JP2015089948A - High tensile steel sheet excellent in gas cutting crack resistance and large heat input weld zone toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile steel having sheet thickness of 50 mm or more and tensile strength of 780 MPa to 930 MPa, excellent in crack property during gas cutting and capable of handling high heat input welding.SOLUTION: There is provided a high tensile steel sheet excellent in crack resistance during gas cutting and toughness during large heat input welding and having tensile strength of 780 MPa, having a predetermined components and satisfying (1) formula in a range of 0.45% to 0.65% and (2) formula in the range of 1% to 2% at the same time. Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 (1). (Cu+Ni)-(Cr+Mo+5*V+10*Nb) (2), where each element symbol expresses mass%.

Description

  The present invention is excellent in cracking properties at the time of gas cutting necessary for manufacturing welded structures such as buildings, bridges, tanks, etc., and has a tensile strength capable of large heat input welding of HT780 MPa class. The present invention relates to a thick high-tensile steel sheet that is 50 mm or more.

High-tensile steel plates having a tensile strength of 780 MPa or more are widely applied as welded structures such as storage tanks, cranes, bridges, and buildings. When manufacturing these structures, processes such as gas cutting, machining, and welding are required. In particular, when a high-tensile steel sheet having a tensile strength exceeding 590 MPa is used, cracks generally occur during gas cutting. Since it is easy to remove, it takes a lot of time to remove it. In addition, there are restrictions such as limiting heat input to prevent the deterioration of the toughness of the weld heat affected zone, which hinders the construction of efficient structures. was there. These restrictions become conspicuous when the 780 MPa class high tension, which has been especially applied recently, and the plate thickness is 50 mm or more, causing a great hindrance to the construction of a wide structure.
In the past, various inventions have been made for high-strength steels in terms of improving gas cutting performance and high heat input weldability. In particular, improvements centered on welding methods have been made, such as omitting welding preheating, improving the toughness of the heat affected zone, and applying high heat input welding.

  For example, Patent Document 1 and Patent Document 2 disclose inventions relating to the production of high-tensile steel with improved gas cutting properties and high heat input weldability. These control the amount of alloy elements having high hardenability such as C, Si, Mn, Cr, Ni, Mo, V, and the like, and control the formation of nitrides such as Ti, N, and B. However, the strength level is limited to the 590 MPa class, and it cannot be applied to the production of the 780 MPa class steel of the present invention.

Patent Document 3 discloses an invention of a steel having a high heat input weldability having a tensile strength of 780 MPa (80 kgf / mm ² ). The feature of the present invention is that it is adjusted with alloy elements such as C, Si, Mn, Cr, Ni, Mo and V so that the carbon equivalent (Ceq) is 0.5% or more. Although an effect can be expected to improve the toughness of the weld heat affected zone during heat input, the upper limit of the carbon equivalent being implemented is 0.56, and the applied plate thickness is 40 mm, No mention is made of gas cutting properties, and the object of the present invention cannot be achieved.

  On the other hand, Patent Document 4 discloses an invention relating to a manufacturing technique of a steel plate of 590 MPa or more that is excellent in toughness of a high heat input weld. The invention includes C: 0.025 to 0.050%, Mn: 1.20 to 2.50%, Cr: 1.50 to 3.50%, Mn + 0.4Cr: 2.50 to 3.00%, etc. However, it does not mention anything about the gas cutting property, and the object of the present invention is achieved. It is not possible.

  Furthermore, Patent Document 5 discloses an invention of a thick-walled high-tensile steel plate having a tensile strength of 935 MPa or more and excellent gas cutting properties. The present invention provides a component range that can improve gas cutting performance by adjusting the alloy elements so that 2Ni + Cr ≦ 3.25, Cr + 2.5Mo ≦ 1.80, 2Mo + Ni ≦ 2.0, and Pcm ≦ 0.30. presentation. Although the present invention aims to reduce the roughness of the gas cutting surface and is certainly effective in improving the gas cutting property, the object of the present invention is the cracking property of the gas cutting surface and the toughness of the weld heat affected zone. Is not mentioned at all, and the strength level is also high, and the present invention cannot solve the problem that is the object of the present invention.

JP 2005-36295 A (Gas cutting + large heat input 60 k) JP 2006-193810 A (〃) JP 61-6616 (Large heat input 80k) JP 2012-241214 A (Large heat input 60k or more) JP-A-8-27538 (gas cutting property 100k) Japanese Patent Laid-Open No. 5-186721

  Accordingly, an object of the present invention is to provide a high-tensile steel having a thickness of 50 mm or more and a high strength steel having a tensile strength of 780 MPa or more and 930 MPa or less. It is to provide a steel plate.

  Generally, in order to achieve a strength of 780 MPa class, a high-strength steel containing B is often used. These B-containing steels utilize the fact that a hard martensite structure and a bainite structure can be generated at the time of quenching even if the amount of alloy elements is small in order to utilize the hardenability improvement by adding a small amount of B. However, since a hardened structure is formed also by the thermal effect of gas cutting, it is said that gas cut cracks are generated in the hardened structure due to intrusion at the time of cutting or hydrogen present in the steel. The high-strength steel that is currently widely used contains several kinds of alloy elements such as B, C, Ni, Cr, and Mo. As a result, it is difficult to avoid the formation of a hardened structure. Generally, preheating is performed in advance during cutting, and measures are taken to reduce hydrogen by eliminating hydrogen during cooling during cutting.

  On the other hand, it is known that the toughness of the weld heat affected zone of the B-containing steel decreases as the heat input increases. This is because as the heat input increases, the cooling rate after welding decreases and hardenability is lost, so the microstructure of the steel changes from martensite to lower bainite structure and further to upper bainite structure as the cooling rate decreases. And it is understood that it reduces toughness. Therefore, as the countermeasures, as described in Patent Document 3 described above, countermeasures such as increasing the alloy element addition amount, improving the hardenability, and suppressing the formation of the upper bainite structure have been considered. . However, in this method, hardening becomes remarkable at a normal low heat input, resulting in problems such as weld cracking and remarkable cracking during gas cutting. Therefore, a component system that does not largely depend on the cooling rate during welding heat is preferable for the toughness of the weld heat affected zone, and for that purpose, it is desirable to apply a component system that does not contain B.

  Therefore, as a result of studying the production of 780 MPa class high-strength steel not containing B, the present inventor adopted a method using Cu precipitation as the means. In addition, as steel strength steel containing Cu, for example, Patent Document 6 discloses an invention for high strength steel having a tensile strength of 686 MPa or more excellent in elongation characteristics. The feature of the present invention is that B is 0.0003% or less, substantially no addition, and instead, Cu is added in an amount of more than 2 W% to 4%, whereby the strength can be improved. A feature of this steel is that although it does not contain B, strength levels of 720 MPa to 1000 MPa can be achieved by quenching and tempering.

Therefore, the inventors applied this principle and studied by adjusting the amount of other alloy elements including Cu.
FIG. 1 illustrates the value of the formula (1) representing the C equivalent that greatly affects the tensile strength of the component within the scope of the present invention and the component outside the scope of the present invention. These results are obtained for the steels shown in the examples. As is clear from this figure, it was found that the steel having the components in the range of the present invention has a linear influence on the tensile strength according to the equation (1). Thus, even if the plate thickness exceeds 50 mm, the target tensile strength of the present invention satisfies the range of 780 MPa or more and 930 MPa or less. The range of the value of the formula (1) is 0.45 or more and 0.65 or less. I found out.

As disclosed in the previous document, it was also found that high strength can be obtained stably by applying the direct quenching method after rolling, as described in the previous literature. In this case, tempering is applied in principle after performing hot rolling described later.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)

  Next, the present inventors selected a component system containing 1.0% or more of Cu, adjusted other components having a tensile strength of 780 MPa class, and then resisted gas cut cracking and toughness during large heat input. Was examined. The examination result is shown in FIG. As main elements, C: 0.03-0.09%, Si: 0.12-0.42%, Mn: 1.05-2.05%, P: 0.002-0.007%, S: 0.002 to 0.004%, Cu: 1.12 to 2.05%, Ni: 1.15 to 2.10%, Cr: 0 to 0.75%, Mo: 0 to 0.57%, V : 0-0.058%, Nb: 0.015-0.025%, Ti: 0.009-0.019%, Al: 0.025-0.048%, N: 0.0025% -0. 0058%, Mg: Melting steel in the range of 0.0006 to 0.0047%, heating to 1150 ° C, hot rolling with a cumulative reduction ratio of 55% in the temperature range of 820 ° C to 900 ° C, Immediately quenching was performed, and then tempering was performed between 530 and 560 ° C. to produce a steel sheet having a thickness of 75 mm.In the steel plate, propane gas was used to cut the gas from the surface of the steel plate to the length of 600 mm using the nozzle of crater # 5 at room temperature and left for 24 hours. The total length of cut cracks existing in the sample was measured, and L (mm) was used as an indicator of gas cut crack resistance.

  On the other hand, three heat cycle test pieces having a size of 11 × 11 × 65 mm were sampled from a ¼ t part of the steel sheet, and the maximum heating temperature was 1350 ° C. (temperature rising time 60 s, holding 10 s), and the cooling time from 800 ° C. to 500 ° C. A heat cycle of large heat input welding corresponding to a heat input of 80 kJ / mm with a heat input of 450 s was applied, and a Charpy test piece (JIS No. 4 V notch) was processed. Thereafter, three Charpy impact tests were carried out at a test temperature of 0 ° C., and the average was evaluated as vE 0 (J).

The horizontal axis in FIG. 2 represents the following newly introduced index (Cu + Ni) − (Cr + Mo + 5 * V + 10 * Nb) (2)
Thus, the influence of the alloy element is quantified, and the vertical axis represents the total length L (mm) of cut cracks during gas cutting and the absorbed energy vE0 (J) after the thermal cycle Charpy impact test.
In FIG. 2, the meaning of the horizontal axis will be described. Cu and Ni are main elements necessary for strength and toughness. Increasing these values tends to improve the strength of the base material and the toughness of the base material and the heat affected zone. On the other hand, since Cr, Mo, V, and Nb are element groups that easily form carbides and excessively increase the hardenability, increasing their sum increases the hardenability and decreases the gas cutting property. At the same time, the toughness of the weld heat affected zone is also reduced. Therefore, it is assumed that the difference between the two indicates the balance of these alloy elements.

  From the results shown in FIG. 2, the plot indicated by ● ▲ is an index L of cut cracking property. Except for the point indicated by ▲, when the index of (2) exceeds 2, L increases. As a result, it was found that cutting cracks occurred. By the way, although it is a difference between ● and ▲, as a result of examining the sample after the experiment in detail, the steel indicated by ● contains Mg in a range of 0.0005% to 0.0072%, and ▲ It was found that the two steels indicated by the above contained only 0.0002% and 0.0003% or less of Mg. Since these melting furnaces were made of crucibles containing magnesium, it was considered that this Mg was dissolved and mixed with Al and other oxidizing agents during refining. Therefore, if it is considered that the difference between ● and ▲ is caused by the amount of Mg, if Mg is added at least 0.0005% or more, the index value of the formula (2) is set to 2 or more, so that gas cutting It was found that cracking can be prevented.

  On the other hand, let us look at the change in the thermal cycle shock absorption energy indicated by ○ △. The difference between ○ and Δ is the difference in Mg content as in the evaluation of gas cutting cracking property. First, looking at the change in ○ where the Mg content is high, when the index value of the horizontal axis (2) is 1 or more, the absorbed energy increases, and when it exceeds 2, there is a tendency that the dispersion slightly increases and slightly decreases. It was. On the other hand, the Δ steel with a low Mg content is not significantly low, but the absorbed energy is clearly lower than the other steels. That is, when the value of the formula (2) is 1 or more, the high heat input welding heat-affected zone toughness is improved, but when Mg is added to 0.0005% or more, a more stable improvement in toughness is obtained. became.

  As described above, when the index value of the formula (2) is limited to the range of 1 to 2, it is possible that both the gas cutting cracking resistance and the toughness of the high heat input welding heat affected zone can be improved. It has been found that the addition of 0.0005% or more makes it more reliable. In order to clarify the reason why these characteristics are improved by adding Mg, an investigation was conducted. First, the results of investigation on gas cutting cracks are described. As a result of the observation of the fracture surface of the steel (indicated by a triangle) in which the Mg content was low and gas-cutting cracks were observed, a large amount of MnS was observed on the cleaved fracture surface. In general, it is known that the presence of MnS promotes hydrogen cracking. In particular, MnS stretched by hot rolling or the like is harmful. Therefore, as a result of polishing a cross section of the Mg-added 0.0005% or more and observing with a microscope, almost no expanded MnS was observed, and spherical MgS was observed instead. From the above, the content of 0.0005% Mg improves the gas-breaking cracking resistance because the formation of spheroidized MgS reduces S in the steel and suppresses the generation of expanded MnS. This is thought to be due to this.

  Next, the reason why the toughness of the heat-affected zone with high heat input was improved was examined. As a result of observation of the fracture surface, an appropriate steel was selected from among the steels with low Mg indicated by △ and ○, and the fracture surface of the Charpy specimen was observed. Although it was not, it was found that the fracture surface unit was slightly rough for Δ where the absorbed energy was slightly low. Then, as a result of carrying out microstructure observation and comparing both, it turned out that the old austenite grain is slightly fine. In addition, considering that the reduction of MnS described above is also a factor for improving the toughness, the reason why the gas cut cracking resistance and the toughness of the weld heat affected zone were simultaneously improved by MgS was also clarified. It should be noted that the addition of Mg is not mixing from the crucible, but industrially, it is possible to produce Mg alloys and add them as wires and powders so that they can be produced even with conventional steel making facilities.

As described above, an object of the present invention is to provide a thick high-tensile steel having excellent cracking property at the time of gas cutting and having a tensile strength capable of high heat input welding of HT780 MPa class and a thickness of 50 mm or more, and a method for producing the same. The main point is to provide
(1) In mass%,
C: 0.03% or more, 0.10% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 1.00% or more, 3.00% or less,
Cu: 1.00% or more, 2.50% or less,
Ni: 1.00% or more, 2.50% or less,
Nb: 0.005% or more, 0.100% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.005% or more, 0.100% or less,
N: 0.0020% or more, 0.0100% or less,
P: 0.020% or less,
S: 0.010% or less,
Mg: 0.0005% or more, 0.0080% or less,
As a basic ingredient,
further,
Cr: 0.05% or more, 0.80% or less Mo: 0.05% or more, 0.60% or less V: 0.005% or more, containing 0.100% or less,
The balance consists of Fe and inevitable impurities, and the formula (1) satisfies the range of 0.45% or more and 0.65% or less, and the formula (2) satisfies the range of 1% or more and 2% or less simultaneously. A high strength steel sheet having a tensile strength of 780 MPa and excellent in crack resistance during gas cutting and toughness during high heat input welding.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
(Cu + Ni)-(Cr + Mo + 5 * V + 10 * Nb) (2)
In addition, each element symbol shows the mass%.

(2) Furthermore,
Ca: 0.0001% or more, 0.0030% or less,
REM: 0.0001% or more, 0.0030% or less,
Zr: 0.005% or more, 0.100% or less,
The thick high-tensile strength steel sheet having a tensile strength of 780 MPa and excellent in cracking resistance during gas cutting and toughness during high heat input welding according to claim 1, wherein at least one of them is contained.

According to the present invention, when welding a high strength steel having a yield strength of 670 N / mm ² or more and 870 N / mm ² or less and a tensile strength of 780 MPa class (780 N / mm 2 or more, 940 N / mm ² or less), Even if the removal annealing is performed, it is possible to provide a steel plate or a welded joint that can obtain high CTOD characteristics.

It is a graph which shows the relationship between tensile strength and the value of (1) Formula. It is a graph which shows the relationship between the cutting crack total length L (mm) at the time of gas cutting, the absorption energy vE0 (J) after a thermal cycle Charpy impact test, and the value of (2) Formula.

On the other hand, three heat cycle test pieces having a size of 11 × 11 × 65 mm were sampled from a ¼ t part of the steel sheet, and the maximum heating temperature was 1350 ° C. (temperature rising time 60 s, holding 10 s), and the cooling time from 800 ° C. to 500 ° C. A heat cycle of large heat input welding corresponding to a heat input of 80 kJ / mm with a heat input of 450 s was applied, and a Charpy test piece (JIS No. 4 V notch) was processed. Thereafter, three Charpy impact tests were carried out at a test temperature of 0 ° C., and the average was evaluated as vE 0 (J). The horizontal axis of the figure represents the following formula (Cu + Ni)-(Cr + Mo + 5 * V + 10 * Nb) (2)
Thus, the influence of the alloy elements is quantified, and the vertical axis represents the total length L (mm) of cut cracks during gas cutting and the absorbed energy vE ₀ (J) after the thermal cycle Charpy impact test.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the steel components of the present invention will be described.

  C is an element that improves the strength of the base material. In order to achieve the intended strength of the present invention, it is necessary to add 0.03% or more, preferably 0.05% or more. Addition increases the hardness of the weld heat affected zone and at the same time decreases its toughness, so the upper limit is limited to 0.10%.

  Si is added as a deoxidizing element and inevitably allows addition of 0.01% or more, but its upper limit is limited to 0.50% or less. In addition, since it may affect deoxidation if too low, Preferably the lower limit shall be about 0.1%.

  Mn is an element effective for deoxidation and improves the strength, so addition of 1.0% or more is allowed, but excessive addition may impair toughness after stress-relief annealing due to temper embrittlement. Therefore, the upper limit is made 3.0%.

  Cu is a main element in the present invention, and 1.0% or more of addition is necessary for improving the strength. However, if added excessively, cracks on the surface of the steel sheet and toughness of the base metal are caused. Since there is a concern about the decline, the upper limit is set to 2.5%.

  Ni is an element effective for improving toughness and needs to be added in an amount of 1.0% or more. However, if excessively added, the hardenability is excessively increased, and there is a concern that the gas cut cracking resistance is lowered. Therefore, the upper limit is set to 2.5%.

  In the present invention, Nb is added as an element that is remarkably effective in refining crystal grains at the time of slab heating and at the end of rolling, thereby increasing the strength toughness of the base material. Therefore, addition of 0.005% or more is necessary, but excessive addition may form coarse carbonitride and inhibit the base material toughness, so the upper limit is 0.100. %.

  In the present invention, Ti is an element that refines the crystal grains of the slab heating and welding heat affected zone, and the toughness of the base material and the welding heat affected zone is improved. Therefore, the addition of 0.0005% or more is effective. However, if excessively added, coarse precipitates may be formed like Nb to inhibit the base metal toughness, so the upper limit is limited to 0.0300%.

  Al is an element useful for deoxidation, and at the same time is an element effective for forming a nitride and reducing the crystal grain size in quenching. In the present invention, addition of 0.005% or more is necessary, but if added excessively, coarse nitrides are formed, and the toughness of the base metal and the weld heat affected zone may be hindered. The upper limit is made 0.100%.

  N is inevitably mixed as a gas component, but is also an element effective for forming a nitride and reducing the crystal grain size of the weld heat affected zone. Therefore, addition of 0.002% or more is preferable. However, if excessively added, the coarsening of the nitride is caused, and at the same time, the toughness of the weld heat-affected zone as welded is lowered. %.

  As described above, Mg is an element effective for improving both the gas cutting crack resistance and the toughness of the high heat input welding heat affected zone. In particular, 0. However, if more than 0.0080% is added, there is a concern that coarse sulfides may be formed during refining, so this is the upper limit.

  P and S are impurity elements contained in the steel, and the smaller the content, the better. In the present invention, the upper limit of P is 0.02% or less, preferably 0.008% or less. The upper limit of S is limited to 0.01% or less, preferably 0.005% or less.

  Furthermore, in the present invention, two or more of Cr, Mo and V are added mainly for improving the strength.

  Cr is an element effective for improving the strength by improving the hardenability and at the same time by precipitation strengthening during tempering. Therefore, 0.05% or more of addition is effective, but if added excessively, the toughness in the base metal and the weld heat affected zone may be lowered. 80%.

  Mo, like Cr, is an element effective for improving the strength by improving the hardenability and at the same time by precipitation strengthening during tempering, and addition equivalent to the lower limit of Cr is allowed. However, if added excessively, the toughness after stress-relief annealing in the weld heat-affected zone may be lowered, and the gas cutting crack resistance is adversely affected, so the upper limit is made 0.60%.

  V, like Mo, is an effective trace element that contributes to hardenability and precipitation strengthening, and requires addition of 0.005% or more. However, excessive addition may impair the base material toughness and the toughness after stress relief annealing of the weld heat affected zone, so the upper limit is made 0.100%.

  In addition, one or more kinds of Ca, REM, and Zr are allowed to be added.

  Ca has the effect of reducing the influence of MnS, which is harmful to toughness, by spheroidizing the sulfides of the steel sheet, and 0.0001% or more may be added for this purpose. However, there is a concern that the addition of a large amount impairs the weldability, and the upper limit is limited to 0.0030% or less.

  Since REM forms an oxide and is effective in improving the toughness of the weld heat affected zone, 0.0001% or more can be added in the present invention. However, a large amount of addition forms a coarse oxide, which in turn causes a decrease in toughness, so the upper limit is limited to 0.0030% or less.

  Zr forms a nitride in the same manner as Ti, and is an effective element for slab heating and refinement of crystal grains in the weld heat affected zone. Addition of 0.0005% or more is necessary, but 0.1000 If added in excess of%, coarse precipitates may be formed to impair toughness, so the upper limit is made 0.1000%.

  In addition to the limitation of the individual elements described above, in the present invention, the range is limited based on the indices defined by the equations (1) and (2). The expression (1) relates to the strength of the base material as Ceq, and 0.45 or more is necessary to obtain a tensile strength of 780 MPa class. Since there is a concern that the toughness decreases, this is the upper limit. As described in detail with reference to FIG. 2, the index in the formula (2) is limited to 1 or more and 2 or less from the range in which the gas cutting crack resistance and the toughness of the high heat input welding heat affected zone are compatible.

  Next, in order to manufacture the steel which has these components as a steel plate, the manufacturing method of the steel product normally used is used. That is, after refining by a converter method, an electric furnace method, and a secondary refining facility, a slab is manufactured by continuous casting or ingot-making. In the present invention, Mg is added, but since Mg easily generates an oxide, deoxidizing elements such as Si and Al are added prior to the addition of Mg, and then Mg is added to form finer MgS. As a result, the microstructure of the weld heat-affected zone tends to become finer, which is preferable for improving toughness.

  Then, after heating to 900-1250 degreeC normally with a slab heating furnace, it rolls to predetermined plate | board thickness by hot rolling, and is then water-cooled with accelerated cooling equipment etc. There is no restriction on the slab heating, but preferably when the temperature is 1150 ° C. or lower, the crystal grain size is prevented from being coarsened and is easily refined, so that the strength is easily improved. In hot rolling, the final rolling temperature is set to 900 ° C. or lower and the cumulative rolling reduction is controlled to be 40% or higher. This is a process necessary to refine the austenite crystal grains and improve the strength of the microstructure after cooling prior to cooling after completion of hot rolling. Therefore, if it is less than 40%, the tensile strength of the base material decreases. Although it is water-cooled immediately after rolling, the cooling rate at the time of cooling depends on the equipment and the plate thickness and is not limited, but is preferably 2 ° C./s or more, more preferably 5 ° C./s. That's it. In this case, the temperature at which water cooling is started needs to be not lower than the Ar3 transformation point, and is preferably not lower than 800 ° C. The water cooling stop may be carried out until the ferrite transformation is completely stopped, and is set to 150 ° C. or lower.

  In addition, when accelerated cooling cannot be realized, a normal quenching process may be performed after rolling and air cooling. In this case, the quenching temperature is equal to or higher than the Ac3 transformation point. However, if it is too high, the austenite crystal grain size becomes coarse and the toughness of the base material may be lowered. Thereafter, water cooling is performed, and the cooling start temperature in that case is a normal quenching process performed from the Ar3 transformation point or higher, preferably from 800 ° C or higher.

The steel sheet quenched as described above is usually tempered, but may be subjected to intermediate quenching and tempering as necessary. The tempering process is a process of heating and cooling at a temperature below the Ac1 transformation point, and is performed for the purpose of adjusting the strength level of the as-rolled steel sheet. The intermediate quenching treatment and the tempering treatment are treatments aimed at reducing the yield ratio without reducing the tensile strength without impairing the effects of the present invention. Specifically, by performing an intermediate quenching treatment that is heated to an Ac1 transformation point or more and below an Ac3 transformation point, and then water-cooled, and further tempered, a sufficiently softened ferrite produced after the intermediate quenching and an intermediate The purpose is to form a two-phase structure with a hard phase that is partially transformed to austenite during quenching and becomes a hardened structure by cooling. As a result, the strength characteristics after tempering, the tensile strength is the same as the quenched structure, and the yield strength is low, so the effect of reducing the yield ratio required for steel materials applied to building structures can be expected. . Therefore, even if they are applied in the present invention, the effects of the present invention are not inhibited at all.
In addition, performing the quenching treatment generally performed on the steel sheet after the main rolling eliminates the structure mainly composed of fine-grained ferrite (partially, bainite) generated during hot rolling and lowers the tensile strength. It should not be applied.

  The steel pieces obtained by melting the steels A1 to A10 and B1 to B31 having the chemical components shown in Table 1 are compared with the steels of the present invention with test numbers 1 to 11 and test numbers 12 to 44 shown in Table 2. Examples Steel sheets having a thickness of 50 to 100 mm were produced according to the conditions of the respective examples.

In the production, hot rolling was performed so that the cumulative reduction ratio at a heating temperature of 1100 to 1250 ° C. and then 900 ° C. or less was 25 to 60%, and then immediately water-cooled to 150 ° C. or less by accelerated cooling equipment. Thereafter, a tempering treatment (T) or an intermediate quenching treatment (L) and a tempering treatment (T) were performed to obtain a test steel plate.
After that, the No. 14 tensile test piece specified in JIS Z 2201 was taken from the thickness ¼t from all the steel plates, and the tensile test specified in JIS Z 2241 was carried out. From the result of the test, the yield strength or 0.2% proof stress and tensile strength were determined. The obtained tensile strength was determined to be acceptable if it was 780 N / mm ² or more and 920 N / mm ² or less.

  On the other hand, with respect to gas cutting resistance, propane gas was used to cut the gas over a length of 600 mm at room temperature using the nozzle of crater # 5 from the surface of the steel plate, left for 24 hours, and then cut by penetration flaw detection. The total extension of the cutting cracks existing in the central part 500 mm was measured and used as an index of gas cutting cracking resistance as L (mm). Based on these numerical values, the case where L was a slight crack of 5 mm or less was determined to be acceptable, and the case where L was other than that was determined to be unacceptable.

Next, the HAZ toughness of the high heat input welding heat-affected zone was evaluated by a reproducible thermal cycle test so as not to cause a difference in plate thickness. That is, three heat cycle test pieces each having a size of 11 × 11 × 65 mm were sampled from a thickness of 1/4 t part of each steel plate, and the maximum heating temperature was 1350 ° C. (temperature rising time 60 s, holding 10 s), from 800 ° C. to 500 ° C. A thermal cycle of high heat input welding corresponding to a heat input of 80 kJ / mm with a cooling time of 450 s was given, and an impact test piece Charpy test piece (JIS No. 4 V notch) in accordance with JIS Z 2242 was processed. Thereafter, three Charpy impact tests were carried out at a test temperature of 0 ° C., and the average was taken as vE ₀ (J) and used as an indicator of toughness. In addition, in the value, 45J or more was determined to be acceptable, and less than that was regarded as unacceptable.

  In Table 1, the underlined chemical composition, the value of the formula (1), and the value of the formula (2) are outside the scope of the present invention, and the value of the A3 point is shown for reference. In addition, this Ac3 point is calculated by the formula of iron and steel 51st (1965) 11th 2006 page reported by Kunitake et al. In addition, the numerical value indicated by the underline in Table 2 indicates that the manufacturing condition is outside the scope of the present invention, or the characteristic does not satisfy the target value.

  In the test numbers 1 to 11 in Table 2, the steel components and welding conditions are all within the scope of the present invention. All of these steels have good tensile strength of the base metal, gas cut cracking resistance, and toughness of the heat-affected zone having a high heat input weld obtained by a heat cycle test. On the other hand, the examples after test number 12 are comparative examples in which either the chemical component or the production method deviates from the scope of the present invention, and will be described in detail below.

  Test No. 12 is an example in which the chemical component is within the scope of the present invention, but during hot rolling, the cumulative rolling reduction at 900 ° C. or less deviates from the scope of the present invention to 25%. As a result, the tensile strength does not satisfy the target value. Similarly, test number 13 is an example in which the chemical composition is within the present invention, but the start temperature of finish rolling is shifted to 950 ° C. in the manufacturing process. In this case, since the fine graining effect during hot rolling has been lost due to the high rolling temperature, the tensile strength is lower than the target value as in Test No. 12.

  Test numbers 14, 17, 21, 23, 28, 30, 32, 34, and 36 show examples in which the main elements of the present invention are deviated. Among these, test numbers 14, 17, 21, and 28 are examples in which C, Mn, Cu, and Nb are deviated from each other, and none of the tensile strengths satisfies the target. On the other hand, test numbers 23, 30, 32, and 36 are examples in which Ni, Ti, Al, and N deviate from each other. All of these have low toughness in the heat-affected zone of high heat input welding. Furthermore, the test number 34 is an example in which Mg deviates low. In this case, the gas cutting crack resistance does not satisfy the target, and the weld heat affected zone toughness also satisfies the target value, but it cannot be said to be sufficiently high.

  On the contrary, the test numbers 15, 16, 18, 19, 20, 22, 24, 25, 26, 27, 29, 31, 33, 35, and 37 are examples in which any component is added beyond the scope of the present invention. It is. Among these, test numbers 15, 16, 18, 25, 26, 27, and 29 are those in which C, Si, Mn, Cr, Mo, V, and Nb exceeded the scope of the present invention. Although these additions are effective for improving the strength, excessive addition inhibits the weld heat-affected zone toughness at the time of large heat input, and all the absorbed energy does not satisfy the target value. Furthermore, in test numbers 31, 33, 35 and 37, Ti, Al, Mg and N were added at a high level, respectively. Excessive addition of these elements forms coarse precipitates and inclusions and inhibits the toughness of the weld heat affected zone, so that the absorbed energy is low in all steels. Test numbers 19 and 20 are examples in which P and S deviate from each other. Since these are impurity elements, the absorbed energy does not satisfy the target in order to inhibit the toughness of the weld heat affected zone. Furthermore, test numbers 22 and 24 are examples in which Cu and Ni deviate highly. As a result, in both cases, the value of the formula (2) also exceeds the scope of the invention, and as a result, the gas cut crack resistance is not satisfactory, and at the same time, the toughness of the weld heat affected zone is low.

  Next, although individual components are within the scope of the present invention, examples in which the formulas (1) and (2) deviate singly and simultaneously are shown. That is, the test numbers 38 and 39 are examples in which the expression (1) does not satisfy the scope of the present invention, the test number 38 is an example that is not low, and the test number 39 is an example that is not high. As a result, none of the target values of the strength of the base material is satisfied. Test numbers 40 and 41 are examples in which equation (2) deviates from the scope of the present invention. Test number 40 is an example that deviates from a low level, and test number 41 is an example that deviates from a high level. As a result, in the test number 40, the toughness of the weld heat affected zone is low, and in the test number 41, the gas cut crack resistance does not satisfy the target. Furthermore, the test number 42 is an example in which the expressions (1) and (2) deviate simultaneously. As a result, the base metal strength is low and the toughness of the weld heat affected zone is also low. The test number 43 is an example in which the expression (1) is not high and the expression (2) is low. In this case, the base metal strength deviates from the target value and the toughness of the weld heat affected zone is low. Finally, the test number 44 is an example in which the expressions (1) and (2) deviate from each other at the same time. As a result, the base material strength is high and the gas cutting crack resistance is also reduced.

Claims (2)

% By mass
C: 0.03% or more, 0.10% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 1.00% or more, 3.00% or less,
Cu: 1.00% or more, 2.50% or less,
Ni: 1.00% or more, 2.50% or less,
Nb: 0.005% or more, 0.100% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.005% or more, 0.100% or less,
N: 0.0020% or more, 0.0100% or less,
P: 0.020% or less,
S: 0.010% or less,
Mg: 0.0005% or more, 0.0080% or less,
As a basic ingredient,
further,
Cr: 0.05% or more, 0.80% or less Mo: 0.05% or more, 0.60% or less V: 0.005% or more, containing 0.100% or less,
The balance is composed of Fe and inevitable impurities, and the formula (1) satisfies 0.45% or more and 0.65% or less, and the formula (2) satisfies the range of 1% or more and 2% or less simultaneously. A high-tensile strength steel plate with a tensile strength of 780 MPa that has excellent crack resistance during gas cutting and toughness during high heat input welding.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
(Cu + Ni)-(Cr + Mo + 5 * V + 10 * Nb) (2)
In addition, each element symbol shows the mass%. further,
Ca: 0.0001% or more, 0.0030% or less,
REM: 0.0001% or more, 0.0030% or less,
Zr: 0.005% or more, 0.100% or less,
The high strength steel sheet having a tensile strength of 780 MPa which is excellent in cracking resistance at the time of gas cutting and toughness at the time of high heat input welding.