KR101128220B1 - High strength steel plate with 980 or above tensile strength excellent in bending workability - Google Patents

Abstract

The high strength steel plate of this invention is C: 0.1-0.25% (meaning of the mass%, the same as for chemical composition below), Si: 0.1-0.5%, Mn: 0.5-2.0%, Cr: 0.1-1.5%, Mo : 0.1 to 0.5%, Ti: 0.01 to 0.05% and Nb: 0.01 to 0.05%, respectively, V: 0.01 to 0.05% and / or B: 0.0001 to 0.005%, and the balance is iron and inevitable. It is made of impurities, the average particle diameter of the old austenite is 20 µm or less, the standard deviation of the old austenite particle size distribution is 5 µm or less, and the tensile strength is 980 MPa or more. By this structure, bending workability also becomes favorable, maintaining high strength.

Description

High-strength steel sheet with a high bending strength of 980 MPa or more {HIGH STRENGTH STEEL PLATE WITH 980 MPa OR ABOVE TENSILE STRENGTH EXCELLENT IN BENDING WORKABILITY}

TECHNICAL FIELD The present invention relates to a high strength steel sheet excellent in bending workability while maintaining a high strength of 980 MPa or more, and more particularly, to a steel sheet that can be suitably used for a building mechanical structure that needs to be enlarged.

Recently, urbanization is progressing rapidly, mainly in China. As a result, the amount of construction related to civil engineering and construction is increasing, and the demand for construction machinery tends to be remarkable.

On the other hand, it is expected that the improvement of the work efficiency by the building mechanical structure will be important in future urban development, and the enlargement of the building mechanical structure for improving the work efficiency is also progressing. However, due to the response to global environmental problems, the weight constraints of the crane itself are also becoming more severe, and a thicker steel sheet having a higher strength (eg, 980 MPa or more in tensile strength) is required.

In addition, such high strength thick steel sheet is required to have good workability (particularly bending workability) when considering application to a building mechanical structure. However, high strength and workability are characteristics that are opposite, and it is difficult to satisfy both characteristics.

In a high tensile strength steel sheet having a tensile strength of 780 MPa or more, as a technique for improving its uniform elongation, for example, a technique such as Japanese Patent Laid-Open No. 2002-88440 is also proposed. This technique aims to reduce the dislocation of the austenite (hereinafter, abbreviated as " gu? &Quot;) to reduce the particle size (7 or more in the particle size number or 10 m or less in the thickness direction in the thickness direction). By increasing work hardening ability, uniform elongation is improved. However, it was found that the workability was not necessarily improved only by miniaturization of the spherical? Particle size.

This invention is made | formed under such a situation, and the objective is to provide the high strength steel plate with favorable bending workability, maintaining the high strength that tensile strength is 980 Mpa or more.

The high strength steel plate of this invention which could achieve the said objective is C: 0.1-0.25% (meaning of the mass%, the same as for chemical component composition below), Si: 0.1-0.5%, Mn: 0.5-2.0%, Cr 0.1 to 1.5%, Mo: 0.1 to 0.5%, Ti: 0.01 to 0.05% and Nb: 0.01 to 0.05%, respectively, V: 0.01 to 0.05% and B: 0.0001 to 0.005% The remainder consists of iron and unavoidable impurities, the average particle diameter of the old austenite is 20 μm or less, the standard deviation (σ) of the old austenite particle size distribution is 5 μm or less, and the tensile strength is 980 MPa or more. to be.

In the high strength steel sheet of the present invention, it is also effective to further contain Ca: 0.0005 to 0.01%, if necessary, thereby further improving the characteristics of the high strength steel sheet.

According to the present invention, bending properties are maintained while maintaining high strength of tensile strength of 980 MPa or more by appropriately adjusting the chemical composition and controlling the average particle diameter of the sphere γ and the standard deviation (σ) of the sphere γ particle size distribution. Even high strength steel sheet can be realized, and such steel sheet is very useful as a material for large building machinery structures.

According to the present invention described above, it is possible to provide a high strength steel sheet having good bending workability while maintaining a high strength of tensile strength of 980 MPa or more.

MEANS TO SOLVE THE PROBLEM This inventor examined from various angles in order to implement | achieve favorable bending workability in the steel plate which has tensile strength 980 Mpa or more. Histologically, it is proposed that the uniform elongation can be improved by miniaturizing the spherical? Particle size (Japanese Patent Laid-Open No. 2002-88440). However, it has become clear that bending workability is not necessarily improved only by miniaturization of the spherical? Particle size. Therefore, the present inventors examined the cause of such a phenomenon.

As a result, in the steel sheet as described above, the anisotropy of the spherical γ particle diameter is not considered at all, and the anisotropy becomes remarkable according to the chemical composition or the manufacturing conditions, and this causes a gap in the steel sheet material. In addition to generating gaps in strength and toughness, gaps were also generated in uniform elongation, thereby degrading workability.

Therefore, the inventors of the present invention have the idea that the reduction of the gap and the reduction of the gap can be achieved by miniaturizing the spherical γ particle size. The review proceeded. In particular, the cause of the gap was considered to be a nonuniformity in the recrystallized grain growth due to various factors, and this was considered to be the cause of the gap, and the conditions for suppressing such nonuniform recrystallized grain growth were examined.

As a result, the following knowledge was obtained as a setting direction of a chemical component composition. In order to realize a steel sheet having a tensile strength of 980 MPa or more, conventionally, it is useful to add Cu, Ni, or the like having high solid solution strengthening ability. As a result, the recrystallization temperature range is wider than that of the conventional component system, so that the frequency of encroaching the non-uniformly generated recrystallized structure increases, making it easier to obtain a more uniform structure. Moreover, by adding an appropriate amount of Nb and Ti, the pinning effect of the crystal grains by these carbonitrides is exhibited, and it becomes possible to make a spherical (gamma) particle size more fine.

On the other hand, the present inventors were able to obtain the following findings regarding the manufacturing conditions for suppressing the production of non-uniform recrystallized tissue. First, the time between the longest passes at the time of finish rolling shall be 15 seconds or less, and at least three times of the reduction ratio per pass in the rolling of one sheet of steel shall be about 20 to 30%, thereby controlling it uniformly. By suppressing recrystallization which occurs and increasing the nucleation site, it becomes possible to obtain a uniform and fine structure. Moreover, the time between the said path | routes refers to the time until the same position is pressed down in the next path | pass after the end of the steel plate longitudinal direction is pressed down.

This defines the time between passes to control the recrystallization frequency and growth of the recrystallized grains during unrolling, and to control the amount of deformation introduced during rolling by defining the rolling reduction per pass, and to control the frequency at which recrystallization occurs. It was thought that fine homogenization of spherical (gamma) particle diameter was attained by this.

In the present invention, by employing the above means, the spherical? Particle size can be reduced, the gap can be reduced, and the bending workability is also improved. As a criterion for miniaturization of the spherical? Particle size, the average particle diameter of the spherical? Has to be made 20 µm or less. Thereby, the elongation required for workability improvement becomes high. However, only by achieving the refinement of the spherical? Particle size, the object of the present invention is not achieved, and the gap must also be reduced. As an index thereof, the standard deviation σ of the crystal grain size distribution needs to be 5 μm or less. The standard deviation σ is obtained by the following method.

[Measurement method of standard deviation σ]

By observing the microstructure as a 400-fold magnification under an optical microscope, obtaining a vertical and horizontal, respectively five equal parts, each determined by the line segment cutting particles (old γ grains) their distribution from the result of measuring the length (x _i) of the After calculating the average value and the variance by the following equations (1) and (2), the standard deviation is taken as the square root of the variance.

(평균값)

(medium)

(분산)

(Dispersion)

As a factor for improving the bending workability, it is conceivable that the maximum grain size dominates, but in the high strength steel sheet of the present invention, the crystal grains (spherical? Grains) exhibit a distribution close to a normal distribution, and the maximum grain size is almost a problem. It was not thought. Such a maximum grain size becomes a problem in the case of low alloy steel, and in the case of high alloy steel having a strength class of 980 MPa or more, such as the steel sheet of the present invention, the temperature range at which abnormal growth of crystal grains occurs is low and driving force is almost generated. There is nothing to do. For this reason, in this invention, the characteristic can be evaluated only by defining the standard deviation instead of the maximum grain size.

In producing a stable, high-strength steel sheet, a structure with large anisotropy is overwhelmingly disadvantageous because it results in anisotropy of the material. As in the present invention, a steel sheet having a good bendability can be obtained by forming a structure having a uniform crystal grain size. In addition, by miniaturizing the tissue unit, the grain boundary per unit area which becomes the obstacle of dislocation is increased, which improves the elongation. In other words, if the work hardening material is large, even if a local stress is applied at the time of tension, the part exhibits a very high work hardening ability, so that no further deformation occurs at that part, and deformation proceeds at another raw hardened part. Done. As a result, an elongation will be large if it is a structure (fine structure) with a big process hardening ability.

The reason why such a phenomenon occurs by the structure refinement is based on the theory of the pile-up of dislocations in crystal grains, and it is known that the grain boundary functions as a pile-up site of dislocations. In the grain boundary, there are sphere γ grain boundaries, bucket grain boundaries, block grain boundaries, lath grain boundaries, etc., but knowledge that the sphere γ grain boundary with the largest difference in orientation with neighboring crystal grains has a high function as a potential pile-up site is obtained. , The oldest particle was considered to have the highest effect as the governing factor of the kidney.

In addition, in the steel sheet of the present invention, it becomes a tempered martensite structure or tempered bainite structure by the manufacturing method mentioned later, and turns into a steel plate with a tensile strength of 980 MPa or more, even if it becomes either structure, The particle size is also preserved in the final product, and the above-mentioned effect is achieved by miniaturizing the sphere? Particle size.

In the high-strength steel sheet of the present invention, from the viewpoint of miniaturization of the structure, it is necessary to appropriately adjust the content of Ti and Nb, but it is necessary to appropriately control other basic components such as C, Si, Mn, Cr, and Mo. The reason for the range limitation of these elements is as follows.

[C: 0.1 ~ 0.25%]

C is an indispensable element for improving the strength. In the manufacturing method by reheating quenching and tempering (QT method described later), when the C content is less than 0.1%, a large amount of other alloy elements are obtained in order to obtain a high strength of 980 MPa or more. This adds cost. More preferably, it is 0.20% or more. However, when the C content is excessive, the toughness and the weldability are remarkably impaired. Therefore, the C content needs to be 0.25%.

[Si: 0.1 ~ 0.5%]

Si is an indispensable element for increasing the strength and deoxidation of the steel, but if it is less than 0.1%, it is insufficient to exert such an effect. For this reason, Si content needs to be 0.1 to 0.5%. The minimum with more preferable is 0.15% and a more preferable upper limit is 0.35%.

[Mn: 0.5-2.0%]

Mn is effective as an element which raises hardenability of a steel plate and improves strength and toughness. In order to exhibit such an effect, it is necessary to make Mn content into 0.5% or more. More preferably, it is 0.80% or more. However, when Mn is excessively contained, the toughness of the welded portion deteriorates, so the upper limit thereof is made 2.0%.

[Cr: 0.1 ~ 1.5%]

Cr, like Mn, is useful as an element to improve hardenability by addition of a small amount. In order to exhibit such an effect, it is necessary to make Cr content into 0.1% or more. More preferably, it is 0.30% or more. However, when Cr is excessively contained, the toughness of the welded portion deteriorates, so the upper limit thereof is made 1.5%. More preferably, the upper limit is 1.0%.

[Mo: 0.1 ~ 0.5%]

Mo has the effect of securing the strength after annealing. In order to exhibit such an effect, it is necessary to contain 0.1% or more. However, even if Mo is contained in excess of 0.5%, the effect is saturated, and the toughness of the steel sheet is rather lowered. For this reason, content of Mo was prescribed | regulated to 0.1 to 0.5%. More preferably, the upper limit is 0.20%.

[Ti: 0.01 ~ 0.05%]

Ti is easy to form fine carbonitrides, and pinning crystal grain boundaries and dislocations with a small amount of addition forms fine spherical? Particles and has an effect of contributing to reinforcement of the steel sheet. In order to exhibit such an effect, it is necessary to make the content into 0.01% or more, but when Ti content becomes excess and exceeds 0.05%, it will cause toughness deterioration. A more preferable upper limit is 0.03%.

[Nb: 0.01% to 0.05%]

Like Ti, Nb tends to form fine carbonitrides, and pins crystal grain boundaries and dislocations, thereby forming fine spherical? Particles and having an effect of contributing to the strengthening of the steel sheet. In order to exhibit such an effect, it is necessary to make the content into 0.01% or more, but when Nb content becomes excess and exceeds 0.05%, it will cause toughness deterioration. A more preferable upper limit is 0.03%.

[V: 0.01 ~ 0.05% and / or B: 0.0001 ~ 0.005%]

V and B are both effective elements in reinforcing steel materials. Among these, V, like Ti and Nb, is easy to form fine carbonitrides, and by pinning crystal grain boundaries and dislocations, fine particles are formed and contribute to the strengthening of the steel sheet. In order to exhibit such an effect, it is necessary to make V content into 0.01% or more, but when its content becomes excess and exceeds 0.05%, it will cause toughness deterioration. A more preferable upper limit is 0.02%.

On the other hand, B has an effect of improving the hardenability of the steel by adding a very small amount and improving the strength of the steel sheet by solid solution strengthening. In order to exhibit such an effect, it is necessary to make B content into 0.0001% or more. More preferably, it is 0.0005% or more. However, when its content becomes excess and exceeds 0.005%, it will cause toughness deterioration. A more preferable upper limit is 0.002%.

In the high strength steel sheet of the present invention, in addition to the above components, trace components (allowable components) which are iron and unavoidable impurities but are inevitably mixed in solvents may be included (for example, P, S, O, etc.). It is included in the scope of the present invention. Moreover, it is also effective to contain Ca: 0.0005 to 0.01% further as needed for the high strength steel plate of this invention, and the characteristic of a high strength steel plate can be improved further by containing Ca. The reason for range limitation when containing Ca is as follows.

[Ca: 0.0005 ~ 0.01%]

Ca has the effect of improving toughness by controlling the nonmetallic inclusions. In order to exhibit such an effect, it is preferable to contain 0.0005% or more. More preferably, it is 0.0010% or more. However, even if Ca content exceeds 0.01% and the effect becomes saturated, it is preferable to make it 0.01% or less. More preferably, the upper limit is 0.0025%.

In order to manufacture the high strength steel plate of this invention, from the viewpoint of suppressing generation | occurrence | production of a nonuniform recrystallization structure, the time between the longest passes at the time of finish rolling is made into 15 second or less, and in the reduction ratio per pass in one sheet of steel sheet, Although it is necessary to perform uniform reduction of about 20-30% at least 3 times, about other conditions should just follow normal manufacturing conditions. However, in the method of directly quenching-tempering from hot rolling after continuous casting (hereinafter referred to as "DQ-T method"), the sphere γ particle size does not increase, but there is a difference in structure due to the cooling variation in the accelerated cooling process. Since it is easy to do this, the gap of the spherical? Particle size tends to be large, which is undesirable.

From this point of view, a method of hot rolling after continuous casting, then cooling once and then quenching and tempering is preferable. Specifically, the slab slabs obtained by melting in a converter with the chemical composition system as described above are subjected to hot rolling at about 1100 to 700 ° C. At this time, the rolling conditions were rolled to a plate thickness of 120 mm or less with a rough rolling mill, and then rolled with the finish rolling mill under the above conditions (time between passes, rolling reduction rate per time), and the rolling end temperature was recrystallized. The rolling is finished above. The plate thickness at this time is 6-50 mm.

After the said rolling, after cooling to 100 degrees C or less in air | atmosphere, reheating hardening process is performed. Quenching conditions at this time are heated to a heating temperature of 880-930 degreeC, hold | maintain at that temperature for 5 to 15 minutes, and water quenching is performed. In addition, after a hardening process, tempering process is performed after making steel plate temperature into 100 degrees C or less. Tempering conditions at this time are heated to temperature: 300-500 degreeC, hold | maintain at that temperature for about 5 to 15 minutes, and then air-cool in air (Hereinafter, this method is called "QT method.").

Hereinafter, although an Example demonstrates this invention more concretely, this invention is not restrict | limited by the following example of course, Of course, it is possible to add and implement in the range suitable for the meaning of the upper and following, All are included in the technical scope of the present invention.

Example 1

Steel of the chemical component composition shown in the following Table 1 was melted by the conventional method, and various steel sheets were manufactured by the above-described method (QT method). Under the present circumstances, it produced also about the directly quenching-tempered steel plate (DQ-T method), without cooling once after hot rolling (No. 16, 17 of Table 2 below).

The obtained steel sheet was subjected to a tensile test by the following method to measure the mechanical properties (tensile strength TS, total elongation EL) of the steel sheet, and the spherical? Particle size was measured by the following method. Statistical evaluation was quantitative.

[Measurement method of mechanical properties]

The test piece of the tensile test may be a JIS Z 2201 5 test piece (plate thickness: 4.5 to 6 mm), a JIS Z 2201 1A test piece (plate thickness: 6 to 40 mm), or a JIS Z 2201 4 test piece ( Using a sheet thickness of 40-50 mm), the test piece was processed so that the direction perpendicular | vertical to a rolling direction may be made into a longitudinal direction. Using these test pieces, the tensile test was done by the method of JISZ2241, and the tensile strength TS of the steel plate and total elongation EL were measured. At this time, the test piece collection position was made into the position of the plate | board thickness direction 1/4 of a steel plate. In addition, at least 980 MPa or more must be secured for the tensile strength, and for the total elongation EL, the pass criterion is 13%, which is required in order not to cause bending cracks when the plate is processed.

[Measurement Method of Old γ Particle Size]

The cross section parallel to the rolling direction of the steel sheet was observed with an optical microscope. Observation conditions at this time were observation magnification: 400 times, the observation position was 1/4 part of the plate | board thickness direction, and the number of views was made into arbitrary 10 fields. The total observation area is 3 × 10 ² (μm ² ) per sample. At this time, in order to make a sample easy to observe, chemical corrosion was performed with the corrosion liquid containing picric acid, hydrochloric acid, surfactant, etc. Then, the average value and the standard deviation σ of the sphere γ particle diameter were obtained by the above-described method.

The results are shown in Table 2 together with the production method (QT method or DQ-T method), the time between the longest passes, and the reduction ratio (average reduction ratio per one when reduced to almost uniform three times). Based on these results, the relationship between the sphere γ particle diameter (average value) and the total elongation EL is shown in FIG. 1, and the relationship between the standard deviation σ and the total elongation EL is shown in FIG.

From these results, it can consider as follows. First, it is understood that the test Nos. 1 to 15 are steel sheets (inventive steels) that satisfy the requirements specified in the present invention, and both exhibit good elongation with high tensile strength. Moreover, it turns out that high elongation can be achieved by making an average sphere (gamma) particle diameter into 20 micrometers or less, and making the standard deviation (sigma) into 5 micrometers or less.

On the other hand, the test Nos. 16 to 20 are ones (comparative steels) lacking any of the requirements specified in the present invention, and all have a small elongation. Specifically, in Test Nos. 16 and 17, the manufacturing method is based on the DQ-T method, and the spherical γ particle size is controlled to be small. However, since there are gaps in the structure due to cooling variation in the accelerated cooling process, The gap between the sphere γ particle size distribution is generated (the value of the standard deviation σ is large), and the desired elongation cannot be secured.

Test Nos. 18 to 20 are component steel sheets which do not contain Nb or Ti, which affect the refinement of grain size and the uniformity of the structure, and cannot reduce the spherical γ particle size due to carbonitrides of Nb or Ti. Moreover, since the time and the reduction ratio between the longest pass in finish rolling are not adjusted suitably, the difference of the spherical-gamma particle size also becomes large by the difference of recrystallized grain growth, and high elongation cannot be achieved.

The test No. 21 has a lack of strength because Mo and Nb are out of the prescribed range, and the gap between the particle diameters of the old? Is widened because it is out of the condition of time between the longest passes.

1 is a graph showing the relationship between a sphere γ particle diameter (average value) and total elongation EL.

2 is a graph showing the relationship between the standard deviation σ and the total elongation EL.

Claims (2)

C: 0.1% to 0.25% (meaning of mass%, the same as for chemical composition), Si: 0.1% to 0.5%, Mn: 0.5% to 2.0%, Cr: 0.1% to 1.5%, Mo: 0.1% to 0.5%, Ti : 0.01 to 0.05% and Nb: 0.01 to 0.05%, respectively, at least one of V: 0.01 to 0.05% and B: 0.0001 to 0.005%, the balance being made of iron and unavoidable impurities, In addition, while the average particle diameter of old austenite is 20 micrometers or less, the standard deviation ((sigma)) of a former austenite particle size distribution is 5 micrometers or less, Tensile strength over 980 MPa High strength steel plate. The method of claim 1, Further, high strength steel sheet containing Ca: 0.0005 to 0.01%.

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