JP5476763B2 - High tensile steel plate with excellent ductility and method for producing the same - Google Patents

Description

  The present invention relates to non-tempered steel sheets, particularly non-tempered thick steel sheets, and is suitable for use in bridges, buildings, line pipe original plates, shipbuilding, construction machinery, industrial machinery, offshore structures, penstock, etc., and has excellent ductility. The present invention relates to a high-tensile steel plate and a method for producing the same.

  In general, since ductility tends to decrease with increasing strength of steel sheets, there is a strong demand for improvement in ductility. In recent years, even for high-tensile steel sheets of 570 MPa class or higher used in the above technical field, further ductility is required. There is a need for improvement.

  In particular, steel sheets used in the construction field are required to have high plastic deformation ability from the viewpoint of earthquake resistance performance of steel materials in addition to high strength and toughness. Steel sheets used for storage tanks, pressure vessels, etc., recover deformation after forming. Excellent workability with little (spring back) is required. Moreover, a high total elongation value may be calculated | required by the line pipe original plate.

  Conventional steel sheets for welded structures such as tempered high-tensile steel sheets and TMCP steel sheets have high strength, but may have poor ductility compared to steel sheets with low strength. As a method for improving the ductility of a steel sheet having a tensile strength (hereinafter referred to as TS) of 570 MPa class or more, a ferrite and a hard second phase are formed by heat treatment in a plurality of stages including quenching from a (γ + α) two-phase region. A method of generating a mixed structure is common. For example, in Patent Document 1, as a method for improving ductility, a reduction in yield ratio (hereinafter referred to as YR) representing a ratio of an upper yield point to tensile strength is aimed. .

  In addition, as described in Patent Document 2, there is also known a method for reducing YR by providing a time for air-cooling a steel sheet after rolling until the start of water cooling to generate proeutectoid ferrite.

Furthermore, as described in Patent Documents 3 and 4, there is also known a method of controlling to a relatively slow cooling rate of 1 to 15 ° C./sec in cooling from an Ar ₃ point to 400 to 650 ° C. Yes.

In addition, recently, when heating a steel sheet after hot rolling / cooling using an induction heating device or the like, the inside of the steel sheet is controlled to be less than Ac ₁ point, and the steel sheet surface layer is heated to Ac ₁ point or more to control the steel sheet surface layer. A technique for improving the workability while reducing the hardness of the part and thereby securing the strength of the steel sheet is disclosed.

In Patent Document 5, a steel sheet having a relatively low C content of 0.005% to 0.02% is air-cooled to 550 ° C. or less after hot rolling, and then the center of the sheet thickness is made less than Ac ₁ point. Further, it is described that a steel plate having excellent workability can be obtained by heating and air cooling the steel plate surface layer to Ac ₁ point or more.

In Patent Document 6, a steel sheet having a C content of 0.02% to 0.15% is accelerated and cooled to a temperature of less than 400 ° C. after hot rolling, and then the sheet thickness center is less than Ac ₁ point. It is described that a steel sheet having a low yield ratio and excellent deformability can be obtained by heating / air cooling the surface layer to Ac ₁ point or more. Patent Document 7 also describes that a steel sheet having excellent SSC resistance can be obtained by the same treatment.

Japanese Patent Publication No.59-52207 JP-A-5-63525 Japanese Patent Laid-Open No. 1-176027 JP-A-5-214440 JP 2007-2308 A JP 2007-177318 A JP 2007-16302 A

  However, the method described in Patent Document 1 can improve ductility through a significant reduction in YR, but requires a plurality of stages of heat treatment, which increases manufacturing costs.

  In the method described in Patent Document 2, productivity is reduced and manufacturing cost is increased. Also in the methods described in Patent Document 3 and Patent Document 4, productivity is reduced and manufacturing cost is increased. Moreover, according to the Example described in patent document 4, the intensity | strength of the steel plate used as the object manufactured is only 500 MPa class at most.

The methods described in Patent Documents 5, 6, and 7 achieve high strength and high workability by reducing only the hardness of the steel sheet surface layer portion using induction heating. After heating the surface layer part to Ac ₁ point or more by heating, it is air-cooled.

  Therefore, in the part heated to the two-phase region to be air-cooled, the austenitized C-concentrated region is agglomerated and coarsened during cooling, and the hard second phase formed after cooling: pearlite becomes coarse, which adversely affects ductility and toughness Concerns remain.

  Accordingly, the present invention provides a high-strength steel sheet having high strength and excellent ductility and a method for manufacturing the same without causing a decrease in productivity and an increase in manufacturing cost, such as heat treatment in multiple stages and regulation of cooling start temperature. The purpose is to provide.

The inventors have advanced the research on the cause of the ductility reduction and the method of improving the ductility in the direct quenching type high-tensile steel sheet, and obtained the following knowledge.
1. Controlled cooling or direct-quenching type high-tensile steel sheets are hardened in the surface layer part (both surface layer parts including the back side) compared to the inside, and the presence of the hardened layer in the surface layer part causes a decrease in ductility. ing.
2. When there is no hardened layer in the surface layer, the ductility of the steel sheet is improved and the bending workability is also improved. If the surface layer portion is made softer than the inside, further excellent ductility can be obtained.
3. In addition to optimizing the components of the steel sheet, the surface layer portion of the steel sheet was reheated to the Ac ₁ transformation point or higher and the interior was reheated to less than the Ac ₁ transformation point, and then accelerated cooling was carried out, so Up to a depth of -20%, ferrite or bainite or ferrite + bainite structure in which pearlite is dispersed is obtained, and a steel sheet having a surface layer portion that is softer than the inside and excellent in ductility can be obtained.

  In other words, by changing the temperature reached between the surface layer and the interior during reheating, the surface layer and the internal material are created separately, specifically, the surface layer is softened to improve the overall ductility of the steel sheet. The present inventors have found that this is possible.

The present invention has been made by further study based on the obtained results. That is, the present invention
(1) By mass%, C: 0.02-0.15%, Si: 0.01-0.50%, Mn: 0.5-2.0%, Al: 0.01-0.08% , P: 0.050% or less, S: 0.010% or less, N: 0.010% or less, balance Fe and unavoidable impurities, average from thickness to 5% to 25% of thickness A high-tensile steel sheet having excellent ductility, characterized by having a ferrite, bainite or ferrite + bainite structure in which pearlite having a particle size of 5 μm or less is dispersed, and the inside being a ferrite + bainite main structure.
(2) Further, by mass%, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Nb : 0.005-0.045%, Ti: 0.005-0.025%, B: 0.0005-0.0030%, Ca: 0.0030% or less, REM: 0.02% or less, Mg: The high-tensile steel sheet having excellent ductility according to (1), containing one or more of 0.005% or less.
(3) After heating the steel material having the component described in (1) or (2) to 1000 ° C. to 1300 ° C., hot at a rolling reduction temperature of 750 ° C. or higher at a cumulative reduction ratio of 30% or higher at 950 ° C. or lower. After rolling into a steel plate, accelerated cooling is performed until the average temperature in the plate thickness direction is 500 ° C. or lower, and in the subsequent induction heat treatment, the steel plate surface layer portion is at or above the Ac ₁ transformation point and the plate thickness center portion is at the Ac ₁ transformation. A method for producing a high-tensile steel sheet having excellent ductility, characterized in that after reheating so as to be less than a point, the average temperature in the thickness direction is accelerated to 500 ° C. or less.

  According to the present invention, a high-strength steel plate having a surface layer portion that is softer than the inside and that has both high strength and excellent ductility, has multiple heat treatment and cooling start temperatures that cause a decrease in productivity and an increase in manufacturing cost. It can be manufactured economically without any restrictions and is extremely useful in industry.

In the present invention, the component composition and the microstructure are defined.
[Ingredient composition]
The component composition is mainly defined for imparting strength and toughness as structural steel. In the following description, the unit represented by% is mass%.
C: 0.02-0.15%
C is an element that improves the strength of steel. In the present invention, it is necessary to contain 0.02% or more in order to ensure a desired strength even after reheating by induction heating. If it exceeds the upper limit, the surface layer portion of the steel sheet becomes extremely hard, which hinders the effect of softening due to reheating, and also adversely affects toughness. Therefore, C is specified in the range of 0.02 to 0.15%. In addition, Preferably it is 0.05 to 0.12%.

Si: 0.01 to 0.50%
Si is an element that acts as a deoxidizing agent. In the present invention, it is necessary to contain 0.01% or more from the demand on steelmaking for effectively performing deoxidation, but it exceeds 0.50%. If contained, the toughness is reduced. For this reason, Si was prescribed | regulated in the range of 0.01 to 0.50%. In addition, Preferably it is 0.20 to 0.35%.

Mn: 0.5 to 2.0%
Mn is an element that improves the strength of steel, and in the present invention, it is necessary to contain 0.5% or more in order to obtain a desired strength. On the other hand, when the content exceeds 2.0%, the toughness of the welded portion is lowered, so the content is specified in the range of 0.5 to 2.0%.

Al: 0.01 to 0.08%
Al needs to be contained in an amount of 0.01% or more from the demand on steelmaking for effectively performing deoxidation, but if contained in excess of 0.08%, when toughness is reduced and welding is performed, In order to reduce the toughness of the weld metal part, it was specified in the range of 0.01 to 0.08%. In addition, Preferably, it is 0.02 to 0.04%.
P: 0.050% or less, S: 0.010% or less, N: 0.010% or less P, when the content exceeds 0.050%, the ductility and the toughness of the base metal and the welded portion are reduced. If it exceeds 0.010%, the ductility and the toughness of the base metal and the weld are reduced, so S is also suppressed to 0.010% or less. If N exceeds 0.010%, the ductility and toughness are reduced, so it is suppressed to 0.010% or less.

  The above is the basic component composition of the present invention, but contains one or more of Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, REM, and Mg in order to obtain further desired characteristics. can do. All of these elements contribute to improving the strength or HAZ (welding heat affected zone) toughness of steel.

Cu: 1.0% or less Cu is an element that improves the strength of the steel by solid solution strengthening, but if added over 1.0%, the toughness decreases, so if added, 1.0% Hereinafter, it is more preferably 0.05 to 1.0%.

Ni: 2.0% or less Ni is an element that improves the strength while maintaining toughness, but if added over 2.0%, the effect is saturated and disadvantageous in terms of cost. Is 2.0% or less, more preferably 0.05 to 2.0%.

Cr: 0.5% or less Cr is an element that improves the strength of steel, but if it exceeds 0.5%, HAZ (welding heat affected zone) toughness is reduced. When added, it is 0.5% or less, more preferably 0.05 to 0.5%.

V: 0.1% or less V is an element that improves the strength of the steel by precipitation strengthening as V (CN), but if added over 0.1%, the toughness is reduced. , 0.1% or less, more preferably 0.003 to 0.1%.

Nb: 0.005 to 0.045%
Nb is an element that improves the strength of steel. To obtain this effect, Nb needs to be contained in an amount of 0.005% or more. Moreover, it has the effect of precipitating as NbC at the time of reheating and suppressing the softening inside the steel sheet. On the other hand, the content exceeding 0.045% adversely affects the toughness after reheating. Therefore, when added, the content is 0.005 to 0.045%, preferably 0.01 to 0.025%.

Ti: 0.005-0.025%
Ti is an element having an action of effectively exerting the effect of B by forming TiN and fixing N in the steel. In addition, there is also an effect of suppressing the austenite grain growth at the time of slab heating and at the weld heat affected zone to refine the structure. Addition of 0.005% or more is necessary to fully exhibit these effects, but if added over 0.025%, the toughness is lowered. 025%, preferably 0.008 to 0.020%.

B: 0.0005 to 0.0030%
B suppresses the nucleation of ferrite by lowering the prior austenite grain boundary energy by adding a small amount. In order to obtain this effect, 0.0005% or more must be added. However, if it exceeds 0.0030%, the toughness is lowered. Therefore, when added, the content is made 0.0005 to 0.0030%.

Ca: 0.0030% or less Ca is contained in 0.0003% or more, and HAZ (welding heat affected zone) toughness is improved by appropriately controlling the balance with S and O by controlling the form of inclusions. Let On the other hand, since the effect is saturated even if it contains exceeding 0.0030%, when adding, it is 0.0030% or less, Preferably it is 0.0003 to 0.0030%.

REM: 0.02% or less REM forms REM (O, S) and improves HAZ (welding heat affected zone) toughness. Even if such an effect exceeds 0.02%, it saturates, so when added, it is 0.02% or less, more preferably, 0.0003% or more at which the effect is obtained, 0 0.02% or less. REM is a rare earth element, and typical ones are La, Ce, Hf, and the like. Industrially, it can be added as a mixture of misch metal or the like.

Mg: 0.005% or less Mg is an element that improves the strength by refining crystal grains. However, when the content exceeds 0.005%, the effect is saturated. % Or less.
In the present invention, the balance other than the above components is Fe and inevitable impurities.
[Microstructure]
The microstructure is mainly defined from the viewpoint of imparting excellent ductility, and ferrite, bainite, or ferrite + bainite in which pearlite having an average particle size of 5 μm or less is dispersed from the thickness layer to a depth of 5 to 25% of the thickness. It has a structure, from which the inside is a ferrite + bainite-based structure. Here, the internal microstructure “ferrite + bainite-based” means that the total volume fraction of ferrite and bainite is 90% or more of the microstructure, and the remaining structure includes pearlite, martensite, etc. Even if it contains up to 10% in total, the effect of the present invention is exhibited. In the present invention, ductility was evaluated as total elongation.

The preferable manufacturing method of the high-tensile steel plate according to the present invention will be described below. First, molten steel having the above composition is melted in a converter or the like, and is made into a steel material by continuous casting or the like.
[Slab heating temperature]
The steel material (slab) is heated to a temperature range of 1000 to 1300 ° C. to completely austenite the steel material. If heating temperature is less than 1000 degreeC, it will perform hot rolling at low temperature, the load to a rolling mill will increase, and rolling efficiency will fall. On the other hand, when the heating temperature exceeds 1300 ° C., the crystal grains become coarse, the toughness of the steel sheet is lowered, the oxidation loss becomes remarkable, and the yield is lowered.

[Hot rolling]
After the heating, hot rolling is performed at a cumulative reduction ratio of 30% or higher at 950 ° C. or lower and a rolling end temperature of 750 ° C. or higher. Rolling at 950 ° C. or lower corresponds to rolling that includes the austenite non-recrystallization temperature range, and by performing hot rolling at a cumulative reduction ratio of 30% or more in the austenite non-recrystallization region, the area of the austenite grain boundary is increased. A large amount of strain energy due to rolling can be accumulated. If the cumulative rolling reduction is less than 30%, this effect cannot be sufficiently obtained.

  As described above, the accumulation of strain energy by rolling promotes ferrite transformation and bainite transformation from within the austenite grain boundaries and austenite grains. Due to a synergistic effect with rolling in the non-recrystallized region, the ferrite and / or bainite to be formed has a structure with a small grain size or packet size, and good toughness can be obtained as a structural steel plate.

  Hot rolling is performed at a rolling end temperature of 750 ° C. or higher. In the present invention, since rolling at 950 ° C. or lower is specified, the rolling end temperature is 950 ° C. or lower, and the lower the rolling end temperature, the better the toughness.

  However, when the rolling end temperature is less than 750 ° C., the effect is saturated and the rolling efficiency is lowered. Therefore, the rolling end temperature is set to 750 ° C. or higher, preferably 800 to 900 ° C. After the hot rolling is completed, the steel sheet is accelerated and cooled.

[Accelerated cooling]
In accelerated cooling, the cooling end temperature is set to 500 ° C. or lower. When the cooling end temperature exceeds 500 ° C., the transformation does not end, and the strength of the desired steel sheet may not be obtained after the subsequent induction heat treatment. The cooling end temperature is an average temperature in the thickness direction.

[Reheating treatment]
The reheating treatment after the accelerated cooling is performed in order to make the surface layer portion of the steel plate softer than the inside of the steel plate. In the reheating treatment, after the accelerated cooling is stopped, the steel sheet surface layer part is heated to the Ac ₁ transformation point or higher and the plate thickness center part is heated to the Ac ₁ transformation point or less.

When the steel plate surface layer part is reheated only to a temperature below the Ac ₁ transformation point, the surface layer part is not softened, a hardness distribution in which the surface layer part is softer than the inside is not obtained, and ductility is also reduced.

When the steel after the accelerated cooling is reheated to the Ac ₁ transformation point or higher, the hard bainite structure generated by the accelerated cooling is heated to a two-phase region temperature of the Ac ₁ transformation point or higher and lower than the Ac ₃ transformation point, Part is transformed into austenite.

Since the center of the plate thickness is heated to less than the Ac ₁ transformation point, the toughness is restored while suppressing the strength reduction by the tempering effect without transformation to austenite, while ensuring the desired strength as a whole steel plate. Ductility can be improved. When the plate thickness center portion is heated to the Ac ₁ transformation point or higher, the strength of the entire steel plate is greatly reduced.
In addition, Ac ₃ point and Ac ₁ point mentioned later can be calculated | required by the following Formula, for example.
Ac ₃ (° C.) = 854-180C + 44Si-14Mn-17.8Ni-1.7Cr
Ac ₁ (° C.) = 723 + 22Si-14Mn-14.4Ni + 13.3Cr
However, each element symbol is a content in mass%.

  In order to provide the above temperature distribution in the thickness direction of the steel sheet, it is desirable to use an induction heating device. In order to obtain the desired characteristics, the induction heating device may be either on-line or off-line as long as it is on the conveying path in the thick plate rolling line.

  However, from the viewpoint of energy cost, it is preferable to be on-line capable of heating immediately after rolling and cooling.

In addition, when heating with an induction heating device, the relationship of the heating time according to the plate thickness is determined in advance using a model formula or numerical table so that the plate thickness center portion is not heated beyond the Ac ₁ transformation point. It is preferable that each time a material to be reheated is reheated manually or automatically for a heating time determined from the relationship.

[Second stage accelerated cooling]
After the reheating treatment, the second-stage accelerated cooling is performed at a cooling stop temperature: an average temperature in the thickness direction of 500 ° C. or less. By accelerated cooling to 500 ° C. or less, a part of the hard bainite structure transformed into austenite after reheating treatment becomes a fine hard second phase (mainly fine pearlite), and becomes ferrite or bainite or ferrite + bainite structure. Finely disperse. If the stop temperature of the second stage accelerated cooling exceeds 500 ° C., the granular pearlite is agglomerated and coarsened, so that the toughness is lowered.

  Note that the cooling rate of the second stage accelerated cooling is desirably 5 ° C./s or more. When the temperature is less than 5 ° C./s, aggregation and coarsening of granular pearlite due to carbon diffusion occurs, and thus toughness decreases.

  In the above description of the manufacturing method, the temperature of the steel plate is the temperature of the surface of the steel plate unless otherwise specified.

  The molten steels having the respective compositions shown in Table 1 were melted in a laboratory vacuum melting furnace and made 100 mm thick by ingot rolling as a rolling material (slab) (steel symbols A to P) and subjected to various rolling conditions. After hot rolling and cooling to a thickness of 20 mm, reheating was performed with an induction heating device under various conditions. 1-27 test steels were obtained. In the production of the steel sheet, both the temperature of the plate thickness surface and the center temperature were measured by a thermocouple inserted on the side of the plate.

  About the obtained test steel, a full thickness JIS No. 1B tensile test piece was taken so that the longitudinal direction of the test piece was parallel to the rolling direction, a tensile test was conducted, yield point (YS), tensile strength (TS) and Total elongation (tEl) was measured. Further, a JIS No. 4 impact test piece was collected from the center of the plate thickness so that the longitudinal direction of the test piece was parallel to the rolling direction, and a Charpy test was performed to determine the fracture surface transition temperature (vTrs). The scope of the present invention was TS> 570 MPa, tEl ≧ 15%, and vTrs ≦ −20 ° C. The microstructure of the steel sheet was measured with an optical microscope, and the granular pearlite diameter was determined by image analysis.

  Table 2 shows the test results obtained together with rolling conditions and induction heating conditions. The chemical composition and production conditions are within the scope of the present invention. In the steel sheets of 1, 4 to 6, and 14 to 19, the mechanical properties satisfied the scope of the present invention. On the other hand, the chemical composition or the production conditions deviate from the scope of the present invention. In the steel sheets of 2, 3, 7 to 13, 20 to 27, any of strength (TS), ductility (tEl), and toughness (vTrs) was out of the scope of the present invention.

Claims (3)

  In mass%, C: 0.02 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Al: 0.01 to 0.08%, P: 0.050% or less, S: 0.010% or less, N: 0.010% or less, balance Fe and unavoidable impurities, average thickness 5 μm from surface thickness layer to depth of 5-25% of thickness A high-tensile steel sheet having excellent ductility, characterized in that it has a ferrite, bainite or ferrite + bainite structure in which the following pearlite is dispersed, and the inside is a ferrite + bainite main structure.   Further, in terms of mass%, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Nb: 0.00%. 005 to 0.045%, Ti: 0.005 to 0.025%, B: 0.0005 to 0.0030%, Ca: 0.0030% or less, REM: 0.02% or less, Mg: 0.005 % High-tensile steel sheet excellent in ductility according to claim 1, wherein the high-tensile steel sheet contains 1 type or 2 types or less. A steel material having the component according to claim 1 or 2 is heated to 1000 ° C. to 1300 ° C., and then hot-rolled at a cumulative reduction ratio of 30% or more at 950 ° C. or less and a rolling end temperature of 750 ° C. or more to obtain a steel plate After that, accelerated cooling is performed until the average temperature in the sheet thickness direction is 500 ° C. or less, and in the subsequent induction heat treatment, the steel sheet surface layer part is at least the Ac ₁ transformation point and the sheet thickness center part is less than the Ac ₁ transformation point. A method for producing a high-tensile steel sheet having excellent ductility, characterized in that after the reheating, the average temperature in the thickness direction is accelerated to 500 ° C. or less.