JP2005113249A - High tensile steel material and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile steel material having little attenuation of reflected echoes when an ultrasonic flaw detection test is performed. <P>SOLUTION: A high tensile steel material which has a prior-austenite grain having a size of over 100 μm in the plate thickness direction by ≤1 grain per 1 mm<SP>2</SP>in any cross section including the plate thickness direction and which shows ≤0.1 (dB/mm) ultrasonic attenuation coefficient α is manufactured by heating a billet produced by continuous casting of a steel composition having ≥680 MPa tensile strength at ≥1,180°C for ≥3 hours, cooling the billet down to ≤800°C at a ≥1 °C/s cooling rate, then again heating and rolling. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-strength steel material having a tensile strength of 680 MPa or more, which is used for buildings, structures, and pressure vessels, and a method for manufacturing the same, and more specifically, a reflection echo when an ultrasonic flaw detection test is performed. The present invention relates to a high-strength steel material having a small attenuation and a tensile strength of 680 MPa or more and a method for producing the same.

  In general, an ultrasonic flaw detection test is performed in order to investigate internal defects in steel materials and defects in welds. In particular, ultrasonic flaw detection tests are performed on strict standards for pressure vessels and steel structures for construction. When this ultrasonic flaw detection test is performed, the reflection echo may be attenuated depending on the steel material, which may hinder the inspection.

  One reason for this is that the steel material has anisotropy ("acoustic anisotropy") in which the attenuation factor of the reflected echo differs depending on the L direction, which is the rolling direction, and the C direction, which is the direction orthogonal to the rolling direction. Is also referred to as “.” Since this acoustic anisotropy causes an error in the defect determination result depending on the inspection direction, various countermeasures have been proposed.

  For example, Patent Document 1 is based on the premise that accurate inspection cannot be performed because the acoustic anisotropy increases due to the texture generated and developed inside the steel sheet, and the steel components are set within an appropriate range. An invention for manufacturing a steel sheet with less acoustic anisotropy by limiting both the rolling conditions and the cooling conditions is disclosed. In addition, the texture is remarkably developed especially in the production of high-strength steel sheets by TMCP, and in order to change the refraction angle and the flaw detection sensitivity in ultrasonic flaw detection tests, particularly oblique flaw detection, the inspection of the soundness of the steel itself or welds It often causes trouble.

  Another reason that the reflection echo is attenuated by the steel material may be attributed to the steel material itself. In other words, if the attenuation of the reflected echo is large, even if the inspection is performed according to a predetermined inspection standard, the reflected echo is affected, so that an accurate inspection cannot be performed.

  As for the attenuation of the reflection echo caused by the steel material itself, it has been known so far that an accurate inspection cannot be performed by a normal ultrasonic flaw detection test due to, for example, a cast structure. However, it has not been known so far that attenuation of reflected echo occurs even in ordinary steel, and particularly in high-tensile steel exceeding 570 MPa class, for example, high-tensile steel having a tensile strength of 680 MPa or more, the usage application is steel or Since it is a pressure vessel and the like, the standard of ultrasonic flaw detection tests for welds is strict, and has recently been regarded as a problem.

  In addition, attenuation of the reflected echo is regarded as a problem due to the oblique flaw detection method itself used in the ultrasonic flaw detection test of the welded portion in order to efficiently inspect the entire area of the welded portion. That is, in the oblique flaw detection method, a transverse wave is used unlike a vertical flaw detection method in which a longitudinal wave is generally used in general. The longitudinal wave has a high sound speed and a short wavelength, so the attenuation of the reflected echo is not a problem. However, the transverse wave has a short wavelength because the sound speed is low, and the ultrasonic echo is greatly attenuated depending on the steel structure.

JP-A-6-316724

  For example, since the invention described in Patent Document 1 improves acoustic anisotropy, it cannot prevent the reflection echo of the steel material itself from being attenuated. As described in Patent Document 1, for example, the texture that develops remarkably in TMCP is not a direct factor for attenuating the reflected echo, so it is assumed that the texture anisotropy can be reduced and the acoustic anisotropy can be improved. However, the attenuation of the reflected echo cannot be directly suppressed.

  When the ultrasonic flaw detection test specified by JIS is applied, if the attenuation of the reflected echo is large, it is difficult to obtain a guarantee that the steel itself is sound even if it is healthy. There is even a risk of missing. Furthermore, since an inspection performed for an ultrasonic flaw detection test that does not specify the attenuation amount of the reflected echo is complicated, a great amount of man-hours are required for this inspection.

  The present invention has been made in view of the problems of such conventional techniques, and it is an object of the present invention to reliably provide a high-tensile steel material having a tensile strength of 680 MPa or more with low ultrasonic attenuation and a method for producing the same. To do.

The present invention has a tensile the strength is not less than 680MPa, at any thickness direction cross-section in the steel, the size of prior austenite grains is a is one less / mm ² in excess of 100 [mu] m in the thickness direction And an ultrasonic attenuation coefficient α defined by the following equation (1) is 0.1 (dB / mm) or less.

α = 20 {Log (V1 / V2)} / (W2-W1) (1)
In this equation (1), the symbol Vl is the gain value (dB) when the bottom echo height V1 = 50% in one skip by the V transmission method, and the symbol V2 is the bottom surface in two skips by the V transmission method. This is the gain value (dB) when the echo height V2 = 50%, the symbol Wl is the beam path length (mm) at the time of one skip by the V transmission method, and the symbol W2 is at the time of two skips by the V transmission method The beam path length (mm). Note that the symbols and definitions in equation (1) are the same as in JIS Z 3060 (2001).

  From another point of view, the present invention relates to a method in which a continuously cast steel slab is heated at 1180 ° C. or more for 3 hours or more and then cooled to 800 ° C. or less at a cooling rate of 1 ° C./s or more. It is a manufacturing method of the high-tensile steel material characterized by heating and rolling again.

  In these high-strength steel materials or methods for producing the same according to the present invention, the high-strength steel materials or the continuously cast steel pieces have a C content of 0.01% or more and 0.20% or less (in this specification, “%” unless otherwise specified). Means “mass%”), Si is 0.5% or less, Mn is 0.6 or more and 2.0% or less, P: 0.015% or less, S: 0.005% or less, Ti is 0.005% or more and 0.03% or less, sol. Contains Al 0.005% to 0.08%, N 0.008% or less, Cr: 0.01% to 1.0%, Mo: 0.01% to 1.50% and V: 0.005% to 0.10%, the balance being Fe and inevitable It has a steel composition consisting of mechanical impurities.

In this case, the high-strength steel material or the continuously cast steel slab further contains Cu 0.05% or more and 1.0% or less, whereby the strength can be further increased.
In these cases, the high-tensile steel material or the continuously cast steel slab can further improve the low temperature toughness by containing 0.05% or more and 4.0% or less of Ni.

In these cases, the high-tensile steel material or the continuously cast steel slab further contains Nb in an amount of 0.005% to 0.08%, thereby preventing toughness from being lowered.
Further, in these cases, the high-strength steel material or the continuously cast steel slab further contains B in an amount of 0.0002% to 0.020%, thereby improving hardenability and increasing strength.

  According to the present invention, a steel material with less ultrasonic attenuation can be reliably obtained, and an ultrasonic flaw detection test can be performed with high accuracy.

The inventors of the present invention have made extensive studies on influencing factors of ultrasonic attenuation and have obtained basic findings (1) to (3) listed below.
(1) The dominant factor of ultrasonic attenuation is not the texture or central segregation but the size of the microstructure.
(2) The coarsening of prior austenite grains in the microstructure affects the ultrasonic attenuation.
(3) By heating and cooling the steel material in advance before heating and rolling the steel material, the coarsening of the prior austenite grains can be effectively prevented.

Here, the above items (1) and (2) will be described.
The present inventors cut out a cubic sample 1 as shown in FIG. 1 representing a method for investigating the scattering state of ultrasonic waves from a steel material having a large ultrasonic attenuation and a steel material having a low ultrasonic attenuation. The state of ultrasonic scattering was investigated using a tentacle 2 (5 MHz, 12.7 mm diameter). Moreover, the microstructure was observed in order to investigate the cause of the attenuation of the steel material having a large ultrasonic attenuation. FIG. 2 is a graph showing the attenuation amount of the reflection echo of each of the two types of steel materials in this investigation. B2 to B6 on the horizontal axis in FIG. 2 indicate the second reflected echo B2 to the sixth reflected echo B6 when B1 = 100%, and the vertical axis represents the height of each reflected echo B2 to B6. Show.

  The graph shown in FIG. 2 shows a comparison of a microstructure in which prior austenite grains are coarsened and not. In steel materials with large ultrasonic attenuation, the prior austenite grains were coarsened, indicating that the coarsening of the prior austenite grains is a significant influencing factor for ultrasonic attenuation.

Furthermore, in order to examine the means for suppressing the coarsening of the prior austenite grains, the following experiment was conducted.
The test piece cut out from the continuously cast slab was heated to 1150 ° C., and the microstructure of the steel material produced when the holding time was changed to 1, 5 or 10 hours was observed. As a result, when the holding time is 1 hour, the structure is substantially fine uniform austenite grains, whereas the longer the holding time is 5 hours or 10 hours, the coarser the prior austenite grains in the fine grain structure. It was found that grains formed and this rate also increased. It was also found that the generation of coarse particles becomes extremely noticeable especially when the holding time exceeds 5 hours. The results are shown graphically in FIG. The coarse austenite grains are also affected after rolling.

  In addition, although the detailed mechanism which the coarse grain of a prior austenite grain produces | generates is unknown, it is guessed that it is the pinning effect by a precipitate. That is, when heated for a long time in a heating furnace, the precipitate grows, and the pinning effect of the precipitate is weakened, so that coarse grains of prior austenite grains are generated.

  FIG. 4 is an explanatory diagram showing an example of the microstructure in the thickness direction. Here, the size of the prior austenite grains refers to the distance between points A and B where the straight line drawn in the thickness direction in FIG. 4 intersects the prior austenite grain boundaries.

The method for observing the prior austenite grains with an optical microscope is not particularly limited, and any known appropriate method can be used, but the methods listed below are preferred.
First, a saturated picric acid solution and 10% ferric chloride were mixed at a mass ratio of 90: 5 as an etching solution for clearly revealing the prior austenite grain boundaries, and a surfactant was further added dropwise. create. Then, the sample to be observed is immersed in the prepared etching solution for about 10 seconds. Thereafter, the sample is taken out, immersed in a reaction stop solution, washed with water and dried, and observed with an optical microscope. The observation magnification is preferably about 100 to 500 times.

If there are two or more old austenite grains larger than 100 μm per 1 mm ² among the observed samples, the refraction angle and the flaw detection sensitivity change during the ultrasonic flaw detection test, particularly the oblique flaw detection test. Therefore, the soundness of the steel material itself or the welded part cannot be properly evaluated.

When there is only one old austenite grain with a size exceeding 100 μm per 1 mm ² , the degree of change in the refraction angle and the flaw detection sensitivity is preferable in the ultrasonic flaw detection test, particularly the oblique flaw detection test. However, since ultrasonic attenuation still occurs when the ultrasonic wave passes through this portion, the size of the prior austenite grains exceeding 100 μm is more preferably one per 1 mm ² above (1) and ( From the findings in Section 2), it is clear that the cause of ultrasonic attenuation is coarse grains of prior austenite grains, and further, by conducting the following experiment, the coarsening of prior austenite grains is prevented. Obtained knowledge.

  A small sample is cut out from a continuously cast slab, heated to 1200 ° C and held for 5 hours as a pretreatment for the slab before production, and then cooled to a surface temperature of 800 ° C or lower at a cooling rate of 5 ° C / s. The treated sample was heated in a laboratory experimental furnace for 1, 5 and 10 hours and compared with the sample without slab pretreatment (see FIG. 3 described above).

  As a result, the average old austenite grain size becomes coarser as the heating time becomes longer for those not subjected to slab pretreatment, whereas those for which slab pretreatment is performed are either 1, 5 or 10 hours. In addition, it can be seen that there is no change in the average prior austenite grain size, and the coarsening of the prior austenite grains can be prevented.

Although this mechanism is not clear, it can be considered as follows.
First, the pinning force of particles that suppress the growth of austenite grains can be expressed by the following equation (2). In the equation (2), the symbol f indicates the volume fraction of the precipitate, and the symbol r indicates the particle radius of the precipitate.

Z = 3f / 4r (2)
That is, in order to suppress the growth of austenite grains, it is important to disperse many precipitates having a small particle radius r.

  In fact, in the cooling process such as continuous casting, the precipitates grow to a relatively large size, and the precipitates are further grown by Ostwald during the heating and holding time in the subsequent rolling process. As a result, some of the particles are likely to generate coarse particles. However, if an appropriate heating and cooling cycle is applied before the rolling process, the pinning force due to the precipitated particles is dramatically improved. In other words, once the precipitate is dissolved, and then cooled at a faster cooling rate, it is possible to disperse a small number of fine precipitates more than the original state. Generation of coarse grains can be prevented.

  Appropriate slab pretreatment in this way will ensure that a high pinning force is achieved, which is ultimately detrimental to ultrasonic attenuation and is a coarse product with dimensions greater than 100 μm. It is possible to prevent the grains from remaining.

In addition, after performing a slab pretreatment, it is not necessary to perform the operation under special conditions, and reheating, rolling and cooling may be performed under normal operation conditions.
Next, the ultrasonic attenuation coefficient α defined in the present invention will be described.

  FIG. 5 is an explanatory diagram schematically showing a situation in which ultrasonic attenuation is investigated by scanning the steel material 5 with the ultrasonic wave transmitter 3 and the ultrasonic wave receiver 4 of the oblique angle probe. In the example shown in FIG. 5, in order to eliminate the influence of the surface state of the steel material 5, the bottom echo at the second skip at the position B is measured after measuring the bottom echo height at the first skip at the position A in the V transmission method. Measure height.

  In this embodiment, the ultrasonic attenuation coefficient α, which is a parameter focused on the ultrasonic attenuation amount from the first skip to the second skip, is used instead of the echo height. This ultrasonic attenuation coefficient α is defined by α = 20 {Log (V1 / V2)} / (W2−W1), which is the equation (1).

In this equation (1), the symbol Vl is the gain value (dB) when the bottom echo height V1 = 50% with respect to the steel plate 5 and V2 is the bottom echo height with two skips on the steel plate 5. The gain value (dB) when V2 = 50%, the symbol Wl is the beam path length during one skip (mm, l ₁ + l ₂ in FIG. 5), and the symbol W2 is the V transmission method 2 skip The beam path length of time (mm, l ₁ +... + L ₄ in FIG. 5).

  If the ultrasonic attenuation coefficient α obtained in this way exceeds 0.1 dB / mm, it will deviate significantly from the attenuation coefficient of the standard specimen specified in JIS Z3060, and the standard specimen cannot be used for sensitivity adjustment according to the provisions of JIS Z3060. In addition, when the ultrasonic attenuation coefficient α varies depending on the flaw detection location, it is necessary to change it partially, which hinders quick or accurate inspection.

Therefore, in the present invention, the ultrasonic attenuation coefficient α is desirably 0.1 (dB / mm) or less.
Next, the desirable composition of the high-strength steel material of the present embodiment will be described. Even for a high-strength steel material having a composition other than the composition exemplified below, the continuously cast steel slab is kept at 1180 ° C. or higher for 3 hours. If it is a high-tensile steel material with a tensile strength of 680 MPa or more, which is manufactured by heating the steel slab to 800 ° C or less at a cooling rate of 1 ° C / s or more and then heating and rolling the steel slab again. In an arbitrary cross section including the plate thickness direction, the number of old austenite grains exceeding 100 μm in the plate thickness direction is suppressed to 1 or less per 1 mm ² . The amount of attenuation of the reflected echo is small, and the present invention is not limited to the composition exemplified below.

  The old austenite grains remain in the product when the structure is a bainite or a mixed structure of bainite and martensite, and the structure is limited to the following if the tensile strength is 68 MPa or more and has such a structure. It is not something.

C: 0.01-0.20%
C is an element effective in increasing the strength by containing 0.01% or more. However, if the C content exceeds 0.20%, it becomes difficult to ensure toughness and prevent deterioration of weld crack resistance. Therefore, in the present embodiment, the C content is limited to 0.01% or more and 0.20% or less. From the same viewpoint, the lower limit of the C content is preferably 0.03%, and the upper limit is preferably 0.17%.

Si: 0.5% or less
Si is an element necessary for deoxidation. However, if the Si content exceeds 0.5%, the toughness of the heat affected zone decreases. Therefore, in this embodiment, the Si content is limited to 0.5% or less. From the same viewpoint, the upper limit of the Si content is desirably 0.3%.

Mn: 0.6 to 2.0%
Mn is an element effective in increasing the strength by containing 0.6% or more. However, if the Mn content exceeds 2.0%, the toughness decreases. Therefore, in this embodiment, the Mn content is limited to 0.6% or more and 2.0% or less. From the same viewpoint, the lower limit of the Mn content is preferably 0.7%, and the upper limit is preferably 1.8%.

P: 0.015% or less, S: 0.005% or less It is desirable that both P and S are impurities and their content is low. However, since significant reduction is accompanied by a corresponding increase in processing cost, P in this embodiment is P The content is limited to 0.015% or less, and the S content is limited to 0.005% or less.

Ti: 0.005 to 0.03%
Ti is an element effective for refinement of crystal grains by containing 0.005% or more. However, if the Ti content exceeds 0.03%, the toughness deteriorates. Therefore, in this embodiment, the Ti content is limited to 0.005% or more and 0.03% or less. From the same viewpoint, the lower limit of the Ti content is preferably 0.007% and the upper limit is preferably 0.02%.

sol.Al: 0.005 to 0.08%
Al is an element that is effective as a deoxidizer and also for refining crystal grains, so 0.005% or more is contained as sol.Al. However, if the sol.Al content exceeds 0.08%, excessive inclusions are generated that are harmful to the properties of the steel. Therefore, in the present embodiment, the sol.Al content is limited to 0.005% or more and 0.08% or less. From the same viewpoint, the lower limit of the sol.Al content is preferably 0.01%, and the upper limit is preferably 0.05%.

N: 0.008% or less N produces nitride together with Al, and is effective for refinement of crystal grains. However, if the N content exceeds 0.008%, the toughness of the weld is impaired. Therefore, in the present embodiment, the N content is limited to 0.008% or less. From the same viewpoint, the upper limit of the N content is preferably 0.006%.

Cr: 0.01-1.0%
Cr is also an element effective in increasing the strength. For that purpose, Cr is contained in an amount of 0.01% or more. However, if the Cr content exceeds 1.0%, the toughness is lowered. Therefore, in this embodiment, the Cr content is limited to 0.01% or more and 1.0% or less.

Mo: 0.01 to 1.50%
Mo is also an element effective in increasing the strength. For that purpose, Mo is contained in an amount of 0.01% or more. However, if Mo is contained in an excessive amount exceeding 1.50%, the toughness is lowered. Therefore, in the present embodiment, the Mo content is limited to 0.01% or more and 1.50% or less.

V: 0.005 to 0.10%
V is also an element effective for increasing the strength. For that purpose, V is contained in an amount of 0.005% or more. However, if V is contained in an excessive amount exceeding 0.10%, the toughness is lowered. Therefore, in this embodiment, the V content is limited to 0.005% or more and 0.10% or less.

As an optional additive element, one or more of Cu: 0.05 to 1.0%, Ni: 0.05 to 4.0%, Nb: 0.005 to 0.08%, and B: 0.0002 to 0.020% . In addition to the above-described elements, one or more of each element described below can be contained as an optional additive element.

  Cu is an element effective for increasing the strength. Therefore, it is desirable to contain Cu by 0.05% or more. However, if Cu is contained in an excessive amount exceeding 1.0%, the toughness is lowered. Therefore, when Cu is added, its content is desirably limited to 0.05% or more and 1.0% or less.

  Ni is an element effective for improving low-temperature toughness. For that purpose, it is desirable to contain 0.05% or more of Ni. However, even if Ni is contained in an excessive amount exceeding 4.0%, it is not possible to expect an increase in strength and improvement in toughness commensurate with the cost increase due to the addition of Ni. Therefore, when Ni is added, its content is desirably limited to 0.05% or more and 4.0% or less.

  Nb is an element effective for refining crystal grains. To that end, Nb is desirably contained in an amount of 0.005% or more. However, if the Nb content exceeds 0.08% and is contained in an excessive amount, the toughness deteriorates. Therefore, when Nb is added, its content is desirably limited to 0.005% or more and 0.08% or less.

  Further, when B is added in an amount of 0.0002% or more, it is effective for improving the hardenability and the accompanying strength increase. However, if the B content exceeds 0.020%, the toughness deteriorates. Therefore, when B is added, its content is preferably limited to 0.0002% or more and 0.020% or less.

The balance other than the above is Fe and inevitable impurities.
Next, the heating condition and cooling condition of the slab as the slab pretreatment described above will be described.
If the heating temperature of the slab is less than 1180 ° C, the main precipitates such as NbC do not dissolve, the number of aggregates during solidification is small, and the precipitate does not change with a large particle size. Due to the shortage, the prior austenite grains become coarse. Therefore, the heating temperature of the slab is limited to 1180 ° C or higher. From the same viewpoint, the heating temperature of the slab is more preferably 1200 ° C. or higher. The heating temperature of the slab is preferably 1260 ° C. or less from the viewpoint of suppressing coarsening due to crystal grain growth.

  Moreover, when the heating time of the slab is less than 3 hours, the inside of the slab is not sufficiently heated, and the precipitate remains in an insoluble state in the inside, and this causes the prior austenite grains to become coarse. . Therefore, the slab heating temperature is limited to 3 hours or more. Further, the heating time of the slab is desirably 12 hours or less in order to suppress coarsening due to crystal grain growth. More desirably, it is 3 hours or more and 10 hours or less.

Next, slab cooling is performed at a cooling rate Vr of 1 ° C./s or higher until the surface temperature of the slab becomes 800 ° C. or lower. Here, the slab cooling rate Vr is defined as follows: the surface temperature of the slab before descaling is T ₀ ° C, the surface temperature of the slab after cooling is stopped is T ₁ ° C, and the cooling time is t seconds. The cooling rate is defined by Vr = (T ₀ −T ₁ ) / t.

This cooling rate Vr is important in the present invention. If the cooling rate Vr is less than 1 ° C./s, the target dispersion state of the small number of precipitates cannot be achieved, and coarse particles may be generated. Further, the cooling rate Vr of the slab is desirably 15 ° C./s or less from the viewpoint of sufficiently depositing the precipitate. More preferably, it is 1 to 10 ° C./s. Here, the cooling means is not particularly limited as long as the cooling rate Vr described above is obtained. In the present embodiment, the amount of cooling water and the like are appropriately adjusted so that the cooling rate Vr in the above-described range can be obtained when passing through the descaling device on the outlet side of the heating furnace a plurality of times.

  According to this embodiment, it is possible to reliably manufacture a high-tensile steel material with a small amount of reflection echo attenuation when an ultrasonic flaw detection test is performed.

Furthermore, the present invention will be described more specifically with reference to examples.
C: 0.10%, Si: 0.25%, Mn: 1.00%, P: 0.006%, S: 0.002%, Cu: 0.20%, Ni: 1.50%, Cr: 0.50%, Mo: 0.50%, V: 0.04%, Ti: 0.015%, Nb: 0.015%, B: 0.001%, sol.Al: 0.03%, using a slab (thickness, 300mm x width 2300mm) with a steel composition consisting of Fe and unavoidable impurities. High-strength steel sheet manufactured by reheating and rolling after performing slab pretreatment that heats and cools repeatedly (example of the present invention, thickness 34 mm × width 3000 mm), and manufactured without slab pretreatment (comparison) For example), the ultrasonic attenuation coefficient α was compared. For comparison, the ultrasonic attenuation coefficient α of the standard specimen STB-A2 defined in JIS Z2345 was also measured.

  In the case of heating twice, the first heating was performed under conditions of 1200 ° C. × 5 Hr, and then cooled to 800 ° C. or lower at 3 ° C./s. The second heating was performed under the conditions of 1000 ° C. × 5 Hr, then rolled, finished at 820 ° C., and cooled with water from 730 ° C. to 200 ° C.

In the case of one-time heating, the second heating condition is the same as the second heating condition, and the subsequent rolling and cooling conditions are also the same.
The ultrasonic flaw detection conditions were as listed below.

Nominal refraction angle: 60 degrees Probe size: 10 x 10mm
Wedge material: Acrylic Measurement position: intersection of 300mm pitch grid in the width and length directions.

  Furthermore, the size of the prior austenite grains in the thickness direction is measured at 10 locations in the 1 mm x 1 mm range on the ultrasonic beam path at the position where the ultrasonic attenuation coefficient α is measured, and exceeds 100 μm. The number of prior austenite grains was counted.

  The results are summarized in Table 1.

  Thus, it is clear that the slab pretreatment has a smaller attenuation coefficient than the slab pretreatment.

It is explanatory drawing showing the investigation method of the scattering state of an ultrasonic wave. It is a graph which shows the attenuation amount of the reflective echo of each of 2 types of steel materials. It is a graph which shows the relationship between a heating holding time and the fraction of a coarse austenite grain. It is explanatory drawing which shows an example of the microstructure of a plate | board thickness direction. It is explanatory drawing which shows typically the condition which scans the steel material with the ultrasonic wave transmitter and ultrasonic wave receiver of an oblique angle probe, and investigates ultrasonic attenuation.

Explanation of symbols

1 Sample 2 Vertical Transverse Wave Probe 3 Ultrasonic Transmitter 4 Ultrasonic Receiver 5 Steel

Claims (2)

With a tensile strength of more than 680MPa, at any thickness direction cross-section in the steel, the size of prior austenite grains is a is one less / mm ² in excess of 100 [mu] m in the thickness direction, and the following ( 1) A high-strength steel material characterized in that the ultrasonic attenuation coefficient α defined by the equation is 0.1 (dB / mm) or less.
α = 20 {Log (V1 / V2)} / (W2-W1) (1)
here,
Vl: Gain value (dB) when the bottom echo height is V1 = 50% in one skip by the V transmission method.
V2: Gain value (dB) when bottom echo height V2 = 50% with 2 skips by V transmission method
Wl: Beam path length in 1 skip by V transmission method (mm)
W2: Beam path length in 2 skips by V transmission method (mm)
Indicates.   After the continuously cast steel slab is heated at 1180 ° C or higher for 3 hours or more and then cooled to 800 ° C or lower at a cooling rate of 1 ° C / s or higher, the steel slab is heated again and rolled. A method for producing high-tensile steel with a characteristic tensile strength of 680 MPa or more.

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