JPH0737647B2 - Method for producing low yield ratio H-section steel excellent in fire resistance and toughness - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a low yield ratio H-section steel having excellent fire resistance and toughness, which is used for various structures in the fields of construction and civil engineering.

[Conventional technology]

As is well known, as a building construction material in the fields of construction and civil engineering, rolled steel for general structure (JIS G3101), rolled steel for welded structure (JIS G3106), weather resistant hot rolled steel for welded structure (JIS G3114), etc. Is widely used.

The well-known steel material is obtained by dephosphorizing and desiliconizing the hot metal obtained by a normal blast furnace and then refining it in a converter, making it into a steel piece in a continuous casting or agglomeration process, and then hot plastic working it to obtain desired properties. Be commercialized as equipped.

By the way, in the case of using the well-known steel materials in buildings, offices, houses, etc., which are closely related to daily life, among various buildings, it is obliged to apply sufficient fireproof coating in order to ensure safety in case of fire. However, building laws stipulate that the temperature of steel products should not exceed 350 ℃ in case of fire. In other words, when using the above-mentioned known steel materials for buildings, 35
Since the yield strength will be 60 to 70% at room temperature at about 0 ° C and may cause the building to collapse, the fireproof coating is carefully applied so that the steel material will not lose its loading capacity due to thermal damage during a fire. Since it is used in this way, the cost of fireproof coating is higher than the cost of steel material, and the cost is rising significantly.

In recent years, as the number of buildings has risen, the fireproof design method was reviewed due to the improvement of design technology and its high reliability, and the "new fireproof design method" was legalized in 1987. Along with this, steel materials and manufacturing methods that can secure high-temperature strength at 600 ° C of two-thirds or more of room-temperature strength stipulated by the Building Standards Act have come to be proposed. For example, the present applicant previously proposed in Japanese Patent Application No. 63-143740 a method of performing composite addition of Nb and Mo to raise the rolling end temperature to a high temperature of 800 ° C. or higher.

However, the steel material targeted by this manufacturing method is a steel plate, and it is difficult to apply this manufacturing method to shaped steel such as H-shaped steel in order to secure appropriate room temperature / high temperature strength, high toughness, and low yield ratio. Is. That is, when a steel containing a certain amount of Mo and Nb is applied to the production of, for example, H-section steel, the temperature difference between the flange and the web in the H-section steel, the fillet portion which is the root of the flange, and the web and the fillet portion is: At the end of the intermediate rolling, the temperature is close to 200 ℃, so when the flange is finished in the temperature range of 800 to 1000 ℃, the room temperature strength of each part is 10kg.
There is a variation of about f / mm ² . Furthermore, the absolute value of the intermediate rolling end temperature of the web must be low, so
In the case of steel with Nb added, Nb mainly works for precipitation strengthening,
As a result, the room temperature strength and the yield ratio become too high. Since the temperature of the fillet portion is higher than that of other portions at the end of the intermediate rolling, the structure becomes coarse grains, the hardenability is increased, and the bainite structure fraction is increased. Therefore, the strength at room temperature sharply increases and the toughness significantly decreases.

[Problems to be Solved by the Invention]

An object of the present invention is to provide a method for producing a low yield ratio H-section steel having excellent fire resistance and toughness in the as-rolled state.

[Means for Solving the Problems]

The present invention overcomes the above-mentioned problems and achieves the object, and the gist thereof is, in terms of weight ratio, C: 0.03 to 0.20.
%, Si: 0.55% or less, Mn: 0.2 to 1.6%, Cr: 0.2 to 0.60%, M
o: 0.4-0.7%, Ti: 0.005-0.03%, N: 0.001-0.010%,
Al: 0.1% or less, the balance consisting of Fe and unavoidable impurities is reheated in a temperature range of 1100 to 1350 ° C, and then rolling is started, and only the outer surface of the flange is water-cooled during the rolling process. In the method for producing a low yield ratio H-section steel having excellent fire resistance and toughness, the rolling is completed within the range of 720 to 850 ° C. to refine the microstructure.

[Action]

Now, the feature of the present invention is that after reheating a steel slab having a composition composition in which Mo and Cr are added to Si-Mn steel at a high temperature, only the outer surface of the flange is water-cooled during rolling, and the temperature of each of the flange and the web is reduced. Hot rolling is completed at a relatively low temperature of 720 to 850 ° C., and the H-section steel manufactured by the present invention has appropriate room temperature strength, low yield ratio and high toughness in each cross section, and It has the characteristic that the yield strength at 600 ℃ is more than two-thirds of the room temperature strength specified by the Building Standards Law.

Next, the characteristic component elements related to the H-section steel produced according to the present invention and the addition amounts thereof will be described.

Mo increases the high temperature strength by solid solution strengthening, but according to the findings so far (Japanese Patent Application No. 63-143740), in order to secure the high temperature strength at 600 ° C, Nb and Mo are added in combination and The main point is to form carbonitrides.
In steel materials such as shaped steels where the temperature of the steel material at the end of the intermediate rolling greatly differs from site to site, the web that is hot-rolled only at a relatively low temperature has room temperature strength due to precipitation strengthening of Nb,
This leads to a large increase in the yield ratio, and the performance specified for rolled steel for welded structures (JIS G3106) and the performance as a building steel cannot be satisfied at room temperature. Therefore, in order to secure the strength at high temperature without adding Nb, it is necessary to improve the hardenability of the steel material by adding Mo and Cr in combination, and to secure the bainite structure fraction in the room temperature structure to some extent. . Therefore, it is necessary to set the lower limit of the amount of addition of Mo to 0.4%, and the rolled steel for welded structure (JI
In order to satisfy the performance specified in S G3106), the upper limit is
It should be 0.7%.

Cr is an element that enhances the strength of the base metal, but if it exceeds 0.6%, the performance specified in the rolled steel for welded structures (JIS G3106) cannot be satisfied at room temperature, and the weldability also decreases. Was set to 0.6%. Further, if less than 0.2%, the effect of improving strength cannot be expected, so the lower limit was made 0.2%.

Further, since Nb is not added, it is possible to manufacture a web whose hot rolling is completed only at a relatively low temperature, with an appropriate room temperature strength and a low yield ratio.

The reasons for limiting the other basic components will be described below.

The lower limit of the amount of C, 0.03%, is the minimum amount for ensuring the strength of the base material. However, if the amount of C is too large, the toughness and weldability of the base material deteriorate, so the upper limit was made 0.20%.

Si is an element contained in the deoxidized upper steel, and the toughness decreases as the Si content increases, so the upper limit was made 0.55%.

Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.2%. However, if the amount of Mn increases, the hardenability increases and the target room temperature strength of the base metal cannot be obtained, so the upper limit was made 1.6%.

Addition of 0.005 to 0.03% of Ti forms Ti oxides and carbonitrides to improve the weldability, but if it is less than 0.005%, there is no effect, and if it exceeds 0.03%, the weldability is adversely affected. .

Al is generally an element contained in deoxidized upper steel, but since Si is also deoxidized, the lower limit of its content is not specified. However, if the amount of Al increases, the cleanliness of steel deteriorates,
Since the toughness decreases, the upper limit was made 0.1%.

N is generally contained in steel as an unavoidable impurity and forms a carbonitride with Ti in a content range of 0.001 to 0.010% to improve weldability. However, if it is less than 0.001%, it is not effective, and if it exceeds 0.010%, the weldability is adversely affected.

The steel piece to which the present invention is applied contains P and S as unavoidable impurities. However, since P and S have little influence on the high temperature strength, their contents are not particularly limited.

Next, the manufacturing method of the present invention will be described.

In the case of H-section steel, when water is not cooled during rolling and steel is rolled by normal rolling, the temperature of the steel at the end of the intermediate rolling greatly differs from site to site, causing a difference in bainite structure fraction and Variations occur and the performance specified for rolled steel for welded structures (JIS G3106) cannot be obtained at each part. Therefore, in the present invention, as compared with the web portion, only the flange outer surface where the intermediate rolling is finished at a relatively high temperature is water-cooled during rolling, and the temperature of each portion of the flange and the web is 720 to 850.
By ending the rolling within the temperature range of ° C, the variation in the bainite composition fraction is reduced. Below the transformation point of less than 720 ° C, the ferrite structure undergoes processing, so the yield ratio increases. In addition, at a temperature higher than 850 ° C, the structure becomes coarse grains and the variation of the bainite structure fraction becomes large, so the rolling end temperature was set in the range of 720 to 850 ° C in the present invention.

In order to increase the high temperature strength by adding Mo to the steel slab, which is a feature of the present invention, it is necessary to sufficiently solutionize Mo during reheating, so the lower limit of the reheating temperature was set to 1100 ° C. Further, if the reheating temperature is too high, the steel billet is significantly oxidized or deformed, so the upper limit was made 1350 ° C.

〔Example〕

The H-section steels with the steel components shown in Table 1 are manufactured by the well-known converter, continuous casting, and rolling process, and the room temperature strength, toughness, and 600
The high temperature strength at ℃ was investigated.

The chemical components of Examples of the present invention are shown in A to C of Table 1, and the chemical components of Comparative Examples are shown in D to I. Next, Table 2 shows the material characteristics of the example of the present invention and the comparative example.

All of the examples of the present invention from A to C in Table 2 have good room temperature and high temperature characteristics. On the other hand, in Comparative Examples D to I, since water cooling is not performed during rolling, the finishing temperature of the flange portion exceeds 850 ° C., and as a result, the room temperature strength is the performance and toughness characteristics specified in the rolled steel for welded structure (JIS G3106). Not satisfied. Further, when Nb was added, the yield ratio of the web portion exceeded 80%. In addition, the measurement site in Table 2 is 1/2 of the flange width B, as shown in FIG.
1/4 part and 1/4 part of web height H.

[Effect of the invention] The H-section steel manufactured according to the present invention is an H-section steel having a yield strength at 600 ° C of two-thirds or more of the yield strength stipulated by the Building Standards Law, for example, 40 kg class H-section steel. If there is 16 kgf / mm ² or more, and 50 kg class H-section steel has 22 kgf / mm ² or more, and also has excellent toughness characteristics, the present invention has an industrially very useful yield yield ratio H. A method for manufacturing a shaped steel is provided.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the measurement site of mechanical properties in H-section steel.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kazufumi Hirose Inventor, 1 No. 1 Tsukiko Hachiman-cho, Sakai City, Osaka Prefecture Nippon Steel Co., Ltd. Sakai Works (56) Reference JP-A-2-209421 (JP, A) Kai 63-176425 (JP, A) JP 63-286517 (JP, A)

Claims (1)

[Claims] 1. By weight ratio, C: 0.03 to 0.20%, Si: 0.55% or less, Mn: 0.2 to 1.6%, Cr: 0.2 to 0.60%, Mo: 0.4 to 0.7%,
Ti: 0.005 to 0.03%, N: 0.001 to 0.010%, Al: 0.1% or less, the balance is a steel slab consisting of Fe and unavoidable impurities.
Rolling is started after reheating in the temperature range of 00 to 1350 ° C, only the outer surface of the flange is water-cooled in the middle of rolling, and the rolling finishes when the temperature of each part of the flange and the web is 720 to 850 ° C A method for producing a low yield ratio H-section steel excellent in fire resistance and toughness, characterized in that: