JPS6323248B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-strength cold-rolled steel sheets having high tensile strength and bake hardenability.
This invention relates to a method for manufacturing high-strength cold-rolled steel sheets with a high grade ² value of 50 to 60 Kg/mm. Recently, there has been a growing trend to use high-strength steel for automobile parts to reduce weight, and tensile strength steel is used for outer panels.
High tensile strength steel plates of up to 40Kg/mm class ^{2 and} higher are now used for inner plates. Therefore, the value
Excellent workability of 1.3 or higher and tensile strength of 50
Although it is thought that a high tensile strength steel plate of Kg/mm class ² or above is required, there is currently no steel plate that corresponds to this, and furthermore, for the above applications, it is suitable for slow aging at room temperature.
It is also desired that the baking hardenability is 2 kg/mm ² or more. Conventionally, it has been considered effective to reduce the amount of C in the steel composition in order to increase the value. However, in high-tensile cold-rolled steel sheets, it is necessary to increase the strength while avoiding an increase in cost, so C: 0.03% or more has conventionally been used in many cases. On the other hand, in order to obtain a steel plate with a high value, it is known that the best method is to gradually heat it by batch annealing, but in this case, it is difficult to obtain a tensile strength of 50 kg/mm ² or more, and it is to increase strength
There was an unresolved problem that temper color tends to occur when Mn or Si, especially Si, is added in an amount of 0.5% or more, which is not desirable in practice. The object of the present invention is to eliminate the drawbacks of the prior art for obtaining a steel plate with a high value of 1.3 or more and a tensile strength of 50 Kg/mm ² or more,
Furthermore, it has slow aging properties at room temperature and bake hardenability of 2.
The present invention provides a method for producing a high tensile strength cold rolled steel sheet having a tensile strength of Kg/mm ² or more. The gist of the present invention is as follows.
That is, in terms of weight ratio, C: 0.01 to 0.029%, Si:
0.3-1.8%, Mn: 0.1-1.0%, P: 0.04-0.20%
and solAl: 0.01 to 0.20%, and (Si+
Steel having a value expressed by 4P)/Mn in the range of 1 to 13, with the remainder consisting of Fe and unavoidable impurities, is subjected to a hot rolling process and a cold rolling process, followed by a continuous annealing line at 650 to 900℃. 2-150 in temperature range
Second-annealed and cooled coils are placed in a bell-shaped annealing furnace.
A method for producing a high-strength cold-rolled steel sheet having a high value and bake hardenability, characterized by comprising a step of performing an overaging treatment of heating in a temperature range of 250 to 500°C for 0.5 to 10 hours. The numerous experiments that the present inventors conducted to arrive at the present invention and the important findings that are essential to the present invention obtained from these experiments will be described. First, regarding the reduction of C as a means of increasing the value, recently the use of bottom blowing converters has made it possible to relatively easily lower the C in molten steel to around 0.01%, reducing C. The disadvantage of significantly higher manufacturing costs due to the use of silicon is being resolved, and the problem of temper color generation is being resolved by adopting a continuous annealing method, which makes it difficult to generate temper color even when 1% or more of Si is added. It no longer inhibits strength. As described above, some of the conventional problems have been solved by recent advances in steel manufacturing technology, and it was decided to make full use of these advantages to achieve the object of the present invention. Therefore, the first goal is to increase the C content by setting it in the range of 0.01 to 0.029%, and instead of reducing the C content, add or increase Mn, P, and Si to ensure tensile strength. I experimented with the aim of First, C is 0.01 to 0.03%, P, Mn and Si
0.25% or less, 0.1-1.5% and 2.0% respectively
Aluminum-killed steel with various changes in the following range was produced, and the obtained steel ingot was hot-rolled and then cold-rolled to a thickness of 0.8 mm. This cold-rolled sheet was continuously annealed at 800°C for 1 minute and cooled to room temperature at a cooling rate of approximately 50°C/sec, and the influence of each element on the mechanical properties was investigated. The findings obtained as a result are as follows. (b) P without adding Si and keeping Mn constant at about 0.5%
Examining the influence of the content, it was found that even if approximately 0.20% of P was added, a tensile strength of 50 Kg/mm ² or more could not be obtained, and the ductility was drastically reduced, so it was meaningless to increase the amount of P any further. However, the value increases as the P content increases, so adding an appropriate amount of P of 0.20% or less is one of the essential requirements of the present invention. (b) Without adding Si and keeping P constant at about 0.10%
Examining the influence of Mn content, when the Mn content exceeds 1.0%, the tensile strength becomes 50 Kg/mm ² or more, but
The upper limit of Mn is
Should be 1.0%. However, if the Mn content is less than 0.1%, it is difficult to obtain a tensile strength of 50 Kg/mm ² or more.
The amount of Mn is 1.0%, which is within the range that does not significantly reduce the value.
The amount below should be kept at an appropriate level, and the lack of tensile strength should be solved by adding a third element. as the third element
Si is preferred. (c) Si with P constant at about 0.10% and Mn constant at about 0.5%.
As a result of investigating the effect of addition, the value and elongation decrease as the Si content increases, but the degree of decrease is small, and a tensile strength of 50 Kg/mm ² or more can be secured with an Si content of at least 0.3%. . Therefore, at least 0.3% Si is one of the essential requirements of the present invention.
However, if Si exceeds 1.8%, the value deteriorates significantly, so the upper limit should be set at 1.8%. (d) As a result of many more experiments, in order to satisfy all of the requirements of high value, high ductility, and high strength with tensile strength of 50 Kg/mm ² or more, C: 0.01 to 0.029% Si: 0.3 to 1.8% Mn: 0.1 It has been found that aluminum killed steel containing ~1.0% P:0.04~0.20% is an essential composition in the present invention. Based on the knowledge of (a), (b), (c), and (d) above, the steel plate with the above composition is melted, hot-rolled, cold-rolled, and then annealed for a short time on a continuous annealing line, and the mechanical properties are as follows: Although satisfactory, the aging properties are extremely poor, and the aging index (hereinafter abbreviated as A and I) decreases to 4 kg/mm ² or less no matter how the hot rolling, cold rolling conditions, and continuous annealing conditions including overaging treatment are changed. I didn't. Therefore, since such a steel sheet is aged at room temperature, stretcher strain occurs during press working, and even if it has excellent mechanical properties, it cannot be used for the purpose of the present invention. Another problem is that the yield point is high and the yield ratio reaches 80 to 90%, so the work hardening index n value is low. In order to solve the above two problems, we melted various steels with different compositions and repeated experiments. As a result, we were able to solve the above two problems at the same time by satisfying the following requirements. Ta. (a) In addition to satisfying the above essential requirements, the composition of the steel should include Si, P, and Mn so that the A value expressed by Si+4P/Mn=A is in the range of 1 to 13, preferably 1.5 to 8. adjust. (b) After continuous annealing, the steel plate or steel strip (hereinafter collectively referred to as steel plate) is over-aged in a bell-type annealing furnace, and the treatment conditions are set to a temperature range of 250 to 500℃.
It is assumed that the temperature is maintained for 0.5 to 10 hours, and the composition is controlled so that the higher the A value, the higher the temperature and the longer the temperature, or the higher the temperature, or the longer the temperature is maintained. The bell-shaped annealing furnace was used for annealing.
Although the process takes a long time of 0.5 to 10 hours,
This is because it is not suitable for continuous annealing furnaces. The above two requirements (a) and (b) are also essential requirements of the present invention. In addition to the above-mentioned limited amounts of C, Si, Mn, and P, regarding the composition of the high-strength cold-rolled steel sheet according to the present invention,
The content of solAl is required to be 0.01 to 0.20%, and the reasons for limiting these chemical components are as follows. C: It is well known that the lower the value of C, the higher the value, but for the purpose of the present invention, it is at least 0.029.
% or less. However, reducing it to less than 0.01% requires degassing treatment, which increases manufacturing costs, so it is set at 0.01 to 0.029%. Si: As mentioned earlier, Si is an element necessary to achieve both high strength and high ductility, and for this purpose, at least 0.3% is required, but if it exceeds 1.8%, the value will deteriorate significantly, so Si is an element necessary to achieve both high strength and high ductility. It was limited to a range of ~1.8%. Mn: Mn requires at least 0.1% or more for desulfurization and is an effective element for increasing strength, but if it exceeds 1.0%, the value decreases, so 0.1% or more
It was limited to a range of 1.0%. P: P is at least
It is necessary to contain 0.04% or more, but since it has the disadvantage of reducing ductility, the upper limit is set at 0.20%.
and the range was 0.04% to 0.20%. sol, Al: Al is an essential element for fixing N in steel as AlN, preventing N aging, and deoxidizing, and for this purpose, at least 0.01% as sol, Al.
However, if it exceeds 0.20%, alumina clusters will form and the surface quality will deteriorate.
It was limited to a range of 0.01-0.20%. In addition to the above limiting components, Si+4P/Mn=A between Si, Mn, and P... (1) The value of A expressed in equation (1) is 1 to 13, preferably
As already stated, it is an essential requirement that the value be 1.5 to 8. The steel having the above-described limited composition may be produced by an ingot-forming method or by a continuous casting method. The hot rolling conditions may be the usual conditions for rolling high-strength cold-rolled steel sheets, but the coiling temperature after hot rolling should be higher, and 650-750°C is preferable than the usual 500-600°C. . Next, there is no need to particularly limit the cold rolling conditions, but it is effective to increase the cold rolling rate to about 80% in order to obtain a high value. Next, the reason for limiting the annealing process will be explained. In the continuous annealing process, it is necessary to perform recrystallization and sufficient crystal grain growth in a short time. for that purpose
A temperature range of 650-900℃ is an essential requirement;
Below ℃, the growth of crystal grains is insufficient and the value is low.
Also, if the temperature exceeds 900℃, the value will actually decrease.
The temperature range was limited to 650-900℃. Also, the soaking time is set to 2 to 150 seconds for the same reason as above. In order to ensure a high cooling rate of the steel plate after continuous annealing, it is desirable to cool the steel plate as fast as possible economically without forming a mixed structure of ferrite and martensite. The cooled steel plate is rolled up and transferred to the overaging treatment in a bell furnace, which is an essential requirement, but there is no need to pass it through the overaging zone in the continuous annealing line; A simple continuous annealing line without overaging zones may be used. The heat pattern of overaging treatment and continuous annealing is the first
There are differences as shown in the figure. That is, continuous annealing is annealing at a high temperature for a short time and has a high cooling rate, whereas overaging treatment is a long time treatment at a low temperature of 250 to 500°C for 0.5 hours or more and 10 hours or less, and the cooling rate is low. The purpose of overaging treatment is to reduce the AI to 4Kg/mm ² or less,
The purpose is to provide baking hardenability of 2.0Kg/mm ² or more. Therefore, the holding temperature during overaging treatment is
If the temperature is less than 250℃, the temperature is too low and overaging will not proceed, and even if the holding temperature exceeds 500℃,
Since the AI obtained is almost the same as in the case of °C, the upper limit was set at 500 °C to save energy. In addition, as for the holding time in the above temperature range, at the temperature of 400 to 500℃ where overaging is most likely to proceed, at least 0.5 hour is required to keep AI below 4Kg/ ^mm2 , and more than 10 hours. If held for a long time, the baking hardenability would be lost or the degree of decrease in AI would be extremely small, so the time range was limited to 0.5 to 10 hours in consideration of economic efficiency. Here, the present inventors have found that the relationship between AI and bake hardenability, and the holding temperature and holding time in overaging treatment is affected by the component composition, and in particular, A We found that there is a close relationship with the value. That is, Si+
The relationship between the A value expressed by 4P/Mn=A, AI, and the amount of baking hardening is as shown in Figure 2, using the overaging treatment conditions as parameters, P 1 and P ₁ , respectively.
A relationship exists as shown by the P ₂ and Q ₁ , Q ₂ curves. The above four curves P ₁ , P ₂ and Q ₁ , Q ₂ are respectively 250% within the range of overaging treatment conditions.
Low temperature and short time of ℃×0.5h and high temperature of 500℃×10h.
It shows the long-term limit line. As the characteristics of the steel sheet of the present invention, it is required that the AI is 4 kg/mm ² or less and the bake hardening amount is 2 kg/mm ² or more. ~13, particularly preferably
It is an essential requirement that it be within the range of 1.5 to 8. Therefore, in actual operation, in the case of a composition with a large A value, overaging treatment is performed at high temperature and for a long time.
When the A value is small, low-temperature and short-time processing is also possible. As described above, the aging property of the steel sheet having the composition of the present invention can hardly be improved by short-time overaging treatment in a continuous annealing line, and the aging property can only be improved by overaging treatment in a bell furnace. . The reason for this has not yet been elucidated, but it is thought to be as follows. That is, the progress rate of overaging generally changes depending on the degree of C supersaturation and the number of precipitation sites. However, in the range of 0.01 to 0.029% of the C content of the present invention, the amount of cementite that serves as a precipitation site is small, and overaging is inherently difficult to progress, but when Si and P are small, overaging occurs as conventionally known. proceed. However, in the present invention, Si and P are contained in large amounts, and especially when the A value is 1 or more, the solid solubility limit of C in ferrite is high and the degree of supersaturation of C is lower. This is thought to be due to the fact that diffusion and precipitation of C takes a long time. Example Twelve types of steel having the compositions shown in Table 1 left were melted in a bottom blowing converter and hot-rolled at a finishing temperature of 890°C and at a coiling temperature.

[Table] Hot rolled at 710°C to a thickness of 3.2mm, then cold rolled.
Finished with a thickness of 0.75mm. Continuous annealing is performed by holding the temperature at 850°C for 60 seconds, then cooling it to near room temperature by forced air cooling, and winding it into a coil.
The temperature and time shown in the table were maintained, and the mixture was cooled in the furnace. After this
The mechanical properties and aging characteristics of products subjected to a 1.5% skin pass are shown in the right half of the same table.
In Table 1, conditions or characteristics outside the limitations of the present invention are underlined. No. 1 and No. 2 steels do not have compositions based on the present invention, and are comparison steels that either lack tensile strength or have low tensile strength, and exceed the low temperature and short time limit conditions of 250°C x 0.5 h. The baking hardenability has been lost due to aging treatment. No. 3 to No. 5 steels are the steels of the present invention, and the values are
It is high at 1.4-1.6, and the baking hardening amount is 3.5-4.7Kg/
It has mm ² . No. 6 steel has the composition of the present invention.
It passed through an over-aging treatment zone at 400℃ for 5 minutes, had a high AI of 7.2Kg/ ^mm2 , and was aged at room temperature. No.7
Steel also has a composition included in the present invention, but contrary to No. 6 steel, this is an example in which the overaging treatment was performed at high temperature for a long period of time, so that the bake hardenability was almost completely lost. No.
Steels No. 8 and No. 9 are steels of the present invention, and the amount of bake hardening is larger than steels No. 3 to No. 5. No. 10 steel is also a steel of the present invention, and has a tensile strength of 60 Kg/mm ^2nd grade and a large amount of baking hardening. No.11 and No.12 steel have tensile strength of 60
Kg/mm ² grade, but the value is low, and the AI does not fall below 4 Kg/mm ² even after over-aging treatment at high temperatures for a long time. The steel of the present invention has high values of 1.3 or more, has a large elongation despite its high tensile strength, has a yield ratio of about 70%, and has an AI of 4.
Kg/mm ² or less and bake hardenability is 2 Kg/mm ² or more, and it is possible to produce a high-strength cold-rolled steel sheet that satisfies these requirements only by the method of the present invention. As is clear from the above examples, in the method for manufacturing high-strength cold-rolled steel sheets according to the present invention, C, Si,
In addition to limiting the component compositions of Mn, P and solAl, the A value expressed by Si+4P/Mn was limited to a range of 1 to 13, preferably 1.5 to 8, and continuous annealing conditions were regulated. By performing low-temperature, long-term overaging treatment in a bell furnace, it has become possible to produce high-strength cold-rolled steel sheets with high values and bake hardenability that have the following excellent effects. (a) is always stable and has excellent workability of 1.3 or higher,
And the aging index (AI) is 4Kg/ ^mm2 or less, slow aging at room temperature, and the baking hardenability is 2Kg/mm2 or ^more .
It has become possible to produce high-strength cold-rolled steel sheets with a tensile strength of 50 to 60 kg/mm ² . (b) The fact that the steel of the present invention has a yield ratio of about 70% also proves its excellent workability. (c) Currently, the proportion of cold-rolled steel sheets manufactured by continuous annealing lines is increasing, and as a result, there is a fear that the conventionally used bell furnaces will become idle or the operating rate will decrease. It paved the way for its use. (d) In the present invention, an ideal process combination is used: high temperature and short time for recrystallization and grain growth, and low temperature and long overaging treatment for improving aging properties and yield ratio. It can be implemented by

[Brief explanation of the drawing]

Figure 1 is a time-temperature curve showing the heat pattern of continuous annealing and overaging treatment in the manufacturing process of high-strength cold-rolled steel sheets according to the present invention, and Figure 2 is one of the requirements of the present invention, Si + 4P / Mn = A A represented by
FIG. 3 is a correlation diagram showing the influence of overaging treatment conditions on the aging index (AI) and the amount of baking hardening.

Claims (1)

[Claims] 1 C: 0.01-0.029%, Si: 0.3-1.8 by weight ratio
%, Mn: 0.1-1.0%, P: 0.04-0.20% and
Contains solAl: 0.01~0.20% and (Si +
Steel having a value expressed by 4P)/Mn in the range of 1 to 13, with the remainder consisting of Fe and unavoidable impurities, is subjected to a hot rolling process and a cold rolling process, followed by a continuous annealing line at 650 to 900℃. 2-150 in temperature range
Second-annealed and cooled coils are placed in a bell-shaped annealing furnace.
A method for producing a high tensile strength cold-rolled steel sheet having a high value and bake hardenability, the method comprising the step of performing an overaging treatment by heating in a temperature range of 250 to 500°C for 0.5 to 10 hours.