JPH08291368A - Cold rolled steel sheet for porcelain enamelling and its production - Google Patents

Abstract

PURPOSE: To improve the formability and enamelling performance of a steel sheet by incorporating specified amounts of C, Mn, Cu, P, N, Ti, B, Nb, Al and Fe therein. CONSTITUTION: The compsn. contg., by weight, <=0.003% C, 0.1 to 0.5% Mn, 0.02 to 0.1% Cu, 0.005 to 0.05% P, 0.005 to 0.015% N and <=0.1% Al, also satisfying Cu/P; 1.5 to 4, Ti; 0.6×(48/14)N to (48/14)N, B; 11[N-(14/48)Ti]/14 or above to <=0.006, and Nb; (93/12)(C-0.001) to (93/12)C, and the balance substantial Fe is prepd. The steel having the same compsn. is formed into a slab by a continuous casting method, which is subjected to hot rolling, is coiled at >=630 deg.C and is thereafter subjected to cold rolling. The steel sheet for porcelain enamelling excellent in bubbling resistance, fish scale resistance and adhesion can be obtained.

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 excellent in press formability, foam resistance, nail popping resistance, and adhesion, and a method for producing the same.

[0002]

2. Description of the Related Art Enamel products such as kettles, pots, sinks, and baths are manufactured by pressing enamel steel plates, welding them if necessary, and then forming a glassy powder called enamel glaze and water on the surface. It is manufactured by applying a mixture of and and baking it at a high temperature of 800 ° C or higher.

Therefore, the steel sheet for enamel must have not only the press formability but also the enamel characteristics such as enamel adhesion, bubble / black spot defect resistance, and nail jump resistance. Conventionally, decarburized capped steel has been used as a steel plate for enamel having the above-mentioned characteristics, but continuous casting steel is currently used because of its high manufacturing cost.

One of such continuous cast steels is Ti-added steel, and if the C content is 0.005 wt% (hereinafter simply referred to as%) or less, excellent press formability can be obtained.
No. 8 and Japanese Patent Publication No. 44-18066. In addition, such Ti-added steel forms Ti carbides, nitrides, and sulfides in the steel, and these precipitates trap hydrogen in the steel, which is the cause of popping. Is provided with Japanese Patent Publication No. 45-40655 and Japanese Patent Laid-Open No. 53
-131919, JP-A-56-9357, etc.

However, such a Ti-added steel is disclosed in JP-A-61-
The adhesion is poor as disclosed in Japanese Patent No. 276958. Similarly, Ti-added steel has, as disclosed in JP-A-60-110845, not only adhesion to enamels but also foam resistance,
It is said that it is inferior to decarburized capped steel in terms of black spot defects.

On the other hand, JP-A-4-235226 discloses that by grinding the surface layer of a continuous cast slab by 1 to 5 mm, it is possible to improve the bubble resistance and the black spot defect resistance, but the slab maintenance will increase the cost. Unavoidable.

As described above, the Ti-added steel is resistant to bubbles and black spot defects,
It is not clear why the enamel adhesion is inferior to the decarburized capped steel, but it is known that it can be improved by decreasing the Ti content. However, if the amount of Ti is reduced, the amount of Ti-based precipitates is reduced, and nail skipping occurs. In order to solve such a problem, for example, Japanese Patent Publication No. 5-59968 and Japanese Patent Publication No. 5-59969 have been proposed.

In the former publication, TiS is positively generated by press-forming by increasing the amount of Ti, such as adding (48/14) N <Ti so that the amount of Ti is larger than N equivalent. Resistance and nail popping resistance are improved, and at the same time TiN is also sufficiently precipitated to improve nail popping resistance, and solid solution N
Therefore, the press formability is improved.

In the latter publication, (48/14) N + (48/32) S + (4
8/12) By adding Ti in an amount equal to or more than each equivalent of N, S, and C such that C ≦ Ti, TiS is positively generated, press moldability is improved, and at the same time solid solution B is dissolved. By its existence, B is added to secure the grain boundary strength.

However, in the case of a Ti-added steel and an extremely low C steel sheet, it was difficult to improve both press formability and enamel properties (nail jump resistance, bubble resistance, and adhesion) to a practically satisfactory level.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel sheet for enamel which is excellent in press formability and is excellent in enamel properties, especially nail resistance and foam resistance and adhesion, and a method for producing the same. That is.

[0012]

Means for Solving the Problems As a result of investigating the cause of the inferior enamel properties (adhesion, bubble resistance, black spot defect resistance) of an ultra-low C steel sheet which is a Ti-added steel, the present inventors Point became clear.

(1) The dissolution rate during pickling is high, and it is difficult to control the pickling loss. (2) For this reason, if the product is pickled too much, a large amount of iron oxide-based residue is generated on the surface of the steel sheet, and many bubbles are generated during firing. (3) On the contrary, if the pickling is slight, sufficient unevenness cannot be obtained on the surface of the steel sheet, so that the adhesion is deteriorated.

Therefore, based on these, the effect of Ti and Nb on the pickling property was further investigated, and as a result, the following points were further found. (4) As the Ti content increases, the dissolution rate with sulfuric acid increases. (5) Nb has the same effect as solid solution Ti, but is much weaker than Ti.

That is, in the case of a Ti + Nb-added steel in which a part of Ti is replaced with Nb, the dissolution rate by pickling can be slowed or the effect on it can be substantially eliminated, and the amount of pickling reduction can also be controlled. It may be easier and the foam resistance may be improved.

By the way, the action of Ti is further
In order to clarify whether it is the effect of Ti or the effect of Ti existing in the acid as ions, sulfuric acid in which metallic Ti is dissolved and dilute sulfuric acid are mixed at various ratios to form a pickling solution, and the pickling is performed. The change in weight loss was investigated. As a result, it was clarified that with the increase of Ti ions in the pickling solution, the grain boundaries were preferentially corroded during pickling, and at the same time, the dissolution rate by pickling decreased.

Further, as a result of investigating the Ti ion source, the following points were found. (6) Compounds of Ti, Nb and Mn in steel sheet (TiN, TiS, Ti
Of C, MnS, NbN, and NbC), except for TiS and MnS, they should hardly dissolve in dilute sulfuric acid. (7) As a result, the source of Ti ions in the sulfuric acid solution becomes a solid solution.
Must be Ti and sulfide Ti.

That is, by reducing the solid solution Ti and Ti sulfide, it is possible to eliminate the factors that affect the dissolution rate due to pickling and improve the adhesion. It was also found that Mn ions eluted in sulfuric acid have almost no effect on pickling performance, unlike Ti ions.

(8) However, the nail-pitching resistance cannot be improved unless a sufficient amount of precipitates (carbides, sulfides, nitrides) are present. However, not only TiN but also BN can be added by adding B. If it is deposited, hydrogen in the steel will be sufficiently trapped, and there is a possibility that the nail popping resistance will be improved.

Therefore, based on the above findings, Ti: 0.6 × (48/14) N to (48/14) (wt%), B: 11 [N− (14/48) Ti] / 14 or more And 0.0060 wt% or less, Nb: (93/12) (C-0.0010) to (93/12) C for cold-rolled steel sheets for enamels, whose steel composition is specified, press formability, nail pop resistance, and adhesion, When the foam resistance was evaluated, it was found that all of these characteristics were satisfied to a satisfactory degree, and the present invention was completed.

The present invention was made based on the above findings, and the gist thereof is as follows. (1) C: 0.0030 wt% or less, Mn: 0.10 to 0.50 wt%, Cu: 0.
020 to 0.10 wt% or less, P: 0.005 to 0.050 wt%, and Cu
/ P (weight ratio): 1.5 to 4.0 N: 0.005 to 0.015 wt%, Ti: 0.6 x (48/14) N to (4
8/14) (wt%), B: 11 [N- (14/48) Ti] / 14 or more and 0.0060 wt% or less, Nb: (93/12) (C-0.0010) to (93 /
12) C sol.Al: 0.10 wt% or less, balance: Cold rolled steel sheet for enamel having a steel composition consisting of inevitable impurities.

(2) The steel described in (1) above is formed into a slab by the continuous casting method, then hot-rolled and wound at 630 ° C. or higher, and then cold-rolled steel sheet for enamel by a conventional method. Manufacturing method. Thus, the present invention is characterized in that, in terms of steel composition, the Ti content is limited to an amount smaller than N equivalent, a small amount of Nb is allowed to coexist, and B is further added.

When Ti is contained in an amount equal to or more than N equivalent, TiS is formed, and during pickling which is a pretreatment for enameling, TiS is dissolved in the acid and the grain boundaries are pickled preferentially, and The dissolution rate due to pickling will decrease significantly. For this reason, a good pickled skin cannot be obtained, and the adhesion after enamel treatment becomes poor. Note that TiN does not dissolve in acid. Nb addition is C in steel
This is for fixing and precipitating as NbC. Also, B
Is precipitated in the form of BN and is added in order to ensure moldability and nail-flying resistance, and is characterized by positively utilizing the precipitation of BN.

[0024]

Next, the reason why the steel composition and manufacturing conditions are limited as described above in the present invention will be explained together with its operation. In the present specification, "%" defining the steel composition is "% by weight" unless otherwise specified.

C: C causes a bubble defect called a pinhole during baking of enamel, and if it exceeds 0.003%, this bubble defect is likely to occur. It is preferably lower than the viewpoint of press formability. Therefore, the upper limit is made 0.003%. It is preferably 0.0015% or less.

Mn: Mn is added to fix S in the steel sheet as MnS. In order to reliably fix S as MnS, it is necessary to add 10 times or more of S. Usually, S is contained as an unavoidable impurity in an amount of about 0.10%, so the lower limit of the amount of Mn is set to 0.10%. If the amount of Mn added exceeds 0.50%, the steel sheet becomes hard and press formability deteriorates. Therefore, the addition amount of Mn is set to 0.10% or more and 0.50% or less. Preferably,
It is 0.10 to 0.30%.

Cu: Cu is added in order to make fine unevenness on the surface of the steel sheet during the pretreatment (pickling) of enamel. further,
Since it has a side effect of reducing the pickling weight loss, a sufficient pickling weight loss cannot be obtained with a normal pickling time. Therefore, in order to adjust the pickling weight loss, it is added in balance with P. When the Cu content is less than 0.020%, pickling does not cause unevenness on the steel plate surface. On the other hand, if the Cu content exceeds 0.10%, Cu that is not oxidized when the slab is heated in the hot rolling process is concentrated on the surface of the slab, causing defects called glaze cracking. At this time, it is necessary to add Ni in order to increase the melting point of the Cu concentrated layer, which increases the cost. Therefore, the Cu content is set to 0.020% or more and 0.10% or less. Preferably,
It is 0.02 to 0.06%.

P: P is an impurity which is inevitably contained in the steel, but is positively added to increase the amount of pickling reduced by Cu added to obtain a good pickling surface. It is an element. Therefore, although it is controlled by the balance with the above-mentioned Cu content, if P is less than 0.005%, a sufficient pickling weight loss cannot be obtained, and the adhesiveness deteriorates. On the other hand, P is 0.05%
If it exceeds, the weight loss due to pickling is too large and bubbles are generated frequently. In addition, it becomes hard and press formability deteriorates. Therefore, it is set to 0.005% or more and 0.05% or less.

Cu / P: In the enameling pretreatment (acidic) in which the acid concentration, the liquid temperature and the pickling time were constant, the acid ratio was changed by changing the weight ratio (Cu / P) of the Cu and P contents. The washout weight can be adjusted. If this value is less than 1.5, the pickling speed will be too fast and the pickling weight loss will be too large, resulting in poor enamel adhesion and frequent foaming. On the other hand, if this value exceeds 4.0, the pickling rate will be too slow, and the pickling weight loss will be too small under the usual enamel pretreatment conditions, and this will also result in poor adhesion. Therefore, the ratio of the Cu and P contents is set to 1.5 or more and 4 or less.

N: In the present invention, N is deposited as TiN and BN, and this is used as a hydrogen trap site to prevent nail popping. Therefore, although N is usually an unavoidable impurity contained in steel, it is positively added in the case of the present invention. However, if it is less than 0.005%, its effect cannot be expected. Also 0.01
If it exceeds 5%, bald marks frequently occur on the surface of the steel sheet.
% Was set as the upper limit. Preferably, it is 0.007 to 0.012%.

Ti: According to the invention, Ti is added to fix N as TiN. As the temperature decreased in the hot rolling process, Ti in the steel formed in the order of TiN, TiS, and TiC and remained.
Ti dissolves. As described above, in order to make the solid solution Ti and sulfide Ti that affect the pickling rate zero, Ti /
The N atomic ratio must be 1 or less. On the other hand, if Ti is too small, the amount of B added, which will be described later, increases and the formability deteriorates. Therefore, the lower limit is 0.6 × (48/14) N (wt%) and the upper limit is (48/1
4) N (wt%) It is preferably less than (48/14) N%.

B: In the present invention, since Ti is added in an amount equal to or less than N, N which is not fixed by Ti remains as it is. B is positively added in order to fix this N as BN and make it a hydrogen trap site. Further, since solid solution B is effective for securing the secondary work embrittlement resistance, the B addition amount is "atomic ratio not fixed by TiN" or more. That is, the lower limit is (11/1
4) [N- (14/48) Ti] (%). Also, excessive addition deteriorates moldability. Therefore, the upper limit was made 0.006%. The preferable upper limit is 0.005%.

Nb: Nb is added to fix C as NbC and reduce the amount of fixed C. In order to suppress the strain aging due to solid solution C within a range where there is no practical problem, it is necessary to suppress the amount of solid solution C given by the following equation to less than 0.001%.

Solid solution C amount = (total C amount) − (C amount precipitated as NbC) On the other hand, when Nb is added in an amount more than necessary for forming NbC, it becomes solid solution Nb. Soluble Nb also dissolves Ti
Although it is weaker than the above, the pickling speed is increased, so it is necessary to prevent solid solution Nb from remaining. Therefore, the amount of Nb added is
(93/2) x (C-0.001) to (93/12) x C.

Acid soluble Al (sol.Al): Al is added as a deoxidizer. When it exceeds 0.10%, the deoxidizing effect is saturated and inclusions such as Al ₂ O ₃ are generated, and the content of these inclusions increases, causing surface scratches. It is preferably 0.05%.

Next, according to a preferred embodiment of the present invention, a steel having such a steel composition is made into a slab by a continuous casting method,
After that, hot rolling is performed, winding is performed at 630 ° C., and cold rolling is performed to obtain a cold rolled steel sheet. The reason for defining such manufacturing conditions is as follows.

Winding temperature: In order to sufficiently precipitate NbC, the winding temperature is preferably 630 ° C. or higher. The annealing may be continuous annealing or box annealing. Next, the working effects of the present invention will be described more specifically by way of examples.

[0038]

Example A slab having the composition shown in Table 1 was melted in a practical furnace, and then continuously cast into a slab (slag).
After that, hot rolling (finishing temperature 890 to 930 ° C, winding temperature: 620 to 670 ° C) was performed, and after pickling, cold rolling was performed to 0.7 mm.

The cold-rolled steel sheet thus obtained is then subjected to continuous annealing (burning: 840 ° C. × about 60 s) or box annealing.
(Burning: 720 ° C x 4 hours) was performed, and then 0.8% temper rolling was performed.

The mechanical properties of each of the steel sheets thus obtained were investigated, and the enamel coating was applied directly once, and the nail skipping, the number of pinholes, and the PEI adhesion were investigated.

The results are shown in Table 3. The treatment conditions for the enamel coating are shown in Table 2. The mechanical properties were measured with JIS No. 5 test pieces, and the r value was [r (0 °) + 2 × r
(45 °) + r (90 °)] / 4.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

As is clear from Table 3, the test steel Nos. 1 to N
In o.6, the formability and enamel are both good, but in Comparative Steel No. 7 which is a comparative example, the formability is poor because the C content exceeds the upper limit, and pinholes are also formed.

In the sample steel No. 7, since the C content exceeds the upper limit, the formability is poor and pinholes are generated. In the sample steel No. 8, the N content is less than the lower limit, and thus the nail flying resistance is poor.

In the sample steel No. 9, the Cu content and the Cu / P value were less than the lower limits, so that the adhesion was poor and pinholes were generated. In sample steel No. 10, the adhesion is poor because the Cu / P value exceeds the upper limit.

In the sample steel No. 11, since the amounts of Ti and B are less than the lower limits, the formability and the resistance to popping are poor. In sample steel No. 12, the Ti content exceeds the upper limit, so the adhesion is poor and pinholes are also formed.

In the sample steel No. 13, the formability was poor and pinholes were generated because the Nb content was less than the lower limit. Test Steel No.14
Then, since the amount of B exceeds the upper limit, the moldability is poor. In sample steel No. 15, pinholes occurred because the Nb content exceeded the upper limit.

[0050]

The cold-rolled steel sheet for enamel according to the present invention is
It has excellent moldability and good enamel properties such as adhesion, nail proofing resistance, foam resistance, etc., and it can meet the needs of diversifying enameled products and bring extremely useful effects in industry. is there.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroyuki Nakagawa 4-53-3 Kitahama, Chuo-ku, Osaka Sumitomo Metal Industries Co., Ltd.

Claims (2)

[Claims] 1. By weight%, C: 0.0030% or less, Mn: 0.10 to 0.50%, Cu: 0.020 to 0.
10%, P: 0.005 to 0.050%, and Cu / P (weight ratio): 1.5 to
4.0 N: 0.005 to 0.015%, Ti: 0.6 x (48/14) N to
(48/14) N (%), B: 11 [N- (14/48) Ti] / 14 or more, 0.0060% or less, Nb: (93/12) (C-0.0010) to (93/12) C sol.Al: 0.10% or less, balance: Cold rolled steel sheet for enamels having a steel composition consisting of inevitable impurities. 2. A steel having the steel composition according to claim 1 is formed into a slab by a continuous casting method, and then hot-rolled to 630 ° C.
After being wound as described above, cold rolling is performed,
Method for manufacturing cold rolled steel sheet for enamel.