JPS6314817A - Production of high-strength thin steel sheet having excellent bending characteristic - Google Patents

Abstract

PURPOSE:To produce a high-strength thin steel sheet having high tensile strength and excellent bending characteristic by subjecting a specifically composed steel contg. C, Si and Mn to adequate cold rolling to soften the same after hot rolling, the subjecting such steel sheet to specific annealing and cooling. CONSTITUTION:The steel contg. 0.03-0.20% C, 0.3-1.5% Si, 0.5-2.6% Mn, contg. further 0.01-0.25% Ti and/or 0.01-0.3% Nb at need and consisting of the balance Fe and unavoidable impurities is hot rolled at the finishing temp. above the A3 transformation point. The hot rolled sheet obtd. in such a manner is heat-treated at >=650 deg.C and is thereby softened prior to coiling at a high temp. of >=650 deg.C and/or cold rolling and thereafter, the steel sheet is subjected to the cold rolling at 40-80% cold rolling draft. The cold rolled sheet is held for 1sec-5min in the temp. range of the A3 transformation point - 900 deg.C in an annealing stage and is cooled down to 100-500 deg.C at 100-500 deg.C average cooling rate. The steel sheet is then held for 1-20min at 200-500 deg.C and is cooled. The high-strength thin steel sheet having 80-150kgf/mm<2> tensile strength and excellent bending characteristic is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a high-strength thin steel plate having a tensile strength of 80 to 150 kgf/fl'' and excellent bending properties.

(Prior Art) In recent years, high-strength steel plates have been increasingly used in the automobile industry due to the need to reduce the weight of vehicle bodies to improve fuel efficiency and to ensure the safety of passengers in the event of a collision.

In particular, from the standpoint of safety in the event of a collision, the tensile strength is 80 kgf/
There is a demand for a steel plate with an unprecedentedly high tensile strength of more than m''.

Automotive steel sheets do not just need to have high strength; they also need workability and weldability due to their intended use.
A steel plate having a tensile strength of 80 k+rf/wm" or higher is required to have good bending properties among its workability.

Generally, the workability of high-strength thin steel sheets is determined by the balance between strength and elongation, and steel sheets with high strength and good elongation are considered to be excellent as high-strength thin steel sheets. However, high-strength thin steel sheets for automobiles are often processed by bending, and it can be said that steel sheets with excellent bending properties are excellent as high-strength thin steel sheets.

On the other hand, regarding the production of high-strength thin steel sheets having a tensile strength of 80 ktrf/mm" or more, Japanese Patent Application Laid-Open No.
As shown in Japanese Patent No. 22327, a method using water cooling is used, or when the cooling rate is slow as in box annealing, a method of increasing the amount of alloying elements added to increase the strength is used.

In the water cooling method, the cooling rate is too fast, so a large amount of strain accumulates in the steel plate during the cooling process, making it impossible to obtain excellent bending properties. In order to obtain a tensile strength of 80 kgf/w" or more, it is necessary to add a large amount of alloying elements, which impairs weldability and makes the steel expensive economically. Moreover, the steel plate before cold rolling Due to the hardness and capacity of cold rolling mills, it is difficult to manufacture thin steel sheets, and conventional methods have been unable to produce steel sheets with sufficient bending properties for automobiles.
It has not been possible to produce a high-strength thin steel sheet having a tensile strength of 0 kgf/m" or more.

(Problems to be Solved by the Invention) The present invention improves the above-mentioned drawbacks and
The present invention provides a method for manufacturing a high-strength thin imvi having a tensile strength of 1.2 and 2.0 and excellent bending properties.

(Means for solving the problems) The present invention has C: 0.03 to 0.20%, Si: 0.3
~1.5%, Mn: 0.5~2.6%, the balance F
A steel consisting of E and unavoidable impurities is hot rolled at a finishing temperature of A3 transformation point or higher, coiled at a high temperature of 650°C or higher, and/or heat treated at a temperature of 650°C or higher before cold rolling. After making it soft before inter-rolling, the cold rolling rate is 40 to 80.
% cold rolling, and in the annealing step A, held at a temperature range of 900°C above the transformation point for 1 second to 5 minutes, and then heated to 100 to 500°C at an average cooling rate of 100 to 500"c/sec.
80 to 150 kg characterized by cooling to
f/+n'' tensile strength and excellent bending properties, and in the above method, additionally Ti: 0.01 to 0.25% and NbO, 01 to 0.3%.
(Contains 711 types or 2 types, Alui is B: 0
．． 0003 to 0.01%, the method is for producing a thin steel sheet with excellent bending properties and a tensile strength of 80 to 150 kgf/mm2. It is possible to economically produce high-strength thin steel sheets.

The present invention will be explained in detail below.

The present inventors have developed a method with excellent bending properties of 80 to 150 kgf/m.
As a result of intensive studies on the manufacturing method of high-strength thin steel sheets with a tensile strength of m2, we found that after hot rolling, the steel sheets are rolled up at a temperature of 650°C or higher and/or heat treated at a temperature of 650°C or higher. After making it soft, the cold rolling rate is 40-80%.
Cold rolling is performed to reduce the thickness of the steel sheet, and in the subsequent annealing process, annealing is performed in the T single phase region of point A3 or higher to make the structure homogeneous, resulting in a high-quality steel sheet with excellent bending properties. It was discovered that a strong thin steel plate could be obtained.

Figure 1 shows C: 0.12%, St: 0.5%, Mn
: 2.2%, Ti: 0.05%, B: 0.00
A steel containing 12% was melted and hot rolled according to a conventional method to a finishing temperature of 850°C and a cooling rate of 50°C from finishing to coiling.
℃/sec, and the coiling temperature was changed from 500℃ to 750℃ to obtain a hot-rolled steel sheet with a thickness of 3R.Then, it was cold-rolled at a cold rolling rate of 60% according to a conventional method to obtain a sheet with a thickness of 1. 2*m steel plate, then heated to 800°C below the A3 transformation point and A,
The temperature was maintained at 850°C, which is above the transformation point, for 40 seconds, and then cooled to 150°C at a cooling rate of 200°C/sec.
It is a diagram investigating the relationship between the hot-rolled coiling temperature and the tensile strength of the hot-rolled steel sheet, the tensile strength after annealing, the microstructural heterogeneity index, and the limit bending radius when the hot-rolled steel sheet is cooled after being held at 00°C for 3 minutes. . Here, the steel sheet surface hardness (HrC1 test load: 150 kg) was measured at 5 points as an index showing the heterogeneity of the structure, and the standard deviation thereof was taken as the index of the heterogeneity of the structure. In addition, the limit bending radius was determined by bending at 90°, which was the minimum bendable radius.

From the figure, when the annealing temperature is A, 850°C above the transformation point,
It can be seen that when the hot-rolling coiling temperature is 680°C or higher, the tensile strength of the hot-rolled steel sheet becomes low, making cold rolling easier, and the tensile strength after annealing becomes as hard as that of the steel sheet coiled at a low temperature. . Moreover, the structural heterogeneity index is small, the critical bending radius is small, and the bending properties are improved.

On the other hand, when the annealing temperature is A, which is 800° C. which is lower than the transformation point, the index of structural heterogeneity is large despite the fact that the tensile strength after annealing is low, the critical bending radius is large, and the bending properties are poor. Furthermore, when the annealing temperature is 850°C, which is the A3 transformation point or higher, the structure contains martensite, which is said to be bad for bending properties, but the fact that the bending properties are good means that it is important to make the structure homogeneous. It shows. moreover,
It can be seen that being soft before cold rolling, having good texture after annealing, and having good bending properties can be achieved by a combination of high-temperature winding during hot rolling and high-temperature annealing during annealing.

Figure 2 shows C: 0.12%, Si: 0.5%, Mn=
2.2%, Ti: 0.05%, B: 0.0012%
A steel containing 20% is melted and hot-rolled according to a conventional method at a finishing temperature of 850°C and a cooling rate of 50°C/s from finishing to coiling.
ec, the coiling temperature was set to 550°C, the steel sheet was rolled into a hot-rolled steel sheet with a thickness of 3. After pickling, it was heat-treated at 700°C for 5 hours in a box-type annealing furnace, and the cold rolling rate was 60% according to the usual method. After cold rolling to obtain a steel plate with a thickness of 1.2 fi, it was held at a temperature of 800°C below the transformation point and 850°C above the transformation point for 40 seconds, and then rolled at 200°C/sec. After cooling to 150°C at a cooling rate and holding at 200°C for 3 minutes, the hot-rolled coiling temperature and tensile strength of hot-rolled steel sheet, tensile strength after annealing, microstructural heterogeneity index, limit bending radius This is a diagram examining the relationship between From the figure, it can be seen that, similar to the high-temperature rolled material in FIG. 1, cold rolling is easily performed by heat treatment before cold rolling, and the bending properties are ultimately improved.

In addition, the fact that tensile strength increases while maintaining good bending properties by rapid cooling from high-temperature annealing means that C
The amount of reinforcing elements added, including , can be reduced, and as a result, weldability can be improved and it is also economically advantageous.

In the present invention, the reason why the chemical composition of the target steel is limited as described above is as follows.

C is an element necessary to obtain strength by utilizing precipitation strengthening and transformation strengthening. If the content is less than 0.03%, precipitation strengthening and transformation strengthening cannot be fully utilized and the desired tensile strength cannot be obtained, so the lower limit is set at 0.03%. Furthermore, if the content exceeds 0.2%, segregation of C occurs, resulting in an uneven structure, deterioration of bending properties, and marked deterioration of weldability, so the upper limit is set at 0.2%.

St is a solid solution strengthening element, and the lower limit is set to 0.3% in order to obtain strength. On the other hand, the upper limit is set to 1.5% because if it exceeds this value, scale will occur significantly in the hot rolling process and the surface quality of the steel sheet will deteriorate.

Mn is an important element for obtaining strength using transformation strengthening, but if its content is less than 0.5%, the amount of martensite produced is small and the desired tensile strength cannot be obtained, so the lower limit is is set to 0.5%. On the other hand, if it exceeds 2.6%, it not only significantly impairs weldability but also impairs the uniformity of the structure, which is a feature of the present invention, and deteriorates the bending properties, so the upper limit is set at 2.6%.

Ti and Nb are elements necessary for obtaining strength by utilizing precipitation strengthening, are essential elements for effectively utilizing B, and are effective elements for improving weldability. B is an element that easily combines with N, and if there is solid solution N in steel, it will precipitate as BN and B cannot be used effectively. Therefore, in order to use B effectively, N must be converted into a nitride of other elements. It needs to be fixed. Since Ti and Nb have the ability to fix N as a nitride, they are important elements from the above-mentioned point of view as well as their role as strength-improving elements.

If the Ti content is less than 0.01%, the above-mentioned N fixation effect and weldability improvement will be insufficient, so the lower limit is set at 0.01%, and if the Ti content exceeds 0.25%. Even if the amount is increased, the effect will be saturated, so the upper limit is set at 0.25%. Similar to Ti, if the content of Nb is less than 0.01%, the effect of fixing Nb and the improvement of weldability are insufficient, so the lower limit is set at 0.01%, and if the content exceeds 0.3%, Even if it is contained, the effect is saturated, so the upper limit is set at 0.3%.

B is a hardening-strengthening element caused by rapid cooling in the annealing process, but if the content is less than 0.0003%, the effect disappears, so the lower limit is set to 0.0003%, and on the other hand, 0.0
If the content exceeds 1%, flaws are likely to occur during the hot rolling process and the surface quality of the steel sheet will be significantly impaired, so the upper limit is set at 0.01%.

Next, the conditions of the manufacturing process of the present invention will be described.

The finishing temperature of the hot rolling process is set to A, the transformation point or higher, because if it is lower than that, rolling distortion will remain and U\8 cannot be made uniform.

The cooling rate from finishing to coiling in the hot rolling process is as fast as possible to make the structure uniform, preferably 50°C/sec.
The reason for setting the zero winding temperature at 650° C. or higher after cooling to a temperature higher than c or higher is to make the steel sheet soft before cold rolling to facilitate cold rolling.

From this, it is better to set the coiling temperature at a high temperature, and it is desirable that the coiling temperature exceeds the transformation point.

In addition, the reason why the heat treatment before cold rolling is set at 650°C or higher is that if the coiling temperature is high, the pickling property deteriorates. This is to make the steel sheet softer and facilitate cold rolling, and to enhance the effect by using it alone or in combination with high-temperature coiling.

The reason why the cold rolling rate is set to 40 to 80% is to reduce the final plate thickness, and the reason why the lower limit is set to 40% is that when reducing the final plate thickness, the thickness of the steel plate is determined based on the capacity of the hot rolling mill. This is because it exceeds the limit of how thin it can be. Further, the upper limit is set at 80% because no matter how much the steel plate is made soft by high-temperature coiling or heat treatment, the thickness of the steel plate exceeds the limit of the thickness of the steel plate due to the capacity of the cold rolling mill.

In the annealing temperature range below the A3 transformation point, α + γ are mixed,
Since the structure is not uniform and the bending properties are poor, the lower limit is A.
There are 3 metamorphosis points. The A transformation point is approximately 810 to 860 t' in the composition range of the steel of the present invention.

Further, since it becomes difficult to pass the plate in the continuous annealing process at a temperature exceeding 900°C, the upper limit is set to 900°C.

If the cooling rate after annealing is less than 100"C/sec, B
Since the effect of the alloy cannot be used effectively and the martensitic transformation due to rapid cooling does not occur, the amount of alloying elements added must be increased to obtain the desired tensile strength, resulting in deterioration of the bending properties. Therefore, the lower limit is 100℃/
sec.

Furthermore, if it exceeds 500 C/sec, the amount of strain accumulated in the steel plate during cooling increases and it is difficult to obtain good bending properties, so the upper limit is set to 500'' C/sec.

After cooling the temperature to 100-500℃, 200-5℃
The reason why the temperature is maintained at 00° C. is that after martensitic transformation, it is tempered to form bainite, which reduces the difference in hardness from ferrite and improves bending properties. Temperature after cooling is 1
If the temperature is less than 00°C, the hardness of martensite will increase and the bending properties will deteriorate, and it will be necessary to heat the martensite to 200 to 500°C, which is economically disadvantageous. Therefore, the lower limit of the temperature after cooling is set at 100°C. On the other hand, if the temperature after cooling exceeds 500°C, it is disadvantageous to martensitic transformation and it is difficult to obtain the desired strength, so the upper limit is set at 500°C.

The reason why the temperature is maintained at 200 to 500° C. after cooling is to improve the bending properties by tempering after martensitic transformation and converting it into bainite. The reason why the lower limit is set to 200°C is to temper it to form bainite, and the reason why the upper limit is set to 500°C is because if it exceeds that temperature, it will be too tempered and it will be difficult to obtain the desired strength.

According to the present invention, by making the structure homogeneous, it is possible to obtain a high-strength thin steel sheet that has excellent secondary workability, stretch flangeability, and fatigue properties in addition to bending properties, and also has the same tensile strength. Since only a small amount of C is required to obtain the steel, it is possible to obtain a high-strength thin steel sheet with excellent weldability.

Therefore, according to the present invention, 80 to 150 kgf/u+"
It is possible to economically produce high-strength thin steel sheets with excellent bending properties and a tensile strength of .

(Example) Next, the present invention will be explained in detail based on examples.

Example 1 Steel shown in Table 1 manufactured by the ingot method or continuous casting method was continuously hot rolled, pickled, cold rolled, and annealed under the manufacturing conditions shown in Table 2. The tensile strength and bending properties of steel sheets obtained after annealing, including those heat-treated before rolling, were investigated.

As can be seen from Table 2, the desired tensile strength and bending properties could not be obtained using the comparative methods other than those according to the present invention, whereas the desired tensile strength and bending properties could be obtained according to the present invention.

(Effects of the Invention) As explained above, according to the present invention, 80 to 150
Since it is possible to economically produce a high-strength thin steel sheet with excellent bending properties and a tensile strength of kgf/mm2, the present invention has great industrial benefits.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the hot-rolling coiling temperature and the tensile strength of the hot-rolled steel sheet, the tensile strength after annealing, the microstructural heterogeneity index, and the critical bending radius. Figure 2 shows the heat treatment temperature before cold rolling. FIG. 2 is a diagram showing the relationship between the tensile strength before cold rolling, the tensile strength after annealing, the microstructural heterogeneity index, and the limit bending radius.

Claims (3)

[Claims] (1) C: 0.03-0.20%, Si: 0.3-1.
5%, Mn: 0.5 to 2.6%, the balance consisting of Fe and unavoidable impurities is hot rolled at a finishing temperature of A_3 transformation point or higher, coiled at a high temperature of 650°C or higher, and / Or heat treated at a temperature of 650°C or higher before cold rolling to make it soft before cold rolling, then cold rolled at a cold rolling rate of 40 to 80%, and in the annealing process, A_3 transformation point or higher 900 ℃ temperature range for 1 second to 5 minutes, then cooled to 100 to 500 degrees Celsius at an average cooling rate of 100 to 500 degrees Celsius/sec, then held at 200 to 500 degrees Celsius for 1 to 20 minutes, and then cooled. Features 80-150kgf
A method for manufacturing a high-strength thin steel plate with excellent bending properties and a tensile strength of /mm^2. (2) C: 0.03-0.20%, Si: 0.3-1.
5%, Mn: 0.5 to 2.6% as basic components, and further T.
i: 0.01-0.25% and Nb: 0.01-0.3
% or two, with the balance consisting of Fe and unavoidable impurities, hot rolled at a finishing temperature of A_3 transformation point or higher, coiled at a high temperature of 650°C or higher, and/or before cold rolling. After being heat treated at a temperature of 650°C or higher to make it soft before cold rolling, cold rolling is performed at a cold rolling rate of 40 to 80%, and in the annealing process, the temperature range is 1 to 900°C above the A_3 transformation point. Average cooling rate 100 seconds after holding for 5 minutes
Flexural properties having a tensile strength of 80 to 150 kgf/mm^2 characterized by cooling to 100 to 500 °C at ~500 °C/sec, then holding at 200 to 500 °C for 1 to 20 minutes, and then cooling. A method for producing excellent high-strength thin steel sheets. (3) C: 0.03-0.20%, Si: 0.3-1.
5%, Mn: 0.5 to 2.6% as basic components, and further T.
i: 0.01-0.25% and Nb: 0.01-0.3
In addition to one or two types of % B: 0.0003 to 0.01
%, with the balance consisting of Fe and unavoidable impurities, hot rolled at a finishing temperature of A_3 transformation point or higher, and heated to 650°C.
650 or more before high temperature coiling and/or cold rolling.
After heat treatment at a temperature of ℃ or above to make it soft before cold rolling, cold rolling is performed at a cold rolling rate of 40 to 80%, and in the annealing process, the temperature range is 900 ℃ above the A_3 transformation point for 1 second to
After holding for 5 minutes, average cooling rate 100-500℃/s
Cool to 100-500℃ with EC, then 200-5℃
A method for producing a high-strength thin steel sheet having a tensile strength of 80 to 150 kgf/mm^2 and excellent bending properties, which comprises holding the steel plate at 00°C for 1 to 20 minutes and then cooling it.