JP5369639B2 - High strength steel material excellent in welding heat-affected zone toughness and HIC resistance and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel to be used for a steel pipe for a line pipe, which has strength in an X80 grade or more in the API standards, and also has toughness of a weld-heat affected zone without impairing its HIC (Hydrogen Induced Crack) resistance. <P>SOLUTION: The high strength steel has a composition containing, by mass, 0.02 to 0.08% C, 0.01 to 0.5% Si, 0.5 to 2.0% Mn, 0.0005 to 0.003% Ca, 0.01 to 0.03% Ti, 0.04 to 0.05% Nb, &le;0.07% Al, &le;0.5% Cu, &le;0.5% Ni, &le;0.5% Cr and &le;0.007% N, and the balance Fe with inevitable impurities. In the steel sheet, carbon equivalent Ceq is &lt;0.41, Ti is comprised by a value obtained by adding 0.003 mass% to a value 3.4 times the N content, also, Ti in precipitates with a size of &lt;20 nm is comprised by &ge;10 mass ppm to the whole of the steel, and the content of Nb is &ge;140 mass ppm to the whole of the steel. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-strength steel material that is used for a steel pipe for a line pipe and the like and that involves welding during manufacture and construction.

  In addition to strength, toughness and weldability, line pipes used to transport crude oil and natural gas containing hydrogen sulfide have resistance to hydrogen-induced cracking resistance (hereinafter referred to as HIC resistance) and stress corrosion cracking resistance (hereinafter referred to as Various characteristics such as SCC resistance) are required. In particular, in recent years, with the development of international large-scale line pipe business, the need for high-strength steel sheets for line pipes exceeding X65 grade such as American Petroleum Institute (hereinafter referred to as API) standard X80 grade is increasing. .

  When increasing the strength of this type of steel that involves welding during manufacturing and construction, it is important not to deteriorate the toughness, particularly the toughness of the heat affected zone (hereinafter referred to as HAZ). Because there is a constraint condition that this strength and HAZ toughness are compatible, conventionally, material design that achieves the required strength mainly by solid solution strengthening, grain refinement, and multiphase structure has been performed, and precipitation strengthening is toughness. The current situation is that it is not actively used because it is likely to damage

  On the other hand, there is a problem in that the HIC resistance is deteriorated when a higher alloy addition amount is required to achieve the strength of API standard X65 grade or higher without impairing the HAZ toughness with the prior art. In HIC of steel, hydrogen adsorbed on the steel surface due to corrosion reaction penetrates into the steel sheet as atomic hydrogen, and diffuses and accumulates around non-metallic inclusions such as MnS and hard second phase structure in the steel. The internal pressure is considered to cause cracks. As a method to prevent HIC, by adding appropriate amount of Ca and Ce to the amount of S, the formation of acicular MnS is suppressed, and the shape is changed to finely dispersed spherical inclusions with small stress concentration and cracking There is known a method for suppressing the propagation of the occurrence of this (for example, Patent Document 1). For high strength steel of X80 grade with excellent HIC resistance, while controlling the form of inclusions by adding Ca with low S, it suppresses center segregation as low C, low Mn, and the accompanying strength reduction is Cr, Ni The method of supplementing by adding such as and accelerated cooling is known (Patent Documents 2 to 4). In addition, high strength steels that improve SCC resistance and HIC resistance by making the microstructure a ferrite single phase structure and utilize precipitation precipitation of carbide obtained by adding a large amount of Mo or Ti are also disclosed (for example, Patent Document 5). A high-strength steel sheet for line pipes with excellent HIC resistance and HAZ toughness characterized by the fact that the metal structure is substantially a two-phase structure of ferrite and bainite and precipitates containing Ti and Mo are dispersed. A manufacturing method is also disclosed (for example, Patent Documents 6 and 7).

  However, the methods for improving the HIC resistance described in Patent Documents 1 to 4 are all about the center segregation part. High strength steel sheets exceeding the X65 grade, such as API X80 grade, are often manufactured by accelerated cooling or direct quenching, so the surface of the steel sheet with a high cooling rate is hardened compared to the inside, and hydrogen-induced cracking occurs from the vicinity of the surface. . Moreover, the microstructure of these high-strength steel sheets obtained by accelerated cooling is a relatively high cracking susceptibility of bainite or acicular ferrite not only to the surface but also to the inside, and measures against HIC at the center segregation part were taken. Even in this case, it is difficult for high strength steel of about API X80 grade to eliminate HIC starting from sulfide or oxide inclusions.

  According to our research, in order to stably secure the strength of X80 grade by the methods of Patent Documents 2 to 4, carbon equivalent Ceq (Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5) must be set to 0.41 or higher. However, if this carbon equivalent Ceq is set to 0.41 or more, the maximum hardness of the surface layer and circumferential welds will be high and it will not be possible to stably secure HAZ toughness, so this method balances steel sheet strength and HAZ toughness. It is not considered a possible technology.

  The fine TiC used in the high-strength steel described in Patent Document 5 is effective for precipitation strengthening, but it tends to coarsen after melting and reprecipitation at the time of welding, so that there is a problem that HAZ toughness is likely to deteriorate.

In Patent Documents 6 and 7, which focus on Ti for improving HAZ toughness, Ti is defined in the range of 50 to 250 mass ppm, but N that preferentially forms nitride with Ti is in the range of 40 to 60 mass ppm. Therefore, it can be predicted that 140 to 200 mass ppm of Ti added will precipitate as TiN. Therefore, with the technique described in the same document, even though it can be expected to improve HAZ toughness using the refinement of prior austenite by fine dispersion of TiN, composite carbides containing Ti, Mo, Nb, and V can be precipitated. It is expected that only solid solution Ti is difficult to secure stably.
JP 54-110119 A JP-A-5-9575 JP-A-5-271766 JP-A-7-73536 JP 7-70697 A JP 2005-60836 A JP 2005-60837 A

  The object of the present invention is to solve such problems of the prior art and use it for steel pipes for line pipes, etc. that have both the strength of API standard X80 grade or higher and the toughness of weld heat affected zone without impairing HIC resistance. It is to provide a high-strength steel material that can be manufactured.

  As a result of intensive research on material design that can achieve both steel strength and HAZ toughness, the present inventor has found that the Nb content that causes precipitates coarsened by HAZ and the Ceq that impairs HAZ toughness are not increased. In addition, we have found a unique method for effectively realizing precipitation strengthening.

Specifically, (1) component design to ensure solute Ti in the slab heating stage, and after rolling, reheat treatment, contained as if pulled by the precipitation of Ti-based precipitates The amount of Nb precipitates increases dramatically (hereinafter referred to as the “Ti precipitation traction effect”), and (2) Nb is richer than Ti and 20 nm in size during the precipitation process. It was found that the strength can be effectively improved without increasing the Nb content and the carbon equivalent Ceq if a certain amount or more of fine composite precipitates is secured.
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] C: 0.02 to 0.08 mass%, Si: 0.01 to 0.5 mass%, Mn: 0.5 to 2.0 mass%, Ca: 0.0005 to 0.003 mass%, Ti: 0.01 to 0.03 mass%, Nb: 0.04 to 0.05 mass% , Al: 0.07 mass% or less, Cu: 0.5 mass% or less, Ni: 0.5 mass% or less, Cr: 0.5 mass% or less, N: 0.007 mass% or less, the balance being Fe and inevitable impurities steel material, carbon equivalent Ceq is less than 0.41, Ti contains 3.4 times N or more than 0.003 mass%, and Ti in precipitates less than 20 nm in size is 10 mass ppm or more, and Nb is the whole steel. High strength steel with excellent weld heat affected zone toughness and HIC resistance, characterized by being 140 mass ppm or higher.
[2] In the above [1], Mo in the precipitate further including any one or more of Mo: 0.05 to 0.5 mass%, V: 0.005 to 0.1 mass% is less than 20 nm in size relative to the whole steel material. The high-strength steel material excellent in weld heat-affected zone toughness and HIC resistance according to claim 1, characterized in that it is 50 mass ppm or more and / or V is 5 mass ppm or more with respect to the whole steel material.
[3] A steel containing the chemical component described in [1] or [2] above is hot-rolled at a heating temperature: 1050 to 1250 ° C. and a rolling end temperature: Ar 3 temperature or higher, and then a cooling rate: Accelerated cooling to 300-600 ° C at 5 ° C / sec or more, left to cool for 0.5-3 minutes at the cooling stop temperature, and then the highest temperature: 600-700 ° C at a heating rate of 0.5-2.0 ° C / sec A method for producing high-strength steel with excellent weld heat-affected zone toughness and HIC resistance, which is characterized by cooling immediately after reheating up to a cooling rate higher than air cooling.
In this specification, “%” and “ppm” indicating the components of steel are mass% and mass ppm, respectively. The Ar3 temperature is the ferrite transformation start temperature during cooling, and can be determined from Ar3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo.

  By applying the present invention, it is much easier to achieve both the strength of API standard X65 grade or higher and the toughness of the weld heat affected zone with a low alloy component that does not deteriorate the HIC resistance.

  The present invention uses a component design that ensures solid solution Ti in the slab heating stage and the precipitation traction effect of Ti during reheating, so that a certain amount of fine precipitates with Nb richer than Ti and less than 20 nm in size are obtained. Precipitation as described above is that strength can be increased without increasing the Nb content and the carbon equivalent Ceq, which are concerned about HAZ toughness and HIC resistance.

1) First, chemical components of the high-strength steel material used in the present invention will be described.
C: Set to 0.02 to 0.08 mass%.
C is an element that contributes to precipitation strengthening as a carbide, but if it is less than 0.02 mass%, sufficient strength cannot be secured, and if it exceeds 0.08 mass%, the toughness and HIC resistance deteriorate, so the C content is within this range. preferable. More preferably, it is 0.03-0.06 mass%.

Si: 0.01 to 0.5 mass%.
Si is contained for deoxidation, but if it is less than 0.01 mass%, the deoxidation effect is not sufficient, and if it exceeds 0.5 mass%, the toughness and weldability are deteriorated, so the Si content is preferably in this range. More preferably, it is 0.01-0.3 mass%.

Mn: 0.5 to 2.0 mass%.
Mn is contained for strength and toughness, but if it is less than 0.5 mass%, the effect is not sufficient, and if it exceeds 2.0 mass%, the weldability and HIC resistance deteriorate, so the Mn content is preferably within this range. More preferably, it is 0.5 to 1.5 mass%.

Al: 0.07 mass% or less.
Al is contained as a deoxidizer, but if it exceeds 0.07 mass%, the cleanliness of the steel is lowered and the HIC resistance is deteriorated, so the Al content is preferably within this range. More preferably, it is 0.01-0.07 mass%.

Cu: 0.5 mass% or less.
Cu is an element effective in improving toughness and increasing strength. In order to acquire the effect, it is preferable to contain 0.1 mass% or more, but when it exceeds 0.5 mass%, weldability will deteriorate, and when it contains, Cu content is prescribed | regulated to this range.

Ni: 0.5 mass% or less.
Ni is an element effective in improving toughness and increasing strength. In order to acquire the effect, it is preferable to contain 0.1 mass% or more, but when it exceeds 0.5 mass%, the HIC resistance is deteriorated.

Cr: 0.5 mass% or less.
Cr, like Mn, is an effective element for increasing the strength. In order to acquire the effect, it is preferable to contain 0.1 mass% or more, but if it exceeds 0.5 mass%, the weldability tends to deteriorate, so when it is contained, the Cr content is specified within this range.

Ca: 0.0005 to 0.003 mass%.
As already mentioned, Ca is an element effective for improving HIC resistance by controlling the form of sulfide inclusions, but if it is less than 0.0005 mass%, the effect is not sufficient, and if it exceeds 0.003 mass%, the cleanliness of the steel The HIC resistance may be deteriorated due to a decrease in the thickness, so this range is specified.

N: 0.007 mass% or less.
If N exceeds 0.007 mass%, solid solution N that cannot be fixed with Ti even if the Ti content is increased to 0.03 mass% adversely affects toughness. Further, from the viewpoint of securing solid solution Ti in the slab heating stage in order to produce fine composite carbide containing Ti effective for precipitation strengthening, the smaller the N content, the better.

Nb: 0.04 to 0.05 mass%.
Nb improves toughness by making the structure finer, but at the same time, in the present invention, it functions as a carbide-forming element having a size of less than 20 nm that is particularly effective for increasing the strength. However, if it is less than 0.04 mass%, the effect of precipitation strengthening tends to be insufficient. On the other hand, if it exceeds 0.05 mass%, it coarsely precipitates during slab heating and causes deterioration of the base metal toughness, and also coarsely precipitates in the HAZ and causes deterioration of HAZ toughness. Therefore, Nb content is prescribed | regulated to this range. A composite carbide may be formed together with Ti, Mo, and V described later.

Ti: 0.01 to 0.03 mass%.
In order to use the Ti precipitation traction effect on Nb as described above, the effect is insufficient if it is less than 0.01 mass%, and if it exceeds 0.03 mass%, it coarsely precipitates in HAZ and causes deterioration of HAZ toughness. The amount is preferably within this range. More preferably, it is 0.015-0.025 mass%.

Mo: 0.05 to 0.2 mass% is preferable.
Mo forms a fine composite carbide with Ti, Nb, etc., and contributes to an increase in strength. In addition, Mo carbide has a growth rate slower than that of Ti or Nb carbide, and thus has an action of suppressing the coarsening of the composite carbide. Containing 0.05 mass% or more effectively works for precipitation strengthening while suppressing pearlite transformation during cooling after hot rolling. However, if the content exceeds 0.5 mass%, a hard phase such as martensite is formed and the HIC resistance tends to deteriorate. Furthermore, from the viewpoint of HAZ toughness, 0.2 mass% or less is preferable, and when it is contained, the Mo content is specified within this range.

V: 0.005 to 0.1 mass% is preferable.
V, like Mo, forms fine composite carbides with Ti and Nb and contributes to an increase in strength. Further, the carbide of V has an action of suppressing the coarsening of the composite carbide, like the carbide of Mo. However, if 0.005 mass%, the effect cannot be obtained, and if it exceeds 0.1 mass%, the HAZ toughness tends to be deteriorated.

  In addition, Ti, Nb, Mo, and V are all in a total molar concentration within the molar concentration of C so as to form precipitates of MC type (equal molar ratio of metal element to C). Contained. At this time, it is sufficient that at least one of Mo and V is contained.

  The balance other than the above consists of Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, particularly toughness, so it is preferably reduced to 0.003 mass% or less. Moreover, in this invention, you may contain P in 0.01 mass% or less and S in the range of 0.002 mass% or less as a trace element which does not impair the effect of this invention.

  In addition to the above, Ti contains at least 3.4 times the N content plus 0.003 mass%. Ti is an element that tends to form nitrides, so if it contains less than 3.4 times the N content, solid solution Ti will form nitrides in the slab heating stage, and precipitate carbides in the subsequent process. Cannot be secured. Therefore, even when the entire amount of N is nitrided in the slab heating stage, in order to precipitate Ti so as to contain Ti of 10 mass ppm or more with only precipitates having a size of less than 20 nm, solute Ti must be at least N having a N content of 3.4. Since it is preferable to secure a value obtained by adding 30 mass ppm (0.003 mass%) to the double, the lower limit is defined in this way.

2) The carbon equivalent Ceq is less than 0.41.
As already mentioned, the carbon equivalent Ceq is an important factor for increasing the strength, but if it is 0.41 or higher, the maximum hardness of the surface layer and circumferential welds will be high, and it will not be possible to stably secure toughness. To do.
The carbon equivalent Ceq is calculated by C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (each element is mass%).

  3) Next, the precipitate and structure of the high-strength steel material of the present invention will be described.

  Ti in precipitates with a size of less than 20 nm should be contained in an amount of 10 mass ppm or more with respect to the entire steel material. Even if such fine precipitates effective for precipitation strengthening are formed, if the amount is less than 10 mass ppm with respect to the whole steel material with a Ti content, it is difficult to obtain a sufficient effect, so it is specified in this way.

  In addition, Nb in precipitates with a size of less than 20 nm is 140 mass ppm or more with respect to the entire steel material. Generally, the precipitation strengthening amount is determined by the size of the precipitated particles and the number density thereof, and it is known that the smaller the size and the higher the number density, the more effective the precipitation strengthening amount. Therefore, if it is difficult to realize the precipitation strengthening amount of a certain amount or more with only Ti of about 10 mass ppm for the whole steel material, the Ti precipitates must be formed very finely and with high density. There is. If the precipitate to be used is formed into a composite precipitate and Nb can be used to form the precipitate, it is much easier to increase the number density of the precipitate and realize a precipitation strengthening amount of a certain amount or more. According to the inventor's research, it was found that the Nb content in the precipitate having an effective size of less than 20 nm at that time is preferably 140 mass ppm or more with respect to the entire steel material. To do.

  Furthermore, it is preferable that the precipitates having a size of less than 20 nm contain 50 mass ppm or more of Mo with respect to the whole steel material and / or 5 mass ppm or more of V with respect to the whole steel material. If not only Ti and Nb but also Mo and V can be used for the formation of the composite precipitate, it becomes easier to increase the number density and realize a precipitation strengthening amount of a certain amount or more. In addition, since carbides of Mo and V have a slower growth rate than Ti and Nb carbides, they have an action of suppressing the coarsening of the composite precipitate. Therefore, Mo and V are effective in increasing the number density of the composite precipitate particles and improving the precipitation strengthening amount. Even when the composite precipitate is once dissolved in HAZ, it is coarse during reprecipitation. It also has an effect of suppressing the deterioration of toughness. According to the inventor's research, Mo in precipitates having an effective size of less than 20 nm is preferably 50 mass ppm or more with respect to the entire steel material, and V is preferably 5 mass ppm or more with respect to the entire steel material. Stipulate in this way.

  The content of Ti, Nb, Mo, V contained in the precipitate having a size of less than 20 nm can be confirmed by the following method.

  After the sample is electrolyzed in a predetermined amount in the electrolytic solution, the sample piece is taken out of the electrolytic solution and immersed in a solution having dispersibility. Next, the precipitate contained in the solution is filtered using a filter having a pore diameter of 20 nm. The precipitate that has passed through the filter having a pore diameter of 20 nm together with the filtrate has a size of less than 20 nm. Next, the filtrate after filtration is analyzed by appropriately selecting from inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, etc. Obtain the content of Ti, Nb, Mo, and V for the entire steel.

Examples of a method for setting the Ti content of precipitates having a size of less than 20 nm to 10 mass ppm or more with respect to the entire steel material include the following methods.
First, the steel melted with the above components is heated at a slab heating temperature in the range of 1050 to 1250 ° C to form a solution, and then rolled in a temperature range above the Ar3 temperature (austenite single-phase temperature range) up to a predetermined thickness. And accelerated cooling from 300 to 600 ° C. at a cooling rate of 5 ° C./sec or more from the same temperature range to the bainite transformation range. This suppresses the growth of coarse precipitates from the high temperature range when it is gradually cooled. Next, during transportation to the reheating equipment, after cooling for 0.5 to 3 minutes at the cooling stop temperature, reheating to a maximum temperature of 600 to 700 ° C at a heating rate of 0.5 to 2.0 ° C / sec, immediately ( In other words, cooling is performed at a cooling rate equal to or higher than the cooling rate during air cooling (without providing a holding time). By this reheating, the metal structure becomes a two-phase structure of ferrite and bainite, and composite precipitates having a size less than 20 nm, which is Nb richer than Ti, can be dispersed and precipitated even in a molar ratio. Although the essence of this Ti precipitation traction effect has not been fully clarified, the thermodynamic driving force by which Ti forms carbides (or carbonitrides) is larger than that of Nb, Mo, and V. It is presumed that because it functions as a precipitation nucleus for composite precipitates, it eliminates the need for energy barriers (nucleation barriers) that must be overcome when Nb, Mo, and V individually precipitate, thereby promoting composite precipitation. The
In addition, rolling in the temperature range of the austenite single phase means that the temperature at the end of rolling is higher than the Ar3 temperature (ferrite transformation start temperature), Ar3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo Can be obtained from Here, the unit of Ar3 temperature is ° C., and the concentration of each component is mass%. The bainite transformation range is 300-600 ° C.
When the heating rate during reheating is less than 0.5 ° C / sec, it takes a long time to reach the target temperature, so that the production efficiency is poor, and pearlite transformation occurs and competes with the precipitation of the composite precipitate. Therefore, the required amount of fine composite precipitates cannot be ensured. In addition, it is difficult to secure the necessary amount of fine composite precipitates even when the rate of temperature rise exceeds 2.0 ° C./sec. This is presumably because the precipitation rate of fine precipitates cannot follow this rate of temperature increase.
It is not necessary to set the holding time at the reheating temperature from the viewpoint of suppressing strength reduction due to coarsening of fine precipitates having a size of less than 20 nm. As the reheating device, it is preferable to use a gas combustion furnace or an induction heating furnace capable of rapid heating of the steel sheet.

Next, examples of the present invention will be described below.

  The test materials (signs A to K) having the components shown in Table 1 were dissolved to form slabs, which were processed under the rolling conditions and heat treatment conditions shown in FIG. First, slab heating was performed at 1150 ° C. for 2 hours. After that, the slab was rolled 75% at 950 ° C with a hot rolling mill and finished at 860 ° C to a plate thickness of about 20mm. Then, the cooling rate was 30 ° C / sec using a water-cooled accelerated cooling facility from 790 ° C. And accelerated cooling to 500 ° C. Thereafter, a part of the test material was air-cooled as it was (heat treatment condition 1: comparative example). The remaining test pieces were left to stand at 500 ° C. for 1 minute for precipitation strengthening, and then reheated to 650 ° C. at a heating rate of 1 ° C./sec using a gas combustion furnace, and then air-cooled ( Heat treatment condition 2: Example of the present invention).

  With respect to the obtained steel materials, the content of each element contained in precipitates of less than 20 nm in size contained in each test material by the method shown below (when the total of the components in Table 1 plus Fe is 100 mass% Of Ti, Nb, Mo, and V), tensile strength, fracture surface transition temperature in Charpy impact test (vTrs), and HIC resistance were evaluated.

B) Measurement of the content of Ti, Nb, Mo, and V contained in precipitates with a size of less than 20 nm for the entire steel material The above-obtained steel material was cut to an appropriate size, and 10% AA electrolyte (10 vol. % Acetylacetone-1 mass% tetramethylammonium chloride-methanol) was subjected to constant current electrolysis at a current density of 20 mA / cm ² .
After the electrolysis, remove the sample piece with deposits on the surface from the electrolyte and immerse it in an aqueous solution of sodium hexametaphosphate (500 mg / l) (hereinafter referred to as the SHMP aqueous solution) to apply ultrasonic vibration. The precipitate was peeled off from the sample piece and extracted into an aqueous SHMP solution. Next, the SHMP aqueous solution containing the precipitate is filtered using a filter with a pore size of 20 nm, and the filtrate after filtration is analyzed using an ICP emission spectroscopic analyzer. Ti, Nb, Mo, and V in the filtrate are analyzed. The absolute amount of was measured. Next, the absolute amounts of Ti, Nb, Mo, and V were divided by the electrolytic weight to obtain the contents of Ti, Nb, Mo, and V contained in the precipitate having a size of less than 20 nm. In addition, the electrolysis weight was calculated | required by measuring a weight with respect to the sample after deposit peeling, and subtracting from the sample weight before electrolysis.

B) Tensile strength A full thickness test piece in the vertical direction of rolling was measured as a tensile test piece. It was judged that precipitation strengthening was effectively realized when the tensile strength of heat treatment 2 exceeded the value of heat treatment 1 by 30 MPa or more. ΔTS is the difference in tensile strength between heat treatment 2 and heat treatment 1 for the same steel type (sign).
Further, it was confirmed by the electron diffraction pattern of the extracted precipitate that the precipitate having a size of less than 20 nm that worked effectively for the precipitation strengthening was MC type. In addition, it was confirmed by the content ratio measurement method that the molar ratio of Nb in the metal component was richer than Ti. As an example, the average composition of precipitates having a size of code B less than 20 nm measured by the above method is (Ti _0.07 Nb _0.43 Mo _0.28 V _0.22 ) C.

C) Measurement of vTrs After heat treatment reproducing HAZ shown in Fig. 2, fracture surface transition temperature (vTrs) was measured by Charpy impact test. When this temperature was less than 10 ° C, it was judged to be acceptable.

D) Measurement of HIC resistance HIC resistance was measured in a light sour environment of pH 3.0, 0.1 atm H ₂ S according to NACE Standard TM-02-84. The case was judged to be good because it was judged to have good HIC resistance.

  The results obtained as described above are shown in Table 2.

  From Table 2, the strength of the present invention example is 30 MPa or more higher than that of the comparative example and the strength of API standard X80 grade or higher is ensured without degrading the HIC resistance and the toughness of the heat affected zone. That is, it can be seen that the low alloy component that does not deteriorate the HIC resistance property satisfies both the strength of API standard X65 grade or higher and the toughness of the weld heat affected zone.

  FIG. 3 is a plot of the tensile strength in Table 1 against the carbon equivalent Ceq. It is clear that the tensile strength is effectively improved with the Ceq of less than 0.41 carbon equivalent in the inventive examples where precipitation strengthening is performed with precipitates of less than 20 nm. Since the tensile strength TS of API X80 targeted by the present invention is 650 MPa or more, it can be seen that the method of heat treatment condition 2 is effective. That is, the tensile strength is high when the Ti content of the precipitate having a size of less than 20 nm is 10 mass ppm or more with respect to the entire steel material and the Nb content is 140 mass ppm or more with respect to the entire steel material. In addition, what was obtained on the heat processing conditions 2 has favorable HAZ toughness.

  FIG. 4 shows the results of plotting vTrs (reproduced HAZ) in Table 2 against Ti content. It can be seen that when the Ti content is 0.03 mass%, vTrs can be suppressed within a favorable range.

  The steel sheet of the present invention is used in an environment where HIC resistance is required, and can be suitably used as a steel material for a line pipe that achieves both the strength of API standard X65 grade or higher and the toughness of the heat affected zone.

It is a schematic diagram of rolling conditions and heat processing conditions. It is the schematic of the heat processing conditions which reproduced the thermal cycle of HAZ. It is a figure which shows the relationship between carbon equivalent Ceq and tensile strength. It is a figure which shows the relationship between Ti content in a reproduction HAZ part, and a fracture surface transition temperature.

Claims (2)

  C: 0.02-0.08 mass%, Si: 0.01-0.5 mass%, Mn: 0.5-2.0 mass%, Ca: 0.0005-0.003 mass%, Ti: 0.01- 0.03 mass%, Nb: 0.04 to 0.05 mass%, Al: 0.07 mass% or less, Cu: 0.5 mass% or less, Ni: 0.5 mass% or less, Cr: 0.5 mass% or less, N: 0.007% by mass or less, the balance being a steel material composed of Fe and inevitable impurities, the carbon equivalent Ceq is less than 0.41, and Ti contains not less than the value obtained by adding 0.003% by mass to 3.4 times N, and The weld heat-affected zone toughness is characterized in that Ti in the precipitate having a size of less than 20 nm is 10 mass ppm or more with respect to the whole steel material, and Nb is 140 mass ppm or more with respect to the whole steel material. High strength steel with excellent HIC resistance. The steel containing the chemical component according to claim 1 is hot-rolled at a heating temperature of 1050 to 1250 ° C. and a rolling end temperature of Ar 3 temperature or higher, and then a cooling rate of 300 to 300 ° C. at 5 ° C./sec or higher. Accelerated cooling is performed to 600 ° C., the mixture is allowed to cool at a cooling stop temperature for 0.5 to 3 minutes, and then the temperature rise rate is 0.5 to 2.0 ° C./sec. Immediately after heating, cooling is performed at a cooling rate equal to or higher than air cooling. Ti in a precipitate having a size of less than 20 nm is 10 mass ppm or more with respect to the entire steel material, and Nb is 140 mass ppm or more with respect to the entire steel material. A method for producing a high-strength steel material excellent in weld heat-affected zone toughness and HIC resistance.

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