JP2013057125A - High strength/high toughness thick steel plate excellent in cutting-crack resistance and dwtt characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength/high toughness thick steel plate, which is excellent in preventing the generation of cracking on the cutting surface in cutting with a shearing work and excellent in DWTT characteristics.SOLUTION: The thick steel plate has a component composition comprising, by mass%, 0.03 to 0.12% C, ≤0.5% Si, 1.5 to 3.0% Mn, 0.01 to 0.08% Al, 0.01 to 0.08% Nb, 0.005 to 0.025% Ti and 0.001 to 0.01% N and further, one or two or more kinds of Cu, Ni, Cr, Mo, V, B, and if necessary, one or two or more kinds of Ca, REM, Zr, Mg and the balance Fe with inevitable impurities, and includes bainite or martensite in a microstructure, and the average particle diameter of cementite present in their structures is ≤0.5 μm.

Description

  The present invention relates to a high-strength and high-tough steel plate, and in particular, has excellent prevention of cracking and DWTT properties at the cut surface during shearing, and has a tensile strength of 650 MPa used for transportation of natural gas and crude oil. The present invention relates to a material suitable for the above-described high strength and high tough steel plate for line pipe.

  In recent years, line pipes used for transportation of natural gas and crude oil have been strengthened year by year in order to improve transport efficiency by increasing pressure and to improve local welding work efficiency by reducing wall thickness, and already have X100 grade lines based on API standards. Pipes have been put into practical use, and the demand for an X120 grade exceeding a tensile strength of 900 MPa is realized.

  Regarding such a method for producing a thick steel plate for welded steel pipes for high-strength line pipes, for example, in Patent Document 1, two-stage cooling is performed after hot rolling, and the second-stage cooling stop temperature is set to 300 ° C. or lower. A technique for achieving high strength is disclosed. Patent Document 2 discloses a technique for achieving high strength by Cu precipitation strengthening by accelerated cooling + aging heat treatment conditions.

JP 2003-293089 A Japanese Patent Laid-Open No. 08-311548

  However, when high strength is achieved by lowering the cooling stop temperature and introducing a hard bainite or martensite structure that generates a low-temperature transformation as described in Patent Document 1, shearing the steel plate as it is cooled to the required size When cutting at, cracks parallel to the plate surface (hereinafter referred to as cutting cracks) occur due to diffusible hydrogen remaining in the steel.

  On the other hand, as described in Patent Document 2, when heat treatment is performed after accelerated cooling, hydrogen in the steel is sufficiently diffused and cutting cracks can be suppressed, but cementite precipitates and coarsens in bainite or martensite during the heat treatment process, resulting in a decrease in toughness. In particular, the DWTT (Drop Weight Tear Test) characteristic for evaluating the brittle crack propagation stop characteristic deteriorates.

  An object of the present invention is to provide a method for producing a high-tensile thick steel plate having a thickness of 10 mm or more, which has improved toughness, particularly resistance to cut cracking without deteriorating brittle crack propagation stopping characteristics.

As a result of intensive research on cutting cracks in high-strength steel as cooled, the present inventors,
1. In order to prevent diffusible hydrogen in steel from being trapped at each trap site, heat treatment for dehydrogenation at least at 300 ° C. or more is necessary.

2. After cooling was stopped, reheating was started immediately, and when the steel sheet temperature was raised to 300 ° C. or higher, hydrogen diffusion was promoted. As a result, it was found that the amount of hydrogen remaining in the steel was below the crack initiation limit.

  In addition, regarding the coarsening behavior of cementite in bainite or martensite, which causes the deterioration of DWTT characteristics, cementite does not coarsen even when heated to a temperature range of 300 to 500 ° C. when the heating rate during reheating is increased. It was found to be suppressed to 0.5 μm or less, and the deterioration of DWTT characteristics was suppressed.

The present invention has been made on the basis of the above findings and further studies, that is, the present invention,
1. % By mass
C: 0.03-0.12%
Si: ≦ 0.5%
Mn: 1.5 to 3.0%
Al: 0.01 to 0.08%
Nb: 0.01 to 0.08%
Ti: 0.005-0.025%
N: 0.001 to 0.01%
Furthermore,
Cu: 0.01-2%
Ni: 0.01 to 3%
Cr: 0.01 to 1%
Mo: 0.01 to 1%
V: 0.01 to 0.1%
B: 0.0005 to 0.005%
In the composition comprising one or more of the following, the balance Fe and inevitable impurities, the microstructure contains bainite or martensite, and the average particle size of cementite present in these structures is 0.5 μm or less A high-strength, high-tough steel plate with excellent cut cracking resistance and DWTT characteristics.
2. In addition to the component composition,
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.02%
Zr: 0.0005 to 0.03%
Mg: 0.0005 to 0.01%
1. A component composition containing one or more of the above, wherein the microstructure includes bainite or martensite, and the average particle size of cementite present in these structures is 0.5 μm or less High-strength, high-tough steel plate with excellent cut cracking resistance and DWTT properties.

  According to the present invention, a steel plate suitable for use in a high-strength and high-toughness line pipe having a tensile strength for transportation of natural gas or crude oil of 650 MPa or more, which is excellent in prevention of cut cracking due to shear and DWTT properties, is extremely useful industrially It is.

  The present invention defines the slab heating temperature, hot rolling conditions, cooling conditions, and reheat treatment conditions in the component composition and manufacturing conditions of the steel sheet. The reason for limitation will be described below.

[Ingredient composition]
C: 0.03-0.12%
C contributes to an increase in strength by being supersaturated in a low temperature transformation structure. In order to obtain this effect, addition of 0.03% or more is necessary. However, if added over 0.12%, the hardness of the circumferential welded part of the pipe is remarkably increased and cold cracking is likely to occur. Therefore, the upper limit is made 0.12%.

Si: ≦ 0.5%
Since Si strengthens the solid solution irrespective of the transformation structure, it is effective in increasing the strength, so it is added. However, if added over 0.5%, the toughness is significantly reduced, so the upper limit is made 0.5%.

Mn: 1.5 to 3.0%
Mn acts as a hardenability improving element, and the effect can be obtained by addition of 1.5% or more. However, in the continuous casting process, the concentration in the central segregation part is remarkably increased. Therefore, the upper limit is set to 3.0%.

Al: 0.01 to 0.08%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with addition of 0.01% or more, but if added over 0.08%, the cleanliness in the steel is lowered and the toughness is deteriorated, so the upper limit is 0.08%. To do.

Nb: 0.01 to 0.08%
Nb has an effect of expanding the austenite non-recrystallized region at the time of hot rolling, and in order to make the non-recrystallized region up to 950 ° C., addition of 0.01% or more is necessary. On the other hand, if added over 0.08%, the toughness of HAZ is significantly impaired, so the upper limit is made 0.08%.

Ti: 0.005-0.025%
Ti forms nitrides and is effective in reducing the amount of solute N in the steel. Precipitated TiN prevents the austenite grains from coarsening by the pinning effect, thereby contributing to the improvement of the toughness of the base material and HAZ.

  Addition of 0.005% or more is necessary to obtain the necessary pinning effect, but if added over 0.025%, carbides are formed, and the toughness deteriorates remarkably due to precipitation hardening. Is 0.025%.

N: 0.001 to 0.01%
N is usually present as an inevitable impurity in steel, but is defined to form TiN that suppresses austenite coarsening by adding Ti as described above.

  In order to obtain the required pinning effect, it is necessary to be present in the steel in an amount of 0.001% or more, but when it exceeds 0.01%, it was heated to 1450 ° C or more in the vicinity of the weld, particularly in the vicinity of the melting line. The upper limit is set to 0.01% because TiN decomposes in HAZ and the negative effect of solute N is significant.

  In the present invention, one or more of Cu, Ni, Cr, Mo, V, and B are further added. Cu, Ni, Cr, Mo, V, and B all act as hardenability improving elements, and by adding one or more of these elements, high strength can be achieved in a thick steel plate having a thickness of 10 mm or more. It becomes possible.

Cu: 0.01-2%
When Cu is added in an amount of 0.01% or more, it contributes to improving the hardenability of the steel, and when added in an amount of 0.8% or more, precipitation strengthening is remarkable during aging heat treatment, thus preventing softening of the heat affected zone. Contribute. However, if 2% or more is added, toughness deteriorates, so the upper limit is 2%. If added, 0.01 to 2%.

Ni: 0.01 to 3%
Ni contributes to improving the hardenability of steel by adding 0.01% or more. In particular, even if added in a large amount, it does not cause toughness degradation, so it is an effective element for toughening but is an expensive element, and even if added over 3%, the increase in strength is saturated, so the upper limit is 3% And when adding, it is made into 0.01 to 3%.

Cr: 0.01 to 1%
Cr also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is 1%, and if added, 0.01 to 1%.

Mo: 0.01 to 1%
Mo also contributes to improving the hardenability of steel by adding 0.01% or more. On the other hand, if added over 1%, the toughness deteriorates, so the upper limit is 1%, and if added, 0.01 to 1%.

V: 0.01 to 0.1%
V forms precipitation strengthening by forming carbonitride, and contributes especially to the softening prevention of a weld heat affected zone. This effect can be obtained by addition of 0.01% or more, but if added over 0.1%, precipitation strengthening significantly reduces toughness, so the upper limit is 0.1%. 0.1%.

B: 0.0005 to 0.005%
B segregates at the austenite grain boundaries, and in particular, addition of 0.0005% or more suppresses ferrite transformation and contributes to prevention of strength reduction. However, since the effect is saturated even if added over 0.005%, the upper limit is 0.005%, and in the case of adding 0.0005% to 0.005%.

  Although the basic component composition of the present invention is as described above, when further improving toughness, one or more of Ca, REM, Zr, and Mg can be added.

  Ca, REM, Zr, and Mg form MnS, which is a non-metallic inclusion in the steel, or form oxides or nitrides, and mainly suppress austenite grain coarsening in the weld heat affected zone by a pinning effect.

Ca: 0.0005 to 0.01%
Ca is an element effective for controlling the form of sulfide in steel, and the addition of 0.0005% or more suppresses the generation of MnS harmful to toughness. However, if added over 0.01%, a CaO-CaS cluster is formed and the toughness deteriorates, so the upper limit is 0.01%, and when added, 0.0005-0.01%. To do.

REM: 0.0005 to 0.02%
REM is also an element effective for controlling the form of sulfide in steel, and by adding 0.0005% or more, the generation of MnS harmful to toughness is suppressed. However, since it is an expensive element and the effect is saturated even if added over 0.02%, the upper limit is 0.02%, and when added, the content is made 0.0005 to 0.02%.

Zr: 0.0005 to 0.03%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses the coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.03%, the cleanliness in the steel is remarkably lowered, and the toughness is lowered. The upper limit is 0.03%, and when added, the content is 0.0005 to 0.03%.

Mg: 0.0005 to 0.01%
Mg is produced as fine oxides in the steel during the steelmaking process, and in particular, has a pinning effect that suppresses the coarsening of austenite grains in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.01%, the cleanliness in the steel decreases, and the toughness decreases, The upper limit is 0.01%, and when added, 0.0005 to 0.01%.

[Production conditions]
The reason for limiting the manufacturing method will be described.
Slab heating temperature: 1000-1200 ° C
The minimum temperature for austenizing the slab is 1000 ° C. On the other hand, when the steel slab is heated to a temperature exceeding 1200 ° C., even if TiN pinning is performed, the austenite grain growth is remarkable and the base material toughness deteriorates, so the upper limit is set to 1200 ° C.

Hot rolling: Cumulative reduction at 950 ° C or lower ≥ 67%
950 ° C. or less is an austenite non-recrystallized region by Nb addition. By carrying out cumulative large pressure reduction in the temperature range, the austenite grains are expanded, particularly in the thickness direction, and the toughness of the bainite steel obtained by accelerated cooling is improved.

  If the reduction amount is less than 67%, the effect of refining is insufficient and the toughness of bainite steel cannot be improved, so the lower limit of the cumulative reduction amount is set to 67%. In addition, the suitable range for aiming at a remarkable toughness improvement is 75% or more.

Cooling start temperature of cooling ≧ 600 ° C.
In the air cooling process after hot rolling until cooling starts, pro-eutectoid ferrite is generated from the austenite grain boundaries, and most of the microstructure becomes ferrite and the strength of the base metal decreases. Is 600 ° C.

Cooling rate of cooling: 20-80 ° C / s
Cooling is performed at 20 ° C./s or more in order to suppress ferrite transformation that causes strength reduction. On the other hand, when the cooling rate exceeds 80 ° C./s, martensitic transformation occurs in the vicinity of the steel plate surface, and the strength of the steel plate increases, but the upper limit of the cooling rate is set to 80 ° C./s due to significant deterioration in toughness, particularly Charpy absorbed energy. .

Cooling stop temperature: ≦ 250 ° C
The cooling stop temperature for cooling is defined in order to increase the strength of the steel sheet by making it a bainite or martensite structure. When the cooling stop temperature exceeds 250 ° C., an upper bainite structure inferior in strength and toughness is obtained, so the cooling stop temperature is set to 250 ° C. or less.

Reheating treatment The reheating treatment is performed to prevent cutting cracks. In steel sheets that have been strengthened by low-temperature transformation by cooling, diffusible hydrogen may remain in the steel after air cooling, causing cutting cracks.

  If the time until reheating is long, hydrogen is less likely to diffuse due to a decrease in temperature during the air cooling process during the reheating process. Therefore, the reheating process should be performed immediately after stopping cooling, preferably within 300 seconds, more preferably within 100 seconds. To do. The reheating method may be either furnace heating or induction heating, and is not particularly defined in the present invention.

Heating rate during reheating: ≧ 5 ° C / s
By immediately reheating the steel whose cooling has stopped, the supersaturated solid solution of carbon in the bainite or martensite transformed by cooling is precipitated homogeneously and finely as cementite.

  And it aggregates and coarsens from the temperature range exceeding 300 degreeC, and has a bad influence especially on DWTT characteristic.

  However, as a result of studies by the inventors, it has been found that the faster the temperature rise rate during heating, the more the aggregation process is suppressed and the cementite is not coarsened.

  And since the cementite was able to maintain the fine state almost immediately after precipitation by setting a temperature increase rate to 5 degrees C / s or more, a temperature increase rate shall be 5 degrees C / s or more.

Reheating temperature: 300 ° C to 500 ° C
When the reheating temperature is less than 300 ° C., hydrogen does not diffuse sufficiently in the steel and cutting cracks cannot be prevented. Therefore, the reheating temperature is set to 300 ° C. or higher. On the other hand, if the temperature is higher than 500 ° C., the upper limit is set to 500 ° C. because the strength decreases significantly due to softening by tempering.

[Microstructure]
In order to obtain a high strength containing bainite or martensite and having an average particle size of cementite present in these bainite or martensite of 0.5 μm or less and a tensile strength of 650 MPa or more, bainite or martensite is added to the microstructure. Need to include.

  Any area ratio of bainite, martensite, bainite and martensite is desirably 60% or more, and it is allowed to contain less than 30% of ferrite or pearlite as other structures.

  Further, when cementite in these structures becomes coarse, the DWTT characteristics deteriorate, so the average particle diameter of cementite is 0.5 μm or less. Preferably it is 0.2 micrometer or less.

  In the present invention, the steel making method is not particularly limited. From the economical point of view, it is desirable to cast steel pieces by a steelmaking process by a converter method and a continuous casting process.

Steel sheets having chemical compositions shown in Table 1 were used under the conditions of hot rolling / cooling and reheating shown in Table 2
~ K was made. The reheating treatment was performed using an induction heating type heating device installed on the same line as the cooling (water cooling) facility.

First, each steel plate was cut at 20 locations with a shearing machine, and the cut end faces were examined by magnetic particle flaw detection to determine the number of cut end faces at which cut cracks were observed. Even when a plurality of cracks could be confirmed in one end face, the number of occurrences of cut cracks was set to 1 because there was only one end face. The number of occurrences of cut cracks was defined as 0 when no cut cracks were observed at all cut locations.
Next, a full thickness tensile test piece and a DWTT test piece in accordance with API-5L were collected from each steel plate, and a JIS Z2202 (1980) V-notch Charpy impact test piece was taken from the center of the plate thickness, and a steel plate tensile test was performed. A DWTT test and a Charpy impact test were performed to evaluate strength and toughness.

  A sample for observing the microstructure was taken in parallel with the cross section in the rolling direction, and after mirror polishing, nitric alcohol etching was performed and the microstructure was observed with an optical microscope.

  Next, after mirror polishing again, speed etching treatment was performed, and cementite was observed with a scanning electron microscope, and the equivalent circle diameters of the cementite particles observed in 10 random fields were averaged and calculated.

  In addition, reproducible thermal cycle test specimens were collected from each steel plate and given a thermal cycle with a maximum heating temperature of 1400 ° C. simulating the coarse-grained heat-affected zone in the vicinity of the melting line. An impact test piece was collected and a Charpy impact test was performed to evaluate the toughness of the weld heat affected zone.

  Furthermore, a y-type weld cracking test was performed in accordance with JIS Z 3158 (1993). The test atmosphere was a temperature of 30 ° C. and a humidity of 80%, and a test bead was welded to a test body at a preheating temperature of 100 ° C. using a hand-welded rod for 100 kgf class high-strength steel left in the atmosphere for 1 hour. Weld crack susceptibility was evaluated by the cross-sectional crack rate of the cross section orthogonal to the test beads.

  Table 3 summarizes the results of the shear cutting test on the steel sheet, the results of the strength / toughness investigation of the base metal, the results of the toughness investigation of the weld heat affected zone by the reproduced thermal cycle, and the evaluation results of the weld crack sensitivity.

The scope of the present invention is that there is no crack in the shear cutting test, the base material has a tensile strength of 650 MPa or more, a yield strength of 550 MPa or more, a base material toughness vE- ₃₀ 200 J or more, DWTT SA- ₃₀ 85% or more, and the toughness of the weld heat affected zone vE- ₃₀ 100J or more, y-type weld crack test cross section crack rate (%) 0% was within the scope of the present invention.

  Inventive Examples 1 to 8, 17 and 18 in which the chemical composition and rolling / cooling / reheating conditions were within the scope of the present invention exhibited no high strength and high toughness without causing cracking.

  On the other hand, although the chemical composition is within the scope of the present invention (steel type C), Comparative Examples 9 to 12 whose production conditions are outside the scope of the present invention are inferior in cutting resistance and the like as compared with the inventive examples.

  In Comparative Example 9 in which the accelerated cooling stop temperature after hot rolling deviated from the upper limit, the strength was significantly reduced as compared with Invention Example 3.

  Further, Comparative Example 10 in which the heating rate during reheating after cooling was lower than the lower limit, the base material strength and the weld heat affected zone toughness were equivalent to those of the present invention example, but the cementite particle size was 0.5 μm. Excessive coarsening occurred, and the DWTT characteristics were remarkably lowered.

  In Comparative Example 11 in which the reheating temperature was lower than the lower limit, hydrogen cracks were not sufficiently diffused, so that a cut crack occurred. In Comparative Example 12 in which the reheating temperature exceeded the upper limit, the base material strength decreased.

  In Comparative Example 13 with steel type G in which the C content of the steel exceeded the upper limit, the base material and the weld heat affected zone toughness were low, and cracks occurred in the weld crack test. In Comparative Example 14 with steel type H in which the amount of Mn exceeded the upper limit, cracks also occurred in the weld cracking test.

  Moreover, the comparative example 15 by the steel type J in which the Nb amount exceeded the upper limit and the comparative example 16 by the steel type K in which the Ti amount exceeded the upper limit both have extremely low toughness of the base material and the weld heat affected zone.

Claims (2)

% By mass
C: 0.03-0.12%
Si: ≦ 0.5%
Mn: 1.5 to 3.0%
Al: 0.01 to 0.08%
Nb: 0.01 to 0.08%
Ti: 0.005-0.025%
N: 0.001 to 0.01%
Furthermore,
Cu: 0.01-2%
Ni: 0.01 to 3%
Cr: 0.01 to 1%
Mo: 0.01 to 1%
V: 0.01 to 0.1%
B: 0.0005 to 0.005%
In the composition comprising one or more of the following, the balance Fe and inevitable impurities, the microstructure contains bainite or martensite, and the average particle size of cementite present in these structures is 0.5 μm or less A high-strength, high-tough steel plate with excellent cut cracking resistance and DWTT characteristics. In addition to the component composition,
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.02%
Zr: 0.0005 to 0.03%
Mg: 0.0005 to 0.01%
A component composition containing one or more of the above, wherein the microstructure includes bainite or martensite, and the average particle size of cementite present in these structures is 0.5 μm or less. 1. A high-strength, high-tough steel plate excellent in cut crack resistance and DWTT properties according to 1.