JP2000192142A - Production of low yield ratio fire resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably mass-produce steel at a low cost without deteriorating the productivity by subjecting a slab having a specified compsn. to hot rolling under specified conditions, thereafter executing accelerated cooling and waiting under specified conditions and successively executing further accelerated cooling. SOLUTION: This steel contains, by weight, 0.04 to 0.20% C, 0.05 to 1.0% Si, 0.5 to 2.0% Mn, 0.2 to 0.8% Mo, 0.002 to 0.1% Al and <=0.020% N. The slab having this compsn. is heated to >=100 deg.C and is thereafter hot-rolled at >=Ar3 at a draft of >=50%. Next, it is subjected to accelerated cooling to the temp. range of Ar3-100 deg.C at >=2 deg.C/sec, and waiting is executed in this temp. range for a waiting time(t) (sec) satisfying the following inequality: the inequality: 1.3-0.006×ΔT<=logt<=log150, where ΔT( deg.C) denotes the temp. difference between Ar3 and the average temp. T( deg.C) of the steel at the time of the waiting. Then, the waited steel is subjected to accelerated cooling to the temp. range of 400 to 650 deg.C average temp. at >=2 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for producing a low yield ratio refractory steel material used for a steel structure such as a building or a bridge which is likely to be in a high temperature state for several hours due to a fire or the like. It is.

[0002]

2. Description of the Related Art In recent years, with the increase in height and size of buildings, members used for the same have been required to be thicker and have higher tensile strength, and high-strength steel plates having a tensile strength of 490 N / mm ² or more have been developed. It is becoming popular. In addition, today's high-rise buildings are subject to a critical state design method that emphasizes human safety, which is to absorb large amounts of seismic energy by plastic deformation of columns and beams to avoid large collapses when a huge earthquake is hit. Applied. Therefore, the column / beam member used in the limit state design method has a low yield ratio (YR) as a measure of high plastic deformability,
In other words, it is desired that the material has a low yield ratio, and it is said that a material having a lower yield ratio has better plastic deformability.

In order to reduce the yield ratio, a ferrite as a soft phase and a bainite as a hard phase are generally used, as represented by a method of performing an intermediate heat treatment of heating in a two-phase region between quenching and tempering. Alternatively, it is known that this is achieved by a ferrite + hard phase structure in which martensite is mixed. Conventional techniques for obtaining the ferrite + hard phase structure include the above-described quenching, two-phase quenching, and tempering treatments, and air cooling to a two-phase region of ferrite and austenite after hot rolling, followed by accelerated cooling. And the like.

[0004] On the other hand, with respect to fires in buildings, a review of fire-resistant design has made it possible to reduce fire-resistant coatings by using fire-resistant steel excellent in high-temperature strength.
Because of the effects of shortening the construction period, reducing the construction cost, expanding the effective area in the building, and the like, its application is becoming active.

[0005]

SUMMARY OF THE INVENTION Low yield ratio steels having excellent fire resistance are disclosed in Japanese Patent Application Laid-Open Nos. 3-130319 and 4-56721. However, since the cooling after the rolling is air-cooled, a low yield ratio cannot be obtained depending on the sheet thickness.

[0006] Further, JP-A-5-339633 and JP-A-5-339644 disclose, after hot rolling,
A method is disclosed in which direct quenching or reheating quenching is performed, followed by two-phase quenching and tempering. However, these require complicated heat treatment, which not only increases the production cost but also inevitably lowers productivity.

Japanese Patent Laid-Open Publication No. Hei 3-6322 discloses a method in which after hot rolling, air cooling is performed to a two-phase region of ferrite and austenite, followed by accelerated cooling. However, in this method, air cooling before the start of accelerated cooling has a long time, and productivity is greatly reduced.

The present invention solves the above-mentioned problems and provides a method for stably producing low-yield-ratio refractory steel for use in steel structures such as buildings and bridges at low cost and in large quantities without impairing productivity. The purpose is to develop.

[0009]

Means for Solving the Problems As a result of intensive studies from the above-mentioned viewpoints, the present inventors have found that, in the hot rolling step of a steel slab containing an appropriate amount of an element such as Mo, which increases the high-temperature strength, an appropriate amount is obtained. By applying the rolling conditions and the two-stage accelerated cooling, a steel material manufacturing technique that can achieve the above object was developed. The gist of the present invention is as follows.

The method for producing a low yield ratio refractory steel material according to the first aspect of the present invention is characterized in that, by weight%, C: 0.04 to 0.20%, Si:
0.05-1.0%, Mn: 0.5-2.0%, M
o: A steel slab containing 0.2 to 0.8%, Al: 0.002 to 0.1%, and N: 0.020% or less,
After heating to 0 ° C or higher, reduction rate 50% in a temperature range of Ar ₃ or higher
And performing hot rolling of the above, the hot rolled steel, the Ar ₃ temperature above, Ar ₃ -100 from Ar ₃
Cooling at a cooling rate of 2 ° C./sec or more to a temperature range of 2 ° C., and performing a standby time of t (seconds) satisfying the following expression (1) in the temperature range: 1.3-0. 006 × ΔT ≦ logt ≦ log150 (1) Here, ΔT (° C.): temperature difference between Ar ₃ and the average temperature T (° C.) of the steel material during standby (= Ar ₃₎ -T) t (second): standby time A step of accelerating and cooling the standby steel material to a temperature in the range of 400 to 650 ° C. at a cooling rate of 2 ° C./second or more. Have

[0011] The method for producing a low yield ratio refractory steel material according to claim 2 is the method according to claim 1, wherein
By weight%, Cu: 0.03 to 1.0%, Ni: 0.03
0.5%, Cr: 0.03 to 0.6%, characterized by containing one or more selected from the group.

The method for producing a refractory steel material having a low yield ratio according to a third aspect of the present invention is the method according to the first or second aspect, wherein the steel slab component is expressed by weight% and Nb: 0.005 to 0.05.
%, V: 0.01 to 0.1%, Ti: 0.003 to 0.
It is characterized by containing one or more selected from the group of 1%.

According to a fourth aspect of the present invention, there is provided a method for producing a low yield ratio refractory steel material according to the first or second aspect of the present invention, wherein the steel billet component is expressed by weight% and Ca: 0.0005 to 0.005.
5%, REM: one or more selected from the group of 0.001 to 0.02%.

According to a fifth aspect of the present invention, there is provided a method for producing a low yield ratio refractory steel material according to the first or second aspect of the present invention, wherein the steel slab component is expressed by weight% and Nb: 0.005 to 0.05.
%, V: 0.01 to 0.1%, Ti: 0.003 to 0.
One or more selected from the group of 1%, and C
a: 0.0005 to 0.005%, REM: 0.001
It is characterized by containing one or more selected from the group of ~ 0.02%.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have studied a technique for producing a low yield ratio refractory steel material without performing a long-time standby and heat treatment before accelerating and cooling a hot-rolled high-temperature steel material. . As a result, a refractory steel material having a tensile strength of 490 N / mm ² or more and a low yield ratio of 80% or less as-rolled by designing a chemical composition of a specific composition and appropriately controlling rolling conditions and accelerated cooling conditions. Could be obtained.

The components of the present invention and the reasons for limiting the components will be described below. The composition of the billet is as follows. In the present specification, hereinafter, all the component compositions of steel are% by weight.

(1) C C is added in an amount of 0.04% or more to secure the strength of the steel. However, if it is contained in a large amount exceeding 0.20%,
The toughness or weldability deteriorates. Therefore, the C content is in the range of 0.04 to 0.20%.

(2) Si Si needs to be added in an amount of 0.05% or more to deoxidize steel. However, if it exceeds 1.0%, the toughness and weldability of the heat affected zone of the steel material deteriorate. Therefore, the Si content is in the range of 0.05 to 1.0%.

(3) Mn Mn is required to be 0.5% or more to improve the strength and toughness of the steel material and to suppress the production of FeS. But 2.0%
The addition of a large amount exceeding the above causes an increase in the hardenability of the steel, a hardened phase is formed at the time of welding, and the crack susceptibility is increased. Therefore, the Mn content is in the range of 0.5 to 2.0%.

(4) Mo Mo is an element effective for increasing the strength of steel by improving hardenability, precipitation strengthening, etc., and is extremely effective especially for medium-high temperature strength. However, when the Mo content is 0.2
%, It is difficult to obtain the effect. on the other hand,
Even if it is added in excess of 0.8%, not only the effect corresponding to the addition cost is not seen, but also the weldability is deteriorated. Therefore, the Mo content is in the range of 0.2 to 0.8%.

(5) Al Al is an element necessary for deoxidizing steel. However, when contained in a large amount, the cleanliness of the steel is deteriorated. From the above viewpoint, the Al content is in the range of 0.002 to 0.1%.

(6) NN is an element contained in steel as an unavoidable impurity.
If contained in a large amount, the toughness of the base metal and the weld heat affected zone deteriorates. From the above viewpoint, the range is set to 0.020% or less and includes 0%.

In the present invention, the above components are used as basic components. On the other hand, Cu, Ni and Cr have a common effect of solid solution in ferrite and have an effect of improving the strength of steel. Cu, Ni
By adding one or more of Cr and Cr, the properties of the steel material of the present invention are further improved.

(7) Cu Cu is a very effective element for increasing strength and improving toughness. If the Cu content is less than 0.03%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 1.0%, precipitation hardening is remarkable, and cracks are easily generated on the surface of the steel material. Therefore, the Cu content is in the range of 0.03 to 1.0%.

(8) Ni Ni has the effect of improving the strength and toughness of the base material, but if the content is less than 0.03%, a sufficient effect cannot be obtained. On the other hand, the addition of Ni exceeding 0.5% leads to an increase in cost. Therefore, the Ni content is 0.03-0.
The range is 5%.

(9) Cr Cr is an element effective for improving hardenability. However, if the content is less than 0.03%, the effect is small, while
If it exceeds 0.6%, the weldability and the toughness of the heat affected zone are deteriorated. Therefore, the Cr content is in the range of 0.03 to 0.6%.

An action common to Nb, V, and Ti is a grain refinement action, and an action common to Ca and REM is shape control of inclusions, all of which have an effect of improving the toughness of steel. In the present invention, the properties of the steel material of the present invention are further improved by adding one or more of Nb, V, Ti, Ca and REM.

(10) Nb Nb is an element that is effective for increasing toughness and room-temperature strength and high-temperature strength by adding a small amount. However, if the content is less than 0.005%, the effect cannot be sufficiently exhibited. On the other hand, when Nb exceeds 0.05%, the toughness of the welded portion is deteriorated and the reduction of the yield ratio is hindered. Therefore, the Nb content is in the range of 0.005 to 0.05%.

(11) V V is an element effective in increasing the strength at normal temperature and high temperature. However, if the content is less than 0.01%, the effect cannot be sufficiently exhibited. On the other hand, if the content of V exceeds 0.1%, the toughness of the weld is deteriorated. Therefore, the V content is in the range of 0.01 to 0.1%.

(12) Ti Ti is an element effective for improving toughness and strength at room temperature and high temperature by adding a small amount of Ti. However, the content is less than 0.1.
If it is less than 003%, the effect cannot be sufficiently exhibited. On the other hand, if the Ti content exceeds 0.10%, the toughness of the weld is deteriorated. Therefore, the Ti content is 0.0
The range is 03 to 0.10%.

(13) Ca Ca is an element effective for improving mechanical properties and lamella tear resistance. However, its content is 0.000
If it is less than 5%, the effect cannot be sufficiently exhibited. On the other hand, if the content of Ca exceeds 0.005%, coarse inclusions are generated and the ductility is deteriorated. Therefore,
The Ca content is in the range of 0.0005 to 0.005%.

(14) REM REM, like Ca, is an element effective for improving mechanical properties and improving lamella tear resistance. However, if the content is less than 0.001%, the effect cannot be sufficiently exhibited. On the other hand, when the content of REM exceeds 0.02%, coarse inclusions are generated, and the ductility is deteriorated. Therefore, the REM content is in the range of 0.001 to 0.02%.

Next, the reasons for limiting the hot rolling condition and the cooling condition of the steel slab having the above-mentioned composition are as follows.

(15) Heating temperature of steel slab If the heating temperature of the steel slab is less than 1000 ° C., good hot workability cannot be obtained. Therefore, the heating temperature of the billet is 1000 ° C.
Above.

(16) Hot Rolling Completion Temperature If the hot rolling completion temperature is less than Ar ₃ , acoustic anisotropy occurs which adversely affects the measurement accuracy of the ultrasonic flaw detection test for steel materials.
Therefore, the hot rolling end temperature is set to a temperature equal to or higher than Ar ₃ .

[0036] (17) rolling reduction at a reduction ratio Ar ₃ or more at Ar ₃ or more is less than 50%, recrystallization tissue coarsened by heating is insufficient, particularly toughness is degraded. Therefore, the rolling reduction at Ar ₃ or more is set to 50% or more.

(18) Accelerated cooling start temperature The accelerated cooling start temperature is Ar_ThreeIf less than, before the start of accelerated cooling
Coarse ferrite is formed, and toughness is deteriorated.
Ar_ThreeCause the cooling wait time to drop below
As a result, productivity decreases. Therefore, accelerated cooling
The starting temperature is Ar _ThreeAbove.

(19) Accelerated Cooling Rate If the accelerated cooling rate is less than 2 ° C./sec, coarse ferrite is generated during accelerated cooling, and the toughness is deteriorated. Therefore, the accelerated cooling rate at this time is set to 2 ° C./sec or more.

(20) Temperature during Cooling Standby and Standby Time In order to produce a low-yield-ratio high-strength steel material, the present inventors have studied a mixed structure of ferrite as a soft phase and bainite as a hard phase. In order to find out the rolling and cooling conditions for obtaining the ferrite + bainite structure as described above, further tests and examinations were repeated.

The steel A having the composition shown in Table 1 falls within the range of the present invention.
Using the above-mentioned heating temperature of the slab, hot rolling end temperature,
Tests were conducted in which the rolling reduction at Ar ₃ or higher, the accelerated cooling start temperature, and the accelerated cooling rate were all varied under various conditions of cooling standby temperature and standby time under conditions that satisfy the range of the present invention. . The test level satisfies the tensile strength of 490 N / mm ² or more and the yield ratio of 80% or less, and furthermore, the standby time satisfies the above-mentioned tensile strength and yield ratio in a relatively short time so as not to impair the productivity. As described above, the area between the standby temperature and the standby time is determined.

Then, a part of austenite is transformed into a required amount of ferrite during standby, and the remaining austenite is transformed into bainite by accelerated cooling after the standby (second stage accelerated cooling) to obtain a ferrite + bainite structure. Achieve the above target value of steel properties.

FIG. 1 shows the results of a test conducted in accordance with the above policy. The figure shows the test results of the effects of the standby temperature and the standby time on the steel material properties. In the figure, the shaded area is an area where both the strength and the yield ratio achieve the target values.
This is a region having a value of 0 N / mm ² or more and a yield ratio of 80% or less. From this result, the present inventors have steel after the rolling, the Ar ₃ temperature above, Ar ₃ -100 from Ar ₃
Temperature range up ° C. to accelerate cooling at a cooling rate higher than 2 ° C. / sec, at a temperature range from the Ar ₃ to Ar ₃ -100 ° C., the standby time t (second), the following equation (1) 1.3 −0.006 × ΔT ≦ logt ≦ log150 ---------- By waiting for a time that satisfies (1), a tensile strength of 49
It has been found that a hot-rolled steel material having a yield ratio of not less than 0 N / mm ^{2 and not} more than 80% can be obtained. Here, ΔT (° C.):
Temperature difference (= Ar ₃ −T) between Ar ₃ and the average temperature T (° C.) of the steel material during standby, where ΔT ≦ 100 ° C.

The above results are explained as follows.

If the standby temperature T is higher than Ar ₃ ,
No ferrite is formed during standby, and a low yield ratio of 80% or less cannot be obtained. On the other hand, when the standby temperature T is lower than Ar ₃ -100 ° C., ferrite is excessively generated even during a short standby period, so that it is difficult to secure strength. Therefore,
The standby temperature is suitably in the range of Ar ₃ to Ar ₃ -100 ° C. On the other hand, even if the standby temperature is set in a temperature range from Ar ₃ to Ar ₃ -100 ° C., the standby time t (second) is 1.3-0.006 × ΔT> logt. --------------- (2) In the case of (2), the standby time is too short and the required amount of ferrite is not generated in the required amount, so the strength is secured, but the low yield ratio 80% or less is not secured. On the other hand, the upper limit of the waiting time is desirably 150 seconds or less so as not to impair productivity. Therefore, the standby time t (second) is suitably a time that satisfies 1.3-0.006 × ΔT ≦ logt ≦ log150 (1).

(21) Accelerated cooling rate after standby If the accelerated cooling rate after standby is less than 2 ° C./sec, transformation from untransformed austenite to bainite after standby is unlikely to occur, and a low yield ratio of 80% or less is obtained. I can't get it. Therefore, the accelerated cooling rate after standby is set to 2 ° C./sec or more.

(22) Accelerated cooling stop temperature after standby When the accelerated cooling stop temperature after standby is set to less than 400 ° C./sec, martensite is generated by accelerated cooling and toughness is deteriorated. On the other hand, when the accelerated cooling stop temperature is higher than 650 ° C., bainite transformation of untransformed austenite does not proceed sufficiently, and it is difficult to secure high tensile strength. Therefore, the accelerated cooling stop temperature after standby is 400 to
The range is 650 ° C.

As described above, when the above-described composition of components and the rolling and cooling conditions are applied, the productivity is not impaired.
It is possible to produce a refractory steel having a strength of 90 N / mm ² or more and a yield ratio of 80% or less.

[0048]

Next, the present invention will be described in more detail with reference to examples, and the effects of the present invention will be proved.

Production tests of low yield ratio refractory steels were conducted by changing the composition of the billets and the conditions of hot rolling and cooling. A test material was sampled from each of the obtained steel materials, and a tensile test at normal temperature, a Charpy impact test, and a tensile test at 600 ° C. were performed.

Table 1 shows the component composition and the Ar ₃ transformation point of the test slab. Steel Nos. A to L have composition compositions within the scope of the present invention, and steel Nos. M to V have composition compositions outside the scope of the invention. Table 2 shows the manufacturing conditions and the mechanical properties of the manufactured steel slab in the hot rolling step of the test steel material.

[0051]

[Table 1]

[0052]

[Table 2] Both the component composition and the production conditions of the hot rolling step were within the scope of the present invention, and Examples 1 to 12 were all 490 N / mm ^2.
It has a sufficient tensile strength (TS) and a yield ratio (YR) of 80% or less, and has a high-temperature strength (0.2% at 600 ° C.) of 2/3 or more of the normal temperature strength (YS) as a refractory steel material characteristic.
% Proof stress). Also having both excellent toughness (absorbed energy _V E ₀ in the Charpy test at 0 ° C.). In addition, stable operation was performed without any loss in productivity. Thus, in the embodiments, the object of the present invention has been completely achieved.

On the other hand, none of Comparative Examples 1 to 10 including at least one constituent element outside the scope of the present invention could completely achieve the object of the present invention. That is, Comparative Example 1 has an accelerated cooling stop temperature after standby higher than the range of the present invention, Comparative Example 5 has an average temperature of the steel material during cooling standby lower than the range of the present invention, and Comparative Example 6 has Was lower than the range of the present invention. Therefore, in any case, the tensile strength is 490 N / mm ² or less,
The strength as high strength steel was not obtained. In Comparative Example 3, the rolling end temperature was lower than the range of the present invention,
In Comparative Example 9, the average temperature of the steel material during cooling standby was higher than the range of the present invention, and in Comparative Example 10, the Nb content was higher than the range of the present invention. Therefore, in each case, the yield ratio is 8
It exceeded 0%, and a low yield ratio was not obtained. In Comparative Example 7, the rolling reduction at a temperature of Ar ₃ or more was lower than the range of the present invention, and in Comparative Example 8, the accelerated cooling stop temperature after standby was lower than the range of the present invention. Therefore, in each case, the toughness is inferior to other steel materials. In Comparative Examples 2 and 4, the Mo content was lower than the range of the present invention. Therefore, in each case, the 0.2% proof stress at 600 ° C. did not reach ２ of the yield strength at room temperature, and did not have the performance as fire-resistant steel.

[0054]

As described above, according to the present invention,
Low-yield ratio refractory steels used for steel structures such as buildings and bridges can be manufactured stably at low cost and in large quantities without reducing productivity due to waiting on a steel production line before accelerated cooling or heat treatment. can do. A method for producing such a low yield ratio refractory steel material can be provided, and an industrially useful effect is provided.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of a standby temperature and a standby time after accelerated cooling performed after rolling on steel material properties.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Minoru Suwa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term in Nihon Kokan Co., Ltd. 4K032 AA01 AA04 AA05 AA08 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 AA40 BA01 CA02 CB02 CC03 CD02 CD03

Claims (5)

[Claims] C: 0.04 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.5 to 2.0%, Mo: 0.2 to 0% by weight%. 8%, Al: 0.002 to 0.1%, N: 0.020% or less After heating a steel slab to 1000 ° C or more, a reduction of 50% or more in a temperature range of Ar ₃ or more. A step of performing hot rolling, and removing the hot-rolled steel material from a temperature of Ar ₃ or more to Ar ₃
From Ar to a temperature range of Ar ₃ -100 ° C. by accelerated cooling at a cooling rate of 2 ° C./sec or more, and waiting in the temperature range for a waiting time: t (sec) satisfying the following expression (1): 0.3−0.006 × ΔT ≦ logt ≦ log150 ---------- (1) Here, ΔT (° C.): the temperature between Ar ₃ and the average temperature T (° C.) of the steel material during standby. Difference (= Ar ₃ −T) t (second): standby time A step of accelerating and cooling the standby steel material to a temperature in the range of 400 to 650 ° C. at a cooling rate of 2 ° C./second or more. A method for producing a low yield ratio refractory steel material. 2. As a billet component, from the group of: Cu: 0.03 to 1.0%, Ni: 0.03 to 0.5%, Cr: 0.03 to 0.6% by weight%. The method for producing a low-yield-ratio refractory steel material according to claim 1, comprising one or more selected materials. 3. The steel slab component further includes: Nb: 0.005 to 0.05%, V: 0.01 to 0.1%, Ti: 0.003 to 0.1% by weight%. The method for producing a low-yield-ratio refractory steel material according to claim 1, wherein the method comprises one or more selected from the group. 4. As a billet component, one or more selected from the group consisting of Ca: 0.0005 to 0.005% and REM: 0.001 to 0.02% by weight. The method for producing a low-yield-ratio refractory steel material according to claim 1, comprising: 5. The steel slab component further includes: Nb: 0.005 to 0.05%, V: 0.01 to 0.1%, Ti: 0.003 to 0.1% by weight%. One or more selected from the group, and Ca: 0.0005 to 0.005%, REM: 0.001 to 0.02%, containing one or more selected from the group The method for producing a low-yield-ratio refractory steel material according to claim 1, wherein: