JP3360624B2 - Cold rolled steel sheet for enamel and its manufacturing method - Google Patents

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 an enamel used for kitchen appliances, electric appliances, building materials and the like, and a method for producing the same.

[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. In general, enamel products are twice enameled in which firing is performed twice or more, but there is also a single enamel in which an upper glaze is directly baked on a steel sheet without a lower glaze.

[0003] The enameled steel sheet is basically made extremely low carbon since the glaze reacts during firing to generate CO gas to generate bubble defects and to cause firing distortion due to transformation. And among the defects that occur during the production of enamel products,
It is considered that the base steel sheet also has the cause, such as poor adhesion of the enamel layer, bubbling, nail jump, black spots, poor surface,
There is firing distortion and the like. In addition, many enamel products are formed into various shapes by press working and enameled, so that such defects do not occur and the steel sheet must have excellent press workability.

Conventionally, steel sheets having excellent enamelness have been made of rimed steel or capped steel, and decarburized by controlling the annealing atmosphere 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. Due to the presence of the rim layer, the surface is beautiful and has good adhesion to glaze, and microcavities and internal inclusions formed by removing carbon in cementite by decarburizing annealing trap hydrogen causing nail jump. As a result, nail jumps are less likely to occur. Therefore, although the workability was somewhat unsatisfactory, it was an optimal steel plate for enamel.

However, the production process of cold-rolled steel sheets has been rationalized, and the slab for rolling has been changed from casting and slab rolling of steel ingots to continuous cast slabs. Has changed to For this reason, an enameled steel sheet, which is one of the cold rolled steel sheet products, has to be manufactured by this manufacturing process, and various studies have been made to obtain a steel sheet having excellent enamelness by this process. In continuous casting, it is impossible to cast the same as rimed steel or capped steel. Therefore, first, C, which deteriorates the enamelness of the steel sheet, is decarburized in the steelmaking stage to extremely low C, and further fixes C more sufficiently. For this reason, steels containing Ti have been developed.

[0006] This steel was excellent in press formability and had good adhesion to enamel glaze, but was inferior in nail resistance and, furthermore, was difficult to rework press-formed products. Improvements have been made such as using Nb as a carbonitride forming element or adding B. After that, in order to improve the anti-claw flying resistance, which is the most important for enameled steel sheets, a high oxygen enameled steel sheet with a large amount of oxide-based inclusions was developed, in which oxygen was increased at the time of extremely low C, which can be used for continuous casting. Was done. Increasing the number of inclusions increases the number of places where hydrogen is trapped and improves the anti-claw flying resistance. However, the increase in inclusions results in poor workability of the steel sheet.

[0007] An enameled steel sheet made of a slab obtained by continuous casting, and continuously annealed after cold rolling, has a very low carbon content at the steel making stage, but is sufficiently dealloyed with the addition of Ti, Nb or B. It can be broadly divided into those based on acidified steel and those based on high oxygen steel. There is also an intermediate one containing Nb and having high oxygen. And in each case,
Developments such as controlling the contents of S, P, Cu, etc. to further improve the enamel adhesion, utilizing further addition of other elements, and controlling the manufacturing conditions to improve the performance are being advanced. Those that have no practical problem are manufactured. However, depending on the application, it is hard to say that a satisfactory product has been obtained.

The evaluation of the enamel properties of a steel sheet usually focuses on the adhesion and the anti-claw flying property. However, depending on the pretreatment conditions of the steel sheet and the glaze, bubbles and black spots may occur, which may be a problem. Foam and black spots caused by the steel plate are alleviated because they are covered with the upper glaze when the enamel is applied twice, but directly affect the appearance of the product when the enamel is applied once. Since such defects are likely to occur in deoxidized steel to which Ti, Nb, B, etc. are added, when these are concerned, high oxygen steel is applied. However, even with high oxygen steel,
Bubbles and black spots may be generated in a portion that comes into contact with a stainless steel basket or jig that holds an article during pretreatment, which is a problem.

[0009] On the other hand, for example, Japanese Patent Application Laid-Open No. 5-5128 discloses that O contains 0.050% to 0.065% of high oxygen and (P + S) / C
An invention is proposed in which steel containing 0.010 to 0.040% of Ni in which u is controlled to 0.5 to 1.0 does not generate such bubbles and black spots. However, such high oxygen steel is inferior in press workability due to increased inclusions even if defects can be suppressed,
It is considered that some parts cannot be adequately adapted.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process which can be carried out by continuous casting and continuous annealing.
An object of the present invention is to provide an enameled steel sheet which is free from defects such as bubbles and black spots, which are easily generated from a portion in contact with a holder or a jig during pretreatment of an enamel such as a flash, and which has excellent workability. .

[0011]

Means for Solving the Problems The inventors of the present invention have developed a steel sheet manufactured by continuous casting and continuous annealing, which is mainly used for enamelling once, and which is used for pretreatment such as pickling or Ni flash. It was necessary to cope with the generation of bubbles and black spots at the jig mark at the contact portion with the holder. These are likely to occur particularly when processing a large number of parts with limited equipment. Such defects are considered to be more common in deoxidized steel containing Ti, Nb or B. As a result of changing the steel plate used to a high oxygen type enameled steel plate, such defects can be greatly reduced. found.

Further, as a result of examining the workability of the steel sheet, the adhesion of the enamel, the anti-claw flying property, etc., when the oxygen content is high,
It has been found that the addition of a small amount of Cr is effective. This is C
r has the effect of trapping hydrogen by forming inclusions in steel as oxides, improving the adhesion of enamel by forming oxides on the surface, and combining with nitrogen to provide strain aging resistance Tend to be slightly improved. The addition of Ni also has the effect of improving the adhesion when Ni flash is performed.

However, many of the target parts are drawn products in the form of a square tube, such as drawers for a system kitchen, and cracks are generated from the corners. There is a drawback that the drawability of the product is insufficient. As an index for evaluation of the drawability of steel sheets for deep drawing, the average value of elongation and r-value obtained by a tensile test of each specimen taken in the directions of 0 °, 45 ° and 90 ° with respect to rolling is often used. Can be Examination of the elongation values by the tensile test in each direction shows that the high oxygen-based steel sheet has a particularly high elongation value in the 45 ° direction tensile test in the other direction compared to the case of deoxidized steel containing Ti or Nb. Significantly lower than.

Usually, a blank is drawn from a drawn and formed rectangular tube with the sides of the rectangular tube parallel or perpendicular to the rolling. For this reason, at the time of press working, the corners of the rectangular tube undergo relatively severe processing than other parts, but this part
It is believed that the specimens taken in the ° direction will undergo similar deformation to the tensile test. That is, when a rectangular tube-shaped part is press-formed using a high oxygen enameled steel sheet, the direction with the least elongation is subjected to the most severe processing. At this time, it is also known that if the blank is sampled at an angle of 45 ° in the rolling direction, it is possible to suppress cracking of the rectangular tubular pressed part, but the yield of the molded product is greatly reduced.

Thus, the high oxygen enameled steel sheet is
Although the press workability is generally inferior due to the large number of inclusions, it was considered that if the elongation in the 45 ° direction could be improved with respect to rolling, it could be sufficiently applied to a rectangular cylindrical molded part. Therefore, the manufacturing conditions were examined to improve the elongation in the tensile test in the 45 ° direction, and to improve the drawability of the steel sheet, especially in the high oxygen steel. As a result, it was found that it is effective to lower the coiling temperature in hot rolling and increase the rolling reduction in cold rolling. The gist of the present invention completed based on the above findings is as follows.

(1) In weight%, C: 0.0005 to 0.0030%,
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%, O: 0.040 to 0.075
%, Cr: 0.01 to 0.08%, and Cu (%) / P
(%): 1.0 to 4.0 P (%) / S (%): 0.6 to 2.0%, and Ni:
0-0.06%, the balance has a chemical composition consisting of Fe and unavoidable impurities, the tensile strength in the direction of 45 ° to the rolling direction, the tensile strength is 280-320 MPa, and the elongation is 41% or more. Cold rolled steel sheet for enamel.

(2) A continuous cast slab having the chemical composition described in (1) is soaked at 1100 to 1250 ° C.
Winding at ~ 600 ° C, cold rolling at a rolling rate of 80-90% after pickling, and then annealing by continuous annealing at a heating temperature of 800-900 ° C and a heating time of 120 seconds or less. The method for producing a cold-rolled steel sheet for enamel according to the above (1).

[0018]

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

The content of C is set to 0.0005 to 0.0030%.
If the content of C 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%.

The content of Si does not significantly affect the enamel properties. However, when the content increases, the surface of the steel sheet is scratched or deteriorated in appearance, and the hardness is increased to deteriorate the press formability. Therefore, the content is limited to 0.2% or less. Since it is not a particularly required element, the lower limit may be 0, and the presence of ordinary impurities may be sufficient.

In order to prevent hot brittleness due to S, Mn is usually contained in steel at about 7 times or more the S content. 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. The lower limit is desirably about 0.05% or more.

P is one of the inevitable impurities in steel, but tends to increase the melting speed of the steel sheet during pickling. If the content is too large, the amount of acid pickling is excessively reduced, so that enamel foam defects are likely to occur. 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 set to 0.005 to 0.020%.

The content of S is set to 0.005 to 0.025%. S, like P, is an unavoidable impurity in 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 to 0.025% from the viewpoint of improving the workability of the steel sheet, and the lower limit is set to 0.005% from the point of enamelness.
I do.

Both P and S are elements that increase the dissolution rate during pickling, that is, the weight loss during pickling. As a result of investigating the effect on the adhesion, 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 is presumed to be related to not only the weight loss of the pickling but also the surface state after the pickling.

Al is used as a deoxidizing agent at the steelmaking stage to ensure the soundness of the slab, but since it contains O, it should be kept in the steel as little as possible. Therefore, its content is set to 0.01% or less.

N does not affect the enamel properties, but causes strain aging and deteriorates the workability of the steel sheet. Therefore, the content of N is set to 0.0030% 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, it is necessary to control the ratio of Cu and P to improve the adhesion. It has been found that there is an optimum range, and that adhesion is inferior if the range is larger or smaller than the range. That is, C
u (%) / P (%) needs to be regulated within the range of 1.0 to 4.0.

The content range of O (oxygen) is set to 0.040 to 0.075%. 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. Although this effect is recognized even when the content is about 0.02%, in order to surely obtain a sufficient effect, the content is made 0.040% or more. However, as the number increases, the surface flaws increase and the workability also deteriorates.
Up to 075%.

Cr is contained in an amount of 0.01 to 0.08%. This is because an oxide is formed on the surface of high oxygen steel to improve the adhesiveness of the enamel, and also has the effect of preventing nail jump. Another effect is to reduce strain aging. To obtain these effects, the content of at least 0.01% is necessary. However, if the content is too large, the press workability of the steel sheet is deteriorated. Therefore, the content is preferably at most 0.08% or less.

Ni may not be added, but has the effect of improving the adhesion and the effect of suppressing the generation of bubbles and black spots.
It is preferred to add up to 06%. When adding
If the content is less than 0.01%, the effect will not appear, while 0.06%
The effect is saturated even if it is contained in excess, and further addition is wasted.

The press workability of a steel sheet can be evaluated by elongation and r value in a tensile test. Generally, when the elongation is large, the r value is often large. In addition, it is usually judged by an average value of values obtained from tensile test pieces taken in directions of 0 °, 45 ° and 90 ° with respect to rolling. However, in the case of a high oxygen enameled steel sheet, the difference between the measured values in these three directions is large, and the 45 ° direction tends to be particularly low. However, the difference in tensile strength is small. In addition, the shape of the component is generally a rectangular tube shape, and processing in the 45 ° direction may be severe. Accordingly, in the high oxygen enameled steel sheet, if the elongation in the 45 ° direction can be made sufficiently large, the formability will be sufficiently excellent. From such a viewpoint, as a result of applying and examining steel sheets for various actual parts, it has been found that sufficient workability can be guaranteed if the elongation in the 45 ° direction is 41% or more in steel sheets having a tensile strength of 280 to 320 MPa. Was. Therefore, the target properties of the steel sheet are that the tensile strength is 280 to 320 MPa and the elongation in the 45 ° direction is 41% or more.

The press formability of a steel sheet is greatly affected by the conditions of each step in the manufacturing process, mainly steps after hot rolling, in addition to its chemical composition. The heating temperature of the hot-rolled slab does not significantly affect the workability,
00 to 1250 ° C. If the temperature is less than 1100 ° C., the rolling load of rolling increases, making it difficult to roll. If the temperature exceeds 1250 ° C., the form of the inclusions changes, resulting in a decrease in nail fly resistance and an increase in scale due to oxidation. The finishing temperature of the hot rolling is preferably immediately above the Ar ₃ transformation point, and is preferably about 850 to 910 ° C. If it is higher than this, the press formability deteriorates, and if it is too low and falls below the Ar ₃ transformation point, the elongation in the 45 ° direction and the r-value of the steel sheet obtained by annealing after cold rolling become poor.

The winding temperature after hot rolling is set at 450 to 600 ° C. This is because the obtained steel sheet has poor press formability both below and below this temperature range. In the case of the above-mentioned chemical composition, it is considered that a trace amount of solute carbon remains at a winding temperature lower than 450 ° C., and that the annealing after cold rolling inhibits the formation of a crystal orientation favorable for press formability when the temperature is raised. On the other hand, at a winding temperature exceeding 600 ° C., since there is no element that suppresses the crystal grain growth as a chemical composition of the steel, the crystal grain size before cold rolling becomes large, and the obtained steel sheet has a 45 ° elongation and r This results in poor values and poor press workability. Desirable is 470-550 ° C.

The rolling reduction of the cold rolling is limited to 80 to 90%. In the ultra-low carbon and high oxygen steel of the present invention, it is important to increase the rolling reduction. By setting the rolling reduction to 80% or more, the elongation and r-value especially in the 45 ° direction with respect to the rolling direction are improved. In addition, the press formability of the obtained steel sheet is improved. However, if the rolling exceeds 90%, not only does the rolling load increase sharply, but these improvement effects are saturated.

The annealing after the cold rolling is preferably performed by a continuous annealing method. In that case, the condition
800 ~ 900 ℃, the holding time of the steel sheet within this temperature range
It should be 120 seconds or less. The continuous annealing method is preferred because the steel plate manufacturing process may be shortened, but the nail enamel fly resistance is improved. This is presumably because micro-cracks formed in the cold rolling process near the inclusions remain in the steel sheet without filling and disappearing due to diffusion due to soaking in a short time, and trap hydrogen. If the temperature is lower than 800 ° C., the annealing may be insufficient. If the temperature is higher than 900 ° C., a tendency such as deterioration of the press formability due to coarsening of the crystal grains and a reduction in the effect of improving the nail flying resistance is exhibited. The holding time in this temperature range of 800 to 900 ° C. can achieve the purpose of annealing in about several seconds or more, but as long as the temperature exceeds 900 ° C., there is no effect such as improvement in nail fly resistance. 12
0 seconds or less.

In the annealing method, in the steel having the chemical composition of the present invention, a sufficient effect of improving enamelness and workability can be obtained by conventional coil annealing or open coil annealing. However, the continuous annealing method is applied. Is more preferred.

[0038]

[Example 1] A steel slab having the chemical composition shown in Table 1 was cast by a continuous casting method, and hot-rolled to a thickness of 4.5 mm at a slab heating temperature of 1200 ° C and a finishing temperature of rolling of 880 to 900 ° C. Then 5
Wound at 30 ° C. After pickling descaling, cold rolling 0.7m
m thickness (reduction rate 85%), heating temperature 850 by continuous annealing method
Annealing was performed at a holding time of 100 seconds in a temperature range of 800 ° C. or higher, and temper rolling was performed at an elongation of 0.8%. The resulting steel sheet was subjected to a tensile test, and the enamelness was examined.

[0039]

[Table 1]

The enamel is 75 mm wide and 200 m long from the steel plate.
A sample of m was cut out, subjected to two kinds of pretreatments as described below, and then enameled once.

Pretreatment 1 Pickling: 17% sulfuric acid, an iron concentration of about 8%, in a 78 ° C. aqueous solution,
Soak for 4 minutes.

[0042] Ni flash: 12g / l NiSO ₄ · 7
Immersion for 4 minutes in 82 ° C aqueous solution of H ₂ O, iron concentration 0.5%, pH 2.8.

Pretreatment 2 Pickling: 12% in a 78 ° C. aqueous solution containing 17% sulfuric acid and about 8% iron.
Soak for a minute.

[0044] Ni flash: 12g / l NiSO ₄ · 7
Immersion for 12 minutes in 82 ° C aqueous solution of H ₂ O, iron concentration 0.5%, pH 2.8.

However, a 1 mm-thick stainless steel plate (SUS316L) was bent into a V-shape on each test piece, and the cut end face was brought into contact with the test piece.
Perform i-flash and evaluate after enameling the contact area here as a tool mark.

Enamelling Glaze 1: 1553B glaze manufactured by Nippon Fellow Co., Ltd. was sprayed to a thickness of about 100 μm per side and glazed on both sides.

Baking 1: 820 ° C., 6 minutes.

Glaze 2: Same glaze as 1 above Firing 2: Same bake as above 1 (corresponding to reworking and re-firing) Reduction in pickling during enameling by pretreatment 1, Evaluation of enamel adhesion by PEI method, Nail The presence or absence of the occurrence of flying and bubbling (visual appearance observation) was performed, and the pretreatment 2 was performed to evaluate the occurrence of bubbling and black spots at the tool contact mark after the same enamelling.

Table 2 shows the results of the tensile test and the enamelling test. Test No. 1-12 steel sheet whose chemical composition is within the range defined by the present invention, 45 ° to the rolling direction
While the tensile test elongation in the direction exceeds 41%
The enamel has good adhesion, no nails fly, and no defects such as foaming are observed. Numbers 13 and 14 are thought to be due to good elongation but rather poor adhesion and no Cr content. In No. 15, the elongation is poor and bubbling is observed. This is because C is too high. No. 16 is considered to be due to nail skipping, and the O content was too low below the range defined by the present invention. Number 17 may be due to poor growth and too much oxygen. Numbers 17 and 18
In this case, bubbling occurred, which seems to be affected by the Nb content. The reason why the PEI of No. 19 is low and the enamel adhesion is not good is that the P / S value is higher than the range defined in the present invention. Nos. 20 and 21 indicate that the Cu content or the value of Cu / P deviates from the range specified in the present invention, which affects the condition of the steel sheet surface after pickling, resulting in poor adhesion or foaming. It seems to have been made.

[0050]

[Table 2]

Example 2 A slab having the composition of steel numbers B, H and J shown in Table 1 was used, and the slab heating temperature was set to 1200.
℃, hot-rolling finishing temperature 880-900 ℃, change the hot rolling winding temperature, cold rolling reduction, annealing temperature and annealing time, temper rolling elongation 0.8%, sheet thickness 0.7mm Was manufactured. The obtained steel sheet was subjected to a tensile test and an enamelling test in the same manner as in Example 1. In this case, the pretreatment of the enamel was performed with one. Table 3 summarizes these conditions and the results of the tensile test and the enamel test of the obtained steel sheet.

[0052]

[Table 3]

As is clear from the above, even if the chemical composition falls within the range of the present invention, a steel sheet having sufficient characteristics may not be obtained when the production conditions are not favorable. That is, in Test No. 37, the winding temperature was too high and the elongation was poor, and in Test No. 38, the elongation in the 45 ° direction was not good because the cold rolling reduction was low. In the case of No. 39, skipping of the nails occurred because the annealing time was too long. Number 40 or 41
Has poor elongation in the 45 ° direction because the annealing temperature is too high or too low.

[0054]

The cold-rolled steel sheet according to the present invention is used for parts made of enamel, has good press formability, has good adhesion to the enamel, has excellent nail flying resistance, and has foaming and black spots even at the jig mark. There is no occurrence. This steel sheet can be easily manufactured in a general cold-rolled steel sheet manufacturing process by continuous casting and continuous annealing, and the occurrence of defects during enameling can be reduced, which is effective in reducing the cost of enamel parts.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-102222 (JP, A) JP-A-10-183300 (JP, A) JP-A-7-118755 (JP, A) JP-A 8- 27522 (JP, A) JP-A-8-120406 (JP, A) JP-A-10-204581 (JP, A) JP-A-10-195541 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [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%, O: 0.040 to 0.075
%, Cr: 0.01 to 0.08%, and Cu (%) / P
(%): 1.0 to 4.0 P (%) / S (%): 0.6 to 2.0%, and Ni:
0-0.06%, the balance has a chemical composition consisting of Fe and unavoidable impurities, the tensile strength in the direction of 45 ° to the rolling direction, the tensile strength is 280-320 MPa, and the elongation is 41% or more. Cold rolled steel sheet for enamel. 2. Using a continuous cast slab having the chemical composition according to claim 1, heating at a temperature of 1100 to 1250 ° C., hot rolling, winding at 450 to 600 ° C., and a rolling reduction of 80 after pickling. ~ 90% cold rolling, then heating temperature 800 ~ 900 ℃, heating time 12
The method for producing a cold-rolled steel sheet for enamel according to claim 1, wherein the annealing is performed by a continuous annealing method within 0 second.