JPH093596A - Low yield ratio type high tensile strength steel plate excellent in weld cracking resistance and having >=440n/ mm2 yield ratio and >=590n/mm tensile strength and its production - Google Patents

Abstract

PURPOSE: To produce a low yield ratio type tensile strength steel plate excellent in weld cracking resistance by specifying the compsn. constituted of C, Si, Mn, P, S, Al, Mo, V, Nb and Fe and furthermore specifying the content of ferrite having high hardness. CONSTITUTION: As the one having a compsn. contg., by mass, 0.05 to 0.15% C, 0.05 to 0.50% Si, 0.30 to 1.80% Mn, <=0.015% P, <=0.005% S, 0.005 to 0.10% Al, 0.05 to 1.00% Mo, 0.005 to 0.070% V and 0.006 to 0.040% Nb, furthermore contg. prescribed amounts of Cu, Ni, Cr, B, Ca, Ti and rare earth metals, and the balance Fe with inevitable impurities, and in which PCM=C+Si/30+Mn/20+ Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B<=0.22%, and furthermore, ferrite in which microhardness in 10kg loads is regulated to >=180HV is contained by 10 to 50 vol.%, the steel plate having about <=80% yield ratio, >=440N/mm<2> yield strength and >=590N/mm<2> tensile strength and excellent in weld cracking resistance can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly applied to a super high-rise building structure and has a yield strength of 440N which is excellent in weld crack resistance.
/ mm ² or more, to a tensile strength 590N / mm ² or more low yield ratio type high tensile steel.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for higher-rise building structures and larger spans.
From 490N / mm ² class steel plate, higher-strength tensile strength 570N / mm ²
The movement to use high-grade high-strength steel sheets has increased. Steel materials for building structures are used to ensure the safety of structures during earthquakes.
Excellent plastic deformability with a yield ratio of 80% or less is required, so it has become necessary to reduce the yield ratio to 80% or less even for tensile strength 570 N / mm ² class high-strength steel sheets. It was

High-strength steel sheets with a tensile strength of 570 N / mm ² grade have a yield ratio generally because the strength is secured by using martensite and bainite produced by quenching and tempering.
High as 90% or more. Therefore, in order to reduce the yield ratio to 80% or less, the two-phase region temperature (Ac ₁ point or more and less than Ac ₃ point) is applied between the two heat treatments of conventional quenching (Q) and tempering (T).
A new heat treatment method (Q-
Q'-T method) yield strength 80% or less tensile strength 570N / m
The m ² class high strength steel plate was developed. According to this method, a soft and ductile ferrite phase formed by Q ′ during holding in a two-phase region and in the subsequent cooling process is mixed into a microstructure composed of a hard phase of bainite or martensite. Therefore, a low yield ratio can be obtained.

However, since this method uses soft ferrite for reducing the yield ratio, it is necessary to increase the amount of the additive element in order to secure a predetermined strength. P than class high-strength steel
There is a problem that _CM (welding crack susceptibility composition) becomes high and weld crack resistance is poor.

Further, the mechanical properties of low yield ratio type high strength steel sheets have been conventionally required in a form corresponding to JIS standard SM570, but standardization (SA440: 19 to 100 mm) is currently underway. The following points were different in yield strength and tensile strength. The yield strength is 16 depending on the plate thickness.
450 N / mm ² or more above 40 mm and below 43 above 43 above 40 mm
0N / mm ² or more, 75 mm in exceeded what was defined as 420N / mm ² or more, and 440 N / mm ² or more in Zen'itaAtsu range and the tensile strength was defined as 570N / mm ² or more , 590N
/ mm ² or more. That is, in the case of thick-walled materials having a plate thickness of 40 mm or more such that a low-yield ratio type high-strength steel sheet is generally used, in order to satisfy the standards for both the yield strength and the tensile strength, it is necessary to add 10 A strength increase of ~ 20 N / mm ² or more was required. Yield strength will be referred to as yield strength in the following, but if it is not clear and the yield point cannot be confirmed, it means 0.2% proof stress.

[0006]

The present invention secures a yield ratio of 80% or less, and at the same time, has excellent weld cracking resistance equal to or higher than that of a conventional high-yield ratio type high-strength steel sheet by the Q-T method. Yield strength of up to 440 N / mm ² and tensile strength of 590 N up to 100 mm
An object of the present invention is to provide a low yield ratio type high strength steel sheet having a yield strength of / mm ² or more.

[0007]

The present inventors have found that the yield strength is
440N / mm^TwoAbove, tensile strength 590N / mm^TwoSecuring high strength above
While ensuring a yield ratio of 80% or less and good weldability
In addition, it is the same as the conventional high-yield ratio type high-strength steel sheet by the Q-T method.
Low P_cmTo maintain strength and low yield ratio
We conducted a thorough research on the dan. As a result, realize these
The conventional low yield ratio is 570N / mm^TwoGrade high strength steel plate
Similarly, in hard phase bainite and martensite structure
It is important to mix ferrite, which is a soft phase, in
But P _cmSolid solution strengthening by alloying elements to reduce
Bainite and Martensite
In addition to transformation strengthening by using, another means of securing strength is utilized.
Precipitation strengthening is an effective means of achieving this.
Among them, especially P_cmIs an element that does not affect
Precipitation strengthening by Nb is expected to increase strength without degrading weldability.
It was found to be very effective as a means to be carried out. This
By utilizing the new knowledge of
Low yield ratio by T method P lower than high strength steel plate_cmChemical composition
Excellent melting resistance as the required strength can be secured in minutes
The crack resistance can be secured.

Further, in the conventional Q-Q'-T method of manufacturing a low yield ratio type high-strength steel sheet, it is necessary to carry out the heat treatment off-line three times, so that the heat treatment is carried out for the purpose of increasing the production efficiency. For the purpose of reducing the number of times, we examined the use of online cooling method.

As a result, by utilizing the precipitation strengthening of Nb which does not affect the P _CM , the conventional off-line Q-
Online with slow cooling rate instead of Q'-T method
Applying the C-T method or the DQ-T method with a slow cooling rate,
The present invention has been completed based on the finding that higher strength can be achieved than in the case of the QQ'-T method while maintaining a low yield ratio.

[0010] Incidentally, AcC refers to accelerated cooling after hot rolling, and DQ generally refers to obtaining a hard phase of martensite and bainite from austenite by direct quenching, but the cooling rate here is slow. DQ refers to a treatment in which the cooling rate is slowed down to a room temperature at a cooling rate that makes it possible to form a soft phase ferrite in addition to the above hard phase during cooling.

By applying the AcC-T method with a slow cooling rate and the DQ-T method with a slow cooling rate, the conventional Q-Q '
Since strength than -T method is increased, it is possible to reduce the amount of additive element, resulting in the P _CM can be reduced to a high yield ratio type high tensile steel and equivalent or lower levels by the conventional Q-T method. Therefore, it is possible to improve the weld crack resistance of the low yield ratio type high strength steel sheet. Furthermore, in the case of the Q-Q'-T method, the off-line heat treatment, which was required three times, is performed only once in the tempering, and the productivity can be significantly improved.

That is, the gist of the present invention is (1) mass%
, C: 0.05 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.30 to 1.
80%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.
10%, Mo: 0.05-1.00%, V: 0.005-0.070%, Nb: 0.00
Ferrite containing 6 to 0.040%, balance Fe and unavoidable impurities, P _CM obtained by the following formula is 0.22% or less, and a micro hardness of HV180 or more at a load of 10 g is 10 to 50 by volume fraction. % resistance to weld cracking resistance containing high yield strength 440 N / mm ² or more, a tensile strength 590N / mm ² or more low yield ratio type high tensile steel. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%)

(2) C: 0.05 to 0.15% by mass%, Si: 0.05
~ 0.50%, Mn: 0.30 ~ 1.80%, P: 0.015% or less, S: 0.0
05% or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V:
0.005 to 0.070%, Nb: 0.006 to 0.040%, Cu: 0.05 to 0.50%, Ni: 0.05 to 2.00%, Cr: 0.05 to 1.
00%, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.010%, T
i: 0.005 to 0.025%, REM: 0.001 to 0.010%, containing 1 or 2 or more selected, consisting of the balance Fe and inevitable impurities, and P _CM obtained by the following formula is 0.22% or less, Furthermore, it contains 10 to 50% by volume of ferrite with a microhardness of HV180 or more at a load of 10 g and excellent weld crack resistance. Yield strength 440 N / mm ² or more, tensile strength 590 N / mm ²
The above is a low-yield ratio type high-strength steel sheet. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%)

(3) C: 0.05 to 0.15% by mass%, Si: 0.05
~ 0.50%, Mn: 0.30 ~ 1.80%, P: 0.015% or less, S: 0.0
05% or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V:
0.005 to 0.070%, Nb: 0.006 to 0.040%, balance F
A steel slab consisting of e and unavoidable impurities and having a P _CM of 0.22% or less obtained by the following formula is heated to a temperature of 1100 to 1200 ° C, and hot-rolled to a finishing temperature of 850 to 950 ° C. , After the hot rolling, the cooling rate is 1 ~ 5 ℃ / s, the stop temperature is
Performs online cooling at 400 to 500 ℃, then reheats to 450 to 650 ℃ offline and air cools to precipitate Nb carbonitrides. Yield strength 440N with excellent weld crack resistance.
/ mm ² or more, a manufacturing method of a tensile strength 590N / mm ² or more low yield ratio type high tensile steel. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%)

(4) C: 0.05 to 0.15% by mass%, Si: 0.05
~ 0.50%, Mn: 0.30 ~ 1.80%, P: 0.015% or less, S: 0.0
05% or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V:
0.005 to 0.070%, Nb: 0.006 to 0.040%, Cu: 0.05 to 0.50%, Ni: 0.05 to 2.00%, Cr: 0.05 to 1.
00%, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.010%, T
i: 0.005 to 0.025%, REM: 0.001 to 0.010%, containing 1 or 2 or more selected, consisting of balance Fe and inevitable impurities, and P _CM obtained by the following formula is 0.22% or less The billet is heated to a temperature of 1100 to 1200 ° C and hot rolled to a finishing temperature of 850 to 950 ° C. After the hot rolling, the cooling rate is 1 to 5 ° C / s and the stop temperature is 400 to On-line cooling at 500 ° C is performed, then offline 450
Yield strength with excellent weld cracking resistance, characterized by performing Nb carbonitride precipitation treatment by reheating to ~ 650 ° C and air cooling.
A method for producing a high yield steel sheet having a low yield ratio and a tensile strength of 0 N / mm ² or more and a tensile strength of 590 N / mm ² or more. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%)

(5) After the hot rolling is completed, the Nb carbonitride is cooled online by cooling it to room temperature at a cooling rate of 1 to 5 ° C / s, and then offline reheating to 450 to 650 ° C and air cooling. the performing process (3) or (4) yield strength 440 N / mm ² or more is excellent in resistance to weld cracking of a tensile strength 590N / mm ² or more low yield ratio method for producing a high tensile steel plate.

[0017]

[Function] First, the reason for limiting P _CM will be described. To improve the weld crack resistance, it is effective to reduce the amount of alloying elements added, and P _CM is proposed and widely used as an index. In the present invention, excellent weld crack resistance equivalent to that of a high yield ratio type high strength steel sheet having a tensile strength of 590 N / mm ² or more by the conventional QT method, that is, crack prevention preheating in a y-type weld crack test by covered arc welding. temperature in order to have the property that 50 ° C. or less, P _CM is less 0.22%.

[0018] Next, P _CM is 80% or less of yield ratio in the range of chemical composition of 0.22% below the yield strength 440 N / mm ² or more, a method of securing tensile strength 590N / mm ² or more high strength simultaneously description To do. As a method for ensuring high strength and low yield ratio, by making the microstructure a structure mainly composed of bainite and martensite, which are hard phases, as in the conventional high-strength steel plate with low yield ratio by the QQ'-T method. It is necessary to secure the strength by utilizing transformation strengthening and solid solution strengthening by adding alloy elements in the range of allowable chemical components, and to secure the low yield ratio by mixing ferrite which is a soft phase. In addition, making the microstructure a bainite or a mixed structure of martensite and ferrite also makes the structure finer and is an extremely effective means in terms of ensuring excellent toughness. However, only the solid solution strengthening and transformation strengthening, yield strength 440 N / mm ² or more, can not secure a tensile strength 590N / mm ² or more high strength, attempts to utilize the precipitation strengthening as a means of further increasing strength.

Here, the present inventors conducted the following experiments. Using steel types A and B with chemical composition shown in Table 1, plate thickness 80
It was hot-rolled to mm and subjected to the heat treatment shown in FIG. 1 to examine the tensile properties and microstructure. Here, the difference between the steel types A and B is the presence or absence of Nb. The heat treatment applied is a two-phase heat treatment Q '
Q-Q'-T method and accelerated cooling (AcC)
It is the AcC-T method utilizing the method. Table 2 shows the survey results.

Here, Q: reheating and quenching from a temperature of _three or more points of Ac, AcC: water cooling immediately after completion of rolling online, 450 ° C.
Stop cooling at (cooling rates are 2 ° C / s and 7 ° C / s) Q ': Reheat quenching from two-phase region temperature (Ac ₁ point or more and less than Ac ₃ points), T: Temperature range of less than Ac ₁ point It is tempered in.

[0021]

[Table 1]

[0022]

[Table 2]

From Table 2, regarding the yield ratio, steel types A and B
When the Q-Q'-T method and the AcC-T method with a cooling rate of 2 [deg.] C./s are applied to, the ferrite content is 10% or more and the yield ratio is 80% or less. On the other hand, in the case of the AcC-T method with a cooling rate of 7 ° C / s,
Since the cooling rate is high and the ferrite content is only 3%, the yield ratio of 80% or less cannot be satisfied. It can be seen that can secure 440 N / mm ² or more yield strength and 590N / mm ² or more tensile strength only in the case of applying the ACC-T method steels A containing Nb for strength. This is because it is difficult to secure high strength unless precipitation strengthening is used as described above.

Next, as shown in Table 3, the above-mentioned steel type A
For steel types C, D, and E with different Nb contents, hot rolling was performed to a plate thickness of 80 mm, an ACC-T heat treatment was performed at a cooling rate of 2 ° C / s, and the T temperature was varied to affect the yield strength. The effect of microhardness (load 10g) of was investigated. The result is shown in FIG.

[0025]

[Table 3]

As is apparent from FIG. 2, in order to secure the yield strength of 440 N / mm ² or more, the micro hardness of ferrite is used.
It turns out that more than HV180 is necessary. Therefore, 440N
In order to secure the yield strength of / mm ² or more, the micro hardness of ferrite is limited to HV180 or more. However, the micro hardness of ferrite referred to in the present invention is 10% at the position of 1/4 of the plate thickness.
12 points were measured in each visual field for 10 visual fields of 0 μm × 100 μm, the average value of 10 points excluding the maximum value and the minimum value was taken as the hardness in that visual field, and the average value in the 10 visual fields was taken as the microhardness.

Next, with respect to steel types C, D, and E also shown in Table 3, hot rolling was performed to a plate thickness of 80 mm, and an ACC-T heat treatment with a different cooling rate was performed to obtain a ferrite volume fraction and a yield ratio. The relationship between yield strength and tensile strength was investigated. The result is shown in FIG.

As is clear from FIG. 3, in order to satisfy the yield ratio of 80% or less, the volume fraction of ferrite is 10% or more,
440 N / mm ² or more yield strength, it can be seen that require it is 50% or less at a volume fraction of the ferrite in order to ensure 590N / mm ² or more tensile strength. Therefore, the micro hardness is H
Limit the volume fraction of ferrite with V180 or more to 10 to 50%. However, the volume fraction of ferrite referred to in the present invention is
The volume fraction is defined as the average value of 10 fields of view, excluding the maximum and minimum values, measured in 12 fields of 100 μm × 100 μm at the position of 1/4 of the plate thickness.

Next, the reasons for limiting the manufacturing method will be described. The present inventors have made a steel having the chemical composition shown in Table 4
Hot rolling to a plate thickness of 80 mm, various heat treatments were performed, and the influence of the manufacturing method on the strength and yield ratio was investigated. The chemical components shown in Table 4 are those obtained by adding Nb, which is a precipitation element, to the chemical components of the low yield ratio type high strength steel sheet by the conventional QQ'-T method.
Table 5 shows the heat treatment conditions. The heat treatment applied is the conventional Q-Q'-T method, the AccC-T method or the DQ-T method using the accelerated cooling (AcC) method and the direct quenching (DQ) method in which the cooling rate is changed. The outline is shown in FIG. The results of the investigation of tensile properties are also shown in Table 5.

Here, Q: reheating and quenching from a temperature of Ac ₃ points or more, Ac C: water cooling immediately after completion of rolling online, 450 ° C.
Cooling stopped at, DQ: Immediately after completion of rolling, water-cooled and cooled to room temperature ((Cooling rates of AcC and DQ are 2 ° C / s and 7 ° C / s) Q ': Two-phase region temperature (Ac ₁ point or more Ac Reheating and quenching from less than ₃ points) and tempering in the temperature range of T: Ac less than ₁ point.

[0031]

[Table 4]

[0032]

[Table 5]

From Table 5, AcC at a cooling rate of 7 ° C./s
The yield ratio is over 80% in the -T method and the DQ-T method, while the Q-Q'-T method including heat treatment in the two-phase region, the cooling rate
Both the AcC-T method and the DQ-T method at 2 ° C / s can obtain a low yield ratio of 80% or less. Among them, the accelerated cooling (AcC) method and the direct quenching (DQ) method are used. In the case of the AcC-T method and DQ-T method, QQ '
It can be seen that higher strength is obtained than in the case of -T method.

The reason for this is that precipitation is strengthened by Nb carbonitrides which are fine and match the matrix. However, Q-Q '
Although the precipitation of Nb carbonitrides is observed even when the -T method is applied, these are precipitated during the heating process during air cooling after rolling and during quenching, and coarsening occurs, and the conformity also collapses and becomes inconsistent. Therefore, it does not contribute to precipitation strengthening.

Further, the cooling rate not including the two-phase region heat treatment 2
As with the QQ'-T method including heat treatment in the two-phase region, the low yield ratio of 80% or less can be obtained in the AcC-T method and the DQ-T method in the case of C / s because the cooling rate is slow. This is because ferrite is formed therein, and as a result, ferrite is formed as if the two-phase region heat treatment was performed. Therefore, when the above heat treatment method is applied, the strength can be increased and the two-phase region heat treatment can be omitted, so that the productivity can be greatly improved.

As described above, Ac at a cooling rate of 2 ° C./s
The C-T method and the DQ-T method are very effective methods for increasing the strength while ensuring a low yield ratio. However, it goes without saying that this is only limited to the case where Nb is contained.

By utilizing the above new knowledge, the present inventors have attempted to improve the weld crack resistance of a high yield steel sheet with a low yield ratio type of 590 N / mm ² or higher. As shown in Table 6, the P _CM of the low yield ratio type 590 N / mm ² steel sheet by the conventional Q-Q'-T method was 0.
To the steel reduced from 23% to 0.20%, the AcC-T method and the DQ-T method with a cooling rate of 2 ° C / s were applied under the same conditions as in Fig. 4. The tensile properties are shown in Table 7.

[0038]

[Table 6]

[0039]

[Table 7]

As is clear from Table 7, in the case of the AcC-T method and the DQ-T method with a cooling rate of 2 ° C./s, a sufficient strength for precipitation strengthening of Nb was obtained despite the low P _CM. Has been obtained. On the other hand, the use of precipitation strengthening cannot be expected Q-
The Q'-T method, 440 N / mm ² or more yield strength, it can not be ensured with 590N / mm ² or more tensile strength. For the above reasons, the manufacturing method is limited to the AcC-T method and the DQ-T method in the present invention.

Next, the reason for limiting the temperature range in the above heat treatment and heating / rolling will be described. The heating temperature of the billet shall be 1000 ° C or higher. The reason for this is that the rolling end temperature must be 850 ° C. or higher in order to completely dissolve Nb in solid solution and to secure excellent acoustic anisotropy, which will be described later. However, if the temperature exceeds 1200 ° C, the austenite grains become significantly coarse, so the upper limit is made 1200 ° C.

In the subsequent rolling, the finishing temperature is limited to 850 ° C. or higher. The reason is as follows. The steel sheet of the present invention is mainly intended for a steel sheet for construction. In the case of steel plates for construction, the acoustic anisotropy in ultrasonic flaw detection is severely limited. This is because when ultrasonic flaw detection is performed on a welding defect, it is difficult to accurately grasp the defect if the acoustic anisotropy is large. This acoustic anisotropy is related to the rolling finishing temperature, and tends to increase as the finishing temperature decreases. Therefore, the lower limit of the finishing temperature is set to 850 ° C to ensure good acoustic anisotropy. Further, in order to secure good toughness, it is necessary to refine the structure. Therefore, the finishing temperature should be as low as possible and the upper limit should be 950 ° C.

After completion of rolling, water cooling is continued. The purpose of this is to bring Nb to room temperature in a solid solution state. However, if water-cooled to below the precipitation temperature, Nb
Since it can be brought to room temperature in a solid solution state, the cooling is accelerated cooling (AcC) that stops the cooling halfway.
Both direct quenching (DQ) for cooling to room temperature and room temperature may be used. In the case of accelerated cooling, the upper limit of the stop temperature is 500 ° C because it is necessary to cool down to the precipitation temperature or lower,
The lower limit is 400 ° C so that straightening for the purpose of improving flatness can be sufficiently performed after cooling is completed.

The cooling rate needs to be a rate at which ferrite is produced during cooling and Nb is not precipitated because the heat treatment in the two-phase region is omitted. Therefore, the steel having the components shown in Table 6 was used as the test steel, the AcC-T method was applied under the same conditions as in FIG. 4, and the tensile properties of the steel sheet having a thickness of 80 mm and varying the cooling rate were investigated. The result is shown in FIG.

As shown in FIG. 5, the cooling rate is 5 ° C./s or less to secure a yield ratio of 80% or less, and the cooling rate is 1 ° C./s to secure a yield strength of 440 N / mm ² or more. It turns out that it is necessary. Therefore, the cooling rate after finishing rolling is
Limit to 1 to 5 ℃ / s.

Next, Nb is reheated to 450 to 650 ° C. and air cooled.
Carry out carbonitride precipitation treatment. This is to precipitate fine Nb carbonitrides matching the matrix for precipitation strengthening. The upper limit of the heating temperature for precipitation treatment is 650 because if the temperature is too high, the strength of Nb precipitates becomes coarse and the strength decreases.
℃. The lower limit is the lower limit of Nb carbonitride precipitation temperature.
Set to 0 ° C.

Finally, the reasons for limiting the chemical components in the present invention will be described. C is an element necessary to secure the strength as a high-strength steel sheet, but if the content is less than 0.05%, it is difficult to obtain a tensile strength of 590 N / mm ² or more, and 0.
Addition in excess of 15% deteriorates weld crack resistance and is not preferred. Therefore, the C content should be in the range of 0.05 to 0.15%.

Si is an element necessary for deoxidation, but its content is
If it is less than 0.05%, this effect is small, and if it is added in excess of 0.50%, the weldability and toughness are deteriorated, which is not preferable. Therefore, the Si content is in the range of 0.05 to 0.50%.

Mn is an element necessary for improving the hardenability of steel and ensuring the strength, and in order to obtain such an effect, it is necessary to add 0.30% or more. However, 1.80%
If added excessively in excess of the above, toughness and weldability are deteriorated. Therefore, the Mn content is in the range of 0.30 to 1.80%.

P impairs the toughness and weldability and also causes the generation of hot cracks during welding. Therefore, the P content is 0.01
5% or less.

S forms inclusions in the form of Mn and MnS and is extended by rolling to deteriorate bending workability and toughness. Therefore, the S content should be 0.005% or less.

Al is a deoxidizing element, but if it is less than 0.005%, such an effect is small, and if it exceeds 0.10%, toughness is deteriorated. Therefore, the Al content is
The range is 0.005 to 0.10%.

Mo is an element effective in increasing the strength due to precipitation strengthening and preventing temper softening, but if the content is less than 0.05%, a sufficient effect cannot be obtained. It deteriorates the property and increases the cost. Therefore, the Mo content is set to 0.05 to 1.00%.

Similar to Mo, V is also an element effective in increasing the strength and preventing temper softening, but if the content is less than 0.005%, its effect is not fully exhibited, and if it exceeds 0.070% and is excessively added. Deteriorates toughness and weldability. Therefore,
The V content should be in the range of 0.005 to 0.070%.

Nb is effective for increasing strength and refining the structure due to the precipitation of carbonitrides, but in order to obtain such an effect, 0.006% or more must be added. But,
If added in excess of 0.040%, the toughness deteriorates, and the absorbed energy in the Charpy test at 0 ° C cannot satisfy SA440 standard of 47J or more. Therefore, the Nb content should be in the range of 0.006 to 0.040%.

In addition to these, Cu, Ni, Cr, B, Ca, Ti and
One or more REMs should be added depending on the plate thickness, target strength, toughness and workability level.

Cu is an element effective for improving the hardenability, solid solution strengthening, and strength increasing by precipitation strengthening, but its content is 0.
If it is less than 01%, such an effect is not sufficiently exhibited, and if it is added in excess of 0.50%, hot workability, toughness and weldability are deteriorated. Therefore, the Cu content is 0.
The range is from 01 to 0.50%.

Ni is an element effective in securing hardenability and improving toughness by increasing stacking fault energy, but its content is
If it is less than 0.05%, such an effect is not sufficiently exhibited, and if it is added in excess of 2.00%, the cost increases. Therefore, the Ni content is in the range of 0.05 to 2.00%.

Cr is an element effective for increasing the strength, but if the content is less than 0.05%, such an effect is not sufficiently exhibited, and if it is added in excess of 1.00%, the weldability deteriorates. Therefore, the Cr content is in the range of 0.05 to 1.00%.

B is an element effective in increasing the hardenability and increasing the strength by adding a trace amount, but if the content is less than 0.0003%, the effect is not sufficiently obtained, and if it exceeds 0.0030%, it is excessively added. This deteriorates the toughness. Therefore, the B content should be in the range of 0.0003 to 0.0030%.

Ca has a spheroidizing effect on non-metallic inclusions and is an element effective for improving bending workability and toughness, but if the content is less than 0.0005%, its sufficient effect cannot be obtained, and If it is added in excess of 0.010%, it becomes an excessive addition for spheroidization, and excess Ca becomes an inclusion and deteriorates toughness.
Therefore, the Ca content should be in the range of 0.0005 to 0.010%.

Ti is an element effective for grain refinement by suppressing the growth of austenite crystal grains, but 0.005% or more is required to obtain such an effect. But,
Excessive addition exceeding 0.025% deteriorates toughness.
Therefore, the Ti content should be in the range of 0.005 to 0.025%.

REM has an effect of controlling the morphology of inclusions and is an element effective for improving bending workability, toughness and ductility in the plate thickness direction, but if the content is less than 0.001%, its effect is not sufficiently exhibited, Further, if added in excess of 0.010%, the toughness deteriorates. Therefore, the REM content is 0.001-0.
The range is 010%.

[0064]

EXAMPLES Hereinafter, the present invention related resistance welding crack excellent in yield strength 440 N / mm ² or more, a tensile strength of 590N / mm ² or more will be described an embodiment of the low yield ratio type high tensile steel, the The invention is not limited to this embodiment.

Example 1 As for the steel sheet to be tested, a steel plate having the chemical composition shown in Table 8 was rolled by the manufacturing method shown in Table 9 and then subjected to various heat treatments to obtain a plate thickness.
It is a finished steel plate of 19 to 100 mm. Specimens were taken from these steel sheets, and the tensile properties, impact properties, weld crack resistance and microstructure were investigated. Table 10 shows the results. The weld crack resistance was judged by JIS Z 3158y type weld crack test.

As is clear from Table 10, all of the steel types A to I of the examples of the present invention have a yield strength of 440 N / mm ² or more and 590 N / mm ^2.
In addition to the above tensile strength and a yield ratio of 80% or less, it has excellent weld cracking resistance of a crack prevention preheating temperature of 25 ° C. or less in a y-type weld cracking test by covered arc welding.

On the other hand, in the steel types J to R of the comparative examples, any one of the chemical composition, the cooling rate, the cooling stop temperature, the precipitation treatment temperature, the micro hardness of ferrite, and the volume fraction of ferrite is within the limit range of the present invention. , The yield strength and tensile strength are low, the yield ratio is high, or the weld crack resistance is poor. That is, the steel types J and Q have a heat treatment of Q-
Since it is Q'-T, the yield strength and tensile strength are low.
Steel type K has a low Nb content from the limited range of the present invention, and steel type P is out of range, so steel type K has low yield strength and steel type P has poor impact properties. The steel type L has a lower cooling rate from the limited range of the present invention, and the steel type O is deviated higher, so that the steel type L has a low yield strength and the steel type O has a high yield ratio of 82%. Steel type M has a high Nb carbonitride precipitation treatment temperature.
Low yield strength and tensile strength. Since steel type N has a high cooling stop temperature, it has low yield strength and tensile strength. Since the steel type R has a high P _CM , the preheating temperature for preventing weld cracking is high.

[0068]

[Table 8]

[0069]

[Table 9]

[0070]

[Table 10]

Example 2 As for the steel sheet to be tested, a steel piece having the chemical composition shown in Table 11 was rolled by the manufacturing method shown in Table 12 and then subjected to various heat treatments to obtain a sheet thickness.
It is a finished steel plate of 19 to 100 mm. Specimens were taken from these steel sheets and the tensile properties and weld crack resistance were investigated. The results are also shown in Table 12. The weld crack resistance was determined by JIS Z 3158 y-type weld crack test.

As is clear from Table 12, steel type A of the present invention
~I are both 440 N / mm ² or more yield strength, with a 590N / mm ² or more tensile strength and 80% or less of yield ratio, cracking prevention preheating temperature in the y-type weld cracking test by shielded metal arc welding 25 It has excellent weld cracking resistance of ℃ or less.

On the other hand, in the steel types J to R of the comparative examples, any of the chemical composition, the cooling rate, the cooling stop temperature and the precipitation treatment temperature is out of the limited range of the present invention. Is low, the yield ratio is high, or the weld crack resistance is poor. That is, steel type J has a high yield strength and low tensile strength because the precipitation treatment temperature of Nb carbonitride is higher than the limit range of the present invention and steel type M is lower than the range. Steel type K has a higher cooling rate than the limited range of the present invention,
Steel type R has a lower deviation, so steel type K has a yield ratio of 83%.
And steel type R has low yield strength and tensile strength. Since the steel type L has a high cooling stop temperature, the yield strength and the tensile strength are low. Since the heat treatments of the steel types N and P are Q-Q'-T, the yield strength and the tensile strength are low. Steel type O has a low yield strength and tensile strength because the Nb content deviates slightly from the range of the present invention. Steel type Q has a high P _CM , so
Weld crack prevention preheating temperature is high.

[0074]

[Table 11]

[0075]

[Table 12]

[0076]

As is apparent from the above description,
According to the present invention, the yield strength is 440 N / mm ² or more, and the tensile strength is 590 N.
It is possible to obtain a high-strength steel sheet with a low yield ratio and a yield ratio of 80% or less of / mm ² or more, and is also superior in weld cracking resistance to the high yield steel sheet with a high yield ratio by the conventional Q-T method. Can be secured. Therefore, the resistance to weld cracking resistance of the present invention superior yield strength 440 N / mm ² or more, a tensile strength of 590N / mm ² or more low yield ratio type high tensile steel is widely used in high-rise architectural structure Long Span be able to.

[Brief description of drawings]

FIG. 1 is a schematic view of a heat treatment method used in an experiment.

FIG. 2 is a diagram showing the relationship between the microhardness of ferrite and the yield strength.

FIG. 3 is a diagram showing the relationship between the volume fraction of ferrite and the yield ratio, yield strength, and tensile strength.

FIG. 4 is a schematic diagram of a heat treatment method used in an experiment.

FIG. 5 is a diagram showing the relationship between cooling rate and tensile properties.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/58 C22C 38/58

Claims (5)

[Claims] 1. C: 0.05-0.15%, Si: 0.05-
0.50%, Mn: 0.30 to 1.80%, P: 0.015% or less, S: 0.005
% Or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V: 0.0
From 05 to 0.070% Nb: containing from 0.006 to 0.040%, and a balance of Fe and unavoidable impurities, and calculated by the following formula P _CM
Is 0.22% or less, and the micro hardness at a load of 10 g is H
Yield strength 440N with excellent weld crack resistance, characterized by containing 10 to 50% by volume of ferrite of V180 or more
/ mm ² or more, a tensile strength of 590N / mm ² or more low yield ratio type high tensile steel. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%) 2. In mass%, C: 0.05-0.15%, Si: 0.05-
0.50%, Mn: 0.30 to 1.80%, P: 0.015% or less, S: 0.005
% Or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V: 0.0
05-0.070%, Nb: 0.006-0.040%, and C
u: 0.05 to 0.50%, Ni: 0.05 to 2.00%, Cr: 0.05 to 1.00
%, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.010%, Ti:
It contains one or more selected from 0.005 to 0.025% and REM: 0.001 to 0.010%, consists of the balance Fe and unavoidable impurities, and the P _CM obtained by the following formula is 0.22% or less, and the load yield strength microhardness at 10g and excellent resistance to weld cracking, characterized in that it contains 10-50% ferrite is HV180 or more in a volume fraction 440 N / mm ² or more, a tensile strength of 590N / mm ² or more low Yield ratio type high strength steel plate. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%) 3. In mass%, C: 0.05-0.15%, Si: 0.05-
0.50%, Mn: 0.30 to 1.80%, P: 0.015% or less, S: 0.005
% Or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V: 0.0
From 05 to 0.070% Nb: containing from 0.006 to 0.040%, and a balance of Fe and unavoidable impurities, and calculated by the following formula P _CM
Of 0.22% or less is heated to a temperature of 1100 to 1200 ° C and hot rolled to a finishing temperature of 850 to 950 ° C.
After hot rolling, the cooling rate is 1 to 5 ° C / s, the stop temperature is 400
Yield strength 440N / with excellent weld cracking resistance characterized by performing online cooling at ~ 500 ° C, then reheating offline to 450 ~ 650 ° C, and air cooling Nb carbonitride precipitation treatment. mm ² or more, a tensile strength of 590N / mm ² or more low yield ratio method for producing a high tensile steel plate. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%) 4. In mass%, C: 0.05-0.15%, Si: 0.05-
0.50%, Mn: 0.30 to 1.80%, P: 0.015% or less, S: 0.005
% Or less, Al: 0.005-0.10%, Mo: 0.05-1.00%, V: 0.0
05-0.070%, Nb: 0.006-0.040%, and C
u: 0.05 to 0.50%, Ni: 0.05 to 2.00%, Cr: 0.05 to 1.00
%, B: 0.0003 to 0.0030%, Ca: 0.0005 to 0.010%, Ti:
Steel piece containing one or more selected from 0.005 to 0.025% and REM: 0.001 to 0.010%, the balance Fe and unavoidable impurities, and the P _CM obtained by the following formula is 0.22% or less Is heated to a temperature of 1100-1200 ° C and hot-rolled to a finishing temperature of 850-950 ° C. After the hot rolling, the cooling rate is 1-5 ° C / s and the stop temperature is 400-500 ° C. On-line cooling is performed and then offline 450 ~
Yield strength 440N with excellent weld crack resistance characterized by performing Nb carbonitride precipitation treatment by reheating to 650 ° C and air cooling
/ mm ² or more and tensile strength 590 N / mm ² or more, low yield ratio type high-strength steel sheet manufacturing method. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5 × B (%) 5. After completion of hot rolling, the material is cooled to room temperature at a cooling rate of 1 to 5 ° C./s online and then offline.
The yield strength 440N / mm ² or more and the tensile strength 590N which are excellent in weld crack resistance according to claim 3 or 4, characterized in that the precipitation treatment of Nb carbonitride is performed by reheating to 450 to 650 ° C and air cooling.
/ mm ² or more low yield ratio type high strength steel sheet manufacturing method.