JPS63230848A - Cold-rolled steel sheet excellent in workability and its production - Google Patents

Abstract

PURPOSE:To inexpensively manufacture a cold-rolled steel sheet excellent in workability, by hot-rolling a continuously cast slab of dead-soft carbon steel, winding the slab, cold-rolling the slab to the final sheet thickness, and then applying continuous annealing to the sheet under specific conditions. CONSTITUTION:A molten steel having a composition containing, by weight, <0.0030% C, 0.09-0.80% Mn, 0.06-0.12% sol. Al, and 0.0050-0.0110% N is formed into a slab by continuous casting and the slab is hot-rolled at 1,200 deg.C into a plate of 3.2mm thickness, which is wound up in a coil at <=560 deg.C. This hot-rolled steel plate is cold-rolled at 75% draft so as to be formed into a cold- rolled steel sheet of about 0.8mm final sheet thickness. At the time of applying continuous annealing to this steel sheet, continuous annealing is carried out under the conditions of 1-20 deg.C/sec average temp.-rise rate at a temp. between 400-700 deg.C and also of 700-900 deg.C maximum heating temp. By this method, the cold-rolled steel sheet having superior workability equal to that prepared by box annealing be manufactured under the efficient production system of continuous casting and continuous annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a cold-rolled steel sheet with extremely excellent workability obtained by the most efficient process using continuous casting and continuous annealing, and a method for manufacturing the same.

(Conventional technology and its problems) Cold-rolled steel sheets, which are used for automobile exterior panels and require good workability, used to be produced by ingot-making, blooming, hot rolling, cold rolling, and box annealing (batch). It was manufactured using a process called annealing. In recent years, with the spread of continuous casting, the former ingot making-blooming process has been greatly streamlined, and a highly efficient technique of continuous annealing is being adopted for annealing after cold rolling.

However, when looking at the quality of cold-rolled steel sheets, especially their workability, the current continuous annealing technology is insufficient, and box annealing, an inefficient technology, still accounts for nearly half of all cold-rolled steel sheet production. applied to.

The main reason why the workability of continuously annealed cold rolled steel sheets is inferior to that of box annealing is that in the former, the effect of controlling the recrystallization texture of AQN (aluminum nitride) cannot be utilized. In other words, in the case of box annealing, the rate of temperature increase during heating is much slower by 10~b, so the AQN solutionized during hot rolling is sufficiently precipitated during this temperature increase process, and the precipitated MN is Only crystals with a specific orientation are developed during recrystallization. When crystal grains with this specific orientation grow, the r value, which is an index of deep drawability, improves.

On the other hand, in the case of continuous annealing, the heating temperature increase rate is 1 to b Weave control effect cannot be used.

For these reasons, i! Since successively annealed materials have a lower r value than box annealed materials, box annealing has no choice but to be used, even if it is inefficient, in order to produce cold rolled steel sheets with high formability.

Attempts have also been made to produce cold-rolled sheets with high formability comparable to box annealing by continuous annealing.

For example, the invention disclosed in Japanese Patent Publication No. 51-6610 is one such invention, which contains a large amount of SO of 0.13 to 0.33%.
l.

By including 8Q, it is expected that 8QN will precipitate even in the continuous annealing process. However, adding such a large amount of AQ not only increases material cost but also increases the hardness of the product steel sheet, which is not practical.

Furthermore, the invention disclosed in Japanese Patent Publication No. 53-20446 is to maintain the temperature in the temperature range of 450 to 650°C for a certain period of time (soaking) during the temperature raising process of continuous annealing.

It is true that this method provides a chance for MN to precipitate, which helps improve the r-value, but usually in a continuous annealing process where the total time from the time the steel plate enters the furnace to the time it exits is around 3 to 8 minutes. , Holding for several minutes during the temperature raising process greatly reduces annealing efficiency and increases equipment costs due to the length of the line.

The present invention, while undergoing a highly efficient process of continuous annealing,
The present invention provides a cold-rolled steel sheet with excellent workability comparable to box-annealed steel and an innovative method for producing the same.

(Means for Solving the Problems) The gist of the present invention is the following cold-rolled steel sheet and method for manufacturing the same.

(1) In weight%, C: 0.0030% or less, Mn
: 0.09-0.80%, sol, AQ: 0.
06-0.12%, N: 0.0050-0.

0110%, the balance being Pe and unavoidable impurities, and is continuously annealed and has an elongated grain structure with excellent workability.

(2) %, C: 0.0030% or less, Mn:
0.09-0.80%, sol, Af2: 0
．． 06-0.12%, N: 0.0050-0.
A continuous cast steel slab consisting of 0110%, balance Fe and unavoidable impurities is hot-rolled and coiled at a temperature of 560°C or less, and after cold rolling by a conventional method, the average temperature increase rate between 400 and 700°C is 1 to b. A method for producing a cold-rolled steel sheet with excellent workability, characterized by continuous annealing at a temperature of 700 to 900°C.

The expanded grain structure in the present invention is defined by JIS GO55.
The degree of elongation e defined by 2 is 2 or more.

(Function) The present invention produces remarkable effects as will be described in detail later through an organic combination of the composition of the steel sheet used as the raw material, the winding conditions after hot rolling, and the temperature rise-heating conditions during continuous annealing. However, first, the results of the research that formed the basis of the present invention will be explained.

Figure 1 is a microscope &[I-weave (X100)] of a cold rolled steel plate after cold rolling and annealing. (C) in the same figure is a uniformly fine grain with a small grain size, and the degree of elongation is 1.2; (a) (b) is a relatively coarse expanded grain with a degree of expansion of 3.5, and (b) is a relatively fine expanded grain with a degree of expansion of 2.
, 1. The reason for the elongated grains shown in (a, b) is that AQN precipitates during the annealing process, which promotes the growth of specific grains during recrystallization. Improved processability. However, in the rapid heating process of continuous annealing, equiaxed fine grains (c) are usually obtained, and the r value thereof is only around 1.3. To obtain the expanded grain structure of (a, b) while using continuous annealing,
It is necessary to comprehensively consider all processes of hot rolling, cold rolling, and annealing, starting from the structure of the steel material.

The present inventor conducted basic experiments using low carbon AQ killed steels A1 to A5 shown in Table 1. The conditions for creating the test material are as follows.

Slab (501 thickness) heating temperature...1200℃ hot rolling finishing temperature...920℃ hot rolled steel plate thickness...
......3u coiling temperature...400℃ and 600℃ cold rolling reduction ratio...75% annealing conditions... From 400℃ using an infrared heating furnace The heating rate ('C/sec) up to 700°C was changed to 0.4.2.10.40, lOO, and the temperature was raised to 750°C, held at this temperature for 40 seconds, and then at 10°C/second. Cool to room temperature.

Table 2 shows the results of 11 microstructures of the cold-rolled steel sheets obtained above.

Table 1 Table 2 Table 1 In Table 1, ○ marks indicate expanded grain structures as shown in Figure 1 (a, b).
The X mark indicates a uniformly fine grained structure as shown in (C).

Looking at this result, in order to obtain an expanded grain structure with a high r value,
Basically, it is low carbon and sol like 3 and 4 to steel.
It can be said that it is necessary to increase the contents of AQ and N, (1) lower the hot rolling coiling temperature, and (2) lower the temperature increase rate. The present invention further advances the basic research described above, and defines the composition of the product steel sheet and the various conditions of the manufacturing process as described above.

Each condition will be explained below.

First, the components of the steel plate (C, Mn, sol, AQ+N)
The reason for limiting the content of is described below.

C: Since C has a great influence on the workability of cold rolled steel sheets,
It is important to keep the content within an appropriate range. In the present invention, when CM is reduced, the grains become elongated after continuous annealing, and r
The above-mentioned knowledge that the value improves is the basis. That is, in order to improve workability, it is better to have a smaller C content. If C exceeds 0.0030%, not only the formation of AflN becomes slow and elongated grains cannot be obtained, but also the room temperature aging property increases and stretcher strain is likely to occur during press molding. Therefore, the upper limit of the C content is suppressed to 0.0030%.

However, if bake hardenability (a phenomenon in which the strength of the steel plate increases when the paint is baked) is required, 0.0030%
Choose a higher C in the range below.

Mn: Mn has the effect of promoting the formation of AQN, so it is better to have a higher amount, and on the other hand, it is better to have a lower amount because it increases the strength and lowers the r value. Therefore, the content should be selected depending on the use of the steel plate, but in the present invention, which is aimed at providing the steel plate with excellent workability, it is less than 0.
It is contained in an appropriate amount in the range of 0.09 to 0.80%. 0.80%
If the Mn
A large amount of addition also increases the cost of the material. If Mn is less than 0.09%, there is a risk of edge cracking of the hot rolled sheet due to hot brittleness.

One of the major features of the present invention is that the amount of Q added to sol, M: is larger than that of ordinary AQ killed steel.

As Ml increases, the formation of Al2N accelerates, and if the timing matches the nucleation period of recrystallization, recrystallization aggregates and [l weave], which are favorable for improving deep drawability, are formed. For this, sol
．． Al: 0.06-0.12% is required. 0.0
If it is less than 6%, the formation of AQN is slow (the texture cannot be controlled. On the other hand, if it exceeds 0.12%, the formation of AQN is too fast and AQN is formed during the hot rolling stage, making it difficult to control the recrystallized texture. cannot contribute, resulting in a low r value.

It is also an important point of the present invention to actively utilize N in relation to the N:AQO amount. If the N content is not 0.0050% or more, the amount of 8QN will be insufficient, and the control force of the aggregated Mn weave will be weakened. However, the N content is 0.011
When it exceeds 0%, AQN becomes too large and elongation decreases.

Besides these components, the remainder is iron and unavoidable impurities. Typical impurities are P and S, and these are the permissible amounts of ordinary cold-rolled steel sheets, P: 0.03% or less, S:
It may be 0.02% or less.

Next, hot rolling conditions will be described.

The heating temperature for hot rolling and the rolling process can be under the conditions of general hot rolling.Continuously cast slabs are used as the raw material slab, but direct rolling, where hot rolling is performed directly from a continuous casting line, or high temperature Hot charge technology, in which continuous cast slabs are charged into a heating furnace, can also be adopted.

The heating temperature and rolling conditions may be within the range of hot rolling conditions for general low carbon steel, however, it is better to use a slightly higher heating temperature in order to dissolve AIN. Therefore, from the point of view of energy efficiency, the above-mentioned direct rolling and hot charging are preferable.On the other hand, the finishing temperature is 85% because the carbon content is small.
A temperature of 0°C or higher is desirable.

In the finish rolling of hot rolling, the effect of the present invention becomes even more remarkable when high-speed rolling is performed at the highest possible reduction rate in the latter stage, and quenching is performed immediately after rolling. However, these conditions are not absolute, as the optimal conditions vary depending on plate thickness and equipment constraints.

It is important that the upper limit of the coiling temperature after hot rolling is 560°C. If coiling is carried out at a higher temperature than this, AQN will precipitate during slow cooling after coiling, and the above-mentioned AQN precipitation during annealing and its effect cannot be expected, so there is no need to theoretically limit the lower limit temperature for zero coiling. However, if the temperature is too low, the cooling water in the coil will not evaporate, causing red rust, so in actual operation, the lower limit would be about 100°C.

The hot-rolled steel sheet thus obtained is subjected to a scale treatment such as pickling, and then subjected to cold rolling. The cold rolling itself may be performed under normal conditions. The rolling reduction ratio is preferably in the range of 60 to 80%.

Annealing is performed by continuous annealing, and the conditions must be appropriately controlled as described below. First, the rate of temperature increase to the maximum heating temperature is important.

The key point of the present invention is to sufficiently precipitate AQN before recrystallization ends during the heating process, but substantially no At2N precipitation occurs at temperatures lower than 400'C.

On the other hand, at a temperature higher than 700°C, recrystallization is completed. Therefore, it is necessary to control the heating rate between 400 and 700°C to promote the precipitation of MN. i.e. 400-700
The average temperature increase rate between 1 and 2[deg.]C is 1:/sec.

If it exceeds 20℃/sec, the temperature increase rate is too fast and the AQ
Recrystallization ends before sufficient N precipitation occurs, making it impossible to obtain the desired elongated grain structure.On the other hand, at a slow heating rate of less than 1°C/sec, continuous annealing for the purpose of high efficiency is not possible. benefits are lost.

The above temperature increase rate can be achieved in a radiant tube type heating furnace for plates having a thickness of about 0.4 to 2.0+am. This can be achieved even in a direct-fired heating furnace by adjusting the line speed and burner position, but it is not desirable from a productivity point of view.
This can be easily carried out by heating so that it takes 15 seconds to 5 minutes to raise the temperature by 300°C to 00°C.

The maximum heating temperature for annealing is 700 to 900°C. 700
If the temperature is lower than ℃, recrystallization will be incomplete and the elongation of the product steel sheet will decrease. On the other hand, when the heating temperature exceeds 900°C, an austenite phase appears and the r value becomes low. It is not necessarily necessary to uniformly maintain the temperature at the maximum heating temperature. It is only necessary for the steel plate to reach this temperature range, and the holding time here does not have a great effect.

Cooling from the maximum heating temperature is not limited to any method, and may be air cooling, slow cooling, or over-aging treatment, and gas chef) cooling, roll cooling, liquid cooling, etc. can be used as the cooling means. It is also optional to perform skin pass rolling after annealing.

Continuous annealing according to the method of the present invention can be carried out not only with dedicated equipment, but also with hot-dip galvanizing line annealing equipment, tinplate, tin free 5teel production equipment, etc. when producing plated steel sheets for deep drawing. It also includes continuous annealing. Even in such continuous annealing in a broad sense, the effects of the present invention are fully exhibited as long as the above-mentioned conditions are satisfied.

Hereinafter, the effects of the present invention will be specifically explained using Examples.

(Example) Table 3 summarizes the composition, hot rolling coiling temperature, and mechanical properties of the steel materials (Bl to B7) used.
8H4li is the increase in tensile strength ft (kgf/wm") when heated under conditions equivalent to baking coating.

micro! ! I is based on the same measurement as in Table 2 above.

Conditions not listed in Table 2 are as follows.

Melting: After converter melting, RH deoxidation, continuous casting slab thickness:
210mm Hot rolling conditions: 1200℃ heating, 900℃ finishing, cooling with water spray to coiling temperature Hot rolled sheet thickness: 3.2mm Cold rolling conditions: Reduction ratio 75%, product sheet thickness 0.81 Continuous annealing conditions: Radiant Using tube heating and gas jet cooling equipment with an overaging zone, two intermediate thermometers are installed in the heating zone, and each temperature is 400 to 450℃, 70℃.
The temperature was heated to 0 to 750°C, and the strip was controlled so that it passed between the two temperature settings in 30 seconds.

Others: Temper rolling with an elongation rate of 0.4% was performed in a continuous annealing line.

In addition, a bake hardening (BH) property test was conducted by applying 2% pre-strain and heating at 170°C for 20 minutes. Tensile test is JIS 5
Depends on the issue.

In Table 3, fragrances 2 to 7 are examples of the present invention.

All of the structures are expanded grains, and the r value is 1.5 or more.

On the other hand, since the hot-rolling and winding temperature of the flavored steel I is too high, it has a uniformly fine grained structure and has an r value of only 1.37.

Flavoring 8 has a high Mn content in the material, so the structure is an expanded grain, but the r value is low.Furthermore, Flavoring 9 has a low N content, so an elongated grain structure cannot be obtained and the r value is low. low.

Steel type B7 has the composition of a general continuous annealing material. Although the processing conditions below hot rolling satisfy those of the present invention,
Because C is high (, sol, 8 Q and N are low), the composition does not have an expanded grain composition and the r value is only 1.29.

From this test result, it is clear that a cold-rolled steel sheet with excellent workability can be obtained only when the composition of the steel material, hot rolling conditions, and annealing conditions all satisfy the conditions specified in the present invention.

Incidentally, the bake hardenability (BH!t) is determined by the C content of the hazo material, regardless of the presence or absence of an expanded grain structure, and the higher the C, the greater the BH31. . Generally BH4] is 0 to 5, O
kgf/mm", but even within the composition range of the steel sheet of the present invention, it is possible to provide any BH property within the above range by adjusting C1.

(Effects) According to the present invention, while following the most efficient production method for cold-rolled steel sheets from continuous casting to continuous annealing, we succeeded in producing a product with high workability that could not be obtained conventionally. This effect was achieved by correlating the composition of the material used, the hot rolling coiling temperature, and the heating conditions during annealing to determine the optimum range. High workability cold-rolled steel sheets, which conventionally had to be manufactured by the inefficient method of box annealing, can be manufactured efficiently and inexpensively by the present invention.

[Brief explanation of the drawing]

Figure 1 is a 'U4 micro-otoscope photograph showing the structure of a cold rolled steel sheet after annealing (a), (b) is an elongated grain structure, and (c) is a uniform grain structure. It is 100 times more.

Claims (2)

[Claims] (1) In weight%, C: 0.0030% or less, Mn: 0.
09-0.80%, sol. Al: 0.06-0.12
%, N: 0.0050 to 0.0110%, the remainder being Fe and unavoidable impurities, and is continuously annealed and has an elongated grain structure with excellent workability. (2) In weight%, C: 0.0030% or less, Mn: 0.
09-0.80%, sol. Al: 0.06-0.12
%, N: 0.0050-0.0110%, balance Fe and unavoidable impurities, a continuous cast steel slab is hot-rolled and coiled at a temperature of 560°C or less, and after cold rolling by a conventional method, it is 400-700%. Average heating rate between 1 and 20℃
A method for producing a cold-rolled steel sheet with excellent workability, characterized by continuous annealing at a maximum heating temperature of 700 to 900°C.