AU611883B2 - Steel suited to cintinuous casting and annealing - Google Patents

Description

COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 I COMPLETE SPECIFICATION FOR OFFICE USE: COMMONWEALTH OF AUSTRALIA Class Int.Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: 0000 0 .Priority: p ."RElated Art: o o0 o o o o o g o 0 :N'me of Applicant: Address of Applicant: o 0 0 oo 0 00 Actual Inventor: 00 0 0 00 JOHN LYSAGHT (AUSTRALIA) LIMITED 50 Young Street, Sydney, 2000 George Mu Sung Timothy William Smythe New South Wales, Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney Complete Specification for the Invention entitled: "STEEL SUITED TO CONTINUOUS CASTING AND ANNEALING" The following statement is a full description of this invention, including the best method of performing it known to me/us:- COMPLETE OF PI0142 dated 2nd February, 1987 1 n n -a olI* i r I This invention relates to the composition of steels and in particular to aluminium killed steels which are suited to continuous casting and continuous subcritical annealing such as occurs in continuous hot dip metallic-coating lines which include an in-line annealing treatment before metallic coating. The invention further relates to hot dip metallic-coated products made from such steel and particularly galvanised aluminium-zinc, or aluminium coated steel products.

Many continuous hot dip metallic coating lines employ a 0000 o rapid subcritical annealing cycle, for example rapid heating o 0 ao to 640-720 0 C, followed by 30-60 seconds hold at a temperature S o 4. 0 00 within the above range. Conventional aluminium killed o 0 0.00. continuously cast steels, when processed by the above method, 0 0 result in products with poorer formability than similar o 0 products made from capped or rimmed ingot steels. This is to o 0o 00 say the former have higher yield strength and lower ductility 0o than the latter.

It is an object of this invention to provide aluminium killed steel compositions which are better suited to o 0 I continuous subcritical annealing so as to produce a product having improved formability characteristics. It is a further object of this invention to provide coated steel products made from such steel which have improved formability characteristics.

In one aspect this invention consists in an aluminium killed steel comprising concentrations by weight of the steel of aluminium and of nitrogen selected so that the product of 2 _L the weight percentage of aluminium and weight percentage of -6 nitrogen does not exceed 40 x j It will be appreciated by those skilled in the art that the aluminium concentration referred to above and hereafter is the concentration of free or soluable aluminium only.

That is, to say the concentration of aluminium not present in the form of aluminium oxides.

In a second aspect this invention consists in a continuously annealed coated steel product formed from k@ aluminium killed steel comprising concentrations by weight of peo the steel of aluminium and of nitrogen selected so that the o o product of the weight percentage of aluminium and weight S-0_6 percentage of nitrogen does not exceed 40 x 0oooo It has been found that such a limitation on the combined level of aluminium and nitrogen results in a product *O °0 0 00 which after a continuous annealing process has superior o0 00 o0000o formability characteristics to previously known aluminium O'0 killed steel products and similar formability characteristics 0 0.; to those of products formed from capped and rimmed steels.

0 20 It is believed that this is because compositions according to 0 0a this invention are such that there is little precipitation of aluminium nitrides during the solidification, hot rolling and annealing of the steel such as may inhibit grain growth in steels having conventional levels of those elements. Grain growth in the steel during annealing is required in order to improve the formability characteristics by reducing the strength of the material and increasing the ductility and plastic strain ratio. In galvanised steel products formed from previously known continuously cast aluminium killed 3 K r 4-' steel compositions the typical grain size has been found to be 5 to 7pm whereas galvanised steel product having good formability characteristics and formed from capped steel has a typical grain size of 7 to 8pm.

It has been found not to be practically possible to reduce the amount of aluminium nitride precipitates by individually reducing the levels of aluminium or nitrogen.

In the case of reducing nitrogen content alone experimental evidence suggested that a nitrogen content of less than 0.001 0oi 0 weight percent of the steel would be required to give grain oo size and hence formability characteristics similar to capped 0 o 0:000: o00 0 steel. This level of nitrogen content is not possible with o current steelmaking technology and practices.

If existing nitrogen contents were retained it would be necessary to reduce the aluminium content to less than 0.005 0 0 weight percent of the steel in order to achieve the grain 0 0 0 o °o g size and thus have the desired formability characteristics.

0 00 a With current steelmaking control this would be possible but 0 0 very difficult. With such aluminium contents, it would be difficult to guarantee that the steel is fully deoxidized so S that during continuous casting, there would be a high incidence of surface pinholes and a high probability of a casting breakout. This latter problem is a rupture of the solidified slab shell by gas evolution during solidification of the molten core resulting in disruption of the casting operation. Lowering of aluminium content alone could also lead to steels containing alumina inclusions which adversely affect surface quality and formability.

4 The following experimental results together with Figures 1 and 2 serve to illustrate, by way of example only, the properties of aluminium killed steels produced according to this invention. Figure 1 is a plot of grain diameter versus aluminium content for steels having from 0.005 to 0.045 weight percent of aluminium after a galvanize annealing treatment and Figure 2 is a plot of grain diameter versus Al x N for steels having 0.005 to 0.045 weight percent aluminium and 0.0015 to 0.0070 weight percent nitrogen after a galvanize annealing treatment.

In conventional continuously cast aluminium killed o 0 steels, aluminium content is typically 0.03 to 0.08 weight percent and nitrogen content is typically 0.002 to 0.006 weight percent. A series of steels with a relatively low 0 0 nitrogen content of 0.002 to 0.003 weight percent and having 0,00, an aluminium content varying from 0.005 to 0.045 weight o.0 percent were produced. In these steels a plot [Figure 1] of grain diameter after a galvanize annealing treatment versus aluminium content shows that grain diameter first decreases slowly w;ith decreasing aluminium content but below about 0 o 0.010 weight percent grain diameter increases rapidly. These changes in grain diameter indicate that the amount of aluminium nitride precipitate is gradually reduced when aluminium content is reduced to 0.010 weight percent. Below this aluminium content, very little or no precipitate is present and grain diameter increases rapidly.

Although there is some suggestion from the above that solely lowering aluminium is beneficial in increasing grain size, this is misleading because the nitrogen content of these steels was quite low. The important factor found in experiments leading to this invention is that by controlling both aluminium and nitrogen contents within certain limits, aluminium nitride precipitates can be minimized and grain size maximized. With current steelmaking technology, a nitrogen content of less than or equal to 0.004 weight percent can be realistically achieved. In addition, with the advent of vacuum degassing and ladle treating stations, more accurate oxygen determinations and better disciplined Ooo0 practices during ladle transfer and continuous casting, it is 0*40 possible to lower the aluminium content to less than or equal 0 o o to 0.010 weight percent and still obtain a steel that is o O fully deoxidized (less than 0.005% oxygen) and satisfactory for continuous casting.

To show the important influence of both aluminium and o o S° nitrogen content of final grain size, a parameter being the o9 product of the weight percentage of aluminium and weight o o 0 percentage of nitrogen Al x N) is used for composition control purposes. This parameter is the product of the 20 aluminium and nitrogen concentrations and is directly related 00, 6 to the solubility temperature of aluminium nitride. Grain sizes of steels with 0.005 0.045 weight percent aluminium and 0.0015 to 0.0070 weight percent nitrogen after a galvanize annealing treatment are plotted against Al x N in Figure 2. It is apparent that to obtain grain diameters greater than 7-8pm, it is necessary to have concentrations of aluminium and of nitrogen selected so that the product of the weight percentage of aluminium and weight percentage of 6

~L

7 I- i i ~II a a nitrogen does not exceed 40 x 10 6 and is preferably no -6 more than 30 x 10 This concentration product parameter indicates that it is possible to obtain the desired grain size by various combinations of aluminium and nitrogen contents. The most common method of achieving a concentration product less than x 10-6 would be to ensure that aluminium content is 0.010 weight percent maximum and nitrogen content is 0.004 weight percent maximum. However, the desired grain size can 64o0 also be obtained with higher aluminium or nitrogen contents providing that the other element is low enough to ensure that 0 0 1-6 S the concentration product does not exceed 40 x 10 That 0 is, a steel with a nitrogen of 0.006% would give satisfactory a oso grain sizes if aluminium is less than 0.0066%.

The tensile properties of galvanised product made from 0 0 o0 0 low aluminium low nitrogen slab cast steel according to this o invention are shown in Table 1 together with the properties °o0 of similar product formed from conventional aluminium killed slab cast steels and ingot cast capped steels. The ZO galvanised product was made in a substantially conventional 0 40 a 6 4 hot dip metallic-coating line which included an inline or continuous subcritical annealing treatment before metal coating. The annealing treatment involved rapid heating to from 6400C to 720 0 C followed by a maintenance at a temperature within that range for from 30 to 60 seconds. As can be seen the steel according to this invention has superior formability characteristics to the conventional aluminium killed steel and similar formability 7 characteristics to those of capped steel. Aluminium-zinc and aluminium coated product formed from aluminium killed steel according to this invention can also be produced in the conventional continuous hot dip metallic coating line referred to above and have been found to have similar formability characteristics to the galvanised product.

Table 1.

Tensile Properties of Formable Quality Galvanized Steel 0 000 000 10 0 o s o 0 0 o0 0 0 0 Steel Grade YS

(MP

Aluminium killed slab cast 340 Capped 300 Low aluminium low nitrogen slab cast 305

TS

a) (MPa) 380 360

UE

20 22

TE

34 36 37 360 0 00 o o o oo o 0 0 0 0 00 20 o S 04 00 0 0 A typical composition of aluminium killed steel according to this invention is as follows:- Carbon 0.04 to 0.08 Oxygen up to 0.005 Nitrogen up to 0.005 Aluminium (sol.) up to 0.02 Phosphorous up to 0.003 Manganese 0.15 to 0.35 Sulphur up to 0.03 Silicon up to 0.03 Iron and incidental impurities remainder with the concentrations by weight of the steel of aluminium and of nitrogen selected so that the product of the weight percentage of aluminium and weight percentage of nitrogen does not exceed 40 x -8-

Claims (9)

1. An aluminium killed steel comprising the following composition in weight per cent of the steel: Carbon 0.04 to 0.08 Oxygen up to 0.005 Nitrogen up to 0.005 Free or soluble Aluminium up to 0.02 Phosphorus up to 0.03 o Manganese 0.15 to 0.35 Sulphur up to 0.03 Silicon up to 0.03 S Iron and incidental impurities remainder wherein the concentrations by weight of the steel of free or soluble aluminium and of nitrogen are selected so that the product of their weight percentages does not exceed 40 x 10 6 o

2. A steel as claimed in claim 1 wherein the product of the weight percentage of free or soluble aluminium and the weight percentage of nitrogen does not exceed 30 x 10 6.

3. A steel as claimed in claim 1 or claim 2 wherein the concentration of nitrogen is from 0.001 to 0.004 weight percent of the steel.

4. A steel as claimed in claim 3 wherein the concentration of free or soluble aluminium is from 0.005 to 0.01 weight percent of the steel.

A steel as claimed in any one of claims 1 to 4 wherein the average diameter of the grains of a strip formed from the steel is at least 7 m. ;P I

6. A continuously subcritical annealed metallic coated steel product formed from aluminium killed steel as claimed in any one of claims 1 to

7. A continuously annealed galvanised steel product formed from aluminium killed steel as claimed in any one of claims 1 to

8. A continuously subcritical annealed aluminium-zinc coated steel product formed from aluminium killed steel as claimed in any one of claims 1 to 0 *4 0

9. A continuously subcritical annealed aluminium coated steel product o formed from aluminium killed steel as claimed in any one of claims 1 to 0 0 0 A continuously subcritical annealed product as claimed in any one of claims 7 to 9 which has been annealed by rapid heating to from 6400C to 0 7200C and being maintained at a temperature within that range for from 30 to o 0 0 00 0 60 seconds. 00-0 Dated this third day of April 1991. S 0 JOHN LYSAGHT (AUSTRALIA) LIMITED Attorney: ROBERT G. SHELSTON Fellow Institute of Patent Attorneys of Australia of SMITH SHELSTON BEADLE <Ei^I