JP2013139602A - Low-yield-ratio high-strength steel sheet excellent in ductile fracture resistance performance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet excellent in ductile fracture resistance performance in a biaxial stress state.SOLUTION: The steel sheet has a composition comprising, by mass%, 0.03-0.08% C, 0.01-0.5% Si, 1.2-2.2% Mn, 0.002 or less S, 0.01-0.5% Mo, 0.08% or less Al, 0.005-0.025% Ti, and 0.005-0.07% Nb, with the balance comprising Fe and unavoidable impurities and has a metallographic structure in which the area fraction of bainite is 30-90%, the area fraction of island-like martensite is 2-10%, and the island-like martensite has a circle equivalent diameter of 3 μm or less and an average aspect ratio of 3 or less.

Description

  The present invention relates to a structural steel plate suitable for use in buildings, offshore structures, shipbuilding, bridges, line pipes, and the like, and particularly has a low tensile strength of 550 MPa or more required for steel plates used in earthquake-prone areas. The present invention relates to a high yield strength steel sheet, and particularly to a low yield ratio high strength steel sheet having excellent ductile fracture resistance in a multiaxial stress state.

  In recent years, steel materials used in the fields of buildings, offshore structures, shipbuilding, bridges, line pipes, etc. have improved safety and operational efficiency (for example, increased transport gas pressure in pipelines), Higher strength is being promoted for the purpose of reducing the total cost by reducing the amount of steel used.

  Moreover, since the area where the above steel materials are used has expanded to the harsh areas of the natural environment, for example, structural steel materials used in earthquake-prone areas are different from the conventional required performance. Excellent plastic deformability and ductile fracture resistance have been demanded.

  From such a situation, in Patent Documents 1 to 3, steel materials having improved plastic deformability by reducing the yield ratio, which is the ratio of yield stress to tensile strength, are disclosed in Patent Documents 4 and 5. Similarly, high deformability steel materials having excellent buckling resistance properties by reducing the yield ratio have been proposed.

  However, even if the steel material is excellent in deformability at a low yield ratio, if a ductile crack is generated from a stress-concentrated part such as a defect, and this progresses, before the plastic deformability is exerted In addition, cracks may propagate and cause unstable ductile fracture.

  Therefore, development of high-strength and high-deformability steel materials has been carried out for the purpose of preventing unstable ductile fracture. For example, Patent Document 6 discloses a method for producing high-tensile steel in which absorbed energy is increased by making the metal structure a fine-grain ferrite-based structure, and Patent Document 7 describes that the metal structure is composed of ferrite and martensite. A high-strength steel pipe with improved stopping performance of unstable ductile fracture has been proposed.

  The technologies described in Patent Documents 6 and 7 relate to a steel material with improved ductile crack propagation characteristics in unstable ductile fracture. However, for example, in a gas pipeline operated at high pressure, once a crack has occurred, it may be difficult to stop the propagation of the crack, and as a result, there is a concern that even local destruction may cause serious damage. Has been. For this reason, steel materials used in gas pipelines not only suppress the propagation of ductile cracks, but even if damage occurs, they do not lead to crack generation, or even if cracks occur, gas leakage is minimized. It is desirable that it can be stopped to the limit.

  In order to meet the above requirements, it is necessary to reliably prevent the damage caused by defects inherent in the steel material itself or external factors, or the occurrence of cracks from the reduced thickness portion due to corrosion. Further, steel structures may receive biaxial loads instead of simple tensile loads, such as shape discontinuities in building structures subject to seismic loads and bending deformation of line pipes subject to internal pressure. The steel materials used in the above are required to have fracture resistance in a biaxial state.

  However, there is currently no high-strength steel sheet that satisfies the above characteristics and has excellent fracture resistance in a biaxial state.

JP-A-55-119152 JP 63-223123 A Japanese Patent Laid-Open No. 03-115524 Japanese Patent Laid-Open No. 10-330885 Japanese Unexamined Patent Publication No. 2000-178689 JP 2002-105534 A JP 2004-197191 A

  The present invention has been made in view of the above circumstances, and is a low-yield ratio high-strength steel sheet excellent in ductile fracture resistance, which is required for steel materials used in severe environments such as earthquake-prone areas, particularly biaxial. It aims at providing the steel plate excellent in the ductile fracture performance in a stress state.

  In order to solve the above-mentioned problems, the inventors have made extensive studies on the ductile fracture behavior of a low yield ratio high strength steel sheet in a biaxial stress state. As a result, by dispersing island-like martensite (hereinafter also referred to as MA) in the bainite structure, a low yield ratio and a high strength are achieved at the same time, and the structure of the steel sheet surface layer where ductile cracks occur is a precursor of ductile fracture. It has been found that the occurrence of ductile cracks can be suppressed even in a biaxial stress state by restricting the amount and shape of the hard second layer and inclusions, which are the starting points of voids formed in stages, to a specific range.

  The present invention has been made based on the above findings and further studies, and the gist of the present invention is as follows.

  [1] Component composition is mass%, C: 0.03-0.08%, Si: 0.01-0.5%, Mn: 1.2-2.2%, S: 0.002 or less , Mo: 0.01 to 0.5, Al: 0.08% or less, Ti: 0.005 to 0.025%, Nb: 0.005 to 0.07%, the balance being Fe and inevitable impurities The metal structure has an area fraction of bainite of 30 to 90%, an area fraction of island martensite of 2 to 10%, an equivalent circle diameter of island martensite of 3 μm or less, and an average aspect ratio of A low-yield-ratio high-strength steel sheet excellent in ductile fracture resistance, characterized by being 3 or less.

  [2] Further, by mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, V: 0.1% or less, Ca: 0.001 to 0.003%, The low yield ratio high-strength steel sheet excellent in ductile fracture resistance according to the above [1], characterized by containing one or more selected from among the above.

  [3] The low yield ratio high-strength steel sheet excellent in ductile fracture resistance according to the above [1] or [2], wherein the Charpy absorbed energy is 180 J or more and the yield ratio is 80% or less.

  ADVANTAGE OF THE INVENTION According to this invention, even when it receives big plastic deformation by an earthquake etc., the low yield ratio high strength steel plate which does not generate | occur | produce a ductile crack can be provided. Therefore, the steel plate of the present invention is suitable for use in buildings, offshore structures, shipbuilding, bridges, line pipes, and the like that have high safety requirements.

It is a figure which shows the shape of the cross-shaped tension test piece for evaluating the ductile fracture performance under biaxial stress. It is a figure explaining the method of performing a tensile test in a principal axis direction in the state where stress was applied to the principal axis perpendicular direction of a cross type tensile test piece. It is a figure explaining the microstructure of the steel plate with which it used for evaluation of the ductile fracture performance under biaxial stress. It is a figure explaining the relation between crack generation opening displacement and principal axis perpendicular direction strain by the biaxial tension test result of A steel and B steel of FIG. It is a figure explaining the shape of the parallel part center part and surface notch of a biaxial tensile test piece.

  In order to reproduce the biaxial stress state, the present inventors searched for a steel sheet having excellent fracture resistance in the biaxial state by a biaxial tensile test using a test piece as shown in FIG.

  FIG. 2 is a diagram showing a tensile test method reproducing a biaxial state. In order to apply stress in the direction perpendicular to the main axis in the test piece of FIG. 1, stress was applied by bolting through a jig as shown in FIG. 2, and in that state, the test piece was deformed in the main axis direction by an Amsler tester. . At this time, the fastening force in the vertical direction of the main shaft was adjusted with a hydraulic jack so that the load in the vertical direction of the main shaft became constant. The ductile fracture performance was evaluated by introducing a surface notch into the central part of the test piece of FIG. 1 and measuring the tensile strain in the principal axis direction where ductile fracture occurs from the bottom of the surface notch in the biaxial state.

  FIG. 3 shows mass% of a high-strength steel plate manufactured under different steel plate manufacturing conditions using steel containing 0.05% C-1.7% Mn-0.15% Mo-0.03% Nb. The microstructure is shown. FIG. 3 (A) is a steel manufactured by applying accelerated cooling after hot rolling (hereinafter referred to as “A steel”), the yield ratio is 93%, and the metal structure is a structure mainly composed of bainite. Many hard phases (MA) are observed.

  Fig. 3 (B) shows the production of massive MA by using carbon enrichment in untransformed austenite by hot-rolling steel with the same composition as steel A and reheating it online after accelerated cooling. The yield ratio is as low as 70% in the steel (hereinafter referred to as B steel).

  A biaxial tensile test as shown in FIG. 2 was performed using these steels. The test piece width is 20 mm, the thickness of the test piece central portion is 6 mm, and a surface notch of 16 mm length, 2 mm depth, 0.2 mm width is introduced into the test piece central portion (F in FIG. 2) for the test. Provided. The crack initiation behavior from the notch bottom was observed with a microscope.

  FIG. 4 shows the result of measuring the opening displacement of the surface notch when a crack is generated from the bottom of the notch when a tensile load is applied in the direction of the main axis with a constant strain applied in the direction perpendicular to the main axis. In a condition (uniaxial condition) in which no strain is applied, the crack opening opening displacement of Steel A is smaller than that of Steel B.

  It is considered that this is because the steel A has a higher yield ratio and the strain concentration at the bottom of the notch is suppressed, so that the opening displacement until crack initiation is reduced. In addition, when the strain in the direction perpendicular to the main axis is applied, that is, in the biaxial stress condition, when the strain in the direction perpendicular to the main axis increases, the crack opening opening displacement is further reduced in steel A, whereas in steel B, the strain is greatly increased. It has become. This is because in Steel A in which elongated MA exists, the generation of microvoids that are precursor stages of ductile fracture is promoted in a biaxial stress state.

  On the other hand, in Steel B in which granular MA is dispersed, the generation of ductile cracks is suppressed in a biaxial stress state because the generation of microvoids is small.

  The inventors have further studied the above results, and as a result of appropriately controlling the component composition and production conditions, and further controlling the MA phase fraction and average aspect ratio within an appropriate range, The present inventors have found that a low-yield-ratio high-strength steel sheet having excellent ductile crack initiation characteristics can be obtained, and the present invention has been completed.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. About component composition First, the reason which prescribed | regulated the component composition of the steel plate of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.03-0.08%
C is an element necessary for promoting the production of MA and increasing the strength of the steel sheet. If less than 0.03% is added, the desired phase strength cannot be obtained because the MA phase fraction is low. On the other hand, if over 0.08% is added, the weldability decreases, so the C content is less than 0.3%. The range is 03 to 0.08%. Preferably it is 0.05 to 0.08% of range.

Si: 0.01 to 0.5%
Si is an element added as a deoxidizer and to increase the strength of steel. However, if it is less than 0.01%, the deoxidation effect is not sufficient, and if added over 0.5%, toughness In order to reduce weldability, the Si content is set to a range of 0.01 to 0.5%. Preferably it is 0.05 to 0.45% of range.

Mn: 1.2-2.2%
Mn is added to increase the strength and toughness of the steel, and to enhance the hardenability and promote the formation of MA. However, the addition of less than 1.2% is not effective. Since addition exceeding 2% lowers weldability, the amount of Mn is made 1.2 to 2.2%. Preferably it is 1.4 to 2.1% of range. More preferably, it is 1.4 to 1.7% of range.

S: 0.002% or less S is an element which is inevitably mixed as an impurity, and generally exists as sulfide inclusions in steel and serves as a starting point for void generation during deformation. Therefore, in order to prevent the occurrence of ductile cracks, the S content must be strictly regulated. However, if it is 0.002% or less, the above adverse effect is small. Therefore, the S amount is set to 0.002% or less. Preferably it is 0.001% or less.

Mo: 0.01 to 0.5%
Mo is added to increase the strength and toughness of the steel, and to enhance the hardenability and promote the formation of MA. However, the addition of less than 0.01% is not sufficient in its effect. Addition exceeding 5% decreases weldability and increases the amount of MA, so the Mo amount is in the range of 0.01 to 0.5%. Preferably it is 0.01 to 0.3% of range.

Al: 0.08% or less Al is added as a deoxidizer, but if it exceeds 0.08%, ductility is deteriorated due to a decrease in cleanliness, so the Al amount is 0.08% or less. Preferably, it is 0.06% or less.

Ti: 0.005-0.025%
Ti not only suppresses grain growth during slab heating by forming TiN, but also suppresses grain growth in the weld heat affected zone and improves toughness by making the base material and the weld heat affected zone finer. However, when the Ti content is less than 0.005%, the effect is not obtained. When the Ti content exceeds 0.025%, the toughness is deteriorated, so the Ti content is in the range of 0.005 to 0.025%. Preferably it is 0.007 to 0.020% of range. More preferably, it is 0.008 to 0.015% of range.

Nb: 0.005 to 0.07%
Nb suppresses grain growth during rolling, and improves toughness by making fine grains. Further, it is an element that improves the hardenability and promotes the formation of MA, and if it is added less than 0.005%, there is no effect. Is in the range of 0.005 to 0.07%. Preferably it is 0.005 to 0.04% of range.

  The above is the basic chemical component of the present invention, and for the purpose of further increasing the strength and toughness of the steel, one or more selected from Cu, Ni, Cr, V, and Ca are added as selective elements. Also good.

Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, V: 0.1% or less Cu, Ni, Cr and Mo increase the strength and toughness of the steel and further improve the hardenability. It is an element that improves and promotes the production of MA and can be added according to the required strength,
If Cu exceeds 0.5%, Ni exceeds 1%, Cr exceeds 0.5%, and V exceeds 0.1%, the weldability is deteriorated. It is preferable that 0.5% or less, Ni content is 1% or less, Cr content is 0.5% or less, and V content is 0.1% or less.

Ca: 0.001 to 0.003%
Ca is an effective element for controlling the form of sulfide inclusions and improving ductility. However, if added less than 0.001%, the effect cannot be obtained, and more than 0.003% is added. However, the above effect is saturated, resulting in a decrease in cleanliness and a decrease in toughness. Therefore, when adding Ca, the amount of Ca is preferably in the range of 0.001 to 0.003%.

  In the steel sheet of the present invention, the balance other than the above components consists of Fe and inevitable impurities. However, even if components other than those described above are added in an amount that is normally recognized as an inevitable impurity, it is allowed as long as the effects of the present invention are not impaired.

2. About metal structure Area fraction of bainite: 30 to 90%
Bainite is a high-strength metal structure, and if the area fraction is less than 30%, the required strength cannot be obtained, and if it exceeds 90%, the yield ratio becomes too high, so the area fraction of bainite is 30 to 90%. Range.

Island-like martensite area fraction: 2-10%
Island-like martensite (MA) is necessary to obtain a steel sheet having a low yield ratio and excellent ductile crack initiation characteristics. When the area fraction of island martensite is less than 2%, the yield ratio is high, and when it exceeds 10%, a ductile crack is easily generated. Therefore, the area fraction of island martensite is set to a range of 2 to 10%.

  The remaining structure other than bainite and island martensite (MA) is mostly ferrite, but may contain pearlite, martensite, and retained austenite in addition to ferrite. These phases other than bainite, island-like martensite (MA) and ferrite are preferably 5% or less because the effects of the present invention are not impaired if the area ratio is 5% or less.

Average aspect ratio of island martensite: 3 or less The average aspect ratio of island martensite is 3 or less. If the average aspect ratio of the island martensite exceeds 3, the strain concentrates at the interface with the ferrite phase and bainite phase near the tip of the island martensite elongated during deformation, and becomes the starting point of ductile cracks. This is because the ductile crack initiation characteristics deteriorate. Here, the aspect ratio of the island-shaped martensite is defined as the ratio of the major axis to the minor axis (major axis / minor axis) of the elongated island martensite. The average aspect ratio is an average value of aspect ratios measured with about 100 or more island-shaped martensites obtained by SEM observation.

Equivalent circle diameter of island martensite: 3 μm or less In the state where the size of island martensite is large and sparsely distributed, stress concentration around the island martensite is promoted, so it is ductile in a biaxial stress state. Cracks are likely to occur. However, when the island martensite is small and dispersed, local stress concentration is also dispersed, so that generation of microvoids can be suppressed. Therefore, the size of the island martensite is 3 μm or less in terms of the equivalent circle diameter. Note that the equivalent circle diameter of the MA was obtained by image-processing the microstructure obtained by SEM observation, obtaining the diameter of a circle having the same area as each MA, and obtaining the average value of the diameters.

3. Mechanical properties Charpy absorbed energy: 180J or more Charpy absorbed energy is a property representing resistance to fracture, and a metal structure in which fine and granular island martensite is dispersed as in the present invention is less prone to ductile fracture. High Charpy energy can be obtained. If the Charpy energy is less than 180 J, ductile cracks are likely to occur with respect to fracture under biaxial stress, so the Charpy absorbed energy is 180 J or more. The Charpy impact test is performed at the temperature at which the steel plate is used or at the design temperature.

Yield ratio: 80% or less If there are notches and discontinuities in shape, if the yield ratio of the steel sheet (yield strength / tensile strength) is high, macro stress concentration can easily cause breakage in the notches and discontinuities in shape. . Therefore, the yield ratio is specified to be 80% or less.

  Steel having the chemical composition shown in Table 1 (steel types A to G) is made into a slab by a continuous casting method, then the steel slab is heated, and a steel plate having a thickness of 14 mm or 20 mm is obtained by hot rolling, accelerated cooling, and reheating. Manufactured. Table 2 shows the manufacturing conditions of the steel sheet.

And the metal structure of the said steel plate, the tensile characteristic, and the Charpy impact characteristic were investigated.

  The microstructure of the steel sheet is observed with a scanning electron microscope (SEM) at 1/4 position in the thickness direction of the steel sheet, and the area of ferrite, bainite and island martensite (MA) is analyzed by image analysis from a microstructural photograph of about 5 fields of view. The rate was determined. The aspect ratio of MA was also obtained in the same manner.

  In the tensile test, the tensile strength and the yield ratio were measured using a round bar tensile specimen taken from the center of the plate thickness. Moreover, the Charpy test performed the normal V notch Charpy test at -20 degreeC, and calculated | required the absorbed energy as an average of three.

  In the fracture experiment under biaxial stress, a cross-shaped biaxial tensile test piece shown in FIG. 1 was taken from the same steel plate. At this time, the shape of the biaxial tensile test piece is set such that the width of the arm portion in the main axis direction and the vertical direction of the main shaft is 20 mm, the thickness is 10 mm, and the thickness of the central parallel portion is 6 mm, as shown in FIG. Three slits each having a width of 1 mm and a length of 15 mm were introduced into each part. Further, a surface notch having a length of 15 mm, a width of 0.2 mm, and a depth of 2 mm was introduced into the central parallel portion in the direction perpendicular to the main axis.

  2 in the state perpendicular to the main axis through the jig shown in FIG. 2 and with a strain of 0.5% in the vertical direction of the main axis added by the strain gauge attached to the back side of the parallel part. A tensile load was applied with an Amsler type tensile tester. At this time, a tensile test was performed while adjusting the strain in the direction perpendicular to the main axis to be constant. The ductile fracture resistance was evaluated by the opening displacement of the notch when a ductile crack occurred from the bottom of the surface notch. The occurrence of a ductile crack from the notch bottom was identified by a potential difference change by a direct current potential difference method, and the opening displacement of the notch was measured by a clip gauge.

  Table 2 shows the microstructure of the test steel, the mechanical properties, and the crack opening displacement in the biaxial tensile test. No. 1-4 have the chemical composition and metallographic structure of the test steel within the scope of the present invention, the tensile strength is high strength of 650 MPa or more, the low yield ratio of 80% or less, and the high absorbed energy of 200 J or more. It has high crack displacement at biaxial tension and excellent ductile fracture resistance.

  On the other hand, no. The chemical component 5 is within the scope of the present invention, but the metal structure is out of the scope of the present invention, and the ductile fracture resistance under biaxial stress is low. No. Nos. 6 and 7 have low Charpy absorbed energy because the chemical components are outside the scope of the present invention or low ductile fracture performance under biaxial stress because the metal structure is outside the scope of the present invention.

A parallel part B main axis direction arm part C main axis vertical direction arm part D slit E thread part F surface notch introduced at the center of the parallel part of the cross-shaped tensile test piece G holder for holding fastening force by nut H from two parts Bolts and nuts for fastening the holder

Claims (3)

  Component composition is mass%, C: 0.03-0.08%, Si: 0.01-0.5%, Mn: 1.2-2.2%, S: 0.002 or less, Mo: 0.01 to 0.5, Al: 0.08% or less, Ti: 0.005 to 0.025%, Nb: 0.005 to 0.07%, the balance is made of Fe and inevitable impurities, The metal structure has an area fraction of bainite of 30 to 90%, an area fraction of island martensite of 2 to 10%, an equivalent circle diameter of island martensite of 3 μm or less, and an average aspect ratio of 3 or less. A low yield ratio high strength steel plate with excellent ductile fracture resistance.   Furthermore, from mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, V: 0.1% or less, Ca: 0.001 to 0.003% The low yield ratio high-strength steel sheet excellent in ductile fracture resistance according to claim 1, comprising one or more selected.   The low yield ratio high-strength steel sheet having excellent ductile fracture resistance according to claim 1 or 2, wherein Charpy absorbed energy is 180J or more and the yield ratio is 80% or less.