JPH04136123A - Production of cold rolled sheet for deep drawing having high baking hardenability - Google Patents

Abstract

PURPOSE:To produce the cold rolled steel sheet for deep drawing having high baking hardenability and deep drawing property by using an extra-low carbon steel having the specific compsn. which is regulated in the contents of S and N and is further added with B and Ti, and optimizing the cooling patterns in recrystallization annealing. CONSTITUTION:The steel blank material which contains 0.0005 to 0.006wt.% C, <=1.5% Si, <=1.5% Mn, 0.005 to 0.10% Ti, 0.0004 to 0.0015% B, 0.005 to 0.100% Al, <=0.15 P, <=0.0030% S, and <=0.0040% N, satisfies the conditions under which the effective component Ti(Ti*) given by the formula Ti*%=([Ti%]-48/14[N%]-48/32[S%]) attains the range of 4 to 20 times the [C%] at <=0.0050% total of the contents of the S and N, and further contains the balance iron and unavoidable impurities, is subjected to hot rolling then to cold rolling. This steel sheet is thereafter heated up to a temp. range of 730 to 900 deg.C at >=1 deg.C/S heating rate. The steel sheet is thereafter subjected to the recrystallization annealing under the conditions under which the steel sheet is rapidly cooled at >=20 deg.C/S cooling rate down to <=550 deg.C temp.

Description

Detailed Description of the Invention (Industrial Application Field) It is important to have high ductility (El), ultra-deep drawability (T value), excellent press formability, and high bake hardenability. It is required for hot-dip galvanized and alloyed hot-dip galvanized steel sheets, including surface-treated steel sheets, including inter-rolled steel sheets.

Incidentally, in recent years, reducing the weight of automobiles is useful for improving environmental problems on a global scale by improving fuel efficiency, and as a measure for this purpose, there has been an increasing demand for higher strength steel sheets for automobiles.

On the other hand, in terms of press formability, it must have the following characteristics: low yield stress, high ductility, and excellent deep drawability.

In order to satisfy these contradictory requirements, we have developed a property that is soft during press forming and increases yield strength and tensile strength after the subsequent paint baking. There is a strong desire to develop steel sheets for deep drawing with high efficiencies.

(Prior art) Regarding the manufacturing method of cold-rolled steel sheets having BH properties, Japanese Patent Laid-Open No. 5
No. 3-114717 discloses Ti-added steel, JP-A-57-70258 discloses Nb-added steel, and JP-A-59-31827 discloses composite addition steel of Ti and Nb.

In all of these, BH properties can be imparted without deteriorating the material by controlling the amount of Ti and/or Nb added and the cooling rate during annealing.

However, if we try to control the amount of Ti and Nb added in this way to ensure that solid solution C remains, subtle changes in the amount added will greatly change the properties of the steel sheet, especially the ductility and drawability, resulting in poor quality. It becomes unstable, and there is also the problem that BH properties deteriorate if TiQNb is added in excess.

In order to avoid such problems, there is a technique that limits the content of S and N that combine with Ti, or JP-A No. 61-267.
The proposal is disclosed in Publication No. 57.

However, according to this technology, although B and Nb are added as the same effective ingredients to improve the T value and El, no special consideration is given to the manufacturing conditions, so the ultra-deep drawability and 3kgf /mm2 to 5kgf/
Although a bake hardening amount (simply referred to as Bt) of mm2 has been obtained, it has not yet been possible to stably secure a high BH amount exceeding 5 kgf/mm2 without deteriorating deep drawability.

(Problem to be solved by the invention) This invention is based on ultra-low carbon steel with T1 added, and by optimizing the component composition and annealing conditions, a high B
The purpose of the present invention is to produce a cold-rolled sheet for deep drawing having H properties.

Here, cold-rolled sheets include not only cold-rolled steel sheets, but also cold-rolled sheets.
This includes hot-dip galvanized steel sheets and base sheets for alloyed hot-dip galvanized steel sheets that have been immersed in liquid zinc and subjected to plating treatment.

(Means for Solving the Problems) As a result of various experiments, the present invention specifies the annealing conditions in addition to the rules for the content of S and H and the addition of B in carbon steel with T1 addition. It was discovered that by doing so, a consistently high BH property can be obtained.
% or less, Si: 1.5 wt% or less, Mn: 1.5 wt% or less, Ti: 0.005 wt% or more, 0.10 wt% or less, B: 0.0004 wt% or more, 0.0015
wt% or less, Al: 0.005 wt% or more, 0
．． 100 wt% or less, P: 0.15 wt% or less, S: 0.0030 wt% or less, and N・0.00
40 wt% or less, including 0.0050 wt% or less of the total content of S and N, and a range of 4 to 20 times the effective Ti (Ti") (Cwt%) given by the following formula: A steel material that satisfies the following conditions and the remainder consists of iron and unavoidable impurities is subjected to hot rolling,
Then, after performing cold rolling, this steel plate was subjected to l″C/SC
/3 degrees or more, raise the temperature to a temperature range of 730°C or more and 900°C or less, and then raise the temperature to a temperature of 550°C or less for 20 minutes.
This is a method for producing a cold-rolled sheet for deep drawing having high bake hardenability, characterized in that recrystallization annealing is performed under conditions of rapid cooling at a cooling rate of .degree. C./S or more.

Here, in recrystallization annealing, the temperature is 730°C or higher and 900°C.
This includes a case in which, after raising the temperature to a temperature range of .degree. C. or lower, the temperature is soaked for a short period of about 30 seconds.

(Function) First, the contents of the experiment that led to this invention will be explained.

(Experiment 1) C: 0.0021wt%, Si: 0.01wt%
, Mn: 0.12wt%, Al: 0.046
wt%, P2 containing 0.011 wt%, B, S.

By changing the N content, effective Ti (w
Vacuum melted steel adjusted to 7 to 8 times C (wt%) or C (wt%) was prepared in the laboratory, hot rolled to a thickness of 3.5 mm, and then cold rolled to a thickness of 0.7 mm. Rolled. This rolled thin plate was heated to 795°C at a heating rate of 5°C/S.
A continuous annealing type thermal cycle was performed in which the material was soaked for 5 seconds, then rapidly cooled to room temperature at 35° C./S, and then temper rolled with an elongation of 0° and 7%.

Regarding the cold-rolled sheet obtained in this way, the elongation (EN)
, the amount of BH was investigated.

Figure 1 shows the relationship between the S, N, and S contents and the amounts of EA and Bl.

Here, El is parallel to the rolling direction (Ezo), 45°
This is a value obtained by averaging the measurement results of test pieces taken in the direction (EJ?ns) and the 90° direction (Ef9°) using the following formula, and the amount of BH is 2% as shown in Figure 2. After applying tensile strain, a considerable heat treatment was performed at 170°C for 20 minutes, and the increase in yield point at this time was measured.

From Figure 1, as the sum of S and N contents decreases, E
l, the amount of BH shows an increasing tendency, and the effect of B addition is
It is noticeable in quantity. In other words, when comparing the sum of the S and N contents, which is 50wtppm (0,005wt%), BH content is 3kgf/mm2 for B-free steel, whereas it is 5kgf/mm2 for S content of l1wt PPm.
If the BH content is as high as mm2, or the S content further increases to 20wtp1)m, the BH amount is 3kgf/mm2.
and decreases.

For steels with S content (llwtl)pm and the sum of S and N contents less than 20wtppm, "kgf/mm2
This shows a very high amount of BH.

On the other hand, even if the S content is Ilwtppm, in steel with an S content of 40wtppm or steel with an N content of 45wtppm, the total S and N content is 4kgf even though the total content is 50wt pf)m. The amount of BH less than "/mm" was obtained.

(Experiment 2) The same contents of C, Si, Mn, and A1 as in Experiment 1 were used. P and effective Ti (wt%) or C (wt%) of 8
From 10 times to 10 times, S: 0.0015wt% to 0.
002wt%, N: 0.0018wt% to 0.0
0.0029wt of B in the component system containing 022wt%
In Experiment 1, steels (including B-free steels) added between
Steel plates were manufactured using the same process as above, and the BH content of these steel plates was measured. The relationship between the amount of B added and the amount of BH is shown in the third
As shown in the figure. As is clear from Figure 3, the amount of B added is 4
Between wtppm and 15wtl)pm, the amount of BH is 5k
gf/mm2 or more.

From the results of Experiments 1 and 2, the contents of S, N, and B, as well as the sum of the contents of S and N, greatly affect the amount of BH. By optimizing the cooling rate of It became clear that a high BH amount of /a+m2 or more could be obtained.

Regarding these results, the reason why the BH properties are improved by regulating the content of S and N is that due to the reduction of TiS and TiN precipitates, the number of TiC precipitate nuclei is reduced, and the formed TiC is This is thought to be due to the fact that it becomes stable and solid solution C tends to remain.Also, it is not clear why BH properties are improved by adding B in the range of 4 wtppm to 15 Wtppm, but B tends to segregate at grain boundaries. This is thought to be due to the fact that it has a high tendency to stabilize solid solution C because it is an element, and the amount of B added is 15wtppm.
It is presumed that the phenomenon in which the BH property deteriorates again when the temperature exceeds 100% is due to part of B forming nitrides and increasing the effective amount of Ti.

Next, the reason for limiting the range of chemical components in the steel composition of this invention will be described below.

C: In order to obtain good deep drawability and ductility, the smaller the amount, the more advantageous. If it exceeds 0.0060 wt%, Ti
Even if the amount is increased, good ductility and deep drawability cannot be obtained.

On the other hand, if it is less than 0.0005 wt%, the high BH properties that are the object of this invention cannot be obtained.

Therefore, its content is 0.000005 wt% or more.
0060wt% or less.

Si, Mn: These are elements effective for increasing the strength of the steel sheet without significantly deteriorating the deep drawability. However, if large amounts are added, the processability is immeasurable and the weldability and surface treatment properties are also deteriorated, so the upper limit of their content should be set to S
Let l be 1.5 wt% and Mn be 1.5 wt%.

T1. If the content is too low, it cannot be expected to improve the 7 value, elongation, etc., and if it is too high, problems such as nozzle clogging during the steel casting stage, deterioration of surface properties, and increased costs will occur.

Therefore, its content is 0.005 wt% or more.
10wt% or less, and since Ti is added to fix S, N, and C, in relation to these contents, the effective Ti amount expressed as: C (wt%) 4 times or more 20
It is necessary to satisfy the condition that the range is less than or equal to

In addition, conventionally, in order to manufacture steel sheets for deep drawing, S.

In order to completely fix N and C, the effective Ti or C(w
t%), that is, the atomic ratio is 1 or more, but in this case, since there is no solid solution C, it has excellent BH properties, apart from deep drawability. I couldn't get it. However, in this invention, even if the effective Ti or C (wt%) is four times or more, a steel sheet can be obtained that has excellent Bl (Bl) properties as well as deep drawability.

B: This is the most important component in this invention, and as is clear from the results of Experiment 1 and Experiment 2 described above, 5 kgf
In order to obtain a high amount of B)I exceeding /mm2, the content should be 0.0004wt% or more and 0.0015w
Is it necessary to keep it below t%?

A1: Is it necessary for deoxidizing steel? Excessive addition not only saturates the effect but also deteriorates the surface quality.

Therefore, its content is 0.005 wt% or more.
゜100 wt% or less.

P: Like Si and Mn, it is an effective element for increasing the strength of steel sheets without significantly deteriorating deep drawability, but excessive addition deteriorates workability, weldability, and surface treatment.

Therefore, its content should be 0.15 wt% or less.

S, N: Regulation of these components is important for this invention. From the results of Experiment 1 and Experiment 2 described above, 5 kg
In order to obtain a high Bl content exceeding f/mm2, their content should be set to an upper limit of 0,0030 wt% for S, respectively.
, N should be 0.0040 wt%, and the sum of the S and N contents should be 0.0050 wt%.

Next, the manufacturing method will be described.

After melting the steel having the above composition, it is made into a slab by a conventional method, hot rolled, then cold rolled, and then 1'C
Recrystallization annealing is performed by heating at a heating rate of /S or higher and then rapid cooling.

The heating rate in recrystallization annealing is
The heating temperature must be at least 1°C/S to prevent the decomposition of
Requires 0°C. However, as the heating temperature increases, r
The value and elongation may improve, but if the temperature exceeds 900°C, the r value may deteriorate due to transformation, and the press formability may significantly decrease due to coarsening of crystal grains. Therefore, the upper limit is set at 900°C.

After heating to a temperature range of 730°C to 900°C, it is rapidly cooled, including a short soaking period. If the cooling rate at this time is slow, precipitation of TiC will proceed during cooling, making it impossible to secure a high B) amount of 5 kgf/m+n2 or more.

Therefore, the cooling rate is limited to 20°C/S or more.

In addition, the precipitation rate of Ti-based carbides is negligibly slow below 550°C, so the maximum temperature at which quenching ends is 550°C.
C, and there is no need to particularly limit the cooling pattern thereafter, and it can also be applied to a hot-dip galvanizing process in which plating is performed at 450"C to 550"C after recrystallization annealing, and alloying heat treatment is performed as necessary. .

Furthermore, the cold-rolled steel sheets and hot-dip galvanized steel sheets obtained by the method of this invention also have excellent resistance to secondary work brittleness due to the presence of solid solution C and the addition of B. There is.

(Examples) Example 1 Ten types of compatible steel of the present invention and seven types of comparative steel were melted in a converter and an RH degassing process to form continuous cast slabs. After heating this slab to 1200°C, it is made into a hot rolled coil at a finish rolling temperature of 910'C and a coiling temperature of 590°C.
After descaling, cold rolling was performed at a reduction rate of 78%, and the plate thickness was 0.
．． It was made into a 7 m cold rolled plate. Then, it was heated to 5°C in a continuous annealing furnace.
Heated to 820 °C at a heating rate of /S, held for 20 seconds, cooled to 400 °C at a cooling rate of 35 °C/S,
Below 0°C, after cooling slowly at a cooling rate of 5°C/S,
6% temper rolling was performed.

The chemical compositions of these steels are shown in Table 1, and the tensile properties of the cold rolled sheets after temper rolling are shown in Table 2.

Table 2 The YS, TS, El, and r values shown in Table 2 are in the rolling direction (Z,), 45° direction (z4.) with respect to the rolling direction, and 90′ direction with respect to the rolling direction, respectively, within the plate surface. This is an average value calculated using the following formula using the measured values of (z, e).

According to Table 2, the compatible side steels bi to 7 of this invention are 5 kg
It shows high Bl (amount of f/mm2 or more), elongation of 51 or more, and good deep drawability with an r value of 1.9 or more.
The compatible side steel Nα8. of the present invention has a high tensile strength by increasing the content of Si, Mn, and P. 9.10 also shows good deep drawability with a high BHi of 5 kgf/ai2 or more and an r value of 2.1 or more despite the high tensile strength.

On the other hand, comparative example steel N (Lll) with a large amount of B, comparative example steel Nα12 with a large amount of N, comparative example steel Nα13 with a large amount of S+N, B
Comparative example steel N [lI4.15. , and comparative example steel N (with a content of more than 20 times the effective Ti (Cwt%))
II6 is inferior, with Bl (amount) being less than 5 kgf/rnm2.

In addition, Comparative Example Steel N (117) with a large amount of C has elongated and deteriorated.

In addition, in these steel plates according to the present invention, no elongation at yield point occurred and no deterioration in elongation properties was observed even after 3 months of use in the apparatus.

Example 2 A steel slab having the chemical composition of steel Nαl shown in Table 1 was used to form a cold rolled plate with a thickness of 0.7 mm under the same process conditions as in Example 1 up to the cold rolling process, and then heated in a continuous annealing furnace. at 7°C
After annealing at a heating rate of /S to 820°C and holding for 30 seconds, various cooling patterns were used, and some of the samples were also hot-dip galvanized. 0,6% for these steel plates
Temper rolling was carried out, and the tensile properties were investigated in the same manner as in Example 1.

Table 3 shows the cooling pattern after annealing and hot-dip galvanizing conditions, and Table 4 shows the results of the investigation of tensile properties.

From Table 4, sample Nα3 whose cooling rate is slower than 20°C/S,
And in the case of sample size 5 where the quenching end temperature is higher than 550°C, is it possible to obtain a BH amount of 5 kgf/mm2?
Under the processing conditions of this invention, the BH amount is stably 5 kgf/11.
This shows that it is possible to secure m2 or more.

Further, even in sample Nα6 subjected to alloying hot-dip galvanizing treatment, a BH amount of 5 kgf/mm” or more was obtained without deterioration of the material properties such as elongation and 7 value.

(Effect of the invention) This invention regulates the content of S, N, and further controls the content by adding B, and produces an extremely low carbon material containing effective Ti and C in an atomic ratio of 1.0 or more. It has been discovered that by using steel and optimizing the cooling pattern during recrystallization annealing, high bake hardenability can be obtained along with good deep drawability. It is suitable as a cold-rolled steel sheet for drawing, and also as a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of S, N, and B contents on BH content; Figure 2 is a graph showing how to measure BH content;
The figure is a graph showing the influence of the cooling rate after annealing, regardless of the amount of BH.

Claims (1)

[Claims] 1. C: 0.0005wt% or more, 0.0060wt%
Below, Si: 1.5 wt% or less, Mn: 1.5 wt% or less, Ti: 0.005 wt% or more, 0.10 wt% or less, B
: 0.0004wt% or more, 0.0015wt% or less,
Al: 0.005wt% or more, 0.100wt% or less,
P: 0.15wt% or less, S: 0.0030wt% or less, and N: 0.0040wt% or less, and when the total content of S and N is 0.0050wt% or less, the following formula Ti^≠wt %=([Tiwt%]-48/14[Nw
t%]-48/32[Swt%])
A steel material that satisfies the condition that i (Ti^≠) is in the range of 4 to 20 times [Cwt%], and the remainder consists of iron and unavoidable impurities, is hot rolled and then cold rolled. After that, the steel plate is heated to 730°C or more and 900°C at a heating rate of 1°C/S or more.
Deep-drawing cooling with high bake hardenability characterized by recrystallization annealing under the conditions of raising the temperature to the following temperature range and then rapidly cooling to a temperature of 550°C or less at a cooling rate of 20°C/S or more. Manufacturing method of rolled plate.