JPH11236648A - Cold rolled steel sheet for porcelain enameling and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a beautiful steel sheet for two time application porcelain enameling free from defects such as flaws on the surface, good in press formability and excellent in the adhesion of porcelain enameling and fishscale resistance. SOLUTION: This cold rolled steel sheet for >= two time application porcelain enameling has a compsn. contg., by weight, 0.0005 to 0.0030% C, <=0.003% N, 0.015 to 0.060% Cu, 0.010 to 0.050% Nb, 0.0020 to 0.0045% B and 0.006 to 0.015% O, also satisfying Cu(%)/P(%): 1.0 to 3.5, P(%)/S(%): 0.6 to 2.0% B(%)/N(%): 0.77 to 2.31 and Nb(%)/C(%): >=10, and the balance Fe with inevitable impurities. Further, in the method for producing the cold rolled steel sheet for >= two time application porcelain enameling, a continuously cast slab having the compsn. is subjected to hot rolling so as to regulate the rolling starting temp. to 1,100 to 1,250 deg.C, is coiled at 580 to 720 deg.C, is pickled, is thereafter subjected to cold rolling at 75 to 90% draft and is subjected to continuous annealing at >=800 deg.C within 120 sec soaking time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet for enamel, which is suitable for two- or more-time enamelling, such as kitchenware, which is strictly processed and particularly required to have a beautiful surface. About.

[0002]

2. Description of the Related Art Enamel products made of a steel plate as a base metal include household items such as pots and kettles, parts for heating appliances and electric appliances, kitchen utensils such as sinks and around water, bathtubs, and building panels. . Ordinary enamel products are prepared by cutting the base steel sheet, shaping it into the required shape, adjusting the part shape by welding, etc., performing degreasing, pickling, applying a lower glaze, drying and firing, firing, and then inspecting the upper glaze. It is baked, baked, and, if necessary, baked with colored glaze.

[0003] Enamel products are generally double enameled by firing twice or more in this way (hereinafter, when enameled twice or more times, they are simply called double enameled). However, there is also a one-time enamel where the upper glaze is directly baked on the steel plate without the lower glaze. In the case of a single enamel, the firing process can be performed only once, so that the work process can be shortened. In addition, depending on the application, the film thickness can be reduced, and a product excellent in thermal shock resistance can be obtained.

[0004] Defects considered to have a cause also in the base steel sheet generated during the production of enamel products include poor adhesion of the enamel layer, foaming, nail skipping, surface defects due to the steel sheet, and firing distortion. Poor adhesion of the enamel layer means that the base material of the steel sheet is exposed, for example, when a steel ball is dropped on the enamel surface and the enamel layer on the surface is destroyed or peeled off. It is said to be due to improper pretreatment of the glazed steel sheet. It is considered that the reason for the steel sheet side is that the roughening of the substrate during pickling is insufficient, and the effect of anchoring the glaze to the steel sheet surface during firing is insufficient. Foaming is a phenomenon in which air bubbles remain on the enamel surface after firing, and mainly due to vaporization components remaining in flaws and the like on the steel sheet surface due to the reaction of mainly the composition C and the glaze in the steel sheet during firing. When the bubbles burst, it becomes a pinhole. Nail fly is also called a fishtail, which is a defect in which the enamel layer breaks into a shape like a cut piece when a crescent-shaped nail is cut after firing and cooling, and from the time immediately after cooling to about one week Occur. This is said to be because the hydrogen that had infiltrated into the steel sheet and dissolved in the process during the process of baking enamel, etc., turned into a gas after cooling and collected at the interface between the steel sheet and the glaze, and the pressure destroyed the enamel layer. . In addition, inclusions and the like inherent in the steel appear on the surface of the base steel sheet and become surface defects, which may affect the surface appearance even after enamelling. The firing strain is a phenomenon in which the transformation temperature of steel containing a small amount of carbon is near the firing temperature of the enamel, and near the transformation temperature, the thermal expansion accompanying the temperature rise becomes non-linear.
The quality of the enamel of a steel sheet is judged by whether or not all these properties are satisfactory for the application.

[0005] In the case of double enamelling, the influence of the steel sheet is reduced by using a glaze having good adhesion to the steel sheet, for example, a glaze containing Co for the lower glaze and a glaze having a dense and beautiful surface for the upper glaze. Can be reduced. That is, by improving the adhesiveness, it acts in the direction of suppressing the nail jump, foaming stops within the range of the lower glaze, and the surface defect of the steel plate increases the concealing effect by thickening the glaze.

[0006] On the other hand, the single enamel has the advantage of requiring only one firing, but has the disadvantage of being strongly affected by the base steel sheet. Defects that occur during the production of enameled products are the same as in the case of double application. However, as the glaze requires a beautiful finished surface, it is not possible to use Co or the like that is deeply colored. Adhesion is inferior to glaze. In addition, since the thickness of the enamel is thin, the surface defects of the steel sheet appear as they are, and since a beautiful and dense enamel layer is formed directly on the steel sheet surface, the permeation of hydrogen is hindered, and a nail jump is likely to occur.

Hitherto, the quality of the enamel of a steel sheet has been judged mainly by whether or not it can be used for this single enamel. This is because once-enameled enamels are more affected by the properties of the steel sheet than twice-enamelled enamels, steel sheets that can be applied to one-time enamels can be used without problems for two-time enamels. Because.
What had excellent enamel properties for the single enamel was a steel sheet made of rimed steel or capped steel and decarburized by an open coil annealing method. This steel sheet has a relatively high-purity layer on the surface from the rim layer of the ingot, but contains a large amount of oxygen due to the need for a rimming action, which forms oxide-based inclusions inside the steel to form a large number. Distributed. C in the steel, which causes firing distortion, pinholes, and air bubbles, is sufficiently reduced by decarburizing annealing, and the surface is beautiful and has good adhesion to glaze due to the presence of the rim layer. Since microcavities and internal inclusions formed by the removal of C trap hydrogen causing nail jump, nail jump hardly occurs, and the steel sheet was optimal for enamel.

[0008] This rimmed steel or capped steel casts a steel ingot and slab-rolls it to produce a slab of hot-rolled material, which results in poor production efficiency and yield. In addition, the casting process requires high heat and poor dust. Due to the working environment, most of the materials used for normal steel plate production have been replaced by slabs produced by continuous casting. However, the continuous casting method cannot produce steel equivalent to rimmed steel or capped steel, and can only obtain a steel sheet having extremely poor enamelness under ordinary production conditions. Therefore, various studies have been made to obtain a steel sheet having excellent enamel properties by a slab of a continuous casting method. First, a steel composition for improving adhesion was selected as a continuous cast material, and as for C, a steel whose carbon was extremely low carbonized by vacuum refining at the time of melting was used. Since the annealing of steel sheets has also shifted from the coil annealing method to the continuous annealing method, decarburization during annealing cannot be sufficiently performed in a short time, and therefore, ultra-low carbon steel is used in this steelmaking stage.

[0009] The most important problem in improving the enamelness of a steel sheet by using a continuous cast material is nail skipping. As described above, nail fly is suppressed by improving the adhesion of the glaze and by additionally trapping hydrogen in inclusions, voids or precipitates in the steel. On the other hand, ordinary continuous cast materials are being refined and cleaned due to advances in smelting technology and demands for better performance and beautiful steel sheet surfaces, as impurities and inclusions are greatly reduced. It has become. For this reason, in the case of enamel use, a method has been developed in which oxygen or the like that causes inclusions is purposely increased. For example, in JP-A-59-35657, C is 0.003% or less and O
Of an enamel steel by continuous casting containing 0.02% or more of Nb and 0.02% or less of the C content except that Nb is present as an oxide. A similar invention is disclosed in JP-A-59-229463. In the official gazette, C is 0.005% or less and O is 0.016% or less.
The invention of a steel in which C is slightly higher than 0.030% and O is slightly lower, and Nb or B is added for the purpose of preventing aging has been proposed. Also, increasing oxygen to increase oxide-based inclusions increases defects such as surface flaws. Zr, REM, Nb, etc. are added via a wire during the downward flow of molten steel in the mold during continuous casting, and electromagnetic stirring is performed. By making it uniform
Japanese Patent Application Laid-Open No. 2-235550 discloses a method for producing a steel for enamel containing 0.020 to 0.080% of O without surface flaws. Furthermore, it is considered that the press formability and the weldability are also excellent.
An enameled steel sheet containing 3 to 0.100% is also disclosed in JP-A-6-57374.
No. in the official gazette.

What is common to the steels in these inventions is that Cu, P, S, etc. are controlled for improving the adhesion of the enamel, and in order to increase the oxygen content, the ultra-low temperature for cold-rolled steel sheets is usually reduced. Al and Ti contained in carbon steel are hardly used. When Al or Ti is not contained, strain aging due to N appears, so B or Nb is added for fixing this nitrogen.

[0011] As described above, the measures by adding oxygen or the like for improving the enamelness increase oxide inclusions, and as a result, increase of surface flaws and press formability of the steel sheet. This results in deterioration. However, in recent years, enamel products, such as kitchenware, have been increasing in pursuit of aesthetic factors, and the press workability of the steel sheet itself has been strongly demanded in order to streamline the manufacturing process. In other words, a steel sheet with excellent enamelness, especially excellent nail-cracking resistance, with no surface flaws, and with excellent press workability is required.However, enameled steel sheets made of continuous cast slabs that can meet this requirement are sufficient. Has not yet been obtained.

[0012]

SUMMARY OF THE INVENTION The present invention is mainly directed to an enameled steel sheet used for double enamelling, and the surface of the steel sheet is beautiful without defects such as flaws, and has good press formability. It is intended to provide a steel sheet having excellent adhesiveness and anti-claw flying resistance.

[0013]

[MEANS FOR SOLVING THE PROBLEMS] In an enamel product,
Claw jumps occur after the final stage of manufacturing,
Since a lot of the lots occur in many cases, and they are almost impossible to repair, if they occur, the damage will be enormous. Therefore, they have been considered as the most important issue for enameled steel sheets. As a countermeasure against the result, as described above, the inclusion in the steel is significantly increased in the steel sheet. However, increasing the number of inclusions in the steel deteriorates the workability, particularly ductility, of the steel sheet, and also increases the surface flaw.

[0014] On the other hand, from the enamel, one time to detect the enamel suitability of the steel sheet remarkably, the beauty of the enamel,
In terms of texture and durability, they are not often used in kitchen utensils and the like, and are mainly products that are repeatedly enameled twice or more times. Also, the steel plate of the material to be enameled is avoided from time-consuming work such as welding, and is formed to the final shape by press working, so that particularly good workability as a steel plate is required. It has become.

It is usually almost impossible to increase the number of inclusions in the steel and further improve the workability of the steel sheet from the common sense of materials. Claw jumps are caused by hydrogen that has collected between the enamel layer and the steel plate,
If the adhesion between the enamel layer and the steel sheet is sufficient, the hydrogen once collected will eventually diffuse and disappear without being able to cause nail jump. The one-time enamel requires the good finish of the enamel at the same time, and it seems that there is a limit to improving the adhesion of the glaze. Can be selected as Therefore, if it is possible to select the double enamel, it is considered that the strictness of the requirement for the steel plate with respect to the anti-claw flying property is greatly reduced.

[0016] From such a viewpoint, the present inventors presuppose that the enamel is applied twice, and in this case, after sufficiently securing the enamel, develop a steel sheet with improved press workability as much as possible. . In the single enamel, a large amount of oxygen is contained to increase inclusions in the steel. However, in order to improve workability by reducing inclusions and reduce surface flaws, oxygen must be reduced as much as possible. Therefore, in the case of double enameling, first, the limit content in which claw jump did not occur was confirmed, and then various measures for improving press workability were examined. As a result, it has been found that it is most effective to combine a low amount of oxygen with an appropriate amount of Nb.

In the case of applying the continuous annealing method after cold rolling of ultra-low carbon steel, the addition of Ti has a remarkable effect on improving the press workability and deep drawability. However, when Ti is added, the oxygen content cannot be increased, and there is a risk that secondary working embrittlement occurs during press molding, and there is a problem that this becomes particularly noticeable when oxygen is high. This secondary work embrittlement is a cracking phenomenon that occurs with a slight deformation when a steel sheet is press-formed and then the formed article is subjected to deformation by rapidly applying a deformation in a different direction from the previous one. If a plate is press-formed into a cylindrical shape like a kettle and processing such as drawing is performed, cracks occur and molding cannot be performed.

Investigation of the use of Nb, which fixes solid solution C like Ti and has the effect of improving deep drawability, showed that the addition of an appropriate amount improves the press workability even if there is some oxygen. It has been found that the effect of (1) is sufficient, the secondary work embrittlement resistance is excellent, and the nail flying resistance is also improved.

Furthermore, if the enamel is applied twice or more, it will be re-heated at a temperature exceeding 800 ° C., but in the case of a very low carbon steel sheet, it will be repeatedly heated at a relatively low workability after pressing. May cause abnormal gray loin. In severe cases, there is one crystal grain per plate thickness, and even a small impact may cause cracking or a pattern that may be affected by giant crystal grains on the enamel surface. The addition of Nb was also effective in suppressing such grayrose.

It is considered that these various advantageous effects were brought about by the properties of Nb. Nb is T
Like i, it is a carbonitride forming element, and it is well-known that C is fixed by adding a small amount to ultra-low carbon steel, thereby significantly improving the press formability of a cold-rolled steel sheet by the steel. However, as compared with Ti, the bonding strength with C is slightly weaker, the affinity with oxygen and nitrogen is slightly lower, and if the content is slightly increased too much, the recrystallization temperature rises, and the annealing temperature must be increased. There are difficulties. However, since the affinity for oxygen is rather low, it is possible to add oxygen at a high level, and the bonding force with C is slightly weak because the solid solution C slightly remains and the secondary processing embrittlement resistance Seems to have improved. In addition, since oxygen is contained in a relatively large amount, in addition to fine NbC precipitates due to bonding with C, N
It is presumed that oxides containing b are mixed in the steel, but these dispersed non-metallic particles effectively act on hydrogen trapping and improve nail fly resistance. In addition, the finely precipitated NbC particles hinder the movement of the crystal grain boundaries and suppress the grain growth, but are considered to be particularly effective in suppressing abnormal graining louse because they are not easily aggregated even at high temperatures.

When Al or Ti having a function of fixing nitrogen is not added in order to secure the oxygen content, nitrogen in a solid solution state causes strain aging or deteriorates workability. Although Nb also has this nitrogen fixing action, it is consumed to form carbides and oxides and its action cannot be expected. Therefore, nitrogen in steel was reduced as much as possible, and a small amount of B was added. As a result, it was found that the addition of an appropriate amount of B has an effect of suppressing strain aging and improving nail fly resistance. Since this steel does not contain Al and other elements for deoxidation and has a high oxygen content, it seems that the addition of a small amount of B, which is usually applied, would oxidize the steel and have no effect.
The effect obtained by adding B to the conventional ultra-low carbon cold rolled steel sheet is exhibited also in this high-oxygen Nb-added steel.
Other factors that affect the adhesion of enamel glaze include Cu,
It is known that elements such as P and S are contained. However, it was confirmed that the addition of an appropriate amount of these elements to this steel is also effective in improving the enamel properties.

The steel sheet as a product is manufactured from the continuous cast slab obtained by adjusting the components in a normal process such as hot rolling, cold rolling, and annealing. Have an effect. In particular, setting the winding temperature of the hot rolling to a higher temperature can improve the anti-claw flying resistance.
This was thought to be due to the fact that the size of the fine particles precipitated during winding grew to a size suitable as a hydrogen trapping site. In addition, continuous annealing after cold rolling is also effective in improving the nail enamel fly resistance.
This is because voids and the like formed around the inclusions during cold working do not disappear by short-time annealing of continuous annealing, and function as a place for trapping hydrogen. Even in continuous annealing, if the soaking time becomes long, the effect of suppressing nail jump may decrease. Also, from the viewpoint of press formability of the steel sheet, in the case of the Nb-added ultra-low carbon steel containing an appropriate amount of oxygen, the hot-rolled coil obtained by increasing the winding temperature has a high reduction ratio in cold rolling. It was also confirmed that the annealing temperature should be set to a high temperature of 800 ° C. or more.

With respect to the above steel composition and the production conditions after hot rolling, the workability is better, the enamelness,
In particular, the present invention has been completed by clarifying the range limit for obtaining an enameled steel sheet having good surface enamelability with good enamelability twice. The gist of the present invention is as follows.

(1) C: 0.0005 to 0.0030% by weight,
Si: 0.2% or less, Mn: 0.5% or less, P: 0.005 to 0.020
%, S: 0.005 to 0.025%, Al: 0.01% or less, N: 0.00
3% or less, Cu: 0.015 to 0.060%, Nb: 0.010 to 0.050
%, B: 0.0020 to 0.0045%, O: 0.006 to 0.015%, and Cu (%) / P (%): 1.0 to 3.5 P (%) / S (%): 0.6 to 2.0% B (%) / N (%): 0.77 to 2.31 Nb (%) / C (%): 10 or more, with the balance being excellent in formability and surface properties characterized by having a steel composition comprising Fe and unavoidable impurities Cold rolled steel sheet suitable for enameling more than twice.

(2) C: 0.0005 to 0.0030% by weight,
Si: 0.2% or less, Mn: 0.5% or less, P: 0.005 to 0.020
%, S: 0.005 to 0.025%, Al: 0.01% or less, N: 0.00
3% or less, Cu: 0.015 to 0.060%, Nb: 0.010 to 0.050
%, B: 0.0020 to 0.0045%, O: 0.006 to 0.015%, and Cu (%) / P (%): 1.0 to 3.5 P (%) / S (%): 0.6 to 2.0% B (%) / N (%): 0.77 to 2.31 Nb (%) / C (%): 10 or more, the remainder being a continuous cast slab having a steel composition composed of Fe and inevitable impurities, and a rolling start temperature of 1100 to 1250. Hot rolling at 580-720 ° C
After pickling, cold-rolled at a rolling rate of 75 to 90%, 800
A method for producing a cold-rolled steel sheet suitable for enameling more than twice, which is excellent in formability and surface properties, characterized by performing continuous annealing with a soaking time of not more than 120 seconds at a temperature of not less than ° C.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In practicing the present invention, the chemical composition and production conditions of a raw steel are regulated. The scope of the regulation and the reason for the regulation will be described. All component contents are% by weight.

C has a content of 0.0005 to 0.0030%.
If the content exceeds 0.003%, bubble defects of enamel tend to occur, and the press formability of the steel sheet also deteriorates. On the other hand, the reason why the content is set to 0.0005% or more is that refining time is too long to reduce the content to a value lower than the value, and it is necessary to increase the amount of oxygen, thereby increasing surface flaws. Desirable is in the range of 0.0005 to 0.0020%.

Although Si does not greatly affect the enamelability, when it is increased, it causes scratches on the surface of the steel sheet or deteriorates the appearance, and increases the hardness to deteriorate the press formability.
Limited to 0.2% or less. The lower limit is not particularly defined, but may be about the level of ordinary impurities.

Mn is necessarily contained in steel at least about 7 times the S content in order to prevent hot brittleness due to S.
However, since the transformation point of the steel is lowered, it is better not to use a large amount for enamel use in order to prevent transformation in the firing temperature range. If the content is too large, the workability of the steel is also deteriorated, so the content is made 0.5% or less.

P is one of the inevitable impurities of steel, but tends to increase the melting speed of the steel sheet during pickling. If the amount is too large, an enamel foam defect is liable to occur due to excessive pickling loss. On the other hand, if the amount is too small, the pickling becomes insufficient and the adhesion is deteriorated. Therefore, the range of the content of P is
0.005 to 0.020%.

The content of S is set to 0.005 to 0.025%. S, like P, is an inevitable impurity of steel and causes cracking during hot rolling. Therefore, Mn is usually added to suppress this. The resulting MnS is a typical inclusion in steel, and the smaller the better, the better the press formability. However, the enamel becomes an inclusion for trapping hydrogen, has an effect of increasing the pickling rate, bringing about moderate surface roughness, and improving the adhesion. Therefore, the upper limit is set in terms of improving the workability of steel sheets.
It is set to 0.025%, and the lower limit is made 0.005% from the point of enamel.

Although both P and S are elements that increase the dissolution rate during pickling, that is, the amount of loss in pickling, investigations on the effect on adhesion have revealed that P and S have an interaction, It was found that if the ratio was too large or too small, the adhesion was poor and bubbles and the like were easily generated. Therefore, P (%) / S (%) is limited to the range of 0.6 to 2.0. this is,
It was presumed that it was related to not only the weight loss of the pickling but also the surface condition after the pickling.

Al is used as a deoxidizing agent at the steelmaking stage to ensure the soundness of the slab, but is contained in steel as much as possible so as to contain O. Therefore, its content is set to 0.01% or less.

N does not affect the enamel property, but causes strain aging and impairs the workability of the steel sheet. Therefore, B is added and fixed as BN. However, B deteriorates the workability of steel and cannot be added much, so N should be reduced as much as possible. Therefore, the content of N is set to 0.003% or less.

Cu reduces the dissolution rate of steel during pickling, but has the effect of improving the adhesion of the enamel. If its content is less than 0.015%, the effect of improving the adhesion is not sufficient, and if it exceeds 0.060%, the dissolution rate is too low, and surface irregularities due to pickling cannot be sufficiently obtained, which also lowers the adhesion. Therefore, its content range is 0.015-0.060%
It is preferable that the content is 0.020 to 0.050%.

In addition, Cu has the effect of reducing the amount of pickling while P has the effect of increasing the amount of acid pickling. As a result of investigating the mutual influence, the improvement of the adhesion can be achieved by controlling the value of the ratio of Cu and P. It has been found that there is an optimum range, and that the adhesion is inferior if the range is larger or smaller. That is, C
u (%) / P (%) needs to be regulated within the range of 1.0 to 3.5.

Nb improves the press formability of the steel sheet by fixing C as NbC. In addition, the generated carbides and oxides trap hydrogen, thereby improving the anti-claw flying resistance. Such an effect is insufficient when the content is less than 0.01%, but when the content is increased to exceed 0.05%, the steel sheet is hardened and the press formability is deteriorated. Therefore, the content is set to 0.010 to 0.050%.

However, if Nb is small relative to the amount of C, the fixing effect is insufficient, causing a reduction in press workability and strain aging. Therefore, it is necessary to add a sufficient amount of Nb to the amount of C. That is, Nb (%) / C (%) is set to 10 or more.

B is added for the purpose of fixing N and making it unaged. In order to combine with nitrogen in a state where a large amount of oxygen is contained, it is necessary to contain at least 0.0020% or more.
However, too much may embrittle the steel, so it is at most 0.0045%. That is, the content of B is 0.0020.
To 0.0045%.

If B is insufficient with respect to N, non-aging becomes insufficient, and if B is too large, press workability is deteriorated. Therefore, it is necessary to regulate the amount of B relative to the content. In the above composition range, a steel sheet obtained by changing only the contents of B and N with substantially the same chemical composition was subjected to a aging test at 100 ° C. for 60 minutes. %) / N
FIG. 1 shows the relationship between the value of (%) and elongation and elongation at yield point. It can be seen that the yield point elongation becomes zero as B / N increases, but the elongation decreases when B / N becomes too large. Therefore, the value of B / N is limited to the range of 0.77 to 2.31.

The content range of O (oxygen) is set to 0.006 to 0.015%. O forms an inclusion mainly composed of an oxide of Mn in the steel, and this inclusion traps hydrogen, so that it is possible to suppress the occurrence of nail jump. In the case of the present invention, N
Although b and B are contained, it is considered that these elements have a strong bonding force with O as much as or more than Mn and affect the state of the oxide.

The effect of the hydrogen trap such as inclusions can be compared by the hydrogen permeation time in the steel sheet. Hydrogen permeation time is 1.
Using a 5% H ₂ SO ₄ aqueous solution containing 4 g / l thiourea as an electrolyte, a current with a current density of 50 mA / dm ² is applied to one surface of the steel plate to generate hydrogen, and hydrogen is observed on the opposite surface Measure the time until it is done. It is said that the longer the hydrogen permeation time, the greater the effect of trapping hydrogen, and the better the enamel's nail resistance. Therefore, the other chemical compositions of the steels were made almost the same, and a steel sheet with no added Nb and B and a steel sheet with added 0.025% of Nb and 0.0030% of B were added in steels with various O contents. Then, the hydrogen permeation time was measured.

The results are shown in FIG. 2. It can be seen that the hydrogen permeation time tends to be longer as O increases, and it becomes even longer when Nb and B are present. Hydrogen permeation time is 5
If it is more than a minute, the nail jump will not occur with the enamel twice,
This can be achieved by including 0.02% oxygen.
However, oxygen should be kept as low as possible, because the increase in inclusions increases the surface flaws and impairs the workability of the steel sheet. The result of FIG. 2 shows that when Nb and B are present,
This indicates that the amount of oxygen for obtaining the same effect may be small. However, even in that case, if the value falls below 0.006%, a nail jump will occur.

As the number of inclusions increases, surface flaws are liable to occur and the workability deteriorates. Therefore, the smaller the oxygen that causes inclusions, the better. From the viewpoint of ensuring excellent workability, the upper limit is preferably up to 0.015% as a range that has substantially no effect even if it is contained.

For the above reasons, the oxygen content is reduced to 0.00
The content is 6% to 0.015%, and more preferably 0.010% to 0.015% in order to achieve both enamelability and workability.

When a steel slab having the above composition is used as a cold-rolled steel sheet for enamel, a conventional hot rolling method is used.
It may be manufactured through the steps of cold rolling and continuous annealing. However, in these steps, if the treatment is desirably performed under the following conditions, a steel sheet having better performance can be obtained.

The heating temperature of the slab is 1100-1250 ° C.
If the temperature exceeds 1250 ° C, the distribution form of precipitates such as NbC and MnS will change, affecting the nail flaw resistance of the enamel and increasing the scale loss due to oxidation.
If the temperature is lower than 1100 ° C., it is difficult to perform rolling due to a decrease in press formability or an increase in rolling load.

The finishing temperature of the hot rolling is preferably from about 850 to 910 ° C. If it is too high or too low, press workability tends to decrease.

The winding temperature is preferably from 580 to 720 ° C. Nb
Carbide, B nitride, etc. are sufficiently precipitated before cold rolling,
A higher winding temperature is better for improving the press formability and the nail fray resistance of the enamel, and it is better to be 580 ° C or higher, but if it is too high, there is a risk of increasing the scale and accompanying deterioration of the surface properties. Therefore, up to 720 ° C.

The cold rolling reduction is preferably at least 75% in order to increase the r-value which is an index of press workability, particularly deep drawability. However, if it is too large, a load is imposed on the rolling, and the r-value also decreases. That is, the rolling reduction of the cold rolling is 75 to 90%.

The annealing is desirably a continuous annealing method from the viewpoint of improving nail flying resistance. This is presumably because micro-cracks and voids formed near the inclusions during cold rolling are not sufficiently filled by the short-time soaking in the continuous annealing method, and serve as trapping sites for hydrogen. In this case, the annealing temperature is 800 to 900 ° C. This is because if the temperature is lower than 800 ° C., the annealing may be insufficient and the workability may be inferior. If the temperature exceeds 900 ° C., the r value may be reduced. Desirable is 840-880C. If the temperature of the material exceeds 800 seconds or more when the temperature of the material is 800 ° C. or more, the purpose of annealing can be substantially achieved. However, over a long period of time, the effect of short-time soaking is lost.
The annealing time in the temperature range exceeding 120 ° C. is preferably 120 seconds or less.

[0052]

EXAMPLES Example 1 A steel slab having the chemical composition shown in Table 1 was cast by a continuous casting method, and a cold-rolled steel sheet was obtained by the following steps and conditions.

1) Hot rolling Slab soaking: 1200 ° C Finishing temperature: 880 to 900 ° C Winding temperature: 680 ° C Finished plate thickness: 4.0mm 2) Pickling 10% HCl for 40 to 50 seconds 3) Cold Rolling Reduction: 83% (thickness: 0.7mm) 4) Continuous annealing Annealing temperature: 850 ° C Soaking time: 100 seconds 5) Temper rolling Elongation: 0.6%

[0054]

[Table 1]

The obtained steel sheet was visually inspected for surface flaws, and after aging treatment at 100 ° C. for 60 minutes, a tensile test and an evaluation of secondary work brittleness were performed. The secondary working brittleness was formed by drawing a cylindrical cup with a diameter of 50 mm with a drawing ratio of 1.8, cooling it to -40 ° C, dropping a 100 kg weight 1000 mm, crushing it from the side, and observing the brittle fracture surface by observing the fracture surface. The presence or absence was checked. For those in which a brittle fracture surface was found, the cooling temperature was increased and crushed, and the limit temperature at which the brittle fracture surface appeared was determined. In this case, the lower the generation limit temperature is, the better.

The enamel was enamelled under the following conditions using a cut piece of steel sheet having a width of 100 mm and a length of 200 mm. For the adhesiveness, an adhesion index was obtained by a PEI method. Was used to check for the occurrence.

Pretreatment After immersion in a 13% H ₂ SO ₄ aqueous solution at 75 ° C. for 5 minutes, neutralized, washed and dried.

Ground enamel glaze Ground coat (# 21269, manufactured by Nippon Fellow Co.) Glaze Sprayed about 100 μm on both sides baked at 820 ° C for 4 minutes The test was not performed in this case, as it would not cause any problems. Table 2 summarizes the results of each of these tests. The steel sheets of test numbers 1 to 5 having the chemical composition specified by the present invention have good elongation, no elongation at the yield point, no surface flaws, and good enamelness. On the other hand, No. 6 has good enamel properties, but surface flaws are recognized and elongation is poor. This is likely due to B and C being too high. No. 7 may be due to the occurrence of nail skipping, and the O content was too low below the range defined by the present invention. However, if the O content is too large, surface flaws appear as in No. 8. Also, Nos. 9 and 10 have poor elongation and elongation at the yield point, and have poor secondary work brittleness. This is presumably because B was not contained, or even if B was contained, B / N was too small to be within the range defined by the present invention, resulting in aging deterioration. Especially number 9
Is that Nb / C is too low below the range defined in the present invention, which may also be the cause of poor elongation. In Nos. 11 and 12, the Cu content or the value of Cu / P deviated from the range defined by the present invention, which affected the condition of the steel sheet surface during pickling, deteriorated adhesion, and caused foaming. It seems to be.

[0059]

[Table 2]

Example 2 Using slabs having the chemical compositions of steels A, B and D shown in Table 1, the slab soaking temperature was 1200 ° C., the rolling temperature was changed, hot rolling was performed, and cold rolling was performed. Cold rolling was performed at a reduction rate of 83%, and after tempering by changing the temperature and time by a continuous annealing method, temper rolling at an elongation of 0.6% was performed. For the obtained steel sheet, in the same manner as in Example 1,
After aging, conduct tensile tests and secondary work brittleness investigations.
The enamel properties were evaluated.

Table 3 shows the coiling temperature, annealing temperature and annealing time of hot rolling, and the results of a tensile test and an enamel test of the obtained steel sheet. As is clear from these, even if the steel has a chemical composition within the range of the present invention, if the production conditions are not favorable, it is understood that a steel sheet having sufficient characteristics cannot be obtained. In other words, Test No. 28 was inferior in elongation because the winding temperature of hot rolling was too low, and nail jumping occurred after enamelling.Test No. 29 caused nail jumping due to too long annealing time, and Test No. 30 was hot rolled. Winding temperature is high and surface flaws are generated. Further, in test No. 31, the annealing temperature was low and the elongation was insufficient, and in test No. 32, the annealing temperature was too high to cause the elongation at the yield point due to aging, and bubbles were formed after enamelling.

[0062]

[Table 3]

[0063]

The cold rolled steel sheet for enamel according to the present invention is beautiful without defects such as flaws on the surface and has good press formability. Good claw flying resistance
Good enamel properties are obtained. By using this steel plate, beautiful enamel parts used for kitchen utensils, system kitchens, home electric parts and the like can be easily manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the content ratio of B and N in steel and the elongation and yield point elongation in a tensile test of a steel sheet after aging treatment.

FIG. 2 is a diagram showing experimental results regarding the relationship between the oxygen content in a steel sheet and hydrogen permeation time, with and without Nb and B added.

[Procedure amendment]

[Submission date] August 7, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0024

[Correction method] Change

[Correction contents]

(1) C: 0.0005 to 0.0030% by weight,
Si: 0.2% or less, Mn: 0.5% or less, P: 0.005 to 0.020
%, S: 0.005 to 0.025%, Al: 0.01% or less, N: 0.00
3% or less, Cu: 0.015 to 0.060%, Nb: 0.010 to 0.050
%, B: 0.0020 to 0.0045%, O: 0.006 to 0.015%, and Cu (%) / P (%): 1.0 to 3.5 P (%) / S (%): 0.6 to 2.0 B (%) / N (%): 0.77 to 2.31 Nb (%) / C (%): 10 or more, with the balance being a steel composition comprising Fe and unavoidable impurities, characterized by having excellent formability and surface properties. Cold rolled steel sheet suitable for enamelling over time.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0025

[Correction method] Change

[Correction contents]

(2) C: 0.0005 to 0.0030% by weight,
Si: 0.2% or less, Mn: 0.5% or less, P: 0.005 to 0.020
%, S: 0.005 to 0.025%, Al: 0.01% or less, N: 0.00
3% or less, Cu: 0.015 to 0.060%, Nb: 0.010 to 0.050
%, B: 0.0020 to 0.0045%, O: 0.006 to 0.015%, and Cu (%) / P (%): 1.0 to 3.5 P (%) / S (%): 0.6 to 2.0 B (%) / N (%): 0.77 to 2.31 Nb (%) / C (%): 10 or more, the remainder being a continuous casting slab having a steel composition composed of Fe and inevitable impurities, and a rolling start temperature of 1100 to 1250 ° C. Hot rolling, 580-720 ° C
After pickling, cold-rolled at a rolling rate of 75 to 90%, 800
A method for producing a cold-rolled steel sheet suitable for enameling more than twice, which is excellent in formability and surface properties, characterized by performing continuous annealing with a soaking time of not more than 120 seconds at a temperature of not less than ° C.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0059

[Correction method] Change

[Correction contents]

[0059]

[Table 2]

Claims (2)

[Claims] [Claim 1] In weight%, C: 0.0005-0.0030%, S
i: 0.2% or less, Mn: 0.5% or less, P: 0.005 to 0.020
%, S: 0.005 to 0.025%, Al: 0.01% or less, N: 0.00
3% or less, Cu: 0.015 to 0.060%, Nb: 0.010 to 0.050
%, B: 0.0020 to 0.0045%, O: 0.006 to 0.015%, and Cu (%) / P (%): 1.0 to 3.5 P (%) / S (%): 0.6 to 2.0% B (%) / N (%): 0.77 to 2.31 Nb (%) / C (%): 10 or more, with the balance being excellent in formability and surface properties characterized by having a steel composition comprising Fe and unavoidable impurities. Cold rolled steel sheet suitable for enameling more than twice. 2. In% by weight, C: 0.0005 to 0.0030%, S
i: 0.2% or less, Mn: 0.5% or less, P: 0.005 to 0.020
%, S: 0.005 to 0.025%, Al: 0.01% or less, N: 0.00
3% or less, Cu: 0.015 to 0.060%, Nb: 0.010 to 0.050
%, B: 0.0020 to 0.0045%, O: 0.006 to 0.015%, and Cu (%) / P (%): 1.0 to 3.5 P (%) / S (%): 0.6 to 2.0% B (%) / N (%): 0.77 to 2.31 Nb (%) / C (%): 10 or more, the remainder being a continuous cast slab having a steel composition composed of Fe and inevitable impurities, and a rolling start temperature of 1100 to 1250. Hot rolling at 580-720 ° C
After pickling, cold-rolled at a rolling rate of 75 to 90%, 800
A method for producing a cold-rolled steel sheet suitable for enameling more than twice, which is excellent in formability and surface properties, characterized by performing continuous annealing with a soaking time of not more than 120 seconds at a temperature of not less than ° C.