WO2015194619A1

WO2015194619A1 - High-strength steel plate and process for producing same

Info

Publication number: WO2015194619A1
Application number: PCT/JP2015/067560
Authority: WO
Inventors: 金子　雅人; 喜臣岡崎; 晴弥川野; 朗伊庭野
Original assignee: 株式会社神戸製鋼所
Priority date: 2014-06-20
Filing date: 2015-06-18
Publication date: 2015-12-23
Also published as: CN108950393A; JP2016008308A; JP6280824B2; KR20170002650A; CN106460120A

Abstract

Provided is a steel plate which has a high strength and stably exhibits excellent low-temperature toughness. The high-strength steel plate is characterized in that the steel plate satisfies a given composition and in that the structure satisfies the following (1) and (2) and the Vickers hardness of a portion thereof present at a depth of 1/4 the plate thickness is 180 or greater. (1) In the portion at a depth of 1/4 the plate thickness, crystal grains that each are surrounded by a large-angle grain boundary, which is a boundary between two adjoining crystals that have a difference in orientation of 15° or greater, have equivalent-circle diameters, the maximum value of which is 30 μm or less. (2) In the portion at a depth of 1/4 the plate thickness, the number density of crystal grains that each are surrounded by a large-angle grain boundary, which is a boundary between two adjoining crystals that have a difference in orientation of 15° or greater, and that have an equivalent-circle diameter of 15 μm or larger is 1.5×10^-3 per μm² or less.

Description

High strength steel plate and manufacturing method thereof

The present invention relates to a high-strength steel plate and a method for producing the same. In particular, the present invention relates to a steel sheet that exhibits high strength and stably exhibits excellent low-temperature toughness, and a manufacturing method thereof.

For example, steel sheets used for the construction of ships, offshore structures, etc. are required to have high strength and excellent toughness at low temperatures. In the unlikely event that an accident occurs in the above-mentioned vessels, human damage and economic damage are significant. High-temperature toughness is required for steel materials used in the above-mentioned ships and the like so that large-scale fracture does not occur. In order to secure the strength of the hull, etc., it is necessary to increase the thickness of the plate or use a high strength material. The application is oriented.

The applicant of the present application has proposed the following techniques as techniques for improving the low temperature toughness of high strength materials. For example, in Patent Document 1, a steel sheet satisfying a prescribed component composition, and an average equivalent circle diameter D of a crystal grain surrounded by a large-angle grain boundary in which an orientation difference between two crystals is 15 ° or more is 35 μm or less, A steel sheet characterized by a random grain boundary fraction R measured from the crystal orientation distribution difference of 50 area% or more is proposed.

Further, in Patent Document 2, when a metal structure of a plane that satisfies the prescribed component composition and the prescribed formula (1) and is parallel to the rolling direction of the steel sheet having a thickness of tmm and perpendicular to the steel sheet surface is observed ( a) The ferrite area ratio is 75% or more, (b) The average equivalent circle diameter of the ferrite grains at the t / 2 position is 20.0 μm or less, and (c) The average aspect ratio of the ferrite grains at the t / 4 position is 2.0 or less. It proposes a steel plate that meets the requirements.

However, in recent years, the required level of safety has further increased, and there is a demand for a steel plate that exhibits high strength and exhibits excellent low-temperature toughness more stably.

JP 2009-228020 A JP 2008-248354 A

The present invention has been made paying attention to the circumstances as described above, and its purpose is to establish a steel sheet that exhibits high strength and stably exhibits excellent low-temperature toughness, and a manufacturing method thereof. is there.

The high-strength steel sheet of the present invention that has solved the above problems is
Ingredient composition is mass%,
C: 0.01 to 0.15%,
Si: more than 0% and 0.50% or less,
Mn: 0.6 to 2.0%,
P: more than 0% and 0.030% or less,
S: more than 0% and 0.025% or less,
Al: 0.02 to 0.07%,
Nb: 0.003% or more and less than 0.05%,
Ti: 0.003-0.03%,
B: 0% or more and 0.005% or less,
N: 0.001 to 0.01%, and Ca: 0.0003 to 0.0060%
The balance consists of iron and inevitable impurities,
It is characterized in that the structure satisfies the following (1) and (2) and the Vickers hardness of 1/4 part of the plate thickness is 180 or more.
(1) In a quarter portion of the plate thickness, the maximum value of the circle equivalent diameter of crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more is 30 μm or less.
(2) The number of crystal grains surrounded by a large-angle grain boundary in which the difference in orientation between two adjacent crystals is 15 ° or more at a quarter part of the plate thickness, and whose equivalent circle diameter is 15 μm or more. The density is 1.5 × 10 ⁻³ pieces / μm ² or less.

The component composition is further mass%,
Cu: more than 0% and 1.0% or less,
Ni: more than 0% and 1.20% or less,
Cr: more than 0% and 0.50% or less,
One or more elements selected from the group consisting of Mo: more than 0% and 0.5% or less and V: more than 0% and 0.1% or less may be included.

The component composition is further mass%,
One or more elements selected from the group consisting of REM: more than 0% and 0.05% or less and Zr: more than 0% and 0.020% or less may be included.

The present invention further includes a method for producing the high-strength steel sheet, characterized in that the steel slab satisfying the component composition is used and the following steps A to F are included in this order. .
Step A: Heating is performed in a temperature range of 900 to 1200 ° C. until the temperature deviation in the plate thickness direction is within 40 ° C.
Process B: The scale of a steel piece surface is removed.
Step C: Rolling with a cumulative reduction ratio of 30% or more is performed in a temperature range where the temperature of ¼ part of the plate thickness becomes the austenite recrystallization temperature.
Step D: Cooling from a temperature range where the temperature of 1/4 part of the plate thickness becomes the austenite recrystallization temperature to a temperature range where the austenite non-recrystallization temperature is reached, when the plate thickness after the step C exceeds 50 mm, It is performed by a method including two or more repetitions of cooling at an average cooling rate of 0.5 ° C./s or more and air cooling. When the plate thickness is 50 mm or less, it is performed by a method other than water cooling.
Step E: Cumulative rolling reduction of 5% when the solid solution B index represented by the following formula (1) is less than 2.0 in the temperature range where the temperature of 1/4 part of the plate thickness is the austenite non-recrystallization temperature When the above rolling is performed and the solid solution B index represented by the following formula (1) is 2.0 or more, rolling with a cumulative reduction ratio of 15% or more is performed.
Step F: Cool from Ar ₃ transformation point to 500 ° C. at an average cooling rate of 5 ° C./s or more. However, the Ar ₃ transformation point is determined by the following formula (2).

In Formula (1), B, N, and Ti show content in steel in the mass% of each element.
Ar ₃ transformation point = 910-310 × C-80 × Mn -20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.35 × (t-8) ··· (2)
In the formula (2), C, Mn, Cu, Cr, Ni, and Mo indicate the steel content in mass% of each element, and t indicates the product thickness expressed in the unit mm.

According to the present invention, it is possible to provide a steel sheet that exhibits high strength and that stably exhibits excellent low-temperature toughness, and a method for manufacturing the steel sheet.

FIG. 1 is a photomicrograph for explaining bainitic ferrite. FIG. 2 is an explanatory view showing the sampling position of the observation specimen in the EBSP (Electron Back Scattering Pattern) method in the example, and the cross section shown by hatching is the observation surface. FIG. 3A is a photograph showing an EBSP measurement result of an example of the present invention in an example. FIG. 3B is a photograph showing an EBSP measurement result of a comparative example in the example.

First, for example, it is known that the development of brittle cracks, which is a problem in marine steel materials, is correlated with the toughness in the L direction, that is, the toughness in the rolling direction, of ¼ part of the plate thickness in the cross section in the plate thickness direction. . It is also known that a grain boundary having an orientation difference of 15 ° or more between two adjacent crystals is effective as a barrier to suppress the development of the brittle crack. In the following, the above-mentioned “grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more” is referred to as “large-angle grain boundary”, and the crystal grain surrounded by this large-angle grain boundary is referred to as “large-angle grain”. There is.

Until now, the size of large-angle crystal grains has been controlled so as to suppress the development of brittle cracks, for example, as shown in Patent Document 1, the average crystal grain size of the large-angle crystal grains has been defined.

However, the present inventors examined the relationship between the size and toughness of the large-angle crystal grains, even if the average crystal grain size of the large-angle crystal grains is less than a constant and the average value of impact energy is less than a constant, When there are few large-sized crystal grains that are coarse or relatively large-sized large grains, when stress is applied from the outside, stress concentrates on these coarse grains and breaks. It became clear that it is easy to become the starting point of

Therefore, the present inventors focused on these coarse crystal grains and the like, and studied to obtain a steel sheet that stably exhibits excellent base material toughness, particularly excellent low temperature toughness of the base material. By satisfying the following requirements (1) and (2) regarding the fine crystal grains, etc., it is possible to suppress the variation in the impact absorption energy value for evaluating the toughness and realize a steel plate that stably exhibits excellent base material toughness The present invention has been completed by conceiving what can be done. Hereinafter, each requirement regarding the organization will be described.

(1) The maximum value of the circle equivalent diameter of a crystal grain surrounded by a large-angle grain boundary whose orientation difference between two adjacent crystals is 15 ° or more at ¼ part of the plate thickness is 30 μm or less. The effect of the maximum value of the equivalent circle diameter on toughness, specifically, the effect on vE- ₄₀ evaluated in Examples described later, its variation, and vTrs was examined. As a result, it was found that when the number density described later is satisfied and the maximum value is 30 μm or less, vE- ₄₀ , its variation, and vTrs satisfy the evaluation criteria and stably exhibit excellent low temperature toughness. . The maximum value is preferably 28.0 μm or less, more preferably 25.0 μm or less, and still more preferably 23.0 μm or less. Although the maximum value is preferably as small as possible, the lower limit of the maximum value is about 10 μm in consideration of the conditions of the specified manufacturing method. In the following description, ¼ part of the plate thickness in the cross section in the plate thickness direction may be referred to as “t / 4 part” and ½ part of the plate thickness may be referred to as “t / 2 part”.

(2) To obtain a steel plate that stably exhibits excellent low-temperature toughness at a number of t / 4 parts, the number density of large-angle crystal grains having an equivalent circle diameter of 15 μm or more is 1.5 × 10 ⁻³ particles / μm ² or less. In addition to the above maximum value, it has been found that the number density of relatively large large-angle crystal grains, that is, large-angle crystal grains having a circle-equivalent diameter of 15 μm or more should be kept constant. Specifically, the influence of the number density on vE- ₄₀ , its variation, and vTrs was examined. If the number density was 1.5 × 10 ⁻³ pieces / μm ² or less, the vE ₋₄₀ Etc. satisfy the evaluation criteria, and excellent low-temperature toughness can be stably obtained. The number density is preferably 1.0 × 10 ⁻³ pieces / μm ² or less, more preferably 0.9 × 10 ⁻³ pieces / μm ² or less. Although preferably as small as possible also the number density, considering the conditions of the manufacturing method of the specified lower limit value of the number density becomes 1.0 × 10 ^-5 cells / [mu] m ² approximately.

The maximum value and the number density are obtained by the method described in the examples.

The present invention does not particularly limit the type of steel structure. For example, the bainitic ferrite may be 10 area% or more, and the other structure may include ferrite, bainite, martensite, or a combination thereof. The bainitic ferrite refers to an intermediate structure between ferrite and lath structure as shown by a circle surrounded by a micrograph in FIG.

In the steel sheet of the present invention, high strength means that the Vickers hardness at t / 4 part is 180 or more. Along with the Vickers hardness, as shown in the following examples, it is preferable that the yield strength, tensile strength, and elongation satisfy the evaluation criteria shown in the following examples.

In order to ensure the high strength of the steel sheet, the toughness of the base material, particularly the low temperature toughness of the base material, and the HAZ (Heat Affected Zone) toughness required for marine steel sheets, etc., as shown below, It is necessary to satisfy the component composition.

C: 0.01 to 0.15%
C is an element indispensable for securing the strength of the steel material, that is, the base material. In order to exert such effects, it is necessary to contain 0.01% or more. C is preferably contained in an amount of 0.03% or more, and more preferably 0.04% or more. However, if the amount of C exceeds 0.15%, a lot of island martensite is generated in the HAZ during welding, which not only deteriorates the HAZ toughness but also adversely affects the weldability. Therefore, the C content is 0.15% or less, preferably 0.10% or less, more preferably 0.060% or less.

Si: more than 0% and 0.50% or less Si is an element that contributes to securing the strength of a steel material by solid solution strengthening. From this viewpoint, Si may be contained in an amount of 0.02% or more, further 0.05% or more. However, if the amount of Si exceeds 0.50%, a lot of island martensite is generated in the HAZ during welding, which not only deteriorates the HAZ toughness but also adversely affects the weldability. Therefore, the Si amount is 0.50% or less. The amount of Si is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.10% or less.

Mn: 0.6 to 2.0%
Mn is an element that contributes to improving the strength of the steel material. In order to exhibit such an effect effectively, it is necessary to contain 0.6% or more of Mn. The amount of Mn is preferably 1.0% or more, more preferably 1.50% or more. However, if the amount of Mn exceeds 2.0%, the weldability of the base material deteriorates. Therefore, the amount of Mn needs to be suppressed to 2.0% or less. The amount of Mn is preferably 1.90% or less, more preferably 1.85% or less, and still more preferably 1.80% or less.

P: more than 0% and 0.030% or less P is an element that easily segregates, and particularly segregates at a grain boundary in a steel material to deteriorate the toughness of the base material. Therefore, P must be suppressed to 0.030% or less. The amount of P is preferably 0.018% or less, more preferably 0.015% or less.

S: more than 0% and 0.025% or less S is a harmful element that combines with Mn to generate MnS, and deteriorates the toughness of the base material and the ductility in the thickness direction. Therefore, the S amount needs to be suppressed to 0.025% or less. The amount of S is preferably 0.012% or less, more preferably 0.008% or less, and still more preferably 0.006% or less.

Al: 0.02 to 0.07%
Al is an element useful for deoxidation, and also contributes to the refinement of crystal grains by forming AlN. In order to exert these effects, the Al content is set to 0.02% or more. However, when the Al amount is excessive, the base material toughness and the HAZ toughness deteriorate, so the Al amount needs to be suppressed to 0.07% or less. The amount of Al is preferably 0.050% or less, more preferably 0.040% or less.

Nb: 0.003% or more and less than 0.05% Nb suppresses coarsening of recrystallized grains by two effects of a solid drag effect by solid solution and a pinning effect by carbonitride precipitation. Contributes to improvement. It also has the function of shifting the transformation start temperature to the low temperature side, which promotes the refinement of the structure. In order to effectively exhibit these actions by Nb, the amount of Nb is made 0.003% or more. The Nb amount is preferably 0.005% or more, more preferably 0.007% or more. However, when the Nb amount is 0.05% or more, the base material toughness and the HAZ toughness deteriorate, and therefore, in the present invention, the Nb amount is set to less than 0.05%. The Nb amount is preferably 0.030% or less, more preferably 0.025% or less, and still more preferably 0.020% or less.

Ti: 0.003-0.03%
Ti is an element that generates nitrides such as TiN and Ti oxide in the steel material and contributes to improvement of HAZ toughness. In order to exhibit this effect, it is necessary to contain 0.003% or more of Ti. The amount of Ti is preferably 0.005% or more, more preferably 0.007% or more, and still more preferably 0.010% or more. However, if Ti is excessively contained, the toughness of the base material deteriorates, so the Ti amount is set to 0.03% or less. The amount of Ti is preferably 0.020% or less, more preferably 0.018% or less.

B: 0% or more and 0.005% or less B is an element that contributes to securing high strength by the effect of improving hardenability. It is also an element that improves the HAZ toughness by suppressing the formation of grain boundary ferrite. In order to exert this effect, 0.0005% or more may be contained, and further 0.0010% or more may be contained. However, if the amount of B exceeds 0.005%, it precipitates as BN at the austenite grain boundary, leading to a decrease in HAZ toughness. Therefore, the B amount is 0.005% or less. The amount of B is preferably 0.002% or less.

N: 0.001 to 0.01%
N is an element that precipitates a nitride such as TiN. Due to the pinning effect, the nitride prevents the austenite grains formed in the HAZ from being coarsened during welding, promotes the ferrite transformation, and contributes to the improvement of the HAZ toughness. In order to exhibit this effect effectively, it is necessary to contain 0.001% or more. The amount of N is preferably 0.0030% or more, more preferably 0.0035% or more, and still more preferably 0.0040% or more. However, if the N amount exceeds 0.01%, the solid solution N amount increases, the base material toughness deteriorates, and the HAZ toughness also deteriorates. Therefore, the N content is suppressed to 0.01% or less. The N amount is preferably 0.0085% or less, more preferably 0.0075% or less.

Ca: 0.0003 to 0.0060%
When Ca is contained, since the TiN generation temperature is lowered, fine TiN is precipitated and the HAZ toughness is improved. Ca also suppresses the formation of coarse TiN that precipitates with Al ₂ O ₃ as a nucleus, and contributes to the suppression of HAZ toughness reduction. In order to exert these effects, the Ca content is set to 0.0003% or more. The amount of Ca is preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0060%, coarse inclusions are precipitated, leading to a reduction in HAZ toughness. Therefore, Ca is made 0.0060% or less. The amount of Ca is preferably 0.0040% or less, and more preferably 0.0030% or less.

The steel material of the present invention contains the above elements, and the balance consists of iron and inevitable impurities. Examples of the inevitable impurities include oxygen, Mg, As, and Se. In particular, oxygen is liable to cause deterioration of characteristics by forming inclusions, and is preferably suppressed to 0.0040% or less. The amount of oxygen is more preferably 0.0020% or less. The present invention further includes, as other elements, steel materials containing Cu, Ni, Cr, Mo, V, and REM and Zr contributing to inclusion shape control, which improve the strength and toughness of the steel material, as shown below. It is.

Cu: more than 0% and less than 1.0%, Ni: more than 0% and less than 1.20%, Cr: more than 0% and less than 0.50%, Mo: more than 0% and less than 0.5%, and V: more than 0% One or more elements selected from the group consisting of 0.1% or less These elements are elements that contribute to improving the strength and toughness of the steel material. Hereinafter, each element will be described.

Cu is an element that increases the strength of steel by solid solution strengthening. In order to effectively exhibit this action, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.15% or more, still more preferably 0.20% or more, and still more preferably 0.25% or more. However, if the Cu content exceeds 1.0%, the toughness of the steel material deteriorates, so the Cu content is preferably 1.0% or less. The amount of Cu is more preferably 0.8% or less, still more preferably 0.5% or less.

Ni is an element that contributes to increasing the strength of the steel material and improving the toughness of the steel material itself. Further, like Nb, it has a function of shifting the transformation start temperature to the low temperature side, which promotes the refinement of the structure. In order to exert these actions effectively, it is preferable to contain Ni by 0.01% or more, more preferably 0.10% or more, still more preferably 0.20% or more, and still more preferably 0.30%. That's it. However, since Ni is an expensive element, for economic reasons, the Ni content is preferably 1.20% or less, more preferably 1.00% or less, still more preferably 0.80% or less, and even more preferably. Is 0.60% or less.

Cr is an element that contributes to increasing the strength of the steel material, and is preferably contained in an amount of 0.01% or more. The amount of Cr is more preferably 0.02% or more, and still more preferably 0.03% or more. However, if the Cr content exceeds 0.50%, the strength of the steel material is excessively increased and the base material toughness is deteriorated, and the HAZ toughness is also deteriorated. Therefore, the Cr content is preferably 0.50% or less. The amount of Cr is more preferably 0.30% or less, still more preferably 0.20% or less, and still more preferably 0.15% or less.

Mo is also an element that contributes to increasing the strength of the steel, and is preferably contained in an amount of 0.005% or more. More preferably it is 0.008% or more, and still more preferably 0.01% or more. However, if it exceeds 0.5%, the strength of the steel material becomes too high, the base material toughness deteriorates, and the HAZ toughness also decreases. Therefore, the Mo amount is preferably 0.5% or less, more preferably 0.3% or less, still more preferably 0.2% or less, and still more preferably 0.10% or less.

V is also an element that contributes to increasing the strength of the steel material, and is also an element that contributes to the improvement of HAZ toughness. In order to exhibit this effect, it is preferable to contain V 0.001% or more. The amount of V is more preferably 0.002% or more, and further preferably 0.005% or more. However, when the amount of V becomes excessive, the precipitated carbonitrides become coarse and the base material toughness deteriorates. Therefore, the V amount is preferably 0.1% or less. The amount of V is more preferably 0.05% or less, still more preferably 0.02% or less.

One or more elements selected from the group consisting of REM: more than 0% and less than 0.05%, and Zr: more than 0% and less than 0.020%. These elements refine inclusions, and improve the base material toughness and HAZ. It works effectively to improve toughness. In order to exert this effect, when REM is used, the REM content is preferably 0.005% or more, more preferably 0.010% or more. When Zr is used, the Zr content is preferably 0.005% or more, more preferably 0.010% or more. However, when these contents are excessive, the oxide becomes coarse and the toughness of the base material and the HAZ deteriorates on the contrary. Therefore, the REM content is preferably 0.05% or less, and more preferably 0.018% or less. Moreover, it is preferable that Zr amount is 0.020% or less, More preferably, it is 0.010% or less. In the present invention, any of scandium, yttrium, and lanthanoid series rare earth elements belonging to group 3 of the periodic table, that is, elements having atomic numbers 57 to 71 can be used as REM.

The thickness of the steel sheet according to the present invention is not particularly limited, and is, for example, 6 mm or more, further 10 mm or more, more preferably 15 mm or more, and 100 mm or less.

Next, the manufacturing method of the steel plate of the present invention will be described. First, the solid solution B index used in the method for producing a steel sheet of the present invention will be described.

High heat input welding is intended for marine steel materials from the viewpoint of improving construction efficiency. In order to increase the HAZ toughness after the welding, generally, Ti, N and B are added to suppress the prior austenite grain growth suppression by TiN generation and intragranular nucleation by BN generation. The above B is divided into two types: B constituting BN and “solid solution B” which does not bond with N and dissolves in steel. These ratios change according to the addition results of Ti, N and B in the steelmaking stage.

The present inventors examined the relationship between the size of the large-angle crystal grains and the element. Among the elements, the above-mentioned B, in particular, the above-mentioned “solid solution B” might cause the coarsening of the large-angle crystal grains. I focused on the point. And from this point of view, as a result of investigating the influence of the amount of solute B on the size of large-angle crystal grains, when solid-solution B in steel increases, some coarse particles are generated as large-angle crystal grains, It was found that the number of coarse large-angle grains tends to increase. The reason why the solid solution B causes the coarsening of the large-angle crystal grains is considered as follows. That is, at the time of transformation from γ to α, the solute B tends to be formed by gathering crystal grains having a crystal orientation difference of less than 15 ° as a result of restricting the selection of transformation orientation, in other words, a crystal orientation difference of 15 ° or more. The large-angle crystal grains are difficult to be formed, and as a result, the crystal grain size having a crystal orientation difference of 15 ° or more is coarse, that is, the size of the large-angle crystal grains is considered to be coarse.

Therefore, for the purpose of suppressing the formation of the coarse large-angle crystal grains, first, the following formula (1) was set in order to calculate the amount of solute B in the steel. The solute B index represented by the following formula (1) is an index representing the amount of solute B in steel, and the larger the value, the greater the amount of solute B in steel. The present inventors have separately confirmed that the larger the solid solution B index, the larger the large-angle crystal grains become, and as a result, the low-temperature toughness is liable to decrease.

In Formula (1), B, N, and Ti show content in steel in the mass% of each element.

The inventors further studied to control the production conditions according to the solid solution B index. As a result, with the solid solution B index = 2.0 as a boundary, particularly when the solid solution B index is 2.0 or more, if the cumulative reduction rate of rolling in the austenite non-recrystallization temperature range is a certain value or more, It has been found that the formation of coarse large-angle crystal grains is suppressed.

Hereinafter, the reason why the manufacturing method is defined in the present invention, including the cumulative rolling reduction ratio of rolling in the austenite non-recrystallization temperature range, will be described.

In the production method of the present invention, a steel slab having the above component composition is cast by a conventional method to obtain, for example, a slab, and thereafter, the following steps A to F are performed in this order. In addition, the temperature prescribed | regulated with the following manufacturing methods means surface temperature unless there is particular notice. Moreover, in the following manufacturing method, unless otherwise indicated, plate | board thickness and t mean the plate | board thickness at the time of a rolling start in the board | plate thickness in each process, and the process including rolling.

[Step A: Heating is performed in a temperature range of 900 to 1200 ° C. until the temperature deviation in the thickness direction is within 40 ° C.]
First, the steel piece is heated to 900 ° C. or higher in order to obtain an austenite single phase. The heating temperature is preferably 1000 ° C. or higher. The upper limit of the heating temperature is 1200 ° C. or less, preferably 1150 ° C. or less from the viewpoint of cost and the like. In addition, the heating temperature here says the atmospheric temperature in a furnace.

The above steel slab is heated until the temperature deviation in the thickness direction is within 40 ° C. The temperature deviation in the plate thickness direction in the step A is a difference between “t / 4 part temperature” and “t / 2 part temperature” obtained by a method shown in an example described later. If the temperature deviation in the sheet thickness direction during heating of the steel slab is large, a deviation occurs in the predicted calculation value of the temperature at t / 4 part, and appropriate rolling cannot be performed in the subsequent process. The temperature deviation is preferably within 35 ° C., more preferably within 30 ° C., and most preferably within 20 ° C.

[Process B: Remove scale on steel piece surface]
In general, when the heating time is prolonged, a scale is easily generated on the surface of the steel sheet. If the scale exists on the surface of the steel slab, calculation of the temperature of t / 4 part calculated from the surface temperature is hindered, and as a result, appropriate rolling of t / 4 part cannot be performed. Therefore, the scale removal on the surface of the steel slab is performed after the step A. As a method for removing the scale, for example, high pressure water is sprayed.

By performing step A and step B, the temperature deviation within the thickness at the start of the following step C, that is, [(temperature of t / 4 part-surface temperature) / plate thickness at the start of step C] is obtained. The numerical value can be suppressed to 1.0 ° C./mm or less.

[Step C: Rolling with a cumulative reduction ratio of 30% or more is performed in a temperature range where the temperature at t / 4 part becomes the austenite recrystallization temperature]
In this step, a reduction of 30% or more in terms of cumulative reduction is applied in the austenite recrystallization temperature range. Strain is accumulated by this reduction, and cooling of the process F, which will be described later, is performed, so that the large-angle crystal grains can be refined. The cumulative rolling reduction is preferably 32% or more, more preferably 35% or more, and the upper limit is about 50%.

[Step D: Cooling from the temperature range where the temperature of t / 4 part becomes the austenite recrystallization temperature to the temperature range where the austenite non-recrystallization temperature is reached, when the plate thickness after the step C exceeds 50 mm, average cooling It is performed by a method including two or more repetitions of cooling at a rate of 0.5 ° C./s or more and air cooling. When the plate thickness is 50 mm or less, it is performed by a method other than water cooling.

When the steel sheet is cooled from the austenite recrystallization temperature range to the austenite non-recrystallization temperature range, it is necessary to cool the steel plate. When the scale is generated, the predicted calculation value of the temperature at t / 4 part is shifted, and appropriate rolling cannot be performed. In order to suppress scale generation, it is conceivable that the steel sheet is cooled only by air cooling. However, when the plate thickness exceeds 50 mm, the steel sheet is not sufficiently cooled. Therefore, when the plate thickness is more than 50 mm, the cooling from the austenite recrystallization temperature range to the austenite non-recrystallization temperature range is performed by cooling at an average cooling rate of 0.5 ° C./s or more and air-cooling. Repeated twice or more. In addition to the above recuperation process, another cooling method may be employed as long as no scale is generated on the steel plate surface.

In the following, cooling at an average cooling rate of 0.5 ° C./s or more is referred to as “intermediate cooling”, and the first intermediate cooling is referred to as “intermediate cooling 1”, and the second intermediate cooling is referred to as “intermediate cooling 2”. There is. The air cooling is referred to as “intermediate air cooling”, and the first intermediate air cooling may be referred to as “intermediate air cooling 1”, and the second intermediate air cooling may be referred to as “intermediate air cooling 2”. The intermediate cooling and the intermediate air cooling may be repeated twice or more, and the temperature range and time of each cooling are not particularly limited.

The average cooling rate of the intermediate cooling is preferably 0.7 ° C./s or more, more preferably 0.9 ° C./s or more. The upper limit of the average cooling rate is about 1.0 ° C./s from the viewpoint of suppressing scale generation.

The average cooling rate is obtained by the method shown in the following examples.

As a cooling method at the above average cooling rate of 0.5 ° C./s or more, for example, water or mist spraying may be mentioned, and water spraying is preferable.

When the plate thickness is 50 mm or less, the cooling method may be any method other than water cooling, as long as it is a method that suppresses the formation of scale. As a cooling method from the austenite recrystallization temperature range to the austenite non-recrystallization temperature range, for example, only air cooling may be performed, or the above-described intermediate cooling and intermediate air cooling may be repeated.

The above step D is carried out, and in particular, when the plate thickness exceeds 50 mm, a temperature recovery within the plate thickness at the start of the following step E, that is, [(t / 4 part temperature−surface temperature) ) / Plate thickness at the start of step E] can be suppressed to 1.0 ° C./mm or less.

[Step E: In the temperature range where the temperature of t / 4 part becomes the austenite non-recrystallization temperature, when the solid solution B index represented by the following formula (1) is less than 2.0, the cumulative rolling reduction is 5% or more. Rolling is performed, and when the solid solution B index represented by the following formula (1) is 2.0 or more, rolling is performed with a cumulative reduction ratio of 15% or more.

When the solid solution B index is less than 2.0, the amount of the solid solution B in the steel is relatively small. Therefore, in the temperature range where the temperature of this t / 4 part is the austenite non-recrystallization temperature, the cumulative reduction ratio is 5%. What is necessary is just to perform the above rolling. The cumulative rolling reduction is preferably 10% or more, more preferably 15% or more. In consideration of productivity and the like, the upper limit of the cumulative rolling reduction is about 50%.

On the other hand, when the solid solution B index is 2.0 or more, as described above, the transformation orientation selection is limited by the solid solution B, and large-angle crystal grains tend to be coarse. Therefore, in the present invention, rolling at a cumulative reduction ratio of 15% or more is performed in a temperature range where the temperature at the t / 4 part becomes the austenite non-recrystallization temperature. By increasing the amount of reduction in this temperature range, dislocations that become nucleation sites during transformation are introduced into the old γ grains, and as a result, orientation restraint during transformation due to solute B is relaxed, and coarse grains Generation is considered to be suppressed. The cumulative rolling reduction is preferably 19% or more, more preferably 20% or more. In consideration of productivity and the like, the upper limit of the cumulative rolling reduction is about 50%.

Strictly speaking, the temperature range where the temperature of t / 4 part becomes the austenite recrystallization temperature and the temperature range where the austenite non-recrystallization temperature is affected by the type of element contained, the content thereof, and the like. These temperature ranges can be obtained from the temperature at which the amount of deformation resistance changes by, for example, a processing for master experiment.

[Step F: Cool from Ar ₃ transformation point to 500 ° C. at an average cooling rate of 5 ° C./s or more. However, the Ar ₃ transformation point is determined by the following formula (2). Note that elements that are not included may be calculated as zero.
Ar ₃ transformation point = 910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.35 × (t−8) (2)
In the formula (2), C, Mn, Cu, Cr, Ni, and Mo indicate the steel content in mass% of each element, and t indicates the product thickness expressed in mm.]

In order to achieve a Vickers hardness of t / 4 part: 180 or more, it is necessary to make a shearing transformation from the viewpoint of dissolving C in solid solution. Therefore, cooling is performed at an average cooling rate of 5 ° C./s or more from the Ar ₃ transformation point at the cooling start temperature: t / 4 part to the cooling stop temperature: 500 ° C. at the surface temperature. The average cooling rate is preferably 6.0 ° C./s or more, more preferably 7.0 ° C./s or more. The cooling start temperature is controlled at a temperature of t / 4 part as described above, but the average cooling rate is calculated using the surface temperature at the start of cooling and when cooling is stopped.

The upper limit of the average cooling rate depends on the plate thickness. For example, in the case of a plate thickness of 65 mm used in the examples described later, the upper limit of the average cooling rate is about 10 ° C./s, but if the plate thickness is thinner than this, the upper limit of the average cooling rate is also high. Become.

In the present invention, at least the range from the Ar ₃ transformation point to 500 ° C. may be cooled at the above rate. That is, the cooling start temperature may be Ar ₃ transformation point + 10 ° C. or higher, more preferably Ar ₃ transformation point + 20 ° C. or higher, for example, a temperature not higher than the finish rolling finish temperature. Further, the cooling stop temperature is further 480 ° C. or lower, more preferably 450 ° C. or lower, and may be a temperature of 400 ° C. or higher, for example.

The cooling to the room temperature after cooling at the above speed is not particularly limited, and examples thereof include air cooling.

This application claims the benefit of priority based on Japanese Patent Application No. 2014-127743 filed on June 20, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-127743 filed on June 20, 2014 are incorporated herein by reference.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Various steel plates were produced under the production conditions shown in Tables 2 and 3 below using various slabs having the composition shown in Table 1 obtained by melting and solidifying steel.

In addition, the temperature range in which the temperature of the t / 4 part is the austenite recrystallization temperature and the temperature range in which the austenite non-recrystallization temperature is obtained were obtained by a processing for master experiment. If it is the component composition range of the steel sheet of the present invention, the temperature range that becomes the austenite recrystallization temperature is 940 ° C. or less and 860 ° C. or more, and the temperature range that becomes the austenite non-recrystallization temperature is 860 ° C. or less and the Ar ₃ transformation point or more. In this example, the above temperature range was adopted. In Tables 2 and 3, the rolling start temperature in the temperature range where the temperature of t / 4 part becomes the austenite recrystallization temperature is “SRT1”, and the temperature in the temperature range where the temperature of t / 4 part becomes the austenite non-recrystallization temperature. The rolling start temperature is “SRT2”, the finish rolling end temperature is “FRT”, the cooling start temperature after rolling in the austenite non-recrystallization temperature region is “SCT”, and the cooling end temperature is “FCT”.

[Temperature measurement method for each part in the plate thickness direction during rolling]
1. Using the process computer, the heating temperature at the position from the front surface to the back surface of the steel slab is calculated based on the atmospheric temperature from the start of heating to extraction, that is, the time in the furnace.
2. Using the calculated heating temperature, a method suitable for calculation such as the difference method of rolling temperature at any position in the plate thickness direction based on rolling pass schedule during rolling and data of cooling method such as water cooling or air cooling between passes Rolling is performed using the calculation.
3. The surface temperature of the steel sheet is measured using a radiation type thermometer installed on the rolling line. However, the theoretical value is also calculated in the process computer.
4). The surface temperature of the steel sheet measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is collated with a calculated temperature calculated from a process computer.
5. If the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, replace the measured surface temperature with the calculated surface temperature and use it as the calculated temperature on the process computer. If it is less than ± 30 ° C, use the calculated temperature calculated from the process computer. Use as is.
6). Using the calculated temperature calculated above, the rolling temperature in the region to be controlled is managed.

[Calculation method of average cooling rate]
The average cooling rate is obtained from the following equation (3).
Average cooling rate: Unit ° C / s = (θs-θf) / τ (3)
In the above formula (3), θs is the temperature at the start of cooling: unit ° C, θf is the temperature at the time of cooling stop: unit ° C, and τ is the cooling time expressed in seconds.

Temperature deviation of SRT1 and SRT2 temperatures in Tables 2 and 3: The unit ° C./mm was obtained from (t / 4 part temperature-surface temperature) / plate thickness at the start of each process.

[Calculation method of cumulative rolling reduction]
Calculated by the following formula.
Cumulative rolling reduction in austenite recrystallization temperature range = (H1−H2) / H1 × 100
Cumulative rolling reduction in the austenite non-recrystallization temperature range = (H2-t) / H2 × 100
In the above, H1 is the plate thickness at the start of rolling in the austenite recrystallization temperature range shown in Table 2, H2 is the plate thickness at the start of rolling in the austenite non-recrystallization temperature range shown in Table 2, t is the product thickness, That is, the thickness is shown in Table 3, and the unit is mm.

Using the steel sheet obtained above, the structure was evaluated in the following manner, and the tensile characteristics, Vickers hardness, and low temperature toughness of the base material were evaluated as characteristics.

[Measurement of large-angle crystal grain size]
At the t / 4 part, the equivalent circle diameter of the crystal grains surrounded by the large-angle grain boundaries in which the orientation difference between two adjacent crystals is 15 ° or more, that is, the size of the large-angle crystal grains, was obtained by the EBSP method. The measurement procedure is as follows.
(1) The above steel plate so that the cross section indicated by diagonal lines in FIG. 2, that is, the plate thickness cross section including the steel plate front and back surfaces parallel to the rolling direction indicated by the double arrow in FIG. A sample was taken from
(2) The observation surface was mirror-finished by polishing using # 150 to # 1000 wet emery polishing paper or polishing using a polishing agent such as diamond slurry as a polishing method having the same function.
(3) Using an EBSP apparatus manufactured by TexSEM Laboratories, at t / 4 part (A) measuring range 200 μm × 200 μm at 0.5 μm pitch, or (B) measuring range 100 μm × 100 μm at 0.25 μm pitch, A boundary having a crystal orientation difference of 15 ° or more was defined as a crystal grain boundary, and the size of a crystal grain surrounded by the crystal grain boundary, that is, a large-angle crystal grain was measured. At this time, measurement points having a confidence index indicating the reliability of the measurement direction smaller than 0.1 were excluded from the analysis target. Further, in this example, the circle equivalent diameter of 2.5 μm or less was considered as noise and deleted.
(4) The equivalent circle diameter of the largest large-angle crystal grain was determined within the above measurement range. Further, the number of large-angle crystal grains having an equivalent circle diameter of 15 μm or more was determined, and the number per 1 μm ² was determined as the number density by dividing by the area of the measurement range, that is, 200 μm × 200 μm or 100 μm × 100 μm.

The obtained steel sheet was cut along the rolling direction, and a structure observation test piece was taken from t / 4 part of the cut surface and observed with an optical microscope at a magnification of 400 times. Was 10 area% or more, and the other structure was a structure made of ferrite, bainite, martensite, or a combination thereof.

[Evaluation of tensile properties]
From t / 4 part, a No. 4 test piece of JIS Z 2201 was sampled at right angles to the rolling direction, and a tensile test was performed according to JIS Z 2241 to determine yield strength, tensile strength, and elongation. The yield strength was 390 MPa or higher, the tensile strength was 530 MPa or higher, and the elongation was 17% or higher. The yield strength is preferably 400 MPa or more, more preferably 415 MPa or more. The tensile strength is preferably 550 MPa or more, more preferably 580 MPa or more. The elongation is preferably 20% or more.

[Measurement of Vickers hardness]
A Vickers hardness test was performed at a load of 98 N at three points at t / 4 part of each steel plate. And the average value of 3 points | pieces of Vickers hardness was calculated | required, and the case where this average value was 180 or more was set as the pass.

[Evaluation of low temperature toughness of base metal]
Three NK U4 test pieces were collected so that the longitudinal direction of the test piece was the L direction, that is, the rolling direction at t / 4 part. And the V notch Charpy impact test was implemented by the method prescribed | regulated to JISZ2242. In the shipbuilding E grade in the NK class, the impact value of the base material is evaluated at the test temperature: -40 ° C, so the energy value of the above three test pieces is measured at the test temperature: -40 ° C as an indicator of the stability of low temperature toughness. (VE- ₄₀ ). Moreover, the said impact test was done and the brittle fracture surface transition temperature (vTrs) was calculated | required from the curve which shows the relationship between test temperature and a brittle fracture surface rate. When the above energy values are 100 J or more, the difference between the maximum value and the minimum value is 100 J or less, and the brittle fracture surface transition temperature satisfies −60 ° C. or less, excellent low temperature toughness is stable. It was evaluated that it was demonstrated.

These results are shown in Table 4.

Tables 1-4 show the following. No. Nos. 1, 6, 8 and 9 use steel satisfying the prescribed component composition, but are not manufactured by the prescribed method. As a result, the structure does not satisfy the prescribed, resulting in poor low-temperature toughness of the base material. It was.

No. No. 1 is the process B, that is, after heating and before descaling before hot rolling is performed, the specified cooling is not performed in the process D, and the cumulative reduction ratio in the processes C and E is small. For this reason, the size of the large-angle crystal grains was increased, resulting in inferior low-temperature toughness of the base material. In addition, this No. In Step 1, since the prescribed cooling was not performed in Step D, the calculation of the temperature at t / 4 part was hindered, and the SRT2 at t / 4 part was increased.

No. No. 6 has a solid solution B index of 8.2. No. 8 has a solid solution B index of 22.0, and it is necessary to set the cumulative reduction ratio in the austenite non-recrystallization temperature range to 15% or more. In both cases, the cumulative reduction ratio was less than 15%. The maximum size of large angle crystal grains is large, and Also increases the number of high-angle grain 8, or 15 [mu] m, as a result, large variation in vE _-40, and vTrs was higher.

No. No. 9 has a solid solution B index of 19.9, and it is necessary to make the cumulative reduction ratio in the austenite non-recrystallization temperature range: 15% or more. However, since the cumulative reduction ratio was less than 15%, The number of large-angle crystal grains having a large maximum size and a size of 15 μm or more was increased. As a result, vE- ₄₀ was small, variation was large, and vTrs was also high.

In addition, the steel sheet No. The EBSP measurement results of No. 2 are shown in FIG. The EBSP measurement result of 6 is shown in FIG. 3B. When these results are compared, the steel plate No. No. 6 shows that the size of the large-angle crystal grains is coarse.

Claims

Ingredient composition is mass%,
C: 0.01 to 0.15%,
Si: more than 0% and 0.50% or less,
Mn: 0.6 to 2.0%,
P: more than 0% and 0.030% or less,
S: more than 0% and 0.025% or less,
Al: 0.02 to 0.07%,
Nb: 0.003% or more and less than 0.05%,
Ti: 0.003-0.03%,
B: 0% or more and 0.005% or less,
N: 0.001 to 0.01%, and Ca: 0.0003 to 0.0060%
The balance consists of iron and inevitable impurities,
A high-strength steel sheet having a structure satisfying the following (1) and (2) and having a Vickers hardness of 1/4 part of the plate thickness of 180 or more.
(1) In a quarter portion of the plate thickness, the maximum value of the circle equivalent diameter of crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystals is 15 ° or more is 30 μm or less.
(2) The number of crystal grains surrounded by a large-angle grain boundary in which the difference in orientation between two adjacent crystals is 15 ° or more at a quarter part of the plate thickness, and whose equivalent circle diameter is 15 μm or more. The density is 1.5 × 10 −3 pieces / μm 2 or less.
The component composition is further mass%,
Cu: more than 0% and 1.0% or less,
Ni: more than 0% and 1.20% or less,
Cr: more than 0% and 0.50% or less,
The high-strength steel sheet according to claim 1, comprising one or more elements selected from the group consisting of Mo: more than 0% and 0.5% or less and V: more than 0% and 0.1% or less.
The component composition is further mass%,
The high-strength steel sheet according to claim 1 or 2, comprising one or more elements selected from the group consisting of REM: more than 0% and 0.05% or less and Zr: more than 0% and 0.020% or less.
It is a manufacturing method of the high strength steel plate according to claim 1 or 2,
A method for producing a high-strength steel sheet, comprising using steel slabs satisfying the above component composition and including the following steps A to F in this order.
Step A: Heating is performed in a temperature range of 900 to 1200 ° C. until the temperature deviation in the plate thickness direction is within 40 ° C.
Process B: The scale of a steel piece surface is removed.
Step C: Rolling with a cumulative reduction ratio of 30% or more is performed in a temperature range where the temperature of ¼ part of the plate thickness becomes the austenite recrystallization temperature.
Step D: Cooling from a temperature range where the temperature of 1/4 part of the plate thickness becomes the austenite recrystallization temperature to a temperature range where the austenite non-recrystallization temperature is reached, when the plate thickness after the step C exceeds 50 mm, It is performed by a method including two or more repetitions of cooling at an average cooling rate of 0.5 ° C./s or more and air cooling. When the plate thickness is 50 mm or less, it is performed by a method other than water cooling.
Step E: Cumulative rolling reduction of 5% when the solid solution B index represented by the following formula (1) is less than 2.0 in the temperature range where the temperature of 1/4 part of the plate thickness is the austenite non-recrystallization temperature When the above rolling is performed and the solid solution B index represented by the following formula (1) is 2.0 or more, rolling with a cumulative reduction ratio of 15% or more is performed.
Step F: Cool from Ar 3 transformation point to 500 ° C. at an average cooling rate of 5 ° C./s or more. However, the Ar 3 transformation point is determined by the following formula (2).

In Formula (1), B, N, and Ti show content in steel in the mass% of each element.
Ar 3 transformation point = 910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.35 × (t−8) (2)
In the formula (2), C, Mn, Cu, Cr, Ni, and Mo indicate the steel content in mass% of each element, and t indicates the product thickness expressed in the unit mm.
It is a manufacturing method of the high strength steel plate according to claim 3,
A method for producing a high-strength steel sheet, comprising using steel slabs satisfying the above component composition and including the following steps A to F in this order.
Step A: Heating is performed in a temperature range of 900 to 1200 ° C. until the temperature deviation in the plate thickness direction is within 40 ° C.
Process B: The scale of a steel piece surface is removed.
Step C: Rolling with a cumulative reduction ratio of 30% or more is performed in a temperature range where the temperature of ¼ part of the plate thickness becomes the austenite recrystallization temperature.
Step D: Cooling from a temperature range where the temperature of 1/4 part of the plate thickness becomes the austenite recrystallization temperature to a temperature range where the austenite non-recrystallization temperature is reached, when the plate thickness after the step C exceeds 50 mm, It is performed by a method including two or more repetitions of cooling at an average cooling rate of 0.5 ° C./s or more and air cooling. When the plate thickness is 50 mm or less, it is performed by a method other than water cooling.
Step E: Cumulative rolling reduction of 5% when the solid solution B index represented by the following formula (1) is less than 2.0 in the temperature range where the temperature of 1/4 part of the plate thickness is the austenite non-recrystallization temperature When the above rolling is performed and the solid solution B index represented by the following formula (1) is 2.0 or more, rolling with a cumulative reduction ratio of 15% or more is performed.
Step F: Cool from Ar 3 transformation point to 500 ° C. at an average cooling rate of 5 ° C./s or more. However, the Ar 3 transformation point is determined by the following formula (2).

In Formula (1), B, N, and Ti show content in steel in the mass% of each element.
Ar 3 transformation point = 910-310 × C-80 × Mn-20 × Cu-15 × Cr-55 × Ni-80 × Mo + 0.35 × (t−8) (2)
In the formula (2), C, Mn, Cu, Cr, Ni, and Mo indicate the steel content in mass% of each element, and t indicates the product thickness expressed in the unit mm.