JP5741016B2 - Method for producing high-strength thick steel plate with excellent weldability and base metal toughness - Google Patents

Description

  The present invention can be widely used as a welded structure such as a building, a bridge, a shipbuilding, an offshore structure, a line pipe, and a construction machine, has a low weld cracking susceptibility in a humid low temperature environment, and is excellent in low temperature toughness. The present invention relates to a method for producing a high strength toughness steel for welded structures having the above tensile strength.

  With the increase in size of welded structures, the demand for high-tensile steel is increasing for the purpose of reducing the weight of the steel material and reducing the cost by increasing the tension of the members. In particular, alloy-saving, high-strength, high-toughness steel has been developed by TMCP (mechanical heat treatment) technology that combines controlled rolling and accelerated cooling. For example, Patent Document 1 discloses a method for manufacturing a steel sheet having excellent strength, toughness, and weldability by a combination of controlled rolling and controlled cooling. Patent Document 2 discloses a method for producing high-strength steel with improved weldability by reducing the amount of C in the steel to the limit.

Japanese Patent Publication No. 01-27128 Japanese Patent Laid-Open No. 08-144019

  However, it has become difficult to cope with the recent severe welding environment and strict requirements for low temperature toughness. For example, in the technique described in Patent Document 1, although the ductile-brittle transition temperature of steel is improved by rolling at a low temperature such as austenite + ferrite two-phase region in order to improve low temperature toughness, in the Charpy impact test Separation occurs on the fracture surface and the absorbed energy is remarkably reduced, so that the requirement for high absorbed energy cannot be satisfied.

  On the other hand, the technique described in Patent Document 2 requires a large amount of alloying elements such as Cu and Ni to achieve high strength instead of lowering C for improving weldability. Becomes higher. Accordingly, an object of the present invention is to provide a high-strength steel excellent in weldability and low-temperature toughness without adding excessive alloying elements.

The inventors pay attention to Nb that exhibits the effect of expanding the non-recrystallized region and improving the hardenability as an alloy element, and intensively researches the behavior of the TMPC material when a large amount of Nb is added. I got the knowledge.
(1) A large number of Nb carbides crystallize in the slab as the amount of Nb added increases, suppressing the growth of austenite grains during slab heating.
(2) Under appropriate slab heating conditions, some Nb carbides dissolve and bring about an effect of expanding the non-recrystallized region. In particular, when 0.10% by mass or more of Nb is added, the expansion effect is remarkable, and an austenite recrystallization suppressing effect is recognized even at 950 ° C. or more.
(3) Further, solid solution Nb brings about a low temperature of bainite transformation during accelerated cooling, and a large amount of expensive alloy elements such as Ni and Mo are added, or a large amount of Cu is added for the purpose of precipitation strengthening. Even without this, the strength of the steel can be easily increased, and the weld crack sensitivity index Pcm value can be greatly reduced.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  1st invention is the mass%, C: 0.04-0.08%, Si: 0.01-0.5%, Mn: 1.0-2.0%, P: 0.02% or less , S: 0.01% or less, Al: 0.005 to 0.08%, Nb: 0.10 to 0.20%, Ti: 0.005 to 0.020%, and Cu: It contains one or more selected from 0.1 to 0.3%, Ni: 0.1 to 0.3%, B: 0.0005 to 0.004%, and the balance Fe and inevitable impurities After reheating the steel having a Pcm value of 0.15 or more and 0.18 or less calculated from the formula (1) to 1100 to 1300 ° C., hot rolling was started, and the steel in the temperature range of 950 to 1000 ° C. Rolling is completed at 900 ° C. or higher, with a cumulative reduction rate of 70% or higher, and accelerated at a cooling rate of 10 to 50 ° C./s from a temperature range of 800 ° C. or higher after the end of rolling Retirement was started a high-strength steel plate manufacturing method having excellent weldability and base material toughness, characterized by subsequently cooled to stop the cooling at 600 ° C. or lower.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5 × B (1)
Here, each component element means mass%.

  2nd invention is the mass%, C: 0.04-0.08%, Si: 0.01-0.5%, Mn: 1.0-2.0%, P: 0.02% or less , S: 0.01% or less, Al: 0.005 to 0.08%, Nb: 0.10 to 0.20%, Ti: 0.005 to 0.020%, and Ca: It contains one or two of 0.0005 to 0.004%, REM: 0.0005 to 0.02%, and consists of the balance Fe and inevitable impurities, and the Pcm value calculated by the formula (1) is 0.15. After reheating the steel of 0.18 or less to 1100 to 1300 ° C, hot rolling is started, and the cumulative reduction in the temperature range of 950 to 1000 ° C is set to 70% or more, and the rolling is finished at 900 ° C or more. Then, after the end of rolling, accelerated cooling is started at a cooling rate of 10 to 50 ° C./s from a temperature range of 800 ° C. or higher, and the temperature is 600 ° C. or lower. In stopping cooling is a manufacturing method of weldability and high strength thick steel plate superior in base material toughness, characterized by subsequent air cooling.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5 × B (1)
Here, each component element means mass%.

  3rd invention is 1st invention characterized by containing the 1 type (s) or 2 types of Ca: 0.0005-0.004% and REM: 0.0005-0.02% by the mass%. Is a method for producing a high-strength thick steel plate having excellent weldability and base material toughness.

  The fourth aspect of the invention is further characterized in that after accelerated cooling is stopped, reheating is performed immediately, and after reaching a temperature range of 600 to 700 ° C., air cooling is performed. This is a method for producing a high-strength thick steel plate excellent in weldability and base metal toughness.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the high strength steel with the tensile strength of 700 MPa class or more which has the excellent low temperature toughness which can be welded in a humid environment, and is very useful industrially.

  The present invention will be described in detail below.

1. Ingredient composition All percentages in the ingredients are mass%.

C: 0.04 to 0.08%
C contributes to an increase in strength by being supersaturated in a bainite structure obtained by accelerated cooling. In order to acquire this effect, it is necessary to contain 0.04% or more, but if it exceeds 0.08%, the risk of weld cracking in a low temperature and wet environment increases, so the C content is 0.8. The range is 04 to 0.08%.

Si: 0.01 to 0.5%
By containing 0.01% or more, Si is strengthened by solid solution regardless of transformation strengthening, and is effective in increasing the strength. However, if the content exceeds 0.5%, the toughness is remarkably lowered, so the Si content is in the range of 0.01 to 0.5%.

Mn: 1.0-2.0%
Mn acts as a hardenability improving element, and by containing 1.0% or more, formation of ferrite during accelerated cooling can be suppressed and a bainite structure can be obtained. On the other hand, if the content exceeds 2.0%, hardening of the central segregation part is promoted and the toughness of the base material is reduced, so the Mn content is set to a range of 1.0 to 2.0%.

P: 0.02% or less P is present as an inevitable impurity in steel. In particular, segregation at the central portion of the plate thickness and in the vicinity thereof is an element that causes an increase in island-like martensite. As a result, the Charpy absorbed energy at the central portion of the plate thickness is significantly reduced, so the P content is 0.02%. The following. Preferably, it is 0.008% or less.

S: 0.01% or less S is also present as an inevitable impurity in the steel. In particular, it exists as inclusions, lowers the cleanliness of steel and adversely affects toughness, so the S content is 0.01% or less. Preferably, it is 0.003% or less.

Al: 0.005 to 0.08%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained by containing 0.005% or more. However, if the content exceeds 0.08%, the cleanliness of the steel including the segregation part is lowered and the toughness is reduced. Is in the range of 0.005 to 0.08%.

Nb: 0.10 to 0.20%
The definition of the Nb amount is the basis of the present invention. Containing 0.10% or more of Nb has an effect of delaying recrystallization of austenite during rolling while suppressing austenite grain growth during slab heating. Thereby, the effect of controlled rolling can be obtained even in a temperature range of 950 ° C. or higher, which could not be controlled rolling conventionally. In general, if hot rolling is completed in an austenite single phase region, for example, a temperature range of 900 ° C. or higher, separation does not occur in the shape of the fracture surface of the Charpy specimen, but it is not always advantageous from the viewpoint of improving low temperature toughness. There wasn't. However, in the present invention, sufficient low-temperature toughness can be obtained even under hot rolling conditions at 900 ° C. or higher where separation does not occur on the fracture surface of the Charpy specimen due to the above-described effects. On the other hand, if Nb exceeds 0.20%, coarse Nb carbides crystallize in the casting stage, and what remains without being completely dissolved during slab heating causes deterioration of the cleanliness of the steel, and toughness decreases. , Nb amount is in the range of 0.10 to 0.20%.

Ti: 0.005-0.020%
Ti forms nitrides and is effective in reducing the amount of solute N in the steel. In addition, the precipitated TiN exerts a pinning effect and prevents the austenite grains from coarsening, thereby improving the toughness of the base metal. Contribute. In order to obtain a sufficient pinning effect, it is necessary to contain 0.005% or more, but if it exceeds 0.020%, carbides are formed, and the toughness is significantly deteriorated by precipitation hardening. The Ti content is in the range of 0.005 to 0.020%.

  The above is the basic component composition of the present invention. When further improving the strength, one or more selected from Cu, Ni, and B, and when improving toughness, one of Ca and REM or Two or more kinds can be added. In the case of improving both strength and toughness, it is possible to add one or more selected from Cu, Ni, and B, and one or more selected from Ca and REM.

Cu: 0.1 to 0.3%
When Cu is contained in an amount of 0.1% or more, it acts as a hardenability improving element and can replace a large amount of Mn addition. However, even if it contains more than 0.3%, the effect is saturated, and also causes Cu cracking on the steel sheet surface, so when containing Cu, the amount is 0.1-0.3% It is preferable to be in the range.

Ni: 0.1 to 0.3%
Ni is also a useful element because it acts as a hardenability improving element and does not cause toughness deterioration when added. In order to acquire this hardenability improvement effect, it is preferable to contain 0.1% or more, but since it is an expensive element, when it contains Ni, the amount is 0.1-0.3%. It is preferable to be in the range.

B: 0.0005 to 0.004%
B segregates at austenite grain boundaries and suppresses ferrite transformation, thereby contributing to prevention of strength reduction. In order to acquire this effect, it is preferable to contain 0.0005% or more, but even if it contains exceeding 0.004%, since the effect is saturated, when it contains B, the amount is 0.0005%. It is preferable to make it into the range of -0.004%.

Ca: 0.0005 to 0.004%
Ca is an element effective for controlling the form of sulfide in steel, and containing 0.0005% or more suppresses the generation of MnS harmful to toughness. However, if the content exceeds 0.004%, a CaO-CaS cluster is formed and the toughness is deteriorated. Therefore, when Ca is contained, the amount is in the range of 0.0005 to 0.004%. It is preferable.

REM: 0.005-0.02%
REM is also an element effective for controlling the form of sulfides in steel, and containing 0.005% or more suppresses the generation of MnS harmful to toughness. However, since it is an expensive element and the effect is saturated even if it contains more than 0.02%, when REM is contained, the amount should be in the range of 0.005 to 0.02%. preferable.

Pcm: 0.15-0.18
Pcm calculated by the equation (1) indicates the crack sensitivity at the time of welding. In particular, in high-tensile steel, Pcm needs to be 0.18 or less in order to prevent cracking without preheating even in a severe environment where the welding environment is a temperature of 10 ° C. and a humidity of 90%. On the other hand, in the case of a low alloy design in which Pcm is less than 0.15, it becomes difficult to ensure the strength of the base material, so Pcm is set to a range of 0.15 to 0.18.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5 × B (1)
Here, the element symbol M indicates the content (mass%) of the element M, and when the element M is not added, it is calculated as 0%.

  The balance other than the components described above consists of Fe and inevitable impurities.

2. Manufacturing conditions Steel having the above-described composition is melted by a conventional method using a melting means such as a converter or an electric furnace, and a steel material such as a slab is formed by a conventional method such as a continuous casting method or an ingot-bundling method. It is preferable to do. The melting method and the casting method are not limited to the methods described above. Moreover, any of a hot piece, a hot piece, and a cold piece can be used for charging the slab into the steel piece heating furnace by the continuous casting method. Thereafter, the product is rolled into a desired shape, and cooled and heated during or after rolling.

(1) Heating temperature: 1100-1300 ° C
When hot rolling is performed, the steel slab is austenitized and heated to 1100 ° C. or higher in order to bring a part of the added Nb into a solid solution state. On the other hand, when the steel slab is heated to a temperature exceeding 1300 ° C., even if pinning is performed with TiN, austenite grain growth is remarkable, and the base material toughness of the finally obtained steel sheet deteriorates. ˜1300 ° C.

(2) Rolling end temperature: 900 ° C. or higher In the present invention, when rolling to less than 900 ° C., Charpy absorbed energy is reduced due to the occurrence of separation, so the rolling end temperature is 900 ° C. or higher.

(3) Cumulative rolling reduction in the temperature range of 950 to 1000 ° C .: 70% or more In the present invention, the austenite non-recrystallization temperature range expands to a higher temperature side than before by increasing the Nb content as described above. Up to a temperature range of 950 to 1000 ° C., rolling in the austenite non-recrystallized region is possible. In order to obtain vTrs of −100 ° C. or less as particularly good low temperature toughness, the cumulative rolling reduction is 70% or more.

(4) Cooling start temperature of accelerated cooling: 800 ° C. or higher After hot rolling, if the temperature at which accelerated cooling is started is low, proeutectoid ferrite is generated from the austenite grain boundaries in the air cooling process, which causes the base material strength to decrease. Therefore, the temperature at which the accelerated cooling is started is set to 800 ° C. or higher.

(5) Cooling rate of accelerated cooling: 10 to 50 ° C./s
In order to obtain a high strength of 700 MPa or higher, the microstructure needs to be a bainite-based structure. For this reason, accelerated cooling is performed after hot rolling. When the cooling rate is less than 10 ° C./s, transformation starts at a relatively high temperature, and thus sufficient strength cannot be obtained.
On the other hand, in the case of a cooling rate exceeding 50 ° C./s, since the effect of increasing the strength by increasing the cooling rate is saturated, the cooling rate of accelerated cooling is set to a range of 10 to 50 ° C./s.

(6) Cooling stop temperature of accelerated cooling: 600 ° C. or less When the cooling stop temperature exceeds 600 ° C., the bainite transformation becomes insufficient, and then the air is cooled with untransformed austenite remaining, and the intermediate transformation structure having low strength. Therefore, the cooling stop temperature is set to 600 ° C. or less from the viewpoint of securing the strength. Preferably, it is 350 degreeC or more and 500 degrees C or less.

(7) Reheating temperature after stopping cooling: 600 to 700 ° C
By reheating immediately after accelerated cooling, fine Nb carbides can be precipitated in the transformed bainite, and the strength can be further increased. When the reheating temperature is less than 600 ° C., the effect of increasing the strength is not observed because the temperature does not reach the precipitation start temperature of Nb carbide. Therefore, the reheating temperature is preferably 600 ° C. or higher. On the other hand, if the reheating temperature exceeds 700 ° C., the bainite transformed by accelerated cooling is austenite again and sufficient strength cannot be obtained. Therefore, when reheating is performed after cooling is stopped, the reheating temperature is set to 600. It is preferable that the temperature is not lower than 700 ° C and not higher than 700 ° C.

  In the reheating, the heating rate of the steel sheet does not particularly affect the mechanical properties, but since it takes a long time to reach the target reheating temperature, the production efficiency deteriorates. / S or more is preferable. In particular, there is no need to set the temperature holding time. In the cooling process after reheating, Nb carbide is obtained regardless of the cooling rate, and thus cooling after reheating is not particularly defined, but basically it is preferably air cooling.

  Here, reheating immediately after stopping cooling refers to performing the above-described reheating treatment within 2 minutes after the steel plate temperature falls below 600 ° C. by accelerated cooling.

  In addition, let the temperature of an above-described steel plate be plate thickness direction average temperature. The plate thickness direction average temperature of the steel plate is determined by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, it can be calculated by calculating the temperature distribution in the plate thickness direction using a difference method. The cooling rate is a value calculated based on the average thickness direction temperature.

  The steel having the composition shown in Table 1 is melted in a converter and made into a slab (steel material) having a thickness of 250 mm by a continuous casting method. The thickness is 15 to 30 mm depending on the hot rolling conditions, accelerated cooling, and reheating conditions shown in Table 2. A steel plate was created. In Table 1, steel types I, J, K, and L are outside the scope of the present invention.

  The No. 4 tensile test piece described in JISZ2201 and the Charpy V-notch test piece described in JISZ2202 are collected from the central portion of the obtained steel plate in the thickness direction in the direction perpendicular to the rolling direction. : A Charpy impact test at −20 ° C. was performed to evaluate strength and toughness. Separately, a y-type weld crack test plate described in JISZ3158 was collected, and a test plate was prepared according to the procedure of JISZ3158. For this y-type welded test plate, the welding environment was set to three levels of 30 ° C. × 60% RH (RH is relative humidity, the same applies hereinafter), 20 ° C. × 80% RH, and 10 ° C. × 90% RH. In each of the constant humidity test tanks, the test welding was repeated three by three after preheating, 50 ° C preheating and 100 ° C preheating.

  The welding method was a coated arc welding method, and the welding rod was a low-hydrogen welding rod described in JISZ3212, which was used after being left in a constant temperature and humidity test tank set in each welding environment for 1 hour. The determination of cracks in the test weld zone was made by observing the cross section of the weld zone 5 in accordance with JISZ3158. And the temperature of the lowest preheating condition in which the cross-sectional crack rate of all five cross sections was 0% about the test plate of 3 each for preheating conditions was made into crack prevention preheating temperature. If no weld cracking occurs even without preheating, the crack preventing preheating temperature is displayed as 0 ° C.

  Table 3 summarizes the results of image analysis of the microstructure of the base metal and the results of the strength, toughness, and weldability investigations.

  Inventive example No. Nos. 1 to 8 have component compositions, rolling, accelerated cooling, and reheating conditions within the scope of the invention. Target tensile strength: 700 MPa or more, Charpy absorbed energy at −20 ° C. vE-20: 350 J or more, and ductility— Brittle fracture surface transition temperature (vTrs): satisfying −100 ° C. or less, crack prevention preheating temperature under any of the conditions of 30 ° C. × 60% RH, 20 ° C. × 80% RH, and 10 ° C. × 90% RH Was 0 ° C.

  On the other hand, No. which is a comparative example. No. 9 lacks the cumulative rolling reduction. No. 10 has a rolling end temperature of less than 900 ° C. No. 11 is that the cooling stop temperature exceeds 600 ° C. 12-No. No. 15 had a component composition outside the scope of the invention, and therefore one or more of strength, toughness, and weldability did not satisfy the target.

Claims (4)

In mass%, C: 0.04 to 0.08%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, P: 0.02% or less, S: 0.01 %: Al: 0.005-0.08%, Nb: 0.10-0.20%, Ti: 0.005-0.020%, and Cu: 0.1-0. 1% or more selected from 3%, Ni: 0.1 to 0.3%, B: 0.0005 to 0.004%, the balance consisting of Fe and unavoidable impurities (1) After reheating the steel having a Pcm value of 0.15 or more and 0.18 or less to 1100 to 1300 ° C., hot rolling was started, and the cumulative rolling reduction in the temperature range of 950 to 1000 ° C. was 70%. As described above, rolling is finished at 900 ° C. or higher, and accelerated cooling is started at a cooling rate of 10 to 50 ° C./s from a temperature range of 800 ° C. or higher after the rolling is finished. High strength thick steel plate manufacturing method having excellent weldability and base material toughness, characterized by subsequently cooled to stop the cooling at a temperature of 350 to 600 ° C..
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5 × B (1)
Here, each element symbol means content (mass%).
In addition, let the temperature of a steel plate be plate thickness direction average temperature, and let a cooling rate be a value calculated based on the said plate thickness direction average temperature. In mass%, C: 0.04 to 0.08%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, P: 0.02% or less, S: 0.01 %: Al: 0.005-0.08%, Nb: 0.10-0.20%, Ti: 0.005-0.020%, and Ca: 0.0005-0. 004%, REM: 0.0005 to 0.02% of one kind or two kinds, comprising the balance Fe and unavoidable impurities, the Pcm value calculated by the formula (1) is 0.15 or more and 0.18 or less A certain steel is reheated to 1100 to 1300 ° C., and then hot rolling is started, the cumulative rolling reduction in the temperature range of 950 to 1000 ° C. is set to 70% or more, the rolling is finished at 900 ° C. or more, and after the rolling is finished, 800 from a temperature range of not lower than ° C. begins the accelerated cooling of the cooling rate of 10 to 50 ° C. / s, cooling at a temperature of 350 to 600 ° C. High strength thick steel plate manufacturing method having excellent weldability and base material toughness, characterized by subsequently cooling down.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5 × B (1)
Here, each element symbol means content (mass%).
In addition, let the temperature of a steel plate be plate thickness direction average temperature, and let a cooling rate be a value calculated based on the said plate thickness direction average temperature.   2. The weldability and mother material according to claim 1, further comprising one or two of Ca: 0.0005 to 0.004% and REM: 0.0005 to 0.02% in mass%. A method for producing a high-strength thick steel plate with excellent material toughness.   Further, after accelerated cooling is stopped, reheating is performed immediately, and after reaching a temperature range of 600 to 700 ° C, air cooling is performed and excellent weldability and base material toughness according to any one of claims 1 to 3 A method for manufacturing high strength thick steel plates.