JP5759109B2 - Steel material excellent in brittle crack propagation stop property and method for producing the same - Google Patents

Description

  The present invention relates to a steel material used for structures such as bridges, buildings, and ships, and more particularly to a steel material in which a brittle crack generated in the steel material is quickly stopped, and a method for manufacturing the same.

  Steel materials used in structures such as bridges, buildings, ships, tanks, offshore structures, and line pipes are required to be difficult to brittle fracture. In order to suppress brittle fracture, it is effective not to cause brittle cracks in the steel material. When brittle cracks occur in the steel material, it is necessary to stop the brittle cracks that have occurred rapidly without progressing (hereinafter referred to as brittleness). It is sometimes called crack propagation stop property).

  It is known that a brittle crack occurs when the stress intensity factor K of a steel material is equal to or higher than a Kca value (K ≧ Kca) measured by a brittle fracture propagation stop characteristic test. The stress intensity factor K is expressed by K = σ√ (π × a) where σ is the stress and a is the crack length. Therefore, brittle cracks are more likely to occur as the strength of the steel material increases and the stress σ increases. In order to prevent the occurrence of brittle cracks, it is effective to reduce the strength of the steel material. However, with the increase in size of structures, the strength required for steel materials is increasing.

  The inventor does not prevent the occurrence of brittle cracks by reducing the stress σ by reducing the stress σ, but increases the allowable range of the stress intensity factor K by increasing the Kca value of the steel material, Japanese Patent Application Laid-Open No. H10-228867 proposed a technique for preventing the occurrence of brittle cracks even when a large stress σ is applied. The thick steel plate proposed in Patent Document 1 is mainly composed of a bainite structure at a depth t / 8 to t / 4 (t is the plate thickness) from the surface, and the orientation difference between two adjacent crystals is 15 ° or more. When the region surrounded by the large-angle grain boundaries is a crystal grain, the average equivalent circle diameter of the crystal grain is controlled to 8 μm or less. The Kca value is increased by refining crystal grains. That is, the refinement of the crystal grains increases the frequency with which the cracks collide with the grain boundaries and stops the cracks from progressing.

JP 2010-1520 A

  Further enhancement of the strength of the steel material is desired, and the present inventor has studied to further improve the brittle crack propagation stop characteristic even after proposing the technique of Patent Document 1.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a steel material further improved in brittle crack propagation stop characteristics and a manufacturing method thereof.

The steel material excellent in the brittle crack propagation stop property according to the present invention that has solved the above-mentioned problems is C: 0.02 to 0.12% (meaning “mass%”, hereinafter the same for chemical components), Si: 0.5% or less (including 0%), Mn: 1-2%, Nb: 0.005-0.04%, B: 0.0005-0.003%, Ti: 0.005-0 0.02%, N: 0.0040 to 0.01%, P: 0.02% or less (not including 0%), S: 0.015% or less (not including 0%), Al: 0.01 -0.06% is satisfied, and the balance consists of iron and inevitable impurities. When the metal structure was observed by the backscattered electron diffraction image method (EBSP method) in the region from the depth t / 8 position of the steel material to the t / 4 position (t is the thickness of the steel material, hereinafter the same), Expressions (1) and (2) are satisfied. However, in Formula (1), D measures the orientation difference of two adjacent crystals by the EBSP method, and the average equivalent circle diameter (μm) of the crystal grains surrounded by the large-angle grain boundary having a crystal orientation difference of 15 ° or more. ). Moreover, in Formula (2), R means the ratio (area%) of the random grain boundary which occupies for the said large angle grain boundary.
D ≦ 8 μm (1)
R ≧ 50 area% (2)

  The steel material preferably has a minimum value of 190 Hv or more when the hardness is measured in a region from the outermost surface to a depth t / 4 position.

The steel material, as another element,
(A) Ni: 0.7% or less (not including 0%), Cu: 0.3% or less (not including 0%), Cr: 1.5% or less (not including 0%), and Mo At least one selected from the group consisting of: 1% or less (not including 0%); and / or (b) V: 0.1% or less (not including 0%),
It may contain.

The steel material excellent in the brittle crack propagation stop property according to the present invention is obtained by heating a steel material having the above component composition to an appropriate temperature (preferably 1050 ° C. or higher) and in a temperature range of Ar ₃ point + 30 ° C. or lower and Ar ₃ point or higher. Rolling is performed at a cumulative rolling reduction of 50% or more, then Ar ₃ point + 30 ° C. (preferably recrystallization temperature −30 ° C. or more) by appropriate means (heating, recuperation, etc., preferably recrystallization temperature + 20 ° C.) Produced by raising the temperature to a temperature range of ℃ or less (preferably less than the recrystallization temperature) and then cooling (preferably cooling from a temperature of Ar ₃ point or higher to 500 ℃ or less at an average rate of 5 ° C / second or more) it can. The steel material may be cooled to the Ar ₃ point + 30 ° C. or less by accelerated cooling after heating. After the temperature increase, the cooling may be performed after rolling.

  In the present invention, paying attention to the metal structure in the surface layer portion of the steel material, in addition to suppressing the average equivalent circle diameter of the crystal grains surrounded by the large angle grain boundaries to 8 μm or less, the ratio of the random grain boundaries in the large angle grain boundaries is Since it is increased to a predetermined value or more, it is possible to provide a steel material with further improved brittle crack propagation stop characteristics.

FIG. 1 is a schematic diagram showing a heat pattern when the recrystallization temperature is measured by a processing for master testing machine. FIG. 2 is an explanatory view showing the shape of a test piece used for evaluating fatigue characteristics. FIG. 3 is a graph showing the relationship between the cumulative rolling reduction during hot rolling and the average equivalent circle diameter D of crystal grains surrounded by large-angle grain boundaries. FIG. 4 is a graph showing the relationship between the presence or absence of temperature rise due to recuperation and the ratio R of random grain boundaries in the large-angle grain boundaries. FIG. 5 is a graph showing the relationship between the average cooling rate after the temperature is raised by recuperation and the minimum value of the hardness in the region from the outermost surface of the steel plate to the depth t / 4 position. FIG. 6 is a graph showing the relationship between the average equivalent circle diameter D of the crystal grains surrounded by the large-angle 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 in the large-angle grain boundaries and the Kca value (brittle crack propagation stopping characteristics) at −10 ° C. FIG. 8 is a graph showing the relationship between the minimum hardness value in the region from the outermost surface of the steel sheet to the depth t / 4 position and the fatigue limit (fatigue characteristics).

  When the tensile strength of the steel material is increased, the stress σ increases, the stress intensity factor K increases, and brittle cracks are likely to occur. For this reason, as disclosed in Patent Document 1, it has been found that the occurrence of brittle fracture cannot be prevented only by suppressing the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries to 8 μm or less.

  Therefore, the present inventor has intensively studied for the purpose of providing a steel material with further improved brittle crack propagation stop characteristics. As a result, it has been found that if the ratio R of the random grain boundaries in the large-angle grain boundaries is 50 area% or more, the progress can be stopped quickly even if brittle cracks occur, and the brittle crack propagation stopping characteristics can be secured.

  That is, it is known that large-angle grain boundaries are roughly divided into “corresponding grain boundaries” with low grain boundary energy and “random grain boundaries” with high grain boundary energy (for example, “material histology”, Nobuo Takagi). Tsuneaki Tsuzaki, Asakura Shoten, page 45). Of these, random grain boundaries with high grain boundary energy became resistant to the development of brittle cracks, and the study was repeated, thinking that the development of brittle cracks could be stopped quickly. As a result, as will be clarified in Examples described later, if the amount of random grain boundaries can be controlled by a predetermined method, and if the ratio R of the random grain boundaries in the large angle grain boundaries is 50 area% or more, brittle cracks It was found that the propagation stop characteristics can be improved.

  The present invention will be described in detail below.

[About the average equivalent circle diameter D of crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more, and the ratio R of random grain boundaries in the large-angle grain boundaries]
The steel material of the present invention needs to satisfy the following formulas (1) and (2) when the metal structure is observed by a backscattered electron diffraction image method (EBSP method). Satisfying both equations can improve brittle crack propagation stopping characteristics.
D ≦ 8 μm (1)
R ≧ 50 area% (2)

  In the above formula (1), D measures the orientation difference between two adjacent crystals by the EBSP method, and when the region surrounded by a large-angle grain boundary having a crystal orientation difference of 15 ° or more is used as a crystal grain, It means the average equivalent circle diameter (μm) of grains. The “equivalent circle diameter” is the diameter of a circle that is obtained by measuring the area of crystal grains and assuming that the areas are equal.

  In the present invention, similarly to Patent Document 1, the D value is set to 8 μm or less in order to improve brittle crack propagation stop characteristics. That is, it is generally known that a brittle crack bends, bypasses, or stops at a large-angle grain boundary having a crystal orientation difference of 15 ° or more. Therefore, by refining the crystal grains surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more, the position where the brittle crack bends, detours, and stops increases, and as a result, the brittle crack progresses. Can be stopped.

  The D value is preferably 7 μm or less, and more preferably 6 μm or less. In addition, although D value is so preferable that it is small, a minimum in particular is not restrict | limited, For example, about 1 micrometer may be sufficient.

  In the above formula (2), R is the ratio (area%) of the random grain boundary occupying the large-angle grain boundary where the crystal orientation difference is 15 ° or more when the orientation difference between two adjacent crystals is measured by the EBSP method. I mean.

  In the present invention, the R value is 50 area% or more. By increasing the number of random grain boundaries having a high grain boundary energy, the resistance to the progress of brittle cracks can be increased, and the brittle crack propagation stopping characteristics can be improved.

  The R value is preferably 53 area% or more, more preferably 55 area% or more. The R value is preferably as large as possible, and the upper limit is not particularly limited, but may be, for example, about 65 area%.

  The D value and the R value are measured by observing the metal structure in the region from the depth t / 8 position to the t / 4 position when the thickness of the steel material is t (mm). Since the brittle crack propagation stop characteristic is affected by energy loss due to the ductile fracture region (shear lip) formed from the surface layer, the D value and R value in the region from the depth t / 8 position to the t / 4 position should be controlled. Can stop brittle cracks.

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

  Incidentally, stress is normally repeatedly applied to the structure. When stress is repeatedly applied, a slip band is generated due to shear stress, which develops with repetition and forms protrusions and penetrations (hereinafter referred to as protrusions) on the surface of the structure. When stress concentrates on this protrusion or the like, fatigue cracks are generated and fatigue fracture occurs. However, in order to ensure safety, the steel material constituting the structure is required to be resistant to fatigue cracks and to have excellent fatigue characteristics.

  In order to prevent the occurrence of fatigue cracks, it is only necessary to suppress slip due to shear stress and prevent formation of protrusions, etc., and it is effective to increase the yield point (YP) and tensile strength (TS) of the steel material. Therefore, in the present invention, in order to increase the yield point and the tensile strength, attention is paid to the hardness in the surface layer portion of the steel material. Since fatigue cracks are generated from the surface of steel, it is thought that if the surface layer of the steel is hardened and the yield point and tensile strength of the part are increased, the occurrence of protrusions can be prevented and the occurrence of fatigue cracks can be suppressed. This is because the. As will be apparent from the examples described later, it has been clarified that the fatigue characteristics can be improved by setting the minimum hardness value of the surface layer portion to 190 Hv or more.

[Minimum hardness]
In this invention, it is preferable that the minimum value of the hardness in the surface layer part of steel materials shall be 190 Hv or more. By hardening the surface layer portion of the steel material, it is possible to prevent the formation of protrusions or the like even when stress is repeatedly applied, so that the fatigue characteristics of the steel material can be improved.

  The minimum value of the hardness is better as it is larger, more preferably 200 Hv or more, and still more preferably 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-based means that the bainite fraction is 60 area% or more when the metal structure is observed with an electron microscope. The bainite fraction is preferably 70 area% or more, more preferably 80 area% or more, still more preferably 90 area% or more, and most preferably bainite 100 area%.

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

  The upper limit of the hardness is not particularly limited, but may be about 260 Hv, for example. This upper limit is approximately equal to the average hardness of the bainite structure.

  The hardness is measured in a region from the outermost surface to a depth t / 4 position where the thickness of the steel material is t (mm). The reason for setting this region is to prevent the occurrence of fatigue cracks because the position where fatigue cracks occur is the outermost surface of the steel material.

  The hardness may be determined by measuring a region from the outermost surface to a depth t / 4 position at equal intervals (for example, 1 mm intervals) and obtaining a minimum value. A specific measurement procedure will be described in the section of Examples described later.

  In the steel material 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 (including 0%), Mn: 1 to 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 (not including 0%), S: 0.015% or less (not including 0%), Al: 0.01 to 0 0.06% must be satisfied. The reasons for setting these ranges are as follows.

  C is an element indispensable for securing the strength of the steel material (base material), and needs to be contained by 0.02% or more. C is preferably contained in an amount of 0.04% or more, more preferably 0.05% or more. However, if C exceeds 0.12%, a lot of island martensite (MA) is generated in the HAZ at the time of welding to cause deterioration of the toughness of the HAZ, and also adversely affect the weldability. Therefore, C is 0.12% or less, preferably 0.1% or less, more preferably 0.08% or less.

  Si is an element that contributes to securing the strength of a steel material by solid solution strengthening. However, if Si exceeds 0.5%, not only does martensite (MA) form in the HAZ during welding to cause deterioration of the HAZ toughness, but also adversely affects weldability. Therefore, Si is 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 Si does not need to contain, in order to add Si and ensure the intensity | strength of steel materials, it is preferable to make it contain 0.02% or more. More preferably 0.05% or more, still more preferably 0.1% or more.

  Mn is an element that contributes to improving the strength of the steel material (base material), and must be contained at 1% or more. Mn is preferably contained at 1.2% or more, more preferably 1.4% or more. However, if Mn exceeds 2%, the weldability of the steel material (base material) is deteriorated. Therefore, Mn needs to be suppressed to 2% or less. Mn is preferably 1.8% or less, more preferably 1.6% or less.

  Nb has the effect of suppressing the coarsening of the recrystallized grains and improving the brittle crack propagation stopping characteristics due to the solution drag effect due to solid solution and the pinning effect due to precipitation of carbonitride. It also contributes to the improvement of the base material toughness. In order to exert such an effect, Nb needs to be contained by 0.005% or more. More preferably, it is 0.007% or more, More preferably, it is 0.009% or more. However, if Nb exceeds 0.04%, the precipitated carbonitride becomes coarse and deteriorates instead of the base material toughness. Therefore, Nb is made 0.04% or less. In particular, in order to improve HAZ toughness, Nb is preferably 0.035% or less, more preferably 0.03% or less, still more preferably 0.025% or less, and particularly preferably 0.02% or less. It is.

  B is an element that improves hardenability and improves strength. B is an element that suppresses the formation of grain boundary ferrite and improves the HAZ toughness. In order to exert the effect of addition of B, it is necessary to contain 0.0005% or more, preferably 0.001% or more, more preferably 0.0015% or more. However, if B exceeds 0.003%, it precipitates as BN at the austenite grain boundary and causes a reduction in HAZ toughness. Therefore, B is 0.003% or less, preferably 0.0025% or less, more preferably 0.002% or less.

  Ti has the effect of preventing the austenite grains from coarsening by finely dispersing nitride (TiN) in the steel and suppressing the coarsening due to austenite recrystallization. It has the effect of enhancing the crack propagation stop characteristics. Further, Ti is an element that generates oxides in addition to nitrides and contributes to improvement of HAZ toughness. In order to exert such an effect, it is necessary to contain Ti by 0.005% or more. Preferably it is 0.007% or more, more preferably 0.01% or more. However, if Ti is added excessively, the toughness of the steel (base material) is deteriorated, so 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 that has the effect of precipitating Ti nitrides and improving the brittle crack propagation stopping characteristics. N is an element that contributes to the improvement of HAZ toughness by preventing the coarsening of austenite grains formed in HAZ during welding and promoting ferrite transformation by the pinning effect of nitride. In order to exhibit such an effect 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 N increases, Ti nitrides are formed and the refinement of austenite grains is promoted, so that it effectively works to improve the toughness of HAZ. However, when N exceeds 0.01%, the amount of solute N increases, the toughness of the base metal itself deteriorates, and the HAZ toughness also decreases. Therefore, N must be suppressed to 0.01% or less. Preferably it is 0.0095% or less, More preferably, it is 0.009% or less.

  P is an element that is easily segregated, and particularly segregates at a grain boundary in the steel material to deteriorate the base material toughness. Therefore, P must 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 generate sulfide (MnS) and degrades the toughness of the base metal and the ductility in the thickness direction. Therefore, S must 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 that acts as a deoxidizer, and is an element that acts to refine crystal grains by forming AlN. In order to exhibit 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, if it is excessive, the base metal toughness and the HAZ toughness are deteriorated, so Al needs to be suppressed to 0.06% or less. Al is preferably 0.04% or less, more preferably 0.035% or less.

  The steel material of the present invention contains the above elements as essential components, and the balance is iron and inevitable impurities (for example, Mg, As, Se, etc.).

It is also effective for the steel material of the present invention to contain an element (Ni, Cu, Cr, Mo) for improving the strength of the steel material and / or an element (V) for further improving the HAZ toughness as other elements. is there. In particular,
(A) Ni: 0.7% or less (not including 0%), Cu: 0.3% or less (not including 0%), Cr: 1.5% or less (not including 0%), and Mo At least one element selected from the group consisting of: 1% or less (not including 0%); and / or (b) V: 0.1% or less (not including 0%),
Etc. are preferably contained. The reasons for setting these ranges are as follows.

[(A) Ni, Cu, Cr, Mo]
Ni, Cu, Cr, and Mo are all elements that contribute to increasing the strength of the steel material, and can be added alone or in combination.

  In particular, Ni is an element that contributes to increasing the strength of the steel material and improving the toughness of the steel material itself. Ni is preferably contained as much as possible, but since it is an expensive element, if it is contained excessively, the cost becomes high. Therefore, the upper limit is preferably 0.7% for economic reasons. More preferably, it is 0.5% or less, More preferably, it is 0.4% or less. Ni is preferably contained in an amount of 0.01% or more in order to effectively exhibit the above-described action. More preferably it is 0.02% or more, and still more preferably 0.03% or more.

  Cu is an element that enhances the strength of the steel by solid solution strengthening. However, if the content exceeds 0.3%, the toughness of the steel material deteriorates, so Cu is preferably 0.3% or less. More preferably, it is 0.28% or less, More preferably, it is 0.25% or less. In addition, in order to exhibit the effect | action mentioned above effectively, it is preferable to make it contain 0.01% or more. More preferably it is 0.02% or more, and still more preferably 0.03% or more.

  Cr acts to increase the strength of the steel material. However, if it exceeds 1.5%, the strength of the steel material (base material) is remarkably increased and the base material toughness deteriorates, so that the HAZ toughness decreases. Therefore, Cr is preferably 1.5% or less. More preferably, it is 1.2% or less, More preferably, it is 1% or less, Most preferably, it is 0.5% or less. In addition, in order to exhibit the effect | action mentioned above effectively, it is preferable to make it contain 0.01% or more. More preferably it is 0.02% or more, and still more preferably 0.03% or more.

  Mo acts to increase the strength of the steel material. However, if it exceeds 1%, the strength of the steel material (base material) becomes extremely high and the base material toughness deteriorates instead, so that the HAZ toughness also decreases. Therefore, Mo is preferably 1% or less. More preferably, it is 0.7% or less, More preferably, it is 0.5% or less, Most preferably, it is 0.05% or less. In addition, in order to exhibit the effect | action mentioned above effectively, it is preferable to make it contain 0.01% or more. More preferably it is 0.02% or more, and still more preferably 0.03% or more.

[(B) V]
V is an element that improves the HAZ toughness. However, if it exceeds 0.1%, the precipitated carbonitrides are coarsened to deteriorate the toughness of the base material. Therefore, V is preferably 0.1% or less. More preferably, it is 0.08% or less, More preferably, it is 0.05% or less, Most preferably, it is 0.01% or less. In addition, in order to exhibit the effect | action mentioned above effectively, it is preferable to make it contain 0.001% or more. More preferably it is 0.002% or more, and still more preferably 0.003% or more.

  Next, a method for producing the steel material of the present invention will be described.

In order to control the average equivalent circle diameter (D value) of the crystal grains surrounded by the large angle grain boundaries and the ratio of the random grain boundaries (R value) in the large angle grain boundaries within a predetermined range, the heated steel is adjusted to the Ar ₃ point. After introducing a strain (dislocation) into the steel material by rolling in the unrecrystallized region immediately above (hereinafter referred to as low temperature rolling), the temperature is raised to the vicinity of the recrystallization temperature (hereinafter referred to as temperature increasing treatment). is important. Even if the temperature is lower than the recrystallization temperature, recrystallization occurs using the introduced strain as a driving force, so that a new grain boundary is generated. A grain boundary generated at a low temperature immediately above the Ar ₃ point becomes a corresponding grain boundary, whereas a grain boundary generated near the recrystallization temperature becomes a random grain boundary. Therefore, the larger the accumulated strain introduced by rolling in the non-recrystallized region, and the closer to the recrystallization temperature due to the subsequent temperature rise, the smaller the D value and the larger the R value.

  The heating temperature of the steel material (the temperature of the steel material means an average temperature. The same applies hereinafter, and the determination method will be described in detail in the column of Examples) is, for example, 1050 ° C. or higher and 1080 ° C. or higher. Is more preferable, and more preferably 1100 ° C. or higher. By heating to 1050 ° C. or higher, the structure of the steel material can be austenite single phase, and Nb can be completely dissolved. However, if the heating temperature is too high, the initial austenite structure becomes too coarse, and it becomes difficult to sufficiently refine the structure after transformation. Therefore, the upper limit of the heating temperature is, for example, 1250 ° C. or lower, preferably 1200 ° C. or lower, more preferably 1150 ° C. or lower.

The heated steel material is subjected to rough rolling and cooling as necessary, and then the low-temperature rolling is performed. The temperature of the low temperature rolling is Ar ₃ point + 30 ° C. or lower (preferably Ar ₃ point + 20 ° C. or lower) and Ar ₃ point or higher. The cumulative rolling reduction in this temperature range is 50% or more, preferably 53% or more, more preferably 55% or more. The upper limit of the cumulative rolling reduction is not limited from the viewpoint of the R value and the D value, but considering the rolling load and production efficiency, the cumulative rolling reduction is, for example, 80% or less, preferably 70% or less, more preferably 65% or less. It is good also as a grade.

The temperature at the Ar ₃ point can be calculated from the following formula (3). In formula (3), [] represents the content (% by mass) of each element, and t represents the finished plate thickness (mm) of the steel material.
Ar ₃ (° C.) = 910−310 × [C] −80 × [Mn] −20 × [Cu] −15 × [Cr] −55 × [Ni] −80 × [Mo] + 0.35 × (t− 8) ... (3)

After the low temperature rolling, the temperature is increased as described above. The temperature range of this temperature rise is Ar ₃ point + 30 ° C. and recrystallization temperature + 20 ° C. or less. As the recrystallization temperature is approached, recrystallization using strain as a driving force proceeds, the R value increases, and the D value decreases. However, if the recrystallization temperature is exceeded, crystal grains begin to grow, making it difficult to suppress the D value. Considering the balance between recrystallization and grain growth, the temperature range of the temperature rise was set to the recrystallization temperature + 20 ° C. or lower. A preferable temperature range is a recrystallization temperature of −30 ° C. or higher (particularly a recrystallization temperature of −20 ° C. or higher) and a lower recrystallization temperature (particularly a recrystallization temperature of −5 ° C. or lower).

  In addition, recrystallization temperature can be measured in the following procedure using a processing for master tester, for example. A cylindrical test piece having a diameter of 8 mm and a height of 12 mm is prepared and processed with the heat pattern shown in FIG. That is, the test piece is heated to 1100 ° C. at a temperature rising rate of 10 ° C./second, held for 1 minute, then subjected to initial processing so that the height becomes 10 mm, and held at 1100 ° C. for 20 minutes. Next, cooling is performed from 1100 ° C. at a cooling rate of 50 ° C./second so that the processing temperature becomes 760 ° C., 780 ° C., 800 ° C., 820 ° C., 840 ° C., 860 ° C., or 880 ° C., and 10 seconds at each processing temperature. Holding → Processing for the first pass → Holding for 10 seconds at the above processing temperature → Repeating the processing for the second pass After finishing the processing for the fourth pass, hold for 10 seconds at the processing temperature and then cool down to room temperature at 50 ° C / Cooled in seconds. In each pass processing, the stroke speed is such that the height of the test piece becomes 9.0 mm for the first pass, 8.0 mm for the second pass, 7.0 mm for the third pass, and 6.5 mm for the fourth pass. The measurement was performed at 15 mm / second. The cooling was quenched with an inert gas. As a comparison object, a sample was prepared that was held for 20 minutes and then cooled to room temperature at a cooling rate of 50 ° C./second (rapid cooling with an inert gas) without processing.

  After processing, the austenite grain size at each processing temperature is measured, the presence or absence of recrystallization is observed, and the recrystallization temperature is determined. The case where the austenite grains are equiaxed was evaluated as recrystallized, and the case where the austenite grains were flat was evaluated as not recrystallized. The austenite grain size was determined by polishing the test piece with wet emery polishing paper of # 150 to # 1000 and then mirror-finishing with a diamond slurry as an abrasive. For example, after etching with 20 g of picric acid, 20 g of sodium dodecylbenzenesulfonate, and 5-10 ml of hydrochloric acid dissolved in 500 ml of distilled water), a field of view of 150 μm × 200 μm was observed at a magnification of 400 × The austenite particle size was measured by analysis.

  The means for raising the temperature is not particularly limited, and for example, either heating (high frequency heating or the like) and recuperation may be used. When using recuperation, it is necessary to perform accelerated cooling (for example, water cooling) until the heated steel material is cold-rolled (particularly immediately before the start of low-temperature rolling). Moreover, when implementing rough rolling, it is necessary to accelerate-cool between rough rolling and low-temperature rolling. Since the temperature difference between the steel surface and the interior can be increased by accelerated cooling immediately before the low temperature rolling, the steel material can be reheated after the low temperature rolling.

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

Control cooling is recommended after the temperature raising process. By controlled cooling, the metal structure can be mainly bainite, the hardness of the surface layer can be increased to a predetermined value or more, and the fatigue characteristics can be improved. In this controlled cooling, for example, cooling is performed at an average rate of 5 ° C./second or more from a temperature of Ar ₃ point or higher to 500 ° C. or lower. When the cooling start temperature is lower than the Ar ₃ point or the average cooling rate is lower than 5 ° C./second, a large amount of ferrite is generated, and it becomes difficult to harden the surface layer portion. The average cooling rate is preferably 7 ° C./second or more, more preferably 9 ° C./second or more. The reason why the cooling stop temperature is set to 500 ° C. or lower is to complete the transformation completely.

  Since the steel material of the present invention is excellent in brittle crack propagation stop characteristics (and fatigue characteristics), it can be used as a material for structures such as bridges, buildings, ships, tanks, offshore structures, line pipes, and the like. . This steel material can prevent toughness deterioration of the weld heat affected zone not only in small to medium heat input welding but also in large heat input welding with a heat input of 50 kJ / mm or more.

  The steel material of the present invention preferably has a tensile strength of 530 MPa or more (particularly 600 MPa or more). Moreover, it is preferable that the minimum value of the hardness of a surface layer part is 160 Hv or more (especially 190 Hv or more).

  The steel material of the present invention is intended for a thick steel plate having a plate thickness of 3 mm or more (particularly 20 mm or more, and further 40 mm or more).

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Steels having the composition shown in Table 1 below (the balance being iron and inevitable impurities) were melted in a converter, and the resulting slab was hot rolled under the conditions shown in Table 2 below. Specific conditions are as follows. After the slab obtained by melting was heated to the temperature shown in Table 1 below, it was accelerated and cooled from 1050 ° C. to the hot rolling start temperature shown in Table 2 below, and from this temperature, the cumulative rolling reduction shown in Table 2 below was applied. Hot rolled.

  After hot rolling, the steel sheet was cooled by raising the temperature by recuperation, and steel sheets having the thicknesses shown in Table 2 below were produced. Table 2 below shows the maximum temperature of the steel material when reheated and the recrystallization temperature (that is, the temperature at which the crystal grains start recrystallization) in each steel type. The recrystallization temperature was measured with a processing for master tester. Table 2 below shows the cooling start temperature and the average cooling rate. In Table 2 below, No. 7 and 8 are examples in which the temperature was raised by recuperation, and then rolled at the rolling reduction shown in Table 2 below and then cooled.

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

《Average temperature》
(1) Using a process computer, based on the atmospheric temperature from the start of heating to the end of heating and the in-furnace time, the heating temperature at an arbitrary position in the thickness direction from the front surface to the back surface of the steel slab is calculated.
(2) Using the calculated heating temperature, based on the rolling pass schedule during rolling and the data of the cooling method (water cooling or air cooling) between passes, the rolling temperature at an arbitrary position in the plate thickness direction is suitable for calculations such as the difference method. Rolling while calculating using the above method.
(3) The surface temperature of the steel slab is measured using a radial thermometer installed on the rolling line. However, the surface temperature is also calculated in the process computer.
(4) The surface temperature of the steel slab measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with the surface temperature calculated by the process computer.
(5) If the difference between the calculated surface temperature and the measured steel slab surface temperature is ± 30 ° C or more, replace the measured steel slab surface temperature with the above calculated surface temperature to obtain the calculated surface temperature on the process computer, ± When the temperature is lower than 30 ° C., the surface temperature calculated by the process computer is used as it is.
(6) Using the calculated calculated surface temperature, an average temperature in the plate thickness direction is obtained.

Table 2 below shows the value of the Ar ₃ point calculated using the above formula (3) based on the component composition shown in Table 1 and the plate thickness (product thickness) of the steel plate.

  Next, the metallographic structure of the obtained steel sheet is observed by the following procedure, and the average equivalent circle diameter D of the crystal grains surrounded by the large angle grain boundaries having a crystal orientation difference of 15 ° or more and the random grain boundaries occupying the large angle grain boundaries The ratio R was determined. The values of D (μm) and R (area%) are shown in Table 3 below.

<< D value >>
(1) A sample cut in parallel to the rolling direction (longitudinal direction) is prepared so as to include both the front and back surfaces of the obtained steel sheet.
(2) Polishing with a wet emery polishing paper of # 150 to # 1000 or a polishing method having the same function as that, and a mirror finish using a polishing agent such as diamond slurry.
(3) The mirror-polished surface is measured in the region from the depth t / 8 position in the thickness direction to the t / 4 position (t is the thickness of the steel sheet) with an EBSP (Electron Back Scattering Pattern) device manufactured by TexSEM Laboratories. The orientation difference between two crystals is measured with a range of 200 μm × 200 μm and a pitch of 0.5 μm, and a boundary where the crystal orientation difference is 15 ° or more is defined as a large-angle grain boundary. The measurement is performed with 5 fields of view in the above region. Note that measurement points whose confidence index indicating the reliability of the measurement direction is smaller than 0.1 are excluded from the analysis target.
(4) In the grain distribution map, the maximum width (usually along the plate thickness direction) and the maximum length (usually along the rolling direction) of the crystal grain surrounded by the large-angle grain boundary having a crystal orientation difference of 15 ° or more. Length), the area of the crystal grains is calculated, the equivalent circle diameter of the crystal grains is calculated, and the average value is obtained.

<< R value >>
(1) The ratio R of the random grain boundaries in the large-angle grain boundaries is a EBSP apparatus manufactured by TexSEM Laboratories, using a sample that has been mirror-finished under the same conditions as when D is calculated. In the region from the depth direction depth t / 8 to t / 4 (where t is the thickness of the steel sheet), the measurement range is 200 μm × 200 μm, the pitch is 0.5 μm, and the orientation difference between the two crystals is measured. The measurement is performed with 5 fields of view in the above region. Note that measurement points whose confidence index indicating the reliability of the measurement direction is smaller than 0.1 are excluded from the analysis target.
(2) Among the measurement results, those having a crystal orientation difference of less than 5.5 ° are considered to be noise and deleted, and the distribution at each orientation difference up to 62.5 ° is obtained.
(3) By associating the crystal orientation difference distribution created in the step (2) with the corresponding grain boundary map (a table in which the number of each corresponding grain boundary is described), the large-angle grain boundary at each plate thickness position The ratio R of the occupied random grain boundary is calculated. Specifically, the distribution of each corresponding grain boundary is obtained by dividing each corresponding grain boundary (Σ1 to 49) by the number of large-angle grain boundaries having an orientation difference of 15 ° or more obtained from the crystal orientation distribution, and summing these distributions. By subtracting from 100%, the ratio R of random grain boundaries to the large angle grain boundaries at each plate thickness position is calculated. The maximum random grain boundary ratio R at each plate thickness position is defined as the random grain boundary ratio R in the rolled material [other than the corresponding grain boundary is set as a random grain boundary (> Σ49)].

  In addition, “TSL OIM Data Collection ver5.2” manufactured by TSL Co., Ltd. was used for the measurement of the corresponding grain boundary, and “TSL OIM Analysis ver5.0” manufactured by TSL Co., Ltd. was used for the analysis.

  Next, a plane parallel to the rolling direction of the steel plate and perpendicular to the surface of the steel plate is exposed from the region from the depth t / 8 position of the steel plate to the t / 4 position (t is the thickness of the steel plate). A sample was cut and polished using # 150 to # 1000 wet emery polishing paper, and then mirror-finished using 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 lath structures other than ferrite were regarded as bainite. The ferrite fraction was determined for 5 fields of view, and the average value is shown in Table 3 below.

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

"Hardness"
The hardness of the steel sheet was measured with a Vickers hardness tester using a sample that was mirror-finished under the same conditions as when D was calculated. The measurement was performed at an interval of 1 mm in the region from the outermost surface of the steel plate to the t / 4 position (t is the thickness of the steel material), the load was 98 N (10 kgf), and the measurement locations were 20 locations. The minimum value among the measurement results is shown in Table 3 below.

《Mechanical properties》
A U14A specimen determined by the NK (Japan Maritime Association) classification is taken from the depth t / 4 part (direction perpendicular to the rolling direction, direction C) of the steel sheet, and subjected to a tensile test according to JIS Z2241, yield point (YP). And tensile strength (TS) was measured. The measurement results are shown in Table 3 below.

  Next, the brittle crack propagation stop characteristics and fatigue characteristics of the steel sheet were evaluated by the following procedure.

《Brittle crack propagation stop characteristics》
The brittle crack propagation stop property was performed in accordance with the “brittle fracture propagation stop test” defined by the steel type certification test method (established on March 31, 2003) published by the Japan Welding Association (WES). In the test, a test piece having the shape shown in FIG. 7.2 of the brittle fracture propagation stop test method is used, and a temperature gradient is applied to the test piece in an arbitrary temperature range selected from the range of −190 ° C. to + 60 ° C. The four Kca values were calculated by the following formula (4). In the following formula (4), c represents the length from the propagation portion entrance to the brittle crack tip, σ represents the length from the propagation portion entrance to the brittle crack tip, and W represents the propagation portion width.

Create a graph showing the correlation between 1 / T and Kca value, where T is the temperature of the brittle crack tip (unit is K), X axis is 1 / T, and Y axis is the calculated Kca value. And the Kca value at -10 ° C. The Kca value at −10 ° C. is shown in Table 3 below. In the present invention, a case where Kca at −10 ° C. is 7000 N / mm ^1.5 or more is regarded as acceptable (excellent in brittle crack propagation stopping characteristics).

《Fatigue properties》
The fatigue characteristics were evaluated by using the miniature tensile fatigue test piece shown in FIG. 2 taken from the t / 4 part of the steel sheet, measuring the fatigue strength at the time of stopping the test as the fatigue limit, with 2 million cycles. . The test conditions are as follows.
<Test conditions>
Test environment: Room temperature, atmospheric test machine load capacity: 10kN
Load mode: Axial force control method: Load control control waveform: Sine wave stress ratio: R = σ _min / σ _max = 0.1
Test speed: 10-20Hz
Breaking repetition number range: 10 ⁴ to 2 × 10 ⁶
Number of measurements: 4 times Test stop condition: Breaking or when the maximum number of repetitions is reached (unbreaked)

  Next, the impact characteristics of the steel sheet and the HAZ toughness when this steel sheet was welded 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 fracture surface transition temperature (vTrs). The measurement was performed in accordance with JIS Z2242 by collecting U4 test pieces defined by the NK (Japan Maritime Association) classification from the t / 4 position. The measurement results are shown in Table 3 below.

<< HAZ toughness >>
In order to evaluate the toughness of the part (HAZ) that is affected by heat during welding, the following welding reproduction test was performed by simulating high heat input welding. In the welding reproduction test, a sample cut from the t / 4 position of the steel plate was heated to 1400 ° C., held at this temperature for 30 seconds, and then given a heat cycle for cooling. The cooling rate was adjusted so that the cooling time from 800 ° C. to 500 ° C. was 300 seconds.

The impact characteristics of the sample after cooling were measured by a V-notch Charpy test. The test was performed at −20 ° C., and the absorbed energy (vE ₋₂₀ ) at −20 ° C. was measured. In the present invention, the case where vE- ₂₀ is 100 J 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 rolling reduction during hot rolling and the average equivalent circle diameter D of crystal grains surrounded by large-angle grain boundaries. FIG. 3 shows that the average equivalent circle diameter D can be made 8 μm or less if the cumulative rolling reduction during hot rolling is 50% or more.

  Next, FIG. 4 shows the relationship between the presence or absence of temperature rise due to recuperation and the ratio R of random grain boundaries in the large-angle grain boundaries. It can be seen from FIG. 4 that the ratio R of random grain boundaries in the large-angle grain boundaries increases to 50 area% or more by reheating after rolling.

  Next, FIG. 5 shows the relationship between the average cooling rate after the temperature is raised by recuperation and the minimum hardness value in the region from the outermost surface of the steel plate to the depth t / 4 position. FIG. 5 shows the numbers in Tables 2 and 3 below. Results from 3 to 12 were plotted. FIG. 5 shows that the minimum value of the hardness of the surface layer portion can be 190 Hv or more by setting the average cooling rate after the temperature rise to 5 ° C./second or more.

Next, FIG. 6 shows the relationship between the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries and the Kca value at −10 ° C. In FIG. 6, ◆ indicates the result that the ratio R of the random grain boundaries occupying the large angle boundaries is 50 area% or more, and ■ indicates the result that the ratio R of the random grain boundaries occupying the large angle boundaries is less than 50 area%. Show. FIG. 6 shows that by suppressing the average equivalent circle diameter D to 8 μm or less, the Kca value at −10 ° C. can be increased to 7000 N / mm ^1.5 or more, and the brittle crack propagation stop characteristic can be improved.

Next, FIG. 7 shows the relationship between the ratio R of random grain boundaries in the large-angle grain boundaries and the Kca value at −10 ° C. In FIG. The results of 1-3, 12, 13, 14, 17, 18 were shown. From FIG. 7, it can be seen that by setting the ratio R of the random grain boundaries to 50 area% or more, the Kca value at −10 ° C. can be made 7000 N / mm ^1.5 or more, and the brittle crack propagation stop characteristic can be improved.

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

  Next, consideration will be given based on Table 3.

  No. 1-3, 5-10, and 12 are examples that satisfy the requirements defined in the present invention, and because the metallographic structure in the surface layer part is appropriately controlled, the brittle crack propagation stopping characteristics can be improved. Moreover, the impact characteristics are also excellent and the toughness of the steel material itself is good. In particular, no. 1-3 and 5-8 are excellent also in HAZ toughness. In addition, No. No. 9 is slightly inferior in HAZ toughness because Nb contained in the steel plate is slightly more.

  Of the examples satisfying the requirements defined in the present invention, No. 1 to 3 and 5 to 9 are excellent in fatigue characteristics in addition to brittle crack propagation stopping characteristics because the minimum value of the hardness in the surface layer portion is 190 Hv or more. On the other hand, no. In Nos. 10 and 12, since the minimum value of the hardness of the surface layer portion is lower than 190 Hv, the fatigue characteristics cannot be improved.

  On the other hand, no. 13 to 18 are examples that do not satisfy the requirements defined in the present invention. Of these, No. In Nos. 13, 15, and 16, since the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries exceeds 8 μm, the brittle crack propagation stopping characteristics cannot be improved.

  No. In Nos. 14, 17, and 18, the average circle equivalent diameter D of the crystal grains surrounded by the large-angle grain boundaries is 8 μm or less, but the ratio R of the random grain boundaries in the large-angle grain boundaries is less than 50 area%. The brittle crack propagation stop characteristic has not been improved.

  No. 10 and 12, the average equivalent circle diameter D of the crystal grains surrounded by the large-angle grain boundaries is 8 μm or less, and the ratio R of the random grain boundaries in the large-angle grain boundaries is 50 area% or more. Although the crack propagation stop property is excellent, the fatigue property cannot be improved because the minimum hardness value of the surface layer is less than 190 Hv.

Claims (9)

C: 0.02 to 0.12% (meaning “mass%”; hereinafter the same for chemical components),
Si: 0.5% or less (including 0%),
Mn: 1-2%
Nb: 0.005 to 0.04%,
B: 0.0005-0.003%,
Ti: 0.005 to 0.02%,
N: 0.0040 to 0.01%
P: 0.02% or less (excluding 0%),
S: 0.015% or less (excluding 0%),
Al: 0.01 to 0.06% is satisfied,
The balance is steel consisting of iron and inevitable impurities,
When the metal structure was observed with an electron microscope in the region from the depth t / 8 position of the steel material to the t / 4 position (t is the thickness of the steel material, hereinafter the same), the bainite fraction was 80 area% or more, A brittle crack having a ferrite fraction of 8 area% or less and satisfying the following formulas (1) and (2) when the metal structure is observed by a backscattered electron diffraction image method (EBSP method): Steel material with excellent propagation stop characteristics.
D ≦ 8 μm (1)
R ≧ 50 area% (2)
[However, in the formula (1), D is an orientation difference between two adjacent crystals measured by the EBSP method, and an average equivalent circle diameter of a crystal grain surrounded by a large-angle grain boundary with a crystal orientation difference of 15 ° or more ( μm).
Moreover, in Formula (2), R means the ratio (area%) of the random grain boundary which occupies for the said large angle grain boundary. ] The steel material is still another element,
Ni: 0.7% or less (excluding 0%),
Cu: 0.3% or less (excluding 0%),
The steel material according to claim 1, comprising at least one selected from the group consisting of Cr: 1.5% or less (not including 0%) and Mo: 1% or less (not including 0%). The steel material is still another element,
The steel material according to claim 1 or 2, containing V: 0.1% or less (not including 0%).   The steel material according to any one of claims 1 to 3, wherein the minimum value is 190 Hv or more when the hardness is measured in a region from the outermost surface of the steel material to a depth t / 4 position. A method for producing the steel material according to any one of claims 1 to 4,
Heat the steel material of the above component composition,
Rolling with a cumulative reduction of 50% or more in a temperature range of Ar ₃ point + 30 ° C. or lower and Ar ₃ point or higher,
Next, after reheating to a temperature range of Ar ₃ point + 30 ° C. and recrystallization temperature + 20 ° C. or lower,
A method for producing a steel material excellent in brittle crack propagation stopping characteristics, characterized in that cooling is carried out at a mean speed of 5 ° C./second or more and 20 ° C./second or less from a temperature of ₃ Ar or higher to 500 ° C. or lower. The heating temperature of the steel material is set to 1050 ° C. or higher, and rolling is performed at a cumulative reduction ratio of 50% or more in a temperature range of Ar ₃ point or higher after the steel material becomes Ar ₃ point + 30 ° C. or lower by accelerated cooling.
The manufacturing method according to claim 5, wherein the temperature is raised to a temperature range of Ar ₃ point + 30 ° C. and recrystallization temperature + 20 ° C. or less by recuperation. The manufacturing method according to claim 5 or 6, wherein the temperature rise is within a temperature range of Ar ₃ point + 30 ° C or more and a recrystallization temperature or less.   The manufacturing method according to any one of claims 5 to 7, wherein the temperature rise is set to a recrystallization temperature of -30 ° C or higher and lower than a recrystallization temperature.   The manufacturing method according to claim 5, wherein the cooling is performed after rolling after the temperature rise.

Images