JPS6235463B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet for deep drawing. BACKGROUND ART In recent years, efforts have been made to reduce the weight of automobiles from the perspective of energy conservation, and for this reason, intensive research is being carried out on manufacturing technology for high-strength automobile steel sheets. Since such steel sheets for automobiles are generally press-formed, they must have excellent not only strength but also press formability. In recent years, so-called dual-phase steel has been developed as a steel sheet for this purpose, which has a two-phase structure of ferrite and martensitic phases, has a low yield ratio and high tensile strength, and has excellent press formability due to its slow aging properties. is in the spotlight. However, this dual-phase steel requires the addition of a considerable amount of alloying elements to create its unique structure, or requires a very fast cooling rate, which increases manufacturing costs. Alternatively, ferrite grains with {111} orientation could not be sufficiently developed, resulting in a low r value. A method of increasing the tensile strength by adding P or N to a high-strength steel plate with a two-phase structure, or by performing open annealing on rimmed steel to appropriately decarburize and denitrify it, and then apply a decarburization and denitrification treatment to the rimmed steel during baking painting after press working. Although methods of increasing strength by utilizing the strain age hardening phenomenon have been considered, none of these methods has been able to fully satisfy press formability, deep drawability, and productivity. The present invention eliminates the drawbacks of the conventional method,
The purpose of this is to provide an improved manufacturing method for high-strength cold-rolled steel sheets for deep drawing.
A low carbon cold rolled steel sheet containing Al0.010~0.080%, P0.10% or less, N0.010% or less, and Nb within the range specified by either of the following conditions (a) or (b), After hot rolling, coiling at a temperature of 300℃ or higher and 750℃ or lower,
A cold-rolled steel sheet for deep drawing, characterized in that after cold rolling, it is heated to a temperature of 900°C or less to undergo recrystallization annealing, and then cooled under any of the cooling conditions shown in (c) and (d) below. The above object can be achieved by the manufacturing method. (a) When the coiling temperature is 600℃ or higher 0.3≦%Nb/7.75 (%C) + 6.65 (0.25-0.023% soluble Al/% total N) (% total N) < 1.2 (b) Coiling temperature Below 600℃ 0.3≦%Nb/7.75 (%C) + 6.65 (0.93-0.073% soluble Al/% total N) (% total N) < 1.2 (c) 50℃/sec or less up to 400℃ Cool slowly at a cooling rate of . (d) After cooling to 400°C at a cooling rate faster than 50°C/sec, slow cooling from 400 to 200°C at a deterioration rate of 10°C/sec or less. Next, the present invention will be explained in detail. The present inventors have studied the manufacturing technology of high-strength cold-rolled steel sheets for deep drawing using a continuous annealing method with good productivity, and have discovered the correlation between the cooling rate and material quality,
Depending on the content of C, N, and Al, Nb is added as a solid solution in the steel sheet within a range that does not affect the aging resistance after continuous annealing.
The present invention has been completed based on the new finding that it is possible to further increase the strength by adding the aluminum to such an extent that it remains, thereby making use of the strain aging hardening phenomenon after press working and baking painting. Next, the present invention will be explained using experimental data. A steel ingot having the composition shown in Table 1 was hot rolled to a thickness of 3.5 mm and then coiled at a high temperature (rolling at 670°C) and at a low temperature (rolling at 525°C).

【table】

[Table] Next, it was cold rolled to 0.7 mm. Figure 1 is a schematic diagram showing the heat cycle of a continuous annealing line.The factors that characterize the annealing conditions are annealing temperature (T _A , °C), annealing time (t _A , sec), and annealing temperature.
Average cooling rate (v ₁ , °C/sec) up to 400 °C and
Average cooling rate from 400℃ to 200℃ (v ₂ ,℃/
sec), the experimental steel plates of the present invention were annealed by changing these factors, and then a 0.7% skin pass was performed. The material and bake hardenability of this steel plate will be described below. First, the amount of Nb is closely related to the amount of C and N in steel, so the composition is Nb (wt%) / {7.75C (wt%) +
6.65N (wt%)}. This value is shown in Table 1.
It is equivalent to Nb/C+N (atomic ratio). For steel with this value of about 0.7, T _A = 830°C, t _A = 40 sec, v ₁ = 6,
FIG. 2 shows the relationship between the material quality and C content after annealing under the conditions of 13°C/sec and v ₂ =20°C/sec. Steel with C0.010% or less has low yield stress (YP), total elongation (El), r
A material with a high value and n value can be obtained, but C>
In steel with 0.010%, YP becomes high and El, n value, and r value decrease significantly. Also, the statute of limitations index (AI, 7.5%
The difference between the deformation stress during tensile deformation and the yield stress when it is aged at 100℃ for 30 minutes) is C.
For steel with ≦0.010%, it is 3 Kg/mm ² or less, and there is no problem in aging resistance as long as the steel plate is used under normal conditions. Furthermore, compared to low-temperature rolled material, high-temperature rolled material has a lower YP, a larger El, and a clear tendency to become softer, and AI also tends to decrease. Figure 3A is a schematic diagram showing the relationship between strain and stress when a steel plate is prestrained and then further strained, where YP is the yield stress when prestrain is applied, and σ _y ′ is The yield stress when strain is applied after applying pre-strain and baking coating treatment, TS′ is the ultimate strength, △σ _y is the difference between σ _y ′ and YP, △σ _w is the increase due to work hardening, △σ _A
is the increase in yield stress purely due to aging. Nb (%) / {7.75C (%) + 6.65N (%)} 0.7
T _A = 830℃, t _A = 40sec, v ₁ = 6℃/
After annealing at sec, v ₂ = 20℃/sec, tensile prestrain of 1% and 5% was added, and a treatment equivalent to baking painting (170℃,
TS′, σ _y ′, △σ of the material when subjected to
The relationship between _y , Δσ _A , Δσ _w and the amount of C is shown in FIG. 3B. From the same figure, TS' is 1 to 4 Kg/mm ² regardless of prestrain.
It can be seen that the level increases. Furthermore, the yield stress σ _y ′ after treatment is approximately 10 Kg/mm ² at 1% pre-strain compared to YP before treatment, as seen from the relationship between Δσ _y and C content.
% prestrain increases by 15-16Kg/mm by ² places. This amount of increase is almost unrelated to the amount of C, but in steels with C0.010% or more, the increase amount due to work hardening (△σ _w ) decreases as the n value decreases, resulting in the amount of increase in yield stress after treatment. tends to decrease slightly. The increase in yield stress (△σ _A ) purely due to aging is 4 to 8 Kg/mm ²
However, low-temperature rolled material tends to be larger. This is expected since the AI of the low-temperature web material is higher than that of the high-temperature web material. Considering this together with the results in Figure 2, if high temperature web material is used, YP will decrease.
It is advantageous for deep drawability such as improved El and r values. However, since the amount of solid solution C and N is smaller than that of the low-temperature rolled material, the degree of increase in yield point due to strain aging becomes smaller. From Figures 2 and 3B, Nb (%)/{7.75C
(%)+6.65N(%)}0.7 ultra-low carbon Al killed steel, it is possible to obtain a material with excellent deep drawability and aging resistance by continuous annealing, and it can be baked and painted after prestraining. It was found that the tensile strength increases by about 1 to 4 Kg/mm ² and the yield point becomes about 35 to 40 Kg/mm ² . However, in this case, the C content is required to be 0.010% or less from the viewpoint of ductility and aging resistance. By the way, Nb (wt%) / {7.75C (wt%) +
6.65N (wt%)}<0.7, the amount of Nb carbonitrides decreases, so it is possible to obtain a soft steel plate even with C>0.010% steel. So Nb
(%) / {7.75C (%) + 6.65N (%)}0.3 steel T _A = 830℃, t _A = 40sec, v ₁ = 6℃/sec, v ₂ =
Figure 4 shows the relationship between the material and the amount of C when annealed at 20°C/sec. Compared to steel with the same amount of C in Figure 3B, YP decreases by 2 to 3 Kg/ ^mm2 , and El decreases by 3%.
The degree increases. However, the decrease in the amount of Nb is caused by solid solute C and N.
By increasing the amount, AI will definitely increase 4
Kg/ ^mm2 or more. Therefore, in steels with C>0.010%, although it is possible to improve ductility such as El by adding a small amount of Nb, the aging resistance deteriorates, making it difficult to obtain the target material of the present invention. Incidentally, not only the amount of C but also the amount of N have a direct influence on the material quality and aging characteristics of the steel material. However, unlike C, N in aluminum killed steel is within the range of about 40 to 80 ppm unless it is intentionally added. Therefore, by continuously annealing ultra-low carbon aluminum killed steel with C≦0.010% and adding Nb in a certain range, a material with excellent deep drawability, aging resistance, and hardening by baking can be obtained. there is a possibility. Therefore, next we will examine the appropriate range of the amount of Nb that should be added to ultra-low carbon aluminum killed steel with C≦0.010%. When considering the appropriate addition range of Nb amount, Nb
, solid solution C in a state where it can combine with Nb to produce Nbc (hereinafter referred to as C ^* ), and solid solution N in a state where it can also combine with Nb to produce NbN (hereinafter denoted as N ^* ) The problem is the atomic ratio Nb/(C ^* +N ^* ). Therefore, it is considered reasonable to use Nb (%)/{7.75×C ^* (%)+6.65×N ^* (%)}...(1) as one parameter. In a continuous annealing line that does not perform overaging treatment, the cooling rate to room temperature after recrystallization annealing is fast, so C cannot be precipitated as Fe ₃ C (or similar iron-based carbides). Therefore, C in equation (1)
^* (%) means the total amount of C. On the other hand, N is in steel
It has a relatively strong affinity with Al. For this reason, some N exists as AlN in the hot-rolled sheet state, although the amount varies depending on the hot-rolling conditions, and even during subsequent annealing, it hardly dissolves and remains as AlN. A part of the N in the solid solution state may also precipitate as AlN during recrystallization annealing after cold rolling. Hereinafter, the amount of N present as AlN after annealing will be abbreviated as N ^A and the total N content will be abbreviated as N ^T . Then, N ^* in equation (1)
(%) is equal to N ^T (%) - N ^A (%). From the above
Equation (1) becomes as follows. Nb (wt%) / {7.75C (wt%) + 6.65 (N ^T (wt%) - N ^A (wt%))} ... (2) The amount of N ^A is greatly influenced by hot rolling conditions. In high-temperature rolled material, most of the N already exists as AlN in the hot-rolled sheet state because it stays for a long time in a temperature range where the precipitation rate of AlN is high. Furthermore, the amount of AlN precipitated is also affected by the amount of Al in the steel even under the same hot rolling conditions.
Therefore, at C0.006%, Nb (wt%)/{7.75C (wt
%) +6.65N ^T (wt%)}0.7 steel T _A = 750,
800, 830℃, t _A = 40sec, v ₁ = 6℃/sec, v ₂ =
N ^A /N ^T and solAl / N when annealed at 20℃/sec
The relationship between ^T (all weight ratios) is shown in Figure 5. In high-temperature rolled material, if solAl/N ^T ≧2 (solAl means soluble Al), more than 80% of N ^T is fixed as AlN. On the other hand, in order for 50% or more of N ^T to be AlN in a low-temperature rolled material, it is a necessary condition that solAl/N ^T ≧6. Paying attention to the results for the high-temperature rolled material and the low-temperature rolled material in Figure 5, it can be seen that if the range is limited to 2≦solAl/N ^T ≦11, N ^A /N ^T and solAl/N are almost independent of the annealing temperature. There is a proportional relationship with ^T. Assuming this relationship to be a linear function, the function coefficients were determined using the least squares method. As a result, N
The relationship between ^A /N ^T and solAl/N ^T is (3) for high-temperature web material.
For low-temperature web material, it can be expressed as equation (4). N ^A /N ^T =0.023(solAl/N ^T )+0.75 …(3) N ^A /N ^T =0.073(solAl/N ^T )+0.07 …(4) Formulas (3) and (4) are This is based on the analysis results obtained when steel in the composition range used for the invention was processed under typical annealing conditions. Therefore, it is a parameter for determining the appropriate addition range of Nb. Equation (2) becomes Equation (5) for high-temperature rolled material and Equation (6) for low-temperature rolled material, and this value will be referred to as Z hereinafter. Z=Nb(wt%)/{7.75C(wt%)+6.65(0.25-0.023solAl/ ^NT ) ^NT (wt%)}...(5) Z=Nb(wt%)/{7.75C( wt%) ₊ 6.65 ( ^{0.93−0.073solAl} / ^N
Material and bake _{hardenability (σ y ′ ,} 5%
Figure 6 shows the pre-distortion) organized using equation (2) as a parameter. AI monotonically decreases as Z increases, almost unrelated to C content and hot rolling conditions. Z<
At 0.3, AI≧4Kg/mm ² and a problem arises in aging resistance. On the other hand, when Z>1.2, AI≦1Kg/mm ² , and as seen from the result of σ _y ', solid solution C and N decrease too much and the amount of bake hardening after pressing becomes small.
Regarding YP and El, if Z≦1.2, there is no problem in press formability. As a result of the above, Nb was added to the ultra-low carbon aluminum killed steel with C≦0.010% within the range shown by the following formula. 0.3≦Z≦1.2 …(7) After hot rolling the steel and coiling it at a predetermined temperature,
By cold rolling and then continuous annealing, a high tensile strength cold rolled steel sheet with excellent deep drawing formability, aging resistance, and baking paint hardening properties can be obtained. In the present invention, the coiling temperature after hot rolling is limited to a range of 300°C or higher and 750°C or lower. This is because winding at temperatures below 300°C is not only difficult in terms of equipment, but also has the disadvantage of significantly damaging the sheet shape.On the other hand, winding at temperatures over 750°C deteriorates descaling properties and This is because the grain size becomes coarse and a surface defect called roughness occurs. Next, we will discuss the effect of annealing temperature on material quality during continuous annealing. N7 steel (C=0.008%,
Nb=0.069%), _tA =40sec, _v1 =6,13℃/
Fig. 7 shows the relationship between the material and the annealing temperature when annealing is performed at sec, v ₂ = 20°C/sec. As the annealing temperature increases up to 900℃, El increases and YP decreases. A.I.
It is less than 4Kg/mm ² up to 900℃. When the temperature exceeds 900°C, Nb (C, N) or AlN begins to dissolve, so the AI increases rapidly. At the same time, El and YP also deteriorate. Therefore, the annealing temperature in the continuous annealing line is required to be higher than the recrystallization temperature and lower than 900°C. Next, we will examine the effects of cooling rates v ₁ and v ₂ on material quality. Cooling rates v ₁ , v ₂ and
Figure 8 summarizes the results of our investigation into the relationship with AI. As is clear from the figure, when v ₁ ≦50° C./sec, good aging resistance of AI≦4 Kg/mm ² was obtained regardless of v ₂ . By the way, if v ₂ > 10℃/sec, then v ₁ > 50℃/sec
In this case, AI>4Kg/mm ² and a problem arises in aging resistance. This is because if the cooling rate from T _A is high, the precipitation of C and N centered on precipitates such as Nb (C, N) or AlN that existed in the hot rolled sheet cannot proceed sufficiently. it is conceivable that. However, if v ₂ ≦10°C/sec and slow cooling in the 200 to 400°C region where the C and N solid solubility limits are relatively low, the precipitation of C and N is promoted.
Even when v ₁ >50°C/sec, AI≦4Kg/mm ² could be obtained. Finally, we will discuss the effect of P addition on improving TS. N6, NP1, NP2, NP3 steel T _A = 830℃, t
FIG. 9 shows the relationship between the material and the amount of P added when annealing was performed at _A = 40 sec, v ₁ = 6°C/sec, and v ₂ = 20°C/sec. TS=35.5 for steel with P=0.008% in high temperature rolled material
Kg/ ^mm2 , but for steel with P = 0.047%, TS is 5
Increases by approximately Kg/ ^mm2 . However, El decreases by 2-3% and YP increases by ^about 2Kg/mm2. Adding 0.10% or less of P can improve TS with relatively little deterioration of YP and El, so it has high utility value as a high-strength steel sheet. Table 2 shows the material properties and bake hardenability of cold-rolled steel sheets actually produced on a continuous annealing line under optimal conditions comprehensively judged from the above results.

【table】

[Table] However, steel plates A to L in Table 2 are steel plates that were annealed at 830°C for 40 seconds and then skin-passed by 0.7% to a plate thickness of 0.7 mm. According to the present invention, sufficient ductility and aging resistance cannot be ensured unless the C content of the target steel is 0.01% or less, regardless of the amount of Nb added.
In addition, deoxidation with Al and Si is essential to increase the yield of Nb addition, and since Al combines with N in steel and has the effect of improving aging resistance and deep drawability, Al≧0.010% is essential. It is necessary to. However, if Al is contained excessively, there are problems such as inclusions or crystal grains becoming too small.
It is necessary to keep Al≦0.080%. Although it is preferable to contain Si, if it is more than 0.20%, the zinc plating properties will be impaired as shown in the examples in Table 2, so the Si content should be 0.20% or less. If the Mn content exceeds 1.0%, the ductility deteriorates and the galvanizing properties deteriorate as shown in the examples in Table 2, so the Mn content must be 1.0% or less. If P exceeds 0.10%, ductility deteriorates, so P
must be below 0.10%. N is an element that has a large influence on aging characteristics, but in aluminum killed steel, unless N is intentionally added, the content is only in the range of 40 to 80 ppm. Furthermore, due to the similarity in behavior of C atoms and N atoms in steel, it is considered that there is no problem as long as the amount of N is within the same range as the amount of C. If N is more than 0.010%, the aging property will be large, so N needs to be 0.010% or less. According to the present invention, the steel having the composition described above is hot-rolled and then coiled at a high temperature (the coiling temperature is 600°C or higher) or at a low temperature (the coiling temperature is 600°C or lower). However, the winding temperature shall be 300℃ or higher and 750℃ or lower. Then, after pickling and cold rolling, it is annealed in a continuous annealing line at a temperature range from the recrystallization temperature to 900°C. Then, cool down to 400℃ at an average cooling rate of 50℃/sec or less, then cool at any cooling rate from 400℃ to 200℃, or cool at 50℃/sec from the annealing temperature to 400℃.
When cooling at an average cooling rate of 400℃ or more
Cools up to 200℃ at an average cooling rate of 10℃/sec or less. According to the present invention, as described above, by continuously annealing Nb-added ultra-low carbon aluminum killed steel, a high-strength cold-rolled steel sheet with excellent three properties of deep drawability, aging resistance, and baking paint hardening properties can be manufactured. I can do it.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the heat cycle of a continuous annealing line, Fig. 2 is a diagram showing the relationship between the carbon content and mechanical properties of a steel plate, and Fig. 3A is a schematic diagram showing the relationship between strain and stress. Figure 3B and Figure 4 respectively show C of steel plate.
A schematic diagram showing the relationship between quantity and mechanical properties. Figure 5 is a diagram showing the relationship between solAl/N ^T and N ^A (N in AlN state)/N ^T (total N) of the steel plate. Nb/ of steel plate
(C+N ^T -N ^A ) That is, a diagram showing the relationship between Z and mechanical properties, Figure 7 is a diagram showing the relationship between the annealing temperature T _A of a steel plate and mechanical properties, and Figure 8 is a diagram showing the relationship between the annealing temperature T A of a steel plate and mechanical properties. FIG. 9 is a diagram showing the relationship between the cooling rates v ₁ and v ₂ and AI, and FIG. 9 is a diagram showing the relationship between the P content and mechanical properties of the steel plate.

Claims (1)

[Claims] 1 C 0.010% or less, Si 0.20% or less, Mn 1.0% or less, Al 0.010 to 0.080%, P 0.10% or less, N 0.010%
Below, a low carbon cold rolled steel sheet containing Nb within the range specified by either of the conditions (a) or (b) below is hot rolled and then coiled at a temperature of 300°C or higher and 750°C or lower. , after cold rolling, it is heated to a temperature of 900°C or less to undergo recrystallization annealing, and then cooled under either of the cooling conditions shown in (c) or (d) below. A method for producing cold-rolled steel sheets. (a) When the coiling temperature is 600℃ or higher 0.3≦%Nb/7.75 (%C) + 6.65 (0.25-0.023%available Al/%total N) (%total N)<1.2 (b) Coiling temperature Below 600℃ 0.3≦%Nb/7.75 (%C) + 6.65 (0.93-0.073%available Al/%total N) (%totalN)<1.2 (c) 50℃/sec or less up to 400℃ Cool slowly at a cooling rate of (d) After cooling up to 400°C at a cooling rate faster than 50°C/sec, gradually cool from 400 to 200°C at a cooling rate of 10°C/sec or less.