JPH0321610B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) Regarding the production of cold-rolled steel sheets as hard tinplate base plates, in particular, using hot-rolled baseplates that have been hot-rolled at low temperatures, continuous annealing can advantageously produce hard tinplates (tain-free steel). This paper aims to provide a method for manufacturing original plates (including (Prior art) Tin-plated steel sheets, that is, tinplates and their original plates, are specified to be expressed by the Rockwell T hardness (HR 30T) value in JIS G3303 for their degree of heat treatment, HR 30T: 46 ~52 (T-1) 50~56 (T-2) 52~58 (T-2 1/2) 54~60 (T-3) In addition to the soft ones, 58~64 (T-4) 62~ Hard materials such as 68 (T-5) and 67 to 73 (T-6) are classified, and the same applies to original plates for stain-free steel. Conventionally, these original sheets are produced by heating a steel slab at a slab heating temperature of approximately 1300°C, then hot rolling to finish at the Ar ₃ transformation point or above, winding it into a hot rolled sheet coil, and then pickling and normal cold rolling. After that, the soft original sheets from T-1 to T-3 were box annealed, and the soft original sheets from T-4 to T-3 were box annealed.
Hard blanks up to -6 have been produced by continuous annealing, respectively. As a method for producing a hard tin plate of T-4 or higher (hereinafter simply referred to as a hard plate), as disclosed in the example of JP-A No. 58-27930, the hot rolling finishing temperature is set to the Ar ₃ transformation point or higher. A method of further adding N as
C as disclosed in the example of Publication No. 27931.
By reducing the hot rolling finishing temperature to as low as 0.01 to 0.04%,
A method of increasing Ar temperature to ₃ or higher is known. As described above, in the conventional manufacturing method, when manufacturing a hard original sheet, it was customary to set the hot rolling finishing temperature to Ar ₃ points or higher. However, when the hot-rolling finishing temperature is set to Ar ₃ points or higher, the time required to maintain the high temperature after hot-rolling before coiling increases, and the oxidation layer that forms on the surface of the steel sheet becomes thicker, resulting in higher oxidation loss and lower yield. There are disadvantages such as: In addition, since the hot-rolled base plate is generally hard, it causes stripping trouble during cold rolling, as well as roll wear, decreased operability due to roll changes, and the rolling load is large, which reduces the power consumption of the rolling mill. Leads to increase. Furthermore, due to poor rolling properties, shape defects are likely to occur. However, on the other hand, as disclosed in Japanese Patent Publication No. 55-48574, the hot-rolling finishing temperature Ar is set to ₃ points or less to make the soft tin plate (hereinafter referred to as a soft blank).
Methods of manufacturing are also known. According to the examples in the publication, it is shown that AlN is very likely to precipitate in steel sheets that have been hot-rolled at 700° C. to ₃ points in Ar, making them soft. However, if the coiling temperature is high, such as 580℃ or higher, the carbides will aggregate and coarsen, which will not only reduce the corrosion resistance of the tin plate but also lead to a decrease in solute C and solute N due to the precipitation of AlN, resulting in a hard base plate. This inevitably reduces the solid solution strengthening elements that are advantageous for manufacturing. As described above, in the conventional method, hot-rolling and low-temperature finishing is advantageous for producing soft blanks but disadvantageous for producing hard tin blanks. (Problem that the invention seeks to solve) As a result of extensive research, the inventors have developed a method for manufacturing a hard original sheet using a low-temperature hot rolling method that has excellent cold rolling properties and is inexpensive to manufacture. It was extremely successful. (Means for solving the problem) This invention provides C: 0.005 to 0.045wt%P: 0.02wt
Less than %, N: 0.002-0.020wt%, Sol・Al: 0.005
Continuously cast slab containing ~0.050wt% at 1050℃
After heating at a temperature above, hot rolling is completed at an Ar of ₃ points or less, coiling is performed at a temperature of 550°C or less, followed by pickling and cold rolling, and then the steel plate is given an Ac of ₁ point at or above its recrystallization temperature. This is a method for manufacturing a hard tin plate, which is characterized by continuous annealing by heating to a temperature in the following range, holding it within 60 seconds, and cooling at a cooling temperature of 30° C./second or higher. By satisfying the above steel composition and manufacturing conditions, the hot rolling finishing temperature can be reduced to Ar ₃ .
It is possible to produce a hard cold-rolled steel sheet even if the temperature is below this point. The experimental results on which this invention is based will be explained in detail below. (Experiment) Mn: 0.25wt% (hereinafter simply expressed as %), Si:
0.010% P: 0.015% S: 0.012% SolAl:
0.043% N: The basic component is 0.0035% and the amount of C is 0.01
% and 0.07% were heated at 1200℃,
Finish hot rolling at a finishing temperature of 700℃~900℃,
It was rolled up at 520℃. After pickling, the hardness (HRB) of the hot rolled sheets was measured and the influence of finishing temperature was investigated. The results are shown in FIG. 1, and the hardness decreases as the hot rolling finishing temperature decreases. Moreover,
The hardness of a 0.01%C hot-rolled sheet is lower than that of a 0.07%C hot-rolled sheet, and the lower the finishing temperature, the lower the hardness of the 0.01%C hot-rolled sheet.
%C, the softening tendency of the hot rolled sheet became greater. From the above results, as shown in FIG. 1, it is possible to keep the rolling load σ during cold rolling low for low-temperature finish-rolled materials. Next, using a soft hot-rolled mother plate that had been subjected to low-temperature hot rolling, we investigated the optimal C content and continuous annealing conditions for producing a hard original sheet. The process will be explained in the next (experiment). (Experiment) Mn: 0.25%, S: 0.008%, P: 0.017%, N:
Plate thickness 200 with basic components of 0.0050% and SolAl: 0.020%, and the amount of C varied within the range of 0.004 to 0.092%.
mm slab was heated at 1000°C, 1200°C and 1250°C (SRT) and hot rolled to a finishing temperature (FDT) of 750°C to form a hot rolled plate with a thickness of 2.5 mm, and then rolled. The sample was kept at a temperature (CT) of 500° C. for 2 hours, and then air cooled to room temperature. Thereafter, it was pickled, cold rolled to a thickness of 0.3 mm, and continuously annealed. The thermal history of this continuous annealing was as follows: heating at a heating rate of 20°C/s, holding at 650°C for 30 seconds, and then cooling at cooling rates (CR) of 10, 15, 30, and 100°C/s.
Subsequently, the plate was subjected to temper rolling at a reduction rate of 15%, then tin plating and hot tin treatment, and the hardness (HR30T) of the tin plate was measured. The results are shown in Figure 2. As a result, even in the low SRT-low FDT rolled material in the C content range of 0.005 to 0.045%, the cooling rate was 30%.
C/s or more satisfies the hardness of T-4 (61±3) or more. (Effect) At present, the reason for the above behavior is not necessarily clear, but it is thought that the coarse grains due to low-temperature finish rolling and the cooling rate during continuous annealing are related and harden. In other words, in steel with a small amount of C, the grain boundaries, which are the precipitation sites for cementite, have a small area because the crystal grains become coarse, and there is also less cementite within the grains, which are also precipitation sites, so the faster the cooling rate , it is thought that the decrease in solid solution C was less likely to occur. In particular, in low-temperature finish-rolled materials, it is presumed that because the ferrite grains become larger, the amount of solid solution C tends to remain, which increases the hardness after cold rolling and continuous annealing. Next, the reason for limiting the steel composition and manufacturing conditions in this invention will be described below. (Ingredients) The reason why C is set at 0.005% to 0.045% is that the amount of C is 0.05%.
As mentioned above, the carbide precipitated during hot rolling becomes large, and the carbide near the surface not only deteriorates the corrosion resistance of the tin plate, but also refines the crystal grains of the hot rolled sheet, impairing cold rollability. The upper limit was set at 0.045% because the hardening effect becomes smaller as the cooling rate increases. The lower limit was set at 0.005% because if it was less than 0.005%, the particle size would become too large and the amount of solid solution C would become too small, causing the material to soften. If P exceeds 0.02%, P will segregate at grain boundaries or on the surface of the original plate, impairing the corrosion resistance of the tinplate.
Less than 0.02%. Like C, N is a major reinforcing element in this invention and must be contained at least 0.002%. In addition, since a large amount of content increases the cost during steel manufacturing, the upper limit is set at 0.02% in this invention. SolAl is effective as a deoxidizing agent in steel making,
At least 0.005% is required. However, N
was fixed as AlN, and its content was set to 0.05% to reduce the amount of solid solution N. In addition to the above, in the present invention, even if the steel contains 0.1% or less Mn, 0.1% or less Si, etc., it does not interfere with the desired performance and is allowed. (Hot rolling) The reason for specifying the slab heating temperature is that if the temperature is lower than 1050°C, precipitated AlN will not melt after pouring molten steel, and the effect of solid solution N in this invention will decrease, so the lower limit is set at 1050°C or higher. And so. The hot rolling finishing temperature needs to be Ar ₃ or lower from the viewpoint of scale loss and cold rolling properties compared to the conventional technology. The reason why the winding temperature is set to 550℃ or less is that 550
When the temperature exceeds ℃, solid solution C and N precipitate,
This is because the material becomes soft after cold rolling and annealing. (Continuous annealing) The reason why the maximum temperature for annealing is set to below Ac ₁ point is because even if the crystal grain size is increased by raising the temperature unnecessarily, threading during continuous annealing becomes difficult.
It is also disadvantageous from that point of view because AlN tends to precipitate, and setting the soaking time to less than 60 seconds is 60%
This is because, in the case of soaking for a long time exceeding seconds, precipitation of AlN is promoted and the amount of solid solution N decreases.
The cooling rate is the main condition of this invention, and as mentioned in the experimental results, the cooling rate of 30
℃/sec or higher is essential. (Experiment example) Molten steel with the composition shown in Table 1 is made into a continuous casting slab with a thickness of 200 mm, heated at a heating temperature of 1000 to 1300 °C, finished in a temperature range of 750 to 870 °C, and rolled at 520 °C. After being made into a rolled coil, it was pickled, cold rolled, and then heated to 650-670℃ at a heating rate of 20℃/s.
Continuous annealing was performed by holding for 30 seconds and then cooling at a cooling rate of 10 to 150°C/s. After that, it was temper rolled with a reduction rate of 1%, tin plated and hot tin treated through an electric tin plating line, and then the material (HR30T) was examined.
It is shown in Table 1.

【table】

[Table] As a result, all steel materials according to the present invention satisfy a hardness of T4 or higher. (Effects of the Invention) According to the present invention, it is possible to reduce descaling costs during hot rolling and advantageously produce a hot rolled mother plate for a hard original sheet with excellent cold rollability.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the influence of hot rolling finishing temperature on hardness and rolling load, and FIG. 2 is a chart showing the relationship between hardness and C content.

Claims (1)

[Claims] 1 A continuous casting slab containing C: 0.005 to 0.045 wt% P: less than 0.02 wt% N: 0.002 to 0.020 wt% Sol/Al: 0.005 to 0.050 wt% at a temperature of 1050°C or higher After heating,
After finishing hot rolling at Ar ₃ points or less, coiling at 550℃ or less, then pickling and cold rolling,
The steel plate is heated to a temperature above its recrystallization temperature and below AC ₁ point and held for 60 seconds or less.30
A method for manufacturing a hard tin plate, characterized by performing continuous annealing by cooling at a cooling temperature of ℃/second or higher.