JPH10265892A - Iron base alloy excellent in ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an iron base alloy excellent in workability and having ductility while its strength is maintained by composing it of a compsn. in which the contents of C and N are regulated to specified value or below, the content of B is regulated to the range of specified value, and the balance Fe with inevitable impurities. SOLUTION: An alloy having a compsn. contg., by weight, <=0.005% C, <=0.005% N and 0.005 to 0.500% B, contg., at need, one or two kinds selected from <=2% Si, <=2% Mn, <=2% Cu and <=2% Ni, and the balance Fe with inevitable impurities is melted. This alloy is formed into a rolling stock by an ingot- making method, which is heated to about <=1,150 deg.C, is subjected to hot rolling, is furthermore subjected to cold rolling, is annealed at about <=880 deg.C and is moreover subjected to skinpass rolling at about <=5% draft to produce an iron base alloy material. In this way, the iron base alloy having strength of about >=240 MPa tensile strength and having ductility suitable for cold working such as bending, drawing or the like can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an iron-base alloy having excellent ductility suitable for use in cold working such as bending and drawing.

[0002]

2. Description of the Related Art An ultra-low carbon steel in which the amount of C in steel is reduced to 50 ppm or less, to which Nb, Ti, B, Zr, etc. are added, a so-called IF (Inte
rstitial Free) steel is manufactured as an ultra deep drawing steel sheet for automobiles through a cold rolling and continuous annealing process. Since Ti is a strong carbon / nitride forming element and also fixes S in the steel as a sulfide, the IF-containing IF steel can obtain excellent deep drawability and ductility stably over a wide component range. However, in a Ti-containing IF steel, Ti is apt to become an oxide, and surface defects due to oxide-based inclusions increase. There is a problem that the grain boundary strength is reduced and secondary working embrittlement occurs.
It is known that the addition of a small amount of B is effective for preventing the brittleness of the secondary working, but there is a problem that the workability is deteriorated.

[0003] On the other hand, the Nb-containing IF steel has excellent deep drawability, but has a problem that the proper addition range is narrower than that of the Ti-containing IF steel. In any case, in IF steel,
The addition of special elements such as Nb, Ti, B, and Zr leads to an increase in material costs and an increase in manufacturing costs due to high-temperature annealing accompanying an increase in recrystallization temperature. For this problem, for example,
Patent No. 2519131 discloses a method of manufacturing a cold-rolled steel sheet which is low in cost and excellent in formability by adding a trace amount of a carbon / nitride forming element and appropriately controlling hot rolling conditions and continuous annealing conditions. It has been disclosed. In the example disclosed in Japanese Patent No. 2519131, the highest ductility was El = 55.2% and the tensile strength at that time was TS = 28.4 kgf / mm ² (278 MPa).

[0004]

Recently, steel materials having high ductility have been demanded in order to further improve the workability. Conventionally, it is known that ductility is improved by reducing impurities as much as possible. However, since strength is reduced, there has been a demand for development of a steel material which maintains strength and has high ductility.

[0005] An object of the present invention is to solve the above-mentioned problems and to provide an iron-based alloy having a tensile strength of 240 MPa or more and high ductility and excellent workability.

[0006]

Means for Solving the Problems The present inventors have conducted intensive studies to improve the workability of a rolled steel material, and first focused on B. Conventionally, B is known to have the effect of improving the hardenability as an additive element of steel materials, and is effective for improving the strain aging resistance of steel and the grain boundary strength. It is also known. However, 0.01wt% of B in steel
It has been said that when added in an amount of not less than%, the workability of the steel material is reduced and the rolling becomes difficult.

The present inventors have studied the effect of the third element on the workability of the iron-based alloy material containing B. As a result, even when a large amount of B was added, the amounts of C and N were kept below appropriate values. It has been found that if the amount is reduced, rolling becomes possible, and further, ductility is remarkably improved. The present invention has been made based on the above findings. That is, the present invention provides
C: 0.005% or less, N: 0.005% or less, B: 0.005%
This is an iron-based alloy having excellent ductility, characterized in that it contains about 0.500% and the balance consists of Fe and unavoidable impurities.

[0008] The present invention also relates to a method for preparing a C: 0.005% by weight.
In the following, N: 0.005% or less, B: 0.005 to 0.500%, Si: 2% or less, Mn: 2% or less, Cu: 2% or less, Ni: 2% or less Or, it is an iron-based alloy having excellent ductility, characterized by containing two or more kinds and being composed of a balance of Fe and unavoidable impurities.

[0009]

First, the reasons for limiting the chemical composition will be described. C: 0.005% or less C precipitates as a solid solution or fine carbide in the matrix,
It is an element that lowers ductility because it hinders the movement of dislocations and reduces it as much as possible. However, C is limited to 0.005% or less because it is acceptable up to 0.005%. Incidentally, the content is preferably 0.0030% or less from the viewpoint of improving ductility.

N: 0.005% or less N, like C, is an element which precipitates as a solid solution or a fine nitride in the matrix and inhibits the movement of dislocations, thereby reducing ductility, and is reduced as much as possible. But 0.005
% Is acceptable, so N was limited to 0.005% or less. Incidentally, the content is preferably 0.0030% or less from the viewpoint of improving ductility.

B: 0.005 to 0.500% B is the most important element in the present invention. B has a low solid solubility limit in iron, like C and N, and when it contains 0.005% or more of B, it precipitates or crystallizes as Fe ₂ B. Many mobile dislocations exist in the matrix (ferrite phase) around the precipitated or crystallized Fe ₂ B. For this reason, plastic deformation is facilitated, and an effect that ductility and workability are improved is obtained. Further, since Fe ₂ B precipitates or crystallizes relatively coarsely in steel, it is considered that the dislocation movement is hardly hindered, which also contributes to the improvement of ductility and workability.

The improvement in ductility and workability is 0.005.
%, The addition is more than 0.500%, but if it exceeds 0.500%, the strength increases and the ductility decreases. Therefore, B is 0.005 to
Limited to the range of 0.500%. Si: 2.0% or less, Mn: 2.0% or less, Cu: 2.0% or less, N
i: One or more selected from 2.0% or less Si, Mn, Cu, and Ni are all elements that increase the strength of the iron-based alloy by solid solution strengthening and hinder the effect of improving the ductility of B. Therefore, it can be added as needed for the purpose of increasing the strength. However, Si, Mn, Cu, and Ni all decrease the ductility when the added amount exceeds 2.0%. Therefore, Si, Mn, Cu, Ni
Are limited to 2.0% or less.

The balance consists of Fe and inevitable impurities. As inevitable impurities, S: 0.020% or less, P:
0.020% or less, O: 0.010% or less is acceptable. Next, a preferred method for producing the iron-based alloy material of the present invention will be described. An alloy having the above composition is melted in a converter, an electric furnace, a vacuum melting furnace, or the like, and then solidified by an ingot casting method or a continuous casting method to obtain a rolled material. It goes without saying that known processes such as ladle refining and vacuum degassing may be performed.

The rolled material is heated and hot rolled to form a hot rolled sheet. The heating temperature of the hot rolling is preferably set to 1150 ° C. or less. When the heating temperature exceeds 1150 ° C, Fe ₂ B melts,
Rolling becomes difficult. The hot-rolled sheet may be used as it is, or may be further cold-rolled and annealed to be a cold-rolled sheet.

[0015] The cold rolled sheet is further annealed. The annealing is preferably either batch annealing or continuous annealing. The annealing temperature is preferably 880 ° C. or lower. Annealing temperature 88
When the temperature exceeds 0 ° C., solid solution of Fe ₂ B is promoted, and ductility is deteriorated. The cold-rolled sheet may be further subjected to temper rolling at a draft of 5% or less.

[0016]

EXAMPLES Steel having the composition shown in Table 1 was melted in an electric furnace and made into a steel ingot by an ingot-making method. This ingot is rolled to 100 mm
As a thick slab, a hot-rolled sheet having a thickness of 5 mm was formed by hot rolling. The hot-rolling heating temperature was 1100 ° C., the finish-rolling end temperature was 950 ° C., and air-cooling was performed after the end of the rolling. Next, the hot-rolled sheet was cold-rolled into a 0.7 mm cold-rolled sheet, and then 830 ° C. × 20se.
Continuous annealing of c was performed to obtain a cold-rolled annealed sheet.

Incidentally, the conventional steel J is a steel having a composition of an ultra-low carbon steel excellent in formability.
The annealing conditions were the same. The tensile properties of these hot rolled sheets and cold rolled annealed sheets were investigated. Tensile properties are JIS Z 2201
Using a No. 13B specimen in accordance with JIS Z 2241, a tensile test was carried out at room temperature in accordance with JIS Z 2241, tensile strength (TS), yield point (YS),
The elongation (El) was measured. Table 2 shows the results.

[0018]

[Table 1]

[0019]

[Table 2]

Each of the hot-rolled sheets A to E of the present invention shows an elongation of 55% or more, has the same tensile strength as that of the conventional example J, and further has excellent ductility and workability. ing. on the other hand,
The hot rolled sheets F, H, and I of the comparative examples out of the range of the present invention have an elongation of 50% or less and high ductility is not obtained. Although the elongation of the hot rolled sheet G of the comparative example is relatively high, the tensile strength is extremely low because it does not contain B.

Further, all of the hot-rolled sheets K to N of the present invention to which an alloy element is added for the purpose of increasing the strength show an elongation of 55% or more and have excellent ductility. Each of the cold-rolled annealed sheets of the present invention shows an elongation of 56% or more, has the same tensile strength as that of the conventional example J, and also has excellent ductility. In addition, as compared with the conventional example J, it is understood that the ductility is improved from the point that the yield point is remarkably lowered as compared with the case where the tensile strength is equal by adding B. Further, in the example of the present invention, the structure is such that Fe ₂ B is precipitated in the matrix (ferrite).

As described above, in the example of the present invention, the same effect as that of the hot-rolled sheet is obtained for the cold-rolled annealed sheet.

[0023]

According to the present invention, a hot-rolled sheet or a cold-rolled annealed sheet having excellent ductility can be easily obtained while maintaining strength by reducing C and N and adding an appropriate amount of B. ,
A special effect can be expected in industry.

Claims (2)

[Claims] 1. An iron base having excellent ductility, containing, by weight, C: 0.005% or less, N: 0.005% or less, B: 0.005 to 0.500%, and the balance being Fe and unavoidable impurities. alloy. 2. In% by weight, C: 0.005% or less, N: 0.005% or less, B: 0.005 to 0.500%, Si: 2% or less, Mn: 2% or less, Cu: 2% or less, Ni: 2%
Contains one or more selected from the following,
An iron-based alloy with excellent ductility, characterized by the balance of Fe and unavoidable impurities.