JPS62287017A - Production of cold rolled steel sheet having excellent deep drawability - Google Patents

Abstract

PURPOSE:To inexpensively produce a cold rolled steel sheet having excellent deep drawability and high quality by specifying the time from a cutter position to hot rolling in the direct hot rolling stage of a steel ingot contg. specific ratios of C, Mn, Si, P, S, and Ti. CONSTITUTION:The steel ingot which contains, by weight %, <=0.006% C, <=1.5% Mn, <=1.0% Si, <=0.100% P, <=0.025% S, and <=0.085% Ti, is of Ti>=3.42N+4C, contains 1 or 2 kinds of 0.0002-0.0020% B and <=0.050% Nb according to need and consists of the balance iron and unavoidable impurities is directly hot rolled. The conveyance time of the ingot is controlled in such a manner that the time (t) from the cutter position of a continuous casting machine to the start of rolling satisfies the equation. The steel sheet is subjected to descaling upon ending of the hot rolling and is then subjected to color rolling at about >=70% draft and annealing at about 725-800 deg.C. The cold rolled steel sheet having excellent deep drawability and good intra-surface anisotropy is thus produced with good productivity.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing cold-rolled steel sheets with excellent deep drawability using ultra-low C steel. The present invention provides a method for manufacturing high-quality cold-rolled steel sheets at low cost by specifying the time until the start of hot rolling when directly rolling a piece.

(Prior Art) Various fin methods have been proposed and manufactured in the past as methods for manufacturing cold-rolled steel sheets for deep drawing. For example, ultra-low C steel has T], ,
So-called IF containing carbonitride-forming elements such as Nb
There's steel. Conventionally, these have been manufactured by box annealing, oven coil annealing, or continuous annealing.

In case of manufacturing by continuous annealing, JP 5B-574,90
As disclosed in Japanese Unexamined Patent Publication No. 57-1.021, the hot rolling heating temperature is lowered to ensure material quality and operational stability.

In addition, in recent years, with the aim of saving energy and improving productivity,
Many attempts have been made to directly roll (hereinafter referred to as CC-DR) continuously cast hot steel billets. For example, when producing cold rolled steel sheets for deep drawing using low C-AQ killed copper, CC
-Unexamined Japanese Patent Publication No. 52-105 aiming at the energy saving effect of DR
No. 520, cc-DT (Special Publication No. 5B-52441, Special Publication No. 52441, 1986) to prevent cracking of steel pieces that sometimes occurs.
There are various publications such as No. 9-1-0846.

However, most of these methods produce steel sheets for deep drawing by continuous annealing. That is, in order to produce a cold-rolled steel sheet for deep drawing using CC-DR and continuous annealing, it is necessary to overcome the following problems.

(Problems to be solved by the invention) When manufacturing deep drawing steel sheets by continuous annealing, CC-DF
(With this method, the material properties are inferior to those produced using conventional processes using steel slabs with the same composition.

That is, the problems with the conventional CC-DRO are as follows.

(j) Workability is inferior to conventional processes.

(2) In continuous annealing, if annealing is performed under the same conditions as in the conventional process, unrecrystallized parts will remain, resulting in significant material deterioration. To ensure the same material quality as the conventional process, the annealing temperature must be significantly higher. This leads to a significant drop in productivity and an increase in costs.

(3) Since it is necessary to perform continuous annealing at high temperatures, the surface quality of the steel sheet deteriorates.

An object of the present invention is to provide a method for producing a cold-rolled steel sheet that solves the above-mentioned drawbacks of CC-DR, has low operating costs, and has excellent deep drawability.

(Means to solve the problem) The gist of the present invention is that C: 0.006 in terms of weight.
0% or less, Mn: ]-, 5% or less, Sl: 0.9
% or less, P: 0.100 ratio or less, S: 0.025 ratio or less, Ti: 0.085% or less, and T1≧3.42 N
+4C.

If necessary, a heated steel billet containing at least B: 0.0002-0.0020% and Nb: 0.051% or less and the balance consisting of iron and unavoidable impurities is heated to Ar.
When hot rolling is carried out by direct rolling or heat retention or heating as necessary without cooling to 3 points or less, the time from the cutter position of the continuous caster to the start of hot rolling satisfies (1), It is characterized by hot rolling, followed by cold rolling and annealing.

In the present invention, the hot rolling start time is theoretically set to the time after complete solidification, but industrially, the complete solidification point is usually controlled near the continuous casting machine, and management The cutter position was selected for ease of use.

The present invention will be explained in detail below.

First, the basis for determining the time at the start of hot rolling 1 will be explained from solidification, which is the most important component of the present invention.

In the laboratory, the steel components listed in Table 1 were hot-rolled at a thickness of 3.-1 by varying the time from solidification to the start of rolling. The material was cold rolled to a thickness of , annealed at 725°C for 1 minute, and the material properties were investigated after temper rolling of ], ]0.

Note that the hot rolling end temperature was constant at 900°C in all cases, and 650°C.
°C! After forced cooling at 650° C. for 30 minutes, the sample was cooled in a furnace to simulate winding.

Figure 1 shows the time from solidification to the start of rolling and the material properties (7
, elongation).

As can be seen from Table 1, in order to obtain a good i value, which is correlated with deep drawability, a certain period of time or more is required from solidification to the start of rolling. It can also be seen that the time required from solidification to the start of rolling to obtain a steel plate with excellent 7 value and elongation differs depending on the steel composition.

Next, various steel components were melted in the same manner in the laboratory, the time from solidification to the rolling start size was varied, hot rolling was performed, and the material properties after cold rolling and annealing were investigated.

A statistical relational expression was determined from the relationship between the steel composition and the time from solidification to the rolling start dimension, where the material properties change. Figure 2 shows the relationship between this relational expression, the difference in time from actual solidification to the start of rolling, and material properties, and Figure 3 shows the relationship with recrystallization temperature.

The manufacturing conditions for the samples shown in FIGS. 2 and 3 are as follows.

([) Steel composition c: 0.0019-0.0060%, Mn: 0.1
0-0.309 Oto 1 >: o, oorji ~ 0.0], 5
%, S'.

0.005-0.0 1. 8φ, u: o, oo” 5~
000 3ro 5 Daotsu, Ti, :
0.0 2 0 ~ 0. 0 4 5
Daro, B 0.0001-0.0005%.

(2) Time from solidification to start of rolling 10 minutes to 150 minutes
(5) Cold rolling: 3.7 to 0.70.4 (6) Annealing near 25°C ] Minutes (7) Skin Pass: ], O%.

The recrystallization temperature was 650 DEG C. to 800 DEG C. at intervals of 0 DEG C., and the annealing was carried out for 1 minute, and the recrystallization temperature was defined as the temperature at which the processed structure completely disappeared when observed under a microscope.

From Fig. 2, the time from solidification to the start of rolling is 30i,
, g((T 1.13-42 N -±2) When the time is longer, the properties correlated with the workability of 7 values and El become better.

As shown in Figure 3, the recrystallization temperature and the time from solidification to the start of rolling are 30710g1 (''], s, a, z+
q5. ．． -5s) ], the longer it becomes, the lower it becomes.

From the facts in Figures 2 and 3, the conditions for obtaining good workability and low recrystallization temperature are the time from the start of the continuous casting machine to the start of heat treatment (1) ≧ 3071og [ □] was determined as a cow.

A preferable range is the time from the continuous caster cutter to the start of hot rolling (1)≧3671og[-m-7'If, as a condition for stably securing a steel plate with good workability.

Theoretically, the time until the start of hot rolling should be the time after complete solidification; The cutter position is determined for ease of use.

If it takes a long time to start hot rolling, the temperature of the slab becomes too low, so it is necessary to charge it into a heating furnace or a fixing furnace. Therefore, from the viewpoint of heating cost, it is preferable to keep the heating time to 1.50 minutes or less. The slab cut with a cutter is
Even if it is naturally cooled or forcedly cooled at the start of hot rolling, or charged into a fixing furnace or heating furnace, it will not break.

Next, the steel components constituting the present invention will be explained.

Setting C to less than 0.001-1 is not economical because vacuum degassing requires a long time. On the other hand, 0.0060
%, even if hot rolled by the method of the present invention, the ductility decreases and the recrystallization temperature increases. The preferred range is
It is in the range of 0.0010 to 0.004.0.

1) 1. +Mn+P is an element effective in increasing the strength of steel sheets. Since the purpose of the present invention is to manufacture soft cold-rolled steel sheets and high-strength cold-rolled steel sheets, Sl is 0.00.
5~], 0%, Mn is 0.05-1.5%,
The component range of p is 0.003-0]00%.

That is, when manufacturing high-strength steel sheets (Ti, these elements are St: 1.0%, Mn: 1.5%,
P: 0.100. Too much S] will deteriorate paintability, too much Mn will make it difficult to produce steel, and too much P will deteriorate spot weldability and cause P to form at grain boundaries. Segregation occurs and secondary workability deteriorates during pressing. For these reasons, fE], M
The ``- limit of n+P was defined.

In addition, when manufacturing cold rolled steel sheets, Mn; 0.05 to 0
．． It is preferable to set it to 50φ. If Mn exceeds 0.5%, the steel becomes hard and cannot produce a highly ductile steel plate. For the same reason, it is preferable that 81 be 0.05% or less and P be 0.02% or less.

It is well known that S is an element that causes hot embrittlement. However, in the present invention, T1. I
There is no need to worry because Jn is added. If there is too much si-, the amount of T1 or Mn added increases, leading to an increase in cost. Therefore, the upper limit of the amount of S is 0.025%, and the preferable range is 0.015% or less. The lower limit of S is 0.00, which is achievable with current technology.
It is 03chi.

In order to prevent surface flaws from occurring due to oxides of Ag, T], the acid-soluble AQ needs to be 0.005% or more.

On the other hand, if the amount is too high, alumina-based inclusions will increase and the surface quality will deteriorate, so the acid-soluble AQ is 0.090.
The upper limit is %. A preferable range is 0.0 as acid soluble AQ.
005 to 0.06.

N is fixed as TiN by T1, but if there is too much N, the cost increases due to the increased amount of Ti added and the workability deteriorates, so it is preferable to have as little as possible. Therefore, the upper limit of N is 0.0060. N
The lower limit of the amount is 0. OO], 0%
One preferred range is 0.0030.

T1 is T1≧3.4.2N + in order to ensure the deep drawability, ductility, and anti-aging properties necessary for cold-rolled steel sheets for deep drawing.
An amount that satisfies 4C is required. On the other hand, the greater the amount of T], the better the processability, but if it exceeds ci, oso%, the effect is saturated, and adding more than this will increase the cost, which is not economical.

B is an effective element for preventing secondary processing cracks caused by P segregated at grain boundaries. It is an element that is effective in lowering the γ-α transformation temperature during hot-rolling, and addition of B enables the hot-rolling temperature range in the present invention to reach a low temperature and ensures stable operating conditions, so B can be added. preferable. ” The range of the amount of B added to make the effect described in item 2 effective is 0.0002 to 0.0020.
The preferred range is 0.0002 to 0. OO],
It is 0%.

Like T1, Nb is a carbonitride-forming element and is effective in improving the drawability of cold-rolled steel sheets. Nt) is added as necessary because it lowers the γ-α transformation temperature, expands the hot rolling temperature range of the present invention, and ensures stable operation. On the other hand, when the amount of Nb added increases, the recrystallization temperature increases and high-temperature annealing becomes necessary.

A preferred range is 0.005-0.020%.

Copper compaction as described above is carried out by the usual method, for example, in a converter furnace.
It is produced by vacuum degassing treatment. Subsequently, the hot slab is continuously cast to become a hot slab, and the hot slab is directly rolled, heat-retained, or heated without lowering the temperature to below Ar 3 points, and hot-rolled.

At this time, the time + t (minutes) from the cutter position of the continuous casting machine to the start of hot rolling is 36 /logc Ti-a, 42N-1
．． 5s/c] or more. Further, even if the present invention is applied to cast slabs that are thin enough to omit rough rolling during hot rolling, the same characteristics of the present invention will not be impaired.

The hot rolling finishing temperature is preferably set to an Ar point of 3 or higher, since if the hot rolling temperature falls below the Ar 3 point, the shape and threadability of the steel sheet will deteriorate due to the difference in deformability between austenite and ferrite. However, even if hot rolling is finished below Ar point 800
As long as the temperature is .degree. C. or higher, the features of the present invention are not impaired.

Although the winding temperature is not specified, if the winding temperature is too high, the variation in the material in the longitudinal direction of the coil will increase due to the difference in cooling after winding, and the scale thickness will increase, which will impede descaling performance. On the other hand, if the temperature is too low, the recrystallization temperature will become high, which is not suitable. In the present invention, 550-7]0℃
This is carried out within the scope of. The preferred range is 550-680
'C.

Subsequently, it is descaled and then subjected to cold rolling. Cold rolling is carried out in a conventional manner. Since the higher the cold rolling rate is, the better the deep drawability is, the cold rolling rate is preferably 70 or higher.

Next, the annealing may be continuous annealing or box annealing, but the effects of the present invention are remarkable in the continuous annealing method.

The annealing temperature must be higher than the recrystallization temperature in order to ensure deep drawability. In the present invention, even if CC-DR is performed, the recrystallization temperature is low, so annealing is mainly performed in the range of 725°C to 800°C. Any method may be used for cooling after recrystallization. The characteristics of the present invention are not impaired even if overaging treatment is performed during cooling or after cooling to room temperature.

The annealed steel plate is subjected to skin pass rolling as required, and then used as a product. The method for manufacturing steel sheets of the present invention can be applied not only to cold-rolled steel sheets, but also to original plate cutting of lead, tin, zinc, aluminum, chromium, tin-lead alloy, etc. plated steel sheets.

Examples The steel shown in Table 2 was melted by converter melting and vacuum degassing treatment, and the cast hot slab was continuously cast under the conditions shown in Table 2, either directly or hot rolled after heat retention. 17. The hot rolling finish temperature is 880~! ] 20℃, and the winding temperature is 600℃.
~660°C/ζ.

After pickling, it was cold rolled to a plate thickness of 0.80, and then... 50
Recrystallization annealing for CXI minutes was performed as continuous annealing. Ko, 0
Mechanical properties were investigated after % skin pass. The results are shown in Table 2.

Coil NαA-1,, B-1 is an example of a soft steel plate with combined addition of T1 and B, and coil N[L C-1,, D
-1,, F-1 is T] and E-] is Ti and Nb.
This is an example of a soft steel plate with composite addition of . Both are high 7
It has value and ductility.

On the other hand, C-2 and F-2, which have the same composition but take a shorter time from solidification to rolling start bowl, both have low 7 values.
The ductility is also poor. -! However, B-2 had an unrecrystallized structure and could not withstand the material property test.

Coils NI G-1, 0-3, H-1, and H-2 are examples of high-strength steel plates, but coils NIG-1, ) T-1, which were made using the method of the present invention, were made using a method other than the present invention. It has better deep drawability than G-2゜(-2), which was carried out by the method of .

Coils NI D-1 and D-2 are laboratory examples using thin cast slabs with a thickness of 40 mm, but coil NαD-1 produced by the method of the present invention has excellent ductility and deep drawing. D-2 has a short time from solidification to rolling start groove.
had an unrecrystallized structure even after annealing.

Coil NαA,-2 is an example manufactured using a conventional reheating process. Although deep drawability and ductility are excellent, Δr
is getting bigger. - Coil N with the same force component
ILA-1 prepared by the method of the present invention not only has excellent deep drawability and ductility, but also has a small Δr and excellent in-plane anisotropy.

(Effects of the Invention) As is clear from the above examples, the present invention provides CC-D
In addition to the energy-saving effect of R, continuous annealing has excellent deep drawability, and it is possible to produce concave cold-rolled steel sheets with in-plane anisotropy, and the recrystallization temperature is low, making continuous annealing highly productive. is of great value.

[Brief explanation of drawings]

Figures 1 and 2 show the time from solidification to the start of rolling and F, E.
, I-. FIG. 3 is a chart showing the relationship between the time from solidification to the start of rolling and the recrystallization temperature. Agent Patent Attorney Tatsuo Chanoki Figure 1

Claims (1)

[Claims] 1% by weight: C: 0.006% or less, Mn: 1.5% or less, Si: 1.0% or less, P: 0.100% or less, S: 0.025% or less , Ti: 0.085% or less, the balance is iron and unavoidable impurities.
When performing direct hot rolling, the time t from the cutter position of the continuous casting machine to the start of rolling is t (minutes) ≧ 30/log [(Ti-3.42N-1.5
A cold rolling method with excellent deep drawability characterized by controlling the conveyance time of a continuously cast slab so as to satisfy S)/C], followed by hot rolling, followed by cold rolling and annealing. Manufacturing method of steel plate. 2 B: 0.0002-0.0020%, Nb: 0.0
A method for producing a cold-rolled steel sheet with excellent deep drawability according to claim 1, which contains 50% or less of one or two types.