US3843417A

US3843417A - Method for hot dipping aluminium-killed steel sheet

Info

Publication number: US3843417A
Application number: US00309890A
Authority: US
Inventors: M Ohbu; K Asakawa
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1971-12-01
Filing date: 1972-11-27
Publication date: 1974-10-22
Anticipated expiration: 1991-10-22
Also published as: DE2258589A1; DE2258589B2; JPS4860028A; JPS5426497B2

Abstract

1. IN A METHOD FOR HOT DIPPING AN ALUMINIUM-KILLED STEEL SHEET IN A HOT DIPPING LINE INVOLVING A REDUCTION ANNEALING PRETREATMENT PROCESS WHICH INCLUDES THE STEPS OF HOT ROLLING SAID STEEL SHEET, DESCALING THE SHEET BY ACID PICKLING, COLD ROLLING THE SHEET, AND THEN DEGREASING THE SHEET IN HOT DIPPING EQUIPMENT, REDUCTION ANNEALING THE SHEET, COOLING THE SHEET TO A SUITABLE DIPPING TEMPERATURE AND IMMERSING THE SHEET IN A HOT DIPPING BATH, THE IMPROVEMENT WHICH COMPRISES WINDING SAID ALUMINIUMKILLED STEEL SHEET AT A TEMPERAUTRE HIGHER THAN 650* C. AFTER THE HOT ROLLING.

Description

Oct. 22, 1974 A 'QHBU em METHOD FOR HOT- DIPPING ALUMINIUM-KILLED STEELMSHEET Filed tlov Z'Z, 1972 I FIG. I

Condition of hot dipping(2endzimir process) Oxidation degreasing 450C Reducing temperature 850C Reducing atmosphere H2 75']: N2 25 /0 Al in hot dipping bath 7 g 0.2% Immersion period in hot dipping bath 1 second Alloy layer thickness Plating adhesiveness good Plating adhesiveness 7 Sheets-Sheet l Alloy layer Q8 thickness Sol Al in steel (1.)

FIG. 2

Plating ad hesiveness good i0 ll l2 Grain size number Plating adhesiveness (ball impact test) 2 Plated layer cracks 3 A part of plated layer peels of 4 Peeling of plated layer is enormous 5 Plated layer peels off totally Oct. 22, 1974 MISAO QHBU EI'AL 3,843,417

METHOD FOR HOT DIPPING ALUMINIUM-KILLED STEEL SHEET Filed NOV. 2'7, 1972 7 Sheets-Sheet FIG. 3

Finishing temperature in hot rolling 890C Plating 4 SoLAl in steel 0.04% cidhesiveness good i l 5,00 550 600 I 650 700 Winding temperature after hot rolling (-C) Winding temperature after hot rolling 650C Plating 4 cidhesiveness good I l l I Tr 0.0! 0.02 0.03 004 0.05 0.06 0.07 008 0.09 0. I 0 j SoLAl in s te'el ('10) Oct. 22, 1974 'MISAQ OHBU ETAL 3,843,417

METHOD FOR n07 DIPPING ALuMINIuwKILLED STEEL SHEET.

Filed Nov. 27, 1972 7 Shuts-Sheet 5 FIG.

Plating amount 150 /m 400 Winding temperature 300 after hot rolling 550C Number of pinholes /dm V Winding temperature/ after hot rolling lOO 650'C O ,.'T"' 'L-' I I l I Tr 0.0l 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0. l0

Sol.Al in steel ('l.)

800 Plating amounflltfl Winding temperature J after hot rolling 550C Number of pinholes poinls Windin lem 1 after mg rolli g 650C O I l I i l i I Tr 0.0l 0020.03 0.04 0.05 0.06 0.07 0.0 0.09 10 SoLAl in steel (1.)

Oct. 22, 1974 MlSAO OHBU ET AL 3,843,417

METHOD FOR HOT DIPPING ALUMINIUM-KILLED STEEL SHEET t -Sheet' t Filed Now 2'7, 1972 7 Shoe s I FIG. 7

Continuous annealing Finishing temperature in hot rolling 890 C Grain size number Batch annealing v (heating rate10 /hr) l l l l Winding temperature after hot rolling ("(1) FIG. 8v

Batch annealing a Finishin tem eratu e Sweating r te10 c/ m hot rguing P r Sol.Al in steel 0.04%

Axial ratio of grains Continuous annealing I l v I Winding temperature after hot roiling(C) SoLAl insteel 0.04%

0a. 22, 1974 A OHB ETAL' 3,343,417

METHOD FOR HOT DIPPING ALUMINIUM-KILLED STEEL SHEET Filed Nov. 27, 1972, '7' Sheets-Sheet FIG. 9

Winding temperature after hot rolling 550C 4 Plating udhesiveness SOLAI in steel 0.04-0.05 good 3 Finishing temperature In hot rolling 890C 2 Winding temperature after hot rolling 650C l r l I l C content ('I..) in Steel Oct. 22, 1974 MISAO OHBU EI'AL 3,843,417

HETHOD FOR 80: DIPPING ALUIINIUILKILLBD STEEL SHEET Filed Nov. 27, 1972 7 Sheets-Shoot 6 Sal. Al: trace in Steel SOLAI: 0.080% in Steel FIGJOG. F|G.|Ob

oLAl: trace. in Steel SoLl: 0.90% in Steel FIGJIEI FIGJlb Oct. 22, 1974 MISAQ QHBU ErAL 3,843,417

METHOD you 80": DIPPING ALUMINIUM-KILLED STEEL SHEET Filed Nov. 27, 1972 7 Shoots-Shoot 7 Sol-Al: trace in Steel Hot Rolling Winding Temp. 650C FIGJZd Sol. AI: 0.045% in Steel Hot Rolling winding Temp. 550c FIG-J2 b SOLAI: 0.0451 in swea Hot Rolling Wmdmg Temp. 650C United States Patent C US. Cl. 14812 9 Claims ABSTRACT OF THE DISCLOSURE A method for hot dipping an aluminium-killed steel sheet in a hot dipping line involving a reduction annealing process as a pretreatment step wherein an aluminiumkilled steel sheet containing less than 0.05% of soluble aluminium and more than 0.02% of carbon is wound up at a temperature higher than 650 C. after hot rolling to obtain a steel sheet composed of crystals having a grain size number less than 9.5 and an axial ratio smaller than 1.5, descaled by acid pickling, cold rolled, and then degreased in hot dipping equipment, subjected to reduction annealing, cooled to a suitable dipping temperature and is then immersed in a hot dipping bath.

The present invention relates to an improvement in the plateability of aluminium-killed steel sheet by hot dipping. In this invention, a detailed investigation was made particularly directed to an improvement in hot zinc dipping, and the result was applied for other hot dippings. Accordingly, problems and the improvement with the hot dipping of aluminium-killed steel with zinc will be explained first.

The object of the present invention is limited only for steel sheets refined by deoxidation with aluminium before casting in a mold. This does not include steel sheets obtained by producing a rimmed layer after casting in a mold and refining the core part with aluminium, because they already possess excellent plateability. The nitrogen content steels for which the present invention is suitable is more than 0.003%.

The method for hot dipping of steel sheet with zinc commonly used in recent years is: a steel sheet is subjected to a so-called pretreatment process, such as, the Sendzimir process or a non-oxidizing furnace process, an oxidation or non-oxidation e.g., degreasing, surface cleansing and a reduction annealing, and then after cooling, to a suitable zinc dipping temperature, is immersed in a zinc dipping bath.

Due to the severe heat treatment, e.g., the rapid heating and cooling in such an instance, an aluminium-killed steel used as a base metal for hot dipping, does not possess the excellent deep-drawing and non-ageing properties which it would possess with cold rolling. Therefore, a rimmed or capped steel, which is cheap and possesses excellent plateability, is used.

Owing to the development of the continuous casting method, the manufacture of cold rolled steel strip and sheet as a base metal for zinc or other plating has become possible in recent years. However, as the manufac ture of rimmed steel by continuous casting is essentially difficult, refining by deoxidation is necessary and a cheap and etfective conventional deoxidizer, such as, aluminium and silicon have been adopted, as before, for this purpose. While the use of other oxidizers may also be considered, various investigations were made in the present invention on the behavior of aluminium-killed steel sheet as a base metal for hot dipping to improve the plateability thereof.

3,843,417 Patented Oct. 22, 1974 For a steel sheet to be suitable as a base metal for hot dipping, the plating must have no pinholes and the sheet must be evenly plated. Further, it is necessary that the adhesiveness of the plating to the base metal be good in order to prevent the peeling of the plating when forming the steel sheet to a desired shape, and that the workability of the base metal be excellent so as to form a desired shape without rupture.

In general, while it is a well known fact that the alloy elements are effective in increasing the reaction velocity between iron and zinc to develop a thick alloy layer when no aluminium is contained in the dipping bath, the effect of alloy elements on the reaction between iron and zinc when aluminium is present in the dipping bath is not yet clear because the behavior is complicated. The evaluation on the zinc plateability of aluminium-killed steel sheet in zinc plating by the Sendzimir process and by others, too, is not certain.

The present inventors found that the plateability of aluminium-killed steel is very inferior in hot dipping, and then studied in detail the behavior of aluminium in steel in hot zinc dipping as well as the effect of the conditions in various processes, after hot rolling, on the adhesiveness of plating. As a result, it was ascertained that, by controlling the conditions in the amount of aluminium in steel as well as in each of the processes after proper hot rolling, the zinc plateability of aluminium-killed steel sheet can be remarkably improved.

It was ascertained in the conventional hot zinc dipping, such as, the Sendzimir process, that, when about 0.2% of aluminium is added in the dipping bath, the growth of the alloy layer is restricted, giving a very thin layer, and accordingly the surface condition of steel sheet before immersing in the dipping bath has a sensitive influence on the plating adhesiveness. On the other hand, the surface condition of steel sheet is influenced by the oxidation-reduction condition in the pretreatment process of the zinc dipping and also by the steel components in the base metal used for hot dipping and by the winding temperature in the hot rolling.

It was found particularly in the present invention that the plateability in hot dipping of aluminium-killed steel is improved remarkably by controlling the winding temperature in the hot rolling adequately. It was ascertained that the degree of crystallization, the condition of the precipitate of AlN, Fe C and others and the amount of AlN precipitated are altered according to the winding temperature in the hot rolling. This results in differences in the surface condition of the steel sheet, and consequently, the winding temperature in the hot rolling has a great influence on the reaction between iron and zinc as well as the adhesiveness of the plating to the base metal.

FIG. 1 is a graph showing the relation between the amount of soluble aluminium (hereinafter referred to as Sol.Al) in the steel sheet, wound up at 550 C., and the adhesiveness thereto as well as the thickness of the alloy layer in hot zinc dipping. FIG. 2 is a graph showing the grain size in the steel as a base metal for hot zinc dipping and the adhesiveness of the plating. FIG. 3 is a graph showing the relation between the winding temperature of the base metal for hot dipping after hot rolling and the plating adhesiveness after hot zinc dipping. FIG. 4 is a graph showing the relation between the amount of Sol.Al in the steel sheet, wound up at 650 C., and the adhesiveness of the plating. FIG. 5 is a graph showing the effects of SoLAl in the steel sheet as well as the winding temperature in the hot rolling on the occurrence of pinholes in the hot dipping with terne metal. FIG. 6 is a graph showing the effects of Sol.Al in the steel sheet as well as the winding temperature in the hot rolling on the occurrence of pinholes in the hot dipping with tin. FIG. 7

shows the relation between the winding temperature in the hot rolling and the grain size in the continuous and batch annealings. FIG. 8 shows the relation between the winding temperature in the hot rolling and the crystal structure. FIG. 9 is a graph showing the relation between the amount of carbon in the steel and the adhesiveness under varying winding temperature. FIG. 10, a and b are the photographs showing the change of alloy layer in hot zinc dipping according to the amount of Sol.Al in the steel sheet. FIG. 11, a and b are the photographs showing the change of alloy layer in the vincinity of base metal in hot zinc dipping according to the amount of Sol.Al in the steel sheet. FIG. 12, a, b and c are the photographs showing the elfects of the amount of Sol.Al in the steel sheet and the Winding temperature on the condition of alloy layer in the hot dipping with terne metal.

FIG. 1 is the experimental result on the effect of Sol.Al in an aluminium-killed steel on the average thickness of the alloy layer and the adhesiveness of the plating to the base metal in hot zinc dipping. The result shows that, while the thickness of alloy layer is nearly constant when the amount of Sol.Al in the steel is less than 0.02%, the average alloy layer becomes thinner in increasing Sol.Al above 0.02%, and there is the tendency that the thickness reaches a nearly constant value when Sol.Al is 0.07- 0.08%.

Such a tendency means that the reactivity between iron and zinc decreases in accordance with the content of Sol.Al in the steel in the range where Sol.Al is larger than 0.02%. It is thought that aluminium dissolved as a solid solution in the steel reacts with the atmospheric gases in the oxidation-reduction process used as a pretreatment of plating, forming aluminium oxide, aluminium nitride and others on the surface of the base metal, which hinder the wettability of the base metal to fused zinc as well as the re action between iron and zinc, preventing the formation of the alloy layer.

On the other hand, as seen from FIG. 1, the plating adhesiveness is excellent in the range where Sol.Al in the steel is less than 0.02%, and deteriorates rapidly with increasing the amount of Sol.Al. The relation is quite reversed against the average thickness of the alloy layer.

FIG. 10 shows the condition of the alloy layers formed when steel sheets containing trace and 0.080% of Sol.Al are hot dipped with zinc (in the photograph, the alloy layer is thicker in accordance with the blackness). While the alloy layer is formed uniformly all over the total surface when Sol.Al is trace, the formation of alloy layer is very scattered, having some portions with almost no formation of alloy layer (white in the photograph), when the Sol.Al is 0.080%.

Therefore, it is easily understood that, although the average thickness of the alloy layer becomes thinner with increasing Sol.Al in the steel in FIG. 1, the base metal is not plated uniformly thin all over the total surface, but has locally unplated parts, and as a result, it is seen that the average thickness of the layer is thin.

It is expected, therefore, that the adhesiveness of the plating can be improved still more when the formation of the alloy layer is accelerated by decomposing the thin film of aluminium oxide, aluminium nitride and others, which is considered to be formed on the surface of steel sheet in the pretreatment process of hot zinc dipping. As a practical means for accelerating the formation of the alloy layer, various treatments such as insufficient cooling of the base metal after reduction annealing and immersion in a hot dipipng bath at a temperature higher than the bath temperature, or reducing the amount of aluminium in the hot dipping bath to control the restricting effect of the alloy layer, may be considered.

However, even if the alloy layer is developed to an average of 1-2,:t by such means, the fluctuation in the adhesiveness of the plating was large. In investigating the reason, it was found that, When the amount of Sol.Al in the steel is large, the condition of the alloy layer in such an instance is essentially different, relating to the adhesiveness of the plating.

As shown in FIG. 11, whereas the alloy layer consisting of fine grains is formed all over the surface when the amount of Sol.Al in the steel is a trace, the alloy layer is of large plate-like crystals when Sol.Al in the steel is 0.080%. The reason may be considered as due to the shortage of active points to initiate the reaction in the formation of the alloy layer.

When the plate-like alloy layer is formed, the adhesiveness of the plating becomes inferior. In order to prevent the formation of such a plate-like alloy layer and to promote the Fe-Zn reaction all over the surface, increasing the active points of the reaction is effective. For this purpose, to elevate the temperature of the steel sheet above 850 C. in the reduction annealing was found to have a tolerable effect, but was insufficient to give a satisfactory adhesiveness suitable to withstand subsequent working.

When the alloy layer is developed to a thickness about 23 the unreacted part in Fe-Zn reaction and the shape effect in the alloy layer are removed, and the plating adhesiveness becomes good. However, when the alloy layer is developed to above 3n, the plating adhesiveness becomes inferior owing to the brittleness of the alloy layer itself. Moreover, to control the alloy layer at 2-3 in the industrial production of galvanized steel sheet is difi'rcult.

It goes without saying, however, that the method is an effective means to improve the adhesiveness of zinc plating to aluminium-killed steel sheet.

Now, as a result of the analysis on the effect of various factors on the plating adhesiveness in galvanized steel sheet using aluminium-killed steel, the present inventors found that the grain size in the base metal for plating is, as shown in FIG. 2, one of the most important factors on the plating adhesiveness, having the tendency of deteriorating the plating adhesiveness with increasing the grain size number.

As for the reason why the increase in grain size (i.e., to reduce the grain size number) is effective for improvin the plating adhesiveness, although the mechanism is not yet known in detail, it is considered on one hand that the alloy layer formed at the brain boundary is poor in adhesiveness and the peeling of plating starts from the rupture in the neighbourhood of grain boundary. In other words, the grain boundary, being the starting point of rupture, is diminished as the grain size becomes larger and consequently the plating adhesiveness is improved. On the other hand the conditions under which growing the grain size is also to be considered. The difference in the winding temperature after hot rolling relates to the difference in the state of the alloy elements in the steel, dissolved as a solid solution or precipitated, particularly in the vicinity of the surface layer. The difference in the high temperature cycle and the holding period in the reduction annealing changes the behavior of the alloy elements in the steel in its precipitation, and thus the activity of steel surface in the Fe-Zn reaction may be improved.

As a proof of above consideration, it is mentioned already that the Fe-Zn reaction proceeds smoothly and the plating adhesiveness becomes good when the reduction annealing temperature is higher than 850 C.

However, it is not recommendable in industrial production to change the grain size remarkably by controlling the condition of oxidation-reduction annealing in the pretreatment process of hot zinc dipping.

The precipitation state or the grain size of alloy elements (particularly aluminium) in the steel can easily be controlled according to the conditions of the processes before the hot zinc dipping, particularly in the hot rolling process. in the conventional manufacture of cold rolled steel strip of aluminium-killed steel, the precipitation of aluminium nitride is controlled by a low temperature winding (below 600 C.) after hot rolling, and the precipitation of aluminium nitride and the formation of pancake grains are accelerated by controlling properly the heating rate and the heating temperature in the annealing process after cold rolling (batch annealing), and thus an excellent workability is obtained.

When aluminium killed steel is used for a continuous annealing, such as, the Sendzimir process, as a rapid heating is done, no pancake grains are obtained (forming tesseral grains), and accordingly there is no need to adhere to a low temperature winding (below 600 C.) in the hot rolling, which has been applied to the manufacture of cold rolled steel strip.

Then, the present inventors investigated, paying attention to the easiness of controlling the grain size by the winding temperature in the hot rolling, the relation between the winding temperature in the hot rolling and the plating adhesiveness. The result is as shown in FIG. 3, From the result, it is obvious that the plating adhesiveness is improved by elevating the winding temperature in the hot rolling, and sufiiciently high adhesiveness suitable for the practical working can be obtained when the winding temperature is above 65 C.

FIG. 4 shows the relation between the amount of Sol.Al in the steel sheet and the plating adhesiveness when the winding temperature after hot rolling is 650 C. To compare the figure with FIG. 1, in which the winding temperature is 550 C., the plating adhesiveness is improved remarkably, and it is clear that sufficient plating adhesiveness is obtained by applying a high temperature winding when the amount of Sol.Al in the steel lies in the range of 0.010.05

As above stated, when an aluminium-killed steel sheet is wound up above 65 0 C. in the hot rolling, the reactivity of the steel sheet in the reaction between iron and zinc is improved remarkably. The theoretical ground for this is not yet certain. Generally, however, it seems that the zinc plateability of aluminium-killed steel sheet has an important relationship to the grain size, and further that the zinc plateability is improved by controlling the winding temperature in the hot rolling as a practical method of controlling the grain size easily.

As a result of investigating the relation between the winding temperature in the hot rolling and the plating adhesiveness after hot zinc plating in various kinds of steel sheet, it was found that the plating adhesiveness of various steel sheets other than aluminium-killed sheets, as shown in Table 1, is improved by elevating the winding temperature in the hot rolling.

However, the effect is not so important for rimmed steel as the steel itself has a good adhesiveness.

TABLE 1 Change of plating adhesiveness to various kinds of steel sheet after hot zinc dipping in varying winding temperature sheet thickness: 1.6 mm. plating amount: 300-330 g./m. plating adhesiveness, cf., remark in FIG. 1

The improvement in the plateability in hot dipping with metals other than zinc is also expected.

FIG. 5 shows the effect of Sol.Al in the steel sheet as well as the winding temperature in the hot rolling thereof on the occurrence of pinholes in the hot dipping with socalled tenne alloy comprising 15% of tin and 85% of lead, in which an aluminium-killed steel sheet is batch annealed (at 700 C., with a heating rate of C./hr. and a holding period of 5 hrs.) and hot dipped by a usual flux method (dipping bath temperature 350 C., immersed for 20 seconds).

Whereas pinholes increase remarkably when 'Sol.Al in the steel exceeds 0.01% in winding at 550 C. after hot rolling, showing that the plateability with terne metal becomes inferior, pinholes diminish in winding at 650 C.,

6 and the effect is particularly distinct in the range where Sol.Al in the steel is less than 0.05%.

As obvious from FIG. 12, in which the condition of the alloy layer in the terne metal dipping is shown, while the alloy layer is fine and compact when the Sol.Al in the steel is a trace, only some large columnar grains are formed in the alloy layer when the steel sheet containing 0.045% of Sol.Al is wound up at a lower temperature (550 C.), showing that the reactivity of the steel sheet with terne metal is poor in this instance. In winding at a higher temperature (650 0), however, the alloy layer obtained is fairly compact all over the surface, showing that the reactivity of the steel sheet is improved.

This is because the plateability with terne metal is improved and pinholes are diminished. By winding at a higher temperature in the hot rolling in this way, an excellent reactivity is given not only in the continuous annealing but also in the batch annealing after cold rolling.

FIG. 6 shows the effect of Sol.Al in the steel sheet as well as the winding temperature in the hot rolling on the occurrence of pinholes in the hot dipping with tin. Whereas pinholes increase remarkably when Sol.Al in the steel exceeds 0.02% in winding at 550 C. after hot rolling showing that the tin plateability is deteriorated, pinholes diminish in winding at 650 C., and the effect is particularly distinct in the range where SoLAl in the steel is less than 0.05%, showing a similar tendency as in the hot zinc and terne metal dipping.

As shown above, by winding at a temperature higher than 650 C. after hot rolling, the plateability of aluminium-killed steel sheet in hot dipping is improved remarkably, irrespective, of batch and continuous annealing is used after cold rolling. The crystal structures of the base metals for hot dipping are as shown in FIGS. 7 and 8. From the result, it is known that the dependency of the heating rate on the grain growth of aluminium-killed steel is reduced remarkably when the winding is carried out above 650 C., and accordingly crystal grains having a grain number smaller than 9.5 can be obtained irrespective of the heating rate, and moreover tesseral crystals can be obtained without the formation of pancake grains even when the heating rate is slow (constant heating rate).

As obvious from FIG. 9, in which the relation between the amount of carbon and the plating adhesiveness is shown, the effect of high temperature winding above 650 C. in the hot rolling becomes distinct when the amount of carbon in the steel is larger than 0.02%. In the low temperature winding (550 C.), while the adhesiveness is satisfactory when carbon is less than 0.02%, it becomes rapidly inferior when carbon content becomes larger. On the other hand, in the high temperature winding, the plating adhesiveness is quite excellent as shown by the broken line in the figure.

From both relationships, it is known that the high temperature winding is effective only when the amount of carbon in the steel is larger than 0.02%, and when the amount of carbon is less than this value, there is almost no difference in the high temperature and low temperature windings.

Therefore, the carbon content in the steel sheet of this invention is restricted to larger than 002%.

While it is not certain whether such a crystal structure as above-mentioned serves directly to improve the plateability in this invention, the crystal structure may possibly be a measure for improving the plateability of aluminiumkilled steel sheet.

Thus, when an aluminium-killed steel sheet obtained by winding up at a temperature higher than 650 C. after hot rolling, is comopsed of nearly tesseral crystals having a grain size number less than 9.5 and an axial ratio smaller than 1.5, the plateability thereof is remarkably improved.

From the standpoint of workability, the draft in the cold rolling is preferably larger than 50%. Below 50%, the workability is apt to be deteriorated owing to the formation of an abnormal structure, such as, mixed grains. From the standpoint of general industrial manufacture, the content of nitrogen in the steel is defined as larger than 0.003%.

To improve the plateability in hot dipping by elevating the Winding temperature after hot rolling is sufficiently applicable, besides hot dippings with said metals, for such but clippings, in which the reaction between the plating metal and the base metal is done by hot dipping after electroplating, as for instance in electro aluminiurnand tin platings.

What is claimed is:

1. In a method for hot dipping an aluminium-killed steel sheet in a hot dipping line involving a reduction annealing pretreatment process which includes the steps of hot rolling said steel sheet, descaling the sheet by acid pickling, cold rolling the sheet, and then degreasing the sheet in hot dipping equipment, reduction annealing the sheet, cooling the sheet to a suitable dipping temperature and immersing the sheet in a hot dipping bath, the improvement which comprises winding said aluminiumkilled steel sheet at a temperature higher than 650 C. after the hot rolling.

2. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 wherein the aluminiumkilled steel sheet contains less than 0.05% of soluble aluminium and more than 0.02% of carbon.

3. The method of Claim 2 wherein said aluminiumkilled steel sheet is composed of crystals having a grain size number less than 9.5 and an axial ratio smaller than 1.5.

4. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 in which the draft in the cold rolling is greater than 5. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 in which said steel sheet is hot dipped with zinc.

6. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 in which said steel sheet is hot dipped with terne alloy.

7. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 in which said steel sheet is hot dipped with tin.

8. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 in which an aluminiumkilled steel is cast continuously.

9. The method for hot dipping an aluminium-killed steel sheet as set forth in Claim 1 in which the annealing after cold rolling is of a continuous annealing or a batch annealing.

References Cited UNITED STATES PATENTS 3,248,270 4/1966 Laidman et al. 148-12 3,260,623 7/1966 Klein 14812.1 3,295,199 1/ 1961 Schrader 117-51 WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 117-5 1