JP3105380B2 - Manufacturing method of cold-rolled steel sheet for deep drawing with excellent dent resistance and surface distortion resistance - Google Patents

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 cold-rolled steel sheet for deep drawing having excellent dent resistance and surface distortion resistance.

[0002]

2. Description of the Related Art An IF steel sheet (Inter) in which Ti and Nb are added to ultra-low carbon steel and solid solution C and N are fixed in the form of carbonitride.
spatial atom free steels
heat) is widely used as a cold rolled steel sheet having excellent deep drawability. However, the weak point of this steel sheet is its softness, so it has excellent surface strain resistance, but its dent resistance is inferior, and when it is applied to an automobile outer plate, etc., its shape easily collapses when an external force is applied after molding. is there. On the other hand, it is known that increasing the yield point is effective for improving the dent resistance, but in this case, the shape freezing property is deteriorated, and the surface distortion is likely to occur. Japanese Patent Application Laid-Open No. 4-143 recently disclosed a method for manufacturing a steel sheet having a high-tens surface layer as a method for manufacturing a steel sheet having dent resistance and surface strain resistance.
No. 227 publication. However, the production method using a multi-layer steel plate is not practical because of a large increase in cost.

On the other hand, as a method for high-tension of only near-surface layer, carburized or well known and Hei 3-243757 and JP-A No. nitriding treatment 4-62
No. 46 discloses a cold-rolled steel sheet having high strength near the surface by nitriding and a method for producing the same. But,
In that invention, the surface layer is made high-tens to prevent dent resistance.
Mentions the possibility of improved dent resistance, but improved dent resistance
There is no example which shows hardening of the surface layer or an increase in the amount of N.
Or not shown. Regarding the manufacturing method,
It should be kept in an atmosphere containing ammonia.
Have been .

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a low-cost production of a cold-rolled steel sheet satisfying both the dent resistance and the surface distortion resistance, which has been possible only with a multi-layer steel sheet which has been accompanied by a large cost. It provides a way to:

[0005]

Means for Solving the Problems The inventors of the present invention have prepared a cold rolled steel sheet satisfying both the dent resistance and the surface strain resistance simultaneously by the continuous annealing.
Development of manufacturing technology by
Obtained. That is, simply hardening the surface layer or N
Increasing the amount does not necessarily increase the dent resistance.
Optimum nitriding to increase dent resistance by continuous annealing
There is a condition. The gist of the present invention is:
C: 0.005% or less, N: 0.01% or less, Mn: 1.5% or less, Si: 1.0% or less, P: 0.1% or less, Al: 0.005% or more by weight ratio 0.2% or less, one or both of Ti and Nb are C / 12 +
N / 14 + S / 32 <0.9 (Ti / 48 + Nb / 9
3) Satisfies the following condition and contains one or both of Ti and Nb in a total amount of 0.02% or more, and if necessary, B: 0.0002% or more and 0.002% or less, and the balance is added. After cold rolling the steel comprising Fe and unavoidable impurities, recrystallization annealing is performed in a continuous annealing furnace, and the concentration (%) of NH _{3 in} a nitriding atmosphere of not less than 600 ° C. and not more than 800 ° C. The product of stay time (sec) is 50 or more, 500
A method for producing a steel sheet for deep drawing excellent in dent resistance and surface distortion resistance characterized by performing nitriding treatment under the following conditions.

Hereinafter, the present invention will be described in detail.
The reasons for limiting the components of the present invention are as follows. Either or both of Ti and Nb are 0.9 (Ti / 48 + N
b / 93)> C / 12 + N / 14 + S / 32, and the addition of either or both of Ti and Nb was limited to 0.02% or more because of the C content in steel.
And N in the form of precipitates, allowing smooth crystal rotation with almost no solid solution of C and N during cold rolling, thereby improving the deep drawability of the product by subsequent recrystallization annealing (111) <112, which is an advantageous orientation for smoothing
>, (554) <225>, and the like, because it is possible to obtain a steel sheet having a texture with a high degree of integration, and to obtain an increase in strength required during nitriding.

On the other hand, C: 0.005% or less, N: 0.0
The addition of C, N and S in excess of 1% and S: 0.02% not only impairs the workability of the product but also satisfies the above condition by adding C, N and S. This is because the amount of Nb increases and the manufacturing cost is unnecessarily increased. In addition, as other components, the components normally contained for improving the strength, that is, the upper limits of Si, Mn, and P are respectively 1.0% or less for Si, 1.5% or less for Mn, and 0.
The reason for setting the content to 15% is that if added more than this, the workability is deteriorated. Further, since the addition of B enhances the secondary workability, it is effective to add 0.0002% or more of B as necessary, but if it exceeds 0.0050%, the workability is significantly deteriorated. The upper limit is set to 0.0050%. Al must be added in an amount of at least 0.005% in order to enable reliable deoxidation in molten steel. However, excessive addition deteriorates workability, so the upper limit was made 0.2%.

Next, the limitation of the manufacturing process will be described. Since the properties of the steel of the present invention do not largely depend on the heating conditions and hot rolling conditions, there is no particular limitation. That is, the slab may be hot-rolled after reheating, or may be hot-rolled directly after casting. Also, a thin cast piece from which hot rolling is omitted may be used. It is not necessary to limit the conditions of the cold-rolling, but a cold-rolling ratio of 70% or more is preferable in order to develop a texture having favorable deep drawability. Regarding the recrystallization annealing conditions, if the steel sheet is exposed to a nitriding atmosphere before recrystallization, recrystallization is remarkably suppressed, so it is necessary to recrystallize before the nitriding atmosphere.

[0009] The most important process conditions nitriding conditions of the present invention, the present invention steel 600 ° C. or higher, the concentration of NH ₃ in nitriding atmosphere at 800 ° C. or less (%) and residence time at this temperature range (in seconds) By performing the nitriding treatment under the condition of an area of 50 or more and 500 or less, a cold-rolled steel sheet satisfying both the dent resistance and the surface strain resistance can be manufactured. Nitriding temperature is 600
The reason why it is not possible to manufacture a cold-rolled steel sheet that simultaneously satisfies the dent resistance and the surface strain resistance at a temperature of less than or equal to ° C is because nitriding is limited to the vicinity of the surface, and it is not possible to realize nitriding to obtain the required characteristics. . On the other hand, when the temperature exceeds 800 ° C., the diffusion is conversely fast, so that it is considered that necessary characteristics cannot be obtained.

When the product of the NH ₃ concentration (%) and the residence time (second) in this temperature range is 50 or less, it is impossible to manufacture a cold-rolled steel sheet satisfying both the dent resistance and the surface strain resistance simultaneously. Although it is not clear, it is considered that nitriding does not always proceed sufficiently, and that the nitriding distribution in the plate thickness direction that enhances dent resistance is not achieved. On the other hand, when the product of the NH ₃ concentration (%) and the residence time (second) in this temperature range is 500 or more, it is impossible to manufacture a cold-rolled steel sheet satisfying both the dent resistance and the surface strain resistance simultaneously. It is considered that the yield strength of the steel sheet becomes too high, the shape freezing property is deteriorated, and a surface strain occurs.

[0011]

EXAMPLES Examples of the present invention will be described together with comparative examples.
Steels having the component compositions shown in Table 1 were nitrided under various conditions. Table 2 shows these conditions and product characteristics. The material here was prepared by heating a continuous cast slab to 1200 ° C.
A 4 mm-thick hot-rolled sheet that was finish-rolled at 30 ° C. was cold-rolled by 80%, recrystallized and annealed at 800 ° C. for 30 seconds in the first half of continuous annealing, and then nitrided. Experiments 1 to 10
Until then, the effects of process conditions were clarified using the same materials. In addition, the effects of steel types were clarified in Experiments 11 to 23. Based on the conventional knowledge that when the yield point (YP) is 250 MPa or more, surface distortion is likely to occur, YP <2
If it is 50 MPa, it is determined that no surface strain occurs. Also,
The index of dent property was represented by the amount of dents remaining after a load was applied to the steel sheet by the experimental device shown in FIG.

[0012]

[Table 1]

[0013]

[Table 2]

Experiment Nos. 3, 4, 5, 6, within the scope of the present invention
8, 11, 12, 13, 14, 15, and 22 have a good surface strain index and a good dent index, and have excellent workability.
On the other hand, in Experiment No. 1, since the nitriding treatment was not performed, the dent resistance was poor. In Experiment No. 2, nitriding was not sufficiently performed, and sufficient dent resistance was not obtained. In Experiment No. 7, the surface strain resistance was hindered due to excessive nitriding. In Experiment No. 9, the nitriding temperature was low, nitriding was not sufficiently performed, and sufficient dent resistance was not obtained. In Experiment No. 11, on the contrary, the nitriding temperature was too high, and before the ammonia was decomposed into active nitrogen and hydrogen on the surface of the steel sheet, decomposition occurred in the atmosphere and the amount of inactive nitrogen increased, so that nitriding was sufficiently performed. As a result, sufficient dent resistance was not obtained.

In Experiment No. 16, since the amount of C was larger than the range of the present invention, a large amount of precipitates were formed over the entire thickness of the sheet, resulting in an increase in YP, and the surface strain resistance could not be satisfied. Experiment numbers 17, 18, and 19 are Si, Mn,
Since P was added in excess of the range of the present invention, the YP after nitriding was increased and the surface strain resistance could not be satisfied. Experiment No. 20 was 0.9 (Ti / 48 + Nb / 9)
3)> (C / 12 + N / 14 + S / 32) is satisfied, but since the amount of Ti is below the range of the present invention, sufficient TiN is not precipitated by nitriding, and the effect of nitriding is not sufficiently obtained. , Sufficient dent resistance could not be obtained.

Experiment No. 21 is 0.9 (Ti / 48 + Nb)
/ 93)> C / 12 + N / 14 + S / 32, the amount of solid solution Ti and Nb before nitriding, which forms precipitates during nitriding, was small, and the effect of nitriding was not sufficiently obtained. No dent resistance was obtained. In Experiment No. 23, since Ti and Nb were not added, the nitriding effect was not sufficiently obtained, and sufficient dent resistance was not obtained.

[0017]

According to the present invention, a steel plate which simultaneously satisfies both the surface strain resistance and the dent resistance, which was not possible without a significant increase in cost, such as a multilayer steel plate, has been manufactured inexpensively. This is an industrially valuable invention.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an outline of an experimental method for measuring dent properties.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 8/26 C21D 8 / 04,9 / 48 C22C 38/00 301 C22C 38/14

Claims (2)

(57) [Claims] 1. A weight ratio of C: 0.005% or less, N: 0.01% or less, Mn: 1.5% or less, Si: 1.0% or less, P: 0.1% or less, Al: 0.005% or more, 0.2% or less, C / 12 + N
/14+S/32<0.9 (Ti / 48 + Nb / 93)
After cold-rolling a steel containing at least one or both of Ti and Nb satisfying the following conditions and containing the balance of Fe and unavoidable impurities, and then performing recrystallization annealing in a continuous annealing furnace, At a temperature of 600 ° C. or more and 800 ° C. or less,
A depth excellent in dent resistance and surface strain resistance, characterized in that nitriding is performed under the condition that the product of NH ₃ concentration (%) and stay time (second) in this temperature range is 50 or more and 500 or less. Manufacturing method of steel sheet for drawing. 2. C: 0.005% or less, N: 0.01% or less, Mn: 1.5% or less, Si: 1.0% or less, P: 0.1% or less, B: B: 0.0002% or more, 0.002% or less, Al: 0.005% or more, 0.2% or less, C / 12 +
N / 14 + S / 32 <0.9 (Ti / 48 + Nb / 9
3) A steel containing one or both of Ti and Nb satisfying the following condition in total of 0.02% or more, and the balance consisting of Fe and unavoidable impurities is cold rolled, and then subjected to recrystallization annealing in a continuous annealing furnace. After that, the nitriding treatment is performed in a nitriding atmosphere of 600 ° C. or more and 800 ° C. or less under the condition that the product of the concentration (%) of NH ₃ and the residence time (second) in this temperature range is 50 or more and 500 or less. A method for producing a deep drawing steel sheet having excellent dent resistance and surface strain resistance.