JPH0548283B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Fields) Cold-rolled steel sheets that have both ultra-deep drawability and bake hardenability are suitable for applications that meet the following requirements. In recent years, there has been an increasing demand for higher strength steel sheets for automobiles, with the aim of improving fuel efficiency by reducing the weight of automobiles. On the other hand, from the perspective of press formability, low yield strength,
Properties such as low tensile strength, high elongation, and high r value are desired. Due to this contradictory background, it exhibits softness and good press formability during press molding, and the yield strength and tensile strength increase during subsequent paint baking.
In other words, a steel plate with bake hardenability is required. The present invention proposes an advantageous method for manufacturing cold-rolled steel sheets for deep drawing that have bake hardenability. (Prior art) Regarding cold-rolled steel sheets having bake hardenability and their manufacturing method, JP-A-53-114717 describes Ti-added steel, and JP-A-57-70258 describes Nb-added steel. In Japanese Patent Application Laid-open No. 59-31827,
Each of Ti and Nb composite additive steels is disclosed. All of these can be achieved by controlling the amount of Ti and Nb added or the cooling rate during annealing.
By adjusting the amount appropriately, bake hardenability is imparted without causing deterioration of the material. However, if an attempt is made to control the amounts of Ti and Nb added in this manner so that solid solute carbon remains, the properties of the steel sheet will be significantly affected by subtle changes in the amounts added. That is, if the amount of Ti or Nb added is insufficient, this will lead to deterioration of the material properties that affect formability, such as elongation and r value, while if the amount added is excessive, the bake hardenability will be lost. Therefore, controlling the amount of added elements is considered to be a key issue in process production. In order to avoid the disadvantages of limiting the amount of carbonitride-forming elements such as Ti and Nb, stable bake hardenability is achieved by limiting the content of S and N that should be combined with Ti. It is extremely advantageous to provide a cold-rolled steel sheet for deep drawing having the following properties (see Japanese Patent Publication No. 63-4899). Regarding limiting the contents of S and N, Japanese Patent Application Laid-open No. 110659/1983 describes that S should be 0.001wt%.
(Hereinafter simply expressed as %) ~0.020%, N0.0035%,
Furthermore, JP-A-58-42752 mentions limiting N to 0.0025% or less, but
The purpose of the former is to prevent surface defects by reducing the amounts of Ti and B added, and the purpose of the latter is merely to improve secondary processability and r-value. (Problems to be Solved by the Invention) Effective solutions based on the content limitations of S and N that should be combined with Ti disclosed in Japanese Patent Application No. 146990/1982
Continuous annealing conditions are BH for extremely low carbon Al-killed cold-rolled steel sheets with appropriate Ti content specified according to Ti (Ti ^* ).
It is an object of the present invention to provide a stable, simple and advantageous manufacturing method for cold-rolled steel sheets for deep drawing having bake hardenability based on the investigation of the influence on the improvement of hardenability. (Means for solving the problem) This invention contains C: 0.0005 to 0.015% and S:
0.003% or less, N: 0.004% or less, and S+N<
0.005%, and the effective value given by the formula below
Using steel whose composition contains Ti (Ti ^* ) in an amount ranging from 4 to 20 times the C content, hot rolling,
A method for producing a cold-rolled steel sheet for deep drawing with bake hardenability, which comprises, after cold rolling, continuous annealing in a temperature range above the recrystallization temperature, including heating and cooling, for a residence time of 300 seconds or less. Ti ^* (%) = ([Ti%] -48/14 [N%] -48/32 [S
%]). In this case, it is recommended that the slab reheating temperature in the hot rolling process be set at 1150°C or higher.
more advantageous. The details of the experimental studies that led to this invention will be explained. C: 0.0020%, Mn: 0.1%, Al: 0.04%, Ti:
Vacuum melted steel containing 0.026%, S: 0.0022%, N: 0.0019%, and therefore Ti ^* /C≒8.1 was melted in a laboratory, hot rolled to a thickness of 3.5 mm, and further Cold rolled to 0.8mm. In this case, the recrystallization temperature of the cold rolled sheet was 660°C. FIG. 1 shows the relationship between the residence time above the recrystallization temperature and the BH property when the cold-rolled sheet was continuously annealed at various soaking times at both heating and cooling rates of 10° C./s. As is clear from FIG. 1, stable high BH properties were obtained by keeping the residence time in the temperature range above the recrystallization temperature within 300 seconds. This is thought to be because TiC precipitation progresses during annealing, and long-time annealing is disadvantageous in securing solid solution C. Therefore, the residence time above the recrystallization temperature, including heating and cooling, needs to be short, and 300 seconds or less is suitable. Here, the BH property is given as a value obtained by measuring the amount of increase in yield point by aging treatment equivalent to baking at 170°C for 20 minutes after giving a 2% prestrain to a cold rolled sheet. For the test steel used in the above experiment, the relationship between the slab reheating temperature before hot rolling and the value of the steel plate after continuous annealing was investigated, and the results shown in FIG. 2 were obtained. Here is the recrystallization temperature in continuous annealing (660℃)
The above residence time was 140 seconds, and the soaking temperature was 800°C. As you can see from the figure, the slab reheating temperature is 1150℃
By doing the above, it is recognized that the value is significantly improved. This is thought to be because reheating the slab at high temperatures changes the distribution and morphology of composite precipitates of TiS and TiC in the hot-rolled sheet, favoring the development of the {111} recrystallized texture during cold-rolling annealing. As a result of subsequent experiments, the slab reheat temperature was 1150
If the temperature is above ℃, a steel plate with extremely high BH properties can be obtained almost regardless of the thermal history of the slab until it is heated, the rolling conditions during hot rolling, and the coil winding temperature. I understood. is the value obtained by averaging the test results of test pieces taken parallel to the rolling direction, 45° direction, and 90° direction using the following formula (1), and the values mentioned later follow formula (2) = r ₀ +r ₉₀ +2r ₄₅ /4 ...(1) =El ₀ +El ₉₀ +2El ₄₅ /4 ...(2) (Function) In this invention, it is not necessarily clear why BH properties appear by limiting the amount of S and N in the steel. However, due to the decrease of TiS and TiN precipitates, TiC becomes unstable, and combined with the restriction of residence time above the recrystallization temperature in the continuous annealing process, the advantageous solid solution C
Furthermore, the reduction of S and N naturally leads to the reduction of precipitates such as TiS and TiN, and this is considered to be suitable for applications that undergo heavy processing such as DI cans. The reason for limiting the ingredients in this invention will be explained. C: The lower C is, the better the material is, 0.015
If it exceeds %, good drawability cannot be obtained even if the amount of Ti added, which will be described later, is increased. On the other hand, if it is less than 0.0005%, the bake hardenability which is the object of this invention cannot be obtained. Therefore, the amount of C is set to 0.0005% to 0.0150%. S, N: The amounts of S and N in steel are the most important components in this invention, and as is clear from the previous experimental results, S≦0.003%, N≦0.004%, and [S%] +
The reason for the limitation is that when [N%]≦0.0050, bake hardenability appears advantageously. Ti: Ti is added to fix S, N and C, but effective Ti (Ti ^* = [Ti%] -48/14 [N%] -48/32 [S%]) is added to C. By setting the atomic ratio to 114 (that is, 4 times the C% in weight%) or more,
Good material quality and bake hardenability can be obtained. However, the addition of excessive Ti leads to deterioration of the surface properties of the steel sheet and is disadvantageous in terms of cost, so the upper limit has to be set.
20×[C%]. Therefore, the amount of Ti added is 4×[C%]≦([Ti%]−
48/14 [N%] - 48/32 [S%])≦20×[C%]. In addition, in this invention, as a general component in steel
Si and Mn are effective in increasing the strength of steel sheets without deteriorating deep drawability, but Si > 1.0% and Mn > 1.0%
%, which may deteriorate the elongation and drawability of the steel sheet, so even if it is added,
It is desirable to keep Si at 1.0% or less and Mn at 1.0% or less. Next, like Si and Mn, P also helps increase the strength of steel sheets without deteriorating deep drawability.
If it exceeds 0.15%, the elongation and drawability of the steel sheet will deteriorate, so it may be added up to 0.15%. Furthermore, Al remains in the steel in an amount of 0.005% or more for deoxidation, etc., but addition of more than 0.10% has a negative effect on the surface quality, so it is preferably kept within 0.10%. In addition to the above, in this invention,
It is also possible to add one or both of Nb and B in combination with Ti, and even in that case, this is a feature of the present invention.
BH properties are not lost and the value is improved. However, for Nb, 3×[C%] and B are
Even if more than 0.0050% is added, the effect will be saturated and the cost will be disadvantageous, so Nb<3×[C
%], B≦0.0050%. Furthermore, 1.0% or less Cr, Cu, V, 0.05% or less
It is of course possible to add Pb and Ca as they do not deteriorate the BH properties and deep drawability. The manufacturing process for cold-rolled steel sheets involves making steel from a converter or electric furnace into a slab using an ingot-blowing or continuous casting method, and then forming a cold-rolled sheet through normal hot rolling or cold rolling. It consists of applying crystal annealing,
In particular, in the continuous annealing according to the present invention, it is essential to limit the residence time in the temperature range above the recrystallization temperature to within 300 seconds, and if this is exceeded, the BH properties will deteriorate significantly. This is because it causes Further, it has been confirmed that the steel sheets manufactured in the following examples have excellent chemical conversion treatment properties and have no problems in hot-dip galvanizing properties and secondary workability. (Examples) Example 1 Steels (Nos. 1 to 3) having the compositions shown in Table 1 were melted in a converter, subjected to vacuum degassing treatment, and then continuously cast to form slabs. These slabs were heated at 1100 to 1220°C, then hot rolled, then cold rolled to form a cold rolled plate with a thickness of 0.8 mm, and then continuously annealed. In this continuous annealing, the residence time above the recrystallization temperature was varied in the cycle of heating and cooling to 820°C. The mechanical properties of the product thus obtained and
Table 2 shows the results of investigating the amount of BH.

【table】

[Table] From Table 2, a high amount of BH was obtained when the residence time above the recrystallization temperature was within 300 seconds, and there were no problems with mechanical properties. By the way, the recrystallization temperature is
650℃ for No.1, 720℃ for Steel No.2, 760℃ for Steel No.3
It was hot. Example 2 Steels (A, B) having the compositions shown in Table 3 were melted in a converter, and after vacuum degassing treatment, were made into slabs using a continuous casting machine. These slabs were heated and soaked (3 to 4 hours) in the range of 1090 to 1330°C and hot rolled. Hot rolling finishing temperature
The hot rolling winding temperature was 910-880°C and 510-600°C. After pickling, the hot rolled steel strip was made into a cold rolled steel strip with a thickness of 0.8 mm, and then subjected to the following continuous annealing. In continuous annealing, the residence time above the recrystallization temperature of the steel plate is in the range of 105 to 172 seconds, and the maximum temperature reached is 790.
It was ~820℃. Table 4 shows the material properties after 0.5 to 0.8% temper rolling.

【table】

[Table] By setting the slab heating temperature to 1210-1330°C, high BH properties were ensured and =2.3-2.6 was obtained. (Effect of the invention) Limiting the amounts of S, N and S+N in steel, and reducing Ti
The present invention provides the necessary and appropriate bake hardenability for ultra-low carbon Al-killed cold-rolled steel sheets with a composition in which effective Ti (Ti ^* ) is in the range of 4 to 20 times the C content, as well as deep drawability. This is advantageously ensured by continuous annealing with controlled recrystallization annealing conditions.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of residence time above the recrystallization temperature on BH properties, and FIG. 2 is a graph showing the relationship of slab reheating temperature on r value.

Claims (1)

[Claims] 1 Contains C: 0.0005 to 0.015wt%, S: 0.003wt% or less, N: 0.004wt% or less, and S+N≦0.005wt%, and furthermore, effective Ti ( Ti ^* ) is C
Using steel with a composition containing Ti in an amount ranging from 4 to 20 times the content, after hot rolling and cold rolling, residence time in a temperature range above the recrystallization temperature including heating and cooling,
A method for manufacturing a cold-rolled steel sheet for deep drawing having bake hardenability, characterized by continuously annealing within 300 seconds. Note Ti ^* (wt%) = ([Tiwt%] - 48/14 [Nwt%] -
48/32 [Swt%]) 2 In the above hot rolling process, the slab reheating temperature is
The method according to claim 1, wherein the temperature is 1150°C or higher.