JP2013145221A - Steel material having low risk of hic crack and central segregation part evaluation method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating and determining the HIC crack sensitivity of a slab and a steel plate by measuring an Mn concentration distribution and taking the influence of content and a segregation degree into consideration.SOLUTION: A method evaluates anti-hydrogen crack sensitivity of a steel plate by using all Q values measured and calculated in each basic measurement area defined by the following expression (1) composed of the element content of a steel material and an Mn segregation degree of a central segregation part. Q=Cf+Cf/6+(Cf+Cf+Cf)/5+(Cf+Cf)/15+Cf/4+2Cf....expression (1). In this case, f represents an Mn segregation degree in a segregation part, and a value defined by (C/C), Crepresents Mn concentration obtained by measuring an Mn concentration distribution of the central segregation part with a cross section of the steel material in a width direction, Crepresents average concentration of Mn concentration, N of frepresents an exponentiation, and Crepresents the content (mass%) of an element i, respectively.

Description

  The present invention relates to a steel material with a low risk of HIC cracking for a high-strength sour line pipe and a method for evaluating the center segregation portion thereof.

  In general, continuous cast slabs of steel (hereinafter sometimes simply referred to as “slabs” or “slabs”) and thick steel plates (also simply referred to as “thick plates” or “steel plates”) made of them are used. In the manufacturing field, it is known that segregation at the center of a slab formed during continuous casting greatly affects product quality. In particular, in steel materials used in line pipes, hydrogen induced cracking (HIC) occurs due to segregation at the center (also simply referred to as “center segregation”), so in order to reduce such center segregation. Numerous technological developments have been made.

  On the other hand, several methods are known for evaluating center segregation. For example, a slab or a thick steel plate is sequentially sliced in the thickness direction, and the components of the chips collected by slicing are obtained. Analyzing and obtaining the concentration distribution in the thickness direction (slicing method), taking a macro print from the longitudinal section of the slab to identify the center segregation region, and drilling from a number of analysis points on this center segregation region After collecting a chip sample, analyzing this chip (drill method), polishing the cut surface of the slab, corroding the segregated portion with a corrosive liquid such as picric acid, and then soaking the ink First, wipe off the ink on the surface, copy the ink remaining in the corroded area onto cellophane paper, etc., and visualize the occurrence of segregation (macro corrosion method). Measure maximum segregation particle size A method (H printing method) or the like that.

  In general, the center segregation of a slab is never uniform in the thickness direction and the width direction when viewed from the C cross section, that is, the entire cross section perpendicular to the casting direction. Therefore, in order to investigate the segregation of a slab or a thick steel plate, it is necessary to evaluate over a wide area of the C cross section.

  Examining the above method from this point, the slicing method takes time to prepare and analyze the sample, and it takes a long time to obtain the result. Therefore, the central segregation evaluation of slabs, thick steel plates, etc. is performed over the entire C section. It is difficult to do. Moreover, in this method, since the sliced chips are analyzed, only an analysis value as an average in the thickness direction can be obtained.

  For this reason, the conventional technique has been used only for evaluating a partial area. In addition, the drill method is faster than the slicing method, but there is a problem that the whole area cannot be evaluated because the chip collection region is further narrower than the slicing method. On the other hand, the macro corrosion method is superior to the above two methods in terms of rapidity. However, the macro-corrosion method has a problem of non-quantitative evaluation because it is determined visually.

  In addition, the H printing method is quantitative, but requires skill for evaluation, takes time, and has a high cost.

  As a method for evaluating the center segregation other than the above, Patent Document 1 measures the hardness of the center segregation part, and the average value of the hardness, the maximum value, and the difference between the maximum value and the minimum value are any one or more. A simple method for evaluating the center segregation using the values has been proposed.

  Patent Document 2 proposes a technique for measuring the area ratio of the concentration of the additive element at the center using an analysis technique such as EPMA (Electron Probe Micro Analyzer).

  Further, for Mn segregation spots, Patent Documents 3 to 5 propose methods for limiting the concentration of P and the concentration of Nb.

JP 09-178733 A JP 2009-236842 A JP-A-6-271974 JP 2002-36389 A JP 2010-209461 A

  However, the method of Patent Document 1 specifies the region of the center segregation part by corrosion, and measures and evaluates the hardness of the center segregation part. When evaluating, since it is necessary to measure all the hardness of a segregation part, much time is required and it is inferior to quickness. Furthermore, since hardness is related to the component composition, structure, etc. of the slab, it cannot be directly evaluated if they are different. Therefore, in order to evaluate the center segregation of various types of slabs, there is a problem that the measurement conditions must be determined in detail.

  Further, in the method of Patent Document 2, the center segregation of continuous cast slabs, thick plates, etc. can be quantitatively and accurately evaluated, and a wide area can be quickly evaluated, but the correlation with the occurrence of HIC cracks is clear. Therefore, there is a possibility of overestimating or underestimating the crack occurrence rate and the degree of segregation. Moreover, although the concentration distribution of the target element becomes clear, it is difficult to evaluate the difference in the addition amount and the difference in segregation strength.

  In Patent Documents 3 and 4, although the form of the Mn segregation spot is defined, the details of the segregation spot discrimination method and analysis method are unclear.

  Regarding Patent Document 5, the form of segregation spots using EPMA is evaluated. However, in order to perform the analysis of the concentrated portion at a high magnification after performing the low-magnification concentration analysis and specifying the concentration position as an analysis method. The evaluation method is complicated. Further, although the degree of segregation is defined, there is a problem that the influence of other additive elements such as Mo, Cu, Ni, and P that affect the segregation part is not taken into consideration. Further, since the influence of the addition amount of Nb and Ti and the degree of segregation are not considered, for example, in Patent Document 5, “Nb addition amount is 0.001 to 0.10% and Nb segregation degree is 4.0. However, if the segregation degree is out of the claim with 4.1% addition within the claimed range of 0.01%, the segregated portion is concentrated 0.04%. Since this is included in the preferred Nb component range of the claims, the relationship between the degree of segregation and the amount added is unclear.

  Therefore, in the present invention, the concentration distribution of Mn, which is a specific element, is measured, and the segregation degree of the element is used to consider the influence of the element content and the degree of segregation. A method for evaluating and judging degree or sensitivity is proposed, and a steel plate for high-strength sour line pipes with good HIC cracking sensitivity is proposed using this evaluation method. Here, in the present invention, the segregation degree of the element M is the ratio of the content (mass%) of the element M in the segregation part of the steel sheet to the average content (mass%) of the element M (= segregation part). Element M content; mass% / average element M content; mass%).

  The inventors map the alloy element concentration at the center of the steel sheet, investigate the degree of segregation of each element, and then sometimes refer to the degree of segregation and HIC (simply “crack” or “HIC crack”). )) Was investigated. Here, the alloy element concentration mapping refers to obtaining planar concentration information by measuring the concentration distribution over the entire measurement region by, for example, EPMA.

  Specifically, an investigation was performed on the correlation of segregation elements such as C and Mn in the segregation part using EPMA. As a result, it has been found that the segregation degree of each element has a certain relationship and can be expressed as a function of the segregation degree and the addition amount. Element distribution information can be obtained by measuring the distribution of one type of additive element. In principle, any element can be measured. However, considering the analysis accuracy and analysis time, it is possible to some extent. It was found that it is effective to use Mn, which is an element with a clear addition amount and segregation form, as a reference.

  Therefore, the area where HIC cracking occurred using samples after various HIC tests of various high-strength sour line pipe steel sheets having an Mn addition amount of 0.8 to 1.5% by mass%. A detailed survey was conducted on areas where cracks do not occur. As a result, it was found that in the state of the steel sheet, there was a Mn concentration spot having a length of 1.0 mm or more in the width direction (direction perpendicular to the rolling direction), and an HIC crack occurred at that location. In addition, since no cracks are observed even in the portion of the Mn concentration spot having such a size, the concentration of the additive element in the Mn concentration spot having a size of 1.0 mm or more is concentrated. When the evaluation was further examined, it was found that the occurrence rate of HIC cracking fluctuates depending on the magnitude of the Q value represented by the following formula (1) and the number of Mn-concentrated spots per unit length. Here, the Mn concentration spot refers to a region where Mn segregates and the concentration is higher than the surroundings. The occurrence rate of HIC cracks is defined by a crack area ratio (CAR) obtained by measuring a material subjected to the NACE test described later by ultrasonic scanning. According to NACE TM0284, the HIC test was carried out by immersing in a standard A solution for 96 hours. The HIC test piece was produced from the center position of the plate. After the test, the crack area ratio (CAR%) of each sample was determined.

  In addition, as a result of Mn mapping analysis using a slab, the difference in the number of spots having a Q value of 1.0 or more and a width of 1.0 mm or more has a strong correlation with the occurrence of HIC cracks after rolling. I found it. Thus, by investigating the correlation between the form investigation of the HIC cracked part after rolling and the number of spots calculated with the shape before rolling in the slab before rolling, the risk of HIC characteristics of the slab (in the case of a steel plate, It was found that the meaning (corresponding to the sensitivity of the HIC characteristics) can be evaluated.

  Further, in evaluating the HIC characteristics, in addition to the spot form, the formation of coarse NbC crystallized material that becomes the starting point of HIC cracking has been found to have a relationship with the segregation degree of slab and the addition concentration, and the relationship Has been found to be defined as a P value.

  Considering the size of the Mn-concentrated spot where HIC cracking occurs as described above, the segregation degree of Mn at that location, and the concentration of added elements, the Q value, the size distribution of the Mn-concentrated spot, and the occurrence of HIC cracking As a result of investigating the rate and the crack cross-sectional ratio, the basic measurement region (basic unit for judgment of cracks is 1 mm × 1 mm for slabs and 0.1 mm × 1 mm in width (thickness direction) for thick steel plates. In the same manner, the distribution of the Q value in the target region is determined, and it has been found that the sensitivity of the HIC characteristic increases as the number of basic measurement regions having a Q value in the region of 1.0 or more increases. Further, at this time, it was found that by obtaining the P value, the risk of formation of coarse NbC that causes HIC cracking in the segregation spot portion can be evaluated, and its HIC sensitivity can be evaluated.

In particular, from the mapping of the Q value at the slab width of 100 mm, among all the Q values measured for each basic measurement region, the Q value in the range of less than 1.3 and 1.0 or more per 100 mm width in the cross section. In the case of 5 or less, when the remaining Q value is Q <1.0 and the P value is 0.35 or less, the crack area ratio (CAR) of the HIC is 1.5% or less. I found out that Here, the HIC cracking area ratio (CAR) is defined by the NACE TM0284-96 standard, and in the present invention, when the ratio is 1.5% or less, it is determined that the HIC characteristics are good.

  Here, a measurement technique for the analysis region will be described. The thickness width described later is determined, and the thickness width is measured in the 100 mm width direction. Specifically, an analysis was performed with respect to a thickness width of 20 mm and a width direction of 100 mm. In the analysis, an electron beam having a diameter of 100 μm was used in EPMA, and the analysis was performed with respect to a step of 100 μm, and a mapping image was created at 200 × 1000 points. Next, the measurement region is divided into a range of 1 mm × 1 mm, an average value of analysis points (10 × 10, 100 points) is calculated to obtain a Q value, and a measurement region of width 100 mm × thickness width 20 mm is 1 mm. A mapping image in the × 1 mm range was produced.

  The Q value is also affected by the chemical composition contained in the material and its content, and the first to fourth parts of the first half of the formula (1) correspond to the carbon equivalent of the concentrated part. it makes sense. In addition, the terms relating to Nb and P in the fifth and sixth terms of the formula (1) are defined by the degree of concentration of Nb and P that have a large influence on the center segregation, and the hardness of the segregated portion and the other portions. It is considered that the Q value is influenced by the difference in the above and coarse precipitates such as NbC, and a correlation with the crack area ratio (CAR) of the HIC occurs. In particular, the P value directly corresponds to a crack caused by NbC, and when this value is high, the risk of cracking due to coarse NbC is evaluated.

  In the present invention, for example, by mapping only the Mn element for the target material, it is possible to determine the Q value including the influence of other added elements of the segregation part, and it is possible to evaluate the HIC cracking susceptibility. The efficiency of this work can also be improved. Even when the components are changed, the materials can be compared based on the degree of segregation. In the above, the evaluation method based on the Mn analysis has been described. However, when the segregation degree of other elements is used, the evaluation can be similarly performed by changing the segregation degree index in the formula.

  That is, the present invention performs concentration mapping analysis of at least one segregating element on the central segregation part of continuous cast slabs and thick steel plates, and evaluates the size and number of concentration spots having a Q value equal to or greater than a threshold value. Further, the steel sheet for a sour line pipe having a high strength having a tensile strength of 550 MPa or more manufactured by using a central segregation evaluation method for predicting the risk of HIC cracking and manufacturing the steel sheet.

[1] The Q value defined by the following formula (1) consisting of the element content of the steel material and the Mn segregation degree of the central segregation part is measured and calculated for each basic measurement region, and all these Q values are used to calculate the steel material. A method for evaluating the resistance to hydrogen cracking.
_{^{Q = C C f 5.4 + C}} Mn f 1/6 + (C Cr f 0.3 + C Mo f 1.3 + C V f 0.6) / 5 +
_{^{(C Cu f 2.8 + C Ni}} f 1.3) / 15 + C Nb f 5/4 + 2C P f 5.4 ···· formula (1)
Here, f is the degree of Mn segregation at the segregation part, and is a value defined by (C _Mn ^C / C _Mn ). C _Mn ^C is the Mn concentration distribution at the center segregation part measured in the width direction of the cross section of the steel material. Mn concentration obtained _{by, C Mn} is the average concentration of the Mn concentration, the N is a power of ^{f N,} at a content of _{C i,} respectively the element i (wt%), i is the element C, Mn, Cr , Mo, V, Cu, Ni, Nb, and P, and when not contained, C _i = 0.

  [2] The chemical composition of the steel material is mass%, C: 0.03 to 0.07%, Si: 0.01 to 0.5%, Mn: 0.8 to 1.5%, Al: 0. 0.07% or less, S: 0.001 or less, P: 0.010 or less, Ti: 0.005 to 0.02%, Nb: 0.005 to 0.07%, Ca: 0.0005 to 0.005 %, N: 0.008% or less, O: 0.005% or less, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5 % Or less, V: containing one or more selected from 0.1% or less, and comprising the balance Fe and inevitable impurities, the hydrogen cracking susceptibility of the steel material according to [1] is evaluated. how to.

  [3] Among all the Q values measured for each of the basic measurement areas of 1 mm × 1 mm of the central segregation portion in the cross section perpendicular to the casting direction of the continuous cast slab, 1.0 ≦ Q <1.3 The number of certain Q values is 5 or less (including 0) per 100 mm width in the cross section, and the remaining Q value is Q <1.0. Steel sheet for high strength sour line pipes with good resistance to hydrogen cracking, characterized in that the piece is made of steel.

[4] All the cross sections in the direction perpendicular to the rolling direction of the steel sheet, the thickness of the central segregation portion of the cross section perpendicular to the rolling direction of the steel sheet being 0.1 mm in the thickness direction and 1 mm in the vertical direction. Among the Q values, the number of Q values satisfying 1.0 ≦ Q <1.3 is 5 or less (including 0) per 100 mm width in the cross section, and the remaining Q value is Q <1.0. A high-strength sour line pipe steel plate having a good resistance to hydrogen cracking, which contains the chemical composition according to [2] above.

[5] The above-mentioned [4], wherein the P value defined by the following formula (2) consisting of the C and Nb element contents of the steel material and the Mn segregation degree of the central segregation part is 0.35 or less. Steel sheet for high strength sour line pipes with good resistance to hydrogen cracking as described.
P = C _C f ^5.4 × C _Nb f ⁵ ... Formula (2)
Here, f is the degree of Mn segregation at the segregation part, and is a value defined by (C _Mn ^C / C _Mn ). C _Mn ^C is the Mn concentration distribution at the center segregation part measured in the width direction of the cross section of the steel material. Mn concentration obtained _{by, C Mn} is the average concentration of the Mn concentration, the N of ^{f N} represents exponentiation. C _C and C _Nb represent the contents (mass%) of C and Nb elements, respectively.

  According to the present invention, the segregation of the central part of a continuous cast slab or a thick steel plate is quantitatively and highly accurate, and based on the evaluation of a material that has been quickly measured over a wide area, the sensitivity of the HIC cracking of the steel material is improved. It becomes possible to manufacture the steel plate for high strength sour line pipes by evaluating.

EPMA analysis area of slab section

  The form for implementing this invention is demonstrated below. First, the reasons for limiting the constituent requirements of the present invention will be described.

  The present invention determines the risk or sensitivity of HIC cracking using the Q value calculated by the following equation.

_{^{Q = C C f 5.4 + C}} Mn f 1/6 + (C Cr f 0.3 + C Mo f 1.3 + C V f 0.6) / 5 +
_{^{(C Cu f 2.8 + C Ni}} f 1.3) / 15 + C Nb f 5/4 + 2C P f 5.4 ···· formula (1)
Here, f is the degree of Mn segregation at the segregation part, and is a value defined by (C _Mn ^C / C _Mn ). C _Mn ^C is the Mn concentration distribution at the center segregation part measured in the width direction of the cross section of the steel material. Mn concentration obtained _{by, C Mn} is the average concentration of the Mn concentration, the N is a power of ^{f N,} at a content of _{C i,} respectively the element i (wt%), i is the element C, Mn, Cr , Mo, V, Cu, Ni, Nb, and P, and when not contained, C _i = 0.

  About the quantitative analysis of each element, what is necessary is just to implement by the usual component-analysis method, and in this case, emission spectroscopic analysis (QV method).

For C _i in this equation, the amount of element added in the material may be determined by a normal method. For this reason, if f which is the degree of segregation of Mn is determined, the Q value is determined. Here, f is the segregation degree of Mn measured in a specific basic measurement region (1 mm × 1 mm for a slab, and 0.1 mm thickness × 1 mm width for a steel plate). The determination method is described below.

Method for Determining Segregation Degree f In the present invention, this value is used to determine the Q value of a steel material (used in the concept including “slab” and “steel plate” in this specification). The decision is important. The element distribution can be measured by measuring the distribution of one kind of additive element. In principle, the distribution of any element can be measured. It is desirable to carry out with C, Mn, Nb, P, etc. with a clear segregation form. Among these, considering the accuracy in short-time analysis, it is preferable to measure the distribution of Mn element having a relatively high content of about 1% with a normal content of mass%. Moreover, since Nb, P, C, etc. are larger in the segregation degree than the base material, these elements are suitable when priority is given to the S / N ratio. Since the accuracy of analytical instruments has been improved in recent years, it is expected that specific methods will be improved and changed, but in principle the spatial resolution is about 0.1 mm for thick steel plate analysis and about 1 mm for slab analysis. Thus, the composition accuracy is preferably 0.05% for Mn and 0.005% or less for other segregating elements.

  Hereinafter, an analysis method based on Mn will be described.

  Concentration mapping analysis of the central segregation part of Mn is performed with an electron dispersive X-ray spectrometer (EDS) and a wavelength dispersive X-ray spectrometer (WDS) attached to an electron probe microanalyzer (EPMA) and a scanning electron microscope (SEM). , Scanning emission spectroscopic analysis, laser ICP, or fluorescent X-ray analysis. Here, one embodiment for EPMA is shown.

  The segregation degree f is analyzed at the central segregation part of the slab or the C section of the steel sheet (in the case of a slab, the section perpendicular to the casting direction, and in the case of the steel sheet, the section perpendicular to the rolling direction). At this time, in the plate width direction, since segregation spots exist at random, it is preferable to analyze with a width of at least 100 mm. Moreover, in the thickness direction, since it is necessary to include a segregation part in the analysis region, it is necessary to adjust. Specifically, in order to identify the segregation part, it is possible to carry out by analyzing the region including the segregation part after confirming the thickness width of the macro segregation part by etching or the like in advance. If the position of the slab thickness tends to be gathered in advance, it can be measured by analyzing the area. Here, the plate thickness width means the length in the thickness direction in the case of slabs and steel plates.

  Regarding the plate thickness width, it is related to the dispersion form of the segregation spot. When the segregation spot at the center segregation portion of the slab spreads with a plate thickness width of about ± 2 mm, the center portion of the slab is ± 5 mm. It is preferable to implement in the region of the above thickness width. In this case, it is preferable to carry out the steel sheet in a region having a thickness width of ± 0.5 mm or more around the central portion.

  For materials with a wide segregation spot in the plate thickness width direction or a material whose width is unknown, the slab is evaluated with a plate thickness width of about ± 20 mm centered on the center portion and a steel plate with a plate thickness width of ± 1.0 mm or more centered on the center portion. It is preferable. However, by grasping the spread of the segregation spot depending on the apparatus and conditions in advance of the measurement work, the analysis thickness width can be made appropriate and the analysis time can be shortened.

_{C Mn} ^C is Mn concentration measurements segregation of C _Mn ^C is, 1mm × 1mm long slab, the Mn concentration of the center segregation area of the basic measurement region having a thickness of 0.1 mm × width 1mm if steel If you can get it. In the analysis using EPMA, in the slab, measurement is performed in a small area of 0.1 mm × 0.1 mm using a beam size of 0.1 mm diameter, and data information of 10 points × 10 points in a 1 mm × 1 mm area is obtained. All may be collected and remapped images may be accumulated and constructed in a basic measurement area of 1 mm × 1 mm using the average value of density.

  Alternatively, mapping may be performed in a small area having a size smaller than this, and redistribution may be calculated in a basic measurement area of 1 mm × 1 mm. Alternatively, an area of 1 mm × 1 mm may be measured by a raster scan to evaluate a quantitative value, a similar analysis may be performed in the width direction and the thickness direction, and concentration mapping may be performed using the values.

  Similarly, in the case of a steel plate, mapping is performed in a measurement unit of a small area of 0.1 mm × 0.1 mm, and remapping of an average value of 1 point in the thickness direction × 10 points in the width direction is performed, and 0.1 mm × 1 mm The concentration distribution in the basic measurement region may be reconstructed.

By dividing the value of C _Mn ^C thus obtained by the content C _Mn , the segregation degree of Mn in the basic measurement region of 1 mm × 1 mm for the slab and 0.1 mm × 1 mm for the steel plate at each location of the segregation part. It is possible to obtain the distribution in the C cross section for f. A mapping image of the Q value on the C cross section can be acquired using f determined in this manner. f ^N of N is a power, but the value of N is different for each unique element, and this value was determined by multiple regression analysis.

Among all the Q values measured for each basic measurement region, the number of Q values satisfying 1.0 ≦ Q <1.3 is 5 or less (including 0) per 100 mm width in the cross section, and the remaining The Q value is Q <1.0.
In the region where the Q value is less than 1.0, almost no HIC cracks are generated, and when the Q value is 1.0 or more, there is a high possibility of starting the HIC cracks. Therefore, when all the Q values measured for all the basic measurement regions do not exceed 1.0, almost no HIC cracking occurs. Accordingly, the Q value is preferably less than 1.0.

  However, when all the Q values measured for each basic measurement region are 1.0 or more and less than 1.3, the number is less than 5 per 100 mm width in the C cross section, and the remaining Q value is Q <1.0. Then, the crack area ratio (CAR) of the segregated portion HIC in that region was 1.5% or less, and it was found that good HIC characteristics can be obtained. 5 or less includes 0, but this means that the Q value does not exist in the range of 1.0 ≦ Q <1.3. In this case, all the Q values are Q <1.0. Means.

  On the other hand, when the number of basic measurement regions having a Q value larger than 1.0 exceeds 5, HIC cracking increases and the HIC resistance is deteriorated. Further, if a basic measurement region having a Q value of 1.3 or more exists, the CAR exceeds 1.5% and the HIC characteristics deteriorate. Further, the P value is a value corresponding to the concentration product at the segregated portion of the Nb and C amounts. If this value is 0.35 or less, the formation of coarse NbC that causes HIC cracking is suppressed, and the HIC characteristics are improved. Therefore, the HIC characteristics can be improved by adjusting the hardness according to the concentration of the entire segregation part by the Q value and controlling the formation of NbC in the segregation part by the P value.

  Adjustment and control of the Q value can be performed by reducing the segregation degree and optimizing the amount of alloy components added. For this reason, even if the same component material is used, the segregation degree is improved and the Q value is lowered and the HIC characteristics are improved when the casting conditions (e.g., cooling rate) and the rolling conditions during slab solidification are changed. Moreover, in the case of the same casting conditions, it can be reduced by further reducing within the range of the appropriate content of the chemical composition to be described later. Q value can be reduced by reducing within the range of appropriate content.

  Further, since the Q value increases as the f value increases, the segregation degree of Mn is desirably 2.0 or less in order to reduce the Q value. In this evaluation, even in the same slab and plate, the difference in segregation degree depending on the location causes a difference in the Q value. Therefore, except for the inferior part of the HIC characteristic, a pipe with excellent HIC resistance is manufactured by manufacturing the pipe. It is also possible to do.

  The steel material evaluated by the evaluation method of the present invention is used to produce a steel sheet by rolling and cooling, or by using the steel sheet evaluated by the evaluation method of the present invention, sour resistance performance is excellent. High strength line pipes can be manufactured.

  The chemical components of the high-strength steel plate will be described below. In the following description, all units represented by% are mass%. Moreover, although a steel plate is demonstrated, it is the same also in the case of a steel raw material.

C: 0.03-0.07%
C is an element that contributes to improving the strength of the steel sheet. However, if it is less than 0.03%, sufficient strength cannot be secured, and if it exceeds 0.07%, the toughness is deteriorated. Therefore, the C content is 0.03%. -0.07%.

Si: 0.01 to 0.5%
Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is 0.01 to 0.00. Specify 5%. Preferably, Si: 0.04 to 0.4%.

Mn: 0.8 to 1.5%
Mn is added for strength and toughness. However, if it is less than 0.8%, its effect is not sufficient, and if it exceeds 1.5%, the center segregation is remarkable and the Q value increases. It is specified to 8 to 1.5%.

Al: 0.07% or less Al is added as a deoxidizer. However, if it exceeds 0.07%, the cleanliness of the steel decreases and the formation of inclusions that become the starting point of HIC cracking increases, so Al is contained. The amount is specified to be 0.07% or less. Preferably, it is 0.01 to 0.05%.

P: 0.010% or less In the present invention, P and S are inevitable impurities, and the upper limit of the amount thereof is specified. P greatly affects the increase of the Q value as shown by the formula (1), and the HIC cracking susceptibility deteriorates. Therefore, the amount of P is set to 0.010% or less. Preferably, the amount of P is 0.005% or less.

S: 0.001% or less If the content of S is large, the amount of MnS produced increases remarkably and the HIC cracking susceptibility deteriorates, so the S amount is 0.001% or less.

Ti: 0.005-0.02%
Ti is an important element that forms TiN and suppresses austenite coarsening during slab heating and improves the toughness of the base metal due to its pinning effect. The effect is manifested when 0.005% or more is added. However, if it exceeds 0.02%, a coarse Ti-based precipitate is generated, which becomes the starting point of HIC cracking, and the HIC cracking susceptibility deteriorates, so the Ti amount is in the range of 0.005 to 0.02%. To do. Preferably, the Ti amount is in the range of 0.009 to 0.015%.

Nb: 0.005 to 0.07%
Nb improves toughness by refining the structure, forms precipitates, and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, a coarse Nb-based precipitate is formed, which becomes the starting point of HIC cracking, and the HIC cracking susceptibility deteriorates. It is specified to be 005 to 0.07%. Nb has a large contribution to increasing the Q value, so the Nb content is preferably 0.05% or less.

Ca: 0.0005 to 0.005%
Ca controls the form of sulfide inclusions to improve the resistance to HIC cracking. The effect appears at 0.0005% or more, and when it exceeds 0.005%, the effect is saturated, and conversely, the cleanliness is lowered to form inclusions as the starting point of HIC. The range is 0.005%.

N: 0.008% or less N is treated as an inevitable impurity, but if the N content exceeds 0.008%, coarse Ti—Nb-based precipitates that form the starting point of HIC cracking are formed. 0.008% or less.

O: 0.005% or less In the present invention, O is an unavoidable impurity and defines the upper limit of the amount thereof. Since O is coarse and suppresses the formation of inclusions that adversely affect HIC resistance, the amount of O is set to 0.005% or less.

  In addition to the above, one or more of Cu, Ni, Cr, Mo, and V shown below are added for the purpose of further improving the strength and toughness of the steel sheet.

Cu: 0.5% or less Cu contributes to the improvement of hardenability of steel, but if added over 0.5%, the toughness of the steel sheet is deteriorated. Is 0.5% or less.

Ni: 1% or less Ni contributes to improving the hardenability of the steel. In particular, Ni does not cause deterioration in toughness even if added in a large amount, so it can be added because it is effective for toughening. However, since Ni is an expensive element, when adding Ni, the amount of Ni is made 1% or less.

Cr: 0.5% or less Since Cr is an effective element for obtaining sufficient strength even at low C as in Mn, it may be added. In order to acquire the effect, it is preferable to add 0.1% or more, but if added excessively, weldability deteriorates, so when added, the Cr content is 0.5% or less.
Mo: 0.5% or less Mo is an element that improves hardenability, and is an element that contributes to an increase in strength by strengthening the MA generation and the bainite phase, and can be arbitrarily added. However, if added over 0.5%, the weld heat-affected zone toughness is deteriorated. Therefore, when added, the Mo content is set to 0.5% or less. It is preferable to make it 0.3% or less.

V: 0.1% or less V is an element that enhances hardenability and contributes to an increase in strength, and may be optionally added. In order to obtain the effect, it is preferable to add 0.005% or more, but if added over 0.1%, the toughness of the weld heat affected zone deteriorates. 1% or less.

  The remainder other than the said component in the steel plate of this invention is Fe and an unavoidable impurity. However, the content of elements other than those described above is not rejected as long as the effects of the present invention are not impaired. For example, from the viewpoint of improving strength and toughness, one or more selected from Mg: 0.02% or less, REM (rare earth metal): 0.02% or less, and B: 0.003% or less can be included.

  It is desirable to use a steel material or steel plate containing the above composition.

  Slabs (steel types 1 to 15) produced by a continuous casting method having the composition shown in Table 1 were used as samples.

  In order to change the segregation degree of the same component, a part of the slab was formed by increasing the casting speed of steel types 1 to 13 to 1.40 m / min and increasing the steel types 14 and 15 to 2.00 m / min. Using these slabs, Mn segregation at the center was evaluated by EPMA. In the evaluation, a sample for analysis is cut out from the slab, the sample is adjusted by polishing, and the range of the slab center part thickness of 10 mm is measured in the width direction of ± 100 mm with the slab width center part (W / 2) as the center. Mapping was performed. The area of EPMA analysis is shown in FIG.

  In addition, for some materials, the intermediate position (W / 4) part between the center part and the end part is also analyzed, and the segregation degrees in the (W / 2) part and (W / 4) part are compared. Went. Mapping was performed using an electron probe having a diameter of 0.10 mm at an acceleration voltage of 25 kV, and mapping of Mn concentration was measured. Since the obtained Mn concentration distribution is a distribution with a size of 0.1 mm × 0.1 mm, the analysis points are averaged at 10 × 10 points, reconstructed as analysis points at 1 × 1 mm, and the segregation degree of Mn (The analysis point after reconstruction when the analysis area is 10 × 100 mm is 1000 points).

  Table 2 shows the maximum value of the degree of segregation of Mn in the Mn analytical amount range. Here, the maximum segregation degree is a value obtained by dividing the maximum value of Mn concentration in the basic measurement region (1 × 1 mm) in the analysis region (20 × 200 mm) as the maximum Mn concentration and dividing the maximum Mn concentration by the amount of added Mn. Defined. Using this maximum segregation value and the amount of each element added, the value calculated was substituted into Equation (1), and the calculated value was taken as the maximum Q value.

  In this experiment, even in the same material, the maximum value of the segregation degree was smaller in the (W / 2) part than in the (W / 4) part, and the segregation degree tended to be good. Using the obtained segregation degree and the component analysis value of each slab, the distribution of the Q value in the analysis region was calculated, and the dispersion form of the Q value was investigated.

  Thereafter, a 30 mm thick steel plate was manufactured by heating, controlled rolling, rapid cooling, and reheating of the slab. All slabs were rolled under the same conditions. Slab heating was performed at 1100 ° C., and hot rolling was performed between 1050 ° C. and 850 ° C. Then, rapid water cooling was performed from 800 ° C. At this time, the cooling stop temperature was 450 ° C. Moreover, after heating to 600 degreeC by induction heating, it manufactured by air cooling. In the present invention, the temperatures in the production conditions are all steel plate average temperatures. The average steel plate temperature is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions and the like. For example, the average temperature of a steel plate is calculated | required by calculating the temperature distribution of a plate | board thickness direction using the difference method. The strength of the thick steel plate thus obtained was in the range of 540 MPa to 680 MPa.

  The HIC test using the steel plate manufactured as described above was performed. The HIC test was carried out by immersing in standard A solution for 96 hours according to NACE TM0284. After the test, the crack area ratio (CAR%) of each sample was measured. Three samples were prepared from the target position (W / 4 part, W / 2 part for some materials) of each plate, and the average CAR was measured.

  The results are summarized in Table 2. Samples having a maximum Q value of 1.0 or less exhibited good HIC characteristics with no HIC cracking (samples 1, 2, 4, 6, 12). Here, in Table 2, a hyphen (-) is used to mean 0. In addition, when the maximum Q value is 1.3 or less and 1.0 to 1.3 or less is 5 or less per 100 mm width, the HIC cracking area ratio (CAR) is less than 1.5%. Yes, showing good HIC characteristics (Samples-3, 5, 7, 8, 9, 10, 11, 14). Moreover, although the maximum value of Q value is 1.3 or less, when the number of spots of 1.0-1.3 or more exceeds 5, the HIC cracking area ratio (CAR) is 1.5% or more. The HIC characteristics are deteriorated (Samples 13 and 15). In addition, the components and the degree of segregation are poor and the maximum Q value exceeds 1.3. , 17, 18, 19, 20).

The P value was larger for sample 18, and therefore NbC clusters were formed and HIC cracking occurred.
It was confirmed that this test method was able to evaluate differences in HIC resistance due to differences in location and segregation in the same material. Moreover, about the steel plate obtained in this way, as a result of carrying out EPMA analysis similarly and measuring Q value, the result similar to evaluation of a slab was obtained, and this analysis can be evaluated also with a steel plate. I knew that there was.

  Next, the slab (steel types 16-20) produced by the continuous casting method of the component composition shown in Table 3 was used as a sample.

  The slab was carried out at a casting speed of 1.1 m / min. In order to change the segregation degree of the same component, partly, the casting speed was 1.8 mm / min and 2.0 m / min, and the reduction amount in the final solidification part was halved (steel type) -18, 20). Using these slabs, Mn segregation at the center was evaluated by EPMA. The evaluation is performed by cutting out a sample for analysis from the slab, adjusting the sample by polishing, and adjusting the thickness of the slab center part to 10 mm in the width direction of ± 100 mm around the slab width 1/4 part (W / 4) part. Mapping was performed for the region. Mapping was performed using an electron probe having a diameter of 0.10 mm at an acceleration voltage of 25 kV, and mapping of Mn concentration was measured. Since the obtained Mn concentration distribution is a distribution with a size of 0.1 mm × 0.1 mm, the analysis points are averaged at 10 × 10 points, reconstructed as analysis points at 1 × 1 mm, and the segregation degree of Mn (The analysis point after reconstruction when the analysis area is 10 × 100 mm is 1000 points). Table 4 shows the maximum value of the degree of segregation of Mn in the analysis amount region of Mn.

  Here, the maximum segregation degree is a value obtained by dividing the maximum value of Mn concentration in the basic measurement region (1 × 1 mm) in the analysis region (20 × 200 mm) as the maximum Mn concentration and dividing the maximum Mn concentration by the amount of added Mn. Defined. Using this maximum segregation value and the amount of each element added, the values calculated were substituted into equations (1) and (2), and the calculated values were taken as the maximum values of the Q value and the P value.

  Thereafter, a thick steel plate having a thickness of 32 mm was manufactured by heating, controlled rolling, rapid cooling, and reheating of the slab. All slabs were rolled under the same conditions (Samples 21 to 25). Slab heating was performed at 1100 ° C., and hot rolling was performed between 1050 ° C. and 850 ° C. Then, rapid water cooling was performed from 800 ° C. At this time, the cooling stop temperature was 480 ° C. In addition, some materials were subsequently heated to 600 ° C. by induction heating and then manufactured by air cooling. The strength ranged from 560 MPa to 660 MPa. These temperatures used the steel plate surface temperature as an index.

  The HIC test using the steel plate manufactured as described above was performed. The HIC test was carried out by immersing in standard A solution for 96 hours according to NACE TM0284. After the test, the crack area ratio (CAR%) of each sample was measured. Three samples were prepared from the target position of each plate, and the average CAR was measured.

  The results are summarized in Table 4. A sample having a maximum Q value of 1.3 or less and a number of spots of 1.0 to 1.3 or more and 5 or less and satisfying the P value has a HIC crack of 1.5%. The following were good HIC characteristics (Samples 21, 22, and 23). The segregation degree was poor, and the Q value of 1.0-1.3 or more of the number of spots exceeded 5 and the P value exceeded 0.40, the HIC characteristics were deteriorated (Sample-24, 25). In particular, when the maximum P value exceeds 0.4, cracking occurs due to the formation of NbC, and the HIC resistance is deteriorated (Sample 24).

  It was confirmed that this test method was able to evaluate differences in HIC resistance due to differences in location and segregation in the same material. Moreover, about the steel plate obtained in this way, as a result of carrying out EPMA analysis similarly and measuring Q value and P value, the result similar to evaluation of a slab was obtained, and this analysis is also in steel plate. It turns out that it can be evaluated.

Claims (5)

The Q value defined by the following formula (1) consisting of the element content of the steel material and the Mn segregation degree of the central segregation part is measured and calculated for each basic measurement region, and the hydrogen resistance of the steel material is calculated using all these Q values. A method to evaluate cracking susceptibility.
_{^{Q = C C f 5.4 + C}} Mn f 1/6 + (C Cr f 0.3 + C Mo f 1.3 + C V f 0.6) / 5 +
_{^{(C Cu f 2.8 + C Ni}} f 1.3) / 15 + C Nb f 5/4 + 2C P f 5.4 ···· formula (1)
Here, f is the degree of Mn segregation at the segregation part, and is a value defined by (C _Mn ^C / C _Mn ). C _Mn ^C is the Mn concentration distribution at the center segregation part measured in the width direction of the cross section of the steel material. Mn concentration obtained _{by, C Mn} is the average concentration of the Mn concentration, the N is a power of ^{f N,} at a content of _{C i,} respectively the element i (wt%), i is the element C, Mn, Cr , Mo, V, Cu, Ni, Nb, and P, and when not contained, C _i = 0.   The chemical composition of the steel material is mass%, C: 0.03-0.07%, Si: 0.01-0.5%, Mn: 0.8-1.5%, Al: 0.07% Hereinafter, S: 0.001 or less, P: 0.010 or less, Ti: 0.005 to 0.02%, Nb: 0.005 to 0.07%, Ca: 0.0005 to 0.005%, N : 0.008% or less, O: 0.005% or less, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, The method for evaluating the hydrogen cracking susceptibility of a steel material according to claim 1, comprising at least one selected from V: 0.1% or less and comprising the balance Fe and inevitable impurities.   Among all the Q values measured for each of the basic measurement areas of 1 mm × 1 mm of the central segregation portion in the cross section perpendicular to the casting direction of the continuous cast slab, the Q value satisfying 1.0 ≦ Q <1.3 3 is 5 or less (including 0) per 100 mm width in the cross section, and the remaining Q value is Q <1.0. A steel sheet for high-strength sour line pipes with good resistance to hydrogen cracking.   A cross section perpendicular to the rolling direction of the steel sheet, and all Q values for each of the basic measurement areas of 0.1 mm in the thickness direction of the central segregation portion of the cross section perpendicular to the rolling direction of the steel sheet and 1 mm in the vertical direction. The number of Q values satisfying 1.0 ≦ Q <1.3 is 5 or less (including 0) per 100 mm width in the cross section, and the remaining Q value is Q <1.0. A steel plate for a high-strength sour line pipe having good chemical resistance to hydrogen cracking and containing the chemical composition according to claim 2. 5. The hydrogen resistance according to claim 4, wherein the P value defined by the following formula (2) consisting of the C and Nb element contents of the steel material and the Mn segregation degree of the central segregation part is 0.35 or less. High strength steel plate for sour line pipes with good crack sensitivity.
P = C _C f ^5.4 × C _Nb f ⁵ ... Formula (2)
Here, f is the degree of Mn segregation at the segregation part, and is a value defined by (C _Mn ^C / C _Mn ). C _Mn ^C is the Mn concentration distribution at the center segregation part measured in the width direction of the cross section of the steel material. Mn concentration obtained _{by, C Mn} is the average concentration of the Mn concentration, the N of ^{f N} represents exponentiation. C _C and C _Nb represent the contents (mass%) of C and Nb elements, respectively.

Images