JPH0524979B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to high-strength cold-rolled thin steel sheets used for forming processing such as automobile bodies, and a method for manufacturing the same. The present invention relates to a high tensile strength thin steel sheet that has a significantly high so-called bake hardenability, in which the hardenability of the steel plate increases significantly, and a method for manufacturing the same. BACKGROUND TECHNOLOGY High tensile strength thin steel plates have been widely used for automobile body exterior panels and the like in order to reduce the weight of the vehicle body. High-strength thin steel sheets for automobiles are subjected to forming processes such as press working, so they must be relatively soft and have good formability. Therefore, recently, steel sheets that are relatively soft during forming and whose strength increases through age hardening during the painting and baking process after forming, that is, bake-hardenable steel sheets, have been developed. Larger steel plates are being used. The amount of bake hardening, which is an index of bake hardenability, is generally measured as follows. That is, first, a pre-strain of about 2%, which corresponds to press forming, is applied, and then a 170°C x 20
Heat treatment for 1 minute. Then, calculate the difference between the deformation stress at 2% pre-strain and the yield stress after heat treatment,
This value is defined as the amount of hardening by baking. By the way, there are various conventional steel plates with bake hardenability, but recently, as a high strength, high ductility steel plate with particularly high bake hardenability, nitrogen ( Quenched and tempered high N steel with increased N) content is known, and for example, "Journal of the Japan Institute of Metals"
19 (1980) P439 or “Bulletin of the Japan Institute of Metals” 19
(1980) P10, "Tetsu to Hagane" Vol68 (1982) No.9,
Dual phase steel sheets such as those described in P1348 are known. Problems to be Solved by the Invention In conventional general steel plates having bake hardenability, the bake hardening amount is only about 5 kgf/mm ² .
The above-mentioned special public relations company 1977- which has particularly high bake hardenability.
Even the steel plate described in Publication No. 48575 is 12 Kgf/mm ² or less, and the high N steel described in that publication has the problem that it is not easy to adjust the N amount, there is large variation in material quality, and the tensile strength (TS) is 60Kgf/
It was only less than mm ² . On the other hand, even with conventional dual-phase steel sheets, the bake hardening amount was less than 10 Kgf/mm ² . Furthermore, in conventional steel sheets, if bake hardenability is to be further improved, the problem of material deterioration due to room temperature aging cannot be avoided. This invention was made against the background of the above-mentioned circumstances, and exhibits higher bake hardenability than conventional ones, specifically, a high bake hardening amount of 15 Kgf/mm ² or more,
Moreover, it is an object of the present invention to provide a high-tensile cold-rolled thin steel sheet that exhibits almost no aging deterioration at room temperature and has good workability. Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have conducted various experiments and studies, and found that the structure of the steel sheet can be improved by controlling the annealing cycle after cold rolling. By using a bainite-based structure, which is not used in thin steel sheets, we can achieve a bake hardening amount of 15 kgf/mm ² that could not be achieved with conventional thin steel sheets.
The inventors have discovered that it is possible to obtain a thin steel sheet that has extremely high bake hardenability, good workability, and little aging deterioration at room temperature, and has led to the present invention. Specifically, the bake-hardenable high-tensile air-cooled thin steel sheet of the first invention of the present application contains 0.08 to 0.20 C by weight%.
%, contains 1.5 to 3.5% Mn, with the balance consisting of Fe and unavoidable impurities, and the steel structure is composed of uniform bainite with a ferrite content of 5% or less or bainite containing some martensite. This is a characteristic feature. In addition, the bake hardenable high tensile strength cold rolled thin steel sheet of the second invention contains C0.08~0.20%, Mn1.5~3.5%, Nb0.01~0.10%, Ti0.01~0.10%, B5 ~
30 ppm, the remainder consists of Fe and unavoidable impurities, and the steel structure is uniform bainite with a ferrite content of 5% or less or bainite containing some martensite as described above. It is characterized by being configured. Furthermore, a third invention provides a method for manufacturing a bake-hardenable high-strength cold-rolled steel sheet, in which steel containing 0.08 to 0.20% C and 1.5 to 3.5% Mn by weight is produced by a conventional method. In applying continuous annealing after hot rolling and cold rolling to the desired thickness,
The thermal temperature for annealing should be ₃ points Ac or more, ₃ points Ac + 100℃ or less, and 400℃ or less in the cooling process after annealing200
It is characterized by rapid cooling at a cooling rate of 20° C./sec or more up to a temperature range of 0.5° C. or higher, followed by gradual cooling at a cooling rate of 0.5° C./sec or lower. Function First, the reason for limiting the steel composition in this invention will be explained. If C is less than 0.08%, the transformation rate of γ → α is high, and it is difficult to reduce the amount of ferrite to 5% or less by continuous annealing. The purpose of this invention, which is to obtain On the other hand, if C exceeds 0.20%, it becomes difficult to spot weld steel sheets for automobiles, which is the main object of this invention, and there is a problem that the spot welding strength decreases. Therefore, the amount of C was set within the range of 0.08 to 0.20%. Mn is an element that suppresses ferrite transformation and facilitates the appearance of bainite, and is an essential element in the steel sheet of this invention. If Mn is less than 1.5%, extremely high bake hardenability cannot be obtained. In this way Mn1.5%
Although it is not yet clear why very high bake hardenability cannot be obtained at lower temperatures, it is probably due to the interaction between Mn and C. On the other hand, Mn is 3.5
%, the bake hardenability will not increase any further, and there is no particular advantage to adding it. Therefore Mn
was within the range of 1.5 to 3.5%. The basic essential components of the steel sheet of this invention are the above-mentioned C and Mn, but in particular, in the second invention, in addition to C and Mn, one or more of Nb, Ti, and B are included. do. The effect and reason for limiting the amount added are as follows. Nb is an effective element for increasing strength while maintaining workability. The hardening becomes noticeable when 0.01% or more is added, but it is almost saturated at 0.10%, so the amount of Nb added was set within the range of 0.01 to 0.10%. Also, the effect of Ti and the reason for limiting the amount added are Nb.
It is similar to B is effective in suppressing ferrite transformation in extremely small amounts, but if added less than 5 ppm, no hardening occurs.
Further, the curing reaches saturation at about 30 ppm. Therefore, when B is added, the amount added is within the range of 5 to 30 ppm. Note that Nb, Ti, and B are two or three of them.
There is no particular problem even if seeds are added in combination, so it was decided to add one kind or two or more kinds. Furthermore, in the steel plate of the present invention, it is extremely important not only to limit the steel components as described above, but also to specify the steel structure to be mainly bainite. That is, it is necessary to have a bainite-based structure with a ferrite content of 5% or less in order to obtain extremely high bake hardenability. This fact was discovered through the following experiments conducted by the inventors. A steel containing 0.11% C, 3.0% Mn, and 0.04% Nb, with the balance consisting of Fe and unavoidable impurities, is hot rolled, pickled, and cold rolled according to a conventional method, and then Steel plates with various amounts of ferrite phase fraction (the remainder is bainite) were obtained by heat treatment under various conditions equivalent to continuous annealing, and the bake hardening amount of each was investigated. It was found that there is a relationship between the amount and the amount as shown in FIG. As is clear from Fig. 1, if the ferrite phase fraction is 5% or less and the remainder is bainite phase, 15Kgf/
A significantly large bake hardening amount of mm ² or more can be obtained.
The reason why such a large amount of bake hardening is obtained in the case of a structure consisting mainly of bainite phase is still not clear, but compared to the ferrite phase, the bainite phase contains a much larger amount of solid solution C, and also has a higher dislocation density. Therefore, the deformation stress after strain aging is also considered to be larger than that of the ferrite phase, and this means that the deformation stress after strain aging is higher in a bainite-based structure than in a ferrite-based structure. It is estimated that the amount of baking hardening increases. It is most desirable that the remainder of the 5% or less ferrite phase be entirely a uniform bainite phase, but a bainite phase partially containing a martensitic phase may be used without any practical problems. However, in the latter case, the martensite phase is preferably 30% or less. In order to obtain the above-mentioned structure consisting mainly of bainite phase, continuous annealing conditions after hot rolling and cold rolling, especially the cooling conditions, are important. Therefore, the reasons for limiting the continuous annealing conditions will be explained next. First, the annealing temperature (soaking temperature after heating temperature rise) is Ac ₃
temperature above the point is required. This is necessary in order to maintain the steel structure as a uniform austenite structure before starting rapid cooling in the cooling process after continuous annealing.
However, if the temperature exceeds the Ac ₃ point + 100°C, the austenite structure becomes coarse, and the structure obtained after cooling also becomes coarse, making the material undesirable.
Therefore, the annealing temperature is Ac ₃ points or more, Ac ₃ points + 100℃
The following was made. After austenitizing in this way, it is necessary to suppress the ferrite transformation and promote the bainite transformation by rapid cooling to a certain level.
It is necessary to perform rapid cooling at a cooling rate of sec or more.
FIG. 2 shows the relationship between cooling rate and various materials.
This experiment contained 0.13% C, 2.8% Mn, and the remainder
Regarding steel made of Fe and unavoidable impurities,
After hot rolling, pickling, and cold rolling to a plate thickness of 1.0 mm according to a conventional method, the plate was soaked at 870°C for 180 seconds, and the cooling rate from that temperature to 300°C was varied.
Subsequently, the amount of bake hardening, elongation, and tensile strength were investigated when the material was slowly cooled at a cooling rate of 0.5℃/sec, and the relationship between the cooling rate up to 300℃ and each material is shown in Figure 2. be. As is clear from FIG. 2, when the cooling rate to 300°C is less than 20°C/sec, the amount of bake hardening is small and the tensile strength is also low. This is because bainite transformation was not sufficiently promoted at temperatures below 20°C/sec. Therefore, after heating and soaking from Ac ₃ points to Ac ₃ points + 100℃,
Rapid cooling is required at a cooling rate of 20°C/sec or higher. In addition, such rapid cooling of 20℃/sec or more
It is necessary to stop within the temperature range of 200℃. If the quenching stop temperature is higher than 400°C, bainite transformation will not be promoted sufficiently, resulting in poor bake hardenability.If the quenching stop temperature is too high, pearlite transformation will occur during subsequent slow cooling, resulting in sintering of the low-temperature transformed phase. Strength decreases due to undoing. On the other hand, if the quenching stop temperature is too low, martenside will be excessive and the self-tempering effect cannot be expected, leading to deterioration of ductility in particular.
This becomes noticeable when the temperature is lower. Therefore 20℃/
Rapid cooling at a cooling rate of sec or more should be performed to a temperature within the temperature range of 400°C or lower and 200°C or higher, and then slow cooling is required. The slow cooling after stopping the rapid cooling must be performed at a cooling rate of 0.5°C/sec or less. This is because if the cooling rate exceeds 0.5°C/sec, the elongation deteriorates significantly and moldability is impaired. There is no particular upper limit to the cooling rate during rapid cooling of 20℃/sec or more after soaking from Ac ₃ points to Ac ₃ points + 100℃, but if the cooling rate is too high, a large amount of martensite will be produced. I get used to it. In order to suppress the excessive production of martensite and create a bainite-based structure, it depends on the ingredients, but approximately 100%
It is preferable to set the cooling rate to approximately C/sec or less. By appropriately setting the continuous annealing conditions as described above, it is possible to achieve a bake hardening amount of 15 Kgf/mm ² or more with a uniform bainite structure with a ferrite content of 5% or less or a bainite-based structure containing some martensite. Furthermore, a high tensile strength cold rolled steel sheet having high tensile strength and good workability can be obtained. Examples Steels having various compositions shown in Table 1 A to H were melted, hot rolled at a finish rolling temperature of 800 to 900°C in accordance with a conventional method, and coiled at 450 to 600°C. pickling,
Further cold rolling was performed to obtain a cold rolled steel plate with a thickness of 1.00 mm. Next, continuous annealing was performed under various conditions shown in Table 2, and the material properties after annealing and cooling were investigated. In addition to examining the ferrite fraction (Vf), we also examined the strength of spot welded joints. The results are shown in Table 2. In Table 1, "primary cooling rate" represents the average cooling rate from the soaking temperature to the primary cooling stop temperature, and "secondary cooling rate" represents the average cooling rate from the primary cooling rate stopping temperature to near room temperature.

【table】

【table】

[Table] In Table 2, steel A whose composition is within the range of this invention (Ac ₃ point temperature approximately 830°C) is subjected to continuous annealing.
In the case of No. 3 applied at a soaking temperature of less than Ac ₃ points and
No. 4 with a low cooling rate of 5°C/sec up to 300°C
In both cases, the ferrite fraction is 80% and 85%.
This was so high that a sufficient amount of baking hardening could not be obtained.
Also, for the same steel A, the secondary cooling rate after quenching was 3.
In the case of No. 2, which had a high temperature of °C/sec, it was found that the elongation was not sufficient and the workability was poor. On the other hand, steel B whose composition is within the range of this invention (Ac ₃ points approximately 820°C)
Regarding No. 6, which was subjected to continuous annealing at Ac ₃ points + soaking temperature exceeding 100°C, it was found that elongation was low and workability was poor. Furthermore, the amount of C in the steel is 0.25
% (No. 9), it was found that the spot welding strength was insufficient even if the manufacturing conditions were within the scope of the present invention. In addition, steel F with a low Mn content of 1.00% (No.10) and steel G with a low C content of 0.05% (No.12)
All of them had a high ferrite fraction, were unable to obtain a sufficient amount of bake hardening, and had low strength. Even when the quenching stop temperature T ₂ of Steel B within the composition range of this invention was higher than 400° C. (No. 11), the ferrite fraction was high and a sufficient amount of bake hardening could not be obtained. On the other hand, if the component composition satisfies the conditions of this invention and the manufacturing conditions also satisfy this invention (No.
1, No. 5, No. 7, No. 8, and No. 13), the steel structure is composed mainly of bainite with a ferrite content of 5% or less, and in all of these cases, the steel structure has a bainite-based structure with a ferrite content of 5% ^{or less} . A good amount of bake hardening was obtained, sufficient elongation and good workability, and high tensile strength of 80 Kgf/mm ² or more was obtained. Furthermore, there was no shortage of strength during spot welding. Effects of the Invention As is clear from the above explanation, the present invention has an extremely large bake hardenability of 15 Kgf/mm ² or more, and has good workability and a hardness of 80 Kgf/mm2.
A high-tensile cold-rolled thin steel sheet with a high strength of f/mm ² or more can be obtained, and therefore it is used for applications such as automobile door impact beams, which require painting and baking after forming, and which also require high strength. It is useful to apply it to

[Brief explanation of the drawing]

Figure 1 shows the ferrite phase fraction (remainder bainite)
Fig. 2 is a correlation diagram showing the relationship between the cooling rate up to 300℃ after soaking during continuous annealing and the material quality (bake hardening amount, elongation, tensile strength). be.

Claims (1)

[Claims] 1. Contains 0.08 to 0.20% of C and 1.5 to 3.5% of Mn by weight, with the balance consisting of Fe and unavoidable impurities, and the steel structure has a ferrite content of 5.
% or less of uniform bainite or bainite partially containing martensite, and has a bake hardening amount of 15 Kgf/mm ² or more. 2 Contains C0.08-0.20%, Mn1.5-3.5% in weight%, and Nb0.01-0.10%, Ti0.01-0.10%, B5
~30ppm, the balance is Fe and unavoidable impurities, and the steel structure is uniform bainite or partial martensite with a ferrite content of 5% or less. Constructed of bainite, baking hardening amount is 15kg
A bake hardenable high tensile strength cold rolled thin steel sheet having f/mm ² or more. 3 After hot rolling and cold rolling a steel containing 0.08 to 0.20% of C and 1.5 to 3.5% of Mn by weight to obtain the required thickness, when continuously annealing the steel, the annealing soaking temperature is be within the range of Ac ₃ points or more and Ac ₃ points + 100℃ or less, and in the cooling process after annealing, rapidly cool to a temperature within the temperature range of 200℃ or more and 400℃ or less at a cooling rate of 20℃/sec or more, and then 0.5 from that temperature
A method for producing a bake-hardenable high-strength cold-rolled steel sheet, characterized by slow cooling at a cooling rate of ℃/sec or less.