JPH0321611B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a cold rolled steel sheet that is applied to parts that require excellent deep drawability such as automobile outer panels, and more particularly to a method for manufacturing a cold rolled steel sheet for deep drawing having a composite structure. In recent years, the strength of automotive steel sheets has been rapidly increased with the aim of reducing the weight and improving safety of automobile bodies. Increasing the strength of automotive steel sheets initially began with parts that did not require much formability, but recently there has been a growing demand for higher strength for outer panels, which are difficult to form even with mild steel sheets. . Conventionally, the following steel plates are known as high-strength cold-rolled steel plates with excellent press formability. (1) So-called refusal steel, which is a low carbon aluminum killed steel with P added. (2) An ultra-deep drawable mild steel sheet made by adding Ti or Nb to ultra-low carbon steel, with solid solution strengthening elements such as P and Si added. (3) A two-phase structure of phalarite and martensite is created by adding large amounts of Mn, Cr, etc. to low carbon aluminum killed steel and performing rapid annealing from the two-phase coexistence region of phalarite (α) and austenite (γ). Composite structure steel sheet. Among the three types of steel plates mentioned above, steel plate (1) has a Rankford value (r value) equal to or higher than that of mild steel plates and has excellent deep drawability, but the yield ratio (yield strength /Tensile strength), it is difficult to apply it completely to automobile exterior panels. In addition, in the case of the steel plate (1), there is a fatal drawback in that sufficient properties cannot be obtained by continuous annealing. On the other hand, in the case of steel plate (2),
It has a high r value, low yield ratio, and excellent ductility, and can be manufactured by continuous annealing, but it has the disadvantage of high steel cost, and in order to achieve a tensile strength of 40 Kgf/mm ² or more, P, There is a drawback that if a large amount of Si or the like is added, the brittleness decreases rapidly. On the other hand, the composite structure steel sheet (3) has an extremely low yield ratio, excellent ductility, and high bake hardenability (BH).
Suitable for use in automobiles. However, the steel sheet (3) requires a large amount of alloying elements, resulting in high steel cost, and has a low r value of about 1, which makes it difficult to apply to parts that require deep drawability. This invention was made in view of the above circumstances, and is a low-cost steel with a small amount of alloy addition, which can be manufactured by a continuous annealing method, has a high r value, and has excellent deep drawability. The purpose of the present invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet with a composite structure that is optimal for. As a result of various experiments and studies to achieve the above-mentioned purpose, the present inventors found that the C content in steel was 0.005%.
In the following ultra-low carbon steels, trace amounts of Nb, B, and Ti are added in combination within a specific range, and soaked and rapidly cooled in the α-γ coexistence region during continuous annealing after hot rolling and cold rolling. The inventors have discovered that a cold rolled steel sheet with a high r value and excellent deep drawability can be obtained by this method, leading to the present invention. Here, the experimental results that formed the basis of this invention will be described. That is, the present inventors calculated 0.004%C - 0.04%
A steel with the basic composition of Al steel and the addition of B (A steel) and a steel with a composite addition of B, Ti, and Nb (B steel) are hot-rolled and cold-rolled to a temperature of 850 to 980 on a continuous annealing line. A tensile test was carried out by soaking at different temperatures within a range of 30°C, followed by rapid cooling to 400°C at a rate of 30°C/sec. Note that temper rolling was not performed. The tensile strength TS obtained by this tensile test,
Average Rankford (value) and Yield Ratio (YR
= yield strength YS/tensile strength TS) is shown in FIG. 1 in correspondence with the soaking temperature in continuous annealing. As is clear from Fig. 1, steel B to which Ti, Nb, and B were simultaneously added in a composite manner differed from steel A to which B was added alone to 890 to Ac ₃ points (around 950℃).
It was found that by soaking in the α-γ coexistence region between the two, the yield ratio YR suddenly decreased, the value suddenly increased, and the tensile strength TS also increased. On the other hand, when soaking was performed in the entire γ region with Ac of ₃ points or more, the values decreased rapidly for all steel types. As mentioned above, the yield ratio was low and the value was high.
890~Ac When we investigated the structure of a Ti, Nb, and B composite steel sheet subjected to soaking treatment at _three points, we found that it had a composite structure consisting of an α phase with extremely low dislocation density and a second phase with localized dislocations. It turned out that there was. This structure is different from conventional composite structure steels consisting of a ferrite phase and a martensitic phase. As a result of further research based on the above experimental results, the present inventors determined the material composition and manufacturing process conditions, especially the continuous annealing conditions after cold rolling, as follows. It has now become possible to produce a composite structure high-strength cold-rolled steel sheet with excellent deep drawability. That is, the method for producing a cold-rolled steel sheet with a composite structure according to the present invention is as follows:
Contains 0.025%, B0.0005~0.0080%, and Ti
The steel material contains Ti (%) / {S (%) + N (%)} in the range of 1 to 5, and the balance is Fe and unavoidable impurities. After performing inter-rolling and cold rolling, α + γ 2
Soaking at phase range temperature and from that soaking temperature
It is characterized in that continuous annealing is performed under conditions of cooling in a temperature range up to 600°C at an average cooling rate of 5°C/sec or more. The method of the present invention will be explained in more detail below. First, the reason for limiting the material steel components will be explained. C: In this invention, the amount of C in the steel material is an extremely important factor. In other words, in steel with a C content exceeding 0.005%, even if trace amounts of Ti, Nb, and B are added, a composite structure will not be formed, and r, which is an index of deep drawability.
The value deteriorates rapidly, and the negative effect on ductility also increases. Therefore, the amount of C needs to be regulated to 0.005% or less. Si, Mn, P: If these elements are Si1.2% or less, Mn1.2% or less, and P0.15% or less, they will have little negative effect on deep drawability and ductility, and will work effectively to increase strength. do. However, if the content exceeds each of the above ranges, not only will the quality of the material deteriorate significantly, but it will also unnecessarily increase the steel cost, which is useless. Therefore Si1.2% or less, Mn1.2
% or less, P0.15% or less. These lower limits are not particularly defined, but usually Si0.005%,
Contains more than 0.01% Mn and 0.005% P. Al: Al has a deoxidizing effect and is effective in precipitating and fixing N as AlN, so it must be contained at 0.005% or more, but if it is added in excess of 0.080%, non-metallic inclusions will be formed rapidly. Al increases the surface quality of the steel sheet, so it
It needs to be in the range of 0.005-0.080%. Nb, B, Ti: In this invention, the combined addition of these three elements is extremely important. It is well known that the deep drawability of cold rolled steel sheets is improved by adding these elements singly or in combination. This effect is mainly due to promoting the development of {111} texture during the cold rolling recrystallization process by precipitating and fixing C or N in the steel as carbonitrides. However, the techniques that have been disclosed so far are not applicable to high-strength cold-rolled steel sheets having a composite structure. Among these elements, B is known to improve the hardenability of steel, but as shown in Figure 1, when B is added alone to ultra-low carbon steel, a composite structure may not be formed. This has been found through experiments conducted by the inventors. On the other hand, in this invention, Nb, B, and Ti are added to ultra-low carbon steel with C0.005% or less.
It is a previously unknown fact that a composite structure with excellent deep drawability can be obtained by adding a small amount of each in an appropriate range.
They discovered a composite addition effect of Nb, B, and Ti, and this composite addition effect is different from the conventionally known
This is completely different from the effect of carbonitride-forming elements such as Nb and the hardenability improving effect of B. The range of each component where a composite structure and high r value can be obtained by adding Nb, B, and Ti is
0.002 to 0.025%, B is 0.0005 to 0.0080%, Ti is 1 x {S (%) + N (%)} or more, 5 x {S (%) + N
(%)}. Even if the amount is lower than this,
Further, even if the amount is high, the above-mentioned effect cannot be obtained, and even when only one or two types are added, the above-mentioned effect cannot be obtained. The metallurgical mechanism by which a composite structure with a high r value is obtained by such a composite addition of Nb, Ti, and B has not yet been clarified. however,
Because the amounts of Nb, Ti, and B are not large enough to sufficiently precipitate and fix C and N in steel, and because it is difficult to imagine that the amounts added can stabilize the γ phase by soaking in the α-γ coexistence region, It is clear that it is difficult to explain the effect using the conventionally known effects of adding these elements. Next, each step of the method for manufacturing a cold rolled steel sheet of the present invention will be explained. First, as for the melting method for obtaining steel having the above-mentioned components, any steelmaking method such as the converter steelmaking method can be applied, but for the melting method of ultra-low carbon steel as the subject of this invention. , decarburization using a degassing device is effective. Further, the steel billet manufacturing method may be either a continuous casting method or a blooming method. Hot rolling of a steel billet may be carried out according to a conventional method. In other words, the steel slab is heated and soaked to 700 to 1300℃,
It may be hot rolled using a normal hot rolling machine and then wound. Here, the hot rolling finishing temperature and coiling temperature are arbitrary, but the finishing temperature is 700 to 900℃, and the coiling temperature is
The temperature is preferably 700°C or lower. Furthermore, a method of directly hot rolling a continuously cast steel billet or sheet bar without using a heating furnace can also be applied. The hot-rolled steel sheet hot-rolled as described above is subjected to pickling, if necessary, and then subjected to cold rolling. Any cold rolling method may be used, but a reduction rate of 50% or more is advantageous in order to improve the r value. The cold rolled steel sheet obtained by cold rolling is subjected to continuous annealing. In the method of this invention, the soaking conditions in the continuous annealing step and the cooling conditions after soaking are extremely important. As a soaking condition, the soaking temperature is α
-γ coexistence region is required. The specific temperature range varies slightly depending on the component, but is usually between 800 and 950.
It is in the range of about ℃. If the soaking temperature deviates from the α-γ coexistence region, a composite structure with excellent deep drawability, which is the object of the present invention, cannot be obtained. Note that α−
The soaking time in the γ coexistence region is preferably about 2 seconds to 2 minutes. On the other hand, the cooling conditions immediately after soaking require rapid cooling from the soaking temperature to 600° C. at an average cooling rate of 5° C./sec or more. Average cooling rate is 5
If it is less than °C/sec, the composite structure targeted by the present invention cannot be obtained. Likewise, when cooling at a cooling rate of 5° C./second or higher is terminated at a temperature higher than 600° C., a composite structure cannot be obtained. As long as the soaking conditions and cooling conditions described above are ensured, the type and model of the continuous annealing furnace may be arbitrary, and it can also be applied to lines that perform molten metal plating after continuous annealing, such as continuous hot-dip galvanizing. Of course. Next, examples of the present invention will be described together with comparative examples. Steels having compositions shown in sample numbers 1 to 13 in Table 1 were melted using a converter and an RH degassing device, and slabs with a thickness of 260 mm were obtained by continuous casting. 1150~ for each slab
It is heated to 1220℃, hot rolled at a finishing temperature of 860 to 900℃ and a coiling temperature of 500 to 680℃, and then the hot rolled steel plate is cold rolled at a rolling reduction of 70 to 79% to a thickness of 0.7 to 0.8. mm
It was made into a cold-rolled steel strip. Continue soaking temperature 880-920℃,
Soaking time 5-100 seconds, cooling rate 3-600℃
Continuous annealing was performed at 120°C/sec. The specific hot rolling/cold rolling conditions and continuous annealing conditions for each sample steel were
Shown in the table. Furthermore, for each cold rolled steel strip that was continuously annealed as described above, JIS No. 5 test pieces were cut out and the mechanical properties were examined, and the results are shown in Table 3.

[Table] *Comparative example; underlined values in the table indicate values outside the range of the present invention.

【table】

[Table] *Comparative example; underlined values in the table indicate values outside the range of the present invention.

[Table] * Comparative example
** 2% tensile prestrain, 170°C x 20 minute aging treatment Amount of strain age hardening From Table 3, it can be seen that the steel used as the raw material is within the composition range specified in this invention, and that it is continuous after hot rolling and cold rolling. The cold-rolled steel sheets of sample numbers 2, 5, 8, 10, 11, and 12, whose annealing conditions were also within the range of this invention, had lower yield ratios YR and lower yield ratios than the cold-rolled steel sheets of other comparative examples. It is clear that the material has a high elongation El and has particularly excellent deep drawability, as well as high strength. Furthermore, since all of the cold-rolled steel sheets obtained by the method of the present invention have high bake hardenability (BH), it is clear that they are advantageous in improving the dent resistance of automobile outer panels and the like. As described above, according to the method of the present invention, it is possible to obtain a high-strength cold-rolled steel sheet with particularly excellent deep drawability.Moreover, since the amount of expensive alloying elements added is small, it can be manufactured at low cost, and furthermore, it can be produced continuously. Since the annealing method is applied, various effects such as high productivity can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a correlation diagram showing the relationship between the soaking temperature in continuous annealing after hot rolling and cold rolling and various mechanical properties of the steel sheet.

Claims (1)

[Claims] 1 C0.005% (weight%, same below) or less, Si1.2
% or less, Mn1.2% or less, P0.15% or less, Al0.005~
0.080%, Nb0.002~0.025%, B0.0005~0.0080%
Contains Ti and Ti (%)/{S (%) + N (%)}
After using a steel material with a value of 1 to 5, with the remainder consisting of Fe and unavoidable elements, and hot rolling and cold rolling the steel material,
A cold-rolled steel sheet with a composite structure characterized by being subjected to a continuous annealing treatment in which it is soaked at a temperature in the two-phase region of α+γ and cooled at an average cooling rate of 5°C/sec or more in the temperature range from the soaking temperature to 600°C. Production method.