JPS6234804B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing cold-rolled steel sheets with good press formability at low cost. Conventionally, to produce cold rolled steel sheets for press forming,
Completely solidified continuous casting slabs are cut, cooled, and then subjected to surface inspection and flaw removal treatment.
The usual process is to charge the coil into a heating furnace maintained at 1300℃, hot-roll it for 30 minutes to 1 hour, and then cold-roll the resulting hot-rolled coil and annealing it. It was hot. However, in recent years, continuous casting methods with extremely good slab surface properties have been developed, but at the same time energy-saving ideas have become more and more pervasive and established. A method has come to be adopted in which the slab is charged into a heating furnace while it is hot without being cooled to room temperature, and the slab is uniformly heated and rolled while reducing heating energy. By the way, in this case, from the point of view of energy saving and work efficiency, it is possible to completely omit charging the slab into the heating furnace for reheating, and simply use the heat from the casting process to continuously process the slab as it is. However, in such a method, the temperature distribution becomes non-uniform because the cooling rate differs between the ends and the center of the slab, which leads to problems with phase transformation. There was a risk that the final product would have different characteristics depending on the part because the method and distribution of the precipitates would be uneven. Therefore, at present, it is not possible to omit the step of charging the slab into a heating furnace and soaking it sufficiently before hot rolling. For example, if you try to directly hot roll a regular continuously cast slab of Al-killed steel, the center of the slab where the slab is cut will maintain a high temperature, so no AlN precipitation will be observed, but the surface At the edges of the plate and the width of the plate, the temperature decreases and AlN precipitates, and a ferrite phase also begins to form. When this is hot-rolled, cold-rolled, and then box-annealed, the center of the product steel sheet becomes an expanded grain structure and has good deep drawability, but AlN precipitates. In the areas where it has been used, an expanded grain structure is not formed and the deep drawability is also poor. In addition, when hot rolling such ordinary Al-killed steel slabs as they are, even if you try to increase the temperature of the part that is cooled by adding a short reheating before rolling, the AlN Re-melting does not occur easily, and once AlN has been produced, re-heating is required for a considerably long time to re-melt it. Furthermore, it is known that if slabs obtained by continuous casting are hot-rolled without being cooled to low temperatures, the workability is extremely poor, causing problems such as edge cracking during rolling. Ta. In other words, when hot rolling is performed by lowering the temperature of a slab to below the Ar ₁ transformation point and then reheating it to the γ single phase region as in the conventional method, crystal grains change during the γ-α transformation or α-γ transformation. becomes finer and has better hot workability, but
If the slab temperature is not lowered below the Ar ₁ transformation point, the γ grains will become coarse and the hot workability will be reduced, and temperature differences among the slab parts due to differences in cooling rate will also occur, making hot rolling difficult. This sometimes causes cracks at the ends. From the above-mentioned viewpoint, the present inventors have developed a cold-rolled steel sheet with good press formability without local variations in material properties, without subjecting the slab to long-term soaking treatment before hot rolling. In order to find a method to manufacture continuously cast slabs by continuously rolling them as they are, we focused on reducing the size of γ grains to ensure good hot workability and achieving good results in deep drawability. As a result of research focusing on the influence of the steel material composition on the [111] plane texture formation after cold rolling and annealing, we found that (a) the C content of the steel was 0.015% (hereinafter referred to as the composition composition); (% indicating the amount is weight %) or less, the δ phase is formed from the molten steel, and then the δ phase is
→In order to generate thin γ grains by causing γ transformation at the lowest possible temperature, and to suppress grain growth during δ-γ transformation and after complete transformation to γ phase, appropriate amounts of Ti, Zr and Nb are added. with 0.0005%
If the above amount of N is added to precipitate TiN, ZrN, NbN, etc., the γ grains of the solidified slab will become extremely fine, and hot workability will be greatly improved. (b) In order to obtain good deep drawability (r value) for the steel sheet after cold rolling and recrystallization annealing, the hot rolling end temperature must be set to Ar ₃
It is necessary to eliminate the orientation of crystal planes when the temperature is higher than 100 mm, but as mentioned above, in direct rolling (rolling performed directly on continuous cast slabs without soaking), the slab temperature at the start of rolling is This tendency is particularly noticeable on the end faces of slabs, making it difficult to finish the end faces or the entire surface with an Ar temperature of ₃ points or higher. However, even in this case, if the relationship between Ti equivalent and C equivalent satisfies the following relationship: (C equivalent) - 1/4 (Ti equivalent) ≦ 0.0010 (%), after cold rolling and recrystallization annealing, 111] Planar texture is sufficiently developed and good deep drawability is obtained. However, if the temperature difference between the end face and the center part is extremely large or the temperature drops extremely, the dimensional accuracy will deteriorate in a normal hot rolling mill, which will also affect the dimensional accuracy after cold rolling. Even in this case, the above-mentioned inconvenience can be overcome by simply performing local auxiliary heating using high-frequency heating or the like. (c) That is, the C and N contents in the steel and the Ti,
By controlling the relationship with the amount of carbonitride-forming elements such as Nb and Zr, it is possible to reduce the It is possible to resolve the problem of poor workability and local variations in material properties of the final product obtained by cold rolling and annealing. We have come to the knowledge shown in (a) to (c) above. This invention was made based on the above knowledge, and contains C: 0.001 to 0.015%, Mn: 0.01 to 1.20%, sol.Al: 0.10% or less, N: 0.0005 to 0.0060%, and Ti: 0.20% or less, Nb: 0.20% or less, Zr: 0.20% or less, or further contains one or more of the following: V: 0.01 to 0.20%, P: 0.03 to 0.10%, Cr: 0.05 to 1.00% , Ni: 0.05 to 1.00%, Si: 0.10 to 2.00%, B: 0.0003 to 0.0040%, and Ti equivalent = Ti (%) + 48/93 Nb (%) + 48/ 91Zr (%) ... (1) C equivalent = C (%) + 12/14N (%) ... (2) (C equivalent - 1/4 (Ti equivalent) ≦ 0.0010 ... (3) Equation (1) above The relationship between the Ti equivalent calculated by the equation (2) above and the C equivalent calculated by the above equation (2) satisfies the above equation (3), and a steel with a composition consisting of Fe + unavoidable impurities: the remainder is cast by continuous casting. After forming a thin slab, it is continuously hot rolled as it is or with supplementary heating, and then cold rolled at a reduction rate of 50 to 90% after descaling, and at 660 to 910℃.
By performing recrystallization annealing at a temperature of It is characterized by its manufacturing. Next, in the method of the present invention, the reason why the chemical composition of the steel and the manufacturing conditions of the cold-rolled steel sheet are limited as described above will be explained. A. Chemical composition C. The smaller the C component, the better the hot workability of slabs and the workability of cold-rolled steel products, so it is preferable; however, if its content is less than 0.001%, melting becomes extremely difficult. On the other hand, if the content exceeds 0.015%, not only will a large amount of carbonitride-forming elements be required, but the amount of carbonitride precipitation will increase, deteriorating the press formability of the final product. The content was set at 0.001-0.015%. Mn The Mn component has the effect of improving the toughness of steel sheets, but if the content is less than 0.01%, hot de-strengthening will occur, while if the content exceeds 1.20%, melting will become difficult. , and also causes cost increases, so its content was set at 0.01 to 1.20%. sol.Al sol.Al is normally contained in order to perform sufficient deoxidation and improve the yield of carbonitride-forming elements, but if sol.Al is contained in excess of 0.10%, Moreover, the upper limit was set at 0.10% because further deoxidizing effects could not be obtained and the cost would be high. The smaller the N content, the less carbonitride-forming elements can be added. However, if the content is less than 0.0005%, the nitridation necessary to suppress the growth of γ grains will be reduced. There is not enough stuff,
Hot workability decreases. On the other hand, its content
If it exceeds 0.0060%, the press formability of the final cold-rolled product will inevitably deteriorate, so the content was set at 0.0005 to 0.0060%. Ti, Nb, and Zr These components have the effect of forming nitrides in the hot slab after solidification and suppressing the growth of γ grains, thereby improving the hot workability of the slab. In steel sheets after hot rolling, fine carbonitrides are formed to improve the press formability of the final product, but even if each content exceeds 0.20%, a further improvement effect can be seen. However, the upper limit is set as follows:
Ti: 0.20%, Nb: 0.20%, and Zr: 0.20% were determined. In addition, the above equations (1) to (3) calculate the amount of solid solution [C+N].
0.0010 (%) or less, and shows a relational expression for precipitating the remaining C+N as carbonitride. In addition, the upper limit of (C equivalent) - 1/4 (Ti equivalent)
The reason why it is set to 0.0010 (%) is that if this upper limit is exceeded, the solid solution [C+N] will increase too much and the press formability of the steel plate will deteriorate. Furthermore, the above components need to be uniformly distributed;
This is possible by applying continuous casting with less segregation. V, P, Cr, Ni, Si, and B These components have a uniform effect of improving the strength of the steel sheet, so they are included as necessary when higher strength is required, but each component are V: less than 0.01%, P: less than 0.03%, Cr: less than 0.05%, Ni: less than 0.05%, Si: less than 0.10% and B:
If the content is less than 0.0003%, the desired strength improvement effect cannot be obtained; on the other hand, V: 0.20% and P: 0.10, respectively.
%, Cr: 1.00%, Ni: 1.00%, Si: 2.00%, and B: 0.0040%, since the weldability and surface quality of the steel plate will deteriorate.
The respective contents are V: 0.01-0.20%, P:
0.03~0.10%, Cr: 0.05~1.00%, Ni: 0.05~
1.00%, Si: 0.10~2.00%, and B: 0.0003~
It was set at 0.0040%. B. Manufacturing conditions Hot rolling of continuously cast slabs After continuous casting of steel with the above chemical composition, it is continuously hot rolled either as is or after auxiliary heating (partial reheating usually within 5 minutes). However, from the standpoint of energy saving, it is desirable for continuous cast thin slabs to be continuously hot rolled after cutting without going through a reheating process. However, if the temperature at the surface or end portions drops too much and the hot workability of the slab is impaired, it is preferable to supplementally heat only such low-temperature portions. This auxiliary heating is not intended to adjust the structure of the steel, but to reduce the difference in deformability between the high temperature and low temperature areas during rolling. When the dimensional accuracy cannot be maintained due to the difference in the slab temperature or the slab temperature is low, continuous heating is applied for up to 5 minutes using high frequency or the like in-line or the like. In other words, for steel with normal composition for press forming, unless it is soaked for more than 5 minutes discontinuously,
There is a risk of edge cracking occurring during hot rolling, but if the ingredients are controlled as in the method of this invention, hot workability will not deteriorate even with a slight temperature difference, and therefore 5.
A sufficient effect can be obtained simply by continuous reheating within minutes, for example, by heating with an edge heater or a surf-s heater. Reduction ratio of cold rolling: If the reduction ratio is less than 50%, not only will it not be possible to obtain a steel plate with good press formability, but the crystal grains will become larger after recrystallization annealing, making the surface rough after press forming. Therefore, the rolling reduction ratio for cold rolling was set at 50% or more. Note that the rolling reduction ratio is preferably 73% or more. However, if the rolling reduction exceeds 90%, a large amount of energy is required for cold rolling, and the properties of the steel sheet are not significantly improved, so the upper limit of the rolling reduction was set at 90%. Temperature of Recrystallization Annealing Although it is preferable to adopt continuous annealing in which rapid heating, short-time soaking, and rapid cooling are performed, box annealing in which slow heating, long-time soaking, and slow cooling are performed may be used. And the annealing temperature in these cases is 660
If the temperature is lower than 660°C, the r value (deep drawability) of the cold rolled steel sheet becomes low and good press formability cannot be obtained, so the recrystallization annealing temperature was set at 660°C or higher. However, when the recrystallization annealing temperature exceeds 910℃,
The upper limit of the recrystallization annealing temperature was set at 910°C because the r value decreased as the γ phase (austenite phase) was formed. Next, the method of the present invention will be explained using examples and comparing with comparative examples. Example 1 C: 0.0040%, Si: 0.010%, Mn: 0.16%,
P: 0.012%, S: 0.004%, sol.Al: 0.028%,
Steel with a composition consisting of N: 0.0015%, Ti: 0.042%, and Fe: the balance was produced by a conventional method. of this steel
The Ti equivalent is 0.042% and the C equivalent is 0.0053%, which satisfy the above formula (3). Next, this steel was continuously cast to a thickness of 200 mm.
After forming a slab with a width of 1240 mm and a length of 6000 mm, it was immediately brought to a hot rolling mill and hot rolled without being reheated. The temperature of the slab just before hot rolling is at the center of the slab width.
The temperature was 1150°C, and the temperature at the width end was 980°C, and the hot-rolled steel sheet was wound into a coil at about 660°C. Note that no cracks were observed in the hot-rolled sheet. Subsequently, after pickling the hot rolled sheet, rolling reduction: 84%
The sheet was cold rolled to a thickness of 0.8 mm, and the cold rolled sheet was subjected to continuous annealing at a temperature of 830° C. for 1 minute. Regarding the resulting cold-rolled steel sheet, JIS No. 5 tensile test pieces were taken from the width edges and the center of the sheet width, and a tensile test was conducted to measure the mechanical properties. The measurement results are shown in Table 1.

[Table] From the results shown in Table 1, the obtained cold-rolled steel sheet has higher tensile strength and lower r value at the edge of the sheet width than at the center, which is somewhat disadvantageous in press formability. Although the values are shown, it is clear that the variation is not large enough to cause a problem in practice. Example 2 After bringing the same slab as in Example 1 to a hot rolling factory and before hot rolling, the width end of the plate was continuously heated for about 30 seconds using a high-frequency heating device, and then heated in the same manner as in Example 1. Hot rolling was carried out. The temperature of the slab just before hot rolling is at the center of the slab width.
The temperature was 1140°C and 1090°C at the width end. Of course, no cracks occurred in the hot-rolled sheet thus obtained. Thereafter, the mechanical properties of the cold rolled steel sheet obtained in the same manner as in Example 1 were as shown in Table 2.

[Table] As shown in Table 2, in this case, the difference in property values depending on location was smaller than in Example 1, and it is clear that a good cold-rolled steel sheet for pressing was obtained. Example 3 C: 0.0020%, Si: 0.010%, Mn: 0.28%,
P: 0.060%, S: 0.006%, sol.Al: 0.09%,
A steel with a composition consisting of N: 0.0058%, Nb: 0.035%, and Fe: the balance was produced by a conventional method. This steel Ti
The equivalent is 0.018%, the C equivalent is 0.0070%, and the above
This satisfies equation (3). Next, this steel was continuously cast to a thickness of 40 mm.
After forming a thin cast slab coil with a width of 1240 mm, it was immediately brought before a hot rolling finish roll and hot rolled to a thickness of 5 mm. The temperature of the thin slab coil at the entry side of the hot rolling roll was 1100°C at the width center and 1040°C at the width end, and the hot rolled steel plate was wound into the coil at about 660°C. No cracks were observed in this hot-rolled sheet. Subsequently, the hot rolled sheet was cold rolled and continuously annealed in the same manner as in Example 1, and its mechanical properties were measured. The measurement results were as shown in Table 3.

[Table] As shown in Table 3, the obtained cold-rolled steel sheet exhibited a high r value both at the width center and at the width end, and had good press formability. Example 4 C: 0.006%, Si: 0.01%, Mn: 0.08%, P:
0.010%, S: 0.001%, sol.Al: 0.05%, N:
Various steels containing 0.004%, varying Ti in the range of 0 to 0.20%, and Fe: the rest were vacuum melted to form thin slabs of thickness: 40 mm, width: 220 mm, and length: 440 mm. After that, hot rolling was immediately performed to a thickness of 4 mm. At this time, the rolling start temperature was about 1080°C, the finishing temperature was about 850°C, and the winding temperature was about 400°C. Next, this hot-rolled sheet was pickled and then cold-rolled at a reduction rate of 80% to form a cold-rolled sheet with a thickness of 0.8 mm, and then maintained at a temperature of 800°C for 90 seconds. Continuous annealing was performed. The r value of the obtained cold rolled steel sheet was determined, and this result was determined in relation to the amount of solid solute C in the cold rolled steel sheet, that is, (C equivalent) - 1/4 (Ti equivalent) shown as the above equation (3). It is shown in Figure 1. As is clear from Figure 1, the value of equation (3) above is
In addition to showing a high r value when it is 0.0010% or less,
It also showed high elongation values. Example 5 C: 0.002%, Si: 0.20%, Mn: 0.45%, P:
0.018%, S: 0.004%, sol.Al: 0.01%, Zr:
At 0.030%, change the N amount from 0.0003 to 0.0035%,
Fe: The remaining steel was melted and hot rolled in the same manner as in Example 4. When the edges of the obtained hot-rolled sheets were observed in detail and the presence or absence of cracks was investigated, the results shown in FIG. 2 were obtained. From FIG. 2, it can be seen that if the N content is 0.0005% or more, the occurrence of cracks is small and there is no problem in practical use. Example 6 Steel having the composition shown in Table 4 was melted, cast, hot rolled, and wound in the same manner as in Example 4 to produce a hot rolled sheet. However, the rolling start temperature is 1080℃.
Two types were used at 980°C, and changes in product characteristics due to changes in temperature conditions during hot rolling were investigated. The hot rolling finishing temperature is approximately 850℃ for the former and approximately 770℃ for the latter.

[Table] It was. The winding temperature was approximately 400°C in both cases. Then, the state of end cracking of the hot rolled sheet at this time was determined. Furthermore, after removing the scale from these hot-rolled sheets, they were cold-rolled at a reduction rate of 80% and at a temperature of 800℃.
Cold rolled steel plates 1 to 8 of the present invention and comparison cold rolled steel plates 9 to 12, each having a thickness of 0.8 mm, were manufactured by performing continuous annealing under the condition of holding for 90 seconds. In addition, comparative cold-rolled steel sheets 9 to 12 all have compositions that are outside the scope of the present invention, and in Table 4, the applicable ones are marked with *. Next, the cold rolled steel sheets 1 to 1 of the present invention obtained as a result
Tensile properties and r values were measured for No. 8 and comparative cold rolled steel sheets No. 9 to No. 12, and the measurement results are also shown in Table 4. As shown in Table 4, cold rolled steel sheets 1 to 1 of the present invention
No. 8 has stable good elongation and high r value even if the hot rolling start temperature fluctuates, that is, good press formability, whereas comparative cold rolled steel sheets No. 9 and No. 10 have (C equivalent) -1/4 (Ti equivalent) is higher than the scope of this invention, which causes cracks to occur in the hot-rolled sheet, and the characteristic values of the product change greatly due to fluctuations in the hot-rolling start temperature. ,
In addition, when the hot rolling start temperature is low, the r value and elongation are particularly low, indicating poor press formability. Comparative cold rolled steel sheet 11 has a low (C equivalent) -1/4 (Ti equivalent), so there is no cracking in the hot rolled sheet, and the characteristic values of the product are stable without being affected by the hot rolling start temperature. However, due to the high C content, the elongation and r value are poor. Furthermore, since the comparative cold-rolled steel sheet 12 is a normal P-added Al-killed steel sheet that does not contain carbonitride-forming elements, the elongation and r value of the product are low. As described above, the method of the present invention has industrially useful effects such as being able to produce cold-rolled steel sheets with good press formability while minimizing energy consumption and at low cost. It is brought about.

[Brief explanation of the drawing]

Figure 1 shows the influence of the value of (C equivalent) -1/4 (Ti equivalent) in cold rolled steel sheets on the elongation and r value of the product, and Figure 2 shows the amount of N in steel sheets and hot rolled sheets. It is a figure showing the relationship between the occurrence of cracks in

Claims (1)

[Claims] 1 Contains: C: 0.001 to 0.015%, Mn: 0.01 to 1.20%, sol.Al: 0.10% or less, N: 0.0005 to 0.0060%, Ti: 0.20% or less, Nb: 0.20% Contains one or more of the following: Zr: 0.20% or less, and Ti equivalent = Ti (%) + 48/93Nb (%) + 48/91 Zr (%) ... (1) C equivalent = C (%) +12/14N (%) ……(2) (C equivalent) – 1/4 (Ti equivalent) ≦0.0010 (%)……(3
) The relationship between the Ti equivalent calculated by the above formula (1) and the C equivalent calculated by the above formula (2) satisfies the above formula (3), Fe + unavoidable impurities: remainder, %) is made into a thin slab by continuous casting, then hot rolled continuously as it is or with supplementary heating, and then after descaling, it is rolled at a reduction rate of 50 to 90%. A method for producing a cold-rolled steel sheet with good press formability, comprising cold rolling and recrystallization annealing at a temperature of 660 to 910°C. 2 Contains C: 0.001 to 0.015%, Mn: 0.01 to 1.20%, sol.Al: 0.10% or less, N: 0.0005 to 0.0060%, Ti: 0.20% or less, Nb: 0.20% or less, Zr: 0.20% Contains one or more of the following, and further includes: V: 0.01-0.20%, P: 0.03-0.10%, Cr: 0.05-1.00%, Ni: 0.05-1.00%, Si: 0.10-2.00%, B : 0.0003 to 0.0040%, also contains one or more of the following, and Ti equivalent = Ti (%) + 48/93Nb (%) + 48/91 Zr (%) ... (1) C equivalent = C (%) +12/14N (%) ……(2) (C equivalent) – 1/4 (Ti equivalent) ≦0.0010 (%)……(3
) The relationship between the Ti equivalent calculated by the above formula (1) and the C equivalent calculated by the above formula (2) satisfies the above formula (3), Fe + unavoidable impurities: remainder, %) is made into a thin slab by continuous casting, then hot rolled continuously as it is or with supplementary heating, and then after descaling, it is rolled at a reduction rate of 50 to 90%. A method for producing a cold-rolled steel sheet with good press formability, comprising cold rolling and recrystallization annealing at a temperature of 660 to 910°C.