JPH10317094A - High formability cold rolled high strength steel sheet with excellent dent resistance - Google Patents

Abstract

(57) [Summary] [PROBLEMS] High strength cold rolling and high tension showing excellent dent resistance and formability by eliminating press failure due to aging deterioration, restriction of heat treatment condition in coating process, and disadvantage of high manufacturing cost. Provide steel sheet. SOLUTION: C: 0.011 to 0.04%, Si: 1.5% or less, M
n: 0.07 to 2.5%, P: 0.005 to 0.10%, S: 0.02% or less, Al: 0.01 to 0.1%, N: 0.008% or less, Ti: (Ti ^*
% / C%) ≧ 4, where Ti ^* <%> = Ti <%> − (48/14)
N <%> − (48/32) S <%> Further, if desired, Nb: 0.005 to 0.05%, B: 0.0001 to
0.0040% of at least one or two, and / or
One or two or more of Cu: 1.0% or less, Cr: 1.0% or less, and Ni: 1.0% or less, with the balance being a steel composition composed of Fe and unavoidable impurities, and preferably the following conditions (1), (1) 2)
Meet. σ _2% −YS ≧ 25 N / mm ²・ ・ ・ (1) σ _{2% + Baking} ≧ 275 N / mm ²・ ・ ・ (2)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high formability cold-rolled high tensile strength steel sheet having excellent dent resistance as a steel sheet for automobiles in response to a demand for high strength for the purpose of improving fuel efficiency by reducing the weight of an automobile body. It relates to a steel plate.

[0002]

2. Description of the Related Art In recent years, in the automotive industry, there has been a general tendency to reduce the thickness of a material by increasing the strength of the material in order to improve fuel efficiency by reducing the weight of a vehicle body. Among them, it is said that steel sheets used for exterior applications such as doors and fenders need not only surface quality but also dent resistance in addition to formability.

Conventionally, steel sheets for automobiles meeting such demands include steel sheets having baking hardening characteristics by controlling the amount of solid solution C, and high-strength IF steels which are completely non-ageable and have high formability and high strength. A compatible steel plate has been proposed.

[0004] For example, in a steel sheet having bake hardenability, as disclosed in Japanese Patent Publication No. 47328/1985, the amount of solid solution C determined by the balance between the amounts of C, N and S and the amounts of Ti and Nb and the addition of B are limited. Japanese Patent Application Laid-Open No. 61-276928 discloses a method for obtaining bake hardenability by dissolving C with N, S, Ti, and Nb based on ultra-low carbon steel.
To obtain bake hardenability by controlling according to
No. 56050 discloses a method of obtaining high bake hardenability by controlling the Mn and S contents of a low carbon Al-killed steel, the hot rolling heating temperature, and the annealing conditions in continuous annealing, respectively.

[0005] Further, as a high strength steel sheet of IF type,
No. 58-18973, based on ultra-low carbon IF steel,
Japanese Patent Application Laid-Open No. 6-17139 discloses a steel sheet which has been strengthened using solid solution strengthening of Mn and P.
A steel sheet which has been strengthened using precipitation strengthening of TiC has been disclosed. However, the steel plates currently used including the above-mentioned steel plates have the following problems.

[0006] First, in the case of a bake hardening steel sheet, since the solid solution C remains, the initial yield stress (YS) of the obtained product becomes high, and for example, molding defects such as surface distortion and press failure due to aging deterioration (cracks, streaks). Char strain, the same applies hereinafter). Further, in order to sufficiently exhibit bake hardening characteristics, it is necessary to restrict the temperature conditions under which the solid solution C diffuses in the coating process to fix mobile dislocations, that is, it is necessary to perform treatment in a specific narrow temperature range and time. As a result, temperature conditions are restricted.

On the other hand, IF type high-strength steel sheets are
No press failure due to aging degradation occurs due to the absence of aging, and there is no temperature restriction in the coating baking process.However, when comparing baking hardened steel sheets of the same strength level with dent resistance, the properties in the large deformation region are equivalent. Nevertheless, there is a problem that the dent characteristics in the small deformation region are inferior to those of the bake hardenable steel sheet. Therefore, it is not possible to apply the IF type steel sheet to avoid the problems of the current bake hardenable steel sheet because the dent resistance is reduced.

[0008] It is qualitatively understood that in order to obtain a dent characteristic equivalent to that of a bake hardening steel sheet, it is necessary to increase the strength of the IF steel sheet, but the IF steel sheet completely precipitates C and N. In order to increase the strength, it is necessary to add Si, Mn, P, etc. in order to increase the strength, despite the fact that the amount of addition of elements such as Ti, Nb, B, etc. is larger than that of the bake hardening steel sheet, and the manufacturing cost is already high.
However, there is a problem that the cost increases due to the addition of a solid solution element such as

In order to ensure dent resistance, Japanese Patent Application Laid-Open No. 8-269621 discloses a method of using both high work hardening and high bake hardening in a low carbon steel Al-killed steel. In addition, the part that depends on baking hardening is largely affected by the heat treatment conditions in the coating process.

That is, in the steel sheets proposed to date, specific solutions for solving the press failure and the temperature constraint in the coating process, which are problems with the current bake hardenable steel sheet, are disclosed. In addition, IF-type high-strength steel sheets that have the potential to solve the problem cannot be put to practical use because conventional steel sheets are insufficient in terms of manufacturing cost and dent resistance.

[0011]

The problems to be solved by the present invention are advantageous in that, for example, press failure due to aging deterioration, restrictions on heating conditions in the coating process, difficulty in securing dent resistance, and increase in manufacturing cost are advantageous. It is to provide a high-strength cold-rolled steel sheet that can be solved.

Specifically, the present invention provides a secondary working brittleness temperature of -30 ° C. or less as evaluated by a vertical cracking at a drawing ratio of 1.8 or less at a load of 22 kg or more at a residual deflection of 0.02 mm, and a YPE after aging of ≦≦ 30 mm.
An object of the present invention is to provide a low-cost, high-strength, cold-rolled, high-tensile steel sheet exhibiting excellent dent resistance of 0.3% and exhibiting excellent formability.

[0013]

Means for Solving the Problems The inventors of the present invention aim at obtaining excellent moldability which shows excellent dent resistance irrespective of the restriction of baking conditions in the coating process but shows no press failure due to aging deterioration. Focusing on defining the steel composition and the mechanical properties of the steel sheet product after production, and as a result of intensive research, as a result, by limiting the steel composition and the properties of the steel sheet product, excellent dent resistance and forming The present inventors have found that a high-strength cold-rolled high-strength steel sheet having heat resistance can be obtained, and have reached the present invention.

Therefore, the present invention relates to a method for preparing C: 0.01% by mass.
1 to 0.04%, Si: 1.5% or less, Mn: 0.07 to 2.5%, P:
0.005 to 0.10%, S: 0.02% or less, Al: 0.01 to 0.1%,
N: 0.008% or less, Ti: (Ti ^* % / C%) ≧ 4,
Ti ^* <%> = Ti <%> − (48/14) N <%> − (48/32) S <%> The balance is characterized by having a steel composition consisting of Fe and unavoidable impurities. High formability cold rolled high tensile steel sheet with excellent dent resistance.

According to a preferred embodiment, the present invention further provides:
The properties of the cold-rolled steel sheet after hot rolling, cold rolling and recrystallization annealing satisfy the following conditions (1) and (2), and have the above steel composition and have excellent dent resistance and high formability. It is.

Initial yield stress: YS Stress at 2% tensile prestrain: σ _2% , Stress after 2% tensile prestrain + Baking (170 ° C. × 20 minutes): σ
_{2% + Baking} σ _2% −YS ≧ 25 N / mm ²・ ・ ・ (1) σ _{2% + Baking} ≧ 275 N / mm ²・ ・ ・ (2) σ _2% −YS = work hardening amount ( WH) The steel composition further comprises, by mass%, Nb: 0.005 to 0.05
%, B: One or two of 0.0001 to 0.0040% may be contained. Further, the steel composition is, for the purpose of corrosion resistance or high strength, Cu: 1.0% or less, Cr: 1.0% or less, Ni: 1.0%
One or more of the following may be contained.

The high-strength steel sheet according to the present invention may be subjected to hot-dip galvanizing. However, in that case, S
It is preferable to limit i: 0.5% or less and P: 0.005 to 0.045%. According to this aspect, it is sufficient that the properties of the steel sheet after the hot-dip galvanization satisfy the above conditions (1) and (2).

[0018]

Next, the present invention will be described in detail. First, the reasons for limiting the numerical values of the components will be described. In this specification, unless otherwise specified, “%” is% by mass.

C: C is one of the important elements in the present invention. In order to obtain 2% strain imparted to the product + yield stress after baking ≧ 275 N / mm ² , it is necessary to increase the strength.
In order to achieve high strength while suppressing manufacturing costs, Si, Mn,
It is necessary to use a method of increasing the strength by using the precipitation strengthening of TiC while suppressing the load of alloy elements such as P. In the present invention, in order to increase the strength, C is set to more than 0.01%, preferably 0.011% or more. On the other hand, if more than 0.04% is added, excessively high strength is caused, so that moldability is deteriorated. Therefore, the content range of C is more than 0.01% and 0.04% or less, preferably
0.011 to 0.04%. More preferably, 0.011 to 0.03
%.

Si: As described above, Si is an element capable of increasing strength by solid solution strengthening. However, if added in excess of 1.5%, a severe squamous scale is formed and cannot be used as an exterior steel plate for doors and the like. Therefore, the upper limit is set to 1.5%. Further, considering cost, it is preferably set to 0.8% or less. The lower limit is not particularly limited, but is usually more than 0.10%. When the solid solution strengthening by Si is actively used, 0.3
% Or more. When the steel sheet according to the present invention is used as a hot-dip galvanized steel sheet, preferably, the Si content is 0.
Limit to 5% or less.

Mn: Mn is an element capable of increasing strength by solid solution strengthening like Si. If Mn exceeds 2.5%, an excessive increase in strength and an increase in cost are caused. If Mn is less than 0.07%,
It is difficult at the current steelmaking level. So the content of Mn
0.07 to 2.5%. Preferably, it is 0.1-1.0%.

P: P, like Si and Mn, is an element capable of increasing the strength by solid solution strengthening. However, if P exceeds 0.1%, there is a problem that the secondary work brittleness is deteriorated. In addition, P is 0.005%
If it is less than the above, there is a problem that the cost of smelting increases. Therefore, the P content range is 0.005 to 0.1%. Preferably, it is 0.005 to 0.08%. When the steel sheet according to the present invention is used as a hot-dip galvanized steel sheet, P is preferably limited to 0.005 to 0.045%.

S: S deteriorates hot brittleness,
Increasing the S content makes it easier to adsorb inclusions due to the interfacial tension, and induces surface flaws such as barbs and the amount of Ti required to fix S as a precipitate increases. %. As a preferred range, S is 0.
015% or less.

Al: Al is a deoxidizing element of steel. If it is less than 0.01%, deoxidation becomes insufficient, while if it exceeds 0.10%, the effect is saturated and the cost is increased. Therefore, the content range of Al
It shall be 0.01-0.1%.

N: N is preferably as small as possible from the viewpoint of workability. If N exceeds 0.008%, the amount of Ti required to fix N as TiN increases and the cost increases, so the upper limit is made 0.008%. In a preferred range, N is 0.005
% Or less.

Ti: Ti is an important element in the present invention. C: C is added to Ti in a relatively large amount in a 0.011-0.04% class steel sheet without precipitation of C as cementite. In order to precipitate as carbide TiC and to reduce solid solution C in steel and to increase the strength, it is necessary to positively add it. Therefore, the amount of Ti added is C
Is defined in relation to, for the case of Ti ^* / C <4, which has a lot of solid solution C is left aging degradation characteristic in the steel, it is possible to generate a pressing problem, Ti ^* / C An amount satisfying ≧ 4 is contained. The upper limit of Ti ^* / C is not particularly limited, but is practically 10.0 or less, preferably 8.0 or less.

Here, Ti * is represented by the following formula. Ti ^* <%> = Ti <%> − (48/14) N <%> − (48/32) S <%> In the present invention, in order to further improve the formability, particularly the deep drawability, Nb , B may be added.

Nb: Like Nb, Nb is an element that fixes C as a carbide. However, in order to increase the r value indicating the deep drawing performance after recrystallization of a cold-rolled steel sheet, it is better to add Nb in combination. .
This is because NbC and TiC have different deposition temperatures.
This is because the hot rolled sheet can be further re-granulated because it is precipitated at a lower temperature than TiC. However, this effect is
If the content is less than 0.005%, the efficacy is small, while if it exceeds 0.05%, the strength is excessively increased, so that the Nb content is 0.005 to 0.05.
%.

B: By adding B, the deep drawability is improved, but the grain boundary strength is further increased and the countermeasure against secondary working brittleness can be achieved. If it is less than 0.0002%, the effect of improving secondary work brittleness does not appear, so 0.0002% or more is added. On the other hand, 0.
If the content exceeds 0040%, the effect is saturated, the cost is increased, and the strength is increased.
40%. Further, in the present invention, at least one of Cu, Cr and Ni may be added for further improving corrosion resistance.

Cu: Since Cu has an effect of improving the corrosion resistance of a steel sheet in a non-plated state, it may be added to improve the corrosion resistance of a non-plated steel sheet. However, excessive addition causes deterioration of hot workability due to Cu checking in the slab, and it is necessary to add Ni in a complex manner to suppress this. Therefore, the upper limit is set to 1.0%. I decided.

Cr: Cr, like Mn, has the effect of stabilizing austenite and improving the corrosion resistance. However, in the steel composition of the present invention, it also acts as an element for increasing the strength. However, if the upper limit exceeds 1.0%, an excessive increase in strength is caused, so the upper limit is made 1.0%.

Ni: Ni is added not only for improving the corrosion resistance but also for suppressing the Cu checking when Cu is added. However, if added in a large amount, the cost is increased, so the upper limit is made 1.0%.

In producing a steel sheet having a steel composition adjusted as described above, a slab having a predetermined steel composition is subjected to hot rolling, cold rolling, and recrystallization annealing to form a cold-rolled high-tensile steel sheet. According to the present invention, the processing conditions for such hot rolling, cold rolling, and recrystallization annealing may be any of ordinary working conditions, and may be any conditions as long as they do not contradict the spirit of the present invention.

Further, according to another aspect of the present invention, such a high-tensile steel sheet may be used as a hot-dip galvanized steel sheet. According to the present invention, hot-dip galvanizing is preferably performed on a steel sheet that has been subjected to a recrystallization annealing treatment.

According to a further preferred aspect of the present invention, the high formability cold-rolled high-tensile steel sheet or high formability hot-dip galvanized high-tensile steel sheet having excellent dent resistance obtained as described above is as follows. With such characteristics, the dent resistance is further improved.

Σ _2% −YS ≧ 25 N / mm ² (1) σ _{2% + Baking} ≧ 275 N / mm ² (2) Next, the reasons for limiting the product characteristics will be described.

In order to investigate the conditions for obtaining the same dent characteristics as those of the currently used bake hardenable steel plate, the present inventors formed a 400 mm (mm) square-shaped panel as shown in FIG. A dent test was performed using a panel to examine the dent resistance of the test steel sheet. It should be noted that the comparative example uses a large amount at present.
A 340 N / mm ² bake hardening steel sheet was used.

The test method was as follows: after forming into a panel (maximum principal strain:
Ex = 2.0%, minimum principal strain: Ey = 1.5% (equivalent to door molding)) and simulated the heat treatment in the painting process, and heat-treated at 170 ° C. × 20 minutes. After that, with the panel restrained, various loads were added to the center with a ball head indenter with a diameter of 50 mm,
The dent resistance was evaluated.

FIG. 2 shows a 340N class bake hardening steel sheet and a 340N class
It is a graph which shows one example of the result of the dent test about IF type steel sheets, and as can be seen, it can be seen that the IF type steel sheets are inferior in dent resistance compared to the baking hardening steel sheets of the same strength level. .

Next, FIG. 3 shows the result of a test for investigating the ultimate σ _{2% + Baking} for obtaining dent resistance equivalent to that of a 340N class bake hardening steel sheet. Test is 2% for various materials
Stress (σ _{2% + Baking} ) reached when heat treatment is performed at 170 ° C for 20 minutes after tensile prestrain is applied, and the panel molded plate is _heated to 170 ° C.
The amount of residual deflection when a load of 22 kg was added after heat treatment for × 20 minutes was investigated.

When σ _{2% + Baking} <275 N / mm ² , 340 N
The result is that the residual deflection is greater than in the case of the baking hardenable steel sheet and the dent resistance is poor.To have properties equal to or higher than that of the 340N bake hardenable steel sheet,
It is understood that _{2% + Baking} ≧ 275 N / mm ² is required.

Further, if 340N grade bake hardenability steel sheet and the initial YS are identical ^{(230N / mm 2), σ} 2% -YS < at 25 N / mm ² sigma
_In order to obtain _{2% + Baking} ≧ 275 N / mm ² , the bake hardening amount is required ≧ 20 N / mm ² , and the heat treatment conditions must be restricted. That is, when the bake hardening amount is ≧ 20 N / mm ² , the bake hardening amount is significantly deteriorated due to the temperature drop.

Therefore, in the present invention, the following formula (1)
It stipulates that the condition of (2) is satisfied. σ _2% −YS ≧ 25N / mm ²・ ・ ・ ・ (1) σ _{2% + Baking} ≧ 275 N / mm ²・ ・ ・ (2)

[0044]

EXAMPLES A steel slab having the chemical composition shown in Table 1 was obtained using a converter and a continuous casting apparatus. After heating these slabs to 1200 ° C. in the usual manner, finish rolling was completed at 920 ° C. and wound at 500 ° C. to form a 3.5 mm hot-rolled steel strip. Thereafter, it was pickled, cold rolled from 3.5 mm to 0.75 mm, and recrystallized and annealed at a soaking temperature of 840 ° C. in a continuous annealing line or a continuous annealing-galvanizing line. In the continuous annealing-hot dip galvanizing line, after recrystallization annealing,
A hot dip galvanizing treatment was performed at a bath temperature of 465 ° C. and an alloying temperature of 500 ° C., with a surface area per unit area of 30/60 (g / m ² ).

Next, a test piece was cut out from the steel sheet thus obtained, and the tensile properties, the BH content and the dent test described above were performed. From the results of the above experiment, 0.02% of the 340 N / mm ² bake hardenable steel sheet was obtained. mm Additional load when residual deflection occurs = 22 kg
Therefore, a load of 22 kg or more was judged to be acceptable.

Further, in order to investigate the aging property of the steel sheet, 100 ° C. ×
Heat treatment was performed for 60 minutes, and aging was evaluated by YPE at that time, and after aging, YPE ≦ 0.3% was regarded as a pass. The secondary work brittleness resistance was evaluated at a temperature at which a vertical crack (brittle crack) occurs at that time after performing a dropping test after forming into a cylindrical shape at a drawing ratio of 1.8. A vertical crack initiation temperature of -30 ° C or less was judged as acceptable.

The material passed through the hot-dip galvanizing line was evaluated by visually observing the wettability of the plating when the line was passed along with the above-mentioned investigation.
After forming at a drawing ratio of 1.8, plating was peeled off with a tape, and the amount of plating adhered to the tape was measured by a gravimetric method. Tables 2 and 3 collectively show the obtained results.

For steel type No. 1, since the C content is outside the scope of the present invention, Y
Since both S and TS are low and σ _{2% + Baking} does not reach the target level, the dent resistance is inferior. Steel type No.2 is
Although the dent resistance is excellent at 27 kg, the yield ratio (YS / TS) is high and the r value is low because the Ti and Ti * / C values are outside the present invention.
1.0% of YPE after aging has occurred
(Cracking, stretcher strain) is a concern.

Further, steel types Nos. 3, 4, and 5 are P, S, respectively.
Since i and Mn are outside the present invention, elongation is inferior. In addition,
Steel type No. 3 also has poor secondary work brittleness resistance. Table 3
Shows the properties of the galvannealed material. The mechanical performance and the dent characteristics are almost the same as those of the cold-rolled material, and it can be seen that the steel of the present invention exhibits excellent characteristics even as a galvannealed steel sheet. Steel type No. 11 with high Si in the steel of the present invention
Only the plating wettability was slightly inferior.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

[0053]

According to the present invention, a high-strength steel sheet excellent in dent resistance can be obtained by solving the problems of the conventional bake hardenable steel sheet, and is of great industrial value. Also,
Even when the steel sheet according to the present invention is subjected to electroplating for improving corrosion resistance in an electroplating line,
The same effect is obtained when hot-dip galvanizing is performed. Further, the steel sheet according to the present invention can be applied to parts requiring dent resistance, such as an automobile door, and can be sufficiently used as a substitute for the existing steel sheet.

[Brief description of the drawings]

FIG. 1 is a schematic view of a panel formed into a kamaboko type panel.

FIG. 2 is a graph showing the relationship between applied load and residual deflection of a 340N class bake hardening steel sheet and a 340N class IF type steel sheet.

FIG. 3 is a graph showing a relationship between a residual deflection amount and σ _{2% + Baking} .

Claims (6)

[Claims] C .: 0.011 to 0.04%, Si: 1.5% or less, Mn: 0.07 to 2.5% by mass.
%, P: 0.005 to 0.10%, S: 0.02% or less, Al: 0.01 to 0.10
%, N: 0.008% or less, Ti: (Ti ^* % / C%) ≧ 4, where Ti ^* <%> = Ti <%> − (48/14) N <%> − (48/32)
S <%> High formability cold-rolled high-strength steel sheet excellent in dent resistance, characterized by having a steel composition in which the balance of components is Fe and unavoidable impurities. 2. The steel composition according to claim 1, further comprising:
The high-formability cold-rolled high-tensile steel sheet excellent in dent resistance according to claim 1, which contains one or two of 005 to 0.05% and B: 0.0001 to 0.0040%. 3. The steel composition further comprises Cu: 1.0% by mass.
The high-formability cold-rolled high-tensile steel sheet excellent in dent resistance according to claim 1 or 2, which contains one or more of Cr: 1.0% or less, and Ni: 1.0% or less. 4. The dent resistance according to claim 1, wherein the properties of the cold-rolled steel sheet after hot rolling, cold rolling and recrystallization annealing satisfy the following conditions (1) and (2). Excellent high formability cold rolled high strength steel sheet. Initial yield stress by tensile test: YS Stress when 2% tensile prestrain is applied: σ _2% , Stress after 2% tensile prestrain + Baking (170 ° C. × 20 minutes): σ
_{2% + Baking} σ _2% −YS ≧ 25 N / mm ²・ ・ ・ (1) σ _{2% + Baking} ≧ 275 N / mm ²・ ・ ・ (2) 5. The high-tensile steel sheet according to claim 1, wherein Si: 0.5% or less, P: 0.005 to 0.045%.
High-formability hot-dip galvanized high-strength steel sheets that are hot-dip galvanized. 6. The properties of the steel sheet after hot rolling, cold rolling, recrystallization annealing and hot dip galvanizing have the following conditions (1):
The hot-dip galvanized high-strength steel sheet having excellent dent resistance according to claim 5, which satisfies (2). Initial yield stress by tensile test: YS Stress when 2% tensile prestrain is applied: σ _2% , Stress after 2% tensile prestrain + Baking (170 ° C. × 20 minutes): σ
_{2% + Baking} σ _2% −YS ≧ 25 N / mm ²・ ・ ・ (1) σ _{2% + Baking} ≧ 275 N / mm ²・ ・ ・ (2)