JPS6119690B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet with good drawability and shapeability. In recent years, there has been a growing trend to reduce the weight of automobile bodies in order to improve their fuel efficiency, and this has led to a demand for higher strength car body panels. Of course, steel sheets that are subjected to severe press processing such as those used for car body panels must have excellent drawability and shapeability. A steel plate that has both good properties and high strength is required. However, no steel sheet has hitherto been found that has both of these properties. That is, the drawability is considered to be better as the r value (Rankford value) in a tensile test becomes higher, and generally the r value tends to decrease as the tensile strength increases. Regarding shape properties, the lower the yield strength YS and yield ratio, the better; however, this property usually deteriorates as the tensile strength increases as the yield strength increases. Taking actual steel as an example, a high-strength steel plate with a general ferrite + pearlite structure has a low r value of around 1.0 and a high yield ratio of 0.65 to 0.85, so naturally the YS
becomes high, resulting in poor drawability and shapeability. Recently, a steel plate in which martensite is finely dispersed in ferrite has been developed by continuous annealing after cold rolling. A cold-rolled steel sheet is heated to a temperature where two phases of ferrite (α) and austenite (γ) coexist, and then rapidly cooled to transform the γ phase into martensite. This steel plate is
Martensite acts as a source of dislocations and uniform deformation can be obtained relatively easily, so yielding is fast and a low yield ratio of 0.60 or less is ensured, resulting in good shape properties despite high tension. have. However, no improvement is seen in the r value, which remains at a low value of 1.0 or less, which is comparable to the ferrite + pearlite structure described above. In addition, this steel sheet requires the addition of a large amount of Mn or Si to obtain martensite, making it expensive and impractical. Therefore, it has been desired to develop a method for obtaining a high tensile strength steel plate with a high r value and a low yield ratio at a low cost. In order to meet this demand, the present inventors focused on steel with the above-mentioned ferrite + martensitic structure, and as a result of various experiments and research on ways to take advantage of its advantages and increase the r value, we found that C, Mn,
By adding appropriate amounts of Cr, SOl.Al, and N and performing box annealing under specific conditions prior to continuous annealing after cold rolling, we succeeded in obtaining a cold rolled steel sheet with the desired properties after continuous annealing. . That is, in the present invention, C0.005-0.04%, Si0.20%
Below, Mn 0.2% or more, Cr 1.8% or less, SOl.Al 0.01
~0.10%, N0.0015~0.0150%, and Mn
%+Cr%=0.8~2.5%, and the remainder is substantially
After hot rolling and cold rolling, steel made of Fe is box annealed at 660 to 780°C, and after cooling, it is heated to 650 to 820°C for a short time in a continuous annealing furnace with a heating zone of 650°C or higher. Drawability characterized by rapid cooling,
The gist of this paper is a method for producing cold-rolled steel sheets with good shape properties. If this method is followed, the r value is 1.2 or more, the yield ratio is
High tensile strength cold rolled steel sheets with a tensile strength of 0.60 or less can be produced. When a cold-rolled steel sheet containing appropriate amounts of SOl.Al and N is box annealed, AlN precipitates during the heating process, forming a recrystallized texture that is favorable for increasing the r value.
This recrystallized texture does not break even after continuous annealing, and contributes to improving the r value. On the other hand, if C, Mn, and Cr are present in steel, when the steel plate is heated to 660 to 780°C during the soaking process of box annealing, C, Mn and Mn will be added to the γ phase present at that time.
and Cr atoms are significantly segregated. This γ phase is likely to transform into a pearlite phase rather than a martensitic phase during box annealing cooling, that is, slow cooling, but when it is subsequently heated during continuous annealing, it returns to the γ phase. Due to rapid cooling during continuous annealing, it finally becomes a martensitic phase,
A ferrite + martensite structure with a low yield ratio is obtained. Since the method of the present invention utilizes the segregation of Cr and Mn in the γ phase, the average amount of Cr and Mn in the steel is small, and therefore the cost is low, and at the same time, the increase in the r value is inhibited. There's nothing to do. Moreover, since the C content is lower than the general 0.04 to 0.08%, a steel plate with stable characteristic values can be obtained. Also, since the method of the present invention involves continuous annealing in the final step, solid solution C remains in the steel plate, and as a result, it is hardened by paint baking, which is extremely effective for increasing the strength of steel plates for car body panels. It has the advantage that favorable properties are also ensured at the same time. Note that the continuous annealing in the present invention can be performed in either a line for general cold-rolled steel sheets or a hot-dip galvanizing line. Next, the reasons for limiting the steel composition and annealing conditions in the present invention will be described. C contributes to the formation of martensite,
It is necessary to contain 0.005% or more, but if it exceeds 0.04%, there is a possibility that a large amount of martensite will be formed, making it difficult to obtain uniform characteristics. It is preferable that the amount of Si is low, and a content exceeding 0.2% will deteriorate the surface condition of the steel sheet after cold rolling. Since Mn and Cr have the same effect on the mechanical properties of the steel sheet, they are regulated by the total amount of Cr amount + Mn amount. In order to form martensite and lower the yield ratio, the amount of Mn + Cr is required to be 0.8% or more,
On the other hand, if it exceeds 2.5%, it is not preferable because it lowers the r value. In this case, there is a risk of hot embrittlement unless the Mn content is 0.2% or more. In addition, Cr does not necessarily need to be added, as it deteriorates the chemical conversion treatment properties of steel sheets.
It is better not to exceed 1.8%. SOl.Al needs to be added in an amount of 0.01 to 0.10% in order to improve the r value by precipitation of AlN. Similarly, N must be added at least 0.0015% to ensure a high r value, but if it exceeds 0.015%, the elongation decreases significantly. Regarding the soaking temperature for box annealing, a low yield ratio is obtained in the range of 660 to 780°C, and at temperatures other than this, the yield ratio increases even if it is lower or higher. In addition, continuous annealing requires heating to 650℃ or higher.
γ phase is not formed and martensite cannot be obtained. On the other hand, when heated above 820℃, the amount of ferrite phase decreases significantly, and the recrystallization texture and Mn and
The Cr segregated phase is destroyed, resulting in a low r value and high yield ratio of the finished product. Next, examples of the present invention will be described. Example 1 Steels (A) and (B) having the components shown in Table 1 were melted in a converter and made into slabs. This slab was hot-rolled to 3.2mm at a heating temperature of 1220℃, a finishing temperature of 860℃, and a coiling temperature of 560℃.
A thick steel plate was obtained. After pickling, it was rolled to a thickness of 0.8 mm by normal cold rolling, and then soaked at a temperature of 40°C/hr, a cooling rate of 40°C/hr, and a soaking time of 16 hr.
Box annealing was performed at various temperatures between 600 and 750°C. The atmosphere was a mixed gas of H ₂ +N ₂ . Thereafter, continuous annealing was performed at a heating rate of 20°C/scc, soaking at 750°C for 1 min, and cooling rate of 10°C/sec.

[Table] JIS No. 5 test pieces were taken from cold-rolled steel sheets after continuous annealing, and tensile tests were conducted. The results are shown in FIG. 1 as the influence of box annealing temperature on yield ratio and r value. In the box annealing temperature range of 660 to 780°C, good results have been obtained with an r value of 1.2 or more and a yield ratio of 0.60 or less. Example 2 C0.01~0.03%, SOl.Al 0.03~0.08%, N0.003
Contains ~0.008%, P0.010~0.020%, Si0.01~0.09%, and further contains Mn 0.1~2.3% and Cr 0.01~2.0%.
After melting the steel with the addition added within the range of 0.8 mm and finishing it into a cold-rolled steel plate with a thickness of 0.8 mm under the same conditions as in Example 1, the heating rate was 40°C/hr, the soaking temperature was 700°C, and the soaking temperature was 700°C. temperature
Box annealing was performed for 16 hours in an atmosphere of _H2 + _N2 , and then continuous annealing was performed in the same manner as in Example 1. Thereafter, a JIS No. 5 test piece was taken from the steel plate and a tensile test was conducted. Figure 2 shows the results, showing the amount of Mn, Cr
Figure 2 shows the effect of quantity on yield ratio and r-value. The figure shows that properties of an r value of 1.20 or more and a yield ratio of 0.60 or less are ensured when the Mn content + Cr content is in the range of 0.8 to 2.5%. Furthermore, if Mn is less than 0.2%, cracking is likely to occur during hot rolling, and Cr is 1.8%.
It can be seen that if the value exceeds 100%, the chemical conversion treatment property deteriorates. Example 3 Steels (C) and (D) having the components shown in Table 2 were melted and prepared in Example 1.
A cold-rolled steel plate with a thickness of 0.8 mm was obtained in the same manner as above. Next, after performing box annealing under the same conditions as in Example 2, continuous annealing was performed by varying the soaking temperature of continuous annealing between 730 and 780°C and changing the soaking time between 30 and 90 seconds. I went.

[Table] Similar to Examples 1 and 2, a tensile test was conducted on the steel plate after continuous annealing. Table 3 shows the results.

[Table] Compared to steel (D) with a high C content outside the range of the present invention,
It can be seen that the steel (C) having the components within the range of the present invention has small variations in each characteristic value, and according to the present invention, the quality of the steel plate is stable.

[Brief explanation of the drawing]

Figure 1 shows the box annealing temperature, yield ratio, and r
It is a chart showing the relationship between values. Figure 2 shows the sample steel plate.
It is a chart showing the relationship between the amount of Mn, the amount of Cr, the yield ratio, and the r value.

Claims (1)

[Claims] 1 C0.005~0.04%, Si0.20% or less, Mn0.20% or more, Cr1.8% or less, SOl.Al 0.01~0.10%,
Contains N0.0015 to 0.0150%, and Mn% + Cr% =
After hot-rolling and cold-rolling a steel containing 0.8 to 2.5% Fe, the remainder being substantially Fe, the steel is heated to 660 to 780°C.
A high-tensile cold rolled product with good drawability and shape, characterized by box annealing, cooling, heating to 650 to 820°C for a short time in a continuous annealing furnace with a heating zone of 650°C or higher, and rapid cooling. Method of manufacturing steel plates.