JP2008121036A - Method for producing high-strength and high-toughness steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength and high-toughness steel sheet having tensile strength of ≥570 MPa, and excellent basic material and HAZ toughness and further, excellent formability by reducing variance in strength in the steel sheet, and a method for producing this steel sheet. <P>SOLUTION: In the method for producing the high-strength and high-toughness steel sheet, the steel containing, by mass, 0.06-0.12% C, ≤0.5% Si, 0.5-2.5% Mn, ≤0.08% Al, 0.005-0.025% Ti and the balance Fe with inevitable impurities, is heated to 1,000-1,200°C and after applying a hot-rolling, an accelerating cooling is performed on condition that the cooling-start temperature is ≤(Ar<SB>3</SB>+10°C) as the steel sheet surface temperature and the cooling-stop temperature is 300-550°C as the steel sheet average temperature, and successively, this steel sheet is heated to 500-700°C as the steel sheet surface temperature and <580°C as the steel sheet average temperature, with an induction-heating. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength, high-toughness steel sheet used in the fields of line pipes, offshore structures, shipbuilding, construction, civil engineering, construction machinery, and the like, and a method for producing the same, and in particular, forming during cold forming. The present invention relates to a high-strength, high-toughness steel sheet with excellent material properties and a small material variation, and a method for producing the same.

  In recent years, pipelines and other steel structures require not only high-strength steel materials to reduce construction costs, but also the development of cold regions. There is a need for excellent steel. Furthermore, in addition to these requirements, steel pipes and steel members with high dimensional accuracy are required in order to increase the work efficiency of circumferential welding of steel pipes and to perform highly accurate structural design. The dimensional accuracy of steel pipes manufactured by cold forming, such as UOE steel pipes, and steel members manufactured by various processes such as cutting and cold forming are strongly affected by material variations and residual stresses of the steel sheets used therefor. In order to improve the dimensional accuracy, it is extremely effective to use a steel plate having a small material variation such as strength and ductility and a small residual stress.

  Conventionally, reduction of material variation has been dealt with in terms of production management, such as strict management of steel plate manufacturing conditions, and material variation when mass-produced, especially variation in strength has been reduced, In order to improve the dimensional accuracy described above, it is necessary to reduce material variations even within the individual steel plates.

  In general, a high-strength steel plate having a tensile strength of 570 MPa or more is manufactured by accelerated cooling, and the longitudinal end portion of the steel plate is easily supercooled, and the strength of the longitudinal end portion is likely to be higher than that of the central portion. Further, due to the fact that the longitudinal end portion of the slab is overheated in the heating furnace before rolling, the strength of the longitudinal end portion may be higher than that of the central portion.

  As a measure to reduce these variations, Patent Document 1 discloses that accelerated cooling is divided into two stages and accelerated cooling is performed, and an air cooling section is provided between the preceding stage and the subsequent stage to reduce temperature unevenness. A method is disclosed. Although this method has a certain effect on the reduction of material variation, the subsequent cooling still causes temperature unevenness in the plate length direction, and thus the material variation cannot be sufficiently eliminated.

  Therefore, conventionally, a method of tempering after accelerated cooling has been employed as a method for reducing the strength variation of a steel plate manufactured by accelerated cooling, particularly for softening a portion where the strength is too high at the longitudinal end of the steel plate. This tempering treatment is carried out by charging a steel plate into a gas combustion furnace and batch-processing it or passing it through a tunnel furnace. In this method, since the amount of softening due to tempering is generally larger in the portion with higher strength before tempering, the difference in strength after tempering is reduced, but there is a problem that the strength of the portion that does not need to be tempered also decreases. Since the tempering process takes time, the carbides are coarsened, resulting in deterioration of toughness, particularly deterioration of DWTT performance.

  As a tempering method that does not degrade toughness, Patent Document 2 discloses a method of performing rapid heating and tempering using an induction heating device installed on the same production line as the accelerated cooling device. According to this method, since the carbides generated during tempering become fine, it is possible to obtain a high toughness steel plate. However, in order to uniformly heat the steel plate, there is an effect in reducing the strength variation in the plate. Absent. Also, by setting the cooling stop temperature in the accelerated cooling process to about 200 ° C. or less and making the microstructure before induction heating martensite, carbide refinement can be achieved by rapid heating and tempering, but the cooling stop temperature is high. In the case of a bainite-based structure, the carbide may be coarsened by tempering and the toughness may be deteriorated.

  In order to solve the above problem, Patent Document 3 proposes a tempering method in which the heating method is changed in the longitudinal direction of the steel sheet when tempering after accelerated cooling is performed by high-frequency induction heating. By this method, the strength of only the portion where the strength is too high, such as the longitudinal end portion of the steel plate, can be reduced, so that the strength variation can be greatly reduced. However, in order to obtain the effect sufficiently, it is necessary to accurately predict the strength distribution in the length direction of the steel plate after accelerated cooling, and to heat only the high strength part to an appropriate temperature. It has been difficult to stably obtain.

As a method for manufacturing a steel plate to which an induction heating device is applied, Patent Document 4 and Patent Document 5 disclose a method for manufacturing a steel plate for a sour line pipe that heats the steel plate surface to a temperature higher than the inside after accelerated cooling. According to these methods, the hardness of the surface layer portion hardened by accelerated cooling can be reduced, and the hardness distribution in the thickness direction of the steel sheet is leveled, but in order to obtain a bainite-based structure that is excellent in sour resistance performance, a comparison is made. Therefore, it was necessary to start accelerated cooling from a particularly high temperature range, and the longitudinal strength variation of the steel sheet remained unimproved.
JP 62-47426 A JP-A-4-358822 JP 2003-27136 A JP 2002-327212 A JP 2003-13138 A

  The present invention has been made in view of the above circumstances, and has a tensile strength of 570 MPa or more, an excellent base material and HAZ toughness, and a high formability with excellent formability by reducing the strength variation in the steel sheet. An object of the present invention is to provide a high-strength, high-toughness steel plate and a method for producing the same.

  As a result of detailed investigations on the causes of material variations in the steel sheet caused by accelerated cooling from the viewpoint of changes in the microstructure, the inventors have obtained the following knowledge. As described above, the increase in strength at the longitudinal end of the steel plate during accelerated cooling is due to supercooling, i.e., due to the decrease in the cooling end temperature. It becomes a structure containing island martensite (MA), and the hardness of the surface layer portion is significantly increased.

  FIG. 1 shows an example in which the thickness distribution in the thickness direction after accelerated cooling of a steel sheet containing 0.07% C-1.7% Mn is measured at the end and center in the longitudinal direction of the steel sheet. The end portion in the longitudinal direction has a higher hardness in the surface layer portion than in the central portion, and as a result, the total thickness strength of the steel plate also increases.

  FIG. 2 shows the result of corrosion of the surface layer portion by two-stage etching and observation by SEM. The surface layer portion has a structure including island martensite. On the other hand, although the island-shaped martensite is seen in the central part of the plate thickness of the steel sheet, the volume fraction is small, and it can be said that the unevenness of the microstructure in the steel sheet is one of the causes of the steel sheet material variation.

  By suppressing the non-uniformity of the microstructure described above, specifically, the amount of island-like martensite in the steel sheet surface layer portion, it is possible to obtain a steel plate with small material variations. It is effective to temper the island martensite generated in the surface layer. However, in the tempering by a general combustion furnace and the tempering by induction heating disclosed in Patent Document 2, the steel plate is heated up to the central portion in the thickness direction of the steel plate. Therefore, the strength variation of the steel sheet is not sufficiently reduced.

  In order to solve this problem, when tempering the steel sheet after accelerated cooling, the tempering temperature of the steel sheet surface layer portion should be higher than that of the central portion in the plate thickness direction of the steel plate, and the heating temperature is set to the steel plate surface layer portion and the plate. By controlling to the optimum range in the central portion in the thickness direction, it is possible to obtain a steel plate having a small microstructure change in the thickness direction of the steel plate, thereby reducing material variations in the steel plate. In order to perform heating at different temperatures between the steel sheet surface layer and the inside as described above, it is effective to use high-frequency induction heating.

  FIG. 3 shows the same component system as in FIG. 1, and the steel sheet heated to about 550 ° C. and the average heating temperature of the steel sheet to 440 ° C. by induction heating after almost the same rolling-accelerated cooling. The thickness direction hardness distribution of is shown. Moreover, the microstructure of the steel plate surface layer part at this time is shown in FIG. The island-like martensite structure as seen in the steel sheet surface layer portion in the accelerated cooling state of FIG. 2 is not seen, and a uniform microstructure is obtained in the steel sheet surface layer portion and the central portion in the plate thickness direction. And as shown in FIG. 3, the difference of the hardness of a surface layer part and a plate | board thickness direction center part is small in both the longitudinal direction edge part and longitudinal direction center part of a steel plate. Further, by limiting the heating temperature at the central portion in the plate thickness direction of the steel plate, it is possible to suppress the strength reduction of the entire steel plate, and to reduce the strength variation in the plate longitudinal direction.

  Also, not only the strength variation in the longitudinal direction of the steel sheet but also the difference in hardness in the thickness direction is small, so the bending workability during cold forming is good, and the residual stress due to internal strain introduced by accelerated cooling However, since it is reduced by subsequent induction heating, the cold formability is good, and the dimensional accuracy after molding is greatly improved.

The present invention has been made based on the above findings,
1st invention is the mass%, C: 0.06-0.12%, Si: 0.5% or less, Mn: 0.5-2.5%, Al: 0.08% or less, Nb: After steel containing 0.005 to 0.025%, Ti: 0.005 to 0.025%, the balance being Fe and unavoidable impurities is heated to 1000 to 1200 ° C. and hot-rolled The cooling start temperature is the steel sheet surface temperature (Ar ₃ + 10 ° C.) or less, the cooling stop temperature is 300 to 550 ° C. with the steel sheet average temperature, and then the steel sheet surface temperature is 500 to 700 ° C. by induction heating. A method for producing a high-strength, high-toughness steel plate, characterized by heating to a steel plate average temperature of less than 580 ° C.

  The second invention is further in terms of mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca : One or more selected from 0.001 to 0.003%. The method for producing a high-strength, high-toughness steel sheet according to the first invention.

  The third invention is a steel plate having a tensile strength of 570 MPa or more manufactured by the method described in the first invention or the second invention, wherein the metal structure of the steel plate surface layer portion is the volume fraction of island martensite. Is a high-strength, high-toughness steel sheet characterized by being 3% or less and the balance being bainite or a mixed structure of bainite and ferrite.

  According to the present invention, it has a high strength of a tensile strength of 570 MPa or more and a high welded portion toughness, and can greatly reduce the strength variation in the longitudinal direction of the steel sheet, and can also reduce the hardness variation in the thickness direction of the steel sheet, so that cold forming Steel sheet with excellent properties can be obtained

The reasons for limiting the respective constituent requirements of the present invention will be described below.
1. About Chemical Components First, the reasons for limiting the chemical components contained in the high-strength, high-toughness steel sheet of the present invention will be described. In addition, all component% means the mass%.

C: 0.06 to 0.12%
C is the most effective element for increasing the strength of the steel sheet produced by accelerated cooling. However, if it is less than 0.06%, sufficient strength cannot be secured, and if it exceeds 0.12%, toughness and weldability are deteriorated. Accordingly, the C content is in the range of 0.06 to 0.12%.

Si: 0.5% or less Si is added for deoxidation, but if it exceeds 0.5%, toughness and weldability are deteriorated. Therefore, the Si amount is set to a range of 0.5% or less.

Mn: 0.5 to 2.5%
Mn is added to improve the strength and toughness of the steel, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.5%, the weldability deteriorates. Therefore, the amount of Mn is made 0.5 to 2.5%.

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. Therefore, the Al content is 0.08% or less.

Nb: 0.005 to 0.025%
Nb suppresses grain growth during rolling, and improves toughness by making fine grains. However, if the Nb content is less than 0.005%, the effect is not obtained, and if it exceeds 0.025%, the toughness of the weld heat affected zone deteriorates. Therefore, the Nb content is in the range of 0.005 to 0.025%.

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, if the amount of Ti is less than 0.005%, the effect is not obtained, and if it exceeds 0.025%, the toughness is deteriorated. Therefore, the Ti amount is set in the range of 0.005 to 0.025%.
In the present invention, in addition to the above chemical components, the following elements can be added as selective elements.

Cu: 0.5% or less Cu is an element effective for improving toughness and increasing strength, but if added over 0.5%, weldability deteriorates. Therefore, when adding Cu, it is 0.5% or less.

Ni: 1% or less Ni is an element effective for improving toughness and increasing strength, but if it exceeds 1%, weldability deteriorates. Therefore, when adding Ni, it is 1.0% or less.

Cr: 0.5% or less Cr is an element effective for increasing the strength by improving the hardenability, but if added over 0.5%, the weldability is deteriorated. Therefore, when adding Cr, it is 0.5% or less.

Mo: 0.5% or less Mo is an element effective for improving toughness and increasing strength, but if added over 0.5%, weldability deteriorates. Therefore, when adding Mo, it is 0.5% or less.

V: 0.1% or less V is an element that increases strength without deteriorating toughness, but if added over 0.1%, weldability is significantly impaired. Therefore, when V is added, the content is made 0.1% or less.

Ca: 0.001 to 0.005%
Ca is an element effective for controlling the form of sulfide inclusions and improving ductility, but if it is less than 0.001%, there is no effect, and even if added over 0.005%, it is effective. Saturates, but rather deteriorates toughness due to reduced cleanliness. Therefore, the Ca content is in the range of 0.001 to 0.005%.
P and S contained as impurity elements are not particularly defined, but in order to obtain higher toughness, the following ranges are desirable.

P: 0.02% or less P is included as an inevitable impurity. However, if the P content increases, the toughness and weldability deteriorate, so the upper limit of the P content is 0.02%.

S: 0.003% or less S is also included as an inevitable impurity. However, when the S content increases, the toughness and ductility deteriorate, so the upper limit of the S amount is set to 0.003%.

  The balance of the steel of the present invention is substantially Fe, and elements other than the above and inevitable impurities can be contained unless the effects of the present invention are impaired.

2. Manufacturing Conditions The present invention is a manufacturing method in which the steel slab containing the above-described chemical components is heated and hot-rolled, then subjected to accelerated cooling, and subsequently tempered by induction heating. Below, the reason for limitation of the manufacturing conditions of a steel plate is demonstrated.

Slab heating temperature: 1000-1200 ° C
When the slab heating temperature is less than 1000 ° C., sufficient strength cannot be obtained, and when it exceeds 1200 ° C., toughness and DWTT characteristics are deteriorated. Therefore, the slab heating temperature is in the range of 1000 to 1200 ° C.

  In the hot rolling process, the lower the rolling end temperature, the better, in order to obtain high base metal toughness, but the rolling efficiency decreases, so the rolling end temperature is arbitrarily set in view of the necessary base material toughness and rolling efficiency. it can. In order to obtain high base metal toughness, it is desirable that the rolling reduction in the non-recrystallization temperature range be 60% or more.

Steel plate surface temperature at the start of cooling: (Ar ₃ + 10 ° C) or less Accelerated cooling is performed after hot rolling. If the steel plate surface temperature at the start of cooling exceeds (Ar ₃ + 10 ° C), the hardness of the surface layer will be Not only does it rise and the formability deteriorates, but the strength variation in the longitudinal direction of the plate tends to increase. Therefore, the steel sheet surface temperature at the start of cooling is set to (Ar ₃ + 10 ° C.) or less.
Here, the Ar ₃ temperature is given by the following formula (1) from the steel components.

Ar ₃ (° C) = 910-310C (%)-80Mn (%)-20Cu (%)-15Cr (%)-55Ni (%)-80Mo (%) (1)
The lower limit of the cooling start temperature is not particularly defined, but if the accelerated cooling start temperature becomes too low, the area ratio of ferrite increases and the strength decreases, so the cooling start temperature is preferably 600 ° C. or higher.

Steel plate average temperature at cooling stop: 300-550 ° C
Accelerated cooling is an important process for obtaining high strength by bainite transformation. However, if the average temperature of the steel sheet when cooling is stopped exceeds 550 ° C., the bainite transformation is insufficient and sufficient strength cannot be obtained. Further, if the average temperature of the steel plate when cooling is stopped is less than 300 ° C., not only the hardness of the surface portion of the steel plate becomes too high, but also the steel plate tends to be distorted and formability deteriorates. Therefore, the steel plate average temperature at the time of cooling stop shall be 300-550 ° C.

  In the present invention, tempering by induction heating is performed following accelerated cooling. Here, the use of the induction heating device enables rapid heating, and furthermore, the heating temperature of the steel sheet, which is an important requirement of the present invention, can be changed between the steel sheet surface layer part and the plate thickness central part. Because.

  FIG. 5 is a schematic diagram showing how the steel plate surface temperature and the steel plate center temperature change with time when the steel plate is heated by an induction heating device. In the induction heating, the surface layer portion of the steel material generates heat due to the induction current, and the temperature inside the steel plate rises due to heat conduction. For this reason, during induction heating, the temperature of the steel sheet surface layer increases greatly compared to the center of the steel sheet, but when induction heating ends, the temperature of the surface layer decreases rapidly due to heat conduction, and the temperature of the steel sheet center increases. And induction heating is complete | finished and the temperature of a steel plate surface and a steel plate center will become substantially equal in several seconds.

  In the present invention, the steel sheet surface temperature at the time of induction heating is an actual measurement value of the steel sheet surface temperature immediately after the induction heating is completed. Further, the steel plate average heating temperature is an actual measured value of the steel plate surface temperature when the steel plate center temperature and the steel plate surface temperature become substantially equal after the end of induction heating.

  In addition, the patent of this application is characterized by heating the steel sheet surface layer part to a temperature higher than the steel sheet center part by induction heating, but when the sheet thickness of the steel sheet becomes thin, the temperature difference between the surface layer part and the center part becomes small, The expected effect cannot be obtained. Therefore, the thickness of the steel sheet is desirably 12 mm or more.

  Below, the reason for limitation of the heating conditions by induction heating is demonstrated.

Steel plate surface temperature: 500-700 ° C
By heating the steel plate surface layer portion, the island martensite generated by accelerated cooling is decomposed, and the hardness of the surface layer portion is reduced. However, when the surface temperature is less than 500 ° C., the island-like martensite is not sufficiently decomposed, so that the hardness is not sufficiently lowered. Much. Therefore, the steel plate surface temperature in induction heating is set to a range of 500 to 700 ° C.

Steel plate average heating temperature: less than 580 ° C. When the steel plate central portion is heated by induction heating, strength reduction due to tempering is caused. However, if the temperature is lower than 580 ° C., the strength is not greatly reduced.

3. Regarding the metal structure In the present invention, the hardness of the steel plate surface portion is reduced by the heat treatment after accelerated cooling, and the hardness distribution in the plate thickness direction is smoothed. The metal structure of the steel plate surface layer portion is defined as follows.

Volume fraction of island martensite in the surface layer: 3% or less Island martensite (MA) is a structure generated by accelerated cooling, and the hardness increases greatly when island martensite is generated. However, if the volume fraction is less than 3%, the difference in hardness from the center of the steel sheet is sufficiently small, so the upper limit of the volume fraction of island martensite is 3%.

  Steel of chemical composition shown in Table 1 (steel types A to K) was made into a slab by a continuous casting method, and a steel plate (No. 1 to 22) having a plate thickness of 20 mm or 30 mm was produced using this.

  The heated slab was rolled by hot rolling. The length of the steel sheet after hot rolling was 38 m for a plate thickness of 20 mm and 25 m for a plate thickness of 30 mm. Immediately after hot rolling, cooling was performed using a water-cooled accelerated cooling facility, and reheating was performed using an induction heating furnace installed on the same line as the accelerated cooling facility. Some steel plates were not reheated for comparison. Table 2 shows the production conditions of each steel plate (No. 1 to 22).

  As for the tensile properties of the steel sheet produced as described above, a tensile test was performed using a full thickness test piece in the rolling direction as a tensile test piece, and the tensile strength was measured. The tensile strength of 570 MPa or more was determined as the strength required for the present invention. Here, in order to confirm the variation in strength in the steel plate, tensile test pieces were collected from the longitudinal direction end and the longitudinal center of the steel plate, and the difference was used as the strength variation.

  Furthermore, about the steel plate longitudinal direction center part, the observation and hardness of the metal structure of the steel plate were measured. The structure of the steel sheet surface layer portion was subjected to electrolytic etching after nital etching (two-stage etching), and the area fraction of island martensite (MA) was measured. In the hardness test, the hardness of the steel plate surface layer portion (1 mm below the surface) and the plate thickness center portion was measured using a Vickers hardness tester with a load of 10 kg. And the difference was made into the variation in hardness. Here, the examples of the present invention were those in which the variation in tensile strength was 20 MPa or less and the variation in hardness was HV 40 or less.

  For the weld heat affected zone (HAZ) toughness, Charpy tests were performed at various temperatures using test pieces to which a thermal history corresponding to a heat input of 40 kJ / cm was added by a reproducible thermal cycle apparatus. The temperature at which the brittle fracture surface ratio was 50% was determined as the fracture surface transition temperature (vTrs). The range of the present invention is that vTrs is 0 ° C. or less.

  In Table 2, all of Nos. 1 to 10, which are examples of the present invention, have chemical components, production methods, and microstructures within the scope of the present invention, high strength of a tensile strength of 570 MPa or more, and strength variation in the steel sheet. As small as 20 MPa or less. Furthermore, since the variation in hardness in the thickness direction is small, it can be said that the bending workability during cold forming is very good. The weld heat-affected zone toughness was also good.

  On the other hand, Nos. 11 to 19 are excellent in welded portion toughness because the chemical components are within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, so that the strength is insufficient, or the surface layer portion. Since the amount of island martensite (MA) exceeds the range of the present invention, the variation in strength and hardness is large. No. 20 to 23 had chemical components outside the scope of the present invention, so that sufficient strength could not be obtained or welding heat affected zone toughness was inferior.

  According to the present invention, a steel sheet having a high tensile strength of 570 MPa or more, high weld toughness and excellent cold formability can be obtained, so that strength toughness, high material homogeneity, and dimensions after cold forming are obtained. It can be applied to line pipes and steel structures where accuracy is required.

It is a figure explaining the thickness direction hardness distribution after accelerated cooling. It is a SEM observation photograph of the steel sheet surface layer part after accelerated cooling. It is a figure explaining the thickness direction hardness distribution after induction heating. It is a SEM observation photograph of the steel plate surface layer part after induction heating. It is a schematic diagram explaining the temperature history of the steel plate surface at the time of induction heating, and a steel plate center part.

Claims (3)

In mass%, C: 0.06-0.12%, Si: 0.5% or less, Mn: 0.5-2.5%, Al: 0.08% or less, Nb: 0.005-0. Steel containing 025%, Ti: 0.005 to 0.025%, the balance being Fe and inevitable impurities is heated to 1000 to 1200 ° C. and hot-rolled, and then the cooling start temperature is the steel plate. The surface temperature is (Ar ₃ + 10 ° C.) or less, the cooling stop temperature is 300 to 550 ° C. at the steel plate average temperature, and then by induction heating, the steel plate surface temperature is 500 to 700 ° C., and the steel plate average temperature is 580 ° C. A method for producing a high-strength, high-toughness steel sheet, characterized by heating to less than   Further, by mass%, Cu: 0.5% or less, Ni: 1% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Ca: 0.001 to 0 The method for producing a high-strength and high-toughness steel sheet according to claim 1, comprising one or more selected from 0.003%.   A steel sheet produced by the method according to claim 1 or 2 having a tensile strength of 570 MPa or more, wherein the surface layer of the steel sheet has a volume fraction of island martensite of 3% or less and the balance is bainite. Or a high-strength, high-toughness steel sheet characterized by having a mixed structure of bainite and ferrite.