JPH02205630A - Production of hot rolled high strength steel plate with low yield ratio for construction use excellent in fire resistance - Google Patents

Abstract

PURPOSE:To produce a hot rolled high strength steel plate with low yield ratio excellent in tire resistance by adding specific amounts of Cu and Ni to a low C-Mn steel and subjecting the above steel to hot rolling, to cooling, and to winding under respectively specified conditions. CONSTITUTION:A slab of a steel which has a composition containing, by weight, 0.02-0.1% C, <=0.5% Si, 0.3-1.5% Mn, <=0.05% P, <=0.1% Al, and 0.6-2.0% Cu and further containing Ni so that the ratio of Ni to Cu is regulated to 0.2-1.0 is cast and then hot-rolled without delay, or, the above steel slab is heated up to <=1150 deg.C and hot-rolled so as to be formed into a metal plate. After finish hot rolling is completed at >=750 deg.C, the above metal plate is cooled at 3-40 deg.C/sec average cooling rate and then wound up at 300-600 deg.C. By this method, the hot rolled high strength steel plate with low yield ratio for construction use having 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 obtained.

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 c 31)
06) Weather-resistant hot-rolled steel plates for welded structures (JIS G
3114), highly weather resistant rolled steel plate (JIS G 31
25), (hereinafter referred to as well-known steel plate), etc. are widely used.

The fire resistance of buildings is important, and a variety of measures are being taken for buildings ranging from large buildings to buildings for general residents. Particularly in general housing, three-story houses are becoming more popular due to rising land prices, and in this case, a certain degree of fire resistance is required. 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 ability of fire-resistant coatings based on the load actually applied to the building, and it has become possible to use steel plates without coating in cases where the high-temperature strength of the steel plate material can be ensured. .

However, Japanese Patent Application No. 63-143470 is the only invention relating to hot-rolled steel sheets with guaranteed high-temperature strength for fire resistance.

The invention of Japanese Patent Application No. 63-143470 has the same object as the present invention, but the 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.

Furthermore, a patent application was filed for a patent application on January 12, 1989 for a technology that applied this thick plate technology to a hot strip mill, but this invention is also based on the addition of Mo, and cannot be applied to high-alloy steel. However, the problem cannot be said to have been completely resolved economically.

The present inventors focused on the superiority of Cu-added steel to ensure high-temperature strength, and filed a patent application on February 4, 1989 (referred to as the prior invention) for an invention related to a technology applied to these uses.

The present invention further improves the properties of the Cu-added steel according to the prior invention.

(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 excellent high-temperature properties, reduce or omit the need for fireproof coating, have high strength at room temperature and low yield ratio, do not require extremely low carbonization as in the prior invention, and are non-Mo based. The purpose of the present invention is to provide a method for manufacturing an economically efficient low yield ratio steel plate or steel strip with a steel composition close to that of steel 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 found that a steel plate with an economical composition system whose yield point strength at 600°C is 60% or more of the normal temperature strength. This led to the invention of a manufacturing method. Furthermore, as a result of studying the strength of steel plates during earthquakes, we found that the yield ratio (yield point strength/tensile strength) at room temperature
It has also been revealed that low yield ratio steel plates with a yield ratio of 80% or less have excellent seismic resistance, and they have also achieved this goal.

The gist of the present invention is as follows: (1) Weight ratio: C: 0.02 to 0.1%, Si≦0
．． 5%, Mn: 0.3-1.5%, P≦0.05%, M
≦0.1%, Cu: 0.6 to 2.0%, and the balance is Fe and unavoidable impurities. After making the steel into a slab, hot rolling is performed immediately or after heating to 1150 ° C. or less, and 750 ° C. After finishing the rolling at a temperature of 300 to 60°C or higher and cooling at an average cooling rate of 3 to 40°C/s,
A method for producing a high-strength hot-rolled steel sheet with a low yield ratio for construction, which has excellent fire resistance and has a yield point strength at 600° C. that is 0.6 or more of the yield point strength at room temperature, characterized by winding at 0° C.
and (2) weight ratio, C: 0.02-0.1%, Si≦0
．． 5%, Mn: 0.3-1.5%, P≦0.05%, M
≦0.1%, Cu: 0.6-2.0%, Ni
After forming a slab of steel containing 0.2 to 1.0 of Ni/Cu and the balance being Fe and unavoidable impurities, hot rolling is performed immediately or after heating to 1150°C or less to 750°C.
Finish rolling at a temperature above and then average cooling rate of 3 to 4
A construction material with excellent fire resistance, characterized in that it is cooled at 0°C/S and then rolled up at 300 to 600°C, and whose yield point strength at 600°C is 0.6 or more of the yield point strength at room temperature. A method for producing a high-strength hot-rolled steel sheet with a high yield ratio.

That is, the gist of the present invention is to use a steel with a low C-Mn basic composition and a large amount of Cu added, and to use a steel with a specific composition that prevents hot cracking due to Cu and sufficiently imparts predetermined properties. Hot rolling is carried out under the following hot rolling conditions, and Ni is 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 is 0.02 to 0.1%. In the present invention, C is an element responsible for strength and toughness. If it is less than 0.02%, it is difficult to impart the necessary strength and toughness. In addition, since intense vacuum degassing is required in steel manufacturing, economic efficiency is impaired. On the other hand, if it exceeds 0.1%, a hardened structure such as martensite tends to occur and the toughness deteriorates.

Next, Mn is added in a range of 0.3 to 1.5%. Below the lower limit, it is difficult to obtain a structure with sufficient strength and toughness.

On the other hand, if it exceeds 1.5%, a hardened structure tends to occur and the toughness deteriorates.

Si is added within 0.5% for the purpose of reinforcing strength.

Exceeding this will deteriorate the toughness and weldability of the steel.

In the case of this component system, P is 0.05% because it deteriorates toughness.
Add within If toughness is strongly required, it is preferable not to add it and to keep the impurity level to within 0.02%. On the other hand, in the case of this component system, it has been found that adding 0.03% or more of P improves corrosion resistance. In this sense, P is set to 0.03 to 0.05%.

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 o, oi%, but if it can be further lowered by deoxidizing with Ti, it may be about 0.003%.

Next, Cu is an extremely important element in the present invention.

That is, it is possible to ensure high-temperature strength, which is the main objective of the present invention, and also to have room-temperature strength and room-temperature yield ratio, and also to have excellent corrosion resistance through interaction with P. Although the strengthening mechanism is not clear, room-temperature strength is due to Cu solid solution strengthening or slight cluster strengthening, and high-temperature strength is due to Cu solid solution strengthening or slight cluster strengthening.
This is thought to be due to uO cluster strengthening or precipitation strengthening.

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 the addition exceeds 2.0%, these effects tend to be saturated, but hot cracking becomes unavoidable, so the upper limit of the addition value is 2.
．． Set to 0%.

In the present invention, Ni is further added depending on the case.

Ni is added to completely eliminate hot cracking.

The amount of Ni added is determined depending on the amount of Cu added, which causes hot cracking. If Ni/Cu is less than 0.2, the effect of reducing hot cracking due to Ni is not recognized, and if Ni/Cu is more than 1.0, Ni is an expensive metal, which is one of the main objectives of the present invention. It hurts economic efficiency.

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.

When heating, the temperature is preferably 1100° C. or lower in order to sufficiently remove Cu. The lower limit of the heating temperature is about 1000° C., which can be achieved with current continuous hot rolling equipment.

Under these conditions, solutionization of Cu is sufficient.

The hot rolling end temperature is 750'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. From this point of view, the hot rolling end temperature is preferably 800°C or higher.

Cooling and winding conditions at the runout table are also determined from the viewpoint of maintaining supersaturation. The former has an average cooling rate of 3~
The temperature is set at 40°C/s. If it is slowly cooled at a cooling rate lower than this, Cu will precipitate during cooling, making it impossible to ensure room temperature strength, room temperature yield ratio, and high temperature strength. On the other hand, the cooling rate is 40
When the temperature exceeds °C/s, a hardened structure is formed and the toughness is deteriorated. In order to stably obtain characteristics, it is preferably 6 to b. The winding temperature is 300 to 600°C.

If the temperature exceeds 600°C, Cu will precipitate due to overaging during slow cooling after winding, making it impossible to obtain the necessary tensile properties. In order to obtain more stable properties considering the variation in winding, it is preferable to set the winding temperature to 520°C or lower.Furthermore, sufficient supersaturated Cu can be obtained over the entire length of the hot-rolled coil to ensure sufficient room-temperature strength and yield. To obtain good ratio, high temperature strength, etc., the coiling temperature should be 450℃.
It is more preferable to set it as below. On the other hand, the winding temperature is 30
If the temperature is below 0°C, a quenched structure tends to occur, which deteriorates the toughness of the steel. The fact that the cooling conditions at the runout table and the winding temperature conditions are wide in this way also makes it possible for the present invention to
This is one of the characteristics of Mo-based fireproof steel.

The steel of the present invention is usually made into steel using a converter, and may be subjected to secondary refining using vacuum degassing or the like. It is then usually continuously cast into slabs. After being made into a hot-rolled coil in a hot strip mill, it is made into a raw material in the form of a coil, either as is, or after being pickled and/or passed through a skin pass or leveler.

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

Next, embodiments of the present invention will be described.

Steel having the components shown in Table 1 was tapped in a converter, then continuously cast into a slab, 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 specimen, the temperature was raised to 600'C at a rate of 150°C/hour, the specimen 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 general company), ○: Minor (shipped product),
∆: Slightly observed (shipping is not possible depending on the destination), ×: Severe occurrence (defective product).

In addition, the toughness of the material is JIS Z 2202 Sharp
The test was carried out using a Notchi test piece according to Z 2242.

However, since the plate thickness was 10 mm or less, a sub-size test piece closest to the original thickness was used.

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 has a tensile strength at room temperature of 402 MPa class, and a yield point strength that satisfies the standard value of 245 MPa or more, and has a yield ratio It has an excellent (yield point strength/tensile strength) of 0.8 or less. Furthermore, 6
The high temperature yield point strength at 00° C. is also sufficiently high, and the ratio to the yield point strength at room temperature satisfies the value of 0.6 or more, and is generally a high value of 0.7 or more.

(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.

Under these circumstances, the present invention enables the production of iron-based materials with excellent high-temperature resistance properties using a Horastrip mill that can supply large quantities, and with a composition similar to that of ordinary steel. It is believed that this will greatly contribute to solving social issues.

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Claims (2)

[Claims] (1) Weight ratio: C: 0.02-0.1%, Si≦0.
5%, Mn: 0.3-1.5%, P≦0.05%, M≦
After forming a slab of steel containing 0.1% Cu, 0.6 to 2.0% Cu, and the balance Fe and unavoidable impurities, it is hot-rolled immediately or after heating to 1150°C or less, and
Finish rolling at a temperature of 0°C or higher, and then reduce the average cooling rate to 3
A construction product with excellent fire resistance, characterized in that it is cooled at ~40°C/s and then rolled up at 300 to 600°C, and whose yield point strength at 600°C is 0.6 or more of the yield point strength at room temperature. A method for producing low yield ratio, high strength hot rolled steel sheets. (2) Weight ratio: C: 0.02-0.1%, Si≦0.
5%, Mn: 0.3-1.5%, P≦0.05%, Al
≦0.1%, Cu: 0.6-2.0%, Ni
After forming a slab of steel containing i/Cu of 0.2 to 1.0 and the balance Fe and unavoidable impurities, hot rolling is carried out immediately or after heating to 1150°C or lower, and then rolling is carried out at a temperature of 750°C or higher. After that, the average cooling rate is 3~40℃/
A low yield ratio and high yield ratio for construction with excellent 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 s and then winding at 300 to 600 °C A method for producing high-strength hot-rolled steel sheets.