JPH01230715A - Manufacture of high strength cold rolled steel sheet having superior press formability - Google Patents

Abstract

PURPOSE:To manufacture a high strength cold rolled steel sheet having superior press formability by successively subjecting a steel sheet contg. specified amts. of C, Si, Mn and Al and obtd. by finish hot rolling at a specified temp. to cold rolling at a specified draft, heating to a specified temp. and cooling at stepwise varied cooling rates. CONSTITUTION:A steel contg., by weight, 0.12-0.40% C, 0.50-2.00% Si, 0.20-2.50% Mn and 0.005-0.10% sol. Al is hot rolled at 700-850 deg.C finishing temp., cold rolled and heat-treated to regulate the ratio of pearlite : retained austenite to (1.5-2.5):1. The resulting steel sheet is pickled and cold rolled at 35-65% draft. This cold rolled steel sheet is heated to 730-900 deg.C, cooled to 600-700 deg.C at 1-10 deg.C/sec cooling rate, further cooled to 200-400 deg.C at 20-200 deg.C/sec cooling rate, held for 2-50sec, further held at 350-450 deg.C for 15sec-10min and then cooled to <=150 deg.C within 30sec. A high strength cold rolled steel sheet having superior press formability can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet with extremely excellent press formability.

(Conventional technology) In recent years, the number of electronic control parts for improving comfort and driving performance and reinforcing members to ensure the safety of passengers has increased with each model change in passenger cars. is also on the rise. On the other hand, the total weight of the car body has remained almost unchanged, but this is offset by the increased strength of the automobile thin steel sheets that are the main constituent material.

It is thought that the trend toward higher quality and higher performance will continue in the future due to improvements in living standards, but on the other hand, there is also a heightened sense of cost performance rooted in the experience of the oil crisis, and this trend will continue in the future. It is expected to strengthen. Therefore, the strength level required for the 31@ plate for automobiles will be a tensile strength of 80 to 100 kgf/- or higher, which has never been thought of before. However, in order to obtain the required member, press working is required, and it is necessary to have various formability such as extensibility, bendability, stretch flangeability, etc. as in the past.

As a steel that tried to meet these demands, the special public
As proposed in Japanese Patent No. 11741, elongation is shared by soft ferrite and strength is shared by hard martensite, and the ferrite-martensite dual phase m (Dual −
phase steel). However, even with this steel, the product of tensile strength and total elongation is at most 2000 kgf/-%, and when the tensile strength level exceeds 100 kgf/mj, not only does the bendability and stretch flangeability deteriorate dramatically, but the characteristics However, the stretchability of the steel sheet deteriorated significantly, and there remained considerable difficulty in meeting the strict formability requirements of thin steel sheets for automobiles. In addition, from the perspective of placing emphasis on bendability, a structure close to uniform bainite is recommended as described in Iron and Steel, 67 (1981L 51181), but in this case, the total elongation for a tensile strength of 100 kgf/mi class is recommended. Since it is as small as lO~15%, it could only be applied to extremely limited molding methods.

However, recently, although it is a simple C-C-5i system that does not contain expensive alloying elements, it has been discovered that it contains more than 15% retained austenite, and its transformation-induced plasticity (Transform-induced plasticity) has increased.
By using cation induced plastic, it has a total elongation of more than 30%, which was previously unachievable, while achieving a total elongation of 80 to 120 kgf/m.
A high-strength steel plate with a tensile strength of J
It has been found that it can be manufactured as disclosed in Japanese Patent No.-43430 and the like. Steel sheets of this type with retained austenite, depending on its amount and stability against deformation, have very good formability due to transformation-induced plasticity. In addition, because it can be manufactured under heat treatment conditions similar to the continuous annealing cycle for mild steel plates, it is expected to be widely used in industry. However, the steel according to the disclosed technology has an extremely high C content and relatively large carbides are densely present in the bainite that exists together with retained austenite. Deterioration is observed, and impact resistance, which is essential when used as a safety and strength member for automobiles, is not excellent.

If the heating temperature range is set to the two-phase coexistence region of ferrite and austenite, as proposed in JP-A-61-157625, even steel with a lower C concentration will contain residual austenite and its TRIP effect will be exhibited. 80～
A tensile strength of 120 kgf/mj class can be obtained,
The bendability also reaches a satisfactory level. However, even when this disclosed technology is used, if the second stage is performed after residual austenite undergoes work-induced transformation into martensite during the first stage, the permissible working range at that time is the so-called duplex steel ( Dual phase steel). Therefore, stretch flangeability, etc. remains at the same level as conventional steel sheets with the same strength, making it difficult to be used in parts that require strict formability, and preventing widespread practical use as thin steel sheets for automobiles. was.

(Problems to be Solved by the Invention) The present invention solves the problems of the prior art as described above, and contains 8 to 25% retained austenite, which improves press forming properties including stretchability, bendability, and stretch flangeability. The present invention provides a method for manufacturing a high-strength cold-rolled steel sheet with excellent properties.

(Means for Solving the Problems) The high-strength cold-rolled steel sheet according to the present invention has a mixed structure mainly composed of ferrite, bainite, and 8 to 25% retained austenite. A high tensile strength exceeding 70 kgf/mj is obtained by the presence of bainite, retained austenite, or martensite generated by deformation-induced transformation, but the transformation of retained austenite, which is stabilized by concentration of C present together with clean ferrite, is induced by the transformation of retained austenite. The large elongation brought about by plasticity provides excellent stretchability that could not previously be obtained with that strength. Here, if the grain size of the retained austenite is made smaller and the stability is increased to make the transformation-induced plasticity more effective, the stretchability will be further improved. At the same time, it is possible to reduce the amount of martensite generated by processing-induced transformation, and in addition to making the grains finer, stretch flangeability can be greatly improved. In addition, the deformation-induced transformation of retained austenite diffuses strain and provides good bendability, but even if the strain associated with deformation-induced transformation is excessive, the presence of clean, highly ductile ferrite can release the strain, which can cause further defects. In this way, a metal structure in which fine retained austenite is uniformly dispersed together with ferrite and bainite is favorable for press formability including stretchability, bendability, and stretch flangeability. The present inventors have focused on the fact that it is important to finely prepare the structure before the start of the heat treatment and at the same time maintain a state close to the distribution of alloying elements in the structure to be finally obtained.

The present invention was based on the discovery that the object can be achieved by controlling the finishing temperature of hot rolling, the metal structure at the time of cooling after coiling, and the rolling ratio of cold rolling within appropriate ranges.

That is, in the present invention, C: 0.12 to 0.40%, S
i: 0, 50-2.00%, Mn: 0.20-2
．． 50%, Sol, /'J:0, OO5~0.1
It consists of ferrite and pearlite obtained by hot rolling steel containing 0% Fe and unavoidable impurities at a finishing temperature of 700 to 850°C, and consists of a series of cycles in which the ratio of pearlite is reduced by cold rolling. A steel plate with a residual austenite ratio of 1.5 to 2.5 times the ratio obtained after completion of heat treatment is pickled and cold rolled at a rolling reduction of 35 to 65%, and then rolled in a two-phase coexistence temperature range of 730 to 900 °C. Heat to
After holding for 15 seconds to 5 minutes, heat to 600 to 700℃ for 1 to 10 minutes.
℃/s, below 20 at a speed of 20 to 200℃/s
Cool to 0 to 400°C, hold within this temperature range for 2 to 50 seconds, and then maintain at 350 to 450°C for 15 seconds to 10 minutes,
A high-strength cold-rolled steel sheet containing 8 to 25% retained austenite and excellent press formability including stretchability, bendability, and stretch-flangeability. This is the manufacturing method.

(Function) First, the reason for limiting the range of components of steel that is the object of the present invention will be described.

First, C is concentrated in austenite, and its Ms point is set to below room temperature. This makes it possible to obtain retained austenite that provides large elongation due to transformation-induced desirability. It is desirable that the amount is low from the viewpoint of weldability and impact resistance, but if it is less than 0.12%, it is impossible to achieve high strength and excellent press formability, so the purpose can be easily achieved. For this reason, it is desirable that the content be 0.15% or more. On the other hand, 0.4
If it exceeds 0%, it is easy to ensure the amount of retained austenite, but the coexisting structure will be mainly composed of bainite in which comparatively large carbides are densely present, resulting in a significant deterioration of impact resistance, making it impractical for practical use. It becomes intolerable.

Since Si does not form a solid solution in cementite, it has the effect of suppressing its precipitation, and in the bainite transformation temperature range of 200 to 450"C, it is possible to further concentrate C into untransformed austenite, which has a supersaturated solid solution. However, within the C content range of the present invention, the effect is recognized when the Si content is less than 0.50%, preferably 0.80% or more. It is necessary to avoid this because it not only tends to cause scale on the surface at high temperatures, but also causes C to precipitate as graphite.Also, since St makes the transformation point of A extremely high, if it is added in a large amount, it will cause continuous Since the heating temperature during annealing must be considerably increased, which increases cost, it is desirable that the content be less than 1.50%.

Further, Mn is concentrated in the austenite forming element, thereby stabilizing the austenite and increasing its strength. It is also necessary to suppress decomposition into pearlite during cooling from the two-phase region to the bainite transformation temperature region. If the content is less than 0.20%, there is a high risk of causing hot brittleness during hot rolling, so it must be avoided, and in order to achieve the objective with a stable cooling rate, the content should be 1.00% or more. desirable. On the other hand, if it exceeds 2.50%, not only will the expected effect be saturated, but it will also cause a significant band structure to be formed, resulting in significant characteristic deterioration, which is not appropriate. In order to improve weldability and suppress increases in alloy costs, it is desirable that the content be 2.00% or less.

Furthermore, sol and Al are used as deoxidizing elements, and AffN
It is necessary to add 0.005 to 0.10% to improve the quality of the material by reducing the grain size of the hot rolled material and suppressing the coarsening of crystal grains during a series of heat treatment steps. If the amount is less than 0.005%, it is difficult to obtain the desired effect and deoxidation becomes insufficient. On the other hand, it is necessary to avoid adding more than 0.10% since it causes deterioration of toughness due to inclusions.

The steel of the present invention has the above-mentioned basic components, but the steel sheet of the present invention contains, in addition to these elements and Fe, impurities such as P, S, N, and other impurities that are generally inevitably mixed into steel.

Also, austenite-forming elements such as Ni, Cu, and Co
It is preferable to add 1% or less of Cr, or 1% or less of Cr, which increases hardenability, to increase the amount of retained austenite and achieve the object of the present invention.

Next, the reasons for the limitations on the process will be explained in detail.

In the hot rolling of steel according to the present invention, the finishing temperature is 700-850.
℃. The reason why hot rolling is performed under such conditions is that the structure after hot rolling has finer grains and better uniformity than when hot rolling is completed under normal conditions, so it is specified in the present invention after cold rolling. This is because retained austenite obtained through a series of heat treatments becomes finer and more uniformly distributed, and stability is also improved, resulting in improvements in both stretch flangeability and bendability. Finishing temperature is 850℃
Even if the temperature exceeds 100%, if an appropriate heat treatment is performed after cold rolling, a high-strength cold-rolled steel sheet with a mixed structure of ferrite, bainite, and retained austenite can be obtained, and the elongation property will be better than ever before. The deterioration of stretch flangeability was observed to be greater than that obtained. On the other hand, if the finishing temperature is less than 700° C., the structure tends to become non-uniform and the desired results cannot be obtained. Also, if the temperature is low, the reduction blade will become enormous, but in order to obtain an effect commensurate with eliminating such difficulties, it is desirable to finish at a temperature of 750° C. or higher.

In the present invention, after finishing hot rolling at such temperature, the structure at the time of cooling after coiling is ferrite and pearlite, and the ratio of pearlite is cold rolling, and the ratio of retained austenite obtained after completion of heat treatment consisting of a series of subsequent cycles. No. 1
．． 5 to 2.5 times.

By creating such a structure, it is possible to perform cold rolling at a predetermined rolling rate without causing cracks, etc. In addition, when heated to a two-phase coexistence temperature range in the subsequent heat treatment, carbides disappear quickly and at the same time. Distribution of alloying elements such as Mn occurs between ferrite and austenite, and through the subsequent heat cycle, reaustenite easily remains. If martensite is mixed into the structure at this point, cold rolling becomes difficult. Furthermore, if the ratio of pearlite is less than 1.5 times the ratio of retained austenite obtained after the completion of heat treatment consisting of cold rolling and a series of subsequent cycles, it will take a long time to obtain a sufficient amount of austenite during heating, resulting in a continuous production line. This makes production difficult. On the other hand, the ratio is 2
．． If it exceeds 5 times, the amount of bainite will be insufficient in the final structure to obtain the desired strength.

In the present invention, a hot-rolled steel sheet having such a metallographic structure is pickled and cold-rolled at a rolling reduction of 35 to 65%. The purpose of this is to make it easier to secure retained austenite and to finally obtain a structure that has both high strength and excellent press formability. When this rolling ratio is less than 35%, the structure is not sufficiently refined, so that a sufficient amount of retained austenite cannot be obtained after the series of heat treatments are completed, resulting in low elongation and poor extrusion properties. Furthermore, since coarse martensite is formed, bendability and stretch flangeability are not excellent. On the other hand, if the rolling reduction exceeds 65%, voids will become noticeable during cold rolling, and will remain even after subsequent heat treatment, resulting in stress concentration during press forming and impairing bendability and stretch flangeability. This should be avoided as it will cause significant deterioration.

In the present invention, after this cold rolling, a heat treatment consisting of a series of cycles is performed. First, the material is heated to a two-phase coexistence temperature range of 730 to 900° C. and held for 15 seconds to 5 minutes. In the steel sheet having the composition system of the present invention, when this heating is performed, most of the carbides exceeding the solid solubility limit disappear, and the austenite becomes approximately 40 to 40%.
A structural state appears in which 80% of the ferrite is present and ferrite occupies the remainder.

C, which has a large diffusion constant, is concentrated in austenite and diluted in ferrite, but by defining the metal structure before heating, a certain degree of distribution of Mn can also be achieved. This is a necessary condition to ensure that 8 to 25% of austenite remains after the subsequent series of heat treatments are completed and brought to room temperature, ensuring high strength and excellent press formability.

If the heating temperature is less than 730°C, there is a high possibility that unmelted carbides will exist if the heating time is set to be industrially practical, and the amount of solid solution C will be insufficient. Therefore, even if a series of subsequent heat treatments are performed, a sufficient amount of retained austenite cannot be secured, resulting in low elongation. on the other hand,
At heating temperatures exceeding 900° C., only a small amount of ferrite, or even no ferrite exists, resulting in a single phase of austenite, so that the overall distribution of alloying elements remains at a dilute level. For this reason, even if alloying elements are intended to be enriched in austenite in the following steps, it can only be achieved insufficiently, and it becomes difficult to secure residual austenite that ultimately contributes to improving elongation, making it difficult to achieve high strength. At the same time, it is not possible to achieve excellent press formability.

If the holding time in this temperature range is less than 15 seconds, undissolved carbides may be present, and recrystallization is not completed and strain caused by cold rolling is not released, resulting in significant deterioration of formability. On the other hand, holding the steel plate for more than 5 minutes not only reduces productivity on a continuous line, but also leads to coarsening of crystal grains, which may deteriorate the mechanical properties of the steel sheet after a series of heat treatments are completed. There is also.

In the present invention, the temperature is continuously increased from 1 to 10°C to 600 to 700°C.
The purpose of this is to ensure a sufficient amount of clean ferrite necessary for effective elongation improvement due to transformation-induced plasticity. In this case, if the cooling rate is less than 1"C/s, austenite will decompose into pearlite, making it impossible to obtain a sufficient amount of residual austenite after the heat treatment is completed.
If it exceeds 1, the cleanliness of the ferrite produced becomes insufficient and its ductility deteriorates, which reduces the effect of the transformation-induced plasticity of retained austenite. These 1 to 10
If the slow end temperature of ``C,/s is higher than 700℃, ferrite will be slightly formed and the concentration of alloying elements in austenite will be insufficient, and if it is lower than 600℃, austenite will be Most of it decomposes into pearlite, resulting in only obsolete mechanical properties.The best balance between strength and press formability is achieved when the temperature is slowly increased from 660 to 700°C at a rate of 1 to 10°C/s. It is.

It was time.

200-4 from the temperature that finished slowing at 600-700℃
In the present invention, the temperature up to 00°C is cooled at 20 to 200°C/s. The purpose of this is to bring the ferrite and austenite present at the end of the slow process to the bainite transformation region where they will be retained and maintained as they are thereafter. If the cooling rate within this temperature range is less than 20° C./s, pearlite will be produced and the strength-ductility balance will deteriorate. On the other hand, when the temperature exceeds 200°C/s, it is extremely difficult to finish the cooling at the desired temperature, and if the entire thin steel plate is not cooled uniformly, its shape becomes unsuitable for practical use. . Also 100℃/
If it exceeds s, needle-shaped ferrite may be formed, which may impair the formability, so the
C/s is the best cooling rate. If this cooling ends at a temperature higher than 400°C, low-strength bainite will rapidly form during subsequent holding, making it extremely difficult to obtain the required strength of the steel sheet, and in some cases, pearlite may also form. retained austenite cannot be secured. Furthermore, when cooled to less than 200° C., austenite transforms into martensite and becomes a ferrite-martensitic dual-phase steel, so that both stretchability, bendability, and stretch flangeability are far from the required levels. If this cooling end temperature is too lower than the subsequent holding temperature, the energy cost required to reach that temperature range will increase, so 250 to 400°C is a practically appropriate range.

This is within an appropriate range.

After finishing this cooling, according to the present invention, the temperature is 2 to 200°C to 400°C.
Hold for 50 seconds, then heat to 350-450℃ for 15 seconds-10
Hold for 30 minutes, then cool to 150°C or less within 30 seconds. The purpose of this is to achieve a state in which 8 to 25% of retained austenite is mixed with ferrite and bainite. In other words, as mentioned above, due to the effect of St contained as an alloying element, the transformation from austenite to bainite separates into two stages, and in the intermediate stage, bainite containing almost no carbide and C discharged from that part.
This results in a state in which concentrated untransformed austenite is mixed, and this austenite remains even after cooling to room temperature. The purpose of the initial holding for a short time of 2 to 50 seconds is to eliminate temperature irregularities that may occur due to rapid cooling, and to achieve a uniform temperature distribution over the entire plate thickness and width. 15 seconds to 10
lower than the retention temperature of
Since the relatively thin part of bainite can be made into higher strength bainite, strength and press formability can be improved at the same time.
The properties of the obtained steel sheet are further improved. If this time is less than 2 seconds, there is a high possibility of reheating, etc., and the purpose cannot be achieved. Furthermore, holding for more than 50 seconds is not practical as it reduces the productivity of continuous lines. Subsequently heated to 350-450℃ for 15 minutes.
If the temperature exceeds 450°C, the austenite decomposes into pearlite, while if the temperature is below 350°C, fine carbides will precipitate quickly. It does not exist, and the balance between strength and press formability is not significantly improved.

In addition, if the holding time is less than 15 seconds, the progress of bainite transformation is insufficient, and austenite whose C content is not sufficiently concentrated becomes martensite during cooling to room temperature, and the resulting steel sheet has high strength but very poor formability. inferior to On the other hand, if the retention time exceeds 10 minutes, a large amount of austenite will precipitate carbides and decompose into bainite, so that less than 8 to 25% will remain when cooled to room temperature. After this 150
The same is true if it takes more than 30 seconds to cool down to below .degree. C., and neither the intended high strength nor excellent press formability can be obtained.

In addition, in the process explained above, the heat temperature for each two-phase region, the temperature held for a very short time after cooling, and the subsequent holding temperature do not need to be constant as long as they are within the specified temperature range. Even if it fluctuates, it will not deteriorate the properties of the final product in any way.

Further, the cooling rate after hot rolling finishing or the winding temperature during winding treatment are not particularly important.

(Example) The steel whose ingredients are shown in Table 1 was hot rolled, cold rolled and heat treated under the conditions shown in Table 2, and after temper rolling of 0.8%,
A JIS No. 5 tensile test piece was taken, and the gauge length was 50 mm.
When a room temperature tensile test was conducted at a tensile speed of 10 mm/min, the tensile strength and total elongation as shown in the table were obtained. In addition, the results shown in the same table were obtained for the minimum bending radius, which is an evaluation index of bendability, and the hole expansion ratio, which is an evaluation index of stretch flangeability. Here, the minimum bending radius is width 40III11
, is the minimum value of the punch tip radius at which no cracks occur when performing a V-bending test with a punch angle of 90° using a test piece with a length of 150 mm.
This is the ratio of the diameter when cracks begin to occur around the hole when a thin steel plate with holes punched out is stretched out to the diameter of the hole immediately after punching.

Sample No. of the present invention, 2.6, 8, 12, 14°16
,19,20,25,28.30,33°36.39,
40, 42, 44, 45.4B.

49.50, the product of tensile strength and total elongation is 2800
Despite its high strength, it has a large elongation and excellent overhang properties, as can be seen from the fact that it exceeds kgf/mm2.%.At the same time, it has a minimum bending radius of 0.5 mm or less and a hole expansion ratio of 1.30. Both of the index values of bendability and stretch flangeability are in good ranges, and it is possible to handle a wide range of severe press forming that could not be achieved with conventional techniques.

On the other hand, steels a, f, and g are outside the composition range of the present invention.

Even if sample i undergoes heat treatment consisting of a series of cycles of hot rolling and cold rolling under conditions considered to be optimal, sample No.
46. 47. 51, even if the composition steel of the present invention is hot-rolled or cold-rolled to satisfy the specified conditions, if there is even one unsuitable part in the heat treatment conditions, sample No. 15.17° 18.21-24,2
6, 27, 29, 31° 32.34, 35, 37.3B
, 41.43, the object of the present invention cannot be achieved because the strength is insufficient or one or more of the stretchability, bendability, and stretch flangeability is poor. Moreover, even when the prescribed heat treatment was performed on the steel of the present invention, samples No. 3 to 5
．． 7.9 to 11.13, when the finishing temperature of hot rolling, the metallographic structure after coiling and cooling, or the cold rolling conditions deviate from the conditions of the present invention, the present invention as explained above cannot be applied. Deterioration is seen in one or more of the properties compared to the case of hot rolling and cold rolling as specified.

(Effects of the Invention) As is clear from the above examples, if the component steel of the present invention is subjected to heat treatment consisting of a prescribed hot rolling/cold rolling and a series of cycles, fine retained austenite will be reduced to 8 to 25%. occupies a volume of It can be manufactured without any practical difficulties and has a remarkable effect in industrial applications.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a heat cycle of heat treatment applied to a steel plate after prescribed hot rolling and cold rolling. Figure 1

Claims (1)

[Claims] C: 0.12-0.40%, Si: 0.50-0.50% by weight
2.00%, Mn: 0.20-2.50%, sol. A
l: 0.005~0.10%, the balance is Fe and unavoidable impurities, and the steel is hot rolled at a finishing temperature of 700~850°C. A steel plate with a retained austenite ratio of 1.5 to 2.5 times the ratio obtained after completion of heat treatment consisting of a series of subsequent cycles is pickled and rolled at a rolling rate of 35.
~65% cold rolling, then heated to a two-phase coexistence temperature range of 730~900°C, held for 15 seconds~5 minutes, and then heated to a temperature range of 600~700°C.
1~10℃/s up to 00℃, 20~200℃ below
Cool to 200-400℃ at a rate of ℃/s, hold within this temperature range for 2-50 seconds, and then cool to 350-450℃ for 15 seconds.
Contains 8-25% retained austenite and has excellent press formability including stretchability, bendability, and stretch-flangeability. A method for producing high-strength cold-rolled steel sheets.