JPS627822A - Manufacture of cold rolled steel sheet for deep drawing having baking hardenability - Google Patents

Abstract

PURPOSE:To manufacture cold rolled steel sheet having both ultradeep drawability and baking hardenability, by applying hot and cold rollings and continuous annealing under specified conditions to steel contg. ultralow carbon, low sulfur and nitrogen and a specified quantity of Ti. CONSTITUTION:Al killed steel slab having compsn. contg. by weight 0.005-0.015% C, <0.003% S, <0.004% N (<=0.005% total content of S and N) and effective Ti quantity (Ti*) given by a formula (I) of 1-20 times of C content is heated to >=1150 deg.C, and hot rolled to plate of 3.5mm thickness. Next, the plate is successively cold rolled to sheet of 0.08mm thickness, then the sheet is held at >= recrystallization temp. range for >=300sec including heating and cooling, and annealed continuously. Ultralow carbon Al killed steel cold rolled sheet having necessary and suitable baking hardenability together with deep drawability is obtd.

Description

[Detailed Description of the Invention] (Field of Industrial Application) A cold-rolled steel sheet that has both ultra-deep drawability and bake hardenability is suitable for use that satisfies 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, in terms of press formability, properties such as low yield strength, low tensile strength, high elongation, and high r value are desired.

Against this background, there is a need for a steel sheet that is soft and exhibits good press formability during press forming, and has characteristics that increase yield strength and tensile strength during subsequent coating baking, that is, has bake hardenability.

The present invention proposes an advantageous method for manufacturing cold-rolled steel sheets for deep drawing that have bake hardenability.

(Prior art) Regarding a cold rolled steel sheet having bake hardenability and its manufacturing method,
JP-A-53-114717 discloses Ti-added steel,
Furthermore, JP-A No. 57-70258 describes Nbg steel processing, and JP-A No. 59-31827 discloses Ti,
Each Nb composite addition steel is disclosed.

In both of these methods, by controlling the amount of Ti and Nb added or the cooling rate during annealing, the amount of solid solute C in the steel can be adjusted to an appropriate level, thereby imparting "bake hardenability" without causing material deterioration. This is what I did.

However, if an attempt is made to control the amounts of Ti and Nb added in order to maintain solute carbon, 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, 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 Ntl, 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 Application No. 14990/1983).

Regarding limiting the contents of S and N, JP-A-58-110659 discloses that
(hereinafter simply expressed as %) ~0.020%, N≦0.003
5%, and in Japanese Patent Application Laid-Open No. 58-42752, N is 0,
0.0025% or less, but the former is only intended to prevent surface defects by reducing the amounts of Ti and B added, and the latter is merely to improve secondary processability and r-value. do not have.

(Problems to be Solved by the Invention) Effective Ti (Ti
In accordance with the investigation on the influence of continuous annealing conditions on the improvement of B)I properties of ultra-low carbon Al-killed cold-rolled steel sheets for which an appropriate T1 content has been specified according to It is an object of the present invention to provide a stable, simple and advantageous manufacturing method for steel plates.

(Means for solving the problem) This invention contains C: 0.0005 to 0.015%,
S: 0.003% or less, N: 0.004% or less, and
Using a steel with a composition in which S+N<0.005% and further containing T1 in an amount such that the effective Ti (Ti") given by the following formula is in the range of 1 to 20 times the C content, the heat Production of cold rolled steel sheet for deep drawing with bake hardenability characterized by continuous annealing in a temperature range above the recrystallization temperature including heating and cooling for 300 seconds or less after rolling or cold rolling. Method.

Record T Bi (%) - ([Ti%] - It is.

In this case, it is more advantageous to set the slab reheating temperature in the hot rolling process to 1150° C. or higher.

The details of the experimental studies that led to this invention will be explained.

C: 0.0020%, Mn: 0.1%, A1: 0,
04%, Tl: 0, including 026%, S: 0.00
22%, N: 0.0019%, therefore Ti”
/ Vacuum melted steel with Cζ8.1 is melted in a laboratory, hot rolled to a thickness of 3.5mm, and then further rolled to a thickness of 0.8mm.
Cold rolled to In this case, the recrystallization temperature of the cold rolled sheet is 66
It was 0°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 precipitation of TiC progresses during annealing, and prolonged annealing is disadvantageous in securing solid solution C.

Therefore, the residence time above the recrystallization temperature, including heating and cooling, must be shortened, and 300 seconds or less is suitable.

Here, the BH property is 17 after giving 2% prestrain to the cold rolled sheet.
It is given as the measured value of the increase in yield point due to aging treatment equivalent to baking for 20 minutes at 0°C.

Regarding the test steel used in the above experiment, the relationship between the slab reheating temperature before hot rolling and the lower value of the steel plate after continuous annealing was investigated, and the results shown in FIG. 5 were obtained.

Here, the residence time above the recrystallization temperature (660°C) in continuous annealing was 140 seconds, and the soaking temperature was 800°C.

As can be seen from the figure, by setting the slab reheating temperature to 1150°C or higher, the lower value is significantly improved. This is thought to be because reheating the slab at a high temperature changes the distribution and morphology of composite precipitates of TiS and TiC in the hot rolled sheet, which is advantageous for the development of the (111) recrystallized texture during cold rolling annealing.

(As a result of experiments, if the slab reheating temperature is 1150°C or higher, the BH property is high regardless of the thermal history of the slab until it is heated, the rolling conditions during hot rolling, and the coil winding temperature. It was found that a steel plate with an extremely high lower value can be obtained with
This is the value obtained by averaging the test results of test pieces taken in the 5° direction and 90° direction using the following formula (1), and the correction that will be mentioned later follows the formula (2). By limiting the amounts of S and N, B
The reason why H-sexuality appears is not necessarily clear, but Tie,
The reduction of TiN precipitates makes TiC unstable,
This is thought to be due to the fact that, in combination with the restriction of residence time above the recrystallization temperature in the continuous annealing process, beneficial solid solution C remains.

A reduction in N naturally leads to a reduction in precipitates such as TiS, TiN, etc., and this is considered to be suitable for applications that are subjected to strong working, such as the 01 can.

The reason for limiting the ingredients in this invention will be explained.

C: The lower the C content, the better the quality of the material, and if it exceeds 0.015%, 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≦0.00
3%, N amount 0.004% and [S%] 10 [N%]≦0
．． The reason for the limitation is that 0050 has favorable bake hardenability.

Good material quality and bake hardenability can be obtained. However, excessive T
Since the addition of I leads to deterioration of the surface properties of the steel sheet and is disadvantageous in terms of cost, the upper limit is set at 20X (C%).

In addition, in this invention, 81°Mn as a film component in steel
is effective in increasing the strength of the steel sheet without deteriorating the deep drawability, but si>1. Addition of (]5%Mn>l, Q%, may deteriorate the elongation and drawability of the steel sheet, so even if it is added, Si should be 1.0% or less, Mn1.
It is desirable to keep it below 0%.

Next, like Si and Mn, P is useful for increasing the strength of steel sheets without deteriorating deep drawability, but if it exceeds 0.15%, it deteriorates the elongation and drawability of steel sheets.
It may be added up to 15%.

Further, 0.005% or more of 8T remains in the steel for deoxidation, etc., but addition of more than 0.10% has an adverse effect on the surface quality, so it is preferably kept within 0.1G%.

In the above-described invention of the basket, Nb,
One or two types of B can be added in combination with Ti, and even in that case, the BH property, which is a feature of this invention, is not lost, and the lower value and correction are improved. However, when adding 3x[C%]t to Nb, the effect is saturated even if it exceeds 0.0050%, and it becomes disadvantageous in terms of cost. It is preferable that it be ≦0.0050%.

Furthermore, 1.0% or less of Cr, Cu, V, 0.05
Of course, it is possible to add Pb and Ca in amounts of less than 20% since they do not deteriorate the BH properties and deep drawability.

The manufacturing process for cold-rolled steel sheets involves turning steel tapped in a converter or electric furnace into a slab using an ingot-blooming or continuous casting method.
The continuous annealing process according to the present invention requires a residence time of 300 min or more in the temperature range above the recrystallization temperature. This is because it is essential for the expression of excellent BH properties to be limited to within seconds, and if this is exceeded, significant deterioration of eh properties will occur.

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.

(Example) Example 1 Steels (Nos. 1 to 3) having the compositions shown in Table 1 were melted in a converter, vacuum degassed, and then continuously cast into slabs.

These slabs were heated at 1100 to 1220° C., then hot rolled, then cold rolled to form cold rolled sheets with a thickness of 0.8 mm, and then continuously annealed.

During this continuous annealing, the residence time above the recrystallization temperature was varied in the cycle of heating and cooling to 820°C. Table 2 shows the results of examining the mechanical properties and BH content of the product thus obtained.゛From Table 2, a high amount of BH can be obtained when the residence time above the recrystallization temperature is within 300 seconds.
Moreover, there were no problems in mechanical properties. By the way, the recrystallization temperature is steel No.

650 ℃ for 1, @No, 720 ℃ for 2, steel N023
The temperature was 760°C.

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 in the range of 1090 to 1330° C. for 3 to 4 hours) and hot rolled. Hot rolling finishing temperature 910
The hot rolling winding temperature was 510 to 600°C.

After pickling the hot-rolled steel strip, a cold-rolled steel strip with a thickness of 0.8 mm was subjected to continuous annealing using a drawing method.

In continuous annealing, the residence time above the recrystallization temperature of the steel plate is 105
~172 seconds, and the maximum temperature reached is 790~82
The temperature was 0°C.

Table 4 shows the material properties after temper rolling of 0.5 to 0.8%.

By setting the slab heating temperature to 1210 to 1330°C, a high B)I property is ensured, and a lower value of -2.3 to 2.6 is obtained.

(Effect of the invention) Limits the amount of S, N and S+N in steel and makes Ti effective T
The recrystallization of this invention provides the necessary and appropriate bake hardenability for ultra-low carbon Al-killed cold-rolled steel sheets having a composition in which i(Ti") is in the range of 1 to 20 times the C content, as well as deep drawability. This is advantageously ensured by continuous annealing with controlled annealing conditions.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the effect 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. Figure 1

Claims (1)

[Claims] 1. Contains C: 0.0005 to 0.015 wt%, S:
0.003wt% or less, N: 0.004wt% or less, and S+N≦0.005wt%, and furthermore, the effective Ti (Ti^*) given by the following formula is in the range of 1 to 20 times the C content. Using steel with a composition containing an amount of Ti, after hot rolling and cold rolling, residence time in a temperature range above the recrystallization temperature including heating and cooling, 30
A method for producing a cold-rolled steel sheet for deep drawing having bake hardenability, characterized by: continuous annealing within 0 seconds. Note Ti^*(wt%)=([Tiwt%]-48/14[
Nwt%]-48/32[Swt%]) 2. Slab reheating temperature is 1150°C or higher in the above hot rolling process 1
, the method described.