JP2011195961A - HIGH TENSILE STRENGTH STEEL SHEET HAVING EXCELLENT WORKABILITY AND TENSILE STRENGTH OF AT MOST 628 MPa - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel sheet having both of high strength and excellent workability, and having tensile strength of ≤628 MPa.SOLUTION: The high tensile strength steel sheet has a composition containing, by mass, 0.005 to 0.02% C, 0.05 to 0.50% Si, 1.0 to 2.5% Mn, 0.01 to 0.08% Al, 0.010 to 0.060% Nb, 0.005 to 0.025% Ti, 0.0010 to 0.0040% B, ≤0.050% P, ≤0.0050% S and ≤0.010% N, and, as necessary, further containing at least one or more kinds selected from ≤1.0% Cu, ≤2.0% Ni, ≤0.5% Cr, ≤0.5% Mo, ≤0.1% V, ≤0.0030% Ca, ≤0.02% Rem and ≤0.005% Mg, and the balance Fe with inevitable impurities, and satisfies the following inequality: (1) hardness of the surface layer part in the steel sheet+15 Hv<hardness of the center part in the sheet thickness of the steel sheet.

Description

The present invention relates to a non-heat-treated thick steel plate, and is particularly suitable for use in bridges, buildings, shipbuilding, construction machinery, industrial machinery, offshore structures, penstock, etc., and has excellent workability with a tensile strength of 628 MPa or less. about the tension thick steel plate.

  Steel sheets used in construction, bridges, storage tanks, pressure vessels, etc. are required to have excellent workability with little deformation recovery (spring back) after forming, in addition to high strength and toughness. In recent years, these steel sheets are required to have higher strength, and high-tensile steel sheets of 550 MPa class or higher are often used. On the other hand, as the strength of these steel plates increases, the workability tends to decrease, and thus there is a strong demand for improvement of the workability.

  Conventional steel sheets for welded structures such as tempered high-tensile steel sheets and TMCP steel sheets have high strength, but are inferior in workability compared to steel sheets having relatively low strength.

  In general, as a method for improving workability, it is conceivable to reduce the yield ratio (hereinafter referred to as “YR”), which represents the ratio of the upper yield point to the tensile strength, in the sense of increasing the plastic deformability. . As a method for reducing the YR of a steel sheet having a tensile strength (hereinafter referred to as “TS”) of 570 MPa or more, ferrite and ferrite are obtained by heat treatment in a plurality of stages including quenching from a (γ + α) two-phase region. A method of generating a mixed structure composed of a hard second phase is common (see, for example, Patent Document 1).

  There is also known a method of reducing the YR by providing a time for air cooling of the steel sheet after rolling until the start of water cooling and generating proeutectoid ferrite (see, for example, Patent Document 2).

Further, in cooling from above A _r3 point to 400 to 650 ° C., a method of controlling a relatively slow cooling rate of 1 to 15 ° C. / sec is also known (e.g., Patent Document 3, Patent Document 4. ).

Alternatively, a method is also known in which accelerated cooling after rolling is stopped just below the _Ar3 point and reheated by induction heating to reduce YR (see, for example, Patent Document 5 and Patent Document 6).

  In the case of processing such as bending, for example, a larger strain is applied as it approaches the surface layer of the steel plate. Therefore, it is conceivable to improve the workability of the steel plate by softening the surface layer portion as much as possible.

As a method of softening the surface layer as much as possible, after cooling is temporarily interrupted, the bainite phase formed in the surface layer is reheated to a point higher than the A _c1 transformation point, thereby transforming it partially into a soft ferrite phase. A method of cooling again is known (for example, see Patent Document 7).

Although completely different from what has been described above, a method for producing a high-strength steel material having excellent weldability and less material variation is also known (see, for example, Patent Document 8). After heating the C content of properly adjusted steel material additive alloy component in terms of the limitation on the range of from 0.001 to 0.025 wt%, the temperature of the A _c3 point to 1350 ° C., hot rolled at a finishing temperature of 800 ° C. or higher And then cooling at 10 ° C./s or less to produce a high-tensile steel material with little material variation. This steel plate is estimated to be excellent in workability because the Vickers hardness (Hv) is as small as 13 or less at the maximum in the thickness direction.

Japanese Patent Publication No.59-52207 JP 59-2111528 A Japanese Patent Laid-Open No. 1-176027 JP-A-5-214440 JP 2003-213332 A JP 2003-213333 A Japanese Patent Laid-Open No. 3-188216 Japanese Patent No. 3465494

  However, the above prior art has the following problems.

  The method described in Patent Document 1 can significantly reduce 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 Document 5 and Patent Document 6 require equipment for performing accelerated cooling.

In the method described in Patent Document 7, the cooling is interrupted until the surface portion once cooled is reheated to a relatively high temperature not lower than the A _c1 transformation point, and the bainite phase is transformed into the ferrite phase. Since a long time is required, the cooling rate at the central portion of the plate thickness is reduced in the initial stage of cooling, and the effect of increasing the strength as in the case of cooling at a high cooling rate cannot be obtained. Further, in the case of recuperating the surface layer portion using the heat inside the steel plate, the strength is inevitably reduced compared to the surface layer portion because the inside remains in a high temperature state.

  Although the method described in Patent Document 8 is superior in that it is a manufacturing method in which the hardness distribution in the thickness direction of the steel sheet is flattened, the surface layer portion is rather than the inside as described later. The softer one is more excellent in workability and has not yet been realized.

The present invention has been made in view of such circumstances, and has a strength and excellent tensile strength of 628 MPa or less 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. and to provide a processability of both high tensile steel plate.

The inventors have advanced research on the cause of workability deterioration and a method for improving workability in a direct-quenching high-tensile steel sheet having a tensile strength of 628 MPa or less . And the high-tensile steel of controlled cooling or direct quenching type has hardened the surface layer part (both surface layer parts including the back side) compared to the inside. I noticed that it was the cause of the decline. As a result of further research, it was found that by eliminating the hardened layer in the surface layer portion, the bending workability of the steel sheet was improved, leading to low YR. It was also found that if the surface layer portion is made softer than the inside, even better processability can be obtained.

However, as disclosed in Patent Document 8, a steel plate in which the surface layer portion is softer than the inside is still obtained by a method in which the difference in hardness in the plate thickness direction is 13 or less in Vickers hardness (Hv). It has not been realized. Therefore, the inventors can manufacture a steel sheet having such a further excellent workability by reheating so that a temperature difference is created between the surface layer portion and the inside of the steel sheet, or in detail the method. Studied. As a result, after optimizing the components of the steel sheet, reheating the surface layer part of the steel sheet to the A _c1 transformation point or more and the inside to the A _c1 transformation point or less makes such a surface layer part softer than the inside. It was found that a steel sheet excellent in workability can be obtained.

The present invention has been completed based on the knowledge obtained as a result of the detailed research as described above, and the features thereof are as follows.
(A) In mass%, C: 0.005 to 0.02%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.5%, Al: 0.01 to 0.08%, Nb: 0.010 to 0.060%, Ti: 0.005 to 0.025%, B: 0.0010 to 0.0040%, P: 0.050% or less, S: 0.0050% or less, N: 0.010% or less, with the balance being Fe and inevitable impurities, satisfying the following formula (1) A high-tensile steel plate with excellent workability with a tensile strength of 628 MPa or less .

Hardness of steel plate surface layer +15 Hv <Hardness of steel plate thickness center (1)
In addition to the components described in (b) and (a), in addition, 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, Ca: 0.0030% or less, Rem: 0.02% or less, Mg: 0.005% or less, containing at least one or two or more selected from the above, with a workability of tensile strength of 628 MPa or less as described in (a) Excellent high strength steel plate.

According to the present invention, the surface layer portion, by softer than the internal, Ru high-tensile steel sheet having both excellent formability and high strength.

It is a figure for demonstrating embodiment of this invention. (A) Top view, (b) Side view, (c) Front view. It is a figure for demonstrating embodiment of this invention.

  First, the range of chemical components of the high-strength steel sheet of the present invention and the reasons for its definition will be described first from the viewpoint of mainly having high strength and toughness. In the following description, all units represented by% are mass%.

C: 0.005-0.02%
C is an element that improves the strength of steel, and in the present invention, it is necessary to contain 0.005% or more in order to ensure a desired strength even after reheating, but if it exceeds 0.02%, the surface layer portion of the steel plate Is significantly hardened and prevents the effect of softening by reheating, so C was specified in the range of 0.005 to 0.02%. In addition, Preferably it is 0.01 to 0.018%.

Si: 0.05-0.50%
Si is an element that acts as a deoxidizing agent, and in the present invention, it is necessary to contain 0.05% or more from the demand on steelmaking for effectively performing deoxidation. Reduce toughness. For this reason, Si was specified in the range of 0.05 to 0.50%. In addition, Preferably it is 0.20 to 0.35%.

Mn: 1.0-2.5%
Mn is an element that improves the strength of steel. In the present invention, it is necessary to contain 1.0% or more in order to obtain a desired strength. On the other hand, the content exceeding 2.5% lowers the toughness of the welded portion. From this, Mn was specified in the range of 1.5 to 2.5%.

Al: 0.01 to 0.08%
Al acts as a deoxidizer, and for this purpose, it needs to be contained in an amount of 0.01% or more.However, if it exceeds 0.08%, the toughness is lowered and, when welded, the toughness of the weld metal part is reduced. Reduce. For this reason, Al was specified in the range of 0.01 to 0.08%. In addition, Preferably, it is 0.02 to 0.04%.

Nb: 0.010 to 0.060%
Nb is an essential component in the present invention, and is an element that lowers the transformation point and improves the strength of the steel by combined addition with B. To obtain this effect, Nb needs to be contained in an amount of 0.010% or more. It also has the effect of precipitating as NbC during reheating and suppressing softening inside the steel sheet. On the other hand, a content exceeding 0.060% adversely affects toughness after reheating. For this reason, Nb was prescribed | regulated in the range of 0.010-0.060%. In addition, Preferably it is 0.02 to 0.045%.

Ti: 0.005-0.025%
Ti is an element having an action of effectively exhibiting 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. However, if the content exceeds 0.025%, the toughness is lowered, so the content is specified in the range of 0.005 to 0.025%. In addition, Preferably it is 0.008 to 0.020%.

B: 0.0010-0.0040%
B functions effectively to suppress the nucleation of ferrite by lowering the prior austenite grain boundary energy by adding a small amount. In order to obtain this effect, addition of 0.0010% or more is necessary. However, if it exceeds 0.0040%, the toughness is lowered, so it is specified in the range of 0.0010 to 0.0040%. In addition, Preferably it is 0.0005 to 0.0025%.

P: 0.050% or less, S: 0.0050% or less, N: 0.010% or less
When the P content exceeds 0.050%, the toughness of the welded portion is reduced, so it is suppressed to 0.050% or less. Similarly, if S exceeds 0.0050%, the toughness of the base metal and the welded portion is lowered, so that S should be suppressed to 0.0050% or less.

  If N exceeds 0.010%, the toughness is lowered, so it is suppressed to 0.010% or less.

  The above is the basic component of the present invention. In the present invention, the following components can be selected and contained in addition to the basic components. They are: 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, Ca: 0.0030% or less, Rem: 0.02% or less, Mg: 0.005% or less These elements are at least one or more selected from among these elements, all of which contribute to improving the strength of the steel or to improve the HAZ (welding heat affected zone) toughness It can be contained alone or in combination as required. The range of each selected component and the reason for its definition will be described below.

Cu: 1.0% or less
Cu is an element that improves the strength of steel by solid solution strengthening. In the present invention, 0.05% or more may be contained, but if it exceeds 1.0%, the toughness is lowered. For this reason, Cu may be contained in a range of 0.05 to 1.0%.

Ni: 2.0% or less
Ni is an element that improves strength while maintaining toughness. In the present invention, 0.05% or more may be contained, but if it exceeds 2.0%, the effect is saturated, which is disadvantageous in terms of cost. For this reason, Ni may be contained in a range of 0.05 to 2.0%.

Cr: 0.5% or less
Cr is an element that improves the strength of steel. In the present invention, 0.05% or more may be contained, but if it exceeds 0.5%, HAZ (welding heat affected zone) toughness is lowered. For this reason, Cr may be contained in a range of 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), and may be contained in an amount of 0.003% or more, but if it exceeds 0.1%, the toughness is lowered. For this reason, you may contain V in 0.003 to 0.1% of range.

Ca: 0.0030% or less
Ca is contained in an amount of 0.0003% or more, and HAZ (welding heat affected zone) toughness is improved by appropriately selecting a balance with S and O by controlling the form of inclusions. On the other hand, even if the content exceeds 0.0030%, the effect is saturated. For this reason, Ca may be contained in a range of 0.0003 to 0.0030%.

Rem: 0.02% or less
Rem forms Rem (O, S) and improves HAZ (welding heat affected zone) toughness. Such an effect is recognized when the content is 0.0003% or more, but even if the content exceeds 0.02%, the effect is saturated. For this reason, you may contain Rem in 0.0003 to 0.02% of range. Rem means a rare earth element, and typical ones are La, Ce, Hf, and the like.

Mg: 0.005% or less
Mg is an element that improves the strength by refining crystal grains. However, since the effect is saturated when the content exceeds 0.005%, Mg may be contained in a range of 0.005% or less.

  The balance other than the above components is Fe and inevitable impurities.

Next, from the viewpoint of having excellent workability, the steel sheet of the present invention needs to satisfy the following formula (1).
Hardness of surface layer portion +15 Hv <Hardness of thickness center portion (1)
This is because excellent bending workability can be obtained when the hardness of the steel sheet satisfies the formula (1). In the formula (1), “the hardness of the surface layer portion” and “the hardness of the center portion of the plate thickness” are measured as Vickers hardness, and the formula (1) is more than the surface layer portion of the steel plate. The center of thickness (the center in the thickness direction of the steel sheet) is harder, and the difference is larger than 15 in Vickers hardness.

  The manufacturing process for obtaining said steel plate is demonstrated 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.

  Next, the steel material is heated to a temperature range of 1000 to 1300 ° C., and the steel material is completely austenitic. When the heating temperature is less than 1000 ° C., the hot rolling is performed at a low temperature, the load on the rolling mill is increased, and the rolling efficiency is lowered. On the other hand, when the heating temperature exceeds 1300 ° C., the crystal grains become coarse, oxidation loss becomes remarkable, and the yield decreases.

  After heating, hot rolling is performed at a cumulative reduction of 30% or more at 950 ° C. or lower and a rolling end temperature of 750 ° C. or higher.

  Rolling at 950 ° C. or lower according to the present invention corresponds to rolling in a non-recrystallization temperature range. By performing hot rolling at a cumulative reduction ratio of 30% or more in the austenite non-recrystallized region, the area of the austenite grain boundary can be increased and a large amount of strain energy can be accumulated by rolling. Thereby, the bainite transformation from the austenite grain boundary and the austenite grain is promoted. Because of the synergistic effect of austenite grain refinement by rolling in a higher temperature recrystallization zone and rolling in the austenite non-recrystallization zone, the bainite produced is separated by large-angle grain boundaries and has a small packet size. It becomes. Thereby, favorable toughness is obtained as a thick steel plate.

  The hot rolling is finished at a rolling finish temperature of 750 ° C. or higher. In the present invention, since rolling at 950 ° C. or lower is specified, the rolling end temperature is necessarily 950 ° C. or lower, but the lower the rolling end temperature, the better the toughness. However, even if the rolling end temperature is less than 750 ° C., the effect is saturated and only the rolling efficiency is lowered. Therefore, in the present invention, the rolling end temperature is set to 750 ° C. or higher. A preferable rolling end temperature is 800 to 900 ° C.

  After the hot rolling is finished, the steel sheet is cooled, but the cooling is air cooling. Cool to 550 ° C or lower. If reheating described below is performed before cooling to 550 ° C. or lower, the transformation has not been completed, and thus the desired strength of the steel sheet may not be obtained. Moreover, the effect | action which makes a surface layer part soft according to this invention rather than an inside may become insufficient.

Now, in order to make the surface layer portion of the steel sheet softer rather than the inside as in the present invention, therefore, after performing the cooling as described above, the steel sheet surface layer portion is moved beyond the A _c1 transformation point, It is necessary to heat the center of the plate thickness below the A _c1 transformation point. When the steel sheet surface layer part is reheated only to a temperature below the A _c1 transformation point, the surface layer part is not softened, and the intended surface layer part does not have a hardness distribution that is softer than the inside. The metallurgical purpose of reheating to a temperature above the A _c1 transformation point is partly by heating the hard bainite structure formed by the transformation to a two-phase region temperature above the A _c1 transformation point and below the A _c3 transformation point. Is transformed into austenite, and then by relatively slow cooling, relatively soft ferrite is produced. Moreover, it is to soften a hard portion that does not transform into austenite by tempering. By these actions, the surface layer portion can be made softer than the inside. On the other hand, in order to ensure a desired strength as a whole steel plate, it is necessary not to reduce the strength inside the steel plate. For this purpose, the component design of the steel sheet and the temperature management during reheating are important, and the component design is as described above. As temperature control at the time of reheating, it is necessary that the inside of the steel sheet does not exceed the A _c1 transformation point, and reheating is performed so that the center portion of the plate thickness is below the A _c1 transformation point. This is because if the thickness center part is heated beyond the A _c1 transformation point, the strength of the entire steel sheet may be significantly reduced. Since there is no other method for measuring the temperature of the steel sheet than measuring the surface temperature with a radiation thermometer or the like, the temperature of the steel sheet specified in the present invention is the same as that of the surface of the steel sheet unless otherwise specified. To represent.

In order to provide the above temperature distribution in the thickness direction of the steel sheet, when the steel sheet is heated in a heating furnace, the surface layer is extracted from the heating furnace at a temperature higher than the inside without taking a sufficiently long heating time. Alternatively, as shown in FIG. 1, a method of intensively heating the surface layer portion of the steel sheet 1 by the induction heating device 10, or as shown in FIG. 2, burners arranged in the width direction of the steel sheet 1 that is the material to be rolled. A method of heating the surface of the steel plate 1 with the burner flame 2 can be used. When using the induction heating device, the arrangement may be on the conveying path in the plate rolling line, that is, online or offline, but from the viewpoint of energy cost, it can be heated immediately after rolling and cooling. It is preferable to do. In addition, when heating with an induction heating device or burner, the relationship of the heating time according to the plate thickness to prevent the center of the plate thickness from being heated beyond the A _c1 transformation point is shown in the model formula and numerical table. It is preferable to pre-determine and reheat manually or automatically for the heating time determined from the relationship each time the material to be reheated comes.

  Molten steel having each composition shown in Table 1 was melted in a converter and used as a steel material (slab) by a continuous casting method (steel symbols A to X). Using these slabs (steel material: 250 mm thick), hot-rolled to a sheet thickness of 40 mm under the rolling conditions shown in Table 2, air-cooled, and then reheated with an induction heating device under the conditions shown in Table 2 , No. 1 to 37 test steels were obtained. The temperature of the plate thickness surface was obtained with a radiation thermometer, and the temperature at the center of the plate thickness was obtained by simulation from the heat value of the surface layer portion obtained from the input power amount of induction heating and the measured value of the surface temperature.

  About these thick steel plates, the full thickness JIS No. 5 tensile test piece was extract | collected, the tensile test was done, and the yield point (YS) and the tensile strength (TS) were measured. In addition, JIS No. 4 impact test piece was sampled so that the 1/4 thickness part (1 / 4t part) was exactly the center of the plate thickness, Charpy test was conducted, and the fracture surface transition temperature (vTrs) Asked. Furthermore, the hardness distribution in the plate thickness direction was measured, the hardness of the surface layer portion and the hardness of the inside (plate thickness center portion) were measured, and the difference (ΔHv) was obtained. The hardness is evaluated by the Vickers hardness measured with the cross section in the plate thickness direction as a surface to which the diamond is applied, and the hardness of the surface layer portion is a measured value at a position of 2 mm from the surface in the plate thickness direction. The results are shown in Table 3. Here, the desired values as the material of the steel plate were YS> 450 MPa, TS> 570 MPa, YR <0.85, vTrs <−20 ° C., ΔHv> 15.

  The chemical composition and production conditions are within the scope of the present invention. The steel plates 1 to 13 were in the above target range for both mechanical properties and hardness distribution. However, the chemical composition or the production conditions are out of the scope of the present invention. In the steel plates 14 to 37, either strength (YS, TS) or toughness (vTrs) was out of the above target range.

1 Steel plate (rolled material)
2 Burner flame 10 Induction heating device 30 Table roller

Claims (2)

In mass%, C: 0.005-0.02%, Si: 0.05-0.50%, Mn: 1.0-2.5%, Al: 0.01-0.08%, Nb: 0.010-0.060%, Ti: 0.005-0.025%, B: 0.0010- Contains 0.0040%, P: 0.050% or less, S: 0.0050% or less, N: 0.010% or less, the balance is Fe and inevitable impurities, and satisfies the following formula (1). Excellent workability High tensile steel plate with a tensile strength of 628 MPa or less .
Hardness of steel plate surface layer +15 Hv <Hardness of steel plate thickness center (1) In addition to the components according to claim 1, 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, Ca: 0.0030% 2. High tensile strength with excellent tensile strength of 628 MPa or less according to claim 1, characterized in that it contains at least one or more selected from Rem: 0.02% or less and Mg: 0.005% or less. steel sheet.

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