JPH01184253A - Cold-rolled steel sheet excellent in workability and its production - Google Patents

Abstract

PURPOSE:To improve workability of a steel sheet by subjecting the steel in which respective contents of C, Mn, Al, N, V and Nb are specified to hot rolling, cold rolling, and heat treatment under respectively prescribed conditions. CONSTITUTION:A steel which has a composition consisting of, by weight, <=0.003% C, 0.09-0.8% Mn, 0.06-0.12% Sol.Al, 0.005-0.011% N, >=0.003% Nb and/or V, and the balance Fe and satisfying 12(Nb/93+V/51)<=C+0.002% is refined. A continuously cast slab of the above steel is hot-rolled, which is wound up at <=560 deg.C. Subsequently, the above steel plate is cold-rolled and the resulting steel sheet is subjected to continuous annealing under the conditions of 1-20 deg.C/sec average temp.-rise rate between 400 and 700 deg.C and also 700-900 deg.C maximum heating temp. By this method, the steel sheet having expanded grain structure and excellent in workability can be obtained.

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, have traditionally been produced by ingot making, blooming, hot rolling, cold rolling, and box annealing. 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 a continuously annealed cold rolled steel sheet is inferior to that of a box annealed steel sheet is that in the former case, the effect of controlling the recrystallization texture of QN (aluminum nitride) cannot be utilized. In other words, in the case of box annealing, the rate of temperature increase during heating is extremely slow by 10~b, so MN that has been dissolved in solution during hot rolling is sufficiently precipitated during this temperature increase process, and the precipitated MN is recrystallized. Only crystals with specific orientations develop at certain times. 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, continuously annealed materials have a lower r value than box annealed materials, so box annealing has no choice but to be used, even if it is inefficient, to produce cold rolled steel sheets with high formability. You don't get it.

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.

Certainly according to this method. This provides a chance for AlN to precipitate, which helps improve the r value. However, in a continuous annealing process where the total time from when the steel plate enters the furnace to when it comes out is normally about 3 to 8 minutes, holding the temperature for several minutes during the temperature raising process will greatly reduce the efficiency of annealing. In addition, the line becomes longer, leading to an increase in equipment costs.

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) Here, from one aspect, the gist of the present invention is that in weight %, C: 0.0030% or less, Fin s 0.09
~0.80%, sol. Al: 0.06-0.12
%, N: 0.0050-0.0110%, further N
It is a cold-rolled steel sheet with excellent workability, consisting of one or both of (b) and (2) with a total balance of Fe and unavoidable impurities, and which is continuously annealed and has an elongated grain structure.

In addition, from another aspect, the gist of the present invention is, in weight %, C: 0.0030% or less, Mn: 0.09 to
0.80%, sol, M: 0.06-0.12%,
N: 0.0050 to 0.0110%, further Nb,
A continuous cast slab of steel containing either one or both of V in total, V, the balance Fe and unavoidable impurities is hot rolled at 560°C.
After being coiled at the following temperature and cold-rolled by a conventional method, 4
This is a method for producing cold rolled steel sheets with excellent workability, characterized by continuous annealing under conditions of an average heating rate of 1 to b seconds between 00 and 700°C and a maximum heating temperature of 700 to 900°C.

The above-mentioned stretched grain m weave 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 (al, (b), (C1) is a microscope set 1ti (X100) of a cold rolled steel plate after cold rolling and annealing. Figure 1 (C) shows uniformly fine grains with small grain size and the degree of elongation is 1.2, same fa
+ indicates relatively coarse expanded grains with a degree of expansion of 3.5, and (b) represents relatively fine expanded grains with a degree of expansion of 2.1. Figure 1 (
The reason why the elongated grains of al and (bl) are formed is that MN precipitates during the annealing process, and this promotes crystal grain growth in a specific direction during recrystallization. With such a structure, the r value is greatly improved. , the workability is improved.However, in the rapid heating process of continuous annealing, the equiaxed grain structure shown in Fig. 1 (fcl) is usually obtained, and its r value is only around 1.3. In order to obtain the elongated grain structure of (al, (b)), it is necessary to comprehensively consider all the steps of hot rolling, cold rolling, and annealing from the structure of the steel material.

The present inventors conducted basic experiments using the low carbon AQ guild steels listed in Table 1. The conditions for creating the test materials were as follows.

Slab (50 mm thick) heating temperature... 1200°C Hot rolling finishing temperature... 920°C Thickness of hot rolled steel plate... 3. Rolling temperature...・400
'C and rolling reduction ratio of 600℃ cold rolling...
75% annealing conditions: The average temperature increase rate (°C/sec) from 400°C to 700°C using an infrared heating furnace is 0.4.2.10.40.10
0 and raised the temperature to 750°C, and at this temperature
Hold for seconds, then cool to room temperature at 10°C/second.

Table 2 shows the observation results of the microstructure of the cold rolled steel sheet obtained above.

Table 1 Table 2 (Note) ○: Stretched grain structure, ×: Equiaxed grain structure In Table 1, ○
The marks in FIG. 1(a) indicate an elongated grain structure such as (bl), and the x marks indicate a uniformly fine grain structure such as (C1).

Looking at these results, in order to obtain a drawn-grain A11 weave with a high r value, basically, you need to use ultra-low carbon and high sol, M, and N contents like steel A3 and A4. It can be said that it is necessary to (1) lower the coiling temperature after hot rolling, and (2) lower the temperature increase rate.

Furthermore, apart from the formation of an elongated grain structure, in order to obtain a high r (i), it is necessary to refine the grains of the hot-rolled sheet. This is because recrystallized grains with an orientation favorable for deep drawability are generated.

Incidentally, the recrystallization process of metals consists of (1) recrystallization nucleation, (2) growth of nuclei, and (2) subsequent grain growth.

The development of an elongated grain structure means that the process of (2) above is controlled, but (2) needs to be controlled by grain refinement of the hot-rolled sheet as described above.

With the steels shown in Table 1, it is difficult to obtain a fine grain structure in hot rolled sheets. Therefore, it is necessary to add elements such as Nb and V that have the effect of making the crystal grains finer.

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+ Mr++ sol, 8Q, N
) content is limited.

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 the amount of C 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 AQN is delayed and expanded 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 MN, 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 strength and lowers the r value. Therefore, the content should be selected depending on the use of the copper plate, but in the present invention, which is aimed at providing excellent workability to the steel plate, the content is 0.0.
It is contained in an appropriate amount in the range of 9 to 0.80%. If it exceeds 0.80%, the above-mentioned undesirable effects become stronger, and a large amount of Mn added 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.

sol, 8Q: One of the major features of the present invention is that the amount of AQ added is larger than that of ordinary AQ killed steel.

When the amount of AQ increases, the formation of AQN accelerates, and when the timing matches the nucleation period of recrystallization, a recrystallized texture favorable for improving deep drawability is formed. For this, sol, A
(!: 0.06-0.12% is required.0.06
If it is less than %, the formation of MN is slow and the texture cannot be controlled. On the other hand, if it exceeds 0.12%, the formation of AQN is too fast and MN is formed during the hot rolling stage, which cannot contribute to the control of the recrystallized texture, resulting in a low r value.

It is also an important point of the present invention to actively utilize N in relation to the amount of N:AQ. If the N content is not 0.0050% or more, the amount of MN will be insufficient and the ability to control texture will be weakened. However, the N content is 0.0110%
If it exceeds this, the AQN will be too large and the elongation will decrease.

Nb, V: These alloy components form carbides, but they re-dissolve in solid solution during slab heating, and remain in solid solution during hot rolling.
This has the effect of suppressing austenite recrystallization and making austenite grains finer, thereby reducing the ferrite grain size after transformation. If the total amount of Nb and V is less than 0.003%, there is no such effect of grain refinement. In addition, since both Nl) and V tend to form nitrides after forming carbides, 12 (Nb/93 + V3Si) is less than C10.00.
If the content exceeds 2%, the amount of A (IN) that is sufficient to control the texture cannot be secured, and the texture cannot be controlled and γ
value becomes lower. Figure 2 shows Nb added in the present invention.
, V's claims are shown on the graph.

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 and rolling process for hot rolling may be under general hot rolling conditions. Continuously cast slabs are used as raw material slabs, but direct rolling, in which hot rolling is performed directly from a continuous casting line, and hot charging, in which high-temperature 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, in order to form a solid solution of 8QN, it is better to heat the mixture at a slightly higher temperature. Therefore, from the viewpoint of energy efficiency, the above-mentioned direct rolling and hot charging are preferable. On the other hand, the finishing temperature is 85% due to the small amount of carbon.
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 the material is wound at a higher temperature than this, MN will precipitate during slow cooling after winding, and the above-mentioned MN precipitation and its effects during annealing cannot be expected. There is no need to limit the lower limit of winding temperature in theory, but if it becomes too low, the cooling water in the coil will not evaporate, leading to red rust, so in actual operation,
The lower limit would be around 100°C.

The hot-rolled steel sheet thus obtained is subjected to descaling treatment such as pickling and then 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. 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 8QN before recrystallization ends during the heating process, but substantially no MN precipitation occurs at temperatures lower than 400°C.

On the other hand, if the temperature is higher than 700°C, recrystallization will be completed. Therefore, it is necessary to control the temperature increase rate between 400 and 700°C to promote precipitation of AQN. i.e. 400-700
The average temperature increase rate between 1 and 20°C/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 a desired elongated grain structure. On the other hand, at a slow heating rate such as less than 1° C./sec, the advantage of continuous annealing aimed at high efficiency is 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 mm. Although this can be achieved with a direct-fired heating furnace by adjusting the line speed and burner position, it is not preferable from a productivity standpoint. Control of heating rate from 400 to 700℃
This can be easily carried out by heating in such a way that it takes 15 seconds to 5 minutes to raise the temperature to 300°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 has no significant effect here.

The method for cooling from the maximum heating temperature does not matter. Any of air cooling, slow cooling, and overaging treatment may be used, and any of gas jet cooling, roll cooling, liquid cooling, etc. can be employed 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 zinc coating line annealing equipment, tinplate, tin free 5teel manufacturing equipment, etc. when manufacturing 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 1 In an experimental vacuum melting furnace, as shown in Table 3, steels with different amounts of carbon, nitrogen, and amounts of AQ and Nb added were melted. These were made into experimental slabs with a thickness of 25 mm by hot forging. Next, it was heated in an electric furnace at 1200°C for 1 hour, and then rolled in a temperature range of 1150°C to 930°C for 3 passes using an experimental hot rolling mill to obtain a hot rolled sheet with a thickness of 4.0 mm. As a simulation of winding, the steel sheet was heated to 200 to 600°C by water spray cooling immediately after hot rolling.
20°C after being cooled to a temperature of
The furnace was cooled at /hr. After pickling, the cold-rolled sheet was cold-rolled to a thickness of 0.8 and heated to 80°C at 10°C in an infrared heating furnace.
After heating to 50°C and holding at that temperature for 40s, 750°C
The mixture was gradually cooled to 3°C, and then cooled to room temperature at 10°C.

After annealing and temper rolling with an elongation rate of 1.2%, JIS5
Tensile test measurements were carried out using No. 1 specimens.

The characteristics are shown in Table 3. These data are summarized and shown in graphs in FIGS. 3 and 4.

As can be seen from FIG. 3, when the coiling temperature after hot rolling and the heating rate during annealing are out of the range of the present invention, the r value becomes low, which is not preferable for deep drawing.

As can be seen from FIG. 4, when the C concentration in the steel exceeds the range of the present invention, the r value becomes low and the steel becomes hard, making it unsuitable for deep drawing. Furthermore, it can be seen that when the amount of nitrogen in the steel or the amount of AQ is lower than the range of the present invention, the r value decreases. Furthermore, it can be seen that there is an optimum addition amount of Nb and ■ for the r value.

Example 2 Steel having the composition shown in Table 4 was melted in a converter,
A part of the slab was subjected to vacuum degassing treatment and a slab was manufactured by continuous casting. These were heated to 1200℃ and hot rolled to 45
It was coiled at 0°C, pickled, and then cold-rolled at a rolling rate of 75% to produce a cold-rolled sheet with a thickness of 0.8 mm, which was further heated to 760°C and then continuously annealed (40 s at 760°C). Baoding and 400
℃ for 3.5 minutes), and the elongation rate was 1.
A cold-rolled steel sheet was manufactured by performing 0% temper rolling.

Table 4 shows the characteristic values of the steel plate thus obtained. C1
The steel C2 steel is the steel of the present invention and has a high r value and good elongation. However, 03 steel, which is outside the scope of the present invention, has an r value of 1.
54, which is low and does not achieve the purpose of the present invention.

[Brief explanation of the drawing]

Figure 1 ta+, fbl, (C1 are all cold rolled -
A microscopic metallographic photograph of a cold rolled steel sheet after annealing; Figure 2 is a graph showing the range of Nb and V added in the present invention; and Figures 3 and 4 collectively show the results of Examples. It is a graph.

Claims (2)

[Claims] (1) In weight%, C: 0.0030% or less, Mn: 0.09 to 0.80%
, sol. Al: 0.06-0.12%, N: 0.00
50 to 0.0110%, and a total of 0.003% or more of any one or both of Nb and V and 12 ((Nb
/93)+(V/51))≦C+0.002%, remainder F
A cold-rolled steel sheet that is continuously annealed and has an elongated grain structure and has excellent workability. (2) In weight%, C: 0.0030% or less, Mn: 0.09 to 0.80%
, sol. Al: 0.06-0.12%, N: 0.00
50 to 0.0110%, and a total of 0.003% or more of any one or both of Nb and V and 12 ((Nb
/93)+(V/51)≦C+0.002%, balance Fe
A continuously cast slab of steel containing unavoidable impurities is hot-rolled and coiled at a temperature of 560°C or less, and then cold-rolled by a conventional method and then heated to 40°C.
A method for manufacturing a cold-rolled steel sheet with excellent workability, characterized in that continuous annealing is carried out under conditions of an average heating rate of 1 to 20°C/sec between 0 and 700°C and a maximum heating temperature of 700 to 900°C.