JPH025812B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides a high-strength cold-rolled steel sheet having a tensile strength of 35 Kg/mm ² or more, excellent formability and seizure hardening ability, and suitable for outer and inner panels of automobiles, and the production thereof. It is about law. For several years now, attempts have been made to apply high-strength cold-rolled steel sheets to the interior and exterior of automobiles in order to reduce fuel consumption and provide safety measures for automobiles, but these have resulted in breakage during forming, surface defects such as wrinkles, and large spring back. However, due to drawbacks such as poor shape and difficulty in spot welding, full-scale adoption was not achieved. Recently, P-added Al-killed steel, ferrite + martensitic steel [so-called dual phase]
Steel sheets with excellent formability have been developed and some of them have come into practical use, but dual-phase steel has the advantage of having a low yield ratio and BH properties. Because of its low thermal resistance, it is hardly ever put into practical use as a forming steel sheet. Therefore, several attempts have been made to improve the deep drawability (value) of dual-phase steel, but (1) when martensite is introduced into ferrite, the value is significantly lowered, and the value is 1.5 or more. A high r value is difficult to obtain. (2) It is difficult to stably obtain grade ² steel of 35-40 kg/mm because of its high strength, but this problem remains unsolved. Therefore, the present inventor has accumulated basic research and detailed experiments on a wide range of composite structure steels without being particular about ferrite + martensitic structure steels, and has found that making the second phase bainite is essential for improving the value. Yes, this also has stretch flangeability,
They discovered that it has excellent BH properties, etc., and completed the present invention. The high strength cold rolled steel sheet according to the present invention is C: 0.02~
Contains 0.1% by weight, Mn: 0.02~0.55% by weight, and further
Si: 0.02-0.5% by weight, P: 0.02-0.09% by weight 1
species or two species, and further Al: 0.01 to 0.1% by weight, B:
Contains 0.0002 to 0.005% by weight of one or two types,
S: It is regulated to 0.01% by weight or less and the balance consists of iron and impurities, and the main feature is that the structure after continuous annealing is a ferrite structure containing bainite in an area ratio of 5 to 50%, and has good deep drawability. It has excellent stretch flangeability and bake hardenability. In addition, when manufacturing such high-strength cold-rolled steel sheets, steel satisfying the above-mentioned composition is cold-rolled, then heated to a holding temperature between the Ac ₁ transformation point and the Ac ₃ transformation point, and then average cooled. The first point is to rapidly cool down to below 500°C at a rate of 50 to 500°C/sec. In this process, some modifications are added, one of which is that the heating rate is 5° C./second or more and the holding time at the holding temperature is 5 minutes or less. Second, in the heating process, the temperature is gradually heated at a rate of 10°C/second or less until the temperature range is higher than the recrystallization temperature and lower than the holding temperature, and finally, the temperature is maintained at 5°C or lower. Part 3 is to cool down from the holding temperature to the Ar ₁ transformation point at a rate of 5 to 40°C/sec, particularly in the cooling process. In the high-strength cold-rolled steel sheet of the present invention, it is important to introduce an appropriate amount of bainite as the second phase. In other words, Figure 1 shows the relationship between the area ratio of the second phase and the value for different types of composite structure steels, one in which martensite is introduced as the second phase and the other in steel with bainite (which may include some martensite). It is a figure showing the result of investigation. This first
As can be seen from the figure, conventional dual-phase steel with martensite introduced as the second phase (marked with ●)
In contrast, in ferrite + bainite steel (marked with Γ) in which bainite is introduced as the second phase, the r value does not deteriorate even with the introduction of the second phase, and the value deteriorates significantly as the amount of martensite increases. Normal ferrite (or ferrite + pearlite) steel
It shows a value comparable to that of 700℃ annealed material, ≧
It can be seen that it exhibits excellent formability exceeding 1.5. Moreover, FIG. 2 is a diagram showing the relationship between the impact transition temperature and the second phase area ratio after deep drawing (drawing ratio 2) for the two types of composite structure steels shown in FIG. As can be seen, the transition temperature after forming of ferrite + bainite steel (marked by Γ) maintains a good value compared to ferrite + martensitic steel (marked by ●) even if the second phase area ratio increases. are doing. In the high-strength cold-rolled steel sheet of the present invention, the area ratio of bainite must be 5% or more, and if it is less than this, it is inherently a composite structure steel with excellent properties such as high strength, low yield ratio, and seizure hardening resistance. characteristics are lost. Furthermore, if the bainite area ratio exceeds 50%, the r value deteriorates and the shock transition temperature increases significantly, so the upper limit is set to 50%. Note that the desirable bainite area ratio is 8 to 20%. On the other hand, in the high-strength cold-rolled steel sheet of the present invention, it is permissible to include a small amount of martensite in addition to bainite, which is desirable in terms of a low yield ratio and improved elongation, but if a large amount is introduced, the above-mentioned It is recommended to limit the amount to 15% or less, preferably less than the amount of bainite, that is, 8% or less, as it may cause deterioration of the through value. As for the state of existence of martensite, it is desirable that martensite is finely dispersed, scattered around bainite, and in direct contact with ferrite. The ferrite referred to in the present invention is preferably a polygonal ferrite, and the bainite includes bainitic ferrite and bainite containing carbides. Next, the chemical composition of the high-strength cold-rolled steel sheet according to the present invention will be described. C is required to be present in an amount of 0.02% by weight or more in order to exhibit the effects of strengthening and improving hardenability, forming bainite, etc., and to make the structure of spot welds sound. However, if there is too much, the value will decrease,
Since it significantly reduces cold workability and significantly hardens spot welds, the upper limit is set at 0.1% by weight.
In addition, when particularly excellent cold workability is required,
The content is preferably 0.07% by weight or less. Mn is an element necessary to prevent red heat embrittlement caused by S and to increase hardenability and obtain a desired structure, so it is necessary to have an amount of 0.02% by weight or more. However, since Mn significantly reduces the value,
The upper limit is set to 0.55% by weight. Note that if a particularly high content is required, it is desirable to set the content to 0.4% by weight or less. Since S deteriorates stretch flangeability and inhibits grain growth during continuous annealing, reducing the value, the upper limit is set to 0.01% by weight, preferably 0.008% by weight. Si and P are suitable elements for promoting the concentration of C in austenite, stabilizing austenite, facilitating the formation of bainite etc. in the heat treatment process, and providing high strength and high ductility. However, if Si is less than 0.02% by weight and P is less than 0.02% by weight, the effect will not be sufficient, and Si
When P exceeds 0.5% by weight and P exceeds 0.09% by weight, the above effects not only reach saturation, but also a large amount of Si deteriorates the surface quality, and a large amount of P makes the steel brittle.
The content of Si is 0.02 to 0.5% by weight, and the content of P is 0.02 to 0.09% by weight, respectively. The present invention is characterized in that it actively contains Si and/or P, which have traditionally been considered unavoidable impurities, within the above range. If the other component is contained in the amount of impurities, the effects of the present invention can be sufficiently exhibited. Both Al and B fix N and help prevent aging, but this effect is not sufficient if Al is less than 0.01% by weight and B is less than 0.002% by weight, while Al is 0.1% by weight.
or B exceeds 0.005% by weight, inclusions increase and elongation and toughness deteriorate. Therefore Al
is 0.01 to 0.1% by weight, and B is 0.0002 to 0.005% by weight. Further, since oxygen forms oxides and inhibits grain growth, the content is preferably 0.05% by weight. Next, the manufacturing conditions for the high-strength cold-rolled steel sheet of the present invention will be explained with reference to the schematic diagram in FIG. 3. In Fig. 3, a cold-rolled steel plate of a predetermined composition is first rapidly heated at a heating rate _h1 to a temperature _T2 in the (α+γ) two-phase region of the _Ac1 to _Ac3 transformation point, and held at the temperature _T2 for t hours. . The purpose of this heating process is to form a recrystallized texture and improve the r value.The reason why the heating rate _h1 of the present invention is set to 5°C/second or more is that if the heating rate is too slow, cementite This is because solid solution occurs and the solid solution carbon prevents the formation of {111} recrystallized texture. Furthermore, the reason why T ₂ was set as the Ac ₁ to Ac ₃ transformation point and t was set as 5 minutes or less is to make austenite appear in this step and prepare for forming a composite structure steel. Note that T ₂ is preferably on the high temperature side of the (α+γ) 2-set range. In addition, in the heating process up to temperature T ₂ , rapid heating is performed at h ₁ =5° C./sec or more as described above until temperature T ₁ is higher than the recrystallization temperature and lower than the holding temperature (T ₂ ), and then T ₁ to
Slow heating between T ₂ at a heating rate h ₂ =10° C./second or less is recommended as a heating control means for obtaining a more suitable recrystallized texture.
The reason for this is that selective growth of {111} recrystallization becomes more possible by slow heating. After maintaining the temperature T ₂ for a predetermined time t, T ₂ or less Ar ₁
Slow cooling is performed at an average cooling rate c ₁ to a temperature T ₃ in the transformation point range. This process concentrates the solute carbon in the ferrite into the remaining austenite to stabilize the austenite, and the ferrite, which has less solid solute carbon, is thereby purified and the ductility is improved. It will be done. This slow cooling is also a preparatory step to obtain the desired proportion of the second phase, c ₁
is 5 to 40°C/sec. Furthermore, instead of this slow cooling process,
T ₂ =T ₃ , that is, the holding time at the holding temperature may be made longer. Alternatively, the temperature may be raised to T ₂ and then slowly cooled to T ₃ . From temperature T ₃ (or T ₂ ) to T ₄ is a rapid cooling process. This is a process for transforming untransformed austenite with concentrated carbon into bainite (or bainite + martensite) as described above, and c ₁
A faster cooling rate is required, but if it is too fast, a large amount of martensite will be produced, so the average cooling rate c ₂ here is set to 50 to 500°C/sec. Also
T ₄ is below 500°C, which is necessary for bainite transformation. Note that after this quenching process, an overaging treatment is performed as necessary. Further, in the quenching process, any method such as a gas jet cooling method, a water cooling roll method, a boiling water injection (or immersion) method, a heat pipe method, etc. can be adopted. Example A 30 mm ^t slab obtained by melting the test materials shown in Table 1 in a vacuum melting furnace and rough rolling was made into a 2.8 mm ^t hot rolled plate in 3 baths. This hot-rolled sheet was cold-rolled to obtain a cold-rolled sheet of 0.8 mm ^t . This cold rolled sheet was subjected to continuous annealing under the conditions shown in Table 2 to obtain steel sheets having various structures. Table 3 shows the results of microstructural observation and mechanical property measurements of this steel plate.

【table】

【table】

[Table] As can be seen from Table 3, both specimens 1 and 2, which are steels of the present invention, have values of 1.5 or more, and have excellent stretch flangeability (hole expansion ratio) and bake hardenability (△ σyBH) also shows a value of 5 Kg/mm ² or more. It has also been confirmed that the steel of the present invention has good spot weldability, fatigue strength, and toughness.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the second phase area ratio and value for ferrite + martensitic steel and ferrite + bainite (+ martensite) steel obtained by continuously annealing low carbon aluminum killed steel sheets with various heat treatment patterns. , FIG. 2 is a diagram showing the relationship between the second phase area ratio and the impact transition temperature after deep drawing (drawing ratio 2) for the steel shown in FIG. 1. Further, FIG. 3 is a schematic diagram showing the heat treatment conditions employed in the present invention.

Claims (1)

[Claims] 1 Contains C: 0.02 to 0.1% by weight, Mn: 0.02 to 0.55% by weight, further Si: 0.02 to 0.5% by weight, and P: 0.02 to 0.5% by weight.
One or two selected from 0.09% by weight, and
Al: 0.01-0.1% by weight, B: 0.0002-0.005% by weight
S: regulated at 0.01% by weight or less, with the balance consisting of iron and impurities, and the structure after continuous annealing is a ferrite structure containing bainite in an area ratio of 5 to 50%. A high-strength cold-rolled steel sheet with excellent deep drawability, stretch flangeability, and bake hardenability. 2 Contains C: 0.02 to 0.1% by weight, Mn: 0.02 to 0.55% by weight, furthermore Si: 0.02 to 0.5% by weight, P: 0.02 to
One or two selected from 0.09% by weight, and
Al: 0.01-0.1% by weight, B: 0.0002-0.005% by weight
Contains one or two selected from S: 0.01% by weight
When performing continuous annealing after cold rolling of steel _consisting of balance iron and impurities according to the following regulations, an average heating rate of ₅ ℃/
Rapidly heat for more than seconds, hold in this temperature range for less than 5 minutes, then cool to 500℃ at an average cooling rate of 50 to 500℃/second
By rapidly cooling to below, the area ratio is 5 to 50%.
A method for producing a high-strength cold-rolled steel sheet with excellent deep drawability, stretch flangeability, and bake hardenability, which is characterized by a ferrite structure containing bainite. 3 Contains C: 0.02 to 0.1% by weight, Mn: 0.02 to 0.55% by weight, furthermore Si: 0.02 to 0.5% by weight, P: 0.02 to
One or two selected from 0.09% by weight, and
Al: 0.01-0.1% by weight, B: 0.0002-0.005% by weight
When carrying out continuous annealing after cold rolling of steel containing one or two selected from the following, S: 0.01% by weight or less and the balance consisting of iron and impurities, between Ac ₁ transformation point and Ac ₃ transformation point. When heating to the holding temperature, rapidly heating to a temperature range above the recrystallization temperature and below the holding temperature at an average heating rate of 5 ° C / sec or more, followed by slow heating to the holding temperature at an average heating rate of 10 ° C / sec or less,
Hold at the holding temperature for 5 minutes or less, then reduce the average cooling rate.
By rapidly cooling to below 500℃ at 50 to 500℃/second, a ferrite structure containing bainite with an area ratio of 5 to 50% is created.It has excellent deep drawability, stretch flangeability, and bake hardenability. A method for manufacturing high-strength cold-rolled steel sheets. 4 Contains C: 0.02 to 0.1% by weight, Mn: 0.02 to 0.55% by weight, furthermore Si: 0.02 to 0.5% by weight, P: 0.02 to
One or two selected from 0.09% by weight, and
Al: 0.01-0.1% by weight, B: 0.0002-0.005% by weight
When continuous annealing is performed after cold rolling of steel containing one or two selected from S: 0.01% by weight or less and the balance consisting of iron and impurities, Ac ₁ ~
Rapid heating to the temperature range of Ac ₃ transformation point at an average heating rate of 5°C/second or more, holding in this temperature range for 5 minutes or less, and then cooling from the holding temperature of Ac ₁ transformation point to Ac ₃ transformation point. Temperature below _Ar A method for producing high-strength cold-rolled steel sheets with excellent deep drawability, stretch flangeability, and bake hardenability, characterized by a ferrite structure containing 50% bainite. 5 Contains C: 0.02 to 0.1% by weight, Mn: 0.02 to 0.55% by weight, further Si: 0.02 to 0.5% by weight, P: 0.02 to
One or two selected from 0.09% by weight, and
Al: 0.01-0.1% by weight, B: 0.0002-0.005% by weight
Contains one or two selected from, and S: 0.01
Ac ₁
~ Ac ₃ When heating to a holding temperature between the transformation points, rapidly heat at an average heating rate of 5°C/second or more to a temperature range above the recrystallization temperature and below the holding temperature, and then heat at an average heating rate of 10°C/second or less to the holding temperature. When slowly heating at a temperature range of 5 minutes or less, and then cooling from a holding temperature of Ac ₁ transformation point to Ac ₃ transformation point,
5 to 40 at temperatures above the Ar ₁ transformation point below the holding temperature
Average cooling rate after slow cooling at an average cooling rate of °C/sec
By rapidly cooling to below 500℃ at 50 to 500℃/second, a ferrite structure containing bainite with an area ratio of 5 to 50% is created.It has excellent deep drawability, stretch flangeability, and bake hardenability. A method for manufacturing high-strength cold-rolled steel sheets.