JP5156453B2 - High strength steel plate with excellent bending workability and tensile strength of 980 MPa or more - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in bending workability while maintaining a high strength of 980 MPa or more in tensile strength, and is particularly suitable for use in a building machine structure that is required to be increased in size. It is related with the steel plate which can be performed.

  In recent years, urbanization has been progressing rapidly mainly in China. As a result, the amount of civil engineering / architecture-related construction work has increased, and there has been a significant increase in demand for construction machinery.

  On the other hand, it is expected that improvement of work efficiency by building machine structures will be important in future urban development, and the size of building machine structures is being increased to improve work efficiency. However, the weight restrictions of the crane itself are becoming stricter due to the response to global environmental problems, and a thick steel plate with higher strength (for example, a tensile strength of 980 MPa or more) is required.

  Such high-strength thick steel plates are also required to have good workability (particularly bending workability) when considering application to building machine structures. However, high strength and workability are contradictory characteristics, and it is difficult to satisfy both characteristics.

As a technique for improving the uniform elongation of a high-strength thick steel plate having a tensile strength of 780 MPa or more, for example, a technique such as Patent Document 1 has been proposed. This technique increases the number of dislocation failures by reducing the grain size of old austenite (hereinafter sometimes referred to as “old γ”) (7 or more in the grain number number or 10 μm or less in the thickness direction thickness average). The uniform elongation is improved by increasing the work hardening ability. However, it has been found that the workability is not necessarily improved only by reducing the old γ grain size.
JP 2002-88440 A

  The present invention has been made under such circumstances, and an object of the present invention is to provide a high-strength steel sheet having good bending workability while maintaining a high strength of a tensile strength of 980 MPa or more.

  The high-strength steel sheet of the present invention that can achieve the above-mentioned object is C: 0.1 to 0.25% (meaning mass%, the same applies to the 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: 0.3%. In addition to each containing 01 to 0.05%, V: 0.01 to 0.05% and / or B: 0.0001 to 0.005%, the balance consisting of iron and inevitable impurities, and It has a gist in that the average particle size of prior austenite is 20 μm or less and the standard deviation (σ) of the prior austenite particle size distribution is 5 μm or less.

  In the high-strength steel sheet of the present invention, it is also effective to further contain Ca: 0.0005 to 0.01% as necessary, whereby the characteristics of the high-strength steel sheet can be further improved.

  According to the present invention, the tensile strength is 980 MPa or more by appropriately adjusting the chemical component composition and controlling the average particle size of old γ and the standard deviation (σ) of the old γ particle size distribution to an appropriate range. A high-strength steel sheet that can be bent while maintaining high strength can be realized, and such a steel sheet is extremely useful as a material for a large-scale construction machine structure.

  This inventor examined from various angles, in order to implement | achieve favorable bending workability in the steel plate whose tensile strength is 980 Mpa or more. Organizationally, it has been proposed that uniform elongation can be improved by refining the old γ grain size (Patent Document 1). However, it has been clarified that the bending workability is not always improved only by reducing the old γ grain size. Therefore, the present inventor examined the cause of such a phenomenon.

  As a result, in the steel sheet as described above, no consideration is given to the anisotropy of the old γ grain size, and the anisotropy becomes prominent depending on the chemical composition and manufacturing conditions, and this is the cause of the steel sheet material. It has been found that these variations cause variations in strength and toughness as well as variations in uniform elongation, thereby impairing workability.

  Therefore, the present inventor has concretely developed the idea based on the idea that if the γ grain size is refined and the variation is reduced, the elongation is improved and the bending workability is also improved. We investigated the means from the viewpoint of chemical composition and production conditions. In particular, the cause of the variation is that the recrystallized grain growth is unevenly caused by various factors, which is considered to cause the variation, and the conditions for suppressing the uneven recrystallized grain growth were examined.

  As a result, the following knowledge was obtained as the setting direction of the chemical component composition. In order to realize a steel sheet having a tensile strength of 980 MPa or more, it is conventionally useful to add Cu, Ni or the like having high solid solution strengthening ability, but the steel sheet of the present invention does not contain any of these elements. Component system. Thus, the recrystallization temperature range becomes wider than that of the conventional component system, and the frequency of phagocytosing the non-uniformly generated recrystallized structure increases, and a more uniform structure can be easily obtained. Further, by adding an appropriate amount of Nb or Ti, the pinning effect of crystal grains by these carbonitrides is exhibited, and the old γ grain size can be made finer.

  On the other hand, the present inventor has obtained the following knowledge about manufacturing conditions for suppressing the formation of a non-uniform recrystallized structure. First, the time between the longest passes at the time of finish rolling is set to 15 seconds or less, and the rolling reduction per pass in the rolling of one steel sheet is controlled to a uniform reduction as about 20 to 30% at least three times, By suppressing recrystallization that occurs non-uniformly and increasing the number of nucleation sites, a uniform and fine structure can be obtained. The time between passes refers to the time from when the leading edge in the longitudinal direction of the steel sheet is reduced until the same position is reduced in the next pass.

  This defines the time between passes, controls the recrystallization frequency and recrystallized grain growth during unrolling, and controls the amount of strain introduced during rolling by defining the rolling reduction per pass. It was considered that fine uniformization of the prior γ grain size was achieved by controlling the frequency of occurrence of recrystallization.

  In the present invention, by adopting the above-described means, it is possible to reduce the variation of the old γ grain size and to improve the bending workability. As a standard for refinement of the old γ particle size, the average particle size of the old γ needs to be 20 μm or less. This increases the elongation required for improving workability. However, the object of the present invention cannot be achieved only by achieving the refinement of the old γ grain size, and it is necessary to reduce the variation. As an index, 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]
The microstructure is observed with an optical microscope at a magnification of 400 times, divided into 5 parts each in the vertical and horizontal directions, and the distribution (distance from the result of measuring the length (xi) of the crystal grains (former γ grains) from which each line segment is cut) After obtaining the average value and variance by the following formulas (1) and (2), the standard deviation is taken as the square root of the variance.

  Although the maximum crystal grain size may dominate as a factor to improve the bending workability, the high-strength steel sheet of the present invention shows that the crystal grains (old γ grains) are close to the normal distribution, and the maximum crystal The particle size was considered to be hardly a problem. Such a maximum crystal grain size becomes a problem in the case of a low alloy steel, and in the case of a high alloy steel having a strength class of 980 MPa or more like the steel sheet of the present invention, a temperature range where abnormal growth of crystal grains occurs. Is low and the driving force is small, it hardly occurs. Therefore, in the present invention, the characteristics can be evaluated only by defining the standard deviation instead of the maximum crystal grain size.

  In producing a steel plate that is stable and has good elongation, a structure with large anisotropy results in anisotropy of the material, which is overwhelmingly disadvantageous. As in the present invention, a steel sheet with good bending workability can be obtained by forming a structure having a uniform crystal grain size. Further, by refining the texture unit, the grain boundary per unit area that becomes an obstacle to dislocation is increased, which improves the elongation. In other words, if the material has a high work hardening, even if a local stress is applied during tension, the part exhibits a very high work hardening ability, and no further deformation occurs in that part. Deformation proceeds at the uncured portion. As a result, if the structure (fine structure) has a large ability to work and harden, the elongation becomes large.

  The reason why the above-mentioned phenomenon occurs due to the refinement of the structure is based on the pile-up theory of dislocations in the crystal grains, and it is known that the grain boundaries function as dislocation pile-up sites. It has been. There are old γ grain boundaries, bucket grain boundaries, block grain boundaries, lath grain boundaries, etc., at the grain boundaries, but the old γ grain boundaries with the largest misorientation between adjacent crystal grains are used as pileup sites for dislocations. As a result, it was considered that the prior γ grain size had the highest influence as the governing factor of elongation.

  In the steel sheet of the present invention, a tempered martensite structure or a tempered bainite structure is finally formed by a manufacturing method described later, and a steel sheet having a tensile strength of 980 MPa or more is used. However, the old γ particle size is preserved even in the final product, and the above-mentioned effect is achieved by making the old γ particle size fine.

  In the high-strength steel sheet of the present invention, it is necessary to appropriately adjust the contents of Ti and Nb from the viewpoint of refinement of the structure, but other basic components such as C, Si, Mn, Cr, and Mo are also appropriately adjusted. Need to control. The reasons for limiting the ranges of these elements are as follows.

[C: 0.1-0.25%]
C is an element indispensable for improving the strength. In the manufacturing method by reheating quenching and tempering (QT method described later), in order to obtain a high strength of 980 MPa or more when the C content is less than 0.1%. In other words, a large amount of other alloy elements is added, resulting in an increase in cost. However, if the C content is excessive, the toughness and weldability are remarkably impaired, so it is necessary to set the content to 0.25%.

[Si: 0.1 to 0.5%]
Si is an indispensable element for increasing the strength and deoxidation of steel materials. However, if it is less than 0.1%, it is insufficient to exert such an effect, and if it exceeds 0.5%, it has toughness. Will be reduced. For these reasons, the Si content needs to be 0.1 to 0.5%.

[Mn: 0.5 to 2.0%]
Mn is effective as an element that improves the hardenability of the steel sheet and improves strength and toughness. In order to exert such effects, the Mn content needs to be 0.5% or more. However, if Mn is contained excessively, the toughness of the welded portion deteriorates, so the upper limit is made 2.0%.

[Cr: 0.1 to 1.5%]
Similar to Mn, Cr is useful as an element that improves hardenability by adding a small amount. In order to exert such effects, the Cr content needs to be 0.1% or more. However, if Cr is excessively contained, the toughness of the welded portion deteriorates, so the upper limit is made 1.5%.

[Mo: 0.1 to 0.5%]
Mo has the effect of ensuring 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%, not only the effect is saturated, but also the toughness of the steel sheet is reduced. For these reasons, the Mo content is defined as 0.1 to 0.5%.

[Ti: 0.01 to 0.05%]
Ti easily forms fine carbonitrides, and by pinning crystal grain boundaries and dislocations with a small amount of addition, it has the effect of forming fine old γ grains and contributing to strengthening of the steel sheet. In order to exert such effects, the content needs to be 0.01% or more. However, if the Ti content becomes excessive and exceeds 0.05%, it causes deterioration of toughness.

[Nb: 0.01 to 0.05%]
Nb is easy to form fine carbonitride like Ti, and has the effect of contributing to strengthening of the steel sheet while forming fine old γ grains by pinning crystal grain boundaries and dislocations. In order to exert such effects, the content needs to be 0.01% or more. However, if the Nb content becomes excessive and exceeds 0.05%, it causes deterioration of toughness.

[V: 0.01 to 0.05% and / or B: 0.0001 to 0.005%]
V and B are both effective elements for strengthening steel. Among them, V, like Ti and Nb, tends to form fine carbonitrides, and by pinning crystal grain boundaries and dislocations, it forms fine old γ grains and contributes to strengthening of the steel sheet. Has an effect. In order to exert such effects, the V content needs to be 0.01% or more. However, if the content is excessive and exceeds 0.05%, it causes deterioration of toughness.

  On the other hand, B exhibits the 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 exert such effects, the B content needs to be 0.0001% or more. However, if the content is excessive and exceeds 0.005%, it causes toughness deterioration.

  In the high-strength steel sheet of the present invention, in addition to the above components, iron and unavoidable impurities can be included, but trace components (acceptable components) that are inevitably mixed during melting can be included (for example, P, S, O)), such steel sheets are also included in the scope of the present invention. In addition, it is also effective to further contain Ca: 0.0005 to 0.01% in the high-strength steel sheet of the present invention, and the characteristics of the high-strength steel sheet can be further improved by containing Ca. Can do. The reasons for limiting the range when Ca is contained are as follows.

[Ca: 0.0005 to 0.01%]
Ca has the effect of improving toughness by controlling non-metallic inclusions. In order to exhibit such an effect, it is preferable to contain 0.0005% or more. However, even if the Ca content exceeds 0.01% and becomes excessive, the effect is saturated.

  In order to manufacture the high-strength steel sheet of the present invention, from the viewpoint of suppressing the formation of a non-uniform recrystallized structure, the time between longest passes during finish rolling is set to 15 seconds or less, and one sheet of steel sheet per pass Of the rolling reduction ratio, it is necessary to perform uniform rolling of about 20 to 30% at least three times, but other conditions may be in accordance with normal manufacturing conditions. However, in the method of direct quenching and tempering from hot rolling after continuous casting (hereinafter referred to as “DQ-T method”), the old γ grain size does not increase, but the structure of the structure caused by cooling unevenness in the accelerated cooling process Since the variation tends to occur, the variation in the old γ particle size tends to increase, which is not preferable.

  From such a viewpoint, after continuous casting, a method of performing hot rolling and then cooling and then quenching and tempering is preferable. Specifically, the slab slab obtained by melting in a converter in the chemical component system as described above is hot-rolled at about 1100 to 700 ° C. The rolling conditions at this time are as follows: After rolling to a sheet thickness of 120 mm or less with a rough rolling mill, rolling is performed with the finish rolling mill under the conditions described above (time between passes, rolling reduction per run), and the rolling end temperature is Rolling ends at the recrystallization temperature or higher. The plate thickness at this time is 6 to 50 mm.

  After the said rolling, after cooling to 100 degrees C or less in air | atmosphere, a reheating quenching process is performed. The quenching conditions at this time are heating to 880 to 930 ° C., and holding at that temperature for 5 to 15 minutes, followed by water quenching. Further, after the quenching process, the steel sheet temperature is set to 100 ° C. or lower, and then the tempering process is performed. Tempering conditions at this time are as follows: temperature: heated to 300 to 500 ° C., held at that temperature for about 5 to 15 minutes, and then air-cooled in the atmosphere (hereinafter, this method is referred to as “QT method”).

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.

[Example 1]
Steels having the chemical composition shown in Table 1 below were melted by an ordinary method, and various steel plates were produced by the method described above (QT method). At this time, a steel plate (DQ-T method) directly quenched and tempered without being cooled once after hot rolling was also produced (Nos. 16 and 17 in Table 2 below).

  About each obtained steel plate, a tensile test is performed by the following method, the mechanical properties (tensile strength TS, total elongation EL) of the steel plate are measured, and the old γ particle size is measured by the following method. Were quantitatively evaluated statistically.

[Measuring method of mechanical properties]
The test piece of the tensile test is a JIS Z 2201 No. 5 test piece (in the case of plate thickness: 4.5 to 6 mm), a JIS Z 2201 No. 1A test piece (in the case of plate thickness: 6 to 40 mm), or JIS Z 2201 4. No. test piece (in the case of a plate thickness of 40 to 50 mm) was used, and the test piece was processed so that the direction perpendicular to the rolling direction was the longitudinal direction. Using these test pieces, a tensile test was performed in accordance with JIS Z 2241, and the tensile strength TS and total elongation EL of the steel sheet were measured. At this time, the test piece collection position was set to a position in the thickness direction 1/4 of the steel sheet. In addition, it is necessary to ensure at least 980 MPa about tensile strength, and about the total elongation EL, 13% required in order not to generate | occur | produce a bending crack when processing a thick plate product was made into the acceptance | permission reference | standard.

[Measurement method of old γ particle size]
A cross section parallel to the rolling direction of the steel sheet was observed with an optical microscope. The observation conditions at this time were observation magnification: 400 times, the observation position was ¼ part in the plate thickness direction, and the number of visual fields was arbitrary 10 visual fields. The total observation area is 3 × 10 ² (μm ² ) per sample. At this time, in order to facilitate observation of the sample, chemical corrosion was performed with a corrosive solution containing picric acid, hydrochloric acid, a surfactant, and the like. And the average value and standard deviation (sigma) of the old gamma particle size were calculated | required by the method mentioned above.

  The results are shown in Table 2 below together with the production method (QT method or DQ-T method), the time between the longest passes, and the rolling reduction (average rolling reduction per one time of rolling almost uniformly three times). Further, based on these results, the relationship between the old γ particle size (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.

  These results can be considered as follows. First, test no. 1 to 15 are steel plates (invention steels) that satisfy the requirements defined in the present invention, and it can be seen that all exhibit good elongation along with high tensile strength. It can also be seen that high elongation can be achieved by setting the average old γ particle size to 20 μm or less and the standard deviation σ to 5 μm or less.

  In contrast, test no. Those of 16 to 20 lack any of the requirements defined in the present invention (comparative steel), and all have small elongation values. Specifically, Test No. 16 and 17, the manufacturing method is based on the DQ-T method, and the old γ particle size is controlled to be small, but because of the variation in the structure due to the cooling unevenness in the accelerated cooling process, The old γ particle size distribution varies (the standard deviation σ is large), and the desired elongation cannot be secured.

  Test No. 18-20 is a steel plate of a component system not containing Nb or Ti that affects the refinement of crystal grain size and the homogenization of the structure, and refinement of the old γ grain size by carbonitride of Nb or Ti. Furthermore, since the time between the longest pass and the rolling reduction in the finish rolling are not properly adjusted, the variation in the old γ grain size becomes large due to the variation in the recrystallized grain growth, so that high elongation cannot be achieved.

It is a graph which shows the relationship between an old gamma particle size (average value) and total elongation EL. It is a graph which shows the relationship between standard deviation (sigma) and total elongation EL.

Claims (2)

  C: 0.1 to 0.25% (meaning by mass, the same applies to the chemical composition), Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, Cr: 0 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: 01-0.05% and / or B: 0.0001-0.005%, the balance is iron and inevitable impurities, and the average grain size of the prior austenite is 20 μm or less, and the prior austenite grains A high-strength steel plate having a tensile strength of 980 MPa or more and excellent bending workability, wherein the standard deviation (σ) of the diameter distribution is 5 μm or less.   The high-strength steel sheet according to claim 1, further comprising Ca: 0.0005 to 0.01%.

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