JP3537477B2 - Cold rolled steel sheet produced by continuous annealing with excellent stretch flangeability and stable paint bake hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is manufactured by continuous annealing using ordinary low carbon Al-killed steel, which has excellent hole expandability and stable paint bake hardenability (hereinafter referred to as BH property). Related to a high-strength cold-rolled steel sheet having a strength level of 300 to 450 MPa.

[0002]

2. Description of the Related Art The strength level of continuous annealing is 300-4.
The production of a 50 MPa class cold rolled steel sheet includes a method using a low carbon Al-killed steel and a method using an ultra low carbon steel. As for the annealing method, there are a box-type annealing method and a continuous annealing method. Further, in order to increase the strength, it is customary to add a solid solution strengthening element such as Si, Mn or the like mainly to P.

[0003] Since these steel sheets are mainly used for panels of automobiles and home electric appliances, excellent workability is required.
Since the deformation mode differs depending on the part, the required processing characteristics change accordingly, but stretch flange forming for enlarging a punched hole or the like may be important. The stretch flangeability of cold-rolled steel sheets has not been studied so far, but precipitates such as cementite are the starting points of cracks during stretch flange forming. It is well known that steel is superior.

[0004] On the other hand, as a technique for increasing the strength after baking of a paint, a BH cold-rolled steel sheet utilizing strengthening of strain aging of C and N is well known. However, it is basically impossible to impart BH properties to ultra-low carbon steel in which C and N are fixed by Ti and Nb. In addition, T of less than the stoichiometric ratio to C and N
Methods are known which aim at adding i, Nb and re-dissolving TiC or NbC by high-temperature annealing. However, in these methods, in order to obtain the BH property stably, it is premised that the components are properly applied in a narrow range and remarkable high-temperature annealing is performed, and it is not always easy and many problems remain.

[0005]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is that, as described above, the continuous annealing using low carbon Al-killed steel does not depend on the extremely low carbon steel having a high melting cost. 1) improving the stretch flangeability of the cold-rolled steel sheet;
2) To provide stable BH property.

[0006]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and expresses C: 0.0 by weight%.
1 to 0.04%, Si: 0.005 to 0.4%, M
n: 0.05 to 0.30%, P: 0.004 to
0.13%, S: 0.003 to 0.02%, Al:
0.015 to 0.15%, N: 0.0040% or less,
B: 0.0002 to 0.0020% if necessary, and the balance is made of steel consisting essentially of unavoidable impurities with Fe, subjected to ordinary hot rolling, cold rolling, continuous annealing, and temper rolling. A steel sheet obtained by a process including the following process, wherein the intragranular cementite density (ρ, ケ / mm ² ) is expressed by the following equation in relation to the P concentration (C _P ): ρ (ke / mm ² ) ≧ 0.5 × 10 ⁵ (1 + 1.2 × C _P ), excellent stretch flangeability with a hole expansion ratio (d / d ₀ ) of 2.1 or more, and stable paint bake hardenability of 30 to 80 MPa. Is a cold-rolled steel sheet having a strength level of 300 to 450 MPa grade produced by continuous annealing.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the numerical values of the component elements will be described. C: The lower limit of C being 0.01% is determined from the viewpoint of aging. That is, if it is less than 0.01%, the degree of supersaturation of C is insufficient, so that carbide precipitation during overaging of continuous annealing is insufficient. On the other hand, if the content exceeds 0.04%, workability such as sufficient overhang property and deep drawability cannot be obtained.

Si: 0.005 to 0.4%. 0.
If it is less than 005%, a remarkable increase in cost will be caused, and if it is more than 0.4%, the chemical conversion property will deteriorate. Mn: As the amount of added Mn is smaller, the deep drawability is improved. However, if it is less than 0.05%, hot embrittlement is caused in the case of a usual amount of S. If it exceeds 0.30%, the deep drawability deteriorates.

P: P is known as an element for effectively solidifying ferrite in solid solution, and in order to obtain a cold-rolled steel sheet having a tensile strength of 300 to 450 MPa class, which is an object of the present invention,
0.004 to 0.13%. If the content is less than 0.004%, there is a problem that the melting cost increases. On the other hand, 0.1
If it exceeds 3%, the strength becomes higher than the desired strength level,
In addition, workability and spot weldability are significantly reduced. S: From the viewpoints of hot embrittlement and workability of the product, S has an upper limit of 0.020%. If it is less than 0.005%, the density of MnS as nuclei of cementite precipitated in grains is excessively reduced, and a desired hole expansion ratio and BH property cannot be achieved.

Al: If the Al content is less than 0.015%, deoxidation may be insufficient, whereas if it exceeds 0.15%, the workability is deteriorated. N: A smaller amount is more preferable. On the other hand, when the content exceeds 0.0040%, workability is remarkably deteriorated. B: P is added as necessary to ensure the secondary workability of the high-strength steel sheet to which P is added. However, if the content is less than 0.0002%, the effect is hardly recognized, and if it exceeds 0.0020%, the workability is significantly impaired. In addition, slab cracks and the like are caused.

Next, the stretch flangeability will be described. The stretch flangeability was evaluated by a hole expansion ratio (d / d ₀ ) in a punched hole expansion test. This method is called the KWI method, and is applied to a steel plate having a side of 80 mm and a diameter of 12 mm.
and a sample of the hole expanding test one punched out holes with a diameter d ₀ due mm punch and the plate thickness of 10% clearance die which gave (one side). In the hole expansion test, the punched hole of the above-mentioned test piece was pushed and expanded with a cylindrical punch having a diameter of 40 mm from the surface of the punched hole without burrs using a press tester. And the cracks are stopped when the crack penetrates the plate thickness. The hole diameter at this time (d) and the original hole diameter (d
₀ )) (d / d ₀ ).

The relationship between d / d ₀ and the intragranular cementite density (ρ) in a cold-rolled steel sheet having a strength level of 300 MPa class will be described with reference to the drawings. The components shown in Table 1, hot rolled,
Using the actual cold-rolled sheet under the cold-rolling conditions as a raw material, a simulation of continuous annealing shown in FIG. 1 was performed in a laboratory to produce materials having various ρ.

[0013]

[Table 1]

[0014] That is, [rho increases with decreasing T _E in FIG. After temper rolling (reduction rate: 1.2%), d / d ₀ was measured by the above-described evaluation method. Also, ρ is 10,000
The magnification was determined using an extracted replica electron micrograph. FIG. 2 shows the relationship between d / d ₀ and ρ. It became clear for the first time that the hole expansion ratio improved with the increase in intragranular cementite density. And, as in the present invention, ρ is 0.5
When it is not less than × 10 ⁵ / mm ² , the d / d of the conventional ultra-low carbon cold-rolled steel sheet used as a comparative material shown in the following examples
₀ (= 2.1) or more, which is significantly better than the box-annealed low-carbon Al-killed steel. Table 2
Table 1 also shows the high-strength cold-rolled steel sheets to which P was added as shown in
The same experiment was performed with the 0.008% P material. As a result, it was found that d / d ₀ increased with ρ, but the critical density increased with the amount of P, and was expressed as in the following equation (1). ρ (q / mm ² ) ≧ 0.5 × 10 ⁵ (1 + 1.2 × C _P ) (1) Here, C _P (wt%) represents the P amount.

The reason why the hole expandability is improved by the presence of intragranular cementite is considered as follows. When cementite is uniformly present in the grains, unlike the case where cementite is present at the crystal grain boundaries in a box-annealed low-carbon Al-killed cold-rolled steel sheet, 1) microcracks are specified as grain boundary cementite at the time of punching. 2) It is presumed that cracks during the hole expansion test also progress uniformly. In addition, when the amount of P increases, ductility deteriorates, so that stretch flangeability generally decreases. Therefore, it is necessary to increase the intra-granular cementite density.

As is well known, the amount of BH is uniquely determined by the amount of dissolved C. When a heat treatment as shown in FIG. 1 is performed using a low-carbon Al-killed steel, a desired amount of solute C can be stably obtained. According to the invention described in JP-A-2-225644, by changing the quenching cooling rate (CR), the end point temperature (T _E ), and the overaging time (t _OA ) among the heat treatment conditions in FIG. , Easily desired solid solution C
It is possible to obtain a stable amount. In the present invention, the target BH amount is in the range of 30 to 80 MPa. here,
After adding a 2% tensile prestrain, the BH amount was 170 ° C.-2
This is the difference between the deformation stress (upper yield stress) before and after the heat treatment when the tensile test is performed again after the heat treatment for 0 min. When the BH amount is less than 30 MPa, the hardening amount of the paint-baked hardened steel sheet is too small. On the other hand, when the BH amount exceeds 80 MPa, the formability between the production of the cold-rolled steel sheet and the press-forming is severely deteriorated. Next, how the present invention is useful as compared with the box-type annealed Al-killed steel and the IF steel will be described in Examples.

[0017]

EXAMPLE A steel having the chemical composition shown in Table 2 described above was tapped, formed into a slab by continuous casting, heated to 1140 ° C., and heated to a finishing temperature of 925 ° C. and a sheet thickness of 4 mm. Cold rolling was performed, followed by ROT cooling at an average cooling rate of 25 ° C./s, and then winding at 710 ° C. After pickling, cold rolling was performed to a thickness of 0.75 mm, followed by continuous annealing. The continuous annealing conditions are as follows: annealing temperature: 810 ° C., soaking: 5
0s, first slow cooling: cooling down to 680 ° C at 5.5 ° C / s, rapid cooling: cooling down to 260 ° C at 90 ° C / s, reheating conditions: 340
Heat to 30 ° C / s at 30 ° C, overaging condition: 270 of end temperature
The temperature was cooled to 200 ° C. with an overage time of 200 s. afterwards,
Temper rolling was performed at a rolling reduction of 1.2%, and the test was performed.

After the temper rolling, tensile and hole expansion tests were performed. The tensile test was performed using a test piece of JIS Z2201, No. 5 according to the method described in Z2241. The hole expansion test was evaluated by the method described above. BH and intragranular cementite density were also determined. The method for evaluating BH and intragranular cementite has already been described.

Further, steels F and G, which are currently mass-produced, are added to Table 2 for comparison. Steel F is a Ti-added ultra low carbon cold rolled steel sheet manufactured by continuous annealing, and steel G is a low carbon Al killed cold rolled steel sheet manufactured by box type annealing. The test results are shown in Table 3.

[0020]

[Table 2]

[0021]

[Table 3]

As is clear from this table, the steels A, B, C, D, and E according to the present invention have almost the same d / d _{0 as} the comparatively annealed ultra-low carbon cold rolled steel sheet (steel F). It turns out that it is equivalent and is superior to the box-type annealed low carbon Al killed cold rolled steel sheet (steel G). Further, there is a feature that the extremely low carbon cold rolled steel sheet and the box-shaped annealed low carbon Al killed cold rolled steel sheet have stable BH property, which is extremely difficult. Further, it is understood that the material of the present invention is excellent also in other tensile property values.

[0023]

The strength level disclosed by the present invention is 300 to
A 450 MPa class cold rolled steel sheet has a hole expansion property equal to or higher than that of a conventional ultra-low carbon cold-rolled steel sheet that has a remarkably high melting cost, and has a stable BH property that is extremely difficult with an ultra-low carbon cold-rolled steel sheet. In addition, it has better hole-expanding properties than conventional box-annealed low-carbon Al-killed cold-rolled steel sheets, and has more stable and high BH properties. The present invention has basically improved the drawbacks of the conventional cold-rolled steel sheet. INDUSTRIAL APPLICABILITY The cold-rolled steel sheet of the present invention is suitable for parts such as panels and frames of automobiles and electric appliances, and also for parts such as undercarriage of automobiles, and can reduce the thickness thereof. It is also suitable as an original sheet for various surface-treated steel sheets such as electrogalvanized, and is expected to have an extremely large industrial effect.

[Brief description of the drawings]

FIG. 1 shows heat treatment conditions for changing the intragranular cementite density.

FIG. 2 shows the relationship between the intragranular carbide density and the hole expansion ratio (d / d ₀ ).

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tosumi Ueda 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Tokutoshi Kimura 1 Kimitsu, Kimitsu City, Chiba Prefecture New Japan (56) References JP-A-2-225644 (JP, A) JP-A-5-287446 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) C22C 38/00 301 C22C 38/06 C21D 8/02

Claims (2)

(57) [Claims] C .: 0.01 to 0.04%, Si: 0.005 to 0.4%, Mn: 0.05 to 0.30%, P: 0.004 to 0.4% by weight. 13%, S: 0.003 to 0.02%, Al: 0.015 to 0.15%, N: 0.0040% or less, with the balance being Fe and substantially unavoidable impurities. , A steel sheet obtained by performing a process including normal hot rolling, cold rolling, continuous annealing, and temper rolling, and having an intragranular cementite density (ρ, ケ / mm ² )
Is related to the P concentration (C _P ) by the following equation: ρ (q / mm ² ) ≧ 0.5 × 10 ⁵ (1 + 1.2 × C _P ), and the hole expansion ratio (d / d ₀ ) Is a cold-rolled steel sheet having an excellent stretch flangeability of 2.1 or more and a strength level of 300 to 450 MPa grade manufactured by continuous annealing stably having a paint bake hardenability of 30 to 80 MPa. 2. C: 0.01 to 0.04%, Si: 0.005 to 0.4%, Mn: 0.05 to 0.30%, P: 0.004 to 0.4% by weight. 13%, S: 0.003 to 0.02%, Al: 0.015 to 0.15%, B: 0.0002 to 0.0020%, N: 0.0040% or less, the balance being Fe A steel sheet obtained by treating a steel substantially consisting of unavoidable impurities with a process including normal hot rolling, cold rolling, continuous annealing, and temper rolling, wherein the intragranular cementite density (ρ, ke / Mm ² )
Is related to the P concentration (C _P ) by the following equation: ρ (q / mm ² ) ≧ 0.5 × 10 ⁵ (1 + 1.2 × C _P ), and the hole expansion ratio (d / d ₀ ) Is a cold-rolled steel sheet having an excellent stretch flangeability of 2.1 or more and a strength level of 300 to 450 MPa grade manufactured by continuous annealing stably having a paint bake hardenability of 30 to 80 MPa.