EP0119088B1

EP0119088B1 - Steel for use as material of cold-rolled steel sheet

Info

Publication number: EP0119088B1
Application number: EP19840301646
Authority: EP
Inventors: Kazuo C/O Kimitsuseitetsusho Koyama; Yasuo C/O Kimitsuseitetsusho Hamamoto; Hiroshi C/O Kimitsuseitetsusho Katoh; Hiromi C/O Kimitsuseitetsusho Toyota
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1983-03-10
Filing date: 1984-03-12
Publication date: 1986-09-03
Also published as: JPS59166650A; DE3460593D1; EP0119088A1; JPS6153411B2

Description

The present invention relates to a steel suitable for use as the material of cold-rolled steel sheet having excellent formability and, more particularly, to a steel of this kind featuring a composition containing Fe, very small amount of C, small amount of N, and O.
Box-annealed low-carbon-AI killed steel and IF steel (Interstitial Free Steel) are known as the steel suitable for use as the material of cold-rolled steel sheet having excellent formability. Rimmed steel and capped steel are also known as the material for such a purpose, although these steels are somewhat inferior to the first-mentioned steels in the formability.
In the production of the low C-Al killed steel, solution treatment of AIN is conducted during the hot rolling and a quenching is effected to prevent any precipitation of the same. Then, during the box annealing after the cold rolling, the steel material is slowly heated to permit the precipitation of the AIN thereby to allow the growth of crystal grains having a crystalline orientation desirable for achieving the high formability. Throughout this process, it is possible to obtain a Lankford value (f value) of 1.5 to 1.8. The AIN content, however, undesirably delays the recrystallization, so that the box annealing is usually conducted taking a long time of about 12 hours at a high temperature of 700°C or so.
In the annealing of a quick heating as in the case of a continuous annealing after cold rolling there cannot occur sufficient precipitation of AIN but such in sufficiently precipitated AIN is rather not preferable. In conducting a continuous annealing following a cold rolling, therefore, it is a common measure to effect a thorough AIN precipitation treatment by coiling the steel at high temperature after the step of hot rolling. The coiling of the steel at high temperature, however, thickens the scale on the steel with the result that the pickling of the steel is apt to be impeded. In addition, the precipitation of AIN cannot be made sufficiently in the inner and outer peripheral parts of the hot-rolled steel coil because these parts are cooled rapidly after the coiling. Consequently, the steel after the annealing following the cold rolling exhibit inferior properties. Thus, the low C-Al killed steel, when it is box-annealed after a cold rolling, causes a problem of too long annealing time which is quite disadvantageous from the economical point of view. In addition, when this steel is treated by continuous annealing after a cold rolling, the quality of the steel is apt to be impaired due to the coiling at high temperature after the hot rolling.
On the other hand, the IF steel is prepared by reducing the C content through a vacuum degassing of the melt and adding such an element having a high affinity to C and N as Ti to decrease the C and N in solid-solution state substantially to zero. This steel exhibits a high value regardless of the rate of temperature rise during the annealing, and is free from the problem of degradation due to strain aging. For these reasons, this steel finds a spreading use as the material of high-grade cold-rolled steel sheet, irrespective of whether it is treated by box annealing or continuous annealing. The IF steel, however, requires the use of Ti which is expensive and, at the same time, requires an intensive deoxidation by adding of AI in order to fully bring about the advantages of Ti. In addition, the IF steel also necessitates an annealing at high temperature, because the recrystallization temperature is raised extremely due to the presence of titanium carbon nitride.
Referring now to the rimmed steel and the capped steel, the cold-rolled steel sheets produced from these steels exhibit only a small elongation value because of the presence of a large amount of oxide inclusions. In addition, since these inclusions impedes the growth of the desirable structure, the values of these steels are not so high.
Further, in the U.S. Patent No. 4,073,643 there is shown a technique of producing a cold-rolled steel sheet having improved formability by reducing the content of Si in steel, however, the formability of the steel obtained by this technique is still insufficient.
Accordingly, it is an object of the invention to provide a low C-0 type steel for use as the material of cold-rolled steel sheet having excellent formability, improved to permit an annealing at low temperature thereby to overcome the above-described problems of the prior art.
To this end, according to one aspect of the invention, there is provided a steel annealable at low temperature and suitable for use as the material of cold-rolled steel sheet having an excellent formability, the steel having a composition comprising by weight, of: 0.001 % to 0.0050% of C, not greater than 0.5% of Mn, not greater than 0.1 % of P, not greater than 0.0050% of N, 0.016 to 0.035% of O and the balance Fe and inevitable impurities, the 0 and at least a part of the Mn existing in the form of fine oxides dispersed substantially uniformly which oxide brings about a structure which imparts the good formability to the steel sheet.
The steel sheet can contain, in addition to these constituents, at least one selected from the group consisting, by weight, of from 0 to 0.0050% of B, from 0 to 0.080% of Nb and from 0 to 0.1% of V.
It is still another object of the invention to provide a method of production cold-rolled steel sheets having an excellent formability, improved to overcome the above-described problems of the prior art.
To this end, the invention provides a method of producing a steel sheet having an excellent formability comprising the steps of: forming by continuous casting a steel slab having a composition comprising, by weight, of: 0.001 to 0.0050% of C, not greater than 0.5% of Mn, not greater than 0.1 % of P, not greater than 0.0050% of N, 0.016 to 0.035% of O and the balance Fe and inevitable impurities; subjecting the steel slab to a hot rolling to form a steel sheet; subjecting the hot-rolled steel sheet to a pickling and then to a cold rolling; and subjecting the cold-rolled steel sheet to a continuous annealing conducted at a low temperature range of between 600 and 770°C.
The steel slab can contain, besides the constituents mentioned above, at least one selected from the group consisting, by weight, of from 0 to 0.0050% of B, from 0 to 0.080% of Nb and from 0 to 0.1% of V.
According to another form of the method of the invention, the above-mentioned continuous annealing can be substituted by a box annealing conducted at a temperature range of between 550 and 680°C.
Thus, according to the invention, the content of C which is impedimental to the formability of cold-rolled steel sheet is decreased as much degree as possible. At the same time, the invention aims at making a positive use of O content, which has been considered as being impedimental, within a specific region. Consequently, in the steel of the invention, the recrystallization and the formation of structure for high formability are promoted by a moderate dispersion of the oxides. That is, in the present invention, in the cold-rolled steel before annealing stage there are evenly distributed oxide inclusions of 0.5-5 pm in size comprising mainly manganese oxide the amount of which oxide inclusions is limited to a value corresponding to 0.016 to 0.035 wt% of oxygen. In these inclusions, relatively large size inclusions act to become nucleus for recrystallization at the time of annealing of cold-rolled steel, while relatively small size inclusions act to become appropriate barrier with respect to grain coarsening caused after the recrystallization to thereby moderately control the grain size of the steel into a range of ASTM grain size No. 7 to 9. These inclusions must be of oxide mainly comprising manganese oxide. In order to obtain such manganese oxide, it is necessary to provide a steel of a low-carbon/manganese/high-oxygen system. Conventional steel contains alumina, which, because it is hard and because it precipitates in a cluster-like state, tends unfavourably to result in line-like defects in the steel. If the carbon content of steel is high, the carbon forms carbon monoxide in the manufacturing stage during which the steel is molten, and that CO in turn remains as blow-hole defects in the solidified steel slab. In the present invention the inclusions are positively utilized to improve the formability of steel. Such inclusions, however, tend to degrade the ductility of steel and, in order to compensate for that, it is necessary to reduce the content of C in the steel, because the C becomes carbides in the final annealing stage, and the carbides in turn tend to prevent the moderate grain growth of cold-rolled steel while the existence of carbides reduces the ductility of the steel.
In the present invention the amount of AI charged to effect deoxidation during the molten stage is kept low in order to achieve a relatively high oxygen content in the steel. All the AI is converted to alumina by reaction with oxygen, and the alumina floats to the surface of the molten steel and is removed, so that the resulting steel contains substantially no aluminium oxide. That is, since aluminium is the most intensive deoxidizing element, in such a condition that oxide inclusions mainly comprising manganese oxide are made to remain in the resulting steel product (i.e., the amount of aluminium for deoxidization is limited to a relatively small value), the charged aluminum floats as alumina on the molten steel surface and are removed from the steel, so that no aluminum in solid-solution state exists in the molten steel stage. Thus, unfavorable precipitation of AIN does not occur after the stage of molten steel, which AIN acts to extremely raise the value of recrystallization temperature of cold-rolled steel as explained above. Since in the steel of the present invention no precipitation of AIN occurs, it becomes easy to control the recrystallization of cold-rolled steel. As deduced from the chemical compositions of the steel of the present invention, there substantially exists no aluminum in the steel of the present invention, and this feature brings about excellent advantageous effect in the steel of the present invention.
Hereinafter, a description will be made as to the reasons of numerical limitations of contents of the chemical composition in the steel of the invention.
It is necessary to decrease the C content as much as possible because C content undesirably impairs the formability. In the steel of the invention, it is intended to improve the formability by an efficient use of oxides. Therefore, the impairment of formability by C is serious particularly in the steel of the invention. For this reason, the C content should be selected to be not greater than 0.0050 wt%. When the decarburization can be conducted without substantial difficulty, it is desirable to maintain the C content not greater than 0.0020% because, by so doing, the C ageing can be prevented without necessitating addition of Nb. In view of present-day steel-making techniques, the lower limit of the C content is about 0.0010 wt%.
Mn combines with oxygen to form oxides which are finely dispersed to form a structure giving excellent formability. Mn is also effective in preventing hot embrittlement by forming MnS upon reaction with S contained by the steel as an impurity. An Mn content exceeding 0.5 wt% should, however, be avoided because such a large Mn content causes a large degradation of elongation. For obtaining excellent formability, it is preferred to keep the S content not greater than 0.005 wt% while maintaining the Mn content not greater than 0.25 wt%. The lower limit for the Mn content depends on the S content.
P is an element which is necessary in the production of a high-strength cold-rolled steel sheet having a tensile strength of between 35 and 40 Kgf/mm² (1 Kgf/mm² equals 9.81 N/mm²). Although P may theoretically be omitted, that is not in practice possible and by adding P it is possible to strengthen the steel without substantially degrading the formability. However, addition of P in excess of 0.1 wt% should be avoided because P makes the steel brittle and degrades the spot-welding characteristics.
For producing a mild cold-rolled steel, the P content should be selected to be not greater than 0.020 wt%. The P content is preferably maintained not greater than 0.01 wt% particularly when there is a demand for high formability and minimization of degradation of impact strength due to high oxygen content.
N is present as an impurity and tends to degrade the quality of the steel through N ageing. Ideally, the N content should be zero, but that is not in practice possible. The N content should, however, be limited to not greater than 0.0050 wt%, because, when N is fixed by means of B or V, a large amount of B or V is required if the N content is large and that is disadvantageous not only from an economical viewpoint but also from the viewpoint of formability, because an excessive amount of B nitrides or V nitrides seriously impairs the formability. Thus, N also is basically an impedimental element and the N content is preferably maintained not greater than 0.0025 wt% when there is a demand for a higher improvement in the formability.
O is an element which reacts with Mn and other elements to form oxides, generally during the initial manufacture of the steel slab. These oxides promote recrystallisation during the recrystallisation annealing process following a cold-rolling, thereby effectively accelerating the growth of crystalline orientation for a high formability. To this end, the amount of oxides should be at least 0.016 wt%, calculated on the weight of oxygen. However, an 0 content exceeding 0.035 wt% undesirably increases the amount of large-size oxide inclusions, which impairs the formability. The formation of such large-size oxide inclusions becomes marked as the 0 content is increased beyond 0.030 wt%. Therefore, when there is a specific demand for a high formability, the amount of oxides is preferably maintained not greater than 0.030 wt%, calculated on the weight of oxygen.
To avoid any problem concerning the ageing degradation due to the presence of C and N in the state of solid solution, it is advisable to add one, two or more of the elements selected from a group consisting of B, Nb and V. Among these elements, B is effective in fixing the N in the state of solid solution. B content not greater than 0.0001 % does not produce any appreciable effect in fixing N, whereas a B content exceeding 0.005 wt% causes defects such as cracking in slab. Nb is added for fixing C in the state of solid solution. For this purpose, the Nb content is selected preferably to range between 0.003 wt% and 0.080 wt%. Addition of Nb by an amount less than 0.003 wt% does not produce any appreciable effect, while Nb content exceeding 0.080 wt% is disadvantageous not only because of a saturation in the effect thereof but also from the view point of economy. It is advisable to add 0.005 wt% to 0.1 wt% of V in order to fix C and N. Any V content less than 0.005 wt% does not produce any appreciable effect, while a V content exceeding 0.1 wt% degrades the formability due to precipitation of V.
The contents of the constituents of chemical composition in the steel of the invention are limited for the reasons as described above. In the steel of the invention having the C and O contents as specified above, no bubbles are formed in the steel in molten state. In order to make the full use of the advantage of the steel composition of the invention, therefore, it is necessary to form the slabs by a continuous casting which ensures a good appearance of the slab surface and a high yield. The degassing of the molten steel may be made by any one of conventional methods such as RH method, DH method and so forth. The slab may be heated and hot-rolled in ordinary way. Alternatively, the warm slab may be heated or the hot slab may be hot-rolled directly. In order to save energy and, hence, to achieve a greater economy, the preheating of the steel prior to the hot-rolling is made at a relatively low temperature of 1150°C or lower, although the invention does not exclude the use of higher preheating temperatures. Preferably, the rolling finishing temperature is selected to be higher than Ar₃ transformation temperature. However, no substantial degradation of the quality is caused after the annealing following a cold rolling, even if the finishing rolling temperature is slightly below the above-mentioned transformation point. Referring now to the temperature at which the steel is coiled up after the hot rolling, it is one of the characteristic feature of the invention that the coiling-up temperature is limited neither to high temperature nor to low temperature. Namely, according to the invention, the steel may be coiled up at ordinary temperature ranging between 500 and 700°C. Namely, when the coiling up of the steel is made at a temperature higher than the above-mentioned temperature range, the pickling is impeded due to a too large scale thickness. On the other hand, the coiling up of the steel at lower temperature inevitably necessitates the reduction in the speed of the hot-rolling to permit a sufficient cooling before the coiling up, with the result that the production efficiency in the hot-rolling step is undesirably lowered. These problems, however, are avoided in the invention because the coiling up of the steel after the hot rolling can be conducted at an ordinary temperature range.
The steel in accordance with the invention after the coiling is subjected to a pickling, cold-rolling and then to an annealing. The roll reduction ratio in the cold rolling may be 50 to 85 wt% which is of a usual value. The annealing after the cold rolling may be conducted either by box annealing or continuous annealing. In order to maximize the advantage of the steel of the invention, however, it is necessary to conduct the annealing at a low temperature. More specifically, the box annealing is conducted at 550 to 680°C, while the continuous annealing is conducted preferably at a temperature range of between 600 and 770°C.
In the conventional continuous annealing of low-C steels, it is a common measure to effect a treatment called "overaging treatment" in which the steel is held at a temperature of between 300 and 500°C during or after the cooling following the annealing, thereby to permit a precipitation of oversaturated C. According to the invention, however, it is not necessary to conduct the overaging treatment, so that the cost of the continuous annealing equipment can be decreased advantageously. This constitutes one of the advantages brought about by the invention.
In the box annealing of cold-rolled low C-Al killed steel, it is also a common measure to use low-AI steel for commercial quality continuous cost steel. The low-AI steel, however, tends to cause the problem of an unusual grain growth to form a so-called "orange peel" on the outer portion of the coiled steel which portion is subjected to a high temperature during annealing. However, even in the box annealing the steel of the invention does not suffer from the unusual grain growth because of both the existence of oxide capable of controlling the grain growth and no existence of AIN impeding secondary recrystallization. It is, therefore, expected that the steel of the invention can find a use also as the material for continuous cast steel in place of conventional AI-killed steel having small C and AI contents.
The steel of the invention can be formed also into a steel strip or cut steel sheet through a temper rolling after the cold rolling and annealing. The band steel or the cut steel sheet may be electrically plated to become plated steel. The steel of the invention may be also passed through a continuous zinc dip-plating line after the cold rolling to become a zinc dip-plated steel sheet. Since in the steel of the present invention the content of AI is substantially zero because of relatively high content of oxygen, there is also such advantage that the steel is usable as a material for carburization and hardening (case-hardening).
The invention will be more fully understood from the following description of embodiments.

Embodiment 1

(Steel treated by continuous annealing)

Steels having chemical compositions as shown in Table 1 were molten and poured from a 300-tonne oxygen top-blowing converter. More specifically, the steels A to H and the steel J after the melting in the converter were subjected to an RH vacuum degassing for removing C and O. Then, AI was added to these steels in the molten state to adjust the oxygen contents as shown in Table 1. Then, suitable amounts of B, Nb, V and Ti were added to some of these steels to obtain a steel of a predetermined composition. These steels were then continuously cast into slabs of 200 mm thick and 1200 mm wide and were hot rolled to a final thickness of 4 mm under the conditions of preheating temperature of 1100°C, finishing rolling temperature of 910°C and coiling temperature of 620°C. After a pickling, the steels were cold rolled into sheets of 0.8 mm thick and 1200 mm wide. The steels I and K are a conventional low C-Al killed steel and a conventional capped steel, respectively. These steels were cast into the form of ingots which were then cut and rolled into slabs. Numerous blow holes were found in the surface of the slab of the steel H, while the slabs of other steels showed smooth surfaces of good appearance. The slab of the steel I was then hot-rolled into the final thickness of 4 mm and width of 1200 mm under the conditions of preheating temperature of 1200°C, finishing temperature of 890°C and coiling temperature of 750°C. On the other hand, the slab of the steel K was hot-rolled into the final thickness of 3 mm and width of 1200 mm under the condition of a preheating temperature of 1200°C, finishing temperature of 890°C and coiling temperature of 700°C. The hot rolled slabs were then cold-rolled down to a thickness of 0.8 mm after a pickling. The cold-rolled steel slabs were then subjected to a continuous annealing conducted under the conditions shown in Table 2 followed by a temper rolling at an elongation of 1.0%, and were then subjected to a test. Test pieces specified as JIS (Japanese Industrial Standard) Z2201, No. 5 test piece were used as the test pieces for tensile test.
Sample Nos. 1 to 10 in Table 2 were prepared from the steels in accordance with the invention, among which the sample Nos. 4 and 5 are high-strength cold-rolled steel having an ultimate tensile strength in the order of 35 Kgf/mm². Sample Nos. 15, 17 and 18 are steel sheets produced from conventional material by conventional process. Namely, steel sheets of sample Nos. 15,17 and 18 were prepared, respectively, from a low C-Al killed steel Ti-IF steel and a capped steel. Steel sheets of sample Nos. 13, 14, 16 and 19 were cold-rolled by the method of the invention from conventional steel materials. Sample Nos. 11 and 12 are comparison steels having compositions which fall out of the composition range limited in accordance with the invention.
The steel sheets of sample Nos. 1 and 2 were made from steels in which C and N existing in solid-solution are not fixed. These steels exhibited, after a temper rolling following an annealing at 750°C, a yield strength of less than 20 Kgf/mm², an elongation not smaller than 45% and a Lankford value r of about 1.35. Thus, these steel sheets of sample Nos. 1 and 2 exhibited higher mildness and formability than the conventional low C-Al killed steel (No. 14) and capped steel (No. 18) annealed at the same temperature. As in the case of the steel sheet of sample No. 3, the steel of the invention was able to exhibit higher mildness and formability than conventional steels even when the annealing temperature was lowered to 650°C. The steel sheet of sample No. 4 also exhibited an excellent ductility and value and can be ranked among the high-strength cold-rolled steel having an ultimate tensile strength in the order of 35 Kgf/mm². As will be understood from a comparison between the steel sheets of sample Nos. 1 and 2, no substantial difference was caused in the mechanical test values of the cold-rolled steel sheet by the employment or omission of overaging treatment in the continuous annealing. Namely, the steel sheet of sample No. 1 exhibited a yield strength of 19.4 Kgf/mm^z in the state immediately after the temper rolling and, after a 30-day aging at 38°C, exhibited a yield strength of 23.8 Kgf/mm², i.e. an increase of 4.4 Kgf/mm². On the other hand, in the steel sheet of sample No. 2 which had not been subjected to overaging in the continuous annealing, the difference of the yield strength between the state immediately after the continuous annealing and the state after a 38°C 30-day aging was about 4.9 Kgf/mm^2. Thus, according to the invention, whether the overaging was conducted in the continuous annealing did not materially affect the mechanical property of the cold-rolled steel sheet. In contrast to the above, in the case of conventional low C-Al killed steels of sample Nos. 13 and 14, the mechanical properties of the cold-rolled steel sheets were largely affected by the overaging treatment in the annealing step. For instance, the steel sheet of sample No. 13 subjected to an overaging treatment exhibited an increase of yield strength by 3.2 Kgf/mm² as a result of a 38°C 30-day aging after the temper rolling, while the steel sheet of sample No. 14 which had not been subjected to overaging exhibited an increase of yield strength by 6.5 Kgf/mm² as a result of the 38°C 30-day aging after the temper rolling. Thus, in the conventional steel material, the formability was largely affected by the omission of overaging. The same applied also to the elongation value.
Steel sheets of sample Nos. 6, 7 and 8 in which either C or N were fixed by Nb, B or the like exhibited still higher mildness and formability than the steel sheets of sample Nos. 1 to 5. Particularly, the amounts of change in the mechanical properties caused by the 38°C 30-days aging in the steel sheets of sample Nos. 6, 7 and 8 were smaller than that in the steel sheets of sample Nos. 1 to 5. The steels with fixed C or N after the 38°C 30-days aging exhibited still higher mildness, i.e., formability, than steels having unfixed C or N. In this regard, the steels having fixed C or N well compared with the conventional low C-AI killed steel treated by a conventional hot annealing.
The steel sheets of sample Nos. 9 and 10 in which both of C and N were fixed exhibited extremely high mildness and high r value, and caused substantially no degradation due to aging. Thus, the steels of sample Nos. 9 and 10 had mechanical properties well comparing with those of high temperature-annealed Ti-IF steel which is shown as sample No. 17.
The steel sheets of sample Nos. 11 and 12 were made of steels having oxygen contents falling out of the range specified by the invention. These steels showed considerably inferior mechanical properties to those of the steels of the invention annealed at the same temperature. In addition, the steel sheet of sample No. 12 had rough surfaces due to blow holes in the slab.

Embodiment 2

(Steel treated by box annealing)

Out of the steel slabs used in the embodiment 1, steel slabs A, C, E, G, H, I, J and K were employed in the embodiment 2. Steel slabs A, C, E, G, Hand J were hot-rolled and cold-rolled under the same conditions as those in the embodiment 1. The steel slab I was hot-rolled to a thickness of 4 mm under the conditions of a preheating temperature of 1270°C, finishing temperature of 890°C and a coiling-up temperature of 550°C, while the steel slab K was hot-rolled to 3 mm thick under the conditions of preheating temperature of 1270°C, finishing temperature of 890°C and coiling-up temperature of 620°C. The hot-rolled slabs were then cold-rolled down to a thickness of 0.8 mm.
The conditions for annealing and the mechanical properties observed through a test are shown in Table 3. The annealing was conducted in the state of tight coil. A temper rolling was conducted at an elongation of 1% after a cooling.
As will be seen from this Table, the steel sheets of sample Nos. 20, 21 and 22 produced from the steels of the invention exhibited, despite the low annealing temperature of 660°C and short annealing time of 6 hours, higher mildness, elongation and value than the comparison steels of sample Nos. 23, 24, 25, 27 and 29 annealed under the same annealing condition.
As will be seen from the above, the steel of the invention does not need any restriction concerning the coiling temperature and overaging in the continuous annealing. In addition, according to the invention, the superior mildness, large elongation and high value of the steel are obtainable through a low-temperature annealing which considerably decreases the energy consumption.
For the case-hardening use, to which the cold-rolled steel sheet is frequently subjected, the steel of the invention is superior also in this regard because it has an excellent carburization and hardening characteristics, that is, even if AI is put into this steel in the molten state, the AI floats in the form of alumina into a slag and is removed, with the result there is substantially no AI content in the product steel. When the steel of the invention is applied to such use, it is not preferred to add Nb.

Claims

1. Steel comprising, by weight:

from 0.001 to 0.005% of C, not more than 0.5% of Mn,

from 0 to 0.1% of P,

from 0 to 0.005% of N,

from 0.016 to 0.035% of O,

from 0 to 0.005% of B,

from 0 to 0.08 of Nb,

from 0 to 0.1% of V,

the balance being Fe and impurities, the O and at least part of the Mn being in the form of oxides dispersed substantially uniformly throughout the steel.

2. Steel as claimed in claim 1, wherein the content of P is not greater than 0.01% by weight.

3. Steel as claimed in claim 1 or claim 2, wherein the content of N is not greater than 0.0025% by weight.

4. Steel as claimed in any one of claims 1 to 3, wherein the content of 0 is from 0.016 to 0.03% by weight.

5. Steel as claimed in any one of claims 1 to 4, wherein the content of B is from 0.0001 to 0.005% by weight.

6. Steel as claimed in any one of claims 1 to 5, wherein the content of Nb is from 0.003 to 0.08% by weight.

7. Steel as claimed in any one of claims 1 to 6, wherein the content of V is from 0.005 to 0.1 % by weight.

8. Steel as claimed in any one of claims 1 to 7, wherein the content of C is not greater than 0.002% by weight.

9. Steel as claimed in any one of claims 1 to 8, wherein S is present as an impurity in an amount not exceeding 0.005% by weight.

10. Steel as claimed in any one of claims 1 to 9, which is in the form of a cold-rolled steel sheet.

11. A method of producing a steel sheet comprising the steps of:

(i) forming by continuous casting a steel slab comprising:

from 0.001 to 0.005% of C, not more than 0.5% of Mn,

from 0 to 0.1% of P,

from 0 to 0.005% of N,

from 0.016 to 0.035% of 0,

from 0 to 0.005% of B,

from 0 to 0.08% of Nb,

from 0 to 0.1% of V,

the balance being Fe and impurities,

(ii) hot-rolling the resulting steel slab to form a steel sheet;

(iii) pickling the hot-rolled steel sheet;

(iv) cold rolling the pickled steel sheet; and

(v) annealing the cold-rolled steel sheet.

12. A method as claimed in claim 11, wherein the annealing is effected by continuous annealing or by box annealing.

13. A method as claimed in claim 11, wherein the annealing is effected by continuous annealing at a temperature within the range of from 600 to 770°C or by box annealing at a temperature within the range of from 550 to 680°C.

14. A method as claimed in any one of claims 11 to 13, wherein the hot-rolling is effected at a temperature not exceeding 1150°C.

15. A method as claimed in any one of claims 11 to 14, wherein the content of P in the steel slab is not greater than 0.01%.

16. A method as claimed in any one of claims 11 to 15, wherein the content of N in the steel slab is not greater than 0.0025%.

17. A method as claimed in any one of claims 11 to 16, wherein the content of O in the steel slab is from 0.016 to 0.03%.

18. A method as claimed in any one of claims 11 to 17, wherein the content of B in the steel slab is from 0.0001 to 0.005%.

19. A method as claimed in any one of claims 11 to 18, wherein the content of Nb in the steel slab is from 0.003 to 0.08%.

20. A method as claimed in any one of claims 11 to 19, wherein the content of V in the steel slab is from 0.005 to 0.1%.

21. A method as claimed in any one of claims 11 to 20, wherein the content of C in the steel slab is not greater than 0.002% by weight.

22. A method as claimed in any one of claims 11 to 21, wherein S is present in the steel slab as an impurity in an amount not exceeding 0.005% by weight.