JPS61117246A - Hot rolled steel sheet for enamel having high strength after baking and superior fish scale resistance and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel sheet for enamel having high strength after baking and superior fish scale resistance by adding specified amounts of C, Mn, N, Ti and B to Fe and forming a steel contg. a specified amount of sol. C. CONSTITUTION:The composition of a steel billet is composed of, by weight, 0.005-0.02% C, 0.05-1% Mn, 0.003-0.015% N, 0.05-0.3% Ti, 0.001-0.01% B and the balance Fe with inevitable impurities. The billet is hot rolled and coiled at 200-550 deg.C to manufacture a hot rolled steel sheet contg. >=0.001wt% sol. C.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention has high strength and does not cause nail skipping defects even after firing, has high strength after firing, and has excellent nail skipping resistance. The present invention relates to a hot-rolled steel plate for use in enamel and a method for manufacturing the same.

[Prior art and its problems]

Steel plates for enameled products are manufactured by fusing a glassy glaze onto the surface of a steel plate at high temperatures.In general, cold-rolled steel plates are often used as the base plate due to their enamel properties. Since steel plates have a limited thickness,
For example, when manufacturing thick enamel products such as enamel silos and hot water tanks, hot-rolled steel sheets must be used as the base plate.

However, hot-rolled steel sheets have a problem in which so-called nail-skipping defects occur frequently, in which the enamel layer peels off in a half-moon shape after sintering.

Therefore, various studies have been conducted to improve the nail-skipping resistance of hot-rolled steel sheets as described above.
Japanese Patent Publication No. 8-1170 and Japanese Patent Publication No. 59-6894 disclose a method of improving the nail chipping resistance by incorporating T1 into the component composition. However, the T1-containing steel plate produced by the above-mentioned conventional method suffers from the problem of being unable to secure sufficient strength after enameling because its strength is significantly reduced during enameling firing at a high temperature.

Therefore, as a method to solve the above problem, for example, Japanese Patent Publication No. 58-36666 discloses a method for sintering by adding T1 to the component composition and increasing the content of C, Si, Mn, etc. Increasing the strength of the hot-rolled steel sheet used as the original sheet.

A method for ensuring strength after firing is disclosed. However, enameled products are manufactured through a process of forming the original plate into a predetermined shape, applying a glaze, and then discarding acid. If it is increased, not only will cold workability deteriorate, making molding difficult, but also, for example, when punching an original plate, problems such as shortening of the life of the punching die will occur.

The present inventors have solved the above-mentioned problems and prevented the occurrence of nail skipping that occurs after rolling firing. The strength after wax firing is high.

We conducted extensive research to develop hot-rolled steel sheets for dipping and their manufacturing method.

The reason why the strength of conventional T1-containing steel sheets decreases after sintering is as follows. That is, in conventional T1-containing steel sheets, all C is T at the stage of hot-rolled steel sheets as original sheets.
It is finely precipitated in the steel as iC.

The finer the Ti, the more it strengthens the steel.
Since C precipitates grow during enameling firing performed at high temperatures and become relatively large TiC precipitates, the above-mentioned T
The strengthening ability of iC precipitates is lost. Since the further grown TiC precipitates do not have the ability to inhibit the growth of ferrite grains, the ferrite grains become coarse.

These causes a decrease in the strength of the steel plate after firing.

Therefore, from the above-mentioned viewpoint, the inventors of the present invention have further investigated the means to prevent the strength reduction after rolling firing. If a part of TiC is made to exist in a solid solution state, and C is combined with T1 during high-temperature firing to precipitate as fine TiC, even after firing, it will remain in the steel. It has been found that the presence of fine TiC precipitates makes it possible to maintain high strength of the steel sheet after sintering.

[Summary of the invention]

This invention was made based on the above knowledge, and the composition of the hot rolled steel sheet is C: 0.005 to 0.02 wt.
, %, Mn: 0.05-1. owt, %, N
: 0.003~O,Ol5wt,%,Ti:o
, o5 ~ 0.3 wt, section, B: 000 engineering ~ 0.010
wt, %, balance: Consists of iron and inevitable impurities,
It is characterized in that it contains solid solution carbon of 0.0% or more.

[Structure of the invention]

The hot rolled steel sheet of this invention has the above-mentioned composition,
At the stage of the hot rolled steel sheet as the original sheet before enamel firing, a part of the C contained in the steel exists in a solid solution state, so when this is enamel fired at a high temperature, the above solid solution state C combines with T1 and precipitates as fine TiC. Therefore, high strength can be maintained even after firing.

In addition, fine TiC, which is present in large quantities even after firing, acts as a trap site for hydrogen, which causes nail skipping, so that the nail skipping resistance can also be improved. Furthermore, as mentioned above, some C in the steel is dissolved in the hot rolled steel sheet as the original sheet, so
As a result of reducing the amount of precipitated fine TiC, which has a large strengthening ability, the original plate can be made softer, and cold workability is improved.

Next, the reason why the chemical composition range of the hot rolled steel sheet for enameling according to the present invention is limited as described above will be described.

(1)C: C has the function of ensuring a predetermined strength.

However, if the C content is less than 5vt% of O,OO, the amount of precipitated TiC is small, so the strength after tX firing cannot be increased. On the other hand, the C content is o,
If it exceeds 0.02 vt,%, TiC tends to precipitate, so even if the coiling temperature of the copper strip after hot rolling is adjusted, all of the C will precipitate as TiC and exist as solid solution carbon in the original plate stage. Therefore, it is not possible to increase the strength of the alloy after firing, and furthermore, it is necessary to increase the content of T1, resulting in a problem of high cost. Therefore, the C content is 0.02 w from O,OO5
t, should be within the range of %.

(2) Mn: Mn has the effect of ensuring a predetermined strength and preventing hot embrittlement caused by S, which inevitably exists in steel. However, if the content of the ratio is o, o s wi,
If it is less than %, the desired effect cannot be obtained from the above-mentioned action;
On the other hand, 1. If it exceeds Ovt,%, the strength of the hot-rolled steel sheet used as the original sheet becomes too high, and the problem arises that it adversely affects the enameling properties. Therefore, the content of Mn is 01
05 to 1. o wi, should be within the range of %.

(3) N: N has the effect of maintaining good nail-skipping resistance. However, if the N content is less than 3 wt.% of O,OO, the desired effects described above cannot be obtained; O1
5wt. %, it becomes necessary to increase the content of T1, resulting in a problem of increased costs.

Therefore, the N content ranges from o, o o 3 to O, Ol
It should be within the range of 5 wt.%.

(4, N T1: T1 has the effect of improving nail-skipping resistance. However, if the content of T1 is less than 0.05 wt.%, the desired effect cannot be obtained from the above-mentioned effect. On the other hand, 0.3
wt, %' (If T7 is exceeded, TiC tends to precipitate,
Even if the coiling temperature of the copper strip after hot rolling is adjusted, all of the C precipitates as TiC, and as a result, it no longer exists as solid solution carbon, making it impossible to increase the strength after rolling firing. occurs.

Therefore, the Ti content is 0.05 to 0.3 wt,
It should be within the range of %.

(5) B: B has the effect of improving nail-skipping resistance. However, if the content of B is less than 0.001 wt.%, the desired effect cannot be obtained from the above-mentioned effect; on the other hand, 0.010
If it exceeds wt.%, weldability deteriorates, causing problems that adversely affect product quality. Therefore, the content of B is 0.
It should be within the range of 0.001 to 0.010 wt.%.

(6) Solute carbon: The amount of solute carbon is the most important point in this invention, and the strength after rolling firing is influenced by the amount of solute carbon present in the hot rolled steel sheet as the original sheet. Ru. That is, if the amount of solid solute carbon is less than 1 wt.% of O,Q O, sufficient strength cannot be obtained after firing. Therefore, the content of solid solute carbon is O, O
Ol vt, should be at least %.

In the method for manufacturing a hot-rolled steel sheet for enameling according to the present invention, the coiling temperature of the hot-rolled steel strip should be within the range of 200 to 550°C. That is, if the coiling temperature of the hot-rolled copper strip is less than 200° C., the product shape will be impaired due to cooling distortion, and a hot-rolled steel sheet with a good shape will not be obtained. On the other hand, when the winding temperature exceeds 550°C, TiC precipitation increases, making it impossible to secure solute carbon in an amount of 0% or more in the hot rolled steel sheet as the original sheet. .

[Embodiments of the invention]

Next, this invention will be further explained in detail with reference to Examples.

Example 1°C: 0.0 1 6 wt, %, Mn:
0.45 wt, %, N: 0. OO57wt
, %, Ti: 0.1 7 2wt, %,
B: A steel billet with a chemical composition of 0.0037 wt.% is hot-rolled at a finish rolling temperature of 900°C, coiled at various temperatures, and air-cooled to produce a hot-rolled steel plate with a thickness of 32 mm. did.

The drawing shows the relationship between the amount of solid solute carbon (measured by internal friction) of the hot-rolled steel sheet produced in this manner and the amount of strength reduction when the hot-rolled steel sheet is fired at 850° C. for 5 minutes. In the drawing, the horizontal axis is the amount of solid solute carbon, and the vertical axis is the amount of strength reduction ΔTS
[(TS of hot-rolled steel sheet) - (T after firing the hot-rolled steel sheet)
S)].

From the drawing, the strength reduction amount ΔTS decreases as the amount of solid solute carbon increases, and l Oppm (Q, OO1wt, %)
It can be seen that the value becomes extremely small when the value exceeds this value. In this way, in a mountain range in which the amount of strength reduction upon firing decreases as the amount of solid solute carbon increases, although the matrix softens during firing, C and T1, which are present in a solid solution state in the original sheet stage before firing, are It is thought that this is because TiC sometimes precipitates as TiC, and this TiC precipitate increases the strength.

Example 2 A steel slab having the chemical composition shown in Table 1 was hot-rolled at a finish rolling temperature of 900°C, coiled at a suitable temperature, and after cooling was pickled to remove scale. A hot-rolled steel plate of 32 pm was manufactured.

Table 1 Table 2 shows the amount of solid solute carbon in the hot-rolled steel sheets produced in this way, the mechanical test values before and after firing, and the amount of snapping. The amount of nail skipping can be determined by degreasing a steel plate, applying a commercially available glaze on both sides, drying, and firing at a temperature of 850°C for 5 minutes in an atmosphere with a dew point of 30°C. It is expressed by the number of nail jumps that occur per unit area on the steel plate.

In Tables 1 and 2, A to D are examples of the present invention, and E to E are comparative examples. In Comparative Examples E and F, the content of C was low and out of the range of the present invention, so the amount of nail skipping was large and the strength after firing was low. Comparative example G is N
Since the content of is so low that it is outside the scope of this invention, the amount of nail skipping is high.

Comparative Example H has a component composition within the range of the present invention, but the coiling temperature is high outside the range of the present invention, so there is no solid solution carbon, and therefore the strength decreases during firing, resulting in poor firing. The strength afterward is not sufficient and some nail skipping occurs. In the comparative example, the content of B is so low that it is out of the range of the present invention, so the amount of nail skipping is large, and the strength after firing is low because there is no solid solution carbon.

On the other hand, Examples A to D of the present invention all had high strength after firing and the amount of nail skipping was zero.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain a hot-rolled steel plate for enameling that has high strength after firing and excellent nail-skipping resistance. In addition, in this invention, the steel billet that is the material of the hot rolled steel plate may be a steel billet manufactured by continuous casting, or a steel billet manufactured by normal ingot making and blooming rolling. A direct hot rolling method may be used, in which a high-temperature steel billet produced by continuous casting is directly hot rolled, or a method may be used in which a steel billet cooled to room temperature is reheated and then hot rolled.

[Brief explanation of the drawing]

The drawing is a graph showing the relationship between the amount of solid solute carbon in a hot rolled steel sheet and the amount of strength reduction after firing of the hot rolled steel sheet.

Claims (2)

[Claims] (1) C: 0.005-0.02wt. %, Mn: 0.
05-1.0wt. %, N: 0.003-0.015wt. %, Ti: 0.05-0.3wt. %, B: 0.001-0.010wt. %, remainder: consisting of iron and inevitable impurities, with a solid solution carbon content of 0.001 wt. % or more, a hot rolled steel sheet for enameling which has high strength after firing and excellent nail chipping resistance. (2) C: 0.005-0.02wt. %, Mn: 0.
05-1.0wt. %, N: 0.003-0.015wt. %, Ti: 0.05-0.3wt. %, B: 0.001-0.010wt. %, balance: A steel billet having a composition consisting of iron and unavoidable impurities is hot rolled, and after the hot rolling is completed, the solid solution carbon is reduced to 0.001 wt. % or more, a method for producing a hot rolled steel sheet for enameling which has high strength after firing and excellent nail chipping resistance.