carbon, manganese, silicon, phosphorus and sulfur in the billet will affect the strength and formability of the subsequent steel sheet. For example, a carbon content of 0.07 wt% to 0.11 wt% and/or a manganese content of 0.5 wt% to 1.0 wt% can produce a steel sheet with higher strength. However, if the strength is too strong, the formability of the steel sheet will be affected. For example, if the carbon content is higher than 0.16 wt%, the formability of the steel sheet will be poor. In addition, the content of copper, nickel and chromium in the billet is mainly controlled to effectively prevent rust without increasing the cost too much. The titanium content must be controlled within the above range to prevent iron jumping. The content of aluminum and nitrogen will affect the precipitated aluminum nitride (AlN), and the precipitation of aluminum nitride helps to make the steel sheet have the effect of softening resistance.

Next, proceed to operation 120, performing a hot rolling operation on the billet to obtain finished rolled steel. In some embodiments, the hot rolling operation includes a step of reheating the steel billet to raise the temperature to 1150°C to 1300°C. Then, the steel billets are subjected to a hot rolling step to obtain finished rolled steel products. The finishing temperature of the hot rolling step is not lower than the Ar3 temperature. It is supplemented that when the Ar3 temperature system steel is cooled, the temperature at which the wasted iron begins to transform into fertile iron. In other words, the hot rolling operation of the present invention contemplates that the hot rolling will be done in the ferrite phase of Worthfield.

Then, proceed to operation 130, performing a coiling operation on the rolled steel material to obtain a hot-rolled steel coil. In some embodiments, the coiling operation has a coiling temperature of 550°C to 650°C. The present invention uses a higher coiling temperature than the conventional coiling operation. It is because if the coiling temperature is too low, the precipitation of aluminum nitride is too small, and the strength of the subsequent steel sheet is too low. Can't reach effect of the present invention.

Next, proceed to operation 140, performing a cold rolling operation on the hot-rolled steel coil to obtain a cold-rolled steel strip. In some embodiments, the reduction rate of the cold rolling operation is 40% to 80%, preferably 60% to 80%. The cold-rolling reduction rate is within the aforementioned range to obtain cold-rolled steel strips that meet the thickness requirements.

The cold rolled steel strip may optionally be subjected to an annealing operation prior to subsequent operation 150 . In some embodiments, the annealing operation can be performed in a continuous annealing device, which includes firstly heating the cold-rolled steel strip for about 500 seconds to about 800 seconds to increase the temperature from room temperature to about 650°C to about 850°C. Next, a temperature holding step is performed at about 650° C. to about 850° C. for 80 seconds to 240 seconds. Then, the cold-rolled steel strip is subjected to a cooling step to drop to room temperature. In this embodiment, the cooling step comprises cooling down to about 450° C. to about 560° C. in about 15 seconds to about 48 seconds; then, cooling down to about 320° C. to about 400° C. in about 12 seconds to about 35 seconds; Then, within about 310 seconds to about 900 seconds, the temperature can be lowered to about 210° C. to about 300° C.; finally, the temperature can be lowered to room temperature within about 40 seconds to about 130 seconds.

Then, after performing operation 140 or the aforementioned optional annealing operation, proceed to operation 150 to perform temper rolling operation on the cold-rolled steel strip to obtain a substrate. In some embodiments, the rolling ratio of temper rolling is 0.5% to 3.0%, preferably 2.0% to 3.0%. When the rolling rate of temper rolling is lower than 0.5%, the extension of the yield point cannot be eliminated, and then the substrate with high yield strength cannot be obtained; on the contrary, if the rolling rate of temper rolling is higher than 3.0%, Then the surface roughness of the obtained substrate is too high, so a smoother steel sheet cannot be obtained subsequently. Compared with the substrate produced by the conventional process, it only has a yield strength not greater than 200 MPa and a tensile strength not greater than 300 MPa. In some embodiments of the present invention, the substrate may have a yield strength not less than 300 MPa and a tensile strength not less than 400 MPa. In other embodiments, the substrate has an elongation of greater than about 20%, preferably greater than about 25%.

Next, proceed to operation 160, performing a heat treatment operation on the substrate to obtain an enamelled steel sheet. In some embodiments, the heat treatment operation includes performing at a temperature of 850° C. to 950° C. for 10 minutes to 20 minutes, and the heat treatment operation may be performed in a general atmospheric atmosphere. In some embodiments, the enamelled steel sheet has a yield strength of not less than 300 MPa and a tensile strength of not less than 400 MPa.

In the conventional process, the yield strength of the enamelled steel sheet obtained after the substrate is subjected to high temperature heat treatment will be greatly reduced, for example, by about 75 MPa or more. However, the difference in yield strength of the substrate of the present invention is not greater than 20 MPa after high-temperature heat treatment. In other words, the enamelled steel sheet of the present invention has good anti-high temperature softening effect.

In some embodiments, the metallographic structure of the enamelled steel sheet produced by the present invention is mainly ferrite phase and high carbon wavelet iron phase, and its grain shape is equiaxed grain. It is supplemented that the isotropic formability of equiaxed crystals is more uniform, so the risk of fracture after forming is lower. In addition, the grain size of the substrate obtained in the above operation 150 is about 11 to 12 (that is, the average grain size is about 5 μm to about 17 μm), while the grain size of the enamelled steel sheet after operation 160 is about 10 to 12. No. 11 (ie, the average grain size is about 10 μm to about 23 μm), that is, the high temperature heat treatment did not cause a significant change in the grain size. Generally speaking, the grain size will affect the strength of the steel, so there is little difference between the strength of the substrate and the yield strength of the enamelled steel sheet, and the similar grain size is due to the pinning of aluminum nitride precipitation on the grain boundary, which effectively slows down Grain growth behavior during high temperature annealing.

In some embodiments, according to ASTM G148 specification, the enamelled steel sheet has a hydrogen diffusion coefficient of 6.0×10 ⁻⁷ to 9.0×10 ⁻⁷ cm ² /s. It should be understood that the smaller the hydrogen diffusion coefficient, the harder it is for hydrogen to pass through the test piece, that is, the harder it is for hydrogen in the metal to diffuse. However, the enamelled steel sheet of the present invention has a relatively small hydrogen diffusion coefficient, which indicates that it has a better hydrogen storage capacity, and therefore should have a better anti-jumping capacity.

Several examples are used below to illustrate the application of the present invention, but it is not intended to limit the present invention. Those with ordinary knowledge in the technical field of the present invention can make various modifications and changes without departing from the spirit and scope of the present invention. retouch. Embodiment one to three

The steel blank used in Examples 1 to 3 contains 0.04 wt% to 0.16 wt% of carbon, 0.01 wt% to 0.30 wt% of silicon, 0.01 wt% to 0.20 wt% of phosphorus, 0.01 wt% to 0.30 wt% of copper, 0.01 wt% to 0.30 wt% chromium, 0.01 wt% to 0.10 wt% titanium, 0.01 wt% to 0.12 wt% aluminum, 0.01 wt% to 0.02 wt% nitrogen, not more than 0.03 wt% sulfur, not more than 0.10 wt% of nickel, the rest is iron and unavoidable impurities. Then, the billets are sequentially subjected to reheating steps, hot rolling steps and coiling operations to obtain hot-rolled steel coils, wherein the temperature in the reheating step is 1160°C to 1240°C, the temperature for finishing rolling is 850°C to 910°C, and the coiling The temperature is 550°C to 650°C.

Next, after cold-rolling the hot-rolled steel coil, carry out continuous annealing operation at room temperature, and perform quenching and tempering rolling after the annealing is lowered to room temperature, so as to obtain the substrate of Example 1, wherein the cold-rolling reduction rate is 50% to 60%, and the quenching and tempering rolling rate is 2.0% to 2.5%. Embodiments 2 and 3 all use the same materials and processes as Embodiment 1.

The substrates of Examples 1 to 3 were subjected to a high-temperature heat treatment at a temperature of about 870°C for about 10 minutes, and the atmosphere was selected as general air, so as to obtain the enamelled steel sheets of Examples 1 to 3 respectively. Comparative Examples 1 and 2

The steel blanks used in Comparative Examples 1 and 2 contained less than 0.01 wt% of carbon, 0.05 to 0.30 wt% of manganese, less than 0.05 wt% of silicon, less than 0.05 wt% of phosphorus, less than 0.05 wt% of copper, less than 0.05 wt% chromium, 0.05 wt% to 0.20 wt% titanium, 0.01 wt% to 0.09 wt% aluminum, 0.005 wt% to 0.02 wt% nitrogen, less than 0.03 wt% sulfur, less than 0.05 wt% nickel, The rest is iron and unavoidable impurities. Comparative Examples 1 and 2 used the same manufacturing process as the above-mentioned Example 1, the only difference being that the coiling temperature was 500° C., and the temper rolling ratio was 1%. The prepared substrates of Comparative Examples 1 and 2 were then subjected to high-temperature heat treatment under the same conditions as in the above-mentioned Example 1 to obtain enamelled steel sheets of Comparative Examples 1 and 2.

Table 1 below shows the results of the yield strength, tensile strength and elongation of the substrates and enameled steel sheets of Examples 1 to 3 and Comparative Examples 1 and 2 respectively after tensile tests.

表1

Table 1

It can be seen from Table 1 above that compared with Comparative Examples 1 and 2, the yield strength and tensile strength of the substrates of Examples 1 to 3 are higher. After the enamelled steel sheet after high temperature heat treatment, the yield strength variation (decrease) of Examples 1 to 3 is all less than 20 MPa, yet, the yield strength variation of Comparative Examples 1 and 2 is greater than 75 MPa. In other words, the yield strength of Examples 1 to 3 did not decrease significantly.

2A and 2B are the metallographic pictures of the substrate and the enamelled steel sheet of Example 1 respectively; and FIG. 2C and 2D are the metallographic pictures of the substrate and the enameled steel sheet of Comparative Example 1 respectively. It can be seen from Figure 2A and Figure 2B that the grain size of the substrate is about No. 11 to No. 12, while the grain size of the enameled steel sheet is about No. 10 to No. 11, that is, the grain size of the substrate in Example 1 after high-temperature heat treatment. The grains will be slightly larger, but the difference is not too large, so the strength performance is similar. In Figure 2C and Figure 2D, the grain size of the substrate of Comparative Example 1 is about No. 8 to No. 9 (that is, the average grain size is about 20 μm to about 50 μm), while the grain size of the enameled steel sheet is about 7 to 8 (that is, the average particle size is about 30 μm to about 70 μm), but in Figure 2D, it can be seen that some areas have obvious abnormal grain growth, such as the area framed by line 200, so the enameled steel of Comparative Example 1 The yield strength of the sheet is quite different from that of the substrate.

(1) 氫擴散係數愈小，代表氫於試片中愈不易通過，即氫愈不易擴散。實施例一至三所計算出的氫擴散係數為6.0×10 ^-7cm ²/s至9.0×10 ^-7cm ²/s；而比較例一及二所計算出的氫擴散係數為1.0×10 ^-6cm ²/s至4.0×10 ^-6cm ²/s。實施例一至三的氫擴散係數較比較例一及二的氫擴散係數小一個數量級以上，顯然實施例一至三的搪瓷鋼片有較佳的儲氫能力，故應具有較佳的抗跳鐵能力。 In addition, the enamelled steel sheets of Examples 1 to 3 and Comparative Examples 1 and 2 were used for hydrogen permeation test, which adopted the ASTM G148 standard, and used the electrochemical hydrogen charging test method to obtain the hydrogen permeation curve and hydrogen diffusion coefficient. FIG. 3 shows the hydrogen permeation curves 310 and 320 respectively obtained from the hydrogen permeation test of the enameled steel sheets of the first embodiment and the first comparative example. It can be seen from Fig. 3 that the hydrogen permeation time (t _b1 ) of Example 1 is longer than that of Comparative Example 1 (t _b2 ), indicating that the time required for hydrogen to diffuse through the thickness of the enamelled steel sheet of Example 1 is longer. According to the following formula (1), the hydrogen diffusion coefficient (DH) can be calculated using the above hydrogen permeation time (t _b ) and the thickness (L) of the enamel steel sheet:

(1) The smaller the hydrogen diffusion coefficient, the harder it is for hydrogen to pass through the test piece, that is, the harder it is for hydrogen to diffuse. The hydrogen diffusion coefficients calculated in Examples 1 to 3 are 6.0×10 ^-7 cm ² /s to 9.0×10 ^-7 cm ² /s; while the hydrogen diffusion coefficients calculated in Comparative Examples 1 and 2 are 1.0×10 ^{- 6} cm ² /s to 4.0×10 ^-6 cm ² /s. The hydrogen diffusion coefficients of Examples 1 to 3 are more than an order of magnitude smaller than those of Comparative Examples 1 and 2. Obviously, the enamelled steel sheets of Examples 1 to 3 have better hydrogen storage capacity, so they should have better anti-iron jumping ability .

According to the above examples, by applying the manufacturing method of the enamelled steel sheet of the present invention, the enameled steel sheet with high temperature softening resistance is obtained by controlling the composition of the steel billet, the coil temperature in the manufacturing process, and the quenching and tempering rolling rate. Therefore, it will not significantly reduce the yield strength due to high temperature heat treatment, so it has sufficient strength. Furthermore, the enamelled steel sheet of the present invention also has a small hydrogen diffusion coefficient, which means it has a good hydrogen storage capacity, so it has a better preventive ability against iron jump defects common in enamel.

Although the present invention has been disclosed as above with several embodiments, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field of the present invention can make various embodiments without departing from the spirit and scope of the present invention. Changes and modifications, so the scope of protection of the present invention should be defined by the scope of the appended patent application.

100: method 110, 120, 130, 140, 150, 160: operation 200: line 310, 320: hydrogen permeation curve t _b1 , t _b2 : hydrogen permeation time

According to the following detailed description and reading together with the accompanying drawings, the aspects of the present disclosure can be better understood. It is to be noted that, as is the standard practice in the industry, many features are not drawn to scale. In fact, the dimensions of many of the features are arbitrarily scaled for clarity of discussion. [ FIG. 1 ] is a flowchart illustrating a method of manufacturing an enamelled steel sheet according to some embodiments of the present invention. [FIG. 2A] and [FIG. 2B] are metallographic pictures of the substrate and the enamelled steel sheet of Example 1, respectively. [Fig. 2C] and [Fig. 2D] are metallographic pictures of the substrate and enameled steel sheet of Comparative Example 1, respectively. [ FIG. 3 ] is a graph showing the hydrogen permeation curves obtained by the hydrogen permeation test of the enameled steel sheets of Example 1 and Comparative Example 1.

Domestic deposit information (please note in order of depositor, date, and number) none Overseas storage information (please note in order of storage country, institution, date, and number) none

100: method

110,120,130,140,150,160: operation

Claims (9)

A method for manufacturing an enamelled steel sheet, comprising: providing a steel billet, wherein based on the steel billet is 100wt%, the steel billet includes: 0.04wt% to 0.16wt% carbon; 0.50wt% to 1.50wt% manganese; 0.01 wt% to 0.30wt% silicon; 0.01wt% to 0.20wt% phosphorus; 0.01wt% to 0.30wt% copper; 0.01wt% to 0.30wt% chromium; 0.01wt% to 0.10wt% titanium; Wt% to 0.12wt% of aluminum; 0.01wt% to 0.02wt% of nitrogen; not more than 0.03wt% of sulfur; not more than 0.10wt% of nickel; and the rest are iron and unavoidable impurities; A hot-rolling operation to obtain a finished rolled steel; performing a coiling operation on the finished rolled steel to obtain a hot-rolled steel coil, wherein a coiling temperature of the coiling operation is 550°C to 650°C; A cold-rolling operation is performed on the rolled steel coil to obtain a cold-rolled steel strip; a temper-rolling operation is performed on the cold-rolled steel strip to obtain a substrate, wherein a rolling ratio of the temper-rolling is 0.5% to 3.0 %; and performing a heat treatment operation on the substrate to obtain the enamelled steel sheet, wherein the heat treatment operation includes performing 10 minutes at a temperature of 850°C to 950°C clock to 20 minutes. The manufacturing method of enamelled steel sheet as described in Claim 1, wherein the hot rolling operation includes: performing a reheating step on the steel billet to raise the temperature to 1150°C to 1300°C; and after performing the reheating step, the The steel billet is subjected to a hot rolling step to obtain the finished rolled steel product, wherein one of the hot rolling step finishing temperature is not lower than the Ar3 temperature. The manufacturing method of the enamelled steel sheet as described in Claim 1 further includes: performing an annealing operation on the cold-rolled steel strip before performing the temper rolling operation. The manufacturing method of enamelled steel sheet as described in Claim 3, wherein the annealing operation includes: performing a heating step on the cold-rolled steel strip to reach 650°C to 850°C; after performing the heating step, the cold-rolled steel strip performing a temperature holding step at 650° C. to 850° C. for 80 seconds to 240 seconds; and after performing the temperature holding step, performing a cooling step on the cold-rolled steel strip to drop to room temperature. The manufacturing method of the enamelled steel sheet according to claim 1, wherein a reduction rate of the cold rolling operation is 40% to 80%. The manufacturing method of the enamelled steel sheet according to claim 1, wherein the difference between the yield strength of the enamelled steel sheet and the yield strength of the substrate is not more than 20 MPa. An enamelled steel sheet produced by the manufacturing method described in any one of Claims 1 to 6. The enamelled steel sheet as claimed in item 7, wherein the enamelled steel sheet has a yield strength of not less than 300MPa and a tensile strength of not less than 400MPa. The enamelled steel sheet as claimed in claim 7, wherein the enamelled steel sheet has a hydrogen diffusion coefficient of 6.0×10 ^-7 cm ² /s to 9.0×10 ^-7 cm ² /s.

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