CA2231760A1 - Cold-rolled steel strip and hot-dip coated cold-rolled steel strip for use as building material and manufacturing method thereof - Google Patents

Abstract

A cold-rolled steel strip or hot-dip coated cold-rolled steel strip for use as a fire-proof building member has composition consisting of 0.01-0.25 wt.% C, up to 1.5 wt.% Si, 0.05-2.5 wt.% Mn, up to 0.1 wt.% P, no more than 0.02 wt.% S, 0.005-0.1 wt.% Al, 0.05-1.0 wt.%
Mo, optionally 0.005-0.2 wt.% one or more selected from Ti, Nb, V
and W, optionally one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B and the balance being essentially Fe except inevitable impurities. The cold-rolled steel strip is manufactured by hot-rolling, acid-pickling, cold-rolling and then annealing at a temperature above its recrystallization temperature but below 950°C . The hot-dip coated cold-rolled steel strip is manufactured in the same way but in-line heating a cold-rolled steel strip at a temperature above its recrystallization temperature but below 950°C and then immersing it into a coating bath. The annealed or hot-dip coated steel strip may be further cold-rolled with such a slight duty to induce a plastic strain of 1-5%.

Description

TITLE OF THE INVENTION
COLD-ROLLED STEEL STRIP AND HOT-DIP COATED COLD-ROLLED STEEL STRIP FOR USE AS BUILDING MATERIAL
AND MANUFACTURING METHOD THEREOF
s BACKGROUND OF THE INVENTION
The present invention relates to a cold-rolled steel strip and a hot-dip coated steel strip useful as building material, and is also concE~rned with a method of manufacturing these steel strips.
to Fire-proof coating has been applied to a surface of a building which needs fire-proof construction, in order to inhibit temperature-up oi-" steel material during a fire or the like. A so-called "fire-proof steel" which exhibits high strength at an elevated temperature is used in these days, so that a building can be kept safe even when the ~5 steel material is heated at an elevated temperature near 600~C. Use of such fire-proof steel enables reduction or omission of fire-proof coating. The high-temperature strength is generally represented by yield strength at a high-temperature.
Such fire-proof steel material which has been used so far as 2o mair.~ structural members for a building is usually a relatively thick hot-rolled steel sheet, although a steel sheet with or without hot-dip coating made from a cold rolled steel strip is partially used for the purpose.
Construction of a building also needs steel material for secondary structural members, roofing and walls in addition to main structural members. Cold-rolled steel sheets and hot-dip coated cold-rolled steel sheets have been often used as such members. When this kind of steel material is improved in fire-proof property by enhancing its high-temperature strength, the same advantages as those of the main structural members can be expected. In this sense, there is a demand for provision of a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip excellent in fire-proof property.
Such a cold-rolled steel sheet is manufactured by subjecting a to hot-rolled steel strip to cold-rolling, annealing, hot-dip coating, etc..
The ;steel sheet is sometimes reformed with a heavy duty in order to shape a building member necessary for an actual use. Therefore, the steel sheet shall have good formability as well as a proper high-temperature strength.
~s SUMMARY OF THE INVENTION
The present invention aims at provision of steel material useful as a fire-proof building member. The steel material may be provided as a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip 2o excellent in high-temperature strength and formability. The exce:~lent high-temperature property is attained by controlling alloying composition of the steel strip and further improved by introduction of a plastic strain to the steel strip.
A newly proposed steel strip useful as a fire-proof building memher essentially consists of 0.01-0.25 wt.% C, up to 1.5 wt.% Si, 0.05-~2.5 wt.% Mn, up to 0.1 wt.% P, up to 0.02 wt.% S, 0.005-0.1 wt.% Al, 0.05-1.0 wt.% Mo and the balance being essentially Fe except inevitable impurities. The steel strip may contain 0.005-0.2 wt.% one or more of Ti, Nb, V and W, andlor one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.%
B.
A cold-rolled steel strip useful as a fire-proof building member is manufactured as follows: A slab having the specified composition 1o is heated at 1000-1250~C%, hot-rolled at 800-950C, coiled at 400-700 C;, acid-pickled, cold-rolled at a reduction ratio of 40-90% and then annealed at a temperature above a recrystallization temperature but below 950 ~C . Either box-type annealing or continuous annealing is applicable.
A hot-dip coated cold-rolled steel strip useful as a fire-proof building member is manufactured as follows: A cold-rolled steel strip processed in the same wary is in-line heated at a temperature above a recrystallization temperature but below 950~C in a continuous hot-dip coating installation and then immersed into a hot-dip coating 2o bath.
Any of the cold-rolled steel strip or the hot-dip coated steel sheet may be further cold-rolled with such a slight duty of 1-5 % to induce a plastic strain to the steel strip, after being annealed or hot-dip coated, respectively. Cold-rolling with a slight duty is the most effect;ive in an industrial point of view, although a plastic strain could be induced into they steel strip by application of a stretching load or leveling. The further cold-rolling advantageously enhances the high-temperature properties of the obtained steel strip.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph which shows a relationship between a cold-rolling reduction ratio after annealing and properties at a room temperature and 600~C .
to I>ETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventors have researched and examined effects of alloying elements on a high-temperature strength necessary for a fire-proof steel member without deterioration of formability.
A hot-rolled steel strip which has been designed for use as a fire-proof building member is improved in high-temperature property by addition of such alloying elements as Mo, W, Ti or Nb which are dissolved in a steel matrix or precipitated as intermetallic compounds. On the other hand, a cold-rolled steel strip or a hot-dip 2o coated cold-rolled steel strip is annealed at a temperature above a recrystallization point after being cold-rolled for improvement of formability, since the steel strip as cold-rolled lacks in formability.
Annealing after cold-rolling effectively improves a high-temperature strength dues to addition of Mo which exhibits the same effecl; as that in a conventional hot-rolled steel strip designed for use as a fire-proof building member. However, a high-temperature strength necessary for the purpose is not often realized.
The inventors suppose the poor high-temperature strength is causE~d by unexpected precipitation or the like in the annealing step after cold-rolling. That is, a cold-rolled steel strip or a hot-dip coated cold-:rolled steel strip is in metallurgical situations different from a hot-rolled steel strip, since it is processed by cold-rolling and annealing. In this sense, it is not practical to simply apply an to alloying design which has been proposed for a hot-rolled steel strip to a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip with the proviso that the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip is merely different in thickness from the hot-rolled steel strip.
~5 Taking into consideration the metallurgical hysteresis of the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip, the inventors have found addition of Mo is the most effective among alloying designs proposed so far, and reached an advantageous alloying design using dissolution and precipitation of Mo in specified 2o composition for enhancement of high-temperature properties. The fire-proof and high-temperature properties may be further improved by combinative addition of such carbide-forming elements as Ti, Nb, V and W. Formability of a steel strip necessary for use as a building member is assured by controlled heat treatment in an annealing or hot-dip coating step.
The high-temperature strength is further enhanced by introduction of a plastic strain of 1-5% to the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip. Introduction of such a slight plastic strain enhances a yield strength of the steel strip at a high temperature near 600 C, so as to offer steel material useful as a fire-proof building member due to its excellent fire-proof properties.
Such a plastic strain is applied to the steel strip in cold state but not hot state. Practically, the plastic strain is applied to the steel strip to by cold-rolling at a small reduction ratio.
The proposed alloying design will be apparent from the following explanation.
C: O.~D1-0.25 wt.%
C is an alloying element to bestow a steel with a required I5 strength. An effect of C on the strength becomes bigger as addition of C in an amount of 0.01 wt.% or more. However, excessive addition of C above 0.25 wt.% would cause deterioration of formability or weld;ability.
Si: up to 1.5 wt.%
2o Si is an alloying element effective in improvement of strength, although excessive addition of Si above 1.5 wt.% would cause hardening of steel and poor ductility. In case of a mother sheet for hot-clip coating) addition of Si in amount above 0.3 wt.% causes remaining of uncoated surface parts. In this regard, Si content shall be controlled lower. Such defects as the remaining of uncoated surface parts can be inhibited by electroplating of Fe or a ferrous alloy to a steel strip and. then passing the steel strip to a hot-dip coating installation, even if the steel strip contains Si in an amount exceeding 0.3 wt.%. In this sense, a steel strip containing Si up to 1.5 wt.% may be processed in the same way.
Mn: 0.05-2.5 wt.%
Mn is added as a deoxidizing agent to a steel in a steel making step and is also effective for inhibition of embrittlement at a high to temperature caused by S included as an impurity. Mn effect becomes remarkable in case of addition of Mn in an amount of 0.05 wt.% or more. However, excessive addition of Mn above 2.5 wt.% would cause poor ductility.
P-ut~ to 0.1 wt.%
P is an element effective for enhancing a strength and also improving corrosion resistance in combination with Cu, although exce;~sive addition of P above 0.1 wt.% would cause embrittlement.
S: no more than 0.02 wt.°/.
S is a harmful element included as an inevitable impurity. Less 2o S content is more preferable for the purpose. Allowable S content in the proposed steel is not more than 0.02 wt.%.
Al: 0.005-0.1 wt.%
A1 is added as a deoxidizing agent to a steel and also effective for stabilization of N as A1N in the steel. This effect is realized by addition of A1 in an amount of 0.005 wt.% or more. However, excessive addition of A1 above 0.1 wt,% would cause deterioration of form<~bility and external appearance.
Mo: (1.05-1.0 wt.%
An additive Mo is dissolved or precipitated as a carbide in a steel matrix, resulting in increase of a high-temperature strength.
This Mo effect becomes remarkable by addition of Mo in an amount of 0.05 wt.% or more, but. saturated at 1.0 wt.%. Excessive addition of M~~ above 1.0 wt.% would rather cause hardening of a steel and poor ductility.
Ti, Nb, V, W: each 0.005-0.2 wt.%
These elements are aptional additives which are precipitated as carbides effective in tensile properties at room temperature as well as a high-temperature strength. The effect on such improvement is realized by addition of Ti, Nb, V or W in an amount of 0.005 wt.% or more. However) the effect is saturated at 0.2 wt.%, and excessive addition of Ti, Nb, V or W above 0.2 wt.% would cause hardening of a steel and poor ductility.
Cu: C1.05-0.6 wt.%
2o Cu is an optional alloying additive which effectively improves corre~sion resistance of a steel in combination with P. Such the effect is remarkable by addition of Cu in an amount of 0.05 wt.%. However, excessive addition of Cu exceeding 0.6 wt.% would rather cause promotion of high-temperature cracking during hot-rolling.

Ni: 0.05-0.6 wt.%
IVi is an optional additive effective in corrosion resistance and inhibition of high-temperature embrittlement. Such the effect is remarkable by addition of Ni in an amount of 0.05 wt.% or more.
However, Ni is such an expensive element to increase a steel making cost, and a favorable effect on such properties is hardly expected regardless increase of Ni content even when Ni is added in an amount above 0.6 wt.%.
Cr: 0.05-3.0 wt.%
to Cr is an optional additive effective in corrosion resistance and also increases a high-temperature strength due to precipitation of carbides. Such effects are remarkable by addition of Cr in an amount of 0.05 wt.% or more. However, excessive addition of Cr above 3.0 wt.% would rather cause -hardening of a steel and poor ductility, but ~5 not fwrther improve the corrosion resistance or the high-temperature strength.
B: 0.~~003-0.003 wt.%
B is an optional additive which effectively strengthens grain boundaries. The B effect is remarkable by addition of B in an amount 20 of 0.0003 wt.% or more, but saturated at 0.003 wt.%.
Steel material having the specified composition is cast to a slab by a conventional continuous casting process and then hot-rolled to a predetermined thickness.
In the hot-rolling step, a slab is soaked, hot-rolled and then coiled.
'The soaking promotes dissolution of alloying elements in the steel matrix and renders the slab to a state capable of hot-rolling, when the slab is heated at a temperature of 1000 C or higher. But, an excessively higher soaking temperature above 1250 C would cause hot embrittlement of the slab.
The hot-rolling is preferably finished at 800-950 C so as to assure oversaturated dissolution of alloying elements in the steel matrix without remaining of work-induced ferritic grains. If the to finish temperature is lower than 800°C, the alloying elements are partially precipitated in the steel matrix resulting in poor high-temperature strength. But, a higher finish temperature above 950 C
would wastefully consume a thermal energy and also put a heavy duty on a heating furnace.
t5 The hot-rolled steel strip is coiled at 400-700°C. The controlled coiling temperature is effective for maintaining the dissolution of alloying elements without growth of intermetallic compounds or the like during coiling. Due to such dissolution, the steel strip after annealing or in-line heating in succession to cold-rolling is improved 2o in high-temperature properties.
Thereafter, the hot-rolled steel strip is acid-pickled before cold-rolling.
The steel strip is then cold-rolled under conventional conditions.
The cold-rolling is preferably performed at a reduction ratio of 40-to 90 % in order to promote recrystallization in the following annealing or continuous hot-dip coating step. Such a controlled reduction ratio is also effective for suppressing growth into coarse crystal grains which would put harmful influences on formability.
In case of manufacturing a cold-rolled steel strip without hot-dip coating, the steel strip after cold-rolled is directly delivered to an annealing step. In the annealing step, the steel strip is heated at a temperature above its recrystallization temperature so as to release strains caused by the cold-rolling and to promote sufficient to recrystallization; otherwise the heat-treated steel would be hard and insufficient of formability. On the other hand, overheating at a temperature above 950 C causes growth into coarse crystal grains, although the steel strip can be softened. The growth into coarse crystal grains would reduce formability and poor external appearance of the heat-treated steel strip.
In case of manufacturing a hot-dip coated steel strip, the steel strip after cold-rolled is in-line heated at a temperature above its recrystallization temperature but below 950 C in a continuous hot-dip coating installation. The in-line heating is performed in a 2o redu~:ing atmosphere to activate a surface layer of the steel strip and to anneal the steel strip.
A temperature for the in-line heating is set above a recrystallization temperature: otherwise release of strains caused by cold-rolling and recrystallization would be insufficient for softening the steel strip. Such low-temperature heating also causes insufficient activation of the steel strip, resulting in remaining of unco~~ted surface parts after hot-dip coating. On the contrary, overheating at a temperature above 950~C causes growth into coarse crystal grains or occurrence of surface defects or the like.
The in-line heated steel strip is then immersed into a hot-dip coating bath in the continuous hot-dip coating installation. The hot-dip coating bath may be Zn, Al or Zn-Al. The steel strip coated with a Zn, A1 or Zn-A1 plating layer is obtained in this way.
1o The cold-rolled steel strip or the hot-dip coated cold-rolled steel strip manufactured in the way as above-mentioned may be further subjected to cold-rolling under such conditions to apply a plastic strain of 1-5 % to the steel strip. The introduction of the plastic strain effectively improves a high-temperature strength of the steel strip.
The effect of the plastic strain on the high-temperature strength is newly discovered by the inventors. The high-temperature strength is increased in response to a degree of the plastic strain whic'!h can be represented by a reduction ratio in such a cold-rolling 2o step with a slight duty as defined above.
Fig. 1 shows the effect of a cold rolling reduction ratio on mechanical properties at a room temperature and 600 C , when a steel consisting of 0.09 wt.% C, 0.05 wt.% Si, 0.55 wt.% Mn) 0.012 wt.% P, 0.006 wt.% S, 0.035 wt.% Al, 0.31 wt.% Mo, 0.07 wt.% V and the balance being Fe except inevitable impurities was hot-rolled, cold-:rolled, annealed 1 min. at 800 C and then further cold-rolled.
It is noted from Fig. 1 that a yield strength at 600 C is increased as increase of a cold-rolling reduction ratio. Such improvement on the mechanical properties can be clearly recognized at a reduction ratio above 1 %. Although a high yield strength at 600 ~ is obtained in case of a reduction ratio above 5%, an elongation at a room temperature is unfavorably reduced. Decrease of an elongation means poor formability of the cold-rolled steel strip to or the hot-dip coated cold-rolled steel strip. From these results, a reduction ratio during cold-rolling after annealing is preferably controlled within a range of 1-5 %.
EXAMPLE
is EXAMPLE 1 Each steel having composition shown in TABLE 1 was melted and cast to a slab. The slab was forged and hot-rolled to a steel strip of 4.0 mm in thickness. The hot-rolled steel strip was then cold-rolled to a thickness of 1.0 mm and annealed under various 2o conditions shown in TABLE 2.
A test peace was cut off each steel strip manufactured in this way and offered to tensile tests at a room temperature and 600'C.
Results are shown in TABLE 2 in combination with an annealing temperature.

It is noted from TABLE 2 that steel strips having specified composition and being annealed at 650-950~C had enough ductility at a room temperature and a higher yield strength at 600 C in comparison with comparative test pieces. Consequently, it is recognized that the steel strips in the scope of the present invention are useful as fire-proof building members excellent in high-temperature properties.

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z Each slab having composition shown in TABLE 3 was prepared by a continuous casting process. The slab was soaked at 1150-1200 C , hot-rolled to 2.3-3.0 mm in thickness at a finishing s temperature of 850-870 C and coiled at 550-580 C . The hot-rolled steel strip was then cold-rolled 1;0 0.8-1.2 mm in thickness.
One group of cold-rolled steel strips were delivered to a continuous annealing line, while the remaining of the steel strips were delivered to a continuous hot-dip coating line. In the continuous annealing line, each steel strip was heated 40 seconds at 820 C and then cooled down to a room temperature with or without overageing treatment. In the continuous hot-dip coating line, each steel strip was in-line annealed 35 seconds at 800~C ) cooled down to 500 C near a temperature of a coating bath. and then immersed into 15 a molten Zn or Zn-5% A1 bath.
A test piece cut off each of the cold-rolled steel strips and the hot-dip coated steel strips was offered to tensile tests at a room temperature and 600~C. Results are shown in TABLE 4.
It is recognized from TABLE 4 that any of the cold-rolled steel 2o strips or the hot-dip coated steel strips having the specified compositions and being annealed in the specified temperature range is useful as a fire-proof building member due to its excellent ductility at a room temperature as well as a high yield strength at 600 C compared with comparative steel strips.
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~z o z l9 Each steel having composition shown in TABLE 5 was melted, cast, forged and then hot-rolled to a steel strip of 4.0 mm in thickness. The hot-rolled steel strip was then cold-rolled to a thickness of 1.0 mm. The cold.-rolled steel strip was annealed by heating 1 min. at 800 C and Gaoling in an opened atmosphere. One group of the annealed steel strips were further cold-rolled with a slight duty to induce plastic strains.
A test piece was cut off each steel strip and offered to tensile to tests at a room temperature and 600~C. Results are shown in TABLE
6. It is noted from TABLE 6 that the steel strips having the specified compositions and being bestowed with plastic strains of 1-5% are useful as fire-proof members due to their excellent ductility at a room temperature as well as high yield strength at 600~C compared with comparative steel strips.

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w _ a x ~ ~ x ~ ~ ~ ~ ~ ~ ~ H H
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W

Each slab having composition shown in TABLE 7 was prepared by a continuous casting process. The slab was soaked at 1180-1210~C, hot-rolled to a steel strip of 2.3-3.0 mm in thickness at a finishing temperature of 840-870 C and then coiled at 530-580 C .
The hot-rolled steel strip was cold-rolled to a thickness of 0.6-1.0 mm.
One group of the cold-rolled steel strips manufactured in this way were delivered to a continuous annealing line, while the to remaining group of the steel strips were delivered to a hot-dip coating line. In the annealing lime, each steel strip was heated 40 sec.
at 800 °C and then cooled down to a room temperature with or without overageing treatment. In the hot-dip coating line, each steel strip was in-line heated 35 sec. at 800~C, cooled down to 500~C near a temperature of a coating bath and then immersed into the coating bath. A molten Zn or Zn-5% Al pool was used as the coating bath.
Each steel strip was cold-rolled with a slight duty after annealing or hot-dip coating, so as to induce a plastic strain.
A test piece was cut off each of the cold-rolled steel strips and 2o the hot-dip coated steel strips and offered to tensile tests at a room temperature and 600 C . Results are shown in TABLE 8. It is noted from TABLE 8 that the steel strips having the specified compositions and being bestowed with plastic strains of 1-5% are useful as fire-proof members due to their excellent ductility at a room temperature as well as high yield strength ;~t 600 C compared with comparative steel strips.

W

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m m m m w A cold-rolled steel strip or a hot-dip coated cold-rolled steel strip according to the present invention is excellent in formability and high-temperature properties necessary for use as a fire-proof building member. Since there is not required any special changes in manufacturing process from a steel making step to an annealing or hot-dip coating step, the steel strip is advantageously manufactured in an industrial point of view. A fire-proof property of the steel strip is further improved by cold-rolling the steel strip with such a slight duty to induce a plastic strain of 1-5 % after annealing or hot-dip to coating.

Claims (11)

1. A cold-rolled steel strip for use as a fire-proof building member consisting of 0.01-0.25 wt.% C, up to 1.5 wt.% Si, 0.05-2.5 wt.% Mn, up to 0.1 wt.% P, no more than 0.02 wt.% S, 0.005-0.1 wt.% Al, 0.05-1.0 wt.% Mo and the balance being essentially Fe except inevitable impurities.

2. The cold-rolled steel strip defined in Claim 1, which further contains 0.005-0.2 wt.% one or more selected from Ti, Nb, V
and W.

3. The cold-rolled steel strip defined in Claim 1, which further contains one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B.

4. The cold-rolled steel strip defined in Claim 1, which further contains 0.005-0.2 wt.% one or more selected from Ti, Nb, V
and W, and one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B.

5. A hot-dip coated cold-rolled steel strip for use as a fire-proof building member consisting of 0.01-0.25 wt.% C, up to 1.5 wt.%
Si, 0.05-2.5 wt.% Mn, up to 0.1 wt.% P, no more than 0.02 wt.%
S, 0.005-0.1 wt.% Al, 0.05-1.0 wt.% Mo and the balance being essentially Fe except inevitable impurities.

6. The hot-dip coated cold-rolled steel strip defined in Claim 5, which further contains 0.005-0.2 wt.% one or more selected from Ti, Nb, V and W.

7. The hot-dip coated cold-rolled steel strip defined in Claim 5, which further contains one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B.

8. The hot-dip coated cold.-rolled steel strip defined in Claim 5, which further contains 0.005-0.2 wt.% one or more selected from Ti, Nb, V and W, and one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B.

9. A method of manufacturing a cold-rolled steel strip for use as a fire-proof building member, comprising the steps of:
preparing a slab having composition consisting of 0.01-0.25 wt.% C, up to 1.5 wt.% Si, 0.05-2.5 wt.% Mn, up to 0.1 wt.% P, no more than 0.02 wt.% S, 0.005-0.1 wt.% Al, 0.05-1.0 wt.% Mo, optionally 0.005-0.2 wt.% one or more selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B
and the balance being essentially Fe except inevitable impurities;
heating said slab at 1000-1250'C ;
hot-rolling the heated slab at a finishing temperature of 800-950'C;
coiling the hot-rolled steel strip at 400-700°C ;
acid pickling the hot-rolled steel strip;
cold-rolling the pickled steel strip at a reduction ratio of 40-90%;
annealing the cold-rolled steel strip at a temperature above its recrystallization temperature but below 950°C; and optionally cold-rolling the annealed steel strip with such a slight duty to induce a plastic strain of 1-5% to said steel strip.

10. The method defined in Claim 9, wherein the cold-rolled steel strip is box-annealed or continuously annealed.

11. A method of manufacturing a hot-dip coated cold-rolled steel strip for use as a fire-proof building member, comprising the steps of:
preparing a slab having composition consisting of 0.01-0.25 wt.% C, up to 1.5 wt.% Si, 0.05-2.5 wt.% Mn, up to 0.1 wt.% P, no more than 0.02 wt.% S, 0.005-0.1 wt.% Al, 0.05-1.0 wt.% Mo, optionally 0.005-0.2 wt.% one or more selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6 wt.% Cu, 0.05-0.6 wt.% Ni, 0.05-3.0 wt.% Cr and 0.0003-0.003 wt.% B
and the balance being essentially Fe except inevitable impurities;
heating said slab at 1000-1250°C;
hot-rolling the heated slab at a finishing temperature of 800-950°C;
coiling the hot-rolled steel strip at 400-700°C;
acid pickling the hot-rolled steel strip;
cold-rolling the pickled steel strip at a reduction ratio of 40-90%;
in-line heating the cold-rolled steel strip at a temperature above its recrystallization temperature but below 950°C in a continuous hot-dip coating installation;
immersing the in-line heated steel strip into a hot-dip coating bath; and optionally cold-rolling the hot-dip coated steel strip with such a slight duty to induce a plastic strain of 1-5% to said steel strip.