JP4705381B2 - Manufacturing method of low yield ratio steel - Google Patents

Description

  The present invention relates to an efficient method for producing a low yield ratio steel material used for a steel structure such as a high-rise building.

  For high-rise buildings in recent years, assuming a case of a huge earthquake, a limit state design that emphasizes human safety by absorbing earthquake energy and avoiding major collapse by plastic deformation of columns and beam members Although the method is applied, it is considered effective to use a steel material having a low yield ratio (YR) indicated by the ratio of yield stress (YP) to tensile strength (TS).

  In general, it is known that a reduction in the yield ratio of steel is achieved by making the microstructure a mixed structure composed of a soft ferrite phase and a hard bainite phase or a martensite phase. As a method for producing a low yield ratio steel material, for example, as described in Patent Document 1, Patent Document 2, and the like, there is a method in which after hot rolling is completed, air cooling to a ferrite + austenite two-phase region is performed, followed by accelerated cooling. . However, in such a manufacturing method, ferrite produced during air cooling is coarsened, so that generally the toughness tends to deteriorate, and in addition, it is difficult to make the material uniform in the longitudinal direction of the steel, and the productivity is also remarkable. There is a problem that it drops.

  As another manufacturing method, as shown in Patent Document 3, there is a method in which heat treatment is performed in a two-phase temperature range of ferrite + austenite after the end of rolling. However, this method also has a problem that the microstructure is coarsened by the two-phase region heating, so that the toughness is deteriorated and the productivity is greatly reduced by the increase of the heat treatment step.

JP-A 63-293110 JP 59-2111528 A Japanese Patent Laid-Open No. 3-219002

  Therefore, the present invention has been made in view of such problems, and the object thereof is to efficiently produce a low yield ratio steel material without requiring a complicated hot rolling process or heat treatment process with low productivity. It is an object of the present invention to provide a new and improved method for producing a low yield ratio steel that can be used.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors set the ferrite fraction to 20% or more by normal heating, rolling, cooling, and heat treatment processes, and then performing light reduction rolling at a low temperature. Thus, it was found that the yield ratio can be easily reduced by reducing only YP without reducing TS and elongation (EL) of the steel material.

The present invention has been made on the basis of such findings and has been further studied, and the means thereof is as follows.
(1) 0.01-0.20 mass% C, 0.02-1.0 mass% Si, 0.20-2.5 mass% Mn, 0.025 mass% or less P 0.020% by mass or less of S, 0.002 to 0.10% by mass of Al, 0.0010 to 0.0080% by mass of N, and the balance being Fe and inevitable impurities, and Low yield, characterized by rolling a steel material with a ferrite fraction of 20% or more at a temperature of 100 ° C or more and at a temperature at which aging does not occur at a rolling reduction of 0.1 to 0.5%. A method for producing a specific steel material.
(2) 0.05 to 1.5 mass% Cu, 0.05 to 1.0 mass% Cr, 0.05 to 0.5 mass% Mo, 0.05 to 0.5 mass% W 0.05 to 0.5 mass% Ta, 0.05 to 3.5 mass% Ni, 0.003 to 0.050 mass% Nb, 0.003 to 0.10 mass% Ti, 0 0.005 to 0.10 mass% V, 0.0003 to 0.0030 mass% B, 0.0003 to 0.0050 mass% Ca, 0.0005 to 0.0060 mass% Mg, and The method for producing a low yield ratio steel according to (1), further comprising one or more selected from the group consisting of 0005 to 0.0060 mass% REM.
(3) The method for producing a low yield ratio steel material according to (1) or (2), wherein the temperature T at which the aging does not occur is a temperature that satisfies the following mathematical formula 1.
T ≦ 250-26000 * [(N%) − (Ti%) / 3.4− (Al%) / 29] (° C.)
(However, if [(N%)-(Ti%) / 3.4- (Al%) / 29] <0, T = 0. If Ti is not included, Ti%) = 0.)
... (Formula 1)

  According to the manufacturing method of the low yield ratio steel material of the present invention, the temperature waiting after the hot rolling and the heat treatment process to reduce productivity can be achieved by reducing only YP by light rolling in a low temperature region where aging does not occur. Low yield ratio steel can be obtained efficiently without the need.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  Based on the knowledge that by introducing movable dislocations in the ferrite of the microstructure of the steel material, it is possible to greatly reduce only YP without reducing TS and EL, The ferrite fraction was examined. As a result, it was found that it is preferable if the ferrite fraction of the steel material is 20% or more. In other words, if the ferrite fraction is less than 20%, even if movable dislocations can be introduced into the ferrite, the absolute amount is small, so the decrease in YP of the steel material is small, and the decrease in YR is small. Is 20% or more, preferably 50% or more.

  Here, in order to obtain a steel material having a ferrite fraction of 20% or more, it is not necessary to precisely control the microstructure by complicated hot rolling or heat treatment that hinders productivity, and final rolling is performed. In the previous stage, the ferrite fraction should be 20% or more. For example, a steel slab heated to 1000 to 1300 ° C. may be rolled at Ar 3 to 1100 ° C. with a cumulative rolling reduction of 50% or more, and then air-cooled, or from a temperature of Ar 3 or higher for higher strength. , Accelerated cooling to a temperature of 20 ° C./s or less, or to a temperature of 400 ° C. or more. Furthermore, heat treatment may be performed at a temperature of 650 ° C. or lower for the purpose of adjusting strength, toughness, elongation, and the like.

  In addition, it is necessary to perform rolling in a light rolling reduction range of 0.1 to 0.5%. This is because if the rolling reduction is less than 0.1%, a uniform and sufficient amount of movable dislocations cannot be introduced into the ferrite, and YR does not decrease. On the other hand, when the rolling reduction exceeds 0.5%, the dislocation density becomes excessive, so that work hardening occurs, and in addition to YP and YR starting to increase, a decrease in EL becomes apparent. Here, the relationship between the rolling reduction of light rolling and YR is shown in FIG. In FIG. 1, the horizontal axis represents the rolling reduction (%), and the vertical axis represents YR (%).

  In addition, regarding the steel material temperature during light rolling, it is necessary to perform rolling within a temperature range in which aging does not occur. This is to avoid the possibility that even if movable dislocations are introduced into the ferrite by light rolling, YP may return to the original due to aging and YR may rise. Here, the difference between the temperature at which the light rolling is performed and the upper limit temperature of Formula 1 and the relationship with YR are shown in FIG. In FIG. 2, the horizontal axis represents the difference (° C.) between the temperature at which the light rolling is performed and the upper limit temperature of Equation 1, and the vertical axis represents YR (%). Further, even in a temperature range where aging does not occur, a temperature of 100 ° C. or higher is preferable because the rolling load can be reduced.

Furthermore, it is preferable to obtain the upper limit T _{max of the} temperature range in which this aging does not occur using the following Equation 1. Here, in Formula 1, (N%), (Ti%), and (Al%) mean numerical values representing the contents of N, Ti, and Al in mass%, respectively.

T _max = 250-26000 * [(N%)-(Ti%) / 3.4- (Al%) / 29] (° C.)
(However, if [(N%)-(Ti%) / 3.4- (Al%) / 29] <0, T _maz = 0)
... (Formula 1)

  Next, the components of the steel material manufactured using the manufacturing method of the present invention are the same as those of a normal steel material, but the reason for limiting the amount added will be described.

  C is an effective component for improving the strength of steel, and the lower limit is set to 0.01%. Excessive addition significantly lowers the weldability and HAZ (Heat Affected Zone) toughness of the steel, so the upper limit is set to 0. 20%.

  Si is an element necessary for deoxidation at the time of melting. If an appropriate amount is added, the matrix is solid-solution strengthened, so 0.02% or more is added. On the other hand, if added over 1.0%, the toughness decreases due to the hardening of the HAZ, so the upper limit was made 1.0%.

  Mn needs to be added in an amount of 0.20% or more as an effective component for securing the strength and toughness of the base metal, but the upper limit is set to 2.5% within an acceptable range of toughness and cracking of the welded part. .

  P and S are desirable as the content is small. However, in order to reduce this industrially, it takes a great deal of cost, so 0.025% and 0.020% were set as upper limits, respectively.

  Since Al is an important deoxidizing element, the lower limit was set to 0.002%. In addition, since the surface quality of the slab deteriorates when a large amount of Al is present, the upper limit was made 0.10%.

  N has the effect of refining austenite by precipitating as AlN. However, if the solid solution N increases due to excessive addition, the HAZ toughness is reduced, so it is limited to a range of 0.0010 to 0.0080%. .

  Moreover, the addition amount of the selective additive element is limited for the following reasons.

  Cu, Cr, Mo, W, and Ta are effective for increasing the hardenability of steel materials, so 0.05% or more is added, but if added in a large amount, weldability and HAZ toughness are reduced. Therefore, the upper limit is 1.5% for Cu, 1.0% for Cr, and 0.5% for Mo, W, and Ta.

  Ni improves the strength and toughness of the steel material, so 0.05% or more is preferably added. However, since increasing the amount of Ni increases the cost, the upper limit is set to 3.5%.

  Nb and Ti function effectively in terms of grain refinement and precipitation strengthening by adding a small amount, so 0.003% or more is added, but if added excessively, the toughness of the welded portion is significantly reduced. The upper limit was 0.050%, and Ti was 0.10%.

  V improves hardenability and contributes to strengthening by forming carbonitride, so 0.005% or more is added, but if added in a large amount, HAZ toughness deteriorates, so 0.10% is the upper limit. It was.

  B is effective in suppressing the growth of grain boundary ferrite and ferrite side plates, which are harmful to HAZ toughness, and increasing the strength. Therefore, B is added in an amount of 0.0003% or more. The upper limit was .0030%.

  Ca, Mg, and REM form oxides and sulfides and are added for the purpose of preventing coarsening of HAZ crystal grains and reducing the anisotropy of the base material. Addition of Ca impairs toughness, so Ca is added in the range of 0.0003 to 0.0050%, Mg: 0.0005 to 0.0060%, and REM: 0.0005 to 0.0060%. Note that REM refers to rare earth elements such as La and Ce.

  Table 1 below shows the chemical composition of the steel material, and Table 2 shows the manufacturing conditions, base material structure and mechanical properties.

  About YP, TS, EL of steel materials, it evaluated using the JIS No. 4 tensile test piece, and YR was computed by YP / TSx100 [%]. The fracture surface transition temperature (vTrs) was evaluated using a JIS No. 4 impact test piece. The test specimens were collected from the center of the plate thickness in a direction perpendicular to the rolling direction.

  No. of the embodiment of the present invention. Since Nos. 1 to 8 were manufactured under the conditions within the scope of the present invention, all had low yield ratios of 75% or less, and had good characteristics as low yield ratio steel materials.

  On the other hand, no. For 1 to 8, the yield ratio was as high as 80% or more because either the manufacturing conditions or the structure deviated from the scope of the present invention. No. In 1-3, since the temperature of the light rolling was high, YP did not decrease and YR did not decrease. No. In Nos. 4 and 5, YR did not decrease because light rolling was not performed or the rolling reduction was small. No. No. 6 had a large rolling reduction, so that YP started to rise and YR became high. No. In No. 7, the YR increased because the temperature of the light rolling was high and the rolling reduction was large. No. In No. 8, since the cooling rate after hot rolling was excessive and the stop temperature was low, the ferrite fraction was low, and YR could not be lowered even when light rolling was performed.

  In addition, No. of the conventional example. For Nos. 1 and 2, the YR was lowered because of the two-phase quenching and heat treatment rather than the light rolling, but the productivity was significantly reduced. No. of the conventional example. In No. 1, since water cooling was started after the temperature dropped to the two-phase region after hot rolling, However, the toughness decreased and the productivity also decreased. No. of the conventional example. No. 2 of Example No. 2 because the two-phase region heat treatment was performed during the hot rolling / accelerated cooling and tempering steps. Although the YR was reduced to the same level as 3, the productivity was significantly reduced.

  As described above, it was confirmed that by applying the method of the present invention, a low yield ratio steel material can be manufactured more efficiently than the conventional method.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

It is a graph which shows the relationship between the rolling reduction of light rolling, and YR. It is a graph which shows the relationship between the temperature which implemented light reduction rolling, the upper limit temperature of Numerical formula 1, and YR.

Claims (3)

0.01 to 0.20% by mass of C;
0.02 to 1.0 mass% Si,
0.20 to 2.5% by mass of Mn,
0.025 mass% or less of P,
0.020 mass% or less of S,
0.002 to 0.10% by mass of Al,
0.0010 to 0.0080 mass% N;
As the balance, Fe and inevitable impurities,
And a steel material having a ferrite fraction of 20% or more,
A method for producing a low yield ratio steel material, characterized by performing light reduction rolling at a reduction rate of 0.1 to 0.5% at a temperature of 100 ° C. or higher and at which aging does not occur.   0.05 to 1.5 mass% Cu, 0.05 to 1.0 mass% Cr, 0.05 to 0.5 mass% Mo, 0.05 to 0.5 mass% W, 0.0. 05-0.5 wt% Ta, 0.05-3.5 wt% Ni, 0.003-0.050 wt% Nb, 0.003-0.10 wt% Ti, 0.005- 0.10 mass% V, 0.0003-0.0030 mass% B, 0.0003-0.0050 mass% Ca, 0.0005-0.0060 mass% Mg, and 0.0005-0 The method according to claim 1, further comprising one or more selected from the group consisting of 0060 mass% REM. 3. The method for producing a low yield ratio steel according to claim 1, wherein the temperature T at which the aging does not occur is a temperature that satisfies the following formula 1.
T ≦ 250-26000 * [(N%) − (Ti%) / 3.4− (Al%) / 29] (° C.)
(However, if [(N%)-(Ti%) / 3.4- (Al%) / 29] <0, T = 0)
... (Formula 1)

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