JPH02205625A - Production of hot rolled steel plate with low yield ratio for construction use excellent in fire resistance - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel plate with low yield ratio excellent in fire resistance by subjecting a slab of a dead-soft carbon steel containing specific amounts of Cu, Ti, Nb, etc., to hot rolling under specific conditions. CONSTITUTION:A slab of a dead-soft carbon steel which has a composition containing, by weight, <=180ppm C, 0.1-0.5% Mn, 0.01-0.1% P, <=0.1% Al, 0.6-2.0% Cu, and 0.008-0.2% Ti and/or 0.008-0.10% Nb or further containing B by 1-30ppm and 1 or Ni so that the ratio of Ni to Cu is 0.2-1.0 is hot-rolled at <=1150 deg.C, and hot rolling is finished at >=800 deg.C, and then, the resulting hot rolled steel plate is cooled at >=3 deg.C/sec average cooling rate and wound up at <=600 deg.C. By this method, the steel plate for construction use relatively reduced in yield ratio as to have a yield point strength at 600 deg.C >=0.6 times the yield point strength at ordinary temp. and excellent in fire resistance can be produced.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a low-yield ratio hot rolled steel with excellent fire resistance for use in lightweight sections for construction, U-columns, and other various buildings in the fields of civil engineering and marine structures. Pertains to the method of manufacturing steel plates.

(Conventional technology) Hot-rolled steel plates for construction include rolled steel plates for shell structures (JIS G
3101), rolled steel plates for welded structures (JIS G 31
06) Weather-resistant hot-rolled steel plates for welded structures (JIS G
3114), highly weather resistant rolled steel plate (JIS G312
5), (hereinafter referred to as well-known steel plate), etc. are widely used.

Fire resistance of buildings is important, and a variety of measures are being taken for everything from large buildings to general residential buildings. However, at present, when fishing on a boat, fire prevention measures are taken by using a fireproof coating as described in Japanese Patent Laid-Open No. 63-47451. As a result, construction costs are rising and the usable space of buildings is becoming smaller.

Recently, fire-resistant design has been reviewed, and in 1986, the new Fire-resistant Design Law for Buildings was enacted, and the previous regulation of allowable steel material temperature (350°C) in the event of a fire was removed, and the high-temperature strength of steel plates was changed. It is now possible to determine the capacity of fire-resistant coatings based on the load actually applied to the building, and it has become possible to use steel plates without coatings in cases where the high-temperature strength of the steel plates is ensured.

However, the invention relating to hot-rolled steel sheets with guaranteed high-temperature strength for fire resistance is limited to the invention of Japanese Patent Application No. 143,740/1983 (referred to as the prior invention).

The prior invention is an invention having the same object as the present invention, but this prior invention mainly relates to thick plates.

However, lightweight steel frames and U-shaped columns in buildings are often made of hot-rolled steel strips or steel plates. Hot-rolled steel strips or steel plates are manufactured by hot strip mills, but in this process, the finishing temperature cannot be unnecessarily lowered or the strip-threading speed must be extremely reduced due to continuous hot rolling.

Furthermore, in order to produce in large quantities, there is a quenching process for the runout table and a winding process. For these reasons, imparting cold tensile properties and high temperature strength properties is significantly different from the plate manufacturing process.

In addition, there is an invention for which a patent application was filed on January 12, 1989 (patent application number not yet received) regarding a technology that applies this thick plate technology to a hot strip mill, but this invention is also a Mo
It is based on additives, and although it is not the case with high-alloy steel, it cannot be said that the problem has been completely solved in terms of economy.

(Problems to be Solved by the Invention) With conventional steels, it is difficult to ensure high-temperature strength due to grain growth, coarsening of precipitates, dissolution of carbides, etc. In addition, high-alloy heat-resistant metals, including iron-based metals, exist, but they are economically disadvantageous as they are consumed in large quantities for construction purposes.

The purpose of the present invention is to provide a non-Mo material which has excellent high-temperature properties, can reduce or omit the need for fire-resistant coating, can be easily processed when forming U-columns, etc., has excellent base material corrosion resistance, and is highly economical.
An object of the present invention is to provide a method for manufacturing a low yield ratio steel plate or steel strip having a steel composition close to that of ordinary steel in the same type using a hot strip mill.

(Means for Solving the Problems) As a result of research on the strength of steel plates in the event of a fire, the present inventors have discovered that steel plates with an economical composition system have a yield point strength of 60% or more of the normal temperature strength at 600"C. Furthermore, as a result of studying the strength of steel plates during earthquakes, it was found that low yield ratio steel plates with a yield ratio (yield point strength/tensile strength) of 80% or less at room temperature have excellent earthquake resistance. This has also been made clear and has been achieved.

The gist of the present invention is as follows: (1) Weight ratio: C≦180ppm SMn: 0.1
~0.5%, P:0.01~0.1%, AI≦0.1%
, Cu: 0.6 to 2.0%, and Ti: 0
．． 008-0.2% or/and Nb: 0.008
~0.10%, with the remainder being Fe and unavoidable impurities, immediately after forming into a slab, or 1150%
600° C. or less, followed by hot rolling, finishing the rolling at a temperature of 800° C. or more, cooling at an average cooling rate of 3° C./s or more, and then winding at 600° C. or less.
The yield point strength at °C is 0 of the yield point strength at room temperature.
．． 6 or more, a method for producing a hot-rolled steel sheet with a low yield ratio for construction with excellent fire resistance, and (2) C≦180 ppm in weight ratio, Mn: 0.
1-0.5%, P: 0.01-0.1%,
8l≦0.1%, Cu: 0.6-2.
In addition to 0%, Ti: 0.008 to 0.2% or/and Nb: 0.008 to 0.10%, and further B: 1 to 30 pp111 or/and Ni to Ni/C
Immediately after making a slab of steel consisting of u of 0.2 to 1.0 and the remainder Fe and unavoidable impurities, or 11
60 characterized by hot rolling after heating to 50°C or lower, finishing rolling at a temperature of 800°C or higher, cooling at an average cooling rate of 3°C/s or higher, and then winding at 600°C or lower.
A method for producing a low-yield-ratio hot-rolled steel sheet for construction with excellent fire resistance, the yield point strength at 0''C being 0.6 or more of the yield point strength at room temperature.

That is, the gist of the present invention is to add Ti or/to ultra-low carbon steel.
In addition, the steel containing Nb and a large amount of Cu is hot-rolled under specific hot-rolling conditions that prevent hot cracking due to Cu and sufficiently impart predetermined properties. In some cases, strength may be adjusted with P, and Ni may be added to strengthen the steel against hot cracking.

The reasons for limiting the numerical values of the constituent elements of the present invention will be described below.

C shall be 180 ppm or less. In the present invention, C is
From the viewpoint of improving formability at room temperature, especially stretch flangeability, and corrosion resistance, so-called IF steel (Intersti
tial Free 5teel). Therefore, the C is set to be extremely low, and Ti and/or Nb are added. If there is a large amount of C, a large amount of Ti and/or Nb is required for IF, which not only impairs economic efficiency but also deteriorates workability and toughness due to these carbides. In this sense, it is preferable that C be 50 ppm or less.

Tt: 0.008-0.2% or/and Nb:
0.008-0.10% forms these carbides and C
It is necessary to fix the If it is less than the lower limit, sufficient IF will not be obtained, and moldability and corrosion resistance will not be achieved. Moreover, when the upper limit is exceeded, the atomic equivalent amount of C is exceeded, which not only impairs economic efficiency but also deteriorates formability due to solid solution of Ti and Nb. Preferably, 0°05≧12/4B (Ti(X)) +12/93
T within the range shown by (Nb(ri) ≧(C(X))
i or/and add Nb.

Next, Mn is added in a range of 0.1 to 0.5%. Below the lower limit, FeS embrittlement tends to occur (and in this component system, solid solution strengthening of Mn cannot be expected to be large, so do not use unnecessarily high (
Doing so hurts economic efficiency.

P is an element with large solid solution strengthening and provides corrosion resistance through interaction with Cu. Therefore, it is necessary to add 0.01%. In order to more stably impart corrosion resistance, it is preferable to add 0.2% or more. On the other hand, 0.
Addition of more than 1% increases embrittlement, so the upper limit is 0.1
%.

M is necessary as a deoxidizing agent, but addition of more than 0.1% increases inclusions and deteriorates the ductility and toughness of the steel.

The lower limit is usually about 0.01%, but if it can be further lowered by deoxidizing with Ti included in the present invention, it may be about 0.003%.

Next, Cu is an extremely important element in the present invention. That is, it secures high-temperature strength, which is the main objective of the present invention, and also has room-temperature strength and room-temperature yield ratio, and also has excellent corrosion resistance due to interaction with P. Although the strengthening mechanism is not clear, it is thought that the room temperature strength is due to solid solution strengthening or some cluster strengthening of Cu, and the high temperature strength is due to cluster strengthening or precipitation strengthening of Cu. If less than 0.6% of Cu is added, the degree of supersaturation of Cu will be insufficient and strength will not be imparted.

This is especially noticeable at high temperatures. Furthermore, if more than 2.0% is added, these effects tend to be saturated, while hot cracking becomes unavoidable, so the upper limit of the addition value is set at 2.0%.

In the present invention, B and/or Ni are further added depending on the case. B is a grain boundary strengthening element, and IF as in the present invention
Steel also has a small amount of solid solute carbon, which is a grain boundary strengthening element, and B is added to compensate for this.

If it is less than 1 ppm, there is no effect, and if it exceeds 30 ppm, the effect is saturated. Further, Ni is added in order to completely eliminate hot cracking. The amount of Ni added is determined depending on the amount of Cu added, which causes hot cracking. When Ni/Cu is less than 0.2, no effect of reducing hot cracking due to Ni is observed;
When i/Cu exceeds 1.0, Ni is an expensive metal, which impairs economic efficiency, which is one of the major objectives of the present invention.

Of course, the effects of the present invention are not brought about by specifying only the above-mentioned component system. That is, hot rolling conditions are also extremely important requirements. In particular, in steels to which a large amount of Cu is added, such as those of the present invention, hot embrittlement called so-called Cu embrittlement occurs, making it impossible to perform sufficient hot rolling. In the present invention, hot rolling conditions are specified as follows.

Hot rolling is done immediately after slab casting (so-called CC-direct rolling)
When heating or heating, the temperature should be 1150°C or lower. If this condition is not met, hot cracking is unavoidable. CC-
When direct rolling is performed, there is no problem in keeping it warm or heating the edges to some extent.

The lower limit of heating temperature is 1000, which can be achieved with current continuous hot rolling equipment.
It is about ℃. Under these conditions, solutionization of Cu is sufficient.

The hot rolling end temperature is 800°C or higher. If rolling is performed at a temperature lower than this temperature, Cu will be strain-induced precipitated by rolling, and it will be useless to ensure high-temperature strength later. That is, one aspect of the hot rolling conditions of the present invention is to keep Cu dissolved in iron in a supersaturated state.

Cooling and winding conditions at the runout table are also determined from this point of view. The former is an average cooling rate of 3° C./s or more. When slowly cooling at a value lower than this cooling rate, C
u precipitates, making it impossible to ensure room temperature strength, room temperature yield ratio, and high temperature strength. Preferably, the rate is 10° C./s or higher in order to better maintain Cu in a solid solution state. Although the upper limit of the cooling rate depends on the plate thickness, the effects of the present invention can be maintained even if the cooling rate is increased to 50°C/s, which can be achieved with the current most powerful equipment.

Further, the winding temperature is 600°C or less. If this temperature is exceeded, Cu will precipitate over aging during slow cooling after winding, making it impossible to obtain the necessary tensile properties. In order to obtain more stable characteristics in consideration of variations in winding, the winding temperature is preferably 520° C. or lower. Further, in order to obtain sufficient supersaturated Cu over the entire length of the hot-rolled coil and obtain sufficient room temperature strength, yield ratio, high temperature strength, etc., it is more preferable that the coiling temperature is 450° C. or lower. There is no particular lower limit to the coiling temperature, and it may be room temperature; however, if the coiling temperature is too low, the IF steel may not be sufficient, and excessive solid solution carbon may remain, impairing the ductility of the steel. From this point of view, the lower limit of the winding temperature is preferably 300°C. The fact that the cooling conditions at the runout table and the winding temperature conditions are wide in this way is also a feature of the present invention.

This is one of the characteristics of fire-resistant steel.

The steel of the present invention is usually made into steel in a converter, and because of its extremely low carbon content, it is usually decarburized by a vacuum degassing method. It is then usually continuously cast into slabs. After being made into a hot rolled coil in a hot strip mill, it can be used as it is, or it can be pickled or/
It is also passed through a skin bath or leveler and made into a coiled material.

Alternatively, it can also be used as a cutting board material.

Next, embodiments of the present invention will be described.

After steel having the components shown in Table 1 was tapped in a converter, it was made into a slab by continuous casting, and then hot rolled immediately or after heating. Hot rolling conditions are shown in Table 2. After pickling the produced hot-rolled coil, a sample for a test was taken by unwinding it on a skin pass line. Tensile test at room temperature is JIS Z 220
The test was conducted in accordance with JIS Z 2241 using a No. 15 test piece. In the high-temperature tensile test, a high-temperature extensometer was attached to the test piece, the temperature was raised to 600°C at a rate of 150°C/hour, the test piece was stretched at this temperature, and the yield point was measured.

In addition, the surface condition caused by so-called Cu curling of the plate was observed over the entire length of the coil when it was unwound using a skin pass line, and the scores were given as follows.

◎: Good (-same as shell material), ○: Slight (product passed for shipment),
∆: Slightly observed (shipping is not possible depending on the destination), ×: Severe occurrence (defective product).

In addition, the workability of the material was evaluated by a hole expansion test, which shows stretch flangeability, which is most required for such hot rolled steel sheets.

The method is to use a conical punch with a 30° apex angle to widen a 20 mm diameter hole until a crack occurs, then divide the hole diameter at that point by the original hole diameter. It is evaluated based on the spread rate.

Evaluation of the base metal corrosion resistance of the material was performed as follows.

After degreasing, the pickled board was exposed outdoors in a coastal area for one year to remove any rust that had formed during this period, and its weight was measured, and the weight was divided by the weight of the original board to determine the corrosion loss rate.

Table 2 shows the hot rolling conditions and the property values of the obtained steel.

The steel according to the present invention has no problem with the degree of Cu sagging at a practical level, and its room temperature tensile strength has a tensile strength of 402 MPa class or 490 MPa class, but the yield point strength exceeds the standard value of 245 MPa or more and 284 MPa or more, respectively. satisfactorily meet the yield ratio (yield point strength/tensile strength)
is 0.8 or less, which is excellent. Further, the hole expansion ratio is also a good value of about 2.5 or more. Furthermore, 600℃
The yield point strength at high temperature is also sufficiently high, and the ratio to the yield point strength at room temperature fully satisfies the value of 0.6 or more, and is generally a high value of 0.7 or more.

Furthermore, the steel according to the present invention has good corrosion resistance and fully exhibits the effects of the present invention. In contrast, wires not in accordance with the invention lack at least one of these characteristic values.

(Effects of the Invention) Building fire prevention measures are a social issue, and high-performance housing is also required for general housing, and fire prevention measures are an important item. According to the present invention, under these circumstances, it is possible to produce iron-based materials with excellent high-temperature resistance properties using hot strip mills that can be supplied in large quantities, and with a composition similar to that of ordinary steel. Therefore, it can greatly contribute to solving the above social issues.

Claims (2)

[Claims] (1) Weight ratio: C≦180ppm, Mn: 0.1-0
．． 5%, P: 0.01-0.1%, Al≦0.1%, C
In addition to u: 0.6-2.0%, Ti: 0.008-0
．． After forming a slab of steel containing 2% or/and Nb: 0.008 to 0.10% and the remainder Fe and unavoidable impurities, hot rolling is performed immediately or after heating to 1150°C or lower, and then hot rolling is performed to 800°C or higher. After finishing the rolling at a temperature of
A method for producing a low-yield ratio hot-rolled steel sheet for architectural use having excellent fire resistance and having a yield point strength at 600° C. that is 0.6 or more of the yield point strength at room temperature, the method comprising: winding the steel sheet as follows: (2) Weight ratio: C≦180ppm, Mn: 0.1-0
．． 5%, P: 0.01-0.1%, Al≦0.1%, C
In addition to u: 0.6-2.0%, Ti: 0.008-0
．． 2% or/and Nb: 0.008 to 0.10%, and further B: 1 to 30 ppm or/and Ni
/Cu, 0.2 to 1.0, and the balance is Fe and unavoidable impurities. After forming the steel into a slab, hot rolling is performed immediately or after heating to 1150°C or lower, and the rolling is finished at a temperature of 800°C or higher. Fire resistance, characterized in that the yield point strength at 600 °C is 0.6 or more of the yield point strength at room temperature, characterized by cooling at an average cooling rate of 3 °C / s or more, and then winding at 600 °C or less. A method for producing low yield ratio hot rolled steel sheets for architectural use.