JPH0548284B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a cold rolled steel sheet having excellent bake hardenability and warp cracking resistance and a high R value.
It has excellent bake hardenability (BH) and warp cracking resistance after deep drawing, especially for cold rolled steel sheets used in parts that require high tensile strength and ultra-deep drawing workability, such as automobile exterior panels. The present invention relates to a method of manufacturing a cold-rolled steel sheet for ultra-deep drawing. [Background of the Invention] Increasing the tensile strength of steel deteriorates workability such as deep drawing and forming. Therefore, methods are being considered to perform forming such as deep drawing in a state with low strength and good workability, and to age harden the steel in a heat treatment process such as baking painting to give it a predetermined high tensile strength. Steel plates and methods for manufacturing the same that can be applied to such methods have also been developed. Manufacturing methods for such steel sheets include (1) using a ferrite + martensitic structure, (2) using phosphorus-added Al-killed steel, and (3) using ultra-low C steel with carbides such as Ti and Nb. The BH properties of steel are improved by adding a small amount of forming elements, nitride-forming elements, or carbonitride-forming elements rather than completely fixing the solid solution C and N in the steel (Japanese Patent Application Laid-Open No. 1989-1999). 114717, Japanese Patent Publication No. 49-130819, Japanese Patent Publication No. 49-130819, Japanese Patent Publication No. 192225-1977), etc. Among these, ferrite + martensitic woven steel strips have a low R value of around 1, which indicates the deep drawability of the material, so they cannot be applied to severe deep drawing parts. Here, R value is the strain in the width direction and plate thickness direction in the tensile test, and the original plate thickness and plate width of the tensile test piece are t0 and w0, respectively, and the plate thickness and plate width after 20% tension are respectively If t20 and w20 are used, the R value is given by the following formula. R=1n(w0/w20)/1n(t0/t20) In addition, phosphorus-added Al-killed steel has a high R value,
Although this steel plate has excellent BH properties, it requires batch treatment during the annealing process, which poses a drawback in terms of manufacturing cost. Further, when the annealing step is performed by a continuous annealing process which is advantageous in terms of manufacturing cost, there is a problem that the range of application of the steel plate is greatly restricted due to a decrease in the R value. On the other hand, carbide-forming elements such as Ni and Nb, nitride-forming elements, and carbonitride-forming elements are dissolved in solid solution C and N in steel.
Steels that improve the BH properties of ultra-low C steels by adding a smaller amount than the amount required to completely fix C have an advantage in terms of manufacturing costs because continuous annealing can be applied. However, there is a problem in that C and N in the steel, which are not fixed by Ti, Nb, etc., degrade the R value of the steel. [Summary of the invention] In weight%, C: 0.0015 to 0.005, Si≦0.5, N≦
0.0040, Mn: 0.05-1.2, P≦0.1, Al: 0.01-0.1
Ti and/or Nb in the range of Ti≦0.05, Nb≧0.025, Ti+Nb≦0.05 and 2×Ti+Nb≦
In a method of manufacturing a steel plate by hot rolling a steel containing Fe and unavoidable impurities within a range of 10×C, followed by cold rolling and annealing, the process involves the completion of rough rolling of the steel. Hot rolling is carried out at a temperature of 1000°C or lower, a time interval of 20 seconds or more from the end of rough rolling to the start of finish rolling, and a coiling temperature of 550°C or higher.
Average cooling rate after being held above 750℃ for 10 seconds or more
It is characterized by cooling from 750°C to 200°C at a rate of 10°C/second or more. Here, we will explain the reasons for limiting the chemical components. The reason for setting C in the range of 0.0015 to 0.005% is that C
If it is <0.0015%, a BH amount of 3Kg/mm2 ^or more cannot be obtained,
This is because when C>0.005, either the R value or the BH amount decreases significantly. The reason for setting Si≦0.5% is as follows. Si is an effective element for increasing the strength of steel and has the effect of improving the ductility of steel through the ferrite cleaning action, but since adding a large amount may deteriorate chemical conversion properties, Si was set at 0.5%. The reason why Mn is set in the range of 0.05 to 1.2% is as follows. 0.05% Mn to prevent red heat embrittlement of steel
However, if more than 1.2% is added, R
This is because a decrease in value is observed. The reason for setting P≦0.1% is that, like Si, P is effective as a steel strengthening element, but if it exceeds 0.1%, the steel will become brittle. The reason why Al is set in the range of 0.01 to 0.1% is as follows. The lower limit is set to 0.01 to ensure sufficient deoxidation.
%. The upper limit was set at 0.1% because no improvement in the deoxidizing effect was observed even if Al was added in an amount of 0.1% or more. The reason for setting N≦0.0040% is that N reacts with Ti.
However, if the amount of N is large, Ti reacts with N.
This is because the amount of Ti increases, and the fixation of C by Ti becomes insufficient, leading to a deterioration of the R value of the steel. Ti and Nb may be added singly or in combination.
Ti and Nb are added to fix C in the steel.
The setting of Ti≦0.05% was determined from the relationship between the weight percent of Ti, the amount of BH, and the R value shown in the drawings. In other words, if it exceeds 0.05%, as shown in the drawing, R
This is because although the value does not worsen, the amount of BH decreases. Nb≦0.025%, Ti+Nb≦0.05%, and 2
The reason why ×Ti+Nb≧10×C is set is that this range is an amount that can sufficiently fix the amount of C in the steel stoichiometrically. In addition, from the perspective of reducing the amount of Ti added necessary to fix N in steel, even if small amounts of elements with strong affinity for N, such as REM, Ce, and Ca, are added, high R
Value and high BH properties are retained. Next, we will discuss the reasons for limiting the manufacturing conditions. The reason why the rough rolling end temperature was set to 1000° C. or lower and the time interval from rough rolling to the start of finish rolling was set to 20 seconds or more is to promote strain-induced precipitation of carbides. Further, the reason why the coiling temperature was set at 550° C. or higher was to further enhance the precipitation reaction during cooling in the coil state. Annealing after cold rolling at an annealing temperature of 750°C or higher,
The reason why it was held for 10 seconds or more was to obtain a high R value and high BH amount in the steel plate. The reason why we decided to cool the steel plate after annealing from 750℃ to 200℃ at a rate of 10℃/second or more is because
This is to give the amount of BH. Note that the cooling method may be any method such as gas jet cooling, water quenching cooling, roll cooling, etc. as long as the above cooling conditions are satisfied. It is also possible to control the amount of room temperature aging and the amount of BH by reheating the steel plate after cooling to a temperature of 250° C. or lower. [Examples of the Invention] Examples of the invention are shown below along with comparative examples. Table 1 shows the chemical composition of the steels and the conditions of each process in Examples and Comparative Examples. The test pieces were made of various ultra-resistance C-ti steels and ultra-low C-Ti steels.
−Nb-based steel is melted in the laboratory and the steel ingot is shaved,
After forging, hot rolling and cold rolling were performed under various conditions shown in Table 1 to obtain a 0.8 mm thick steel plate. These steel plates were annealed under various conditions shown in Table 1, and their mechanical properties were investigated. Mechanical properties are also shown in Table 1. In Table 1, No. 1 to No. 4 are steels with the same chemical composition but different processes such as rolling cooling, among which No. 1 is the example and No. 2 to No. 4 are the comparative examples.
This is No.4. No. 5 and No. 6 have the same chemical composition, but about twice the amount of Ti was added to steel No. 1 to No. 4. The difference between No. 5 and No. 6 is that the conditions of each process are different, of which No. 5 is the example and No. 6 is the comparative example. No.7 steel is No.5,
Approximately twice as much Ti is added to No. 6 steel, and Ti+Nb>
This is a comparative example where the value was set to 0.05. Steel No. 8 is a comparative example in which the amount of C is increased and 2×Ti+Nb<10. No.
9, No. 10 shows an example. No. 11 shows an example in which both Ti and Nb were added. No. 12 shows an example in which only Nb was added. No.13 has the same chemical composition as No.12, but the winding temperature is 550℃ in each process.
A comparative example is shown below. Bake hardenability of steel (BH amount), vertical cracking resistance, R
The values were evaluated by directly measuring the BH properties and R values of various test pieces. In addition, bake hardenability and vertical cracking resistance are closely related to the amount of solid solute C in the steel.
A material with a large amount of solid solute C and excellent bake hardenability will naturally have excellent warp cracking resistance, so the evaluation of warp cracking resistance is based on the bake hardening amount (BH amount).
I decided to represent it. From Table 1, the only steels with an R value of 1.9 or more and a BH value of 2.0 or more are Examples No. 1, No. 5, and No. 9 to No. 12. From this, the steel in the example has a higher R value and BH value than the comparative example, so it is a cold rolled steel plate with excellent bake hardenability and warp cracking resistance and a high R value. I understand. [Effects of the Invention] As explained above, by aiming at sufficient fixation of solid solution C through carbonitride precipitation at the hot rolling stage, and by re-dissolving a certain amount of C into the steel after recrystallization annealing, , has a high R value and is excellent
It becomes possible to produce a steel plate having BH properties and warp cracking resistance at a low manufacturing cost.

【table】

【table】 [Brief explanation of drawings]

The drawing is a graph showing the influence of the amount of Ti on the R value and the amount of BH.

Claims (1)

[Claims] 1% by weight, C: 0.0015-0.005, Si≦0.5, vN
≦0.0040, Mn: 0.05~1.2, P≦0.1, Al: 0.01~
0.1, Ti≦0.05, Nb≦0.025, Ti+Nb≦
0.05 and 2×Ti+Nb≧10×C, by hot rolling a steel containing Ti and/or Nb, the balance being Fe and unavoidable impurities, followed by cold rolling and annealing. In a method for producing a steel plate, the steel is hot rolled at a rough rolling finish temperature of 1000°C or less, a time interval from the end of rough rolling to the start of finish rolling of 20 seconds or more, and a coiling temperature of 550°C or higher, and then cold rolled. Annealing after inter-rolling, annealing temperature
Average cooling rate after being held above 750℃ for 10 seconds or more
A method for producing a cold-rolled steel sheet having excellent bake hardenability and warp cracking resistance and a high R value, characterized by cooling from 750°C to 200°C at a rate of 10°C/second or more.