JPH049850B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a method for manufacturing a cold rolled steel sheet which is used by enameling after pressing. <Prior art and its problems> Steel plates for enameling need to be ultra-low carbon steel and have a clean surface in order to suppress the occurrence of bubble defects and firing distortion. In addition, in order to suppress the skipping defect, which is a phenomenon in which hydrogen stored in the steel sheet during enamel firing leaks between the steel sheet and the enamel layer during cooling and causes the enamel layer to fly away, oxides,
It is necessary to contain a large amount of second phase such as carbides and nitrides in the steel. Decarburized and denitrified annealed capped steel sheets satisfy these requirements and have excellent press workability, and have been widely used as steel sheets for enameling. However, since decarburization and denitrification annealing is required, high costs are unavoidable. In order to solve this problem,
As disclosed in No. 12348 and other publications, attempts have been made to manufacture steel sheets for enameling using Ti-added ultra-low carbon steel with excellent press formability and using continuous casting or box annealing instead of decarburization and denitrification annealing. A lot was done. For example, as disclosed in JP-A-51-98619 and JP-A-54-125117, various methods for producing enameled steel sheets with excellent press formability and resistance to chipping have been proposed. In the case of Ti-added steel, Ti is TiC, TiN or Ti
Sulfides and Ti phosphides are formed in the steel to improve flaking resistance, and at the same time, since C, N, and S are no longer in a solid solution state, press formability is also improved. Therefore, in order to suppress tripping, sufficient
It has been considered necessary for steel to contain elements such as C, N, S, and P along with Ti. However, steel sheets for enameling manufactured by conventional methods have problems such as inferior enamel adhesion compared to decarburized and denitrified capped steel sheets, and the tendency to generate enamel bubble defects. . Because of these drawbacks, Ti-added ultra-low carbon steel is rarely used for direct single-ply enamel, in which over-ply enamel, which has weak adhesion, is directly glazed and fired onto a steel plate, and has poor press formability and flaking resistance. There has been a strong desire for a method to improve the surface texture and enamel adhesion without impairing the enamel. <Object of the invention> The present invention has been made in view of the above-mentioned actual situation, and provides a method for producing a steel plate for enameling that has excellent surface properties and enamel adhesion without impairing press formability and chipping resistance. The purpose is to provide <Structure of the Invention> The present invention will be described in more detail below. The present inventors found that the bubble defects and poor adhesion that occur when Ti-added steel is enameled are caused by pickling as a pre-enameling treatment.
The cause is smut made of Ti, Fe, S, P, O, N, etc., and this smut is formed by Ar ₃ during hot rolling.
We have discovered that by rolling below the transformation point and creating a texture that makes it difficult for smut to adhere only to the surface layer after cold rolling annealing, the bubble defects and enamel adhesion caused by the smut problem can be suppressed, and the enamel adhesion and bubble defects caused by the smut problem can be greatly improved. This led to the present invention. The reasons for limiting the components of the steel plate in the present invention will be described. As the amount of C in the steel plate used increases, press workability deteriorates and enamel bubble defects are more likely to occur. In the steel used in the present invention, in order to suppress the occurrence of smut during pre-enameling treatment, it is necessary to lower the hot-rolling finishing temperature as described later, and as shown in Figure 1, the C content is 0.002wt%. If it exceeds this, the material will deteriorate significantly. Therefore, the amount of C is
It needs to be 0.002wt% or less. In order to obtain chipping resistance as a steel plate for enameling, it is necessary to contain appropriate amounts of C, N, S, and P, but at the same time, in order to obtain good press formability, the amount of C should be lowered, In addition, as will be described later, in order to obtain good enamel adhesion and a good enamel surface, it is necessary to reduce the amounts of S and P as much as possible. Therefore, it is necessary to add a sufficient amount of N to suppress the skipping. FIG. 2 is a graph showing the tendency of jumps in steel sheets in which only the amount of N was varied. C amount is 0.003wt
The conditions for enameling using a steel plate with a
In order to facilitate the occurrence of skipping, the pretreatment was only pickling for 20 seconds, the firing temperature was 850°C, and the dew point in the firing furnace was 40°C. The glaze is commercially available Nippon Fellow L.
The rate of occurrence of flaking per 20 pieces after firing and keeping at 160°C for 16 hours is shown.
As is clear from Figure 2, when the amount of C is low, N
If the amount is less than 0.009wt%, skipping tends to occur. Therefore, the amount of N needs to be 0.009wt% or more. S is contained in steel as an unavoidable impurity, but when the amount of S is large, the amount of smut produced and attached to the surface of the steel sheet increases during pickling of the base steel sheet, which is usually performed as a pre-enameling treatment, especially during pickling. If the amount of smut increases, bubble defects and poor adhesion will occur after enameling. Therefore, the smaller the amount of S in the steel, the better the enameling properties, but in order to improve the enameling properties, it is necessary to keep it at 0.008wt% or less. Like S, P causes the formation of a large amount of smut during pickling in pre-enamel treatment, causing bubble defects and poor adhesion. Therefore, it is preferable to reduce the amount of P as much as possible. From the perspective of improving enamel properties, the amount of P is 0.01wt%.
It is necessary to do the following. Ti is an essential element for improving the press formability of a steel sheet, forming TiC, TiN, Ti sulfide, and Ti phosphide in the steel, and preventing flaking. In order to achieve these two objectives, it is necessary to add 0.05 wt% or more. However, when the amount of Ti added increases, the amount of Ti contained in the smut generated during pickling of the steel sheet increases, causing deterioration of adhesion. It is not clear why Smuts with a high Ti content cause particularly poor adhesion, but it is a component of normal Smuts.
In addition to Fe oxides, sulfates, and phosphates, in the case of Ti-added steels, Ti-containing oxides, sulfates, and phosphates become thermally stable and decompose during enamel firing or enamel layer. After firing, it becomes difficult to dissolve in the steel plate and remains at the interface between the steel plate and the enamel.
It is estimated that the adhesion will deteriorate. In order to prevent such deterioration of adhesion, as mentioned above, the amounts of S and P must be reduced as much as possible, and the amount of Ti must be kept at 0.12 wt% or less. Next, the hot rolling conditions will be described. Hot rolling conditions are particularly important in the present invention. Taking press formability and enameling properties into consideration, steel with the composition range determined above can be made into a slab by either continuous casting or the one-time ingot method, but heating and soaking of the ingot It is preferable to manufacture by a continuous casting method which can omit the process because the cost is low. From the viewpoint of press formability, hot rolling is Ar ₃
Although it is preferable to complete hot rolling at a temperature above the transformation point, from the viewpoint of enameling adhesion, it is necessary to hot-roll below the Ar ₃ transformation point, preferably in the α region. In other words, when hot rolling is carried out above the Ar ₃ transformation point, {111} texture develops significantly in the surface layer after cold rolling and annealing, which makes it easier for smut to accumulate during pickling in pre-enameling treatment, resulting in poor adhesion. deteriorates. However, when the hot rolling finishing temperature is set to ₃ Ar points or less, the texture of the surface layer after cold rolling and annealing becomes stronger with textures other than {111}, such as {110}, {210}, and {100}, resulting in a smooth surface layer. deposition is significantly reduced. Moreover, in steels with controlled compositions such as the steel of the present invention, the texture of the center layer of the steel sheet is less affected by the hot-rolling finishing temperature, and even if the Ar is ₃ points or less, a {111} texture favorable for press formability can be obtained. It will be done. Therefore, for the purpose of improving enamel adhesion, it is sufficient that the temperature of the surface layer during hot rolling is below Ar ₃ points, but it is difficult to obtain a good shape of the steel sheet in such a temperature range. A good shape can be obtained by rolling at ₃ Ar points or less at 820°C or less. Further, when the hot rolling temperature is 750°C or lower, even in the steel of the present invention, the processed structure remains in the hot rolled sheet, and in order to obtain press formability after cold rolling and annealing, the FDT needs to be 750°C or higher. Therefore, in order to satisfy both press formability and enamel adhesion, it is necessary to complete the hot rolling at a temperature of 750°C or higher and 820°C or lower. The purpose of the present invention can be achieved by any conventionally conventional method for winding, descaling, cold rolling, and recrystallization annealing after hot rolling. <Example> The present invention will be specifically explained based on an example. Steel having the composition shown in Table 1 was melted using a converter and an RH degassing process, and was made into a slab using a continuous casting method. Steels Nos. 1 to 9 shown in Table 1 were hot-rolled, then cold-rolled to a thickness of 0.7 mm, and continuously annealed at 800° C. with a soaking time of 30 seconds. Next, 0.8% negative rolling was performed.
Figure 1 shows the relationship between the amount of C in steel and ductility. The tensile test was conducted using a JIS No. 5 tensile test piece. As the amount of C increases, ductility deteriorates. In order to ensure ductility of 50% or more, the amount of C needs to be 0.002wt% or less. FIG. 3 shows how smut formation during enameling pretreatment (sulfuric acid pickling), which is a feature of the steel of the present invention, is influenced by hot rolling finishing temperature. Table 1
Two types of steel, No. 3 and No. 5, have different hot rolling finishing temperatures.
It was divided into cases where the test ended at 820°C or higher (a) and cases where the test ended at 780°C or higher and 820°C or lower (b). Between (a) and (b), there is a large difference in the amount of smut during pickling. This corresponds well to the texture of the surface. Table 2 shows the results of texture measurements. Steel with a large amount of smut has a strong {222} plane, while steel with little smut has a weak {222} plane, and vice versa. {220} plane and {211}
The surface strength is strong and corresponds well to the finish rolling temperature. In order to examine the enamel adhesion of each of the above steel sheets, sulfuric acid pickling (10%
H ₂ SO ₄ , 75°C), Ni bath immersion (2% NiSO ₄ , 65°C)
A commercially available top glaze [1553B (Ti white glaze) manufactured by Nippon Ferro] is applied to the steel plate treated for various times.
After firing at 820°C, adhesion was measured using the adhesion test method (ASTM; C313-59) recommended by PEI (American Enamel Institute). In this test method, an enameled steel plate is deformed and the area ratio of the deformed part where the enamel has not peeled off is measured. An adhesion index of 100% is considered good, and an adhesion index of 0% is considered bad. Figure 4 shows a steel plate with a large amount of smut (No. 3-
This figure shows the enamel adhesion of a) and a small steel plate (No. 3-b) at various pickling losses. In Comparative Example No. 3-a, the adhesion index at the time of pickling loss at one point where smut has not yet occurred is 80.
%, stable adhesion cannot be obtained. In addition, pinholes were noticeable on the surface, and in other pickling loss areas, pinholes and rough skin were also observed. In comparison, the steel plate of Example No. 3-b of the present invention, which had a smaller amount of smut, stably obtained good adhesion over a wide pickling loss range and had good surface quality.

【table】

[Table] <Effects of the Invention> As mentioned above, the present invention makes it possible to manufacture a steel plate that satisfies all the properties necessary for a steel plate for enameling, such as press formability, chipping resistance, enameling adhesion, and surface texture. As has been made clear, this can only be achieved by strictly adjusting the amounts of various components in the steel and appropriately controlling the finish rolling temperature of the slab during hot rolling. According to the present invention, high-grade enameling steel sheets, which were conventionally manufactured by the ingot method, can be manufactured by the continuous casting method, which brings about great advantages in terms of cost and energy saving.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between C content in steel and ductility. Figure 2 shows the N in steel required to suppress tripping.
It is a graph showing the amount. Figures 3 and 4 are
These glasses show the effect of finishing temperature on pickling time on smut amount and the effect of finishing temperature on enamel adhesion, respectively.

Claims (1)

[Claims] 1. When manufacturing a steel plate for enameling with excellent press formability, enameling adhesion, and chipping resistance, C
≦0.002wt%, 0.009wt%≦N, S≦0.008wt%,
After continuous casting of steel consisting of P≦0.01wt%, 0.05wt%≦Ti≦0.12, balance iron and unavoidable impurities to form a slab, the hot rolling finishing temperature is changed from 750℃ to 820℃.
1. A method for producing a steel plate for enameling, which comprises hot rolling in a range of 100 to 100 ml, followed by pickling, cold rolling, and recrystallization annealing.