JP2008240116A - High tensile strength steel sheet having excellent shape freezability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet in which high tensile strength and the securance of shape freezability are made consistent. <P>SOLUTION: The steel sheet has a componential composition comprising, by mass, 0.05 to 0.15% C, ≤0.5% Si, 1.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, 0.2 to 1.5% Al, ≤0.01% N and 0.02 to 0.1% Nb under the regulation of Mn/Al, and the balance iron with inevitable impurities, and has a structure where a ferritic fraction is ≥50% and the average ferritic grain size is ≤3.0 μm, and, provided that the average ferritic grain sizes in the rolling direction and in the sheet thickness direction are defined as dL and dN, respectively, the ratio dN/dL is ≥0.70, and further, provided that the standard deviation of the natural logarithm value of the individual ferritic grain sizes in the rolling direction is defined as σA, the relational inequality of σA≤0.70 is satisfied. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a high-strength steel sheet having rigidity suitable mainly for automobile bodies, for example, having a tensile strength of 590 MPa or more and a sheet width direction Young's modulus of 235 GPa or more, and having a shape freezing property, and a method for producing the same. About. The high-strength thin steel sheet of the present invention is assumed to have a plate thickness sensitivity index of rigidity, that is, a component rigidity, such as an automobile side sill, center pillar, side frame and cross member, that is, the component rigidity is proportional to the product of the Young's modulus of the member and the plate thickness λ. It is widely suited for applications that require rigidity and shape freezing, with columnar structural members having a λ of close to 1 as a typical example.

  In recent years, various measures have been taken, such as exhaust gas regulations being imposed on automobiles in response to increasing interest in global environmental problems. In order to satisfy such regulations, weight reduction of the vehicle body in an automobile is a very important issue. This weight reduction of the vehicle body can be achieved by, for example, increasing the strength of the steel plate and reducing the plate thickness. In recent years, as a result of remarkable progress in increasing the strength of steel sheets, the use of thin steel sheets with a thickness of less than 2.0 mm has increased, and in order to further reduce the weight, to use such thin steel sheets In addition, it has become indispensable to simultaneously suppress a decrease in vehicle body rigidity due to the thinning of the steel plate.

  The structure of the vehicle body has the greatest influence on the rigidity of the vehicle body, but it is not easy to change the basic structure. Therefore, for parts that are spot welded, it is effective to change the welding conditions, such as increasing the number of welding points, switching to weld bonding, or switching to laser welding, but the problem is that the cost increases. Accompanied by.

  In addition, there is a method of attaching a resin or the like to a portion that requires rigidity, but this also increases the cost. Furthermore, although it is effective to change the shape and cross-sectional shape of the part, there are various problems in design and press working, and it is not easy to implement.

  On the other hand, if the rigidity of the material used for the part is increased, the rigidity of the part can be increased without changing the part shape or welding conditions. In particular, for parts that require bending rigidity, it is necessary to increase the bending rigidity of the material, and for that purpose, it is important to increase the Young's modulus. Furthermore, when the strength of the steel plate is increased, maintaining the press shape also becomes a problem. Therefore, when the strength of the steel plate is increased, it is necessary to increase the Young's modulus and also improve the shape freezing property after press working.

  Here, the Young's modulus is largely governed by the texture, and in the case of steel having a body-centered cubic lattice, it is high in the <111> direction, which is the dense direction of atoms, and conversely, is small in the <100> direction where the atomic density is low It is known. In general, it is known that the Young's modulus of normal iron with no crystal orientation and no anisotropy is about 210 GPa, but the crystal orientation has anisotropy and the atomic density in a specific direction By increasing the Young's modulus in that direction. On the other hand, when considering the rigidity of an automobile, a load is applied to each component from various directions, so that it is required that the rigidity be improved not only in a specific direction but also in correspondence with the load in each direction.

Regarding the Young's modulus, various studies have been made on steel sheets whose values in a specific direction are increased by controlling the texture. For example, Patent Document 1, by increasing the Ar ₃ transformation point and the amount of C is added Si and Al to 0.05 mass% or less of low-carbon steel, the hot rolling, the rolling reduction below Ar ₃ transformation point A method for producing a hot-rolled steel sheet is disclosed in which the ratio is set to 60% or more, whereby {211} <110> is developed to increase the Young's modulus in the direction perpendicular to the rolling direction.

  However, Patent Document 1 does not mention anything about the shape freezing property, and it cannot provide an excellent shape freezing property together with the improvement in rigidity. Furthermore, since the high Young's modulus by the technique of Patent Document 1 produces a thin steel plate by hot rolling, a high strength steel plate having a thickness of less than 2.0 mm can be stably produced by low-temperature finish rolling. There was a problem that it was difficult. Further, rolling in the ferrite region causes a problem that crystal grains become coarse and workability is remarkably lowered.

Here, regarding the shape freezing property of the steel sheet, Patent Document 2 discloses a technique in which the structure is a composite structure of ferrite, bainite and martensite, and the yield point is 300 MPa or less.
JP-A-9-53118 JP 2005-281867 A

  In the technique described in Patent Document 2, the shape freezing property is enhanced by lowering the yield point. However, since the C content is low and the strength is low, it is necessary to increase the strength. However, since the yield point naturally increases as the strength is increased, it has been difficult to achieve both high tension and ensure shape freezing property by simply lowering the yield point.

  Then, this invention aims at providing together with the advantageous manufacturing method about the steel plate which made the high tension | tensile_strength and securing of shape freezing property compatible.

  As a result of intensive studies to solve the above-mentioned problems, the inventors have regulated both the amount of Mn and Al and made a ferrite structure under predetermined conditions, so that both high tension and excellent shape freezing properties can be achieved. The inventors have found that this is possible and have completed the present invention.

That is, the gist configuration of the present invention is as follows.
1. C: 0.05 to 0.15 mass%, Si: 0.5 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.2 to 1.5 mass%, N: 0.01 mass% And Nb: 0.02 to 0.1 mass% in a range satisfying the relationship represented by the following formula (1), with the balance having a component composition composed of iron and inevitable impurities, with a ferrite fraction of 50% or more and When the average ferrite grain size is 3.0 μm or less, the average ferrite grain size in the rolling direction and the plate thickness direction is dL and dN, respectively, the ratio dN / dL is 0.70 or more, and the individual ferrite grain size in the rolling direction A high-tensile steel sheet with excellent shape-freezing property that satisfies the relational expression of σA ≦ 0.70, where σA is the standard deviation of the value obtained by taking the natural logarithm.
Record
2.0 ≦ Mn / Al ≦ 12 (1)

2. 1. The high-strength steel sheet having excellent shape freezing property, wherein the component composition further includes Ti: 0.01 to 0.2 mass% and V: 0.01 to 0.2 mass%.

3. In the above 1 or 2, the component composition further comprises at least one of Cr: 0.05 to 1.0 mass%, Ni: 0.05 to 1.0 mass%, Mo: 0.05 to 1.0 mass%, and Cu: 0.1 to 2.0 mass%. A high-tensile steel sheet with excellent shape freezing property, characterized by containing.

4). In said 1, 2 or 3, the said component composition contains W: 0.1-2.0mass% further, The high-tensile steel plate excellent in the shape freezing property characterized by the above-mentioned.

5. C: 0.05 to 0.15 mass%, Si: 0.5 mass% or less, Mn: 1.5 to 3.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.2 to 1.5 mass%, N: 0.01 mass% The following and Nb: 0.02 to 0.1 mass% are contained in a range satisfying the relationship shown in the following formula (1), and the remainder is subjected to hot rolling on a steel material having a component composition composed of iron and inevitable impurities. Finish the finish rolling at 800 to 950 ° C., then wind up at 500 ° C. or higher, pickling, cold rolling at a rolling reduction of 40 to 80%, and heating at 1.0 ° C./s or higher. Heating at a rate of 800 to 900 ° C., performing soaking in the temperature range for 300 s or less, performing annealing, and subsequently cooling at an average cooling rate of up to 620 ° C. at 1.0 to 30 ° C./s. A method for producing high-tensile steel sheets with excellent shape freezing properties.
Record
2.0 ≦ Mn / Al ≦ 12 (1)

6). In 5 above, the steel material is further
Ti: 0.01-0.2 mass% and V: 0.01-0.2 mass%
The manufacturing method of the high-tensile steel plate excellent in the shape freezing property characterized by containing.

7). In 5 or 6, the steel material further includes
Cr: 0.05-1.0mass%,
Ni: 0.05-1.0mass%,
Mo: 0.05-1.0mass% and
Cu: 0.1-2.0mass%
A method for producing a high-tensile steel sheet having excellent shape freezing property, comprising at least one of the above.

8). In said 5, 6 or 7, said steel material is further W: 0.1-2.0 mass%
The manufacturing method of the high-tensile steel plate excellent in the shape freezing property characterized by containing.

  Here, the mechanism which can improve the Young's modulus and shape freezing property of the width direction of a steel plate by this invention is considered as follows. That is, in the hot rolling and the subsequent winding process, ferrite transformation is promoted, and cold rolling (112) [1-10] is developed, which is advantageous for increasing the Young's modulus in the width direction of the steel sheet. Then, Nb suppresses recrystallization during the temperature rising process and promotes austenite transformation from unrecrystallized ferrite, and in the subsequent cooling process, by adjusting the cooling rate and the amount of Mn and Al, The transformation start temperature is adjusted to 800 to 700 ° C, and transformation of 80% or more of all ferrite is completed at 620 ° C or higher. As a result, the grain size and grain size distribution of ferrite can be controlled as described above, and (112) [1-10] can be developed and the shape freezing property can be improved with respect to the orientation of the ferrite after annealing. it can.

  In addition, in the steel plate which this invention makes object, the steel plate which gave surface treatments, such as a hot dip galvanized material and an electrogalvanized material including alloying, is included other than a cold-rolled steel plate.

  According to the present invention, it is possible to improve both high rigidity and excellent shape freezing property, which have been difficult to achieve in the past, and an extremely effective industrial effect is brought about.

Next, the high-tensile steel sheet of the present invention will be described in order from the reasons for limiting each chemical component in the component composition.
C: 0.05-0.15 mass%,
C is an element that stabilizes austenite, and in the cooling process during annealing after cold rolling, it can greatly contribute to high strength by increasing the quenching degree and greatly promoting the generation of a low temperature transformation phase. it can. In order to obtain such effects, the C content needs to be 0.05 mass% or more.

  On the other hand, when the amount of C increases, the fraction of the hard low-temperature transformation phase increases, and the steel becomes extremely strong and the workability deteriorates. Further, the addition of a large amount of C suppresses the development of a texture that is advantageous for improving rigidity in cold rolling and the subsequent annealing process. Further, the addition of a large amount of C causes deterioration of weldability. For the above reasons, the C content needs to be 0.15 mass% or less, preferably 0.11 mass% or less.

Si: 0.5 mass% or less
When Si is contained in a large amount, it deteriorates the weldability of the steel sheet and promotes the formation of firelite on the slab surface during hot rolling, thereby promoting the generation of a so-called red scale surface pattern. Will do. Furthermore, when used as a cold-rolled steel sheet, the Si oxide formed on the surface deteriorates the chemical conversion property, and when used as a hot-dip galvanized steel sheet, the Si oxide generated on the surface is not present. Induces plating. For the above reasons, the Si content needs to be 0.5 mass% or less, preferably 0.3 mass% or less.
On the other hand, Si is an element that stabilizes ferrite, and rigidity can be improved by promoting ferrite transformation in the cooling process in the annealing process after cold rolling. In order to obtain such an effect, it is desirable to add at 0.1 mass% or more.

Mn: 1.5-3.0mass%
Mn is an austenite stabilizing element and greatly contributes to increasing the strength by enhancing the hardenability and greatly promoting the generation of the low temperature transformation phase in the cooling process in the annealing process after cold rolling. Further, by acting as a solid solution strengthening element, it contributes to increasing the strength of steel. Furthermore, the effect of improving the rigidity by texture control is obtained by refining ferrite produced in the cooling process. In order to obtain such an effect, the Mn content needs to be 1.5 mass% or more.

  On the other hand, when a large amount of Mn is added, ferrite transformation is suppressed in the cooling process in the annealing process after cold rolling, so that it becomes impossible to develop a texture that is advantageous for improving the rigidity. Furthermore, the addition of a large amount of Mn also deteriorates the weldability of the steel sheet. Moreover, the rolling load at the time of hot rolling and cold rolling will be increased, resulting in an increase in manufacturing cost. Therefore, the Mn content needs to be 3.0 mass% or less, more preferably 2.5 mass% or less.

P: 0.05 mass% or less P segregates at the grain boundary, lowers the ductility and toughness of the steel sheet, and degrades the weldability. Further, when used as an alloyed hot-dip galvanized steel sheet, the alloying rate is delayed by P. Therefore, the content of P needs to be 0.05 mass% or less. On the other hand, P is an element effective for increasing strength as a solid solution strengthening element, and also has an action of promoting C concentration in austenite as a ferrite stabilizing element. Furthermore, steel added with Si also has an effect of suppressing the occurrence of red scale. In order to obtain such an action, the P content is preferably set to 0.01 mass% or more.

S: 0.01 mass% or less S, in order to remarkably reduce hot ductility, induces hot cracking and remarkably deteriorates surface properties. Further, S hardly contributes to the strength, but also reduces the ductility and hole expandability by forming coarse MnS as an impurity element. Since these problems become significant when the S amount exceeds 0.01 mass%, the S amount needs to be 0.01 mass% or less. Furthermore, from the viewpoint of particularly improving the hole expansibility, it is preferably 0.005 mass% or less.

Al: 0.2-1.5mass%
Al is one of the important elements of the present invention. In other words, Al promotes the transformation of ferrite in the annealing process after cold rolling, reduces its grain size distribution, and increases the rigidity by controlling the texture of ferrite by making the aspect ratio 0.7 or more. Contributes to improved shape freezing. In order to obtain such an effect, the Al content needs to be 0.2 mass% or more.
On the other hand, Al is a strong ferrite stabilizing element, and when added in a large amount, the Ac3 transformation point is greatly increased during the temperature rising process in the annealing process after cold rolling, resulting in texture control utilizing transformation. Therefore, the upper limit of the Al content is 1.5 mass%, preferably 1.0 mass%.

Mn / Al: 2.0-12
In the cooling process of the annealing process after cold rolling, if the ferrite transformation temperature is high, the grain growth of the transformed ferrite is promoted to increase the ferrite grain size and the grain size distribution. And the development of the texture necessary for improving the rigidity is suppressed.

  On the other hand, when the temperature of ferrite transformation is low, ferrite transformation is suppressed and the aspect ratio of ferrite grains described later greatly deviates from 1. Tissue development is suppressed. Therefore, the amount of Mn, which is an austenite stabilizing element, and Al, which is a ferrite stabilizing element, needs to be controlled in balance, and the ratio of Mn / Al needs to be 2.0-12. In this formula, Mn and Al are the contents (mass%) of each element.

  Here, FIG. 1 shows the relationship between Mn / Al and the Young's modulus in the width direction of the steel sheet. As shown in FIG. 1, it can be seen that by controlling Mn / Al within the range of 2.0 to 12, the Young's modulus in the steel sheet width direction can be made 235 GPa or more. In addition, about the steel plate which obtained the evaluation result shown in FIG. 1, it showed to the Example (Specimen No. 1, 2, 4, 5, 6, 12, 19, 21, 23 and 24) mentioned later.

N: 0.01 mass% or less When N is contained in a large amount, there is a risk of surface marks being generated along with slab cracking during hot rolling. Further, the formation of coarse nitrides with Nb at a high temperature reduces the effect of Nb addition, leading to an increase in manufacturing cost. Therefore, the N amount needs to be 0.01 mass% or less, more preferably 0.005 mass% or less.

Nb: 0.02-0.10mass%
Nb promotes austenite transformation from unrecrystallized ferrite by suppressing recrystallization of processed ferrite in the temperature rising process in the annealing process after cold rolling, and further refines austenite grains, and after transformation The ferrite can also be refined to improve the rigidity by texture control. In order to have such an action, the Nb content needs to be 0.02 mass% or more. On the other hand, even if a large amount of Nb is added, the effect of suppressing recrystallization of austenite at the time of hot rolling and annealing at the time of annealing after cold rolling is saturated and the rolling load in hot rolling and cold rolling is increased. Therefore, the Nb content needs to be 0.10 mass% or less.

  In addition, it is preferable that the remainder except an above-described component is Fe and an unavoidable impurity. Furthermore, when improving strength, rigidity, and shape freezing property, one or more of the following components may be added as necessary, in addition to the above-mentioned chemical components.

Ti: 0.01-0.20mass%
V: 0.01 ~ 0.20mass%
Ti and V contribute to an increase in strength by forming fine carbonitrides. Moreover, in the temperature rising process in the annealing process after cold rolling, by suppressing recrystallization of the processed ferrite, the austenite transformation from the unrecrystallized ferrite is promoted and the austenite grains are refined. Furthermore, by reducing the size of the ferrite after transformation, the shape freezing property can be improved and the rigidity can be improved by texture control. In order to have such an action, the Ti and V contents are each preferably 0.01 mass% or more. On the other hand, even if a large amount of Ti and V are added, the effect of suppressing the recrystallization of ferrite at the time of temperature rise after annealing is saturated, so the contents of Ti and V are each 0.20 mass% or less. It is preferable.

Cr: 0.05-1.0 mass%, Ni: 0.05-1.0 mass%, Mo: 0.05-1.0 mass% and Cu: 0.1-2.0 mass%
Cr, Ni, Mo, and Cu are elements that enhance the hardenability and greatly contribute to high strength by greatly promoting the generation of low-temperature transformation phase in the cooling process after soaking in the annealing process. it can. In order to obtain such an effect, it is preferable to add Cr, Ni and Mo in amounts of 0.05 mass% or more and Cu in amounts of 0.1 mass% or more.

  On the other hand, adding a large amount of Cr, Ni, Mo and Cu not only saturates the effect, but also increases the rolling load and alloy costs, so the amount of Cr, Ni and Mo is less than 1.0 mass% respectively. And, the amount of Cu is preferably set to 2.0 mass% or less.

W: 0.1-2.0mass%
W can improve rigidity by existing as a solid solution element or carbide. In order to obtain this effect, the W amount is preferably set to 0.1 mass% or more. On the other hand, the addition of a large amount of W increases the alloy cost, so the W amount is preferably set to 2.0 mass% or less.

Furthermore, it is important that the steel sheet of the present invention has the following organization.
Ferrite fraction: 50% or more Ferrite can be increased in rigidity by controlling texture control. For that purpose, the ferrite fraction needs to be 50% or more. Although there is no particular upper limit for the ferrite fraction, it is advantageous to use the martensite phase to improve shape freezeability and increase strength, and to ensure a martensite fraction of 10% or more. The ferrite fraction is preferably 90% or less.

Average ferrite particle size: 3.0 μm or less Ratio of average ferrite particle size (dL) in rolling direction to average ferrite particle size (dN) in sheet thickness direction dN / dL: 0.70 or more Natural ferrite particle size in rolling direction Logarithmic standard deviation σA: 0.70 or less Ferrite grain size is large, individual ferrite grains vary widely, and the aspect ratio dN / dL of ferrite grain size greatly deviates from 1 and is less than 0.70. In addition to hindering the development of complex textures, when stress acts on a member, stress transmission becomes non-uniform and the stress is concentrated on specific grains, resulting in a decrease in rigidity. Further, during the press working, stress concentrates on coarse grains deviating from the center of the particle size distribution, so that the internal stress increases and the shape freezing property is remarkably lowered.

  For this reason, the average ferrite particle size needs to be 3.0 μm or less, more preferably 2.0 μm or less. No lower limit is provided, but if the particle size is extremely small, the workability deteriorates, so 0.5 μm or more is preferable.

  Similarly, the ratio dN / dL needs to be 0.70 or more, more preferably 0.80 or more. The upper limit is preferably 1.0.

Further, the standard deviation σA of the natural logarithm of the individual ferrite grain sizes in the rolling direction needs to be 0.70 or less, more preferably 0.60 or less.
Here, FIG. 2 shows the relationship between σA and the opening width of the vertical wall portion, which is an index of shape freezing property, for the evaluation results in the examples described later. As shown in FIG. 2, it can be seen that the opening width of the vertical wall portion can be made 19 mm or less by setting σA to 0.70 or less.
In addition, about the steel plate which obtained the evaluation result of FIG. 2, it showed to the Example (Specimen No. 1,2,4-13,16,17,20,21,23,24) mentioned later.

In addition, regarding the ferrite grain size, in the SEM photograph of the cross section in the rolling direction of the steel sheet in a 30 × 30 μm field of view, straight lines are drawn at intervals of 2 μm in the rolling direction and the thickness direction, and the cutting length of each ferrite grain is determined by the cutting method The measurement was made for each rolling direction and sheet thickness direction.
The above measurement was performed for three fields of view to determine the rolling direction average ferrite particle size dL and the plate thickness direction average ferrite particle size dN. Here, the average ferrite grain size in each direction is a value obtained by simply averaging the cutting lengths in each direction. The average philite particle size is an average value of the rolling direction average ferrite particle size dL and the plate thickness direction average ferrite particle size dN. Furthermore, σA is the standard deviation of the distribution when taking the natural logarithm of the individual filite cutting lengths in the rolling direction.

Next, the manufacturing conditions of the present invention will be described.
First, in accordance with the above-described component composition, for example, steel having a chemical component corresponding to a target strength level is melted. This melting method can be applied as appropriate, such as a normal converter method and an electric furnace method. The molten steel is cast into a slab and then heated as it is or after cooling and hot rolling.

  In hot rolling, after finishing at the finish rolling finish temperature of 800 to 950 ° C, the winding temperature is wound at 500 ° C or higher, and then pickled at a normal pickling rate of 40 to 80%. Apply cold rolling. Subsequently, heating is performed at a heating rate of 1.0 ° C./s or higher to 800 to 900 ° C., and a soaking process is performed for 300 s or less in the temperature range, annealing is performed, and subsequently an average cooling rate to 620 ° C. is set to 1.0 to 30 ° C. / Cool as s. After cooling to 620 ° C., in the case of a cold-rolled steel sheet, cooling may be continued and an overaging treatment may be performed in the middle of cooling, or after cooling, the overaging treatment may be performed by reheating. Further, after cooling to 620 ° C. or lower, a plating treatment may be performed to form a plating film on the steel sheet surface. As a kind of plating, for example, galvanizing can be mentioned, and by having a plating film, the corrosion resistance of the steel sheet can be improved.

  When manufactured as a hot dip galvanized steel sheet, it can also be plated by letting it pass through hot dip galvanized steel. Furthermore, when manufactured as an alloyed hot dip galvanized steel sheet, Reheating at a temperature of ℃ or higher can also be performed.

Hereinafter, each manufacturing condition will be described in detail.
Finishing finish temperature: 800-950 ℃
When the finish rolling finish temperature exceeds 950 ° C., the ferrite grain size becomes large in the subsequent transformation, and the development of the orientation advantageous for improving the rigidity is suppressed by cold rolling. Therefore, the finish temperature of finish rolling is 950 ° C. or less.

  On the other hand, when the finish rolling finish temperature is lower than 800 ° C., the rolling load increases in the austenite region and the ferrite grain becomes coarse although the load decreases in the ferrite region. Therefore, the finish rolling finish temperature is 800 ° C. or higher.

Winding temperature: 500 ° C or more When the winding temperature falls below 500 ° C, a low temperature transformation phase is generated, and in the subsequent cold rolling, not only the rolling load increases, but also the development of a texture that improves rigidity. Will not be able to. Therefore, the coiling temperature needs to be 500 ° C. or higher. On the other hand, when the coiling temperature is high, the ferrite grains are coarsened in the hot rolling stage, and the development of the texture that improves the rigidity is suppressed.

Cold rolling reduction: 40-80%
When the cold rolling is performed after the hot rolling process, the rolling reduction can be optimized to the (112) [1-10] orientation effective in improving the rigidity. In order to develop such an orientation, the rolling reduction of cold rolling needs to be 40 to 80%.

Temperature rise rate during annealing: 1.0 ° C / s or more If the temperature rise rate is extremely slow, recrystallization of ferrite proceeds during annealing, so the temperature rise rate during annealing must be 1.0 ° C / s or more. is there. Although there is no particular upper limit for the rate of temperature increase, rapid heating preferably increases the temperature at 30 ° C./s or less because the production cost increases.

Heating temperature during annealing: 800-900 ° C
During annealing, the austenite transformation from unrecrystallized ferrite proceeds, and the transformation strain remains by suppressing the austenite grain growth, and it is developed by cold rolling to release the transformation strain during cooling. The improved rigidity improvement orientation can be further developed. When the heating temperature (also referred to as the soaking temperature) exceeds 900 ° C., the austenite grains become coarse and the transformation strain disappears, and the subsequent cooling process hinders the development of texture that is advantageous for improving the rigidity. The heating temperature during annealing needs to be 900 ° C. or lower.
On the other hand, if the heating temperature is low, transformation is not completed and unrecrystallized ferrite remains, so the heating temperature needs to be 800 ° C. or higher.

Soaking time during annealing: 300 s or less During soaking, it is necessary to suppress grain growth of austenite, so the soaking time needs to be 300 S or less, more preferably 150 s or less. Although there is no specific lower limit, it is desirable that the soaking time be 10 s or longer because the austenite transformation needs to be completed.

Cooling after soaking: 1.0-30 ° C / s to 620 ° C
When the cooling rate after soaking is large, the transformation start temperature of the ferrite is lowered, the ferrite transformation is suppressed, and the aspect ratio of the ferrite grains is greatly deviated from 1, so that the shape freezing property is lowered and the rigidity is improved. The development of the assembly set required for this is suppressed. Therefore, the cooling rate after soaking needs to be 30 ° C./s or less.

  On the other hand, if the cooling rate after soaking is small, the ferrite transformation start temperature becomes high, the ferrite becomes coarse, the particle size distribution becomes large, the shape freezing property is lowered, and the texture necessary for improving the rigidity is reduced. Development is suppressed. Therefore, the cooling rate after soaking needs to be 1.0 ° C./s or more. In addition, ferrite that transforms at a low temperature of less than 620 ° C. has a large driving force for transformation, hardly undergoes transformation mainly due to relaxation of transformation strain during soaking, and it is difficult to develop an orientation that is advantageous for improving rigidity. Therefore, in order to improve the rigidity, it is necessary to complete the transformation for 80 mass% or more, which is the majority of all ferrite by 620 ° C. Therefore, the cooling after soaking should be 1.0-30 ° C./s up to 620 ° C. Need to cool at speed. Here, the cooling rate is an average cooling rate from the soaking temperature to 620 ° C.

  Here, FIG. 3 shows the relationship between the Mn / Al, the cooling rate after soaking, and the opening width of the vertical wall as an index of shape freezing property. As shown in FIG. 3, it can be seen that the opening width of the vertical wall portion can be reduced to 19 mm or less by setting the cooling rate to 1.0 to 30 ° C./s after setting Mn / Al in the above-described range. In addition, about the steel plate which obtained the evaluation result of FIG. 3, it showed to the Example (Specimen No. 1,2,4,5,6,12,13,14,19,21,23,24) mentioned later.

  Table 1 shows the chemical composition, production conditions and characteristic values of the specimens. After melting a steel slab having the chemical composition shown in Table 1, slab heating was performed at 1250 ° C. for 1 hour, after hot rough rolling, finish rolling was performed at the finishing temperature (FT) shown in Table 2, and then Winding was performed at the winding temperature (CT) shown in Table 2. The thickness of the obtained hot-rolled sheet was 1.7 to 8.0 mm. After pickling, the sheet thickness was 1.2 mm by cold rolling at various rolling reductions. After cold rolling, heating and soaking were performed under the conditions shown in Table 2, and cooling was performed at a cooling rate in Table 2 up to 620 ° C. After that, by aging treatment at 350 ° C. during cooling for 150 s to make a cold-rolled sheet, or by hot dip galvanizing at 470 ° C. during cooling and reheating to 500 ° C. or higher, alloying treatment An alloyed hot-dip galvanized steel sheet was obtained.

In the above manufacturing process, the obtained cooling plate or alloyed hot-dip galvanized steel sheet was observed for structure, and mechanical characteristics, part rigidity and shape freezing property were investigated. The results are shown in Table 3.
First of all, the structure was observed by observing a section in the rolling direction of the steel plate with nital corrosion, followed by scanning electron microscope (SEM) observation, taking three photographs of 30 × 30μm area, and then processing the ferrite fraction (ferrite The average particle size, the ratio dN / dL, and the particle size distribution in the rolling direction were determined by the intercept method as described above.

  The mechanical properties were determined by cutting a JIS No. 5 tensile test piece from the sheet width direction, which is the direction perpendicular to rolling, and conducting a tensile test at a tensile speed of 1 mm / min.

  The Young's modulus was measured in accordance with the American Society for Testing Materials standard (C1259) using a transverse vibration type resonance frequency measuring device by cutting a 10 × 60 mm test piece with the plate width direction as the longitudinal direction.

  Furthermore, the part rigidity was evaluated by fabricating a column type part having an inner diameter of 30 × 50 mm and a length of 800 mm. The one-hat part and bottom plate of the column-type parts are connected by spot welding, the longitudinal direction is the direction perpendicular to the rolling direction, flanges are attached to both ends, and the deflection amount when the indentation load is 300 N by the cantilever method is used. It was measured.

  For shape freezing, a specimen of 350 x 80m was cut out with the longitudinal direction as the rolling direction, and a single hat part was pressed with a punch width of 99mm, a die width of 103mm, and a die depth (corresponding to the vertical wall) of 95mm. The opening width of the wall was measured.

It is a figure which shows the relationship between Mn / Al and the Young's modulus of the steel plate width direction. It is a figure which shows the relationship between standard deviation (sigma) A and the opening width of a vertical wall part. It is a figure which shows the relationship between the cooling rate after Mn / Al and soaking, and the opening width of a vertical wall part.

Claims (8)

C: 0.05-0.15 mass%,
Si: 0.5 mass% or less,
Mn: 1.5-3.0mass%,
P: 0.05 mass% or less,
S: 0.01 mass% or less,
Al: 0.2-1.5mass%,
N: 0.01 mass% or less and
Nb: 0.02-0.1mass%
In a range satisfying the relationship expressed by the following formula (1), with the remainder having a component composition composed of iron and inevitable impurities, with a ferrite fraction of 50% or more and an average ferrite particle size of 3.0 μm or less. Yes, when the average ferrite grain size in the rolling direction and the sheet thickness direction is dL and dN, respectively, the ratio dN / dL is 0.70 or more, and the standard value obtained by taking the natural logarithm for each ferrite grain size in the rolling direction A high-tensile steel sheet with excellent shape freezing property, characterized by satisfying the relational expression of σA ≦ 0.70 when the deviation is σA.
Record
2.0 ≦ Mn / Al ≦ 12 (1) The component composition according to claim 1, further comprising:
Ti: 0.01-0.2 mass% and V: 0.01-0.2 mass%
A high-strength steel sheet with excellent shape freezing property, characterized by containing. The component composition according to claim 1 or 2, further comprising:
Cr: 0.05-1.0mass%,
Ni: 0.05-1.0mass%,
Mo: 0.05-1.0mass% and
Cu: 0.1-2.0mass%
A high-strength steel sheet excellent in shape freezing property, characterized by containing at least one of the above. In Claim 1, 2, or 3, the said component composition is further W: 0.1-2.0mass%
A high-strength steel sheet with excellent shape freezing property, characterized by containing. C: 0.05-0.15 mass%,
Si: 0.5 mass% or less,
Mn: 1.5-3.0mass%,
P: 0.05 mass% or less,
S: 0.01 mass% or less,
Al: 0.2-1.5mass%,
N: 0.01 mass% or less and
Nb: 0.02-0.1mass%
In the range satisfying the relationship expressed by the following formula (1), and the balance is finish-rolled at 800 to 950 ° C. when hot-rolling a steel material having a composition composed of iron and inevitable impurities. Then, after picking up at 500 ° C. or higher, pickling, cold rolling at a rolling reduction of 40 to 80%, and then heating to 800 to 900 ° C. at a heating rate of 1.0 ° C./s or higher. And high-tensile steel plate with excellent shape freezing characteristics, characterized by annealing at a temperature range of 300 s or less, annealing, and then cooling at an average cooling rate up to 620 ° C. at 1.0 to 30 ° C./s. Manufacturing method.
Record
2.0 ≦ Mn / Al ≦ 12 (1) The steel material according to claim 5, further comprising:
Ti: 0.01-0.2 mass% and V: 0.01-0.2 mass%
The manufacturing method of the high-tensile steel plate excellent in the shape freezing property characterized by containing. The steel material according to claim 5 or 6, further comprising:
Cr: 0.05-1.0mass%,
Ni: 0.05-1.0mass%,
Mo: 0.05-1.0mass% and
Cu: 0.1-2.0mass%
A method for producing a high-tensile steel sheet having excellent shape freezing property, comprising at least one of the above. 8. The steel material according to claim 5, 6 or 7, further comprising W: 0.1 to 2.0 mass%.
The manufacturing method of the high-tensile steel plate excellent in the shape freezing property characterized by containing.

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