JPH0559445A - Production of cold rolled non-ageing steel sheet excellent in baking hardenability and workability - Google Patents

Abstract

PURPOSE:To obtain the non-ageing cold rolled steel sheet excellent in ageing characteristic at ordinary temp. by specifying cooling conditions in the course of continuous annealing and contained elements, respectively, and regulating the form of carbide precipitation after continuous annealing, the amount of solid-solution C, and the ratio among contained elements, respectively. CONSTITUTION:A steel having a composition consisting of, by weight, 0.005-0.025% C, <=0.08% Si, 0.05-0.80% Mn, 0.02-0.15% P, 0.03-0.10% Sol.Al, <=0.0080% N, and the balance Fe is used. This steel is hot-rolled, pickled, cold-rolled at >=60% draft annealed at a temp. in the range between the recrystallization temp. and 900 deg.C, and cooled down to a temp. between 300 deg.C and room temp. at >=10 deg.C/sec average cooling rate. Successively, this steel sheet is subjected to overageing treatment at 330-450 deg.C and then subjected to continuous annealing in order to regulate the volume percentage of intergranular carbide based on the total carbide precipitated in the course of continuous annealing and overageing treatment to >=80%. By this method, the steel sheet well balanced between baking ageing hardenability and ordinary-temp. ageing characteristic and having high press formability as to withstand working even when the working is done under severe conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a non-aged cold-rolled steel sheet excellent in bake hardenability and workability, more specifically, it is excellent in workability and bake hardenability by continuous annealing, and further at room temperature. The present invention relates to a method for producing a non-aging cold rolled steel sheet which is also excellent in aging.

[0002]

2. Description of the Related Art Generally, the properties required for a processing material such as an outer panel of an automobile are that it has high press formability and shape fixability in order to be processed into a predetermined shape. It has a beautiful appearance without wrinkles called stretcher strain on the surface, and when the material is thinned to reduce the weight of the car body,
It has been mentioned that it has sufficient yield strength so as not to impair dent resistance.

As a measure for satisfying such characteristics at the same time, a cold-rolled steel sheet which is soft at the time of working, which can increase the yield strength in the coating baking process, and which is excellent in normal temperature aging A method for producing the alloy by continuous annealing has been proposed (Japanese Patent Publication No. 59-020733, Japanese Patent Application Laid-Open No. 02-125817, Japanese Patent Application Laid-Open No. 61-1).
17225, JP-A-58-022333,
(See JP-A-56-119734).

In these various proposed technologies,
By specifying the C content and the annealing conditions, the solid solution C content level that obtains a predetermined bake hardening is secured without impairing the room temperature strain aging resistance, and the solid solution hardening of Si, Si, P, etc. is utilized. This controls the intensity level. Further, proposals of positively containing P as an element for improving bake hardenability are described in JP-A-56-119734 and JP-A-02-125817.

However, in these proposals,
As a mechanism for improving the bake hardenability, the amount of solid solution C in iron ferrite is maintained at a predetermined amount, and essentially the presence of a considerable amount of solid solution C was necessary. In addition, various measures have been proposed for the evaluation of room temperature aging, and after room temperature aging (for example, 38 ° C. × 16 days to 30 days; JP-B-59-0207).
33, Japanese Patent Application Laid-Open No. 02-125817, Japanese Patent Application Laid-Open No. 56-119734).

Generally, in order to prevent stretcher strains which impair the appearance quality of press-formed products, an aging index (AI; 10% pre-strain after temper rolling is applied,
It is well known that the difference between the yield point of the aged material after aging at 100 ° C. for 1 hour and the 2% flow stress at the time of prestraining must be 30 N / mm ² or less.

However, when continuous annealing is performed in the range of the C content level (0.001 to 0.03 wt%) according to the various proposals described above, a sufficiently low AI is achieved and a high BH amount (baking) is achieved. It was difficult to set the amount (curing amount), and it was insufficient as a material from the viewpoint of stretcher strain resistance.

Further, R. Stevenson (Met.
Trans. A, 1980, p. 1909), solid solution C in ferrite reduces local elongation and is an effective index value for evaluating the workability of steel sheet.
DH ₀ (AK Ghosh: Met. Eng. Q.,
1974, p. There was a problem of decreasing 30). Also, from the viewpoint of workability, the material manufactured by the conventional method is insufficient.

[0009]

DISCLOSURE OF THE INVENTION In view of various problems of each of the techniques proposed in the related art as described above, the present invention has been earnestly studied and studied by the present inventor. As a result of stacking, in order to maintain both the normal temperature aging resistance and the workability while securing a high bake hardening amount (BH amount), the cooling conditions during continuous annealing were specified, and a certain amount of carbide was included in the grains. It was found that it is preferable to specify the contained element in order to reduce the amount of solid solution C and to obtain a high BH amount at a C level of a low solid solution amount by precipitating. By controlling the morphology, the amount of dissolved C and the content ratio of the contained elements, both the room temperature aging resistance and the bake hardenability are maintained, and the workability of the bake hardenable Al killed steel sheet by the conventional continuous annealing is maintained. Is a solution to the problem of Ri, yield stress (YS) 270
In N / mm ² or less, a tensile strength (TS) 330N / mm ² or more, the total elongation of 40% or more, r value is 1.3 or more, the bake hardenability amount indicating a bake hardenability (BH amount) 40N / mm ² or more ,
Aging index (AI) showing room temperature aging resistance is 30 N / mm ²
Hereinafter, a method for producing a non-aging cold-rolled steel sheet having LDH _0, which is an index of local ductility, of 31 mm or more and having excellent bake hardenability and workability was developed.

[0010]

The method of producing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention is characterized by C 0.005 to 0.025 wt.
%, Si ≤ 0.08 wt%, Mn 0.05 to 0.8
0wt%, P 0.02-0.15wt%, Sol.Al
0.03 to 0.10 wt%, N ≤ 0.0080 wt%
Steel containing the balance Fe and unavoidable impurities,
After hot rolling and pickling, and cold rolling at a rolling reduction of 60% or more, annealing is performed at a temperature of recrystallization temperature or more and 900 ° C or less, and then at an average cooling rate of 10 ° C / sec or more. After cooling to a temperature of 300 ° C. to room temperature and subsequently performing an overaging treatment at a temperature of 330 to 450 ° C., the volume percentage of the intragranular carbide with respect to all the carbides precipitated during continuous annealing and overaging treatment is 80. The purpose is to carry out continuous annealing in order to achieve the above percentage.

The method for producing the non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention will be described in detail below. First, the contained components and component ratios of steel referred to in the method for producing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention will be described.

[0012] C is an element which is limited in view of age hardening, and when the content is less than 0.005 wt%, the degree of C supersaturation in the supercooling process during continuous annealing is low and only the precipitation of carbide in the grain is driven. Since the strength cannot be obtained, the amount of solid solution C in the ferrite after annealing cannot be reduced, and room temperature aging resistance cannot be expected, and the content exceeds 0.025 wt%. If it is included in the alloy, workability will be deteriorated. Therefore, the C content is 0.005 to 0.
It is set to 025 wt%.

If the Si content exceeds 0.08 wt%, the number of precipitation sites of carbides that precipitate in the grains during the supercooling process can be reduced and the amount of solid solution C remaining after annealing can be effectively reduced. It becomes difficult and deteriorates the room temperature aging resistance. Therefore, the Si content is 0.08 wt% or less.

If the content of Mn is less than 0.05 wt%, hot brittleness cannot be expected to be prevented, and 0.80 w
If the content exceeds t%, deterioration of press formability cannot be avoided. Therefore, the Mn content is 0.05 to 0.8.
It is set to 0 wt%.

P is an important element that significantly improves the bake hardenability and improves the press formability, especially the deep drawability. As shown in FIG. 1, if the content is less than 0.02 wt%, the bake effect is not achieved. I can't expect to improve it, and
If it is contained in a large amount exceeding 15 wt%, the ferrite is hardened and the press formability is greatly reduced. Therefore, P
The content is 0.02 to 0.15 wt%. In FIG. 1, ● indicates bake hardenability (BH) and 2% prestrain; 170
The amount of increase in stress after aging at ℃ × 20 min is shown. In addition, ◯ indicates normal temperature aging (AI), 10% pre-strain; 100 ° C. × 1
The amount of stress increase after hr aging is shown.

Sol. Al is an element that has the effect of deoxidizing and fixing N in steel and removing the solid solution N that is extremely harmful to room temperature aging resistance. If the content is less than 0.03 wt%, fixing of N is not possible. Sufficient and 0.10 wt%
If it is contained in an amount exceeding the above range, the workability is deteriorated. Therefore, S
ol. The Al content is 0.03 to 0.10 wt%.

If N is contained in an amount exceeding 0.0080 wt%, workability is deteriorated. Therefore, the N content is set to 0.0080 wt% or less.

Next, manufacturing conditions in the method for manufacturing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention will be described. That is, the steel having the above-mentioned contained components and component ratios is subjected to pickling after hot rolling by a normal process, and subsequently, when cold rolling-continuous annealing is performed, a cold rolling rate, an annealing temperature, and a cooling. The conditions, the overaging temperature and the structure of steel will be specifically described.

If the cold rolling ratio of cold rolling is less than 60%,
Since bake hardenability and press workability cannot be expected to improve, it is necessary to carry out cold rolling at a rolling reduction of 60% or more.

Annealing after cold rolling is performed at a recrystallization temperature of 90 ° C or higher.
Since it is performed at a temperature of 0 ° C. or lower, it is necessary to set the temperature to the recrystallization temperature or higher in view of press workability, and if the temperature is lower than the recrystallization temperature, good deep drawability cannot be obtained.
If the temperature exceeds ℃, austenitization and grain growth are promoted, and press workability and bake hardenability deteriorate. Therefore, the annealing temperature is set to the recrystallization temperature or higher and 900 ° C. or lower.

The cooling rate after annealing must be 10 ° C./sec or more, and if the cooling rate is less than 10 ° C./sec, precipitation of carbides proceeds during cooling and is necessary for carbide precipitation in the subsequent overaging step. The driving force is greatly reduced, the amount of residual solid solution C is increased, and the room temperature aging resistance is deteriorated. Therefore, the cooling rate is 10 ° C./second or more.

In the overaging process, the temperature is from room temperature to 30.
It is necessary to cool to a temperature of 0 ° C. at the above cooling rate (supercooling treatment), which increases the driving force required for precipitation by supercooling, so that the precipitation site moves from the grain boundary to the inclusion interface in the grain and It is to change to holes. Therefore, the nucleation density of the intragranular precipitates is appropriately increased, the mean free path required for the precipitation of the solid solution C is decreased, and the subsequent 330 to 45 are performed.
By overaging treatment at a temperature of 0 ° C., the carbide grows and the absolute amount of residual solid solution C decreases, maintaining good room temperature age hardening resistance and good press workability evaluated by LDH ₀ . can do.

When the supercooling temperature exceeds 300 ° C., the ratio of the intragranular precipitation sites moving from the grain boundaries into the grains is reduced, and the supercooling effect cannot be obtained. Also, the overaging temperature is 3
If the temperature is lower than 30 ° C, the growth of carbide is slow, and it is necessary to lengthen the residence time in the overaging zone. If the temperature exceeds 450 ° C, the effect of improving the bake hardenability and press workability due to overaging is small and the energy loss. Becomes

Further, the structure in which the volume percentage of the intragranular carbide with respect to the total precipitated carbide is 80% or more will be described.

In the conventional manufacturing method for bake hardenable steel sheet, a certain amount of solid solution C is added to maintain bake hardenability.
It is necessary to keep the residual amount of N, and there is no particular need for supercooling. Therefore, sufficient room temperature aging resistance and press workability cannot be obtained. That is, in the conventional bake hardening type Al killed steel sheet by continuous annealing, the room temperature aging property, the baking aging property and the press workability cannot be satisfied at the same time. Had a problem.

Therefore, in the method for producing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention, by supercooling and overaging to generate carbide precipitates mainly in the grains, It was clarified that the amount of solute C can be sufficiently reduced, and further the high baking aging property and press workability can be maintained by adjusting the component ratio.

Further, as a result of investigation, the volume ratio of intragranular carbide to the total carbide in the microstructure of the steel produced under specific conditions, room temperature aging (AI), press workability (LD)
It was found that there is a relationship between H ₀ ) as shown in FIG. That is, the volume percentage of the intragranular carbide needs to be 80% or more, and if it is less than 80%, the room temperature aging resistance and the press workability are significantly deteriorated. The bake-hardening amount (BH amount) of 50 N / mm ² or more is secured. Figure 2
In, AI is 10% prestrain; stress increase after 100 ° C. × 1 hr aging.

[0028]

[Examples] Examples of the method for producing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention will be described.

[0029]

[Practical example 1] A steel having the components and component ratios shown in Table 1 was melted into an ingot, and the finishing temperature was 900 ° C and the coiling temperature was 70 ° C.
After hot rolling at 0 ° C. to a plate thickness of 3.0 mm and descaling by pickling, cold rolling and continuous annealing were performed under the conditions shown in Table 2.

Supercooling was carried out by water-cooled roll cooling controlled to an appropriate temperature or gas jet cooling, and kept at a predetermined supercooling temperature for 5 seconds. Further, the overaging temperature is the maximum temperature in the treatment, and the overaging temperature is maintained for 2 minutes or more, or the residence time at the sound door between the maximum overaging temperature and 100 ° C or less is 2 minutes or more. Gradual cooling so that
The cooling was performed at a cooling rate of not less than / sec.

Next, after temper rolling of 1.0 to 2.0%, various tests were conducted. -Mechanical tests conformed to the method specified in JIS. -The volume fraction of carbide in the grain was measured by a scanning electron micrograph of 1000 times. The LDH0 test was conducted by using a spherical punch having a diameter of 101.6 mm and a die matched to the thickness of the test material, a crosshead speed of 0.42 mm / sec, a wrinkle holding pressure of 33 tons, and oil lubrication. The results are shown in Table 3. In Table 3, AI and BH are as follows. AI; after tempering of 1.0 to 2.0%, 10% pre-strain, 100 ° C
× Difference between yield point after aging for 1 hr and stress at 10% prestrain BH; after tempering of 1.0 to 2.0%, 2% prestrain, 170 ° C ×
Difference between yield point after 20 min aging and stress at 2% prestrain

From Table 3, the steels No. 1 to No. 5 produced by the method for producing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention are all AI ≦ 30.
N / mm ² , BH ≧ 40 N / mm ² , LDH ₀ ≧ 31 m
m, El ≧ 40%, showing excellent room temperature aging resistance,
It can be seen that it has baking age hardening and press workability. Further, a steel sheet manufactured under conditions outside the range of the manufacturing method of the non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention (Comparative Example No. 6 has a low C content, No. No. 7 has a high C content, No. 8 has a high Si content, No. 9 has a high Mn content, No. 10 has a low Mn content, and plate cracking occurs due to hot rolling. P content is low, No.12 P content is high, No.13
Has a low Al content, No. 14 has a high Al content,
No. 15 has a low cold rolling rate, No. 16 has a low cooling rate, No. 17 has a supercooling temperature and an overaging temperature out of the range,
No. 18 has a high supercooling temperature, and No. 19 has a low overaging temperature. It can be seen that in (), although AI is too high even if BH is high, or LDH0 is too low, the balance between aging characteristics and workability is poor.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

As described above, since the method for producing a non-aging cold rolled steel sheet excellent in bake hardenability and workability according to the present invention has the above-mentioned constitution, the produced steel sheet is bake aged. It has an excellent effect that the curability and room temperature aging resistance are well balanced and that it has high press formability that can withstand even severe processing conditions.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of P content on AI and BH contents.

FIG. 2 is a diagram showing a relationship between AI and LDH ₀ and an intra-grain carbide volume ratio.

Claims (1)

[Claims] 1. C 0.005-0.025 wt%, Si
≤ 0.08 wt%, Mn 0.05 to 0.80 wt%, P 0.02 to 0.15
wt%, Sol.Al 0.03 to 0.10 wt%, N ≤ 0.0
Steel containing 080 wt% and the balance Fe and unavoidable impurities,
After hot rolling and pickling, and cold rolling at a rolling reduction of 60% or more, annealing is performed at a temperature of recrystallization temperature or more and 900 ° C or less, and then at an average cooling rate of 10 ° C / sec or more. After cooling to a temperature of 300 ° C. to room temperature and subsequently performing an overaging treatment at a temperature of 330 to 450 ° C., the volume percentage of the intragranular carbide with respect to all the carbides precipitated during continuous annealing and overaging treatment is 80. % Or more, continuous annealing is performed, and a method for producing a non-aging cold rolled steel sheet having excellent bake hardenability and workability.