JPH073429A - Production of cold rolled steel sheet for deep drawing, excellent in dent resistance and surface strain resistance - Google Patents

Abstract

PURPOSE:To inexpensively produce a steel sheet excellent in dent resistance and in surface strain resistance by cold-rolling a steel having a composition where respective contents of C, N, Mn, P, Si, Al, S, and Fe are specified and then subjecting the resultant sheet to annealing and nitriding under respectively specified conditions. CONSTITUTION:A steel, which has a composition consisting of, by weight, <=0.005% C, <=0.01% N, <=1.5% Mn, #1% Si, <=0.1% P, 0.005-0.2% Al, in the amount satisfying the relations in C/12+N/14+S/32<0.9(Ti/48+Nb/93), and/or >=0.02% Nb, and the balance Fe, is refined. This steel is cold-rolled and then subjected to recrystallization annealing in a continuous annealing furnace. Subsequently, nitriding is done under the condition of (NH3 concentration)(%) X(time of stay) (sec)=50 to 500 at 600-800 deg.C.

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 which is excellent in dent resistance and surface strain resistance.

[0002]

2. Description of the Related Art IF steel sheet (Inter steel sheet) 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
stial atom free steels
Heet) is widely used as a cold-rolled steel sheet having excellent deep drawability. However, the weak point of this steel sheet is that it is soft and therefore excellent in surface strain resistance, but inferior in dent resistance, and when applied to automobile outer plates, etc., the shape easily collapses when 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 fixability deteriorates and the surface strain easily occurs. As a method for producing a steel sheet having dent resistance and surface strain resistance, a method for producing a multi-layer steel sheet whose surface layer has been made high tenth has been disclosed recently.
There is a 227 publication. However, the production method using multi-layered steel plate is not practical because the cost increases significantly.

On the other hand, a carburizing treatment or a nitriding treatment is well known as a method for increasing the tensile strength of only the near surface layer. JP-A-3-243757 discloses a cold rolled steel sheet having high strength near the surface by carburizing and nitriding treatment. A manufacturing method thereof is disclosed. However, in order to obtain a balance between excellent strength and workability, dent resistance and surface strain resistance have not been studied at all, and a nitrided steel sheet and a dent resistance and surface strain resistance that simultaneously satisfy Not only is there no disclosure about its manufacturing method,
No suggestive results are presented.

[0004]

DISCLOSURE OF THE INVENTION The present invention inexpensively manufactures a cold-rolled steel sheet satisfying both dent resistance and surface strain resistance, which has been possible only with a multi-layer steel sheet with a significant cost increase. It provides a way to do it.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have investigated a method for producing a cold-rolled steel sheet satisfying both dent resistance and surface strain resistance at the same time, and by optimizing the steel composition and nitriding conditions, continuous annealing Found that it is possible to produce this type of steel. The gist of the present invention is that C: 0.005 by weight.
% 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, one or both of Ti and Nb is C / 12 + N / 14 + S / 32 <
Satisfies the condition of 0.9 (Ti / 48 + Nb / 93),
In addition, one or both of Ti and Nb are contained in a total amount of 0.02% or more, and if necessary, B: 0.0002%.
As described above, 0.002% or less is added, and the steel consisting of the balance Fe and unavoidable impurities is cold-rolled, recrystallized and annealed in a continuous annealing furnace, and then NH in a nitriding atmosphere of 600 ° C or more and 800 ° C or less. Deep drawing with excellent dent resistance and surface strain resistance, characterized by nitriding under conditions where the product of concentration (%) of ₃ and residence time (seconds) in this temperature range is 50 or more and 500 or less. It is in the method of manufacturing steel sheets for use.

The present invention will be described in detail below.
The reasons for limiting the components of the present invention are as follows. One or both of Ti and Nb is 0.9 (Ti / 48 + N
b / 93)> C / 12 + N / 14 + S / 32 and the sum of the addition amounts of either or both of Ti and Nb is limited to 0.02% or more.
By fixing N and N in the form of precipitates and allowing solid crystal C and N to hardly exist during cold rolling, and enabling smooth crystal rotation, good deep drawability of the product in subsequent recrystallization annealing is achieved. It is an advantageous azimuth for training (111) <112
This is because it is possible to obtain a steel sheet having a highly integrated texture such as>, (554) <225>, and to obtain the required increase in strength during nitriding.

On the other hand, C: 0.005% or less, N: 0.0
The content of 1% or less and S: 0.02% or less means that if these amounts are exceeded and C, N, and S are added, not only the workability of the product is impaired but also Ti necessary for satisfying the above conditional expression or This is because the amount of Nb increases and the manufacturing cost unnecessarily increases. As other components, the components that are usually contained for improving strength, that is, the upper limits of Si, Mn, and P are Si: 1.0% or less, Mn: 1.5% or less, and P: 0.
The reason why it is set to 15% is that the addition thereof further deteriorates the workability. Further, since the addition of B enhances the secondary workability, it is effective to add 0.0002% or more of B as required, but if it is 0.0050% or more, the workability is significantly deteriorated. The upper limit is 0.0050%. Al needs to be added in an amount of at least 0.005% to enable reliable deoxidation in molten steel, but excessive addition deteriorates workability, so the upper limit was made 0.2%.

Next, the limitation of the manufacturing process will be described. Since the characteristics of the steel of the present invention do not largely depend on heating conditions and hot rolling conditions, there is no particular need to limit it. That is, it does not matter whether the slab is reheated and then hot-rolled or the direct slab is hot-rolled. Further, a thin cast piece from which hot rolling is omitted may be used. The cold-rolling conditions are not particularly required to be limited, but a 70% or more cold-rolling rate is preferable in order to develop a texture with favorable deep drawability. Regarding the recrystallization annealing conditions, if the steel sheet is exposed to a nitriding atmosphere before being recrystallized, recrystallization is remarkably suppressed. Therefore, it is necessary to perform recrystallization before the nitriding atmosphere is established.

The most important process condition of the present invention is the nitriding condition. The concentration of NH ₃ (%) and the residence time (seconds) in this temperature range in the nitriding atmosphere of the steel of the present invention of 600 ° C. to 800 ° C. By performing the nitriding treatment under the condition that the product is 50 or more and 500 or less, it is possible to manufacture a cold-rolled steel sheet that simultaneously satisfies dent resistance and surface strain resistance. Nitriding temperature is 600
It is thought that the reason why it is not possible to manufacture a cold-rolled steel sheet that simultaneously satisfies dent resistance and surface strain resistance at ℃ or below, because nitriding is limited only to the vicinity of the surface, nitriding to obtain the required properties cannot be realized. . On the other hand, if the temperature is 800 ° C. or higher, the diffusion is rather fast, and it is considered that the necessary characteristics cannot be obtained due to the diffusion.

The reason why the product of the NH ₃ concentration (%) and the residence time (second) in this temperature range is 50 or less and the dent resistance and the surface strain resistance cannot be simultaneously produced cannot be manufactured. Although not clear, it is considered that the reason is that nitriding does not always proceed sufficiently and that a 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 dent resistance and surface strain resistance at the same time. It is considered that the yield strength of the steel sheet becomes too high, the shape fixability deteriorates, and surface strain occurs.

[0011]

EXAMPLES Examples of the present invention will be described together with comparative examples.
Steels having the chemical compositions shown in Table 1 were nitrided under various conditions. Table 2 shows these conditions and product characteristics. The material here is about 9% by heating the continuous cast slab to 1200 ° C.
A 4 mm-thick hot-rolled sheet finish-rolled at 30 ° C. was cold-rolled by 80%, subjected to recrystallization annealing at 800 ° C. for 30 seconds in the first half of continuous annealing, and then subjected to nitriding treatment. Experiment 1 to 10
Up to the same material was used to clarify the effect of process conditions. Further, in Experiments 11 to 23, the influence of the steel type was clarified. Based on the conventional knowledge that surface strain easily occurs when the yield point (YP) is 250 MPa or more, YP <2
If it is 50 MPa, it is assumed that the surface strain does not occur. Also,
The index of dent property was expressed by the amount of dents remaining after a load was applied to the steel sheet by the experimental apparatus shown in FIG.

[0012]

[Table 1]

[0013]

[Table 2]

Experiment numbers 3, 4, 5, 6 within the scope of the present invention
8, 11, 12, 13, 14, 15, and 22 have good surface strain index and dent index, and are also excellent in workability.
On the other hand, Experiment No. 1 is inferior in dent resistance because it is not nitrided. In Experiment No. 2, sufficient nitriding was not performed and sufficient dent resistance was not obtained. In Experiment No. 7, surface nitriding resistance was impaired due to excessive nitriding. In Experiment No. 9, the nitriding temperature was low, the nitriding was not sufficiently performed, and sufficient dent resistance was not obtained. On the contrary, in Experiment No. 11, the nitriding temperature is too high, and the ammonia decomposes in the atmosphere before it decomposes into active nitrogen and hydrogen on the surface of the steel sheet, and the amount of inactive nitrogen increases, so nitriding is sufficiently performed. Therefore, sufficient dent resistance could not be obtained.

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

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

[0017]

According to the present invention, it is possible to inexpensively manufacture a steel sheet satisfying both surface strain resistance and dent resistance, which has been impossible until now without a significant increase in cost, such as a multi-layer steel sheet. It is an invention that is industrially valuable.

[Brief description of drawings]

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

Claims (2)

[Claims] 1. By weight ratio, 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)
The steel containing either or both of Ti and Nb satisfying the following conditions in total of 0.02% or more and the balance Fe and unavoidable impurities is cold-rolled and then recrystallized in a continuous annealing furnace. At temperatures above 600 ° C and below 800 ° C,
Also, the nitriding treatment is performed 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, and the depth is excellent in dent resistance and surface strain resistance. Manufacturing method of steel sheet for drawing. 2. By weight ratio, 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: 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) Steel containing either or both of Ti and Nb satisfying the condition 3) in a total amount of 0.02% or more and the balance Fe and unavoidable impurities is cold rolled and then recrystallized in a continuous annealing furnace. After that, the nitriding treatment is performed 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 in a nitriding atmosphere of 600 ° C or more and 800 ° C or less. And a method for producing a deep-drawing steel sheet excellent in dent resistance and surface strain resistance.