KR800000621B1 - Process of making high strength cold-rolled steel sheet having excellent bake-hardening properties - Google Patents

Abstract

This invention comprises a steel-making stage, a hotrolling stage, a cold-rolling stage and a full continuous annealing stage. At the steel-making stage, C and Mn content in steel are controlled within 0.04 to 0.12 % and 0.1 to 1.60 %. At the hot-rolling stage, a finishing temp. of more than 800≰C and a coiling temp. of less than 700≰C are maintained At the full continuous annealing stage, a travelling strip is heated up within 700-900≰C, held the above temp. for 10-120 seconds and cooled by a jet stream of water to room temp.

Description

Manufacturing method of high tensile cold rolled steel sheet with excellent back-hardening characteristics

1 is a graph showing the change in aging characteristics accelerated by the preheater of a moving strip.

The present invention relates to a method for manufacturing inexpensive high tensile cold rolled steel sheet, in particular, by increasing the yield point by maintaining the tensile strength of 40 ~ 80kg / mm ² and performing heat treatment after rolling molding in practical use, such as the acceleration of significant improvement The present invention relates to a method for providing a cold rolled steel sheet having excellent aging characteristics.

The improvement of the cold rolled steel here is only known for mild steel with so-called low yield point.

However, for the safety of vehicles, especially for the safety of passenger cars, recent demands have turned to cold rolled steel sheets with high tensile strength. Several proposals have emerged and have been put into practice, which can be classified as follows:

1. Complete recrystallization using special elements such as Ti, Nb, Cu, Ni, etc.

2. Controlling annealing conditions and maintaining a partially cold working structure;

3. Cold re-rolling of completely recrystallized steel and increasing strength by work hardening;

4. For the purpose of improving the strength by quenching low carbon steel,

However, these methods have been found to have one or more defects. For example, method (1) is expensive, and method (2) is distributed in the longitudinal direction because of the difference in annealing temperature around the inside and outside of the coil. (3) The method improves the strength, but does not improve the elongation, which has a problem that cannot be solved. On the other hand, the method (4) is suitable because it is not expensive to obtain a high strength cold rolled steel sheet. The proposal is continuously developed until recently as Japanese Patent Nos. 40-3,020, 46-9,541 and 46-9,542. Has been.

The technique shown in Japanese Patent No. 40-3,020 is well known by the BISRA (British) method. This technique involves heating the cold rolled strip to 740-850 ° C., quenching the strip to a temperature of 150-250 ° C., the coil in the same manner and then overaging through the self-annealing of the coil. The steel produced by the above method does not have accelerated aging characteristics such as so-called AA properties in which the strength is increased from the original by heat treatment such as heating the surface after rolling molding, and causes the strength to be too low. Therefore, if the strength is to be increased, it is necessary to increase the carbon content. At the same time, the continuous process is hindered by the above-mentioned self-annealing, resulting in reduced productivity.

The techniques shown in Japanese Patent Nos. 46-9,541 and 46-9,542 were called the INLAND) method (United States of America). The former is composed of fine cementite or granular cementite and ferrite coils up to a temperature of at least _three A to form a complete austenite structure without finally containing pearlite. It is heated and then quenched to obtain a uniform structure composed of martensite. The latter is to make the coil as a partially austenitic and heated at point A ₁ to A ₃ point, then ferrite and thus the same as the e-quenching in order to obtain a martensite structure mix site. The steel produced by these methods has the drawback of lowering the tensile strength to about 15 kg / mm ² by coating and heating after rolling forming, and having a low ductility compared to the strength. This means that it is very difficult to handle steel because it hardens when rolled and softens when finished.

On the other hand, steels with high AA properties whose strength is improved by heat treatment after rolling forming are known as ISLAND STEEL CORPORATION (“Blast furnace and steel plant” March 1971 issue 149-). This technique involves the addition of about 100 ppm of nitrogen, particularly at the beginning of the steelmaking stage, which can be cold rolled by the usual method, annealed by batch type annealing, and then used. The user must perform appropriate rolling forming and heat treatment to improve the strength, and such improvement in strength may contribute to the aging of nitrogen as mentioned above, but such effect will give the initial strength of 5-6kg / It is known that it is automatically restricted to limit the use when using it as 40-50kg / mm ² compared to the value as low as mm, so about 9kg / m in the heating step after rolling. It is thought that a technique which can obtain a safe AA effect at the yield point of m ² is not available at present.

The present invention has been developed to provide a solution composed of a method for solving the above-mentioned problems, and has the feature that the production is carried out by a continuous annealing process. In this technique, heating and soaking is selected in the temperature range of 700-900 ° C. at which the treatment solution of cold rolled steel is reached, and the steel is quenched by injecting water from a temperature higher than room temperature. It is reheated to 150-400 ° C. and subjected to aging at the latter temperature. The aging treatment is not caused by the precipitation of carbon in the steel, but by the partial remaining of solid carbon. The steel is then cooled to room temperature and then wound into a coil. After consideration, a tensile strength of 40-80 kg / mm ² is obtained. When the steel sheet thus obtained is subjected to heat treatment for heat coating of the steel after rolling, its yield strength is increased by about 7-11 kg / mm ² compared with the initial strength before rolling.

It is therefore an object of the present invention to provide a process for producing high tensile steel sheet for the safety of a vehicle which is used by increasing the accelerated aging characteristics of the steel sheet.

It is still another object of the present invention to provide a manufacturing process which saves cost in the continuous annealing of high tensile steel sheets having excellent accelerated aging characteristics without addition of any special elements.

Other objects and advantages will be apparent from the following description and the accompanying drawings.

The steel used in the present invention is composed of 0.04-0.12% carbon and 0.10-1.60% manganese. And there is no need to add other special elements. Various types of steel making furnaces currently known can be used for the present invention, and any process after steel making, for example, ingot, blooming, continuous casting, etc., can be used. After scarping, the steel is heated to a temperature of 1,250 ° C. or higher, hot rolled at a final temperature of 800 ° C. or higher, and the coiled temperature is 700 ° C. or lower. A common cold rolling process is performed after pickling. In subsequent steps the annealing is completed in a continuous annealing process. In this case the strip is heated to 700-900 ° C. and held at this temperature for 10-120 seconds. The heating temperature should be from A ₁ to 850 ° C and must be tightly controlled as it affects the strength of the final product. Carbon in the steel thus obtained becomes a solid solution in the heating and soaking process mentioned above. Hardening from this temperature should be done as soon as possible. Therefore, the use of steam is used because carbon in the steel remains as carbon solids as mentioned above. The moving strip quenched to room temperature is reheated to 150-400 ° C., preferably 180-300 ° C., and must be maintained at this temperature for a suitable time. The appropriate time to maintain the temperature is dependent on the reheating temperature, in particular 15-300 seconds for the reheating temperature selected in the range of 150-180 ° C, and 4-300 seconds, especially 20-120 seconds for the 180-300 ° C. Preferably, 2-200 second is preferable at 300-400 degreeC. This form of cold aging does not completely precipitate solid carbon in the river, but rather remains partially in solid solution. At that time, the moving strip is forcedly cooled to room temperature, coiled, and then soaked. The steel sheet thus obtained will readily have the following properties.

Tensile Strength: 40-80kg / mm ²

Yield strength: 30-60kg / mm ²

Tensile Strength (kg / mm ² ) + Elongation (%)> 73

The value of tensile strength (kg / mm ² ) + elongation (%) is used as an index to measure the balance between the two, since the increase in tensile strength generally lowers the elongation at which it is ductile. The inventive steel thus treated exhibits a remarkable effect of improving the strength, in particular yield point of at least 7 kg / mm ² , when heat-treated between 100-200 ° C. after rolling and molding in a preferred form. Naturally, this contributes to the aging effects of solid carbon, which remains partly due to cold aging. In other words, the molding is easily carried out due to the somewhat low yield point, so that the excellent rolling molding of the product is maintained as it is, and the above-mentioned aging effect of carbon is due to the heat used for molding in the heating process of the product. It becomes clear. However, there is no other object of the present invention, and the safety when such steel is used in a vehicle or the like is almost perfect. The process of the present invention will be described in detail in individual steps. Carbon in steel is an element that plays an important role in the present invention. The lower limit of carbon is set at 0.04% in view of the necessity of maintaining the strength of the steel and the work of the converter used in the steelmaking stage.

The upper limit is 0.12% in terms of roll formability and weldability. However, to be more certain, the upper limit should be chosen within the above range depending on the strength of the steel. It becomes 0.10% because of the red brittleness of manganese. And the upper limit is 1.60% in terms of safe work in steelmaking. In fact, the manganese content is selected in the range of 0.10-1.60 depending on the carbon content. If silicon and aluminum are kept at 0.2-0.02 or less for the purpose of carbonation control, undesirable results will be produced. There is no specific scope for the causal term, but it is recommended to exist at a general level.

In order to obtain a uniform thin rolling structure, the hot rolling conditions should be such that the finishing temperature is 800 ° C or higher, preferably 830 ° C or higher. The coiling temperature should be at the same level as a typical cold rolled plate, ie it should be at a level of 700 ° C or 500-650 ° C. There is no particular problem if normal pickling and cold rolling are performed after hot rolling. This invention is characterized by the annealing process.

The annealing step is continuously performed between the start of work by the cold rolled coil and the end of work by the coiling of the annealed strip. Since the required heat depends on the solid-solution carbon and fine precipitation in the steel and the reinforcement by the steel reinforcing mechanism of the present invention, the strip is required to be heated to a temperature at which a large amount of solid-solution carbon is formed to obtain the required strength. It is done.

In other words, the heating temperature should be at least 700 ° C. or higher, and practically at least A ₁ point. On the other hand, an increase in heating temperature in this way improves the quenching structure of the martensitic system and increases the strength of the steel. But at the same time this lowers the elongation because the hardness difference between the ferrite base and the quenched structure of the second phase particles is very large. This is the reason why the upper limit of the heating temperature is 900 ° C or at least 850 ° C.

The minimum period for fixation of the strip moving at the heating temperature must be sufficient to completely recrystallize the cold rolled strip and to make the carbon in the steel solid solution. This is why the lower limit is 10 seconds. However, the longer the fixed time is, the longer the heat-sinking zone is extended and the working speed decreases, and at the same time, recrystallization occurs and softening by particle growth proceeds. Therefore, the upper limit must be set to 120 seconds or less.

The moving strip thus heat treated and soaked is quenched to room temperature by high speed injection of water. In this case, the high-speed jet of water promotes cooling and gives a complete cooling effect of all parts in "air" or "water". In other words, when heated moving strips are quenched in water, they form a vapor film on the surface of the strip, which impairs thermal ductility and lowers the cooling rate. The high speed jet of water in the present invention is most widely used to remove such vapor films, and the Applicant has found that the cooling rate of 3,000 ° C./sec or more can be easily obtained through implementation. On the other hand, it has been found that the cooling rate obtained simply by water, oil, salt bath or metal bath is only less than 1,000 ° C / sec. The reason for using such a high cooling rate in the present invention is that the solid carbon obtained through the heating and soaking process is maintained at room temperature. As mentioned above, hardening up to room temperature has a decisive influence on the next step of low temperature aging treatment.

The reheating in the continuous process of low temperature aging treatment as mentioned above first formed precipitation nuclei for fine carbides of solid solution carbon in the steel. The fine carbides thus formed have a very significant effect on improving the strength, but will serve to restrain the decrease in strength due to the minimization of solid solution carbon.

This is the first reason to quench the strip to room temperature.

The second reason is that if the steel remains after quenching up to room temperature, the strength is very high at this stage due to the strengthening of the solid solution of carbon. However, it is unavoidable that the above-mentioned solid solution carbon precipitates and causes a decrease in strength by coating-heating (generally 10-20 minutes at 100-200 ° C) performed after rolling forming. In other words, it is the hardest to handle steel because the strength is high when subjected to rolling, but the strength decreases after rolling. For this reason, low temperature aging is accomplished by reheating of the moving strip, which is quenched to room temperature. In this step, the solid solution carbon should be precipitated to some extent so as not to cause a decrease in strength during coating-heating of the molded product. To be more specific, the conditions of heating, soaking and cooling must be appropriate to ensure that all of the dissolved carbon is not precipitated by reheating and some remains in solid solution. In the above-mentioned low temperature aging treatment, a product having excellent AA property was obtained at low reheating temperature, because the tensile strength is high and the yield strength is of course lower in narrow formability in the rolling forming step. However, if the temperature is too low, precipitation of solid solution carbon does not occur, and a decrease in strength is inevitably caused because the solid solution carbon is precipitated by heat treatment in the coating-heating step. Therefore, the minimum reheating temperature is the temperature at which the solid solution carbon partially precipitates. The minimum of the said temperature is 150 degreeC. Specifically, the processing temperature is about 180 ° C. in order to obtain a stable product in view of the heating temperature after rolling forming.

Too high a reheating temperature, on the other hand, tends to soften some of the ferrite in a two-phase structure where ferrite bases and quenched structures are released, which inevitably lowers the strength.

In addition, the difference in hardness between the ferrite and the quenched structure is made so that, despite the low tensile strength, the elongation is restored and the index of the "tensile strength + elongation" is low. Reheating at high temperatures will cause supersaturation of the dissolved carbon precipitated in the aging step to avoid the AA effect. Therefore, the upper limit of reheating temperature becomes 400 degreeC. There is no advantage over 400 ° C. For practical use, the most preferred temperature is 180-300 ° C. The appropriate range of holding time at such a reheating temperature varies with the temperature. If the reheating temperature is between 150-180 ° C, the holding time is 15-300 seconds, and between 180-300 ° C is 4-300 seconds, and between 300-400 ° C is 2-200 ° C. If the low temperature aging treatment is carried out for X hours at such a temperature, all of the above mentioned problems will be solved, and at this stage a portion of the dissolved carbon will easily remain.

By such a process, a decrease in the "tensile strength + elongation" index caused by an increase in the difference in hardness of the two structures can be avoided, and a sufficient AA effect is expected. In practical use, when the holding time for a reheating temperature of 180-300 ° C. is chosen to be 20-120 seconds, the maximum AA characteristic, ΔYP, is assured and in the most stable range. The graph of the accompanying figures shows the variation at this stage. At least AA properties of 7kg / mm ² in the graph (in this case ΔYPkg / mm ²⁾ is shown to be obtained jindago temperature × time eases the stability is selected within this range. It is possible to obtain a value of ΔYP of 11 kg / mm ² under suitable conditions.

In the graph, ΔYP represents an increase in the yield point of the steel subjected to the coating-heating plant before rolling after rolling.

After the above-mentioned low temperature aging treatment has been carried out, no special limitation appears in the cooling step. However, if forced air cooling is used for practical reasons, it is very easy to cool the strip to the appropriate temperature for consideration when the crude mill is connected to a continuous cooling line. After regulating rolling the strip is coiled and transferred to the plant. If the soot rolling mill is not attached, the general line of coiling of the strip and the temper rolling are used simultaneously.

The following table shows a comparison of the properties of the steel of the present invention with those of other forms of steel.

1. Hot Rolling Condition: ◎ indicates comparison with steel after baking.

Finishing temperature 850 ℃

Final plate thickness of the strip: 2.8mm

2. Final plate thickness after cold rolling: 1.2mm

3. Baking conditions after rolling are the same as those generally used in automobile manufacturing.

In the above table, the influence of the cooling method on the steels 1 to 3 was investigated. Steel 1 was quenched by high speed injection of water in accordance with the present invention. Steel 2 was quenched in purified water, and steel 3 was quenched by the Viscola method. The steel of the present invention exhibited a yield strength of 34.2kg / mm ² , a tensile strength of 44.7kg / mm ² and an elongation of 30.9% when transferred to the plant after skin pass, and yielded after coating-heating treatment after rolling. The strength was 44.5 kg / mm ² and the tensile strength was 45.5 kg / mm ² , which showed a significant improvement in ΔYP, ie 10.3 kg / mm ² . This well explained the excellent AA effect of the present invention because of the excellent hardening by the high-speed injection of water. The fact that the "tensile strength + elongation" index is 66.5 in steel 2 and 68.8 in steel 3 is high in Dead Sea Steel 75.6 indicates good rolling formability. Steels 4 to 8 were investigated for the effect of maximum heating temperature, where steels 4 and 8 were comparative steels and steels 5, 6 and 7 were inventive steels. The maximum heating temperature is 8, whereas steel 4 is lower than the inventive steel. The difference is clearly explained in the mechanical properties of the steel. The "tensile strength + elongation" indicators of the comparative steels are all 73 or less, and the indicators of the inventive steel exceed 73.

This indicates that if the maximum heating temperature is outside the scope of the present invention, an undesirable effect is on compressive forming. This at the same time, as that the invention steel is when both indicate the value of 9kg / mm ² or more excellent ΔYP, the comparative steels are explained by the fact that represents a low value of 5.0kg / mm ² and 7.7kg / mm ^2, It affects the AA effect. This fact indicates that even if the quenching treatment is performed by the present invention, if the heating temperature is outside the scope of the present invention, the desired object can hardly be achieved. Therefore, the heating temperature should be selected in the temperature range between 700-900 ° C, more preferably in the temperature range between A ₁ point and 850 ° C.

Steels 9 to 13 were investigated for the effects of reheating temperatures, with steels 9, 10 and 13 being comparative steels, and steels 11 and 12 being inventive steels. Of these steels, steel 9 is not reheated, but has a very low tensile strength, ΔTS -20.1 kg / mm ² , which is an example of a typical steel that is difficult to roll and soften. In steel 10, since the reheating temperature is 100 ° C, the tensile strength at the time of rolling forming is increased to 58.2 kg / mm ² . However, by coating-heating after rolling, the tensile strength is lowered to 47.5 kg / mm ^2, that is, -10.5 kg / mm ² , and even though the increase in tensile strength is 10.1 kg / mm ² , the steel cannot be well-balanced. . Therefore, it can not be said that Li Kang 10 has a good AA effect. Wherein the steel 11 and 12 of the present invention maintains the same tensile strength value as in the case of rolling molding, ΔYP is 10.9kg / mm ² and 7.9kg / mm ² , respectively, "tensile strength + elongation" index is 75.6 and 73.4, respectively Indicates a good value of. It is certain that the rivers are well harmonious and satisfying. It will therefore be understood from the above fact that the reheating temperature should be selected in the range of 150-400 ° C., preferably in the range of 180-300 ° C. In rivers 14 through 17, the effect of holding time on reheating time was investigated. In these steels, steel 17 is a comparative steel, and steels 14 to 16 are inventive steels. As shown in the table, the inventive steels 14 to 16 exhibit well harmonized properties and have sufficient AA effects.

On the other hand, steel 17, which has a long treatment time of 10 minutes, has a low yield strength of 4.0 kg / mm ² , indicating that no AA effect has occurred. The holding time at the reheating temperature is selected in the range of 2-300 seconds depending on the reheating temperature to be used.

Steels 18 to 20 were examined for the effect of emphasis, with steel 20 being a comparative steel and steels 18 and 19 being steels of the present invention.

Steel 20 is subjected to the same treatment, except that the carbon content is 0,15% higher than the inventive steel. As can be seen from the table, river 20 has a “tensile strength plus elongation” index of 71.5. This indicates that there is a difficulty in stable formability and weldability. Therefore, the steel will have to be concerned even if it shows sufficient AA effect after heating. On the other hand, all of the steels within the adjustment range of the present invention exhibit excellent “tensile strength + elongation” index and AA effect and have satisfactory properties. Therefore, it has been found that the carbon content in the steel should be between 0.12-0.04% and vary according to the required strength.

Steels 21 to 23 were investigated for the effect of manganese as the steel of the present invention. It was shown that the strength improved by the increase of manganese does not necessarily lead to a decrease in elongation, nor a decrease in the "tensile strength + elongation" index. Therefore, manganese is a very effective element in the present invention because it does not lower the mechanical properties and directly improves the strength, depending on the level of strength required when the carbon content is determined. Is selected.

As explained in the various examples above, the significant effect of the method of the present invention as compared to the inland steel process of batch type annealing, which achieves the AA effect by nitrogen in the steel, and the conventional bisra process of the semi-continuous process, The fundamental technical differences are as follows.

(1) The difference between the annealing method and the annealing method in which the strip is quenched from the bath to the coiling temperature and the method of the present invention is quenched to the room temperature by high speed spraying of water.

(2) The difference between the Visla method of depositing solid solution carbon by self-annealing at the coiling temperature, and the present invention method of maintaining solid solution carbon by reheating.

(3) The action of nitrogen content in batch annealing of inland steel where no aging effect by carbon content occurs, and the method of controlling solid solution carbon in the continuous annealing process of the present invention (where nitrogen content is an inevitable element in steelmaking) ).

Claims (1)

In the manufacturing method of high strength cold rolled steel sheet having a steelmaking step, a hot rolling step, a cold rolling step, and a completely continuous annealing step, the steelmaking step has a carbon and manganese content in the range of 0.04 to 0.12% and 0.1 to 1.60%, respectively. In the hot rolling step, the finishing temperature is above 800 ° C, the coiling temperature is below 700 ° C, in the complete continuous annealing step, the moving strip is heated to within the range of 700 ~ 900 ° C 10 to 120 seconds above the temperature, and the heated strip maintained at high temperature is rapidly cooled to room temperature by a high-speed spray of water, and the re-heated to the temperature of 150-400 ℃ range, reheat The step is characterized in that it is maintained for a selected time in the range of 2-300 seconds depending on the temperature, and cooling and coiling the strip obtained above. A method of manufacturing high tensile cold rolled steel sheet having a number of accelerated aging characteristics.

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