One-sided enamelable hot-rolled steel sheet and process for producing the same

A hot-rolled steel sheet suitable for enamelling on one side is disclosed. The steel consists essentially of 0.0050-0.07 wt % C, 0.05-1.5 wt % Mn, 0.03 0.15 wt % P, 0.03-0.1 wt % Al, 0.003-0.010 wt % N, provided that the N content is the sum of at least 0.002 wt % of free nitrogen not bound to Al and the content of nitrogen in other states, the nitrogen content as defined above satisfying the condition of Al/N.gtoreq.10 on the basis of weight percents and the balance being Fe and incidental impurities.

FIELD OF THE INVENTION 
The present invention relates to a hot-rolled sheet material suitable for 
enamelling on one side made from a continuously cast steel, and to a 
process for producing such a sheet steel. 
Hot-rolled steel sheets that are primarily intended to be used in 
combustors such as water heaters and boilers and that are enamelled on one 
side are required to have resistance to blistering, copperhead formation 
and warping. In addition, a certain amount of strength is required for 
most of these steel sheets in the finished state used in these enamelled 
products. 
The common steel sheets suitable for enamelling on two sides are required 
to have resistance to fish scale formation. Fish scale is a phenomenon in 
which water in the steel during firing to form hydrogen gas, which 
aggregates at the interface between the applied and fired enamel coat and 
the steel sheet after the enamel coat is fired, and which disrupts some 
areas of the fired enamel coat so that it will spall in the form of fish 
scale. In the case of the steel sheets suitable for enamelling on one side 
which are contemplated by the present invention, fish scale is not a 
problem since hydrogen gas can escape from the steel sheet on the side 
which is not to be enamelled. 
One-sided enamelable hot rolled steel sheets are often used as the material 
of comparatively large and heavy vessels and hence are required to possess 
a certain amount of strength. However, the enamelling process involves a 
heat treatment at temperatures of 800.degree. C. and higher and a decrease 
in the strength of the base metal is inevitable. This means that the 
strength of one-side enamelled hot rolled steel sheets to which the heat 
treatment in the enamelling process has been subjected cannot be ensured 
by simply increasing the strength of the parent steel sheets, and it has 
been difficult to reduce a drop in strength during or after the heat 
treatment in the enamelling process by conventional continuous-cast 
steels. Under these circumstances, nitrogen-bearing ingot-cast steels that 
have comparatively high carbon contents have been used as the starting 
material from which enamelled steel sheets are made. Since ingot-cast 
steels have rimmed-layers and are decarburized at the surface, they are 
free from the problems of blistering and copperhead formation and are 
suitable for use as materials for enamelling steel sheets. Notwithstanding 
this advantage, ingot casting is neither economical nor effective in view 
of the current rapid increase in the number of continuous casting mills. 
The present inventors previously proposed in Japanese Laid-Open Patent 
Publication No. 60-221520 a process for producing one-sided enamelable 
hot-rolled sheet material which, by subjecting a continuously-cast steel 
having a specified composition to heat treatments conducted under 
specified conditions, would ensure that the resultnng steel will have good 
response to enamelling while undergoing no substantial decrease in 
strength. This technique was adaptable to applications of enamelled steel 
sheets such as in boilers and water heaters, and thereby satisfied to some 
extent the requirements of users. However, this technique had certain 
conditions where excessively high temperatures occurred during the firing 
of bases were used as materials of comparatively small vessels and if 
parts thereof were subjected to a strong working, grains in the parts were 
made coarse in the final this technique resulted in a drop in its 
strength. 
SUMMARY OF THE INVENTION 
An object, therefore, of the present invention is to provide a one-sided 
enamelable hot rolled steel sheet from a continuously cast slab that has 
good responsiveness to enamelling and which yet is free from the 
aforementioned problem of decrease in strength due to formation of coarse 
grains. 
Another object of the present invention is to provide a process for 
producing such one-sided enamelable hot-rolled steel sheet. 
The essence of the present invention is summarized below. 
(1) A one-sided enamelable hot-rolled steel sheet that consists essentially 
of 0.0050-0.07 wt % C., 0.05-1.5 wt % Mn, 0.03 -0.15 wt % P, 0.03-0.1 wt % 
Al, 0.003-0.010 wt % N, at least 0.002 wt % of free nitrogen not bound to 
Al, the nitrogen content as defined above satisfying the condition of 
Al/N.gtoreq.10 on the basis of weight percents, and the balance being Fe 
and incidental impurities. 
The term "N" (i.e. nitrogen) means the total nitrogen or all kinds of 
nitrogens that are contained in the steel sheet whereas the term "free 
nitrogen" means the nitrogen which is not in such a state that it has 
combined with any element such as aluminum in the steel sheet. 
(2) A one-sided enamelable hot-rolled steel sheet as defined in (1) wherein 
the content of free N unbound to Al is in the range of from 0.002 wt % to 
0.010 wt %, sum of N (wt %) as nitride and free N (wt %) is no less than 
10. 
(3) A process for producing a one-sided enamelable hot-rolled steel sheet 
consisting of the steps of: 
(a) heating to at least 1,200.degree. C. a continuously cast slab 
containing AlN in precipitated form which consists essentially of 
0.0050-0.07 wt % C., 0.05-1.5 wt % Mn, 0.03 0.15 wt % P, 0.03-0.1 wt % Al, 
and 0.003-0.010 wt % N, provided that Al (wt %)/N (wt %) .degree. 10, and 
the balance being Fe and incidental impurities; 
(b) hot-rolling the heated slab to produce a steel sheet; and 
(c) coiling the resultant steel sheet at a temperature that is at least 
400.degree. C. and which is not higher than the temperature determined by 
the following formula: 
##EQU1## 
where log is the common logarithm and both N and Al are in weight percent. 
(4) A process for producing a one-sided enamelable hot-rolled steel sheet 
which consists essentially of the steps of: 
(a) hot-rolling a continuously cast slab containing AlN in unprecipitated 
form, which slab consists essentially of: 0.0050-0.07 weight percent C., 
0.05-1.5 wt % Mn, 0.03-0.15 wt % P, 0.03-0.1 wt % Al, and 0.003-0.010 wt % 
N, provided that Al (wt %)/N (wt %) .gtoreq.10, and the balance being Fe 
and incidental impurities; and 
(b) coiling the resultant steel sheet at a temperature that is at least 
400.degree. C. and which is not higher than the temperature determined by 
the following formula: 
##EQU2## 
where log is the common logarithm and both N and Al are in weight percent. 
DETAILED DESCRIPTION OF THE INVENTION 
The criticality of each of the components in the one-sided enamelable 
hot-rolled steel sheet of the present invention and that of their 
compositional ranges are described below. 
Carbon is an element that has the important influence on the workability of 
the enamelling steel sheet and phenomena such as warping, blistering and 
copperhead formation. Blistering is a phenomenon in which the carbon in 
the steel sheet reacts during the enamel coat-firing step with the oxygen 
in the enamel or the firing atmosphere so as to cause gasous components 
such as CO and CO.sub.2 to evolve. Copperhead is a defect that is caused 
by the formation of iron oxides due to large bubbles occurring ont the 
surface of the base steel and is harmful not only to the appearance of the 
enamelled product but also to its corrosion resistance. 
As will be understood from the above, carbon is a factor that bears great 
significance in the development of defects occurring in the enamelled 
product. In one-sided enamelable hot-rolled steel sheets where fish scale 
is not a problem, copperhead is the most damaging defect occurring in 
enamelled products. In order to produce enamelled steel sheets intended 
for use in water heaters and boilers, the enamel having hot 
water-resistant is employed but this enamel is highly likely to cause 
copperhead. 
The present inventors conducted intensive studies in order to prevent the 
occurrence of copperheads and provide a desired result with a coat of 
enamel having hot water-resistant being once applied. As a consequence, 
the present inventors have found that if a special enamel containing Co 
(cobalt) content more than in a regular enamel is to be used, the 
occurrence of copperheads can be prevented by controlling the carbon 
content of the steel so as not to exceed 0.07 wt %. 
The present inventors have also found that if one wants to attain desired 
results with a coat of the regular enamel being once applied, the carbon 
content must be controlled to be no more than 0.025 wt %. 
If the upper limit of the carbon content in the steel is 0.07 wt % or 
below, a desired, fired enamel coat can be formed without causing 
copperhead and any other defects such as blistering and warping in the 
enamelled product. Since a minimum level of strength must be ensured, the 
lower limit of carbon content in the steel is set at 0.0050 wt %. A 
preferred range of carbon content is between 0.008 and 0.020 wt % if the 
regular enamel is used, and it is between 0.008 and 0.050 wt % if the 
special enamel is used. 
Manganese is necessary as an auxiliary strengthening element. The absolute 
strength of the enamelled steel sheet in the finished state is ensured 
principally by P, and Mn and C both serve as auxiliary strengthening 
elements. In order to accomplish enamelling without causing any 
deleterious effects such as warping and to ensure that no difficulty is 
encountered in melting the steel in a steel-making process, the upper 
limit of Mn content is set at 1.5 wt %. The lower limit of Mn content is 
set at 0.05 wt % for the purpose of preventing hot-brittleness due to S. 
Phosphorus is the principal strengthening element used to afford the 
absolute strength of the enamelled steel sheet in the finished state. 
Phosphorus is also effective as a component to be incorporated in an 
enamelling steel sheet for providing ease of scale removal. The upper 
limit of P content is 0.15 wt %, beyond which the steel remarkably becomes 
brittle. The lower limit of P content is set at 0.03 wt % for providing 
ease of descaling. 
The condition in which aluminum and nitrogen exist in the steel and their 
contents are the most important factors for the purposes of the present 
invention. One function of aluminum is deoxidation, so it must be present 
in an amount of at least 0.03 wt %. Using aluminum as a deoxidizer does 
not constitute an important aspect of the present invention, in which 
aluminum is incorporated for the principal purpose of ensuring that it 
will be bound to free N in the steel during the enamel coat-firing step to 
form AlN. In order to attain this effect by providing as many sites as 
possible where aluminum binds with free N, the condition that 
Al/N.gtoreq.10 (where N is the total nitrogen containing N as nitrides and 
free N) must be satisfied. The upper limit for the conent of Al is set at 
0.1 wt % in order that Al-derived inclusions such as Al.sub.2 O.sub.3 will 
not cause any deleterious effects on the surface properties of the steel 
sheet or its workability. 
As will be understood from the foregoing explanation, N as well as Al are 
important elements for the purposes of the present invention. Firing the 
enamel coat on a steel plate is a special heat treatment in that it is per 
se but for bonding the enamel to the steel both physically and chemically. 
As a result of the firing step, grains in ordinary steels grow in size to 
cause a drop in their strength. Water heaters, hot water supply systems 
and boilers are pressurized during use, so the enamelled steel sheets of 
which they are made must have a strength great enough to withstand such 
high pressures. research to provide a method for preventing the growth of 
grains during the enamel coat-firing. As a consequences, it was found that 
producing a precipitate in the steel during the enamel coat-firing is 
important for preventing the growth of grains and that AlN is the only 
precipitate such as Ti, Nb and B that form stable precipitates is adverse 
to the purposes of the present invention and such elements should not be 
present in one-sided enamelable hot-rolled steel sheets of the present 
invention. These elements have already formed stable precipitates at the 
stage of hot rolling, and in the subsequent enamel coat-firing step the 
precipitates will simply grow in size and are unable to prevent the 
movement of grain boundaries. On the other hand, AlN is not 
thermodynamically a precipitate that is as stable as Ti, Nb and B and this 
is the only compound that will not precipitate during the steel 
manufacturing process but which can be precipitated in the subsequent 
enamel coat-firing step. 
For the reasons stated above, N must be incorporated in the steel and in 
this case, free N is the most important. The grains in the base steel 
sheet will grow during the enamel coat-firing unless it contains free N in 
a minimum amount of at least 0.002 wt %. Therefore, the total content of N 
including free N in the steel before enamel application and firing must be 
at least 0.003 wt %. In steelmaking operations, if N is supplied in 
excessive amounts, excess N will produce fumes that will either foul the 
environment or cause adverse effects on the health of operators. 
Therefore, in order to avoid the occurrence of such environmental problems 
during melting operations, the upper limit of N is set at 0.010 wt %. 
Having discussed the criticality of the compositional range of each of the 
components in the one-sided enamelable hot-rolled steel sheet, we now 
describe the conditions to be employed in performing hot rolling to 
produce such a sheet. 
A continuous-cast slab having a high temperature is preferably fed into the 
rolling mill directly in the ascast condition if AlN has not precipitated 
during the casting. Otherwise, the slab must be heated to 1,200.degree. C. 
and above so as to form a solid solution compound of AlN. If the heating 
temperature is less than 1,200.degree. C., AlN will precipitate in the 
hot-rolled product and the advantages of the present invention cannot be 
attained. The upper limit of the temperature at which the slab is heated 
is not specified but based on practical considerations, 1,300.degree. C. 
would be a preferable upper limit. 
In order to ensure that AlN will not precipitate during hot rolling, the 
temperature at which the hot-rolled steel is coiled must be specified. To 
achieve this end, as well as to ensure consistent production, the lower 
limit of the coiling temperature is set at 400.degree. C. The upper limit 
depends on the contents of Al and total N and should satisfy the following 
formula: 
##EQU3## 
where log is the common logarithm and Al and N are both in wt %. 
The steel emerging from the last rolling stand is cooled on the runout 
table as it is taken up by the coiler. For maintaining N and Al in the 
rolled steel in better condition, the cooling pattern is preferably such 
that quenching in the first half of the cooling zone is done more rapidly 
than in the second half. 
Other operations on the hot strip mill may be performed by normal 
procedures, and the finishing operations after coiling may be also done by 
the conventional procedures.

EXAMPLE 
Liquid steels having the compositions shown in Table 1 were continuously 
cast into slabs. 
TABLE 1 
______________________________________ 
Classi- 
Steel C Mn P Al N fication 
______________________________________ 
I 0.067 0.17 0.054 
0.05 0.0042 Within 
the scope 
of the 
invention 
II 0.042 0.25 0.073 
0.04 0.0037 Within 
the scope 
of the 
invention 
III 0.021 0.29 0.091 
0.06 0.0049 Within 
the scope 
of the 
invention 
IV 0.013 1.30 0.079 
0.05 0.0032 Within 
the scope 
of the 
invention 
V 0.008 0.54 0.131 
0.05 0.0043 Within 
the scope 
of the 
invention 
VI 0.108 0.32 0.049 
0.06 0.0039 comparison 
VII 0.043 0.29 0.071 
0.02 0.0049 comparison 
VIII 0.041 0.27 0.070 
0.07 0.0016 comparison 
______________________________________ 
The slabs were heated at the temperatures indicated in Table 2, hot rolled 
at a finishing temperature of products having 2.5 mm thick were coiled at 
500.degree. C. 
In Table 2, measurements of the content of free nitrogen, yield point (YP), 
tensile strength (TS) and elongation (E1) were conducted on the resultant 
as-hot rolled sheets, and measurements of yield point, tensile strength 
and elongation were also conducted on the sheets that had been subjected 
to a heat treatment equivalent to enamel coat-firing, and the results are 
shown in Table 2. A regular enamel and a special Co-containing enamel were 
applied to each of the steel sheets and the quality of the fired enamel 
coat was evaluated. The results are also shown in Table 2. Nos. I-1 to 
VIII-1 in Table 2 correspond to Nos. I to VIII in Table 1, and No. II-2 in 
Table 2 corresponds to No. II in Table 1. 
TABLE 2 
__________________________________________________________________________ 
After heat treatment 
Enamel coat 
As hot rolled equivalent to the Special 
Heating enamel coating-firing 
Co-con- 
Classifi- 
Sample 
temperature 
Free N 
YP TS El YP TS El Regular 
taining 
cation No. (.degree.C.) 
(wt %) 
(kgf/mm.sup.2) 
(kgf/mm.sup.2) 
(%) (kgf/mm.sup.2) 
(kgf/mm.sup.2) 
(%) 
enamel 
enamel 
__________________________________________________________________________ 
Steels of the 
I-1 1250 0.0039 
35.1 44.2 33.1 
34.9 43.9 33.2 
x O 
invention 
II-1 
1250 0.0033 
31.0 43.1 33.2 
30.9 42.7 36.4 
x O 
III-1 
1250 0.0045 
35.1 44.8 31.0 
36.0 45.7 30.1 
O O 
IV-1 
1250 0.0029 
33.3 43.4 39.8 
35.1 42.6 41.6 
O O 
V-1 1250 0.0040 
29.9 40.1 41.4 
30.1 39.8 41.4 
O O 
Comparative 
VI-1 
1250 0.0032 
38.4 46.9 29.8 
35.1 42.8 30.9 
x x 
steels VII-1 
1250 0.0045 
30.8 42.9 34.1 
26.6 38.1 36.1 
x O 
VIII-1 
1250 0.0011 
30.3 42.1 34.9 
27.9 34.4 38.9 
x O 
II-2 
1100 0.0006 
30.2 42.9 33.9 
27.1 35.1 38.1 
x O 
__________________________________________________________________________ 
Note: 
O, good; 
x, copperheads formed. 
As Table 2 shows, sample No. VI-1 having a high carbon content was very low 
in its responsiveness to enamelling. Sample Nos. VII-1 and VIII-1 were 
also outside the scope of the present invention in terms of Al and N 
contents, respectively, and experienced significant decreases in their 
strength properties after heat treatment equivalent to the enamel 
coat-firing. Sample No. II-2 which had been heated at an unduly low 
temperature (1,100.degree. C.) had an unsatisfactory low level of free N 
production. 
Sample Nos. II and VIII were subjected to a strong working (i.e. working of 
buckling such as hydrostatic bulging was effected but it was stopped 
immediately before each of the steel species was fractured) and then 
received a heat treatment equivalent to the enamel coat-firing. The 
resulting changes in their strength as a function of the coiling 
temperature are shown in Table . 
TABLE 3 
______________________________________ 
Coiling Steel No. II Steel No. VIII 
tempera- YP TS YP TS 
Sample 
ture (kgf/ (kgf/ El (kgf/ (kgf/ El 
No. (.degree.C.) 
mm.sup.2) 
mm.sup.2) 
(%) mm.sup.2) 
mm.sup.2) 
(%) 
______________________________________ 
2 700 24.9 40.2 40.0 22.7 31.0 41.3 
3 600 25.5 41.0 37.1 23.8 33.2 40.7 
4 550 30.8 43.0 35.8 24.1 33.2 39.2 
1 500 31.1 43.2 35.1 24.3 33.8 39.1 
5 450 32.1 44.1 34.7 25.2 34.2 38.2 
6 400 32.5 45.3 34.0 25.9 35.1 37.8 
______________________________________ 
As Table 3 shows, the strength of sample Nos. 2 and 3 of steel No. II was 
low because of the high coiling temperatures that were used. Steel No. 
VIII which did not have a good balance between Al and total N was 
unsuitable for the strong working under severe conditions. 
As will be understood from the foregoing description, the one-sided 
enamelable hot-rolled steel sheet of the present invention is highly 
responsive to enamelling and yet undergoes no drop in strength as a result 
of the enamel coat-firing.