JPH0747774B2 - Low yield ratio 60 Kgf / mm excellent in weld crack resistance. - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a low yield ratio 60 kgf / mm ² grade steel sheet used in a building structure such as a high-rise building, and more specifically, to a base material The present invention relates to a method for producing a low yield ratio 60 kgf / mm ² grade steel sheet having a yield ratio of 80% or less and having excellent joint toughness in a large heat input welding joint such as electroslag welding and having excellent weld crack resistance.

(Prior art) Steel sheets used for steel frames have become thicker and stronger with the increase in the number of buildings. For example, in a box pillar of a super-high-rise intelligent building, a low yield ratio of 100 mm is used. 60k
gf / mm ² grade steel sheets are being adopted.

Further, as the labor shortage becomes more serious, the adoption of large heat input welding such as two-electrode submerged arc welding and electroslag welding has become strong. However, not only is the embrittlement of the heat-affected zone affected by high heat input welding,
The problem that minute cracks shown in FIG. 1 occur in the box column corner joint weld has become apparent. It is said that this kind of crack is related to non-metallic inclusions and segregation of the steel sheet and hydrogen from the welding material, but the cause has not been clarified.

In addition, the steel sheet for construction is required to have a yield ratio of 80% or less from the viewpoint of the deformability of the structure during an earthquake. Therefore, in order to lower the yield ratio of 60 kgf / mm ² grade steel sheet, in addition to the usual quenching and tempering treatment, after heating and holding in the Ac ₃ to Ac ₁ transformation point temperature range, by cooling, ferrite with a low yield ratio is obtained. Various methods have been proposed for incorporating s into steel.

For example, JP-A-55-97425 discloses that rolling at a cumulative reduction of 25% or more at 950 ° C. or less, cooling at a cooling rate of air cooling or higher after heating and holding in the Ac _{3 to} Ac ₁ transformation point temperature range, Then Ac
It is intended to lower the yield ratio by tempering at a temperature not higher than _one transformation point. Further, JP-A No. 64-52023 discloses that quenching is performed immediately after controlled rolling, followed by heating and holding in the Ac _{3 to} Ac ₁ transformation point temperature range, followed by water cooling, and further tempering at an Ac ₁ transformation point temperature or less. , Which has realized a low yield ratio.

(Problems to be solved by the invention) However, when the extremely thick and high strength steel sheet as described above is manufactured by the conventional quenching and tempering method, Ceq. (Carbon equivalent)
Therefore, it is indispensable to limit the amount of heat input to the welding for preheating to prevent cold cracking in the weld and to secure the toughness of the welded joint, and there is a problem that welding work and work management become very troublesome.

Further, the manufacturing methods proposed in JP-A-55-97425 and JP-A-64-52023 do not take any measures to prevent minute cracks occurring in the corner joint welded portion of the box column. Absent.

The present invention has been made in order to solve the above-mentioned problems, and by controlling chemical components, reducing segregation, and optimizing rolling conditions, it is possible to prevent microcracks that occur in the corner joint welds of box columns. An object of the present invention is to provide a method for manufacturing a low yield ratio 60 kgf / mm ² grade steel sheet having excellent weld cracking properties.

(Means for Solving the Problems) In order to prevent microcracks that occur in the corner joint welds of the box column, the inventors of the present invention control the heating temperature and heating time of the billet to reduce segregation. By utilizing the precipitation strengthening of Nb in quenching and tempering and lowering Ceq., It was found that it is possible to manufacture a low yield ratio 60 kgf / mm ² grade steel sheet with excellent weld crack resistance. It was the invention.

The first invention is C: 0.10 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.
70 to 1.50%, P: 0.010% or less, S: 0.002% or less, Nb: 0.005
~ 0.030%, Ca: 0.0010 to 0.0030%, balance Fe and inevitable impurities, and defined by the following formula
Slabs with Ceq. Value of 0.45% or less, 1150 to 1250 ℃
After being heated for 2.5 × T [T: billet thickness (mm)] for a total of 60% in the recrystallization temperature range of austenite.
With the above secured, the rolling is completed in this recrystallization temperature range, and then cooled to a temperature of 400 ° C. or lower at a cooling rate in the range of 5 to 30 ° C./sec from the temperature of the Ar ₃ transformation point or higher, and , Ac ₃
It is a method for producing a low yield ratio 60 kgf / mm ² grade steel sheet with excellent weld cracking resistance, which comprises heating to the Ac ₁ transformation point temperature range, water cooling, and then tempering at Ac ₁ transformation point temperature or lower.

Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... The second invention is, in addition to the contained elements limited in claim (1), further Cu: 0.30% or less, Ni: 0.50% or less, Cr: 0.30
% Or less, Mo: 0.30% or less, V: 0.05% or less, low yield ratio 60 kgf / mm with excellent weld crack resistance according to claim (1).
^This is a manufacturing method for ^second- grade steel sheets.

(Operation) Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the chemical components in the present invention will be described.

C is an element that contributes to the strength increase, but its content is 0.10
If it is less than 0.1%, it is difficult to secure the strength, and if it exceeds 0.18% in a large amount, the toughness and weldability of steel deteriorate. Therefore, the C content is in the range of 0.05 to 0.20%.

Si is an essential element in steelmaking as a deoxidizing element,
If the content is less than 0.05%, this effect is small, and 0.50
If it is contained too much in excess of%, the toughness of steel deteriorates.
Therefore, the Si content is set to the range of 0.05 to 0.50%.

Mn is an element necessary for deoxidation and securing strength, but if the content is less than 0.70%, such an effect is not sufficiently obtained, and if it exceeds 1.50% and is excessively contained, Deteriorates toughness. Therefore, the Mn content is 0.70 to 1.50.
The range is%.

P promotes segregation, causes a local increase in hardness inside the steel, and easily causes minute cracks in the weld heat affected zone during high heat input welding, so the upper limit is made 0.010%.

S is contained in steel as a non-metallic inclusion, but in particular,
Since MnS becomes the starting point of minute cracks that occur in the weld heat affected zone during high heat input welding, it is desirable to reduce it as much as possible. Therefore, in the present invention, the upper limit of S is 0.002%.

Nb improves the toughness of steel by preventing coarsening of austenite grains during heating, and also increases the strength by fine dispersion precipitation of carbonitride in the tempering process after quenching. In the present invention, a content of at least 0.005% is necessary, but if the content exceeds 0.030% and is excessive, weldability deteriorates. Therefore, the Nb content is set in the range of 0.005 to 0.030%.

Ca is effective in controlling the morphology of sulfide-based inclusions such as MnS, which is the starting point of minute cracks, but if contained in a large amount, it forms huge inclusions in the steel and deteriorates the properties of the steel. Therefore, the Ca content is in the range of 0.0010 to 0.0030%.

Elements such as Cu, Ni, Cr, Mo, and V have the effect of suppressing the decrease in strength to the right of the center of the plate thickness of thick steel plates, but since they are all expensive, the content of each of Cu is 0.30% or less, Ni Is 0.50% or less, Cr is 0.30% or less, Mo is 0.30 or less, and V is 0.05% or less.

However, these elements must satisfy Ceq. Defined by the formula of 0.45% or less within the above-mentioned content range. When Ceq. Exceeds 0.45%, the weldability deteriorates, and hardening cracks during small heat input welding and minute cracks due to hydrogen during large heat input welding tend to occur. Therefore, Ceq. Should be 0.45% or less.

Next, the manufacturing conditions in the present invention will be described.

In the present invention, in order to reduce the microsegregation in the steel that promotes the generation of minute cracks, the heating temperature and holding time of the billet are limited as follows. That is, the steel piece is heated to a temperature range of 1150 to 1250 ° C. and held for 2.5 × T [T: steel piece thickness (mm)]. By this heating and holding, elements that tend to segregate, such as C and P, are diffused, microsegregation is reduced, and abnormal hardened parts in the steel are reduced. It should be noted that heating at a temperature lower than the limited conditions or holding for a short time results in insufficient diffusion of the above elements, while heating at a high temperature or holding for a long time causes coarsening of austenite grains. As a result, the toughness deteriorates.

In the present invention, the reason for rolling in the recrystallization temperature range of austenite is to secure the toughness by making the austenite grains fine recrystallized austenite grains, and in order to secure a stable and good toughness, 60% total reduction in crystallization temperature range
It is necessary to secure the above.

Further, the cooling rate after completion of rolling requires a cooling rate of 5 ° C./sec or more from a temperature of Ar ₃ transformation point or higher to obtain a desired quenched structure, but the cooling rate is 30 ° C./sec. Even if it exceeds, the effect will be saturated. If the cooling stop temperature exceeds 400 ° C, the desired quenched structure cannot be obtained. Therefore, the cooling rate after rolling is in the range of 5-30 ° C / sec.
The cooling stop temperature is limited to 400 ° C or lower.

In the present invention, since ferrite having a low yield ratio is mixed in the microstructure, it is heated to the two-phase region temperature, that is, the Ac _{3 to} Ac ₁ transformation point temperature range, and then water-cooled. The tempering temperature is not higher than the Ac ₁ transformation point temperature in order to reduce the residual stress in the steel sheet caused by the heat treatment in the previous stage and ensure the safety of the structure.

The temperatures of the Ac ₃ transformation point, the Ac ₁ transformation point and the Ar ₃ transformation point are determined by the following equations.

Ac ₃ (℃) = 908-223.7C + 438.5P + 30.5Si + 37.9V-34.
4Mn-23.0Ni Ac ₁ (℃) = 723 + 22.0Si-14.0Mn-14.4Ni + 23.3Cr Ar ₃ (℃) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo However, the content of each alloy element (% ).

(Example) Below, the Example of this invention is described.

The test steel plate has a thickness of 50 to 100 m according to the rolling condition and cooling condition shown in Table 2 of the steel piece having the chemical composition shown in Table 1.
It was finished in m. Samples were taken from these steel plates, tensile test and impact test of base metal, maximum hardness test for confirming weldability, tensile test and impact test of two-electrode submerged arc welded joint were performed, and further, The microcracks were observed. The results are also shown in Table 2. The maximum hardness test is based on JIS Z 3101, and the two-electrode submerged arc welding assumes a box column corner joint.
The heat input was 170 kJ / cm.

Table 1 shows the chemical components, Ceq., Of the inventive methods A to D and Comparative Examples E to H, and Table 2 shows the rolling conditions, cooling conditions, mechanical material of the base metal, weldability and weld joint performance, respectively. .

Comparative Example E in Table 1 is an example in which C, Mn, Ceq. Are high, F and G have a large amount of impurities P or S, and H is an example in which Nb is not added.

In Table 2, steel symbols A1 and A2 mean that different manufacturing conditions were applied to the steel symbol A in Table 1,
Steel symbols B1, B2, and so on.

As is clear from Table 2, the method of the present invention (A1, B1, C1, D
In 1), the tensile strength (TS) is 60 kgf / mm ² or more, the yield ratio (YR) is stable at 70 to 75%, and vTrs is on the low temperature side and the toughness is good. Moreover, the maximum hardness is low, the weldability is good, the weld joint strength and toughness (vE ₀ ) are also good, and no microcracks in the box column are observed.

In Comparative Example A2, since the heating temperature was high and the austenite grains were coarsened, the toughness (vTrs) was deteriorated. Comparative example
In B2, since the heating time of the steel slab is short, the reduction of microsegregation is insufficient, and small cracks in the box column are observed. Comparative Example C2 has a high yield ratio because there is no heating in the Ac ₃ to Ac ₁ transformation point temperature region and subsequent water cooling necessary for ferrite formation. In Comparative Example D2, since the cooling rate after rolling was slow, a quenching structure could not be obtained and the tensile strength was low at less than 60 kgf / mm ² .
Comparative Example E has a high Ceq., And therefore has the highest maximum hardness and poor weldability. Further, in Comparative Examples F and G, a large amount of P or S, respectively, and a large number of hardened parts or non-metallic inclusions due to segregation, which is the starting point of cracks, are present, and therefore microcracks in the box column are recognized. In Comparative Example H, since Nb was not added, fine recrystallized austenite grains were not obtained, the toughness was deteriorated, and the precipitation strengthening action of carbonitride of Nb was not obtained, and the strengthening was also reduced. .

(Effects of the Invention) As described above, the present invention controls the heating temperature and heating time of a steel slab to reduce segregation, and utilizes the precipitation strengthening of Nb in quenching and tempering to obtain Ceq. By lowering it, a low yield ratio 60kgf / mm ² class steel plate is manufactured.
According to the present invention, while maintaining the same strength and toughness as the conventional low yield ratio 60 kgf / mm ² grade steel sheet, and having good weldability,
Joint toughness at high heat input welding is good, good that it is possible to produce a low yield ratio 60 kgf / mm ² class steel sheet weld cracking and excellent in resistance to weld cracking resistance does not occur in the corner joint weld box columns It has an effect.

[Brief description of drawings]

FIG. 1 is a diagram showing microcracks generated in a corner joint weld portion of a box column of a steel frame for construction.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/12

Claims (2)

[Claims] 1. C: 0.10 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.
70 to 1.50%, P: 0.010% or less, S: 0.002% or less, Nb: 0.005
~ 0.030%, Ca: 0.0010 to 0.0030%, balance Fe and inevitable impurities, and defined by the following formula
Slabs with Ceq. Value of 0.45% or less, 1150 to 1250 ℃
After being heated for 2.5 × T [T: billet thickness (mm)] for a total of 60% in the recrystallization temperature range of austenite.
With the above secured, the rolling is completed in this recrystallization temperature range, and then cooled to a temperature of 400 ° C. or lower at a cooling rate in the range of 5 to 30 ° C./sec from the temperature of the Ar ₃ transformation point or higher, and , Ac ₃
~ A method for producing a low yield ratio 60 kgf / mm ² grade steel sheet with excellent weld crack resistance, which comprises heating to an Ac ₁ transformation point temperature range, water cooling, and then tempering at an Ac ₁ transformation point temperature or lower. Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14… 2. C: 0.10 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.
70 to 1.50%, P: 0.010% or less, S: 0.002% or less, Nb: 0.005
~ 0.030%, Ca: 0.0010 to 0.0030%, Cu: 0.30% or less, Ni: 0.50% or less, Cr: 0.30% or less, Mo: 0.30% or less,
V: A steel slab containing 0.05% or less, consisting of balance Fe and unavoidable impurities, and having a Ceq. Value defined by the following formula of 0.45% or less is heated to a temperature range of 1150 to 1250 ° C. , 2.5 × T [T: Billet thickness (mm)], after holding for a total reduction of 60% or more in the recrystallization temperature range of austenite,
Rolling is completed in this recrystallization temperature range, and after that, at a cooling rate in the range of 5 to 30 ° C / sec from the temperature of the Ar ₃ transformation point or higher,
Cooling to a temperature of ℃ or less, further heating in the Ac ₃ ~ Ac ₁ transformation point temperature range, followed by water cooling, and then tempering below the Ac ₁ transformation point temperature, excellent low resistance to welding cracking. method of manufacturing yield ratio 60 kgf / mm ² grade steel. Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14…