JP6248545B2 - Steel plate, steel slab using the same, and method for producing them - Google Patents

Description

  The present invention relates to a steel plate, a steel slab using the same, and a manufacturing method thereof, and more particularly, to a steel plate excellent in fatigue characteristics of a welded portion, a steel slab using the same, and a manufacturing method thereof.

  In recent years, the occurrence of many fatigue damages in steel deck deck plates has become a social issue, and countermeasures for them are an urgent issue. In particular, the fatigue damage that progresses in the thickness direction of the deck plate starting from the joint between the deck plate and the trough rib is a serious problem for road managers because it is difficult to perform flaw detection from the outside.

  Fatigue damage at the joint between the deck plate and the truffle is difficult to detect because cracks occur and grow in the closed section. Although there is a method of using a valve rib type steel slab having an open cross-sectional structure instead of the trough rib, there is a problem that the manufacturing cost becomes high.

  If a crack penetrates the deck plate, it leads to a road surface depression and is extremely dangerous. Therefore, development of a technique for preventing fatigue damage is currently strongly demanded. Various studies have been made so far regarding techniques for suppressing fatigue cracks in steel materials or welds.

  Patent Document 1 describes the fatigue crack growth rate of a steel base material by defining the metal structure of the steel material, the chemical composition, the hardness upper limit value of the soft phase of the duplex stainless steel, and the boundary density between the soft phase and the hard phase. Techniques for suppression are disclosed. Patent Document 2 discloses a long-life structure in which a steel material having excellent fatigue properties is applied to at least one of a steel deck deck and a cross beam web having a steel deck structure.

  Patent Document 3 specifies the hardness distribution of the boundary between the weld metal and the weld heat-affected zone, which defines the metal structure, X-ray diffraction strength, and chemical composition of the steel, and can be a fracture starting point of the joint. A technique for providing a welded joint excellent in fatigue characteristics in a long life region is disclosed.

  Patent Document 4 discloses a technique for suppressing the fatigue crack growth rate of a base material by controlling the metal structure. Specifically, focusing on the surface layer, by dispersing the soft phase in the base made of the hard phase, the hardness difference of the hard phase soft phase is provided to suppress the progress rate.

  Patent Document 5 discloses a technique for suppressing the fatigue crack growth rate of a base material by controlling the metal structure. Specifically, for steel sheets with a thickness of 20 mm or less, focusing on the balance between total elongation characteristics and fatigue crack growth characteristics, suppressing the C content, controlling the carbon equivalent Ceq to a specific range, cooling A technique for lowering the stop temperature is disclosed.

  Patent Document 6 discloses a technique for suppressing fatigue crack growth rate in a base material and improving fatigue crack generation characteristics starting from a weld surplus toe of a welded joint, that is, fatigue strength. Yes. Specifically, the chemical composition of the base metal is appropriately specified, the hardness difference between the hard part and the soft part is specified, and the hardness relationship between the three parts of the base material of the welded part, the weld heat affected zone, and the weld metal Is stipulated.

Japanese Patent Application Laid-Open No. 08-225882 JP 2003-183769 A JP 2010-024467 A JP 2010-084189 A JP 2010-196109 A JP 2012-144780 A

  As described above, various studies have been made on techniques for suppressing fatigue cracks in steel materials or welds, but it is still impossible to suppress the occurrence of fatigue damage starting from the joint between the deck plate and the trough rib. is the current situation.

  The present invention has been made to solve the above-described problems, and is a steel plate capable of suppressing the occurrence of fatigue damage starting from a welded portion, and a steel slab using the same, and a method for producing the same. The purpose is to provide.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.

  Compressive stress is generated at the joint between the deck plate and the trough rib due to the load from the wheels. Nevertheless, fatigue cracks are generated and propagated due to the presence of high levels of tensile residual stress in the weld. After welding, the weld metal shrinks as it cools, while the surrounding base metal part resists and generates a reaction force. Therefore, tensile residual stress is generated at the weld. And the fatigue strength of a welding part falls greatly compared with the steel plate which is a base material by it.

  In order to improve the fatigue strength at the welded portion, it is necessary to relieve the tensile residual stress. As described above, the tensile residual stress in the welded portion is caused by the reaction force of the steel plate that is the base material. Therefore, if the reaction force of the steel sheet can be weakened, the tensile residual stress can be relaxed.

  A repeated stress is applied to the welded portion by a wheel load. If this repetitive stress becomes a driving force and the steel sheet repeatedly softens, the reaction force decreases, so that a high level of residual tensile stress is relaxed. As a result, it becomes possible to remarkably improve the fatigue characteristics of the welded portion. By optimizing the chemical composition and production conditions of the steel sheet, the repeated softening characteristics of the steel sheet can be enhanced.

  The present invention has been made on the basis of the above-mentioned findings, and the gist thereof is the following steel plate, a steel slab using the steel plate, and a production method thereof.

(1) A steel plate used for a deck plate of a steel deck,
Chemical composition is mass%, C: 0.01-0.10%, Si: 0.04-0.60%, Mn: 0.50-2.00%, Al: 0.003-0.06 %, Ti: 0.001 to 0.100%, N: 0.0020 to 0.0120%, O: 0.0005 to 0.0040%, Cr: 0 to 2.0%, Mo: 0 to 1. 0%, Ni: 0 to 1.5%, Cu: 0 to 1.5%, Nb: 0 to 0.1%, V: 0 to 0.1%, B: 0 to 0.0030%, balance Fe And the ratio of Ti and N (Ti / N) is 3.4 or less, Ceq defined by the following formula (i) is 0.25 to 0.40,
The microstructure at a thickness of 1/4 of the plate thickness is composed of a composite structure of a hard structure and a soft structure, and the hardness difference between the hard structure and the soft structure is 150 HV or more,
A steel sheet having a repeated softening parameter of 0.95 or less in the load direction of the steel sheet.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.

  (2) The said chemical composition is the mass%, Cr: 0.05-2.0%, Mo: 0.04-1.0%, Ni: 0.2-1.5%, Cu: 0.2 Steel plate as described in said (1) containing 1 or more types selected from -1.5%.

  (3) The above chemical composition (1) or (2), wherein the chemical composition contains one or more selected from Nb: 0.01 to 0.1% and V: 0.005 to 0.1% by mass. ) Steel sheet.

  (4) The steel sheet according to any one of (1) to (3), wherein the chemical composition is mass% and contains B: 0.0003 to 0.0030%.

(5) Using the steel plate according to any one of (1) to (4) above as a deck plate, a steel slab welded to a closed cross-section rib,
A steel slab in which the hardness of the heat-affected zone of the steel sheet satisfies the following formula (ii) in relation to the hardness of the steel sheet and the weld metal.
1.5 × H _B−W ≧ H _HAZ (ii)
However, H _B-W means the lower value of the hardness of the steel plate and weld metal, and H _HAZ means the hardness of the heat affected _zone of the steel plate.

(6) A heating step of heating the slab having the chemical composition according to any one of (1) to (4) above to a temperature range of 900 to 1250 ° C;
A pre-rolling cooling step in which cooling is performed at a cooling rate of 1 ° C./s or higher after heating
A hot rolling process for performing hot rolling;
After hot rolling, when performing accelerated cooling from a start temperature of 800 ° C. or higher to a stop temperature of 500 ° C. or lower, an average cooling rate from 800 ° C. to 500 ° C. is set to 20 to 60 ° C./s, and recuperation on the steel sheet surface is performed. The manufacturing method of the steel plate provided with the post-rolling cooling process which makes a temperature range 40% or less of accelerated cooling stop temperature.

  (7) When the steel plate according to any one of (1) to (4) is used as a deck plate and welded with a closed cross-section rib to produce a steel deck, the heat input in the welding is 1.0. A method for producing a steel slab of ~ 3.0 kJ / mm.

  (8) When the steel plate manufactured by the method described in (6) above is used as a deck plate and welded to a closed cross-section rib to manufacture a steel slab, the heat input in the welding is 1.0-3. A method for producing a steel slab of 0 kJ / mm.

  By applying the steel material according to the present invention to the deck plate of the steel deck of the road bridge, the fatigue characteristics of the welded portion can be remarkably improved and the fatigue strength soundness can be enhanced.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition of steel sheet The reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.01 to 0.10%
C is an element having an effect of increasing the strength. Therefore, it is necessary to contain 0.01% or more of C. On the other hand, when the C content exceeds 0.10%, the hardness distribution in the welded portion becomes inhomogeneous, and the fatigue strength of the welded portion cannot be ensured. Therefore, the C content is set to 0.01 to 0.10%. C is an inexpensive element, and in order to suppress other additive elements having an effect of increasing the strength and to ensure the strength economically, the C content is preferably 0.03% or more.

Si: 0.04 to 0.60%
Si is an element necessary for deoxidation of steel. If the Si content is less than 0.04%, an appropriate deoxidation effect cannot be expected. On the other hand, if the Si content exceeds 0.60%, the toughness of the steel sheet is impaired, and there is a risk of lack of suitability as structural steel. Therefore, the Si content is 0.04 to 0.60%. The Si content is preferably 0.20% or more, and more preferably 0.50% or less.

Mn: 0.50 to 2.00%
Mn, like C, is an element that is effective in securing the strength of the steel material and improving the fatigue crack growth resistance of the steel plate. Therefore, it is necessary to contain 0.50% or more of Mn. On the other hand, when the Mn content exceeds 2.00%, the deterioration of toughness becomes remarkable. Therefore, the Mn content is 0.50 to 2.00%. The Mn content is preferably 0.80% or more, and preferably 1.50% or less.

Al: 0.003 to 0.06%
Al is an element having a deoxidizing action. In order to deoxidize steel, it is necessary to contain Al by 0.003% or more. On the other hand, if the Al content exceeds 0.06%, a large number of hard island martensites are generated in the welded portion, and the toughness of the welded portion deteriorates. Therefore, the Al content is set to 0.003 to 0.06%. In order to ensure sufficient toughness, the Al content is preferably 0.05% or less.

Ti: 0.001 to 0.100%
Ti is an element effective for improving the fatigue crack growth suppression characteristics of a steel sheet because it forms carbides to reinforce and strengthen the soft part. Therefore, it is necessary to contain Ti 0.001% or more. On the other hand, if the Ti content exceeds 0.100%, not only the effect of improving the fatigue crack growth suppressing property of the steel sheet is saturated, but also the strength of the steel sheet increases excessively, and as a result, the toughness is impaired. Therefore, the Ti content is 0.001 to 0.100%. The Ti content is preferably 0.010% or more, and preferably 0.030% or less.

N: 0.0020 to 0.0120%
N is an important element that combines with Ti to generate TiN and contributes to the refinement of the weld heat affected zone (hereinafter also referred to as “HAZ”). In order to improve the fatigue characteristics in the welded portion through the fine graining of HAZ, it is necessary to contain N in an amount of 0.0020% or more. On the other hand, if the N content exceeds 0.0120%, the toughness is impaired. Therefore, the N content is set to 0.0020 to 0.0120%. The N content is preferably 0.0050% or more, and preferably 0.0090% or less.

  Further, when N is excessively contained with respect to Ti, N that cannot form TiN is present in the steel sheet, and the toughness deteriorates. As a result of performing various fatigue tests on the welded portion, it was found that there is an appropriate region in the ratio of Ti and N, and Ti / N needs to be 3.4 or less. Ti / N is preferably 1.5 or more, and preferably 3.1 or less.

O: 0.0005 to 0.0040%
O is an element that plays an extremely important role in the formation of inclusions. Inclusions may be the starting point for fatigue cracks. Therefore, suppressing the shape and the amount of inclusions is important for improving the fatigue of the welded portion. In the present invention, inclusions are controlled in order to improve fatigue strength, and a smaller O content is advantageous for controlling inclusions. However, in order to reduce the amount of oxygen, many man-hours are required at the steelmaking stage, which is problematic in terms of economy. Therefore, the O content needs to be 0.0005 to 0.0040% from the viewpoint of achieving both improved fatigue characteristics and economy as a structural member. The O content is preferably 0.0010 or more, and preferably 0.0030% or less.

Cr: 0 to 2.0%
Cr is an element that improves corrosion resistance, improves fatigue crack growth suppression characteristics in a corrosive environment, and is effective in controlling the dislocation structure of the soft part and suppressing microscopic plastic deformation. Therefore, you may contain Cr as needed. However, even if Cr is contained in excess of 2.0%, these improvement effects are saturated and the strength is excessively increased, so that the toughness is impaired. Therefore, the Cr content when contained is 2.0% or less. The Cr content is preferably 1.8% or less. In order to obtain the above effect, the Cr content is preferably 0.05% or more.

Mo: 0 to 1.0%
Mo is an expensive element, but is also an effective element for efficiently improving the fatigue characteristics in HAZ. Therefore, you may contain Mo as needed. The fatigue characteristics referred to here are resistance to a fatigue crack starting from the surging toe of the welded joint. A feature of including Mo is that the HAZ notch fatigue strength can be greatly improved without significantly increasing the HAZ hardness. However, even if Mo is contained in excess of 1.0%, these effects are saturated and the strength is excessively increased, so that the toughness is impaired. Therefore, the Mo content when contained is 1.0% or less. From the balance of various material properties, the Mo content is preferably 0.3% or less. When it is desired to obtain the above effect, the Mo content is preferably 0.04% or more, and more preferably 0.05% or more.

Ni: 0 to 1.5%
Ni, like Cr and Mo, improves corrosion resistance, improves fatigue crack growth suppression characteristics in corrosive environments, and is also effective in controlling the dislocation structure of soft parts and suppressing microscopic plastic deformation. Element. Therefore, you may contain Ni as needed. However, even if Ni is contained exceeding 1.5%, these effects are saturated and the strength is excessively increased. Therefore, the Ni content when contained is 1.5% or less. The Ni content is preferably 1.0% or less. In order to obtain the above effect, the Ni content is preferably 0.2% or more.

Cu: 0 to 1.5%
Cu, like Cr, Mo, and Ni, is effective in improving corrosion resistance, improving fatigue crack growth suppression characteristics in a corrosive environment, controlling the dislocation structure of the soft part, and suppressing microscopic composition deformation. . Therefore, you may contain Cu as needed. However, even if Cu is contained in excess of 1.5%, these effects are saturated, the strength is excessively increased, and the toughness is impaired. Therefore, the Cu content when contained is 1.5% or less. The Cu content is preferably 1.2% or less. When it is desired to obtain the above effect, the Cu content is preferably 0.2% or more.

Nb: 0 to 0.1%
Nb is an element effective for improving fatigue crack growth suppression characteristics in a corrosive environment because it combines with C to form carbides to refine and soften the soft part. Therefore, you may contain Nb as needed. However, even if Nb is contained in an amount exceeding 0.1%, this effect is saturated and the strength is excessively increased, so that the toughness is impaired. Therefore, the Nb content in the case of inclusion is 0.1% or less. The Nb content is preferably 0.05% or less. When it is desired to obtain the above effect, the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.

V: 0 to 0.1%
V, like Nb, is combined with C to form carbides, so that the soft part is refined and strengthened. Therefore, V is an effective element for improving fatigue crack growth suppression characteristics in a corrosive environment. Therefore, you may contain V as needed. However, even if V is contained in an amount exceeding 0.1%, this effect is saturated and the strength is excessively increased, so that the toughness is impaired. Therefore, the V content when contained is 0.1% or less. The V content is preferably 0.07% or less. When it is desired to obtain the above effect, the V content is preferably 0.005% or more, and more preferably 0.01% or more.

B: 0 to 0.0030%
B is an element that has the effect of remarkably enhancing hardenability and has the effect of improving strength and fatigue crack growth resistance. Therefore, you may contain B as needed. However, if B exceeds 0.0030%, the toughness deteriorates. Therefore, the B content when contained is 0.0030% or less. In order to obtain the above effect, the B content is preferably 0.0003% or more.

  The steel sheet of the present invention has a chemical composition composed of the above elements C to B, the remaining Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means something. As for the elements from Cr to B, Cr, Mo, Ni, and Cu are 0.03% or less, Nb and V are 0.003% or less, and B is 0.0003% or less, respectively. Can be mixed.

Ceq: 0.25 to 0.40
Ceq means a carbon equivalent and is defined by the following formula (i). In the present invention, as will be described later, the uniformity of hardness in the welded portion when the steel plates are welded is an important factor. When Ceq exceeds 0.40, the hardenability of the steel sheet is increased, so that hardening accompanying welding becomes remarkable, resulting in disturbing hardness uniformity in the welded portion. On the other hand, if Ceq is less than 0.25, it is difficult to ensure the strength of the steel sheet. Therefore, Ceq is set to 0.25 to 0.40.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.

(B) Structure of steel sheet The structure of the steel sheet according to the present invention is a composite structure composed of a hard structure and a soft structure. Here, the hard structure is one or more structures selected from pearlite, bainite, and martensite, and the soft structure is a ferrite structure. Among the above, the hard structure is preferably a bainite structure. Moreover, since the ratio of a hard structure and a soft structure should just be determined according to the strength design of steel, there is no restriction | limiting in particular.

  In the present invention, the hardness difference between the hard tissue and the soft tissue is important. The greater the difference in hardness between the hard and soft tissues, the greater the resistance of fatigue cracks that the boundaries of the above structures will develop, and the more excellent fatigue crack growth characteristics will be exhibited. Therefore, the hardness difference between the hard tissue and the soft tissue needs to be 150 HV or more in the Vickers hardness.

  In addition, about the test force in the measurement of Vickers hardness, there is no restriction in particular. It is desirable to select appropriately according to the crystal grain size of the composite structure. Further, when it is difficult to measure the hardness in the micro Vickers hardness test because the composite structure is extremely fine, a hardness measurement method in a minute region such as a nanoindentation method can be used. Based on the measurement result, it is possible to calculate the hardness difference in Vickers hardness.

(C) Cyclic softening parameter Cyclic softening parameter in the loading direction of steel sheet: 0.95 or less When the repetitive softening parameter of the steel sheet exceeds 0.95, the degree of repeated softening is small, and the welding residual stress of the weld is relaxed by softening of the steel sheet. The improvement mechanism is not working. For this reason, the repeated softening parameter in the load direction of the steel sheet is set to 0.95 or less. It should be noted that the repeated softening parameters can be lowered by positively introducing dislocations into the steel sheet in the manufacturing process, that is, by increasing the initial dislocation density.

  Here, a loading method in evaluating the repeated softening parameters will be described. A round bar test piece is sampled by machining so that the thickness ¼ of the steel plate to be evaluated is closest to the central axis of the test piece. The round bar test piece is not particularly limited in shape and dimensions unless buckling deformation occurs during compression loading. For example, the total length is 70 mm, the test parallel part diameter is 6 mm, the parallel part length is 15 mm, and the parallel part to the gripping part. The diameter increment is connected by a curved line of R20, and the gripping part includes a 20 mm long M12 screw (however, the pitch is 1.25 mm fine). In addition, the sampling direction of the test piece is set to the direction in which repeated loading is received in the actual structure. Generally, it is the rolling direction or the direction perpendicular to the rolling direction.

  The above-mentioned round bar test piece is subjected to repeated tensile and compression swing strain under strain control. To evaluate the repeated softening characteristics, for example, a swinging strain of ± 0.012 (± 1.2%) is applied to the test piece for a long period of time, and the initial state and the repeated load test are advanced and the material response characteristics are stable. Can be considered. When the stress corresponding to the initial state strain +0.012 is 600 MPa and the stress corresponding to the stable state strain +0.012 is 540 MPa, the amount of repeated softening can be expressed as 60 MPa, and the ratio can be expressed as 0.90. The latter ratio obtained at this time corresponds to the repeated softening parameter.

  In the present invention, instead of the constant strain waveform as described above, a strain waveform called an incremental step waveform in which the strain amplitude gradually increases and decreases is employed as the evaluation method of the repeated softening parameters. This is because if an incremental step waveform is adopted, not only the stress corresponding to ± 0.012 but also the stress corresponding to any strain level reaching ± 0.012 can be evaluated at the same time. It is. Specifically, the strain amplitude is gradually increased to ± 0.001, ± 0.002, and ± 0.003, and is gradually increased to ± 0.012 with 12 waves. After reaching ± 0.012, on the contrary, it is gradually decreased to ± 0.011, ± 0.010, ± 0.009, and gradually decreased to ± 0.001 with 12 waves. A set of waveforms consisting of this gradual increase and decrease is named Block. As a repeated softening parameter, a stress σ1 corresponding to a strain of +0.012 in the first block and a stress σ15 corresponding to a strain of +0.012 in the fifteenth block are read, and the ratio between them, that is, σ15 / σ1 was used.

(D) Uniformity of hardness in welded portion The steel deck according to the present invention is formed by welding the above steel plate as a deck plate and a closed cross-section rib. In order to improve the fatigue strength in the welded portion of the steel deck, it is necessary to prevent the occurrence of cracks by avoiding localization of the fatigue damage region.

  Factors that cause stress concentration in the weld include a material notch. The material notch is a stress concentration phenomenon based on a change / distribution of a material in the material, that is, in the case of a welded portion, a change / distribution of strength. The change and distribution of strength can be evaluated by measuring the hardness distribution. In the welded portion, three materials of steel plate, weld metal, and HAZ are continuously present. Usually, among these three materials, HAZ has the highest hardness.

In order to suppress the stress concentration from the viewpoint of the material notch, it is desirable that the hardness of the above three materials is substantially equal and the hardness distribution is substantially uniform. In other words, it is desirable to make the hardness of HAZ having the highest hardness as close as possible to the hardness of the steel plate and the weld metal. Specifically, fatigue characteristics in the welded portion can be improved by defining the hardness of the steel plate, the weld metal, and the weld heat affected zone so as to satisfy the following equation (ii).
1.5 × H _B−W ≧ H _HAZ (ii)
However, H _B-W means the lower value of the hardness of the steel plate and weld metal, and H _HAZ means the hardness of the heat affected _zone of the steel plate.

(E) Manufacture of steel plate Although there is no restriction | limiting in particular about the manufacturing conditions of the steel plate which concerns on this invention, By using the manufacturing method provided with the heating process shown below, the cooling process before rolling, the hot rolling process, and the cooling process after rolling. The steel plate of the present invention can be manufactured.

<Heating process>
It is desirable to heat the slab having the above chemical composition to a temperature range of 900 to 1250 ° C. If the heating temperature at this time is less than 900 ° C., the deformation resistance of the steel sheet is large, and there is a risk of placing an excessive burden on the hot rolling mill. On the other hand, if it exceeds 1250 ° C., the austenite grain size becomes coarse, and the toughness of the final product at the end of rolling may not be secured.

<Cooling process before rolling>
After heating, it is desirable to provide a step of cooling at a cooling rate of 1 ° C./s or more before rolling. If the cooling rate in the pre-rolling cooling step is less than 1 ° C./s, repeated softening of the extreme surface layer may not be promoted. The cooling rate in the pre-rolling cooling step is preferably 15 ° C./s or less. When the cooling rate exceeds 15 ° C./s, not only the material change of the extreme surface layer, but also the cooling effect is exerted on the surface of the steel sheet, and the surface layer is hardened in the hardness distribution in the thickness direction of the entire steel sheet, so that the bending characteristics Etc. may deteriorate.

  Although there is no particular limitation on the cooling stop temperature, it is desirable that the temperature range is T-60 to T-180 ° C. when the heating temperature in the heating step is T (° C.). If the cooling stop temperature in the pre-rolling cooling step exceeds T-60 ° C, there is a possibility that repeated softening in the extreme surface layer cannot be promoted. On the other hand, if the cooling stop temperature is lower than T-180 ° C., the surface layer is hardened in the hardness distribution in the thickness direction of the entire steel sheet, and the bending characteristics and the like may deteriorate.

  The cooling method is not particularly limited. For example, high-pressure descaling or other cooling devices may be used. Providing the above cooling step is preferable because the surface layer can be cooled and the repetitive softening parameter of the surface layer can be stably reduced to 0.95 or less.

<Hot rolling process>
It is desirable to perform hot rolling following the above heating and cooling before rolling. The rolling start temperature is not particularly limited, but it is desirable to start rolling immediately after stopping the cooling in the cooling process before rolling. Moreover, although there is no restriction | limiting in particular about rolling finishing temperature, it is desirable to set it as 800 degreeC or more from the relationship with the below-mentioned post-rolling cooling process.

<Cooling process after rolling>
After rolling, it is desirable to provide a process of performing accelerated cooling from a start temperature of 800 ° C. or higher to a stop temperature of 500 ° C. or lower. In the cooling step, the average cooling rate from 800 ° C. to 500 ° C. is preferably 20 to 60 ° C./s.

  If the start temperature in the above accelerated cooling is less than 800 ° C., there is too much ferrite precipitation from austenite during cooling, and thus there is a possibility that an appropriate final structure cannot be obtained. Also, if the accelerated cooling stop temperature exceeds 500 ° C., the dislocations introduced into the steel sheet are rearranged, so the dislocation density is lowered, and the repeated softening parameters cannot be lowered. There is a possibility that it cannot be secured.

  Moreover, when the average cooling rate from 800 ° C. to 500 ° C. is less than 20 ° C./s, dislocations introduced into the steel sheet are reduced, leading to deterioration of fatigue characteristics. On the other hand, when the average cooling rate exceeds 60 ° C./s, the surface layer is remarkably hardened in the hardness distribution in the thickness direction of the entire steel sheet, which may deteriorate the bending characteristics.

  At that time, if the recuperation temperature width on the steel sheet surface is 40% or less of the accelerated cooling stop temperature, the softening characteristic is repeatedly ensured over the entire thickness, and the appropriate volume fraction and form of the hard and soft tissues are ensured. It is preferable because it can be secured by

(F) Manufacture of steel slab As described above, the steel slab according to the present invention can be manufactured by welding the steel plate as a deck plate and a closed cross-section rib. The closed cross-section rib includes a U-shaped cross-section or a V-shaped cross-section, and the most common closed cross-section rib is a U-shaped cross-section truffle.

  As described above, in order to improve the fatigue strength in the welded portion of the steel deck, it is necessary to increase the uniformity of hardness in the welded portion. When the welding heat input at the time of welding the deck plate and the closed cross-section rib is less than 1.0 kJ / mm, the cooling rate at the welded portion increases, and the hardness increase at the heat-affected zone may become remarkable. On the other hand, if the welding heat input exceeds 3.0 kJ / mm, the cooling rate after welding is remarkably slow, so that the crystal grains in the weld heat affected zone are coarsened and the toughness may not be ensured. Therefore, it is desirable that the welding heat input when welding the deck plate and the closed cross-section rib is 1.0 to 3.0 kJ / mm.

Although there is no restriction | limiting in particular about the method of welding a deck plate and closed cross-section rib, For example, it can carry out by arc welding. Note that, in arc welding, generally, the strength and toughness of the welded portion are reduced by the penetration of gas components such as atmospheric gas into the weld metal. Therefore, with the expectation of a shielding effect by flux or gas, welding is performed by shielded arc welding (SMAW welding: Shielded Metal Arc Welding), MAG welding (MAG welding: Metal Active Gas Arc Welding) or carbon dioxide arc welding (CO ₂ welding). It is preferable to carry out.

  Here, when the amount of the flux or the shielding gas is small, the atmospheric gas component is increased in the weld metal, and the HAZ hardness is increased. Moreover, in the manufacturing process of a steel slab, generally, after welding, the welded portion is allowed to cool in the atmosphere. At this time, the HAZ particle size also increases when the heat of the HAZ is not sufficiently released and the heat is transferred from other regions to the HAZ. Therefore, when manufacturing a steel slab, it is necessary to perform sufficient shielding and to perform sufficient heat management of the HAZ.

  Moreover, there is no restriction | limiting in particular about the steel plate used for a closed cross-section rib. By using the steel plate according to the present invention for the deck plate, there is no restriction on the steel plate used for the closed cross-section rib, and general-purpose steel or the like can be used. However, when there is a concern that fatigue damage also occurs on the closed cross-section rib side, the steel plate according to the present invention can be applied to the closed cross-section rib.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Slabs having the chemical composition shown in Table 1 were produced by melting and casting. Thereafter, under the conditions shown in Table 2, heating, cooling before rolling, hot rolling, and cooling after rolling were sequentially performed to produce a steel plate.

  Thereafter, the structure of the steel sheet was examined and the hardness and repeated softening parameters were measured. For the structural investigation and hardness measurement of the steel plate, the steel plate was embedded in an epoxy resin, cut, subjected to cross-section polishing and etching, and subjected to microscopic observation and micro Vickers hardness test at a 1/4 thickness position. The micro Vickers hardness test was performed according to JIS Z2244 (2009), and the test force was 0.09807N (10 gf).

  In addition, repeated softening parameters were measured as follows. First, a round bar test piece was sampled by machining so that the thickness ¼ of the steel plate was closest to the central axis of the test piece. The round bar test piece has a total length of 70 mm, the diameter of the test parallel part is 6 mm, the parallel part length is 15 mm, and the diameter increment from the parallel part to the grip part is connected by a curved line of R20. The pitch is 1.25 mm fine).

  The above-mentioned round bar test piece is subjected to repeated tensile and compression swing strain. In this test, the above-mentioned incremental step waveform was adopted. Then, the stress σ1 corresponding to the strain of +0.012 in the first block and the stress σ15 corresponding to the strain of +0.012 in the fifteenth block are read, and the ratio between them, that is, σ15 / σ1 is obtained. It was decided to use it as a repeated softening parameter. The results are shown in Table 3.

  Furthermore, a steel slab was produced using the above steel sheet, and fatigue characteristics were investigated at the weld. As a large-sized welded structure model specimen, a steel plate slab specimen (bridge axis direction 450 mm, width direction 1620 mm, height 500 mm, trough rib 1) was prepared, and a test steel plate was applied to the deck plate portion. The thickness of the deck plate portion is 12 mm. General-purpose steel was used for trough ribs. The deck plate and the truffle were welded by mag welding with the welding heat input shown in Table 3. In addition, YGW15 (diameter 1.2 mm) prescribed | regulated by JISZ3312 (2009) was used for the welding material.

  A part of the welded portion of the test body was cut out, embedded in resin, and subjected to a Vickers hardness test for each of the steel plate, the weld metal, and the HAZ. The Vickers hardness test was performed according to JIS Z2244 (2009), and the test force was 9.807 N (1 kgf). The results are also shown in Table 3.

  Thereafter, a compressive load was repeatedly applied directly above the truffles via a 200 mm × 260 mm rubber plate to simulate a wheel load. Then, a strain gauge was attached in the vicinity of the unwelded root portion of the welded portion between the truffle and the deck, and the strain was measured at regular intervals during the fatigue test. The fatigue test was performed under a constant amplitude load, and the difference between the strain at the maximum load and the strain at the minimum load was calculated as a strain range. Further, the stress range Δσ can be derived by multiplying the strain range at the start of the fatigue test by an elastic constant. The stress range Δσ is 223 MPa in all the tests of this example.

  As a result of a fatigue test on each specimen, fatigue cracks occurred from the unwelded root portion of the welded portion between the truffle and the deck, and the fatigue cracks progressed in the thickness direction of the deck. Since the fatigue test was performed under a constant amplitude load, there was a case where a through-thickness crack like a real bridge was not recognized. In particular, under low load conditions, fatigue cracks were often retained without penetrating after the cracks detoured in a direction parallel to the steel sheet surface.

  Therefore, the fatigue life evaluation target was not the deck penetration life, but the fatigue crack initiation life. Since it is a fatigue crack initiation / propagation behavior within a closed cross section starting from the unwelded root part, the occurrence of a fatigue crack cannot be observed directly. Therefore, the fatigue crack initiation life was indirectly measured from the strain range measured by a strain gauge attached in the vicinity of the fatigue crack initiation portion. The measurement method will be described in detail below.

When a fatigue crack is generated in the root portion, the rigidity in the region around the crack is reduced. Therefore, the load is shared by other parts, and the load sharing rate decreases in the crack peripheral region. As a result, in the crack peripheral region, the strain under the same load is reduced and the strain range is reduced. In the present invention, the fatigue crack initiation life is defined as the time when the strain range is reduced by 15% with reference to the strain range at the start of the test. And when the fatigue crack generation life exceeded 7 × 10 ⁵ times under the above test conditions, it was judged that the fatigue characteristics were good.

The fatigue crack initiation life is also shown in Table 3. As can be seen from Table 3, test No. which is an example of the present invention. In Nos. 1 to 7, the chemical composition, structure, and cyclic softening parameter of the steel sheet satisfy the provisions of the present invention, and the hardness uniformity satisfies the provisions of the present invention even in the welded portion of the steel slab manufactured using the steel plate. Therefore, the fatigue characteristics were good. On the other hand, test No. which does not satisfy the provisions of the present invention. In Nos. 8 to 15, the fatigue crack initiation life was 7 × 10 ⁵ times or less, and the fatigue characteristics were inferior.

  By applying the steel material according to the present invention to the deck plate of the steel deck of the road bridge, the fatigue characteristics of the welded portion can be remarkably improved and the fatigue strength soundness can be enhanced.

Claims (8)

A steel plate used for a deck plate of a steel deck,
Chemical composition is mass%, C: 0.01-0.10%, Si: 0.04-0.60%, Mn: 0.50-2.00%, Al: 0.003-0.06 %, Ti: 0.001 to 0.100%, N: 0.0020 to 0.0120%, O: 0.0005 to 0.0040%, Cr: 0 to 2.0%, Mo: 0 to 1. 0%, Ni: 0 to 1.5%, Cu: 0 to 1.5%, Nb: 0 to 0.1%, V: 0 to 0.1%, B: 0 to 0.0030%, balance Fe And the ratio of Ti and N (Ti / N) is 3.4 or less, Ceq defined by the following formula (i) is 0.25 to 0.40,
The microstructure at a thickness of 1/4 of the plate thickness is composed of a composite structure of a hard structure and a soft structure, and the hardness difference between the hard structure and the soft structure is 150 HV or more,
A steel sheet having a repeated softening parameter of 0.95 or less in the load direction of the steel sheet.
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.   The said chemical composition is the mass%, Cr: 0.05-2.0%, Mo: 0.04-1.0%, Ni: 0.2-1.5%, Cu: 0.2-1. The steel plate according to claim 1, containing one or more selected from 5%.   3. The chemical composition according to claim 1, wherein the chemical composition contains, in mass%, one or more selected from Nb: 0.01 to 0.1% and V: 0.005 to 0.1%. steel sheet.   The steel sheet according to any one of claims 1 to 3, wherein the chemical composition contains B: 0.0003 to 0.0030% in mass%. Using the steel plate according to any one of claims 1 to 4 as a deck plate, a steel slab formed by welding with a closed cross-section rib,
A steel slab in which the hardness of the heat-affected zone of the steel sheet satisfies the following formula (ii) in relation to the hardness of the steel sheet and the weld metal.
1.5 × H _B−W ≧ H _HAZ (ii)
However, H _B-W means the lower value of the hardness of the steel plate and weld metal, and H _HAZ means the hardness of the heat affected _zone of the steel plate. A method of manufacturing a steel sheet according to any one of claims 1 to 4,
A heating step of heating the slab having the chemical composition according to any one of claims 1 to 4 to a temperature range of 900 to 1250 ° C;
After heating, when the heating temperature in the heating step is T (° C.), to the temperature range of T-60 to T-180 ° C., the cooling step before rolling for cooling at a cooling rate of 1 ° C./s or more,
A hot rolling process for performing hot rolling;
After hot rolling, when performing accelerated cooling from a start temperature of 800 ° C. or higher to a stop temperature of 500 ° C. or lower, an average cooling rate from 800 ° C. to 500 ° C. is set to 20 to 60 ° C./s, and recuperation on the steel sheet surface is performed. The manufacturing method of the steel plate provided with the post-rolling cooling process which makes a temperature range 40% or less of accelerated cooling stop temperature.   When the steel plate according to any one of claims 1 to 4 is used as a deck plate and welded to a closed cross-section rib to produce a steel slab, the heat input in the welding is 1.0 to 3.0 kJ. / Mm manufacturing method of steel slab. When the steel plate produced by the method according to claim 6 is used as a deck plate and welded to a closed cross-section rib to produce a steel deck, the heat input in the welding is 1.0 to 3.0 kJ / mm. A method for manufacturing steel slabs.