KR101273783B1 - Steel material excellent in brittle crack propagation suspension property and method for producing the same - Google Patents

Abstract

The steel material excellent in the brittle crack propagation stop characteristic of this invention is a following formula when the metal structure is observed by back-scattering electron diffraction (EBSP method) in the area | region from the depth t / 8 position to t / 4 position of a steel material. And 2 are satisfied. However, D measures two adjacent crystal orientation differences by the EBSP method, and the average circle equivalent diameter (micrometer) of crystal grains enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more, and R is the ratio of the random grain boundary which occupies at the said diagonal grain boundary. (Area%).
[Equation 1]
D ≤ 8 μm
&Quot; (2) "
R ≥ 50 area%

Description

STEEL MATERIAL EXCELLENT IN BRITTLE CRACK PROPAGATION SUSPENSION PROPERTY AND METHOD FOR PRODUCING THE SAME}

The present invention relates to steel materials used in structures such as bridges, buildings, ships, and the like, and more particularly, to steel materials in which brittle cracks generated in steel materials are quickly stopped.

Steel materials used in structures such as bridges, buildings, ships, tanks, offshore structures, and line pipes are required to be brittle and difficult to break. In order to suppress brittle fracture, it is effective not to produce brittle cracks in steel materials, and when brittle cracks occur in steel materials, stopping them quickly without advancing the brittle cracks generated (hereinafter referred to as brittle crack propagation stop characteristics). Case) is valid.

Brittle cracks are known to occur when the stress expansion coefficient K of the steel material is equal to or higher than the Kca value (K ≧ Kca) measured by the brittle fracture propagation stop characteristic test. The stress expansion coefficient K is expressed by K = σ√ (π × a) when the stress is σ and the length of the crack is a. Therefore, the higher the strength of the steel and the higher the stress sigma, the more likely the brittle cracking is to occur. In order to prevent the occurrence of brittle cracks, it is effective to lower the strength of the steel. However, as the structure becomes larger, the strength required for steel materials has been increasing.

The present inventors do not reduce the stress expansion coefficient K by reducing the stress sigma to prevent the occurrence of brittle cracks, but widen the allowable range of the stress expansion coefficient K by increasing the Kca value of the steel material, thereby increasing the large stress sigma. Japanese Patent Laid-Open Publication No. 2010-1520 has proposed a technique for preventing the occurrence of brittle cracks even when a load is applied. The thick steel plate proposed therein is a diagonal grain boundary where the structure at the position of depth t / 8 to t / 4 (t is the plate thickness) from the surface is the bainite main body, and two crystal orientation differences adjacent to each other are 15 ° or more. When the enclosed area was made into crystal grains, the average circle equivalent diameter of this crystal grain was controlled to 8 micrometers or less. By miniaturizing the crystal grains, the Kca value is increased. That is, the refinement of the crystal grains increases the frequency at which the cracks collide with the grain boundaries and stops the cracks from advancing.

Since further high strength of steel materials is desired, the present inventors have studied to further improve brittle crack propagation stopping characteristics even after proposing the technique of the above patent.

This invention is made | formed in view of such a situation, and the objective is to provide the steel material which further improved the brittle crack propagation stop characteristic, and its manufacturing method.

The steel material which concerns on the said subject which could solve the said subject is C: 0.02-0.12% (The meaning of "mass%. Hereafter, the same with respect to a chemical component.), Si: 0.5% or less, Mn: 1-2%, Nb : 0.005 to 0.04%, B: 0.0005 to 0.003%, Ti: 0.005 to 0.02%, N: 0.0040 to 0.01%, P: 0.02% or less, S: 0.015% or less, Al: 0.01 to 0.06% Addition of iron and unavoidable impurities. When the metal structure is observed by the backscattered electron diffraction method (EBSP method) in the region from the depth t / 8 position to the t / 4 position (t is the thickness of the steel. 1 and 2 are satisfied. In Equation (1), D denotes an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries whose crystal orientation differences are 15 degrees or more by measuring two adjacent crystal orientation differences by the EBSP method. In addition, in Equation 2, R means the ratio (area%) of the random grain boundary occupying the diagonal grain boundary.

[Equation 1]

D ≤ 8 μm

&Quot; (2) "

R ≥ 50 area%

When the hardness is measured in the region from the outermost surface to the depth t / 4 position, the steel is preferably at least 190 Hv.

The steel material may further include, as another element, at least one selected from the group consisting of (a) Ni: 0.7% or less, Cu: 0.3% or less, Cr: 1.5% or less, and Mo: 1% or less; And / or (b) V: 0.1% or less.

The steel having excellent brittle crack propagation stopping characteristics according to the present invention is heated to an appropriate temperature (preferably 1050 ° C or higher) of the above-described composition, and accumulated in a temperature range of Ar ₃ point + 30 ° C. or lower and Ar ₃ point or higher. Rolling with a reduction ratio of at least 50%, followed by Ar ₃ points + 30 ° C (preferably, recrystallization temperature-30 ° C or more) by appropriate means (heating, reheating, etc., preferably reheating), and recrystallization After heating up in the temperature range of temperature +20 degrees C or less (preferably less than recrystallization temperature), cooling (preferably cooling from the temperature of Ar ₃ or more point to 500 degrees C or less at the average speed of 5 degrees C / sec or more) It can manufacture by. The said steel material can also be cooled to the said Ar ₃ point +30 degrees C or less by accelerated cooling after heating. After the said temperature rising, you may perform the said cooling after rolling.

In the present invention, focusing on the metal structure in the surface layer portion of the steel material, in addition to suppressing the average circle equivalent diameter of the crystal grains surrounded by the diagonal grain boundary to 8 µm or less, the proportion of the random grain boundary occupied by the diagonal grain boundary is increased to a predetermined value or more. Since the brittle crack propagation stop characteristic is improved, the steel can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the heat pattern at the time of measuring recrystallization temperature with a processing poremaster tester.
It is explanatory drawing which shows the shape of the test piece used for evaluating a fatigue characteristic.
3 is a graph showing the relationship between the cumulative reduction ratio during hot rolling and the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries.
4 is a graph showing the relationship between the presence or absence of elevated temperature due to reheating and the ratio R between random grain boundaries at diagonal grain boundaries.
It is a graph which shows the relationship between the average cooling rate after heated up by reheating, and the minimum value of the hardness in the area | region from the outermost surface of a steel plate to the depth t / 4 position.
Fig. 6 is a graph showing the relationship between the average circle equivalent diameter D of grains surrounded by diagonal grain boundaries and the Kca value (brittle crack propagation stop characteristic) at -10 ° C.
FIG. 7 is a graph showing the relationship between the ratio R of random grain boundaries at diagonal grain boundaries and Kca value (brittle crack propagation stop characteristic) at −10 ° C. FIG.
8 is a graph showing the relationship between the minimum value of hardness and the fatigue limit (fatigue characteristic) in the region from the outermost surface of the steel sheet to the depth t / 4 position.

When the tensile strength of the steel is increased, the stress σ becomes large, and thus the stress expansion coefficient K becomes large, and brittle cracking tends to occur. Therefore, as disclosed in Japanese Patent Application Laid-Open No. 2010-1520, it is not possible to prevent the occurrence of brittle fracture only by suppressing the average circle-equivalent diameter D of crystal grains surrounded by diagonal grain boundaries to 8 µm or less. Turned out.

Therefore, the present inventor has earnestly examined with the aim of providing a steel material which further improved the brittle crack propagation stopping characteristic. As a result, it was found that when the ratio R of the random grain boundary occupying the diagonal grain boundary is 50 area% or more, the progress can be stopped quickly even if a brittle crack occurs, and the brittle crack propagation stop characteristic can be secured.

In other words, diagonal grain boundaries are known to be classified into "corresponding grain boundaries" having low grain boundary energy and "random grain boundaries" having high grain boundary energy (for example, "material histology", Takaki Setsuo, and Tsagagi Aki. Tsukasa, published by Asakura Bookstore, page 45. Among them, the random grain boundary having a high grain boundary energy was resistant to the progress of brittle cracking, and the study was repeated with the thought that the brittle crack could stop rapidly. As a result, as is clear from the examples described later, the amount of random grain boundaries can be adjusted by a predetermined method, and the brittle crack propagation stopping characteristic is improved when the ratio R of the random grain boundaries occupies the diagonal grain boundary is 50 area% or more. It turns out that you can.

Hereinafter, the present invention will be described in detail.

[About the average circle equivalent diameter D of crystal grains enclosed by diagonal grain boundaries whose crystal orientation difference is 15 degrees or more, and the ratio R of random grain boundaries occupying diagonal grain boundaries]

When the steel material of this invention observes a metal structure by a backscattering electron diffraction image method (EBSP method), it is necessary to satisfy following formula (1) and (2). By satisfying both equations, the brittle crack propagation stop characteristic can be improved.

[Equation 1]

D ≤ 8 μm

&Quot; (2) "

R ≥ 50 area%

In Equation 1, D measures two adjacent crystal orientation differences by the EBSP method, and when an area surrounded by diagonal grain boundaries having a crystal orientation difference of 15 ° or more is used as the crystal grain, the average circle equivalent diameter (μm) of the crystal grain is determined. It means. The "circle equivalent diameter" is the diameter of the circle which measured the area of a crystal grain, and assumed so that area might become the same.

In this invention, in order to improve brittle crack propagation stop characteristic similarly to the said patent publication 2010-1520, the said D value is made into 8 micrometers or less. That is, it is generally known that brittle cracks bend, bypass, or stop at diagonal grain boundaries with a crystal orientation difference of 15 ° or more. Therefore, by miniaturizing the crystal grains surrounded by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more, the position where the brittle cracks are bent, bypassed, and rectified increases, and as a result, the brittle cracks can be stopped from progressing.

The said D value becomes like this. Preferably it is 7 micrometers or less, More preferably, it is 6 micrometers or less. On the other hand, D value is so preferable that it is small, and a minimum in particular is not restrict | limited, For example, about 1 micrometer may be sufficient.

In Equation 2, R denotes the ratio (area%) of random grain boundaries occupied at diagonal grain boundaries where the crystal orientation differences are 15 ° or more when two adjacent crystal orientation differences are measured by the EBSP method.

In this invention, R value is made into 50 area% or more. By increasing the random grain boundary with high grain boundary energy, the resistance to the growth of the brittle crack can be increased, and the brittle crack propagation stop characteristic can be improved.

Preferably said R value is 53 area% or more, More preferably, it is 55 area% or more. On the other hand, R value is so preferable that it is large, and an upper limit in particular is not restrict | limited, For example, about 65 area% may be sufficient.

When the thickness of steel materials is t (mm), D value and R value observe and measure the metal structure in the area | region from the depth t / 8 position to t / 4 position. The brittle crack propagation stop characteristics are D values and R in the region from the depth t / 8 position to the t / 4 position because the energy loss due to the soft fracture region (shear lip) formed from the surface layer is affected. By controlling the value, brittle cracking can be stopped.

By the above, the brittle crack propagation stop characteristic of steel materials improves. This technique can be used effectively for steel and high strength steel which improved the surface hardness especially.

By the way, the structure is usually repeatedly loaded with stress. When the stress is repeatedly loaded, slip bands are generated by the melt stress, which develops with repetition to form protrusions or grooves (hereinafter referred to as protrusions, etc.) on the surface of the structure. When stress concentrates on this protrusion or the like, fatigue cracking occurs and fatigue fracture occurs. However, in order to ensure safety, fatigue cracking is unlikely to occur in the steel constituting the structure and it is required to have excellent fatigue characteristics.

In order to prevent the occurrence of fatigue cracking, it is effective to suppress the sliding due to the shear stress, to prevent the formation of protrusions, and to increase the yield point YP and the tensile strength TS of the steel. Therefore, in the present invention, in order to increase the yield point and the tensile strength, attention was paid to the hardness at the surface layer portion of the steel. Fatigue cracks are generated on the surface of the steel, so if the surface layer of the steel is hardened to increase the yield point and tensile strength of the portion, it is possible to prevent the occurrence of protrusions and to suppress the occurrence of fatigue cracks. Because I did. And as it turned out in the Example mentioned later, when the minimum value of the hardness in surface layer part was 190 Hv or more, it turned out that a fatigue characteristic can be improved.

[About the minimum value of hardness]

In this invention, it is preferable to make minimum value of the hardness in the surface layer part of steel materials into 190 Hv or more. By hardening the surface layer part of steel materials, even if stress is repeatedly loaded, formation of protrusions and the like can be prevented, so that fatigue characteristics of steel materials can be improved.

The minimum value of the said hardness is so good that it is large, More preferably, it is 200 Hv or more, More preferably, it is 210 Hv or more.

In order to set the minimum value of the hardness to 190 Hv or more, the metal structure of the surface layer portion may be mainly bainite. The bainite principal means that the bainite fraction is 60 area% or more when the metal structure is observed under an electron microscope. The bainite fraction is preferably at least 70 area%, more preferably at least 80 area%, even more preferably at least 90 area%, and most preferably at least 100 area% of bainite.

Ferrite may be a metal structure other than bainite. However, as the ferrite fraction in the metal structure increases, the hardness of the steel tends to decrease. Therefore, the fraction of ferrite in the metal structure is as small as possible, for example, preferably 8 area% or less, more preferably 5 area% or less, and still more preferably 3 area% or less.

Although the upper limit of the said hardness is not specifically limited, For example, about 260 Hv may be sufficient. This upper limit is almost the same as the average hardness of bainite structure.

The said hardness is measured in the area | region from the outermost surface to the depth t / 4 position, when the thickness of steel materials is t (mm). This area is set in order to prevent the occurrence of fatigue cracking because the position where fatigue cracking occurs is the outermost surface of the steel.

The said hardness measures the area | region from the outermost surface to the depth t / 4 position at equal intervals (for example, 1 mm space | interval), and should just calculate | require a minimum value. The specific measurement procedure is demonstrated in the Example part mentioned later.

In the steel of the present invention, the metal structure (preferably metal structure and hardness) in the surface layer portion satisfies the above requirements, and the component composition of the steel material is C: 0.02 to 0.12%, Si: 0.5% or less, Mn: 1-2%, Nb: 0.005-0.04%, B: 0.0005-0.003%, Ti: 0.005-0.02%, N: 0.0040-0.01%, P: 0.02% or less, S: 0.015% or less, Al: 0.01 To 0.06%. The reason for determining this range is as follows.

C is an indispensable element in order to secure the strength of the steel (base metal), and it is necessary to contain C at least 0.02%. It is preferable to contain C 0.04% or more, More preferably, it is 0.05% or more. However, when C exceeds 0.12%, not only the formation of island-like martensite (MA) in the HAZ during welding causes not only the deterioration of the toughness of the HAZ but also adversely affects the weldability. Therefore, C is at most 0.12%, preferably at most 0.1%, more preferably at most 0.08%.

Si is an element which contributes to securing the strength of steel by solid solution strengthening. However, when Si exceeds 0.5%, a large amount of phase martensite (MA) is generated in the HAZ during welding, which not only causes deterioration of the HAZ toughness but also adversely affects the weldability. Therefore, Si is made into 0.5% or less. Preferably it is 0.4% or less, More preferably, it is 0.3% or less, More preferably, it is 0.2% or less. In addition, although it does not need to contain Si, in order to add Si and ensure the strength of steel materials, it is preferable to contain Si2% or more. More preferably, it is 0.05% or more, More preferably, it is good to contain 0.1% or more.

Mn is an element which contributes to the strength improvement of steel materials (base material), and needs to be contained 1% or more. Mn is preferably 1.2% or more, more preferably 1.4% or more. However, when Mn exceeds 2%, the weldability of the steel material (base material) is deteriorated. Therefore, Mn needs to be suppressed to 2% or less. Mn becomes like this. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

Nb has an effect of suppressing coarsening of recrystallized grains and improving brittle crack propagation stopping characteristics by a solute drag effect by solid solution and a pinning effect by depositing carbonitrides. Have It also contributes to the improvement of base metal toughness. In order to exhibit such an effect, it is necessary to contain Nb 0.005% or more. More preferably, it is 0.007% or more, More preferably, it is 0.009% or more. However, when Nb exceeds 0.04%, precipitated carbonitrides coarsen and degrade base metal toughness. Therefore, Nb is made into 0.04% or less. In particular, in order to improve the HAZ toughness, Nb is preferably 0.035% or less, more preferably 0.03% or less, still more preferably 0.025% or less, particularly preferably 0.02% or less.

B is an element which improves hardenability and improves strength. In addition, B is an element which suppresses formation of grain boundary ferrite and improves HAZ toughness. In order to exhibit the effect by B addition, it is necessary to contain 0.0005% or more, Preferably it is 0.001% or more, More preferably, it is 0.0015% or more. However, when B exceeds 0.003%, it precipitates as BN in an austenite grain boundary, and causes the fall of HAZ toughness. Therefore, B is 0.003% or less, Preferably it is 0.0025% or less, More preferably, it is 0.002% or less.

Ti has a function of finely dispersing nitride (TiN) in steel to prevent coarsening of austenite grains and suppressing coarsening due to recrystallization of austenite. Height has a function. In addition, Ti is an element that forms oxides in addition to nitrides and contributes to improvement of HAZ toughness. In order to exhibit such an effect, it is necessary to contain Ti 0.005% or more. Preferably it is 0.007% or more, More preferably, you may be 0.01% or more. However, excessive addition of Ti deteriorates the toughness of the steel (base material), and therefore Ti should be suppressed to 0.02% or less. Preferably it is 0.018% or less, More preferably, it is 0.016% or less.

N is an element which has the effect | action which precipitates Ti nitride and raises brittle crack propagation stop characteristic. In addition, N is an element that prevents coarsening of austenite grains formed in HAZ during welding, promotes ferrite transformation, and contributes to improvement of HAZ toughness due to the pinning effect of nitride. In order to exhibit these effects effectively, it is necessary to contain 0.0040% or more. Preferably it is 0.005% or more, More preferably, it is 0.006% or more. As the N content increases, the Ti nitride is formed to promote the miniaturization of the austenite grains, so that N is effective in improving the toughness of the HAZ. However, when N exceeds 0.01%, the amount of solid solution N increases, the toughness of the base material itself deteriorates, and HAZ toughness also falls. Therefore, N needs to be suppressed to 0.01% or less. Preferably it is 0.0095% or less, More preferably, you may be 0.009% or less.

P is an element which is easy to segregate, and in particular, it segregates at grain boundaries in steel materials and deteriorates base metal toughness. Therefore, P should be suppressed to 0.02% or less. Preferably it is 0.018% or less, More preferably, you may be 0.015% or less.

S is a harmful element that combines with Mn to form sulfide (MnS), which degrades the ductility of the base metal and the ductility in the sheet thickness direction. Therefore, S should be suppressed to 0.015% or less. Preferably it is 0.012% or less, More preferably, it is 0.008% or less, More preferably, it is 0.006% or less.

Al is an element which acts as a deoxidizer, and is an element which forms AlN and acts in refinement | miniaturization of a crystal grain. In order to exert such an effect, Al needs to be 0.01% or more. Al is preferably 0.02% or more, and more preferably 0.03% or more. However, when excessive, the base metal toughness and the HAZ toughness deteriorate. Therefore, Al needs to be suppressed to 0.06% or less. Al is preferably 0.04% or less, and more preferably 0.035% or less.

The steel material of this invention contains the said element as an essential component (except Si, P, S), and remainder is iron and an unavoidable impurity (for example, Mg, As, Se, etc.).

The steel of the present invention is also effective to further contain, as another element, an element (Ni, Cu, Cr, Mo) for improving the strength of the steel and / or an element (V) for further improving the HAz toughness. Specifically,

(a) at least one element selected from the group consisting of Ni: 0.7% or less, Cu: 0.3% or less, Cr: 1.5% or less, and Mo: 1% or less; And / or (b) V: 0.1% or less. The reason for determining this range is as follows.

[(a) Ni, Cu, Cr, Mo]

Ni, Cu, Cr, and Mo are all elements which contribute to raising the strength of steel materials, and can be added individually or in combination, respectively.

In particular, Ni is an element which contributes not only to increasing the strength of the steel, but also to improving the toughness of the steel itself. It is preferable to contain Ni as much as possible, but since it is an expensive element, when it contains excessively, it will become high cost. Therefore, for economic reasons, the upper limit is preferably 0.7%. More preferably, it is 0.5% or less, More preferably, it is 0.4% or less. On the other hand, it is preferable to contain Ni 0.01% or more in order to exhibit the above-mentioned effect effectively. More preferably, it is 0.02% or more, More preferably, it is 0.03% or more.

Cu is an element which solid-solution strengthens and raises the strength of steel materials. However, since the toughness of steel materials deteriorates when it contains exceeding 0.3%, Cu is preferable to be 0.3% or less. More preferably, it is 0.28% or less, More preferably, it is 0.25% or less. On the other hand, in order to exhibit the above-mentioned effect effectively, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more, More preferably, it is 0.03% or more.

Cr acts to increase the strength of the steel, but when it exceeds 1.5%, the strength of the steel (base material) is significantly increased so that the toughness of the base material deteriorates, so the HAZ toughness is lowered. Therefore, Cr is preferably at most 1.5%. More preferably, it is 1.2% or less, More preferably, it is 1% or less, Especially preferably, it is 0.5% or less. On the other hand, in order to exhibit the above-mentioned effect effectively, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more, More preferably, it is 0.03% or more.

Mo acts to increase the strength of the steel, but when it exceeds 1%, the strength of the steel (base material) becomes so high that the base metal toughness deteriorates rather than the HAZ toughness. Therefore, it is preferable to make Mo 1% or less. More preferably, it is 0.7% or less, More preferably, it is 0.5% or less, Especially preferably, it is 0.05% or less. On the other hand, in order to exhibit the above-mentioned effect effectively, it is preferable to contain 0.01% or more. More preferably, it is 0.02% or more, More preferably, it is 0.03% or more.

[(b) V]

Although V is an element which improves HAZ toughness, when it contains more than 0.1%, the deposited carbonitride will coarsen and deteriorate the toughness of a base material. Therefore, V is preferably made 0.1% or less. More preferably, it is 0.08% or less, More preferably, it is 0.05% or less, Especially preferably, it is 0.01% or less. On the other hand, in order to exhibit the above-mentioned effect effectively, it is preferable to contain 0.001% or more. More preferably, it is 0.002% or more, More preferably, it is 0.003% or more.

Next, the method of manufacturing the steel material of this invention is demonstrated.

In order to control the average circle equivalent diameter (D value) of the grains surrounded by the diagonal grain boundaries and the ratio (R value) of the random grain boundaries occupied by the diagonal grain boundaries within a predetermined range, the heated steel is placed in the unrecrystallized region immediately above Ar ₃ . After introducing a strain (potential) into steel materials by rolling (hereinafter referred to as low temperature rolling), it is important to raise the temperature to near the recrystallization temperature (hereinafter, referred to as a temperature raising process). Even below the recrystallization temperature, since recrystallization occurs using the introduced strain as a driving force, new grain boundaries are generated. And while the grain boundary produced at the low temperature just above Ar ₃ becomes a corresponding grain boundary, the grain boundary produced near recrystallization temperature becomes a random grain boundary. Therefore, D value can be made small and R value can be made large, so that the accumulation | accumulation of the deformation | transformation introduced by rolling in a non-recrystallization area | region is large, and closer to recrystallization temperature by subsequent temperature increase.

The heating temperature of the steel material (the temperature of the steel means an average temperature, which is the same below, and the method of determination thereof is described in detail in the examples) is preferably 1050 ° C or more and 1080 ° C or more, for example. Preferably it is 1100 degreeC or more. By heating at 1050 degreeC or more, the structure of steel materials can be made into austenite single phase, and Nb can be fully dissolved. However, when the heating temperature is too high, the initial austenite structure becomes excessively coarse, so that it becomes difficult to sufficiently refine the structure after transformation. Therefore, the upper limit of heating temperature is 1250 degrees C or less, Preferably it is 1200 degrees C or less, More preferably, you may be 1150 degrees C or less.

The heated steel materials are subjected to crude rolling and cooling as necessary, and then subjected to the low temperature rolling. The temperature of low temperature rolling is Ar ₃ point + 30 degrees C or less (preferably Ar ₃ point + 20 degrees C or less), and Ar ₃ point or more. The cumulative reduction ratio in this temperature range is 50% or more, preferably 53% or more, more preferably 55% or more. In view of the R value and the D value, the upper limit of the cumulative reduction ratio is not limited, but considering the rolling load and the production efficiency, the cumulative reduction ratio is, for example, 80% or less, preferably 70% or less, and more preferably 65 It may be about% or less.

On the other hand, the temperature of the Ar ₃ point can be calculated from the following equation (3). In Equation 3, [] represents the content (mass%) of each element, and t represents the finished sheet thickness (mm) of the steel.

&Quot; (3) "

Ar ₃ (° C.) = 910-310 × [C] -80 × [Mn] -20 × [Cu] -15 × [Cr] -55 × [Ni] -80 × [Mo] + 0.35 × (t-8 )

After low temperature rolling, it heat-processes as above-mentioned. The temperature range of this temperature increase is more than Ar <_3> +30 degreeC and recrystallization temperature +20 degreeC or less. The closer to the recrystallization temperature, the more the recrystallization with the strain as the driving force proceeds, the larger the R value and the smaller the D value. However, if the recrystallization temperature is excessively exceeded, crystal grains will start to grow, which makes it difficult to suppress the D value. In consideration of the balance between recrystallization and grain growth, the temperature range of the elevated temperature was set to the recrystallization temperature + 20 ° C or lower. Preferable temperature ranges are recrystallization temperature -30 degreeC or more (especially recrystallization temperature -20 degreeC or more) and below recrystallization temperature (especially recrystallization temperature -5 degrees C or less).

In addition, recrystallization temperature can be measured in the following procedure using a processed pore master tester, for example. A cylindrical test piece having a diameter of 8 mm and a height of 12 mm is prepared and processed into a thermal pattern shown in FIG. 1. That is, after heating a test piece to 1100 degreeC at a temperature increase rate of 10 degree-C / sec, holding for 1 minute, initial processing is performed so that height may become 10 mm, and it hold | maintains for 20 minutes with 1100 degreeC. Next, cooling is carried out at a cooling rate of 50 ° C./sec from 1100 ° C. so that the processing temperature becomes 760 ° C., 780 ° C., 800 ° C., 820 ° C., 840 ° C., 860 ° C. or 880 ° C., and the process is maintained for 10 seconds at each processing temperature → 1 pass. 10th process hold | maintained from the said process temperature, the 10th process holding | maintenance was repeated, and after completion | finish of the 4th process, it hold | maintained for 10 second at processing temperature, and cooled to room temperature at 50 degreeC / sec. Each pass was processed at a stroke speed of 15 mm / sec so that the test piece height became 9.0 mm in the first pass, 8.0 mm in the second pass, 7.0 mm in the third pass, and 6.5 mm in the fourth pass. The cooling was quenched using inert gas. As a comparison object, the sample which hold | maintained for said 20 minutes, and cooled (quenched by inert gas) at 50 degree-C / sec of cooling rate to room temperature was also prepared, without processing.

After processing, the austenite grain size at each processing temperature is measured, the presence or absence of recrystallization is determined, and the recrystallization temperature is determined. It was evaluated that the case where the austenite grains were equiaxed was recrystallized, and that the case where the austenite grain was flat was not recrystallized. The austenitic particle diameter is obtained by polishing the test piece using wet emery abrasive papers of # 150 to # 1000, followed by mirror finish using a diamond slurry as an abrasive, and the mirror polished surface with an extremely low carbon corrosion solution (e.g., For example, 20 g of picric acid, 20 g of sodium dodecylbenzenesulfonate, and 5 to 10 ml of hydrochloric acid were prepared by dissolving in 500 ml of distilled water), followed by etching at a magnification of 400 times to observe a 150 µm x 200 µm image. The austenite particle diameter was measured.

The means for raising the temperature is not particularly limited, and any of heating (high frequency heating and the like) and recuperation can be used, for example. When recuperation is used, accelerated cooling (for example, water cooling) must be performed until the heated steel is cold rolled (especially just before the start of cold rolling). In addition, when performing rough rolling, it is necessary to accelerate-cool between crude rolling and low temperature rolling. Accelerated cooling immediately before the low temperature rolling can increase the temperature difference between the steel surface and the inside, so that the steel can be recovered after the low temperature rolling.

In the temperature range of the said temperature rising, you may roll as needed. By rolling while recrystallizing at an elevated temperature, crystal grains can be made finer and D value can be made smaller. The cumulative reduction ratio of this rolling is, for example, 3% or more (preferably 5% or more, particularly preferably 8% or more), 25% or less (preferably 20% or less, particularly preferably 18% or less). to be.

It is recommended to cool the control after completion of the temperature increase treatment. By controlled cooling, the metal structure can be made mainly of bainite, the hardness of the surface layer portion can be increased to a predetermined value or more, and the fatigue characteristics can be improved. In this controlled cooling, for example, the temperature is at least ₃ ° C to 500 ° C or lower at an average speed of 5 ° C / sec or more. When the cooling start temperature is lower than the Ar ₃ point or the average cooling rate is lower than 5 ° C / sec, a large amount of ferrite is generated, which makes it difficult to harden the surface layer portion. The average cooling rate is preferably 7 ° C / sec or more, more preferably 9 ° C / sec or more. The cooling stop temperature was set at 500 ° C. or lower to completely complete the transformation.

Since the steel materials of the present invention are excellent in brittle crack propagation stop characteristics (and fatigue characteristics), they can be used as materials for structures such as bridges, buildings, ships, tanks, marine structures, line pipes, and the like. This steel can prevent the deterioration of the toughness of the weld heat-affected zone even in small to medium heat input welding, as well as large heat input welding having a heat input amount of 50 kJ / mm or more.

It is preferable that the steel material of this invention is 530 Mpa or more (especially 600 Mpa or more). In addition, it is preferable that the minimum value of the surface layer part hardness is 160 Hv or more (especially 190 Hv or more).

The steel material of this invention is intended for the thick steel plate whose plate | board thickness is 3 mm or more (especially 20 mm or more, Furthermore, 40 mm or more).

Hereinafter, although an Example demonstrates this invention further more concretely, the following Example is not a property which limits this invention, It is also possible to change suitably and to implement in the range which may be suitable for the purpose of the previous and later, It is all included in the technical scope of the present invention.

[Example]

The slab obtained by dissolving steel (remainder and iron and unavoidable impurity) of the component composition shown in following Table 1 by the converter was hot-rolled on the conditions shown in following Table 2. Specific conditions are as follows. After heating the slab obtained by solvent to the temperature shown in following Table 1, it accelerated-cooled at 1050 degreeC to the hot rolling start temperature shown in following Table 2, and hot-rolled at this cumulative reduction ratio shown in following Table 2 from this temperature.

After hot rolling, it heated up by reheating, cooled, and produced the steel plate of the plate thickness shown in following Table 2. Table 2 below shows the maximum temperature of the steel material when reheated and the recrystallization temperature (that is, the temperature at which crystal grains start recrystallization) in each steel type. The recrystallization temperature was measured with the processed pore master tester. In addition, in Table 2, the cooling start temperature and the average cooling rate are shown. On the other hand, No. 7, 8 is an example of cooling after rolling up by the reduction ratio shown in following Table 2 after heating up by reheating.

In this example, all of the above temperatures were managed as average temperatures. The calculation method of an average temperature is as follows.

<Average temperature>

(1) Using a process computer, the heating temperature at any position in the sheet thickness direction from the surface of the steel piece to the back surface is calculated based on the atmosphere temperature and the ashing time from the heating start to the heating end. do.

(2) Using the calculated heating temperature, the rolling temperature at any position in the sheet thickness direction is calculated on the basis of the rolling pass schedule during rolling or the data of the cooling method (water cooling or air cooling) between the passes and the difference method. It rolls, calculating using the method suitable for.

(3) The surface temperature of a steel piece is measured using the radial thermometer provided on the rolling line. However, the surface temperature is also calculated on the process computer.

(4) The surface temperature of the measured steel piece at the time of rough rolling start, the rough rolling end, and the finish rolling start is compared with the surface temperature calculated by the process computer.

(5) When the difference between the calculated surface temperature and the surface temperature of the measured steel piece is ± 30 ° C or more, the surface temperature of the measured steel piece is replaced with the calculated surface temperature to be the calculated surface temperature on the process computer, and is less than ± 30 ° C. In this case, the surface temperature calculated by the process computer is used as it is.

(6) Using the calculated surface temperature, the average temperature in the plate thickness direction is obtained.

Table 2 shows, based on the composition and thickness (product thickness) of the steel sheets shown in Table 1, shows a value of the Ar ₃ point calculated using the above equation (3).

Next, the metal structure of the obtained steel sheet was observed in the following order, and the average circle equivalent diameter D of the crystal grain enclosed by the diagonal grain boundary whose crystal orientation difference is 15 degrees or more, and the ratio R of the random grain boundary occupied by diagonal grain boundary were calculated | required. The values of D (μm) and R (area%) are shown in Table 3 below.

<D value>

(1) The sample cut | disconnected in parallel in the rolling direction (length direction) is prepared so that both the surface and the back surface of the obtained steel plate may be included.

(2) Polishing is performed by wet emery polishing papers of # 150 to # 1000, or by a polishing method having a function equivalent thereto, and mirror finishing is performed by using an abrasive such as a diamond slurry.

(3) The mirror polished surface is an EBSP (Electron Back Scattering Pattern) device manufactured by TexSEM Laboratories, and the measurement range is 200 in the region from the depth t / 8 position to the t / 4 position (t is the thickness of the steel sheet) in the sheet thickness direction. Two crystal orientation differences are measured by making a pitch of 0.5 micrometer x 200 micrometers, and a pitch of 0.5 micrometer, and let a boundary with a crystal orientation difference of 15 degrees or more be diagonal grain boundary. Measurement is carried out at 5 o'clock in the above range. On the other hand, measurement points whose confidence index indicating reliability of the measurement orientation is smaller than 0.1 are excluded from the analysis target.

(4) In the grain distribution map, measure the maximum width (usually the length along the sheet thickness direction) and the maximum length (usually along the rolling direction) of grains surrounded by diagonal grain boundaries with a grain orientation difference of 15 ° or more. Then, the area of the crystal grains is calculated, the circle equivalent diameter of the crystal grains is calculated, and the average value is obtained.

<R value>

(1) The ratio R of the random grain boundary in the diagonal grain boundary is an EBSP device manufactured by TexSEM Laboratories, using a sample subjected to specular finishing under the same conditions as when calculating the above D, and has a depth in the plate thickness direction in the mirror polished surface. In the area | region from t / 8 position to t / 4 position (t is thickness of a steel plate), two crystal orientation differences are measured for 200 micrometers x 200 micrometers in a measurement range, and 0.5 micrometer in pitch. Measurement is carried out at 5 o'clock in the above range. On the other hand, the measuring point whose confidence index showing the reliability of the measuring direction is smaller than 0.1 is excluded from the analysis object.

(2) Among the measurement results, it is considered that noise having a crystal orientation difference of less than 5.5 ° is removed, and the distribution in each direction difference up to 62.5 ° is obtained.

(3) The random grain boundary occupied by the diagonal grain boundary in each plate thickness position by making the crystal orientation difference distribution created in the process of said (2) correspond with the corresponding grain boundary map (the table in which the number of each grain boundary is described). Calculate the ratio R of. Specifically, by dividing each corresponding grain boundary (Σ1-4) by the number of diagonal grain boundaries of 15 degrees or more obtained by the crystal orientation distribution, the distribution of each corresponding grain boundary is calculated | required, and it totals and subtracts from 100%, The ratio R of the random grain boundaries at the diagonal grain boundaries at the sheet thickness positions is calculated. The ratio R of the maximum random grain boundary in each plate | board thickness position is made into the ratio R of the random grain boundary in the rolling material (other than a corresponding grain boundary is set as a random grain boundary (> Σ49)).

On the other hand, "TSL OIM Data Collection ver 5.2" of TSL Corporation was used for the measurement of a corresponding grain boundary, and "TSL OIM Analysis ver 5.0" of TSL Corporation was used for analysis.

Next, in the region from the depth t / 8 position to the t / 4 position (t is the thickness of the steel sheet) of the steel sheet, the sample is cut out so that a surface parallel to the rolling direction of the steel sheet and perpendicular to the surface of the steel sheet is exposed. This was polished using wet emery polishing papers from # 150 to # 1000, followed by mirror finish using a diamond slurry as an abrasive. After this mirror polished surface was etched with a 2% nitric acid-ethanol solution (nital solution), a 150 μm × 200 μm field of view was observed at a magnification of 400 times, and image analysis was performed to measure the ferrite fraction. All non-ferrite RAS tissue was considered bainite. The ferrite fraction was calculated | required about 5 fields, and the average value is shown in Table 3 below.

Next, the hardness in the region from the outermost surface of the steel sheet to the depth t / 4 position (t is the thickness of the steel sheet) and the mechanical properties (yield point and tensile strength) of the steel sheet were measured.

<Hardness>

The hardness of the steel plate was measured with a Vickers hardness tester using a sample mirror-finished under the same conditions as when the above D was calculated. The measurement performed the area from the outermost surface of the steel plate to the t / 4 position (t is the thickness of the steel) at intervals of 1 mm, the load at 98 N (10 kgf), and 20 measurement points. The minimum value of the measurement results is shown in Table 3 below.

<Mechanical characteristics>

The U14A test piece determined by NK (Nihon Maritime Association) distribution was taken from the depth t / 4 site | part of a steel plate (direction perpendicular to a rolling direction, C direction), the tensile test was performed according to JIS Z2241, and a yield point (YP) and tensile strength ( TS) was measured. The measurement results are shown in Table 3 below.

Next, the brittle crack propagation stop characteristic and the fatigue characteristic of the steel plate were evaluated in the following order.

<Fragile Crack Propagation Characteristics>

The brittle crack propagation stop characteristic was performed according to the brittle fracture propagation stop test specified in the steel grade certification test method (established on March 31, 2003) issued by the Nippon Welding Association (WES). The test was carried out using a test piece of the shape shown in Fig. 7.2 of the brittle fracture propagation stop test method, giving the test piece a temperature gradient in an arbitrary temperature range selected from the range of -190 ° C to + 60 ° C. The Kca value was calculated by the following equation (4). In Equation 4 below, c represents the length from the inlet to the brittle crack tip,? Is the length from the inlet to the brittle crack tip, and W represents the width of the propagation section.

&Quot; (4) &quot;

Using T as the temperature at the brittle crack tip (unit: K), X-axis as 1 / T and Y-axis calculated Kca, a graph showing the correlation between 1 / T and Kca values was created. And the intersection point of 273K were made into Kca value at -10 degreeC. Kca values at −10 ° C. are shown in Table 3 below. In the present invention, to pass (is excellent in brittle crack propagation stop characteristics), if the Kca at -10 ℃ 7000N / mm ^{1 .5} or greater.

<Fatigue Characteristics>

Fatigue characteristics were evaluated by measuring the fatigue strength at the time of stopping test as a fatigue limit, using the miniature tensile fatigue test piece shown in FIG. did. The test conditions are as follows.

<Test Conditions>

Test environment: room temperature, air

Testing Machine Load Capacity: 10kN

Load form: axial force

Control method: load control

Control waveform: sine wave

Stress ratio: R = σ _min / σ _max = 0.1

Test speed: 10-20Hz

Break Repeat Range: 10 ⁴ to 2 × 10 ⁶

Number of measurements: 4 times

Test stop condition: at break or at maximum repeat (unbreak)

Next, the impact characteristic of the steel plate and the HAZ toughness at the time of welding this steel plate were evaluated. The evaluation procedure is shown below.

<Shock characteristics>

The impact characteristics of the steel sheet were evaluated by performing a V notch Charpy test and measuring the brittle wavefront transition temperature (vTrs). The measurement was extract | collected U4 test piece which NK (Nihon Maritime Association) distribution determines from t / 4 position, and performed it according to JISZ2242. The measurement results are shown in Table 3 below.

<HAZ Toughness>

In order to evaluate the toughness of the heat affected zone (HAZ) at the time of welding, a high heat welding was simulated to perform a welding reproducing test shown below. The welding reproduction test was heated so that the sample cut out from the t / 4 position of a steel plate might be set to 1400 degreeC, hold | maintained at this temperature for 30 second, and gave the heat cycle to cool. The cooling rate was adjusted so that cooling time from 800 degreeC to 500 degreeC might be 300 second.

The impact characteristic of the sample after cooling was measured by the V notch Charpy test. The test was performed at -20 degreeC and the absorption energy (vE- ₂₀ ) in _-20 degreeC was measured. In the present invention, the case where vE- ₂₀ is 100J or more was evaluated as "excellent in HAZ toughness." The measurement results are shown in Table 3 below.

First, FIG. 3 shows the relationship between the cumulative reduction ratio during hot rolling and the average circle equivalent diameter D of crystal grains surrounded by diagonal grain boundaries. 3 shows that when the cumulative reduction ratio at the time of hot rolling is 50% or more, the said average circle equivalent diameter D can be 8 micrometers or less.

Next, FIG. 4 shows the relationship between the presence or absence of temperature increase due to reheating and the ratio R of the random grain boundary occupying the diagonal grain boundary. It can be seen from FIG. 4 that the ratio R of the random grain boundaries at the diagonal grain boundaries increases to 50 area% or more by reheating after rolling.

Next, FIG. 5 shows the relationship between the average cooling rate after heating up by reheating and the minimum value of the hardness in the area | region from the outermost surface of a steel plate to the depth t / 4 position. 5, No. 2 and Table 3 below. The results of 3 to 11 were plotted. 5 shows that when the average cooling rate after temperature rising is 5 degrees C / sec or more, the minimum value of the hardness of a surface layer part can be 190 Hv or more.

Next, the relationship between the average circle equivalent diameter D of the crystal grain enclosed by diagonal grain boundary, and the Kca value in -10 degreeC is shown in FIG. In Fig. 6, indicates that the ratio R of the random grain boundary occupies the diagonal grain boundary is 50 area% or more, and the result of the ratio R of the random grain boundary occupies the diagonal grain boundary is less than 50 area%. From Figure 6, by inhibiting the average circle equivalent diameter D below the 8㎛, it can be the Kca value of the -10 ℃ to 7000N / mm ^{1 .5} or higher, to improve brittle crack propagation stop characteristics It can be seen that there is.

Next, the relationship between the ratio R of the random grain boundary in a diagonal grain boundary and Kca value in -10 degreeC is shown in FIG. In Fig. 7, the results of Nos. 1 to 3, 11, 12, 13, 16, and 17 are shown. From Figure 7, that the random grain boundary of the ratio R by not less than 50 area%, it is possible to value the Kca at -10 ℃ to 7000N / mm ^{1 .5} or higher, to improve brittle crack propagation stop characteristics It can be seen that.

Next, FIG. 8 shows the relationship between the minimum value of hardness and the fatigue limit in the region from the outermost surface of the steel sheet to the depth t / 4 position. 8 shows that by setting the minimum value of the hardness at the surface layer portion to 190 Hv or more, the fatigue limit can be 400 MPa or more, and the fatigue characteristics can be improved.

Next, consider based on Table 3.

No. 1-11 are examples which satisfy | fill the requirements prescribed | regulated by this invention, and since the metal structure in a surface layer part is appropriately controlled, brittle crack propagation stop characteristic is improved. That is, since the average circle equivalent diameter D of the crystal grains enclosed by the diagonal grain boundary is 8 micrometers or less, and the ratio R of the random grain boundary in a diagonal grain boundary is 50 area% or more, brittle crack propagation stop characteristic is excellent. Moreover, it is excellent in impact characteristics and the toughness of steel material itself is favorable. In particular, 1-8 are also excellent in HAZ toughness. On the other hand, Since 9 has a little much Nb contained in a steel plate, HAZ toughness is inferior slightly.

In the example which satisfy | fills the requirements prescribed | regulated by this invention, No. Since 1-9 are made into the minimum value of the hardness in surface layer part 190Hv or more, in addition to a brittle crack propagation stop characteristic, it is excellent also in a fatigue characteristic. On the other hand, In 10 and 11, the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundary is 8 µm or less, and since the ratio R of the random grain boundary occupies the diagonal grain boundary is 50 area% or more, the brittle crack propagation stop characteristic is excellent. Since the minimum value of the surface-layer part hardness is less than 190 Hv, a fatigue characteristic does not improve.

On the other hand, 12-17 is an example which does not satisfy the requirement prescribed | regulated by this invention. In these examples, the brittle crack propagation stop characteristics are improved because the average circle equivalent diameter D of the crystal grains surrounded by the diagonal grain boundaries exceeds 8 µm, or the ratio R of the random grain boundaries occupying the diagonal grain boundaries is less than 50 area%. Not.

Claims (9)

As steel,
C: 0.02 to 0.12% (the meaning of "mass%", hereinafter the same as for the chemical component),
Si: 0-0.5%,
Mn: 1-2%,
Nb: 0.005 to 0.04%,
B: 0.0005 to 0.003%,
Ti: 0.005 to 0.02%,
N: 0.0040 to 0.01%,
P: 0 to 0.02%,
S: 0 to 0.015%,
Al: satisfies 0.01 to 0.06%,
The balance consists of iron and inevitable impurities,
When the metal structure is observed by the backscattered electron diffraction method (EBSP method) in the region from the depth t / 8 position to the t / 4 position (t is the thickness of the steel. Steel satisfying two.
[Equation 1]
D ≤ 8 μm
&Quot; (2) &quot;
R ≥ 50 area%
[Equation 1] In Equation (1), D denotes an average circle equivalent diameter (mu m) of crystal grains surrounded by diagonal grain boundaries whose crystal orientation differences are 15 degrees or more by measuring two adjacent crystal orientation differences by the EBSP method.
In addition, in Equation 2, R denotes the ratio (area%) of the random grain boundary to the diagonal grain boundary.] The method of claim 1,
As another element,
Ni: more than 0 and less than 0.7%,
Cu: more than 0 and less than 0.3%,
Cr: greater than 0 and less than 1.5% and
Mo: more than 0 and less than 1%
Steel material containing 1 or more types chosen from the group which consists of. The method of claim 1,
As another element,
V: Steel containing more than 0 and 0.1% or less. The method according to any one of claims 1 to 3,
The minimum value is 190 Hv or more when hardness is measured in the area | region from the outermost surface of the said steel to the depth t / 4 position. First carried out to claim 3, wherein heating the steel material of the chemical composition according to any one of items, and Ar ₃ point + 30 ℃ or less, Ar ₃ temperature point or more rolling cumulative rolling reduction of 50% or more in, then Ar ₃ point A method for producing steel, characterized in that the cooling is carried out after the temperature is raised to a temperature range of more than + 30 ° C and a recrystallization temperature of + 20 ° C or less. The method of claim 5, wherein
The heating temperature of the steel is set to 1050 ° C or higher, the steel is reduced to Ar ₃ points + 30 ° C or lower by accelerated cooling, and then the rolling reduction is performed at 50% or more of the cumulative reduction rate at a temperature range of Ar ₃ or more, A method for producing a steel, wherein the temperature is raised to a temperature range of more than ₃ points of Ar + 30 ° C and a recrystallization temperature of + 20 ° C or less. The method of claim 5, wherein
The said elevated temperature is a recrystallization temperature-30 degreeC or more, The manufacturing method of the steel characterized by the above-mentioned. The method of claim 5, wherein
After cooling, the cooling is carried out, followed by rolling. The method of claim 5, wherein
The said cooling is the manufacturing method of the steel materials characterized by the above-mentioned Ar ₃ point or more to 500 degrees C or less to an average speed of 5 degrees C / sec or more.

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