JP2013007101A - Method for manufacturing high strength steel plate excelling in balance of low-temperature toughness and strength, and control method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a high strength steel plate excelling in the balance of low-temperature toughness and strength, and a control method for manufacturing the steel plate with such characteristics with a high yield.SOLUTION: The method for manufacturing the high strength steel plate includes heating, to 1,000-1,250°C, a steel piece containing 0.05-0.15% C, 0.05-0.5% Si, 1.0-2.0% Mn, at most 0.02% P, at most 0.01% S, 0.005-0.045% Al, and 0.005-0.08% Nb with a remaining portion consisting of iron and inevitable impurities, then subjecting the steel piece to hot rolling with a cumulative draft at 1,000°C or lower being 50% or more and with a rolling end temperature being an Arpoint or higher and 900°C or lower, and cooling the steel piece at an average cooling rate of 3-50°C/s from Ar-30°C or higher to a temperature range from a bainitic transformation end temperature +50°C to a bainitic transformation end temperature -50°C, wherein a bainitic area rate to the entire texture in a position of plate thickness 1/4 is 80% or more.

Description

  The present invention relates to a method for producing a steel plate (particularly a thick plate) excellent in balance between low-temperature toughness and strength suitable for shipbuilding, construction, bridges, line pipes, tanks, penstocks, and other large structures, and such characteristics. The present invention relates to a method for controlling manufacturing conditions for obtaining a steel sheet having the following.

  In recent years, with the increase in size of structures, steel sheets used are required to have higher strength. By using a high-strength steel plate, the amount of use of the steel plate can be reduced, so that advantages such as expansion of the space in the structure and reduction in weight can be obtained. Moreover, since it is assumed that a large-sized structure is exposed to a severe environment, it is requested | required that it is excellent also in low temperature toughness from a viewpoint of preventing the damage of a structure. However, since the low temperature toughness tends to deteriorate when the strength of the steel plate is simply increased, it has been a challenge to achieve both.

  Conventionally, when manufacturing a high-strength steel plate, it was common to manufacture by a process that combines hot rolling, accelerated cooling, and tempering treatment. However, performing the tempering process generally causes an increase in manufacturing time, and a decrease in productivity becomes a problem.

  Therefore, a manufacturing process that omits the tempering process itself has been proposed for the purpose of improving the productivity of high-strength steel sheets, and a manufacturing method has been proposed in which accelerated cooling is performed after hot rolling to obtain a high-strength steel sheet as quenched. ing. For example, as a method for producing a steel sheet having a yield strength of 500 MPa or more, which is required as a high-strength steel sheet, accelerated cooling is performed after hot rolling to promote bainite transformation that contributes to increasing the strength, thereby improving the strength. However, island-like martensite (MA structure: Martensite-Austenite Constituent) that adversely affects low-temperature toughness during accelerated cooling is generated and remains in the steel sheet, thereby simultaneously increasing the strength of the steel sheet and improving low-temperature toughness. It was difficult.

  Various manufacturing methods for suppressing the formation of MA structure have been proposed for such problems. For example, Patent Document 1 proposes a method for controlling the MA structure by reducing the content of C, Si, and Al in steel components and adjusting the cooling rate and cooling stop temperature after rolling. . However, in this manufacturing method, although the formation of MA structure can be suppressed, the content of oxysulfide in the steel sheet increases because deoxidation and desulfurization cannot be performed sufficiently as a result of reducing the contents of Si and Al. On the other hand, there has been a problem that the low temperature toughness deteriorates.

  Patent Document 2 proposes a cooling stop temperature and a method for suppressing the MA structure by isothermal holding or slow cooling after the cooling stop. However, this manufacturing method has a problem that it is difficult to apply to a manufacturing process in an actual machine because it is necessary to strictly control the cooling conditions after rolling.

  In Patent Document 3, the ratio of Al (0.05 to 0.5%) and Cr (0.010 to 1%) is controlled, and the metal structure of the steel sheet is controlled to improve the corrosion resistance and the base metal toughness. Techniques to make it have been proposed. However, in this technique, since the Al content is increased in order to exert a sufficient anticorrosive action in the presence of Cr oxide, the amount of Al dissolved in the steel sheet increases and sufficient low temperature toughness is obtained. There is a problem that can not be.

JP 2008-261101 A Japanese Patent No. 2776174 Japanese Patent No. 4444924

  The present invention has been made paying attention to the above circumstances, and its purpose is to suppress the MA structure generated in the manufacturing process of the steel sheet while maintaining high strength, and to balance low temperature toughness and strength. An object of the present invention is to provide an excellent method for producing a high-strength steel plate and a control method for producing a steel plate having such characteristics with a high yield.

The method for producing a high-strength steel sheet excellent in the balance between low-temperature toughness and strength according to the present invention, which has solved the above problems, is C: 0.05 to 0.15% (meaning mass%. The same applies to the components below). , Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, P: 0.02% or less (excluding 0%), S: 0.01% or less (including 0%) No), Al: 0.005 to 0.045%, Nb: 0.005 to 0.08%, and the steel slab composed of the remaining iron and unavoidable impurities was heated to a temperature of 1000 to 1250 ° C. Then, after performing hot rolling with a cumulative rolling reduction at a temperature of 1000 ° C. or lower being 50% or more and a rolling end temperature being Ar ₃ points or higher and 900 ° C. or lower, from a temperature of Ar ₃ −30 ° C. or higher, 3 to 50 Bainitic transformation finish temperature (Bf point) + 50 ° C to bainitic transformation finish at an average cooling rate of ° C / sec The gist is that the bainite area ratio with respect to the entire structure at the 1/4 thickness position is 80% or more, which is characterized by cooling to an end temperature of −50 ° C.

  In the present invention, as other elements, Cu: 1.5% or less (not including 0%), Ni: 3.0% or less (not including 0%), Mo: 0.8% or less (0% ), Cr: 1.5% or less (not including 0%), and V: 0.08% or less (not including 0%), and at least one selected from the group consisting of This is also a preferred embodiment.

  Further, as other elements, B: 0.005% or less (not including 0%), Ca: 0.050% or less (not including 0%), Ti: 0.030% or less (including 0%) N) and N: not more than 0.010% (not including 0%) is also a preferred embodiment.

The present invention is also a method for controlling the cooling step after hot rolling to obtain a high-strength bainite steel sheet having an excellent balance between low-temperature toughness and strength, and C: 0.05 to 0.15%, Si: 0 0.05 to 0.5%, Mn: 1.0 to 2.0%, P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%), Al : 0.005 to 0.045%, Nb: 0.005 to 0.08%, and the steel slab composed of the remaining iron and inevitable impurities is heated to a temperature of 1000 to 1250 ° C, and then 1000 ° C or less. In the cooling step after the hot rolling step in which the cumulative reduction rate at 50 ° C. is 50% or more and the rolling end temperature is _{3 to} 900 ° C., the cooling stop temperature after accelerated cooling is the bainite transformation end temperature ± Low temperature toughness of bainitic steel sheet having a gist in the range of 50 ° C And strength balance improvement method.

In the present invention, it is also a preferred embodiment that the accelerated cooling start temperature in the cooling step is Ar ₃ −30 ° C. or higher and the average cooling rate is 3 to 50 ° C./second.

  According to the production method of the present invention, the production of the MA structure can be achieved by adjusting the chemical composition of the steel material within an appropriate range and controlling the rolling conditions and the cooling conditions after the rolling (especially the cooling stop temperature). A steel sheet excellent in balance between low temperature toughness and strength can be realized while being appropriately suppressed. Moreover, since this invention is a non-tempered manufacturing method which does not perform tempering heat processing after performing accelerated cooling to a hot-rolled steel sheet, productivity can also be improved. Furthermore, according to the control method of the present invention, it is possible to produce a steel sheet having a good balance between low temperature toughness and strength with a high yield. The steel plate produced by the method of the present invention is extremely useful as a material applied to large structures such as bridges and high-rise buildings exposed to harsh environments.

FIG. 1 is a graph plotting the relationship between the cooling stop temperature, the N value, and the strength (YS). FIG. 2 is a graph plotting the relationship between the cooling stop temperature and the low temperature toughness (fracture surface transition temperature) of the base material. FIG. 3 is a graph plotting the relationship between low temperature toughness (fracture surface transition temperature) and strength (YS) of each steel plate (base material) in the examples. FIG. 4 is a graph showing the SS curve, with the horizontal axis representing the logarithm of true strain (Ln (ε)) and the vertical axis representing the logarithm of true stress (Ln (σ)), and a graph showing its primary approximation line. is there.

  In the present invention, “the balance between low-temperature toughness and strength is good” means that the fracture surface transition temperature (vTrs in Charpy impact test) of the base material is low while being high strength. Specifically, as also shown in this example described later, the range of a suitable fracture surface transition temperature (vTrs), which is an index of low temperature toughness, can be changed in relation to the strength (here, yield strength YS). . For example, as shown in FIG. 3, the balance between low temperature toughness and strength can be distinguished by using a certain line (for example, vTrs = 0.506YS-362.5) as a boundary.

  In the present invention, “strength” means that the yield strength (YS) is high, but preferably it means that the tensile strength (TS) is also high in addition to the yield strength (YS).

  As a result of diligent research on a method for manufacturing a steel sheet that can solve the above-mentioned problems, the inventors have particularly determined the cooling conditions after hot rolling (particularly the cooling stop temperature) as the bainite transformation end temperature (Bf point, hereinafter simply referred to as “Bf”). It has been found that a high-strength steel sheet having a good balance between low-temperature toughness and strength can be produced by appropriate control in relation to “point”).

  Specifically, the present invention has the greatest feature in that the cooling stop condition after hot rolling optimum for improving the balance between the low temperature toughness and the strength of the base material has been ascertained. If the manufacturing method of this invention is used, the steel plate excellent in the said balance will be obtained stably with a high yield. The production method of the present invention is useful as a method for improving the balance between the low temperature toughness and strength of the base material (particularly, the production method with a high yield).

  First, derivation of the range of the bainite transformation end temperature (Bf point) ± 50 ° C., which is the cooling stop temperature that characterizes the present invention, will be described based on the course of experiments.

  In general, a steel sheet showing a yield point when a stress-strain curve (SS curve) is drawn (discontinuous yield type or yield point type) has a stress (yield stress) higher than the elastic limit. From the viewpoint of securing the strength of large structures such as bridges, it is said that it is more effective to increase the yield point type than the continuous yield type (round house type) that does not show a clear yield point. . The presence or absence of the appearance of the yield point is related to the magnitude of the initial dislocation density (movable dislocation density), and it is known that the yield point decreases and the yield strength decreases when the movable dislocation density is large. Since the movable dislocation density increases as the area ratio of the MA structure in the steel sheet increases, it is necessary to reduce the MA structure as much as possible in order to suppress a decrease in yield strength. Moreover, since the MA structure is a coarse hard phase and deteriorates the low temperature toughness as a starting point of crack generation, the MA structure should be suppressed from the viewpoint of improving the low temperature toughness.

  Therefore, the present inventors consider that the low temperature toughness and strength of the base material can be improved in a well-balanced manner by suppressing the area ratio of the MA structure relative to the entire structure of the steel sheet (sometimes referred to as “MA structure fraction”). We have earnestly researched the production conditions that can control the process properly. As a result, it was found that it is particularly important to control the cooling stop temperature after hot rolling within the range of the Bf point ± 50 ° C.

  Hereinafter, the reason why the cooling stop temperature is set to the Bf point ± 50 ° C. will be described based on FIG. 1 showing the relationship between the strength (YS) of the steel sheet and the cooling stop temperature. In FIG. 1, the N value (N value is expressed as N and represented by a black triangle) is a work hardening index uniquely set by the present inventors, and stops cooling after hot rolling. It shows the value when the temperature (cooling stop temperature) is varied. In addition, all the measurement results shown in FIG. 1 use the steel type A of the below-mentioned Example. The N value is obtained by calculating the logarithm (Ln (ε) and Ln (σ)) of the true strain (ε) and the true stress (σ) in the range where the strain amount of the steel sheet is 0.5 to 5%. Is the logarithm of the true strain (Ln (ε)), the vertical axis is the logarithm of the true stress (Ln (σ)), and the slope of the primary approximation line (see FIG. 4).

  In the measurement of the N value of the present invention, the amount of strain is set in the range of 0.5 to 5% because the range of the SS curve is defined as the amount of strain in order to ensure the required strength (YS) of the steel sheet. It is a value set based on the examination results of the present inventors that it is effective to be 0.5 to 5% in relation. The yield stress is measured at the upper yield point for the yield point type and 0.2% proof stress for the round house type. This is in the vicinity of the strain region where 0.2% proof stress is measured from the viewpoint of securing the strength of the steel sheet. This is because the shape of the SS curve in FIG. In the line (“▲”) in which the N value is plotted in FIG. 1, the N value starts to decrease from around 410 ° C., starts to increase again to the peak (minimum value) at about 460 ° C. It is shown that it has risen after that (a value equivalent to around 410 ° C.).

  As a result of examining the relationship between the N value and the strength of the steel sheet (YS: “●” in FIG. 1) in detail, the N value starts to decrease as indicated by the line plotting the YS value (“●”) 410. It is shown that the intensity increases from around ℃, reaches a peak (maximum value) at about 460 ℃, becomes equal to the value at the time of increase in strength at around 510 ℃, and thereafter the strength decreases. And it turned out that the relationship between this work hardening index (N value) and intensity | strength (YS value) has shown the completely opposite behavior which makes a peak at 460 degreeC in the range of 410-510 degreeC.

  Further, in FIG. 1, when the temperature indicating the peak of the N value and the YS value was examined, all the peak temperatures (460 ° C. in FIG. 1) were the bainite transformation end temperature (Bf point), and this Bf point ± 50 In the range of ° C. (in the range of 410 to 510 ° C. in FIG. 1), it was found that the MA structure fraction was low and the strength was high. In the present invention, the Bf point is calculated based on a CCT curve prepared by using a processing for master test as will be described later, and is not an absolute value but a chemical composition and production conditions (hot rolling). The value fluctuates depending on the subsequent average cooling rate, strain amount, and the like.

  Next, a description will be given based on FIG. FIG. 2 is a graph showing the relationship between the cooling stop temperature measured using the same steel plate as that used in FIG. 1 and the low temperature toughness of the base material. FIG. 2 shows that the toughness of the base material exhibits extremely high toughness such as −80 ° C. or less at the fracture surface transition temperature (vTrs) in the range of Bf point ± 50 ° C. (410 to 510 ° C.) in FIG. ing.

  Although not shown in the figure, when the relationship between the cooling stop temperature and the MA structure fraction of the steel sheet was examined, the steel sheet manufactured with the cooling stop temperature in the range of the Bf point ± 50 ° C. was Bf point ± 50 ° C. It was found that the MA structure fraction in the metal structure was smaller than that of the steel sheet produced by removing from the above, and the low temperature toughness of the base material was excellent.

  As a result of a detailed study of the effect when the cooling stop temperature is set within the temperature range of the Bf point ± 50 ° C., the inventors of the present invention generate MA when the cooling stop temperature is set within the temperature range of the Bf point ± 50 ° C. The amount of structure is small, and the produced MA structure is further decomposed by cooling to room temperature, and the fraction of MA structure in the metal structure of the produced steel sheet is very small (0 to several area%, for example, 6 area% or less. It was found that the metallographic structure of the steel sheet was mainly bainite contributing to high strength.

  The results of FIGS. 1 and 2 described above are the results when predetermined rolling and cooling are performed using the steel type A in Table 1 described later, but the present invention is not limited to this and is defined by the present invention. It has been confirmed by experiments that the same tendency is observed when the manufacturing method under the conditions is performed.

  From the above examination results, by setting the cooling stop temperature after rolling to the Bf point ± 50 ° C., the formation of MA structure can be suppressed, and bainite steel having a bainite fraction of 80 area% or more can be manufactured with high accuracy and at a low temperature. The present inventors have found that a steel plate having a good balance between toughness and strength can be produced stably, and have led to the present invention. Hereinafter, the present invention will be described in detail.

  First, the chemical component composition of the high-strength steel sheet of the present invention will be described. The component composition of the high-strength steel sheet of the present invention is composed of alloy components that are usually included in various structural steel sheets, but if the steel slab subjected to hot rolling does not satisfy the following component composition, Even if the manufacturing method is adopted, a desired steel plate cannot be obtained. Therefore, it is necessary to make an appropriate adjustment in consideration of the influence on the characteristics required for the steel plate.

C: 0.05 to 0.15%
C is an element essential for ensuring the strength of the steel sheet, and is contained at 0.05% or more. The C content is preferably 0.06% or more, more preferably 0.07% or more. On the other hand, when C is added excessively, weldability is deteriorated and formation of MA structure which is a hard phase cannot be suppressed, and low temperature toughness is deteriorated. Therefore, C needs to be suppressed to 0.15% or less. The C content is preferably 0.13% or less, more preferably 0.12% or less.

Si: 0.05-0.5%
Si is an element that is used for deoxidation of molten steel and acts to improve the strength. To obtain these effects, 0.05% or more is contained. The Si content is preferably 0.07% or more, more preferably 0.10% or more. On the other hand, if the Si content is excessively increased, weldability and low temperature toughness deteriorate, so it is necessary to suppress the content to 0.5% or less. The Si content is preferably 0.45% or less, more preferably 0.40% or less.

Mn: 1.0-2.0%
Mn is an element that contributes to increasing the strength of the steel sheet by increasing the hardenability. In order to exhibit such an action effectively, it is necessary to contain 1.0% or more of Mn. A preferable Mn content is 1.1% or more, more preferably 1.2% or more. On the other hand, if Mn is added excessively, weldability and low temperature toughness deteriorate, so Mn needs to be suppressed to 2.0% or less. The Mn content is preferably 1.9% or less, more preferably 1.85% or less.

P: 0.02% or less (excluding 0%)
P is an element that deteriorates low-temperature toughness, so it is necessary to reduce it as much as possible. In the present invention, P is suppressed to 0.02% or less. In addition, since P is inevitably contained in steel, it is assumed that 0% is not included.

S: 0.01% or less (excluding 0%)
Since S is an element that degrades low-temperature toughness, it is necessary to reduce it as much as possible. In the present invention, S is suppressed to 0.01% or less. Since S is unavoidably contained in steel as well as P, 0% is not included.

Al: 0.005 to 0.045%
Al is an element used as a deoxidizing agent, and is contained in an amount of 0.005% or more in order to sufficiently exhibit a deoxidizing action. Preferably it is 0.008% or more, More preferably, it is 0.010% or more. On the other hand, when Al is contained excessively, coarse oxides increase and low temperature toughness deteriorates, so the content is suppressed to 0.045% or less. The Al content is preferably 0.040% or less, more preferably 0.035% or less.

Nb: 0.005 to 0.08%
Nb is an element that contributes to improvement of low-temperature toughness and strength improvement by precipitation strengthening. In particular, the reason why Nb is an essential element in the present invention is to promote the refining of austenite grains in controlled rolling (cumulative rolling reduction of 50% or more in a temperature range of 1000 ° C. or lower) by lowering the recrystallization temperature of austenite. This contributes to the improvement of low temperature toughness. In order to exhibit such an effect effectively, it is necessary to contain Nb 0.005% or more. A preferable Nb content is 0.008% or more, more preferably 0.010% or more. On the other hand, when Nb is added excessively, low temperature toughness deteriorates, so Nb needs to be suppressed to 0.08% or less. The Nb content is preferably 0.07% or less, more preferably 0.06% or less.

  The steel sheet of the present invention satisfies the above component composition, and the balance is iron and inevitable impurities. Inevitable impurities include, for example, the above-described P and S that may be brought into steel depending on the situation of raw materials, materials, manufacturing equipment, and the like, and trump elements (Pb, Bi, Sb, Sn, etc.). Moreover, it is also possible to positively contain the following elements as other elements as long as the effects of the present invention are not adversely affected.

(A) Cu: 1.5% or less (not including 0%), Ni: 3.0% or less (not including 0%), Mo: 0.8% or less (not including 0%), Cr: At least one selected from the group consisting of 1.5% or less (not including 0%) and V: 0.08% or less (not including 0%);
(B) B: 0.005% or less (not including 0%), Ca: 0.050% or less (not including 0%), Ti: 0.030% or less (not including 0%), and N : May contain at least one selected from the group consisting of 0.010% or less (excluding 0%), and the like. The reason for setting this range is as follows.

(A) Cu: 1.5% or less (not including 0%), Ni: 3.0% or less (not including 0%), Mo: 0.8% or less (not including 0%), Cr: At least one selected from the group consisting of 1.5% or less (excluding 0%) and V: 0.08% or less (not including 0%) is any of Cu, Ni, Mo, Cr and V It is an element useful for securing strength.

Cu: 1.5% or less (excluding 0%)
Cu is an element that enhances hardenability and contributes to increasing the strength of the steel sheet. In order to acquire these effects, it is preferable to make it contain 0.03% or more. The Cu content is more preferably 0.05% or more, still more preferably 0.08% or more. On the other hand, if the Cu content is excessive, the low temperature toughness is deteriorated, so that the content is preferably 1.5% or less. The Cu content is more preferably 1.3% or less, still more preferably 1.2% or less.

Ni: 3.0% or less (excluding 0%)
Ni is an element that simultaneously improves the strength and low temperature toughness of the base material and the weld. In order to exhibit such an action effectively, it is preferable to make it contain 0.03% or more. The Ni content is more preferably 0.05% or more, still more preferably 0.08% or more. On the other hand, if the Ni content is excessive, the cost increases, so it is preferable that the Ni content be 3.0% or less. The Ni content is more preferably 1.5% or less, still more preferably 1.2% or less.

Mo: 0.8% or less (excluding 0%)
Mo is an element effective for improving strength and low temperature toughness. In order to exhibit such an effect effectively, it is preferable to make it contain 0.05% or more. The Mo content is more preferably 0.06% or more, still more preferably 0.08% or more. On the other hand, if the Mo content is excessive, weldability and low temperature toughness (including the low temperature toughness of the welded portion) deteriorate, so 0.8% or less is preferable. The Mo content is more preferably 0.7% or less, still more preferably 0.6% or less.

Cr: 1.5% or less (excluding 0%)
Cr is an element effective for increasing the strength. In order to obtain such an effect, it is preferable to contain 0.05% or more. The Cr content is more preferably 0.06% or more, still more preferably 0.08% or more. On the other hand, if the Cr content is excessive, the low temperature toughness is deteriorated, so it is preferable to keep it to 1.5% or less. The Cr content is more preferably 1.3% or less, still more preferably 1.2% or less.

V: 0.08% or less (excluding 0%)
V is an element contributing to strength improvement, and is preferably contained in an amount of 0.005% or more, more preferably 0.008% or more, and still more preferably 0.010% or more. On the other hand, when the V content increases, weldability and low-temperature toughness deteriorate, so the content is preferably 0.08% or less, more preferably 0.07% or less, and still more preferably 0.06% or less.

(B) B: 0.005% or less (not including 0%), Ca: 0.050% or less (not including 0%), Ti: 0.030% or less (not including 0%), and N : At least one selected from the group consisting of 0.010% or less (excluding 0%) B, Ca, Ti, and N are all elements useful for improving low-temperature toughness.

B: 0.005% or less (excluding 0%)
B is an element that enhances hardenability and contributes to high strength and improves low-temperature toughness (particularly, low-temperature toughness of welds). In order to acquire such an effect, it is preferable to make it contain 0.0003% or more, More preferably, it is 0.0005% or more, More preferably, it is 0.0008% or more. On the other hand, if the B content increases, weldability and low temperature toughness deteriorate, so the content is preferably 0.005% or less, more preferably 0.003% or less, and still more preferably 0.0025% or less.

Ca: 0.050% or less (excluding 0%)
Ca is an element contributing to deoxidation and improving the low temperature toughness of the base material. In order to acquire such an effect, it is preferable to make it contain 0.0005% or more, More preferably, it is 0.0008% or more, More preferably, it is 0.0010% or more. On the other hand, when the Ca content is increased, coarse inclusions such as CaS and CaO are formed to deteriorate the ductility and the low temperature toughness of the base material. Therefore, preferably 0.050% or less, more preferably 0.040% or less, More preferably, it is 0.030% or less.

Ti: 0.030% or less (excluding 0%)
Ti is an element that generates TiN-based precipitates to improve low-temperature toughness (particularly a welded portion) and contributes to high strength. In order to effectively exhibit such an action, the content is preferably 0.003% or more, more preferably 0.005% or more, and still more preferably 0.007% or more. On the other hand, if the Ti content is excessive, coarse Ti precipitates are formed, which causes a decrease in low temperature toughness. Therefore, it is preferably 0.030% or less, more preferably 0.025% or less, and still more preferably 0.8. 022% or less.

N: 0.010% or less (excluding 0%)
N is an element that forms a nitride with an element such as Ti or Al to improve low-temperature toughness. Therefore, N is preferably contained in an amount of 0.002% or more, more preferably 0.003% or more. On the other hand, when the N content is excessive, the amount of dissolved N is increased and the low temperature toughness is deteriorated. Therefore, the N content is preferably 0.010% or less, more preferably 0.009% or less.

Next, a method for producing the steel plate of the present invention using steel pieces satisfying the above component composition will be described. A steel slab (material steel) used in the present invention is a steel slab having a predetermined size obtained by melting a molten steel having the above composition by a known melting method such as a converter, and then by a known casting method such as a continuous casting method. It is preferable to use a slab. After the steel slab cast by a conventional method is heated to a temperature of 1000 to 1250 ° C., the hot rolling is performed so that the cumulative rolling reduction at a temperature of 1000 ° C. or less is 50% or more and the rolling end temperature is Ar ₃ points or more and 900 ° C. or less. Apply. After hot rolling, it can be produced by accelerated cooling at an average cooling rate of 3 to 50 ° C./second from a temperature of Ar ₃ −30 ° C. or higher to a temperature range of bainite transformation end temperature (Bf point) ± 50 ° C. In addition, all the said temperature in the manufacturing method of this invention measures the temperature of a steel slab surface with a radiation thermometer.

Heating temperature: 1000-1250 ° C
The steel slab having the above composition is heated before hot rolling. However, if the heating temperature is too low, Nb in the steel is not sufficiently dissolved, so that the austenite structure cannot be sufficiently refined even after rolling. In addition, since sufficient precipitation strengthening of Nb cannot be obtained, desired low-temperature toughness and strength cannot be ensured. Therefore, the heating temperature is 1000 ° C. or higher, preferably 1030 ° C. or higher. On the other hand, if the heating temperature is too high, the austenite grains become coarse and the low-temperature toughness decreases, so the heating temperature is 1250 ° C. or less, preferably 1200 ° C. or less.

  Next, after the heating, hot rolling is performed. Hot rolling start temperature is not specifically limited, For example, you may start in the range of 1000-1200 degreeC.

Cumulative rolling reduction at 1000 ° C. or lower: 50% or more Subsequently, hot rolling is performed at 1000 ° C. or lower with a cumulative rolling reduction of 50% or more. When rolling in the austenite recrystallization temperature range, the recrystallization of austenite is repeated, the austenite grains are refined and sized, the structure is refined, and the low temperature toughness is improved. When rolling in the austenite non-recrystallization temperature range, the area of the austenite grain boundary can be increased, and strain can be introduced into the austenite grain. Thereby, the transformation from the austenite grain boundaries and the austenite grains can be promoted to refine the structure, and the low temperature toughness is improved. Therefore, the rolling performed in the present invention may be performed in either the austenite recrystallization temperature range or the austenite and non-recrystallization temperature range. The rolling temperature is preferably 730 ° C. or higher, more preferably 750 ° C. or higher, and the upper limit is 1000 ° C. or lower, preferably 950 ° C. or lower, more preferably 900 ° C. or lower in consideration of the recrystallization temperature.

  As for the rolling reduction, if the cumulative rolling reduction at 1000 ° C. or less is less than 50%, the structure is not sufficiently refined and a sufficient low temperature toughness improving effect cannot be obtained. Moreover, since the tempering heat treatment is omitted in the present invention, a cumulative rolling reduction of 50% or more is necessary from the viewpoint of increasing the strength (YS). A preferred cumulative rolling reduction is 55% or more, more preferably 60% or more, preferably 85% or less, more preferably 80% or less.

Rolling end temperature: Ar ₃ points or higher and 900 ° C. or lower The rolling end temperature of the hot rolling described above is Ar ₃ points or higher. If the rolling end temperature is less than Ar ₃ , soft ferrite is generated and a desired strength cannot be ensured, and strain is introduced into the ferrite by rolling the ferrite, resulting in low temperature toughness. On the other hand, when the rolling end temperature exceeds 900 ° C., even if the cumulative rolling reduction at 1000 ° C. or less is 50% or more, the structure becomes coarse and the low temperature toughness decreases.

The above Ar ₃ points are: Ar ₃ points = 868-369 × [C] + 24.6 × [Si] −68.1 × [Mn] −36.1 × [Ni] −20.7 × [Cu] −24 0.8 × [Cr] + 29.6 × [Mo]. In the formula, [] indicates the content (% by mass) of each element, and the content of elements not included in the steel sheet may be calculated as 0% by mass.

  After rolling, accelerated cooling is performed at an average cooling rate of 3 to 50 ° C./second. In the present invention, the time until the start of accelerated cooling is not particularly limited, and may be a normal interval and may be set within a range in which the time until the start of cooling becomes longer and the effect of accelerated cooling does not decrease. For example, it is preferable that the time from the end of hot rolling to the start of accelerated cooling is generally within 10 minutes.

Accelerated cooling start temperature: Ar ₃ −30 ° C. or higher After hot rolling under the above conditions, accelerated cooling is performed at an average cooling rate of 3 to 50 ° C./sec. The accelerated cooling start temperature is Ar ₃ −30 ° C. That is necessary. When the cooling start temperature is lower than Ar ₃ -30 ° C., ferrite (polygonal ferrite) is formed, and a desired structure (bainite fraction of 80% or more) cannot be formed.

Accelerated cooling rate: 3-50 ° C./second (average cooling rate)
In the present invention, accelerated cooling at an average cooling rate of 3 ° C./second or more is performed up to the cooling stop temperature. By performing accelerated cooling at an average of 3 ° C./second or more after hot rolling, a structure having a bainite fraction of 80% or more can be secured, and a desired high strength can be obtained. On the other hand, when the average cooling rate is less than 3 ° C./second, a high-temperature transformation structure such as ferrite or pearlite having low strength is generated, and the bainite fraction is lowered, so that a desired strength cannot be ensured. On the other hand, when the average cooling rate exceeds 50 ° C./second, martensite and retained austenite, which are low-temperature transformation structures, are generated, and it becomes difficult to secure a bainite fraction of 80% or more. A preferred accelerated cooling rate is an average cooling rate of 5 ° C./second or more, more preferably 7 ° C./second or more, preferably 45 ° C./second or less, more preferably 40 ° C./second or less.

Cooling stop temperature: Bf point + 50 ° C. to Bf point −50 ° C.
As described above, the low temperature toughness and strength of the base material can be increased by setting the cooling stop temperature within the range of the Bf point ± 50 ° C. Moreover, if it is in this temperature range, the produced | generated MA structure | tissue is controlled to such an extent that it is decomposed | disassembled by subsequent cooling. Therefore, the obtained steel sheet contains almost no MA structure (preferably 6% or less, more preferably 0%), and the low temperature toughness of the base material due to the MA structure can be suppressed.

  If the cooling stop temperature is less than −50 ° C., the cooling stop temperature is too low, so the MA structure generated in the accelerated cooling process cannot be decomposed in the subsequent cooling process, and the MA structure remaining in the steel sheet increases, resulting in a low temperature. Toughness deteriorates. Preferably it is Bf point -45 degreeC or more, More preferably, it is Bf point -40 degreeC or more.

  On the other hand, when the cooling stop temperature exceeds the Bf point + 50 ° C., too much untransformed austenite is generated, so that a large amount of MA structure is generated by the subsequent cooling, and the MA structure remaining in the steel sheet increases, resulting in low temperature toughness. Gets worse. Preferably it is Bf point +45 degrees C or less, More preferably, it is Bf point +40 degrees C or less.

  After accelerating cooling is stopped at the cooling stop temperature, it is cooled to room temperature. The cooling rate to room temperature is not particularly limited, but if it is about the same as the accelerated cooling rate (average cooling rate of 3 ° C./second or more), the effect of stopping the accelerated cooling at the cooling stop temperature cannot be obtained. It is preferable to cool at an average cooling rate of less than 3 ° C./second, and more preferably to cool (air cool).

In the present invention, the component composition, rolling conditions, and cooling conditions are within the above ranges so that the bainite fraction of the steel sheet obtained by hot rolling and accelerated cooling is 80% or more. It is desirable to make appropriate adjustments. If the bainite fraction is 80% or more, the balance between strength and low temperature toughness can be achieved. That is, if the bainite fraction is 80% or more, it is not necessary to consider the influence on the steel sheet due to the metal structure contained as the remainder. When the bainite fraction of the steel sheet is less than 80 area%, the strength and low-temperature toughness of the steel sheet may be affected by other structures (for example, ferrite and pearlite) existing as the balance. In addition, when the bainite fraction is low, it may be necessary to lower the rolling end temperature to ensure the strength of the steel sheet (for example, Ar ₃ points or less). In addition, the structure of the steel sheet cannot be controlled, and for example, the ferrite, pearlite, and martensite fractions are increased, and the low temperature toughness of the base material may be lowered.

  In addition, the bainite fraction is a ratio with respect to the whole structure | tissue in the plate | board thickness 1/4 position of a steel plate, as explained in full detail in the Example.

  After hot pressure is applied to the steel slab having the above component composition, a metal structure becomes a structure mainly composed of bainite (bainite area ratio of 80% or more) by performing a cooling process in which accelerated cooling conditions are appropriately controlled. In addition, it is possible to obtain a non-tempered thick steel plate having a good balance between low temperature toughness (vTrs) and strength (YS).

Specifically, after heating a steel slab satisfying the above component composition to a temperature of 1000 to 1250 ° C. as described above, the cumulative rolling reduction at a temperature of 1000 ° C. or less is 50% or more, and the rolling end temperature is Ar ₃ points or more. In the cooling step after the hot rolling step of 900 ° C. or less, the bainitic steel sheet having a good balance between low-temperature toughness and strength is obtained by setting the cooling stop temperature after accelerated cooling within the range of Bf point ± 50 ° C. Can be manufactured with good yield.

In this cooling step, it is desirable that the accelerated cooling start temperature is Ar ₃ −30 ° C. or higher and the average cooling rate is 3 to 50 ° C./second as described above.

  The production method of the present invention is particularly suitable for producing a steel plate having a thickness of about 10 to 100 mm, but is not limited thereto.

  Furthermore, the present invention includes a method of controlling a hot rolling step and a cooling step after hot rolling for obtaining a high-strength steel sheet having excellent strength and low temperature toughness. Specifically, high-strength materials that combine low-temperature toughness and strength in a well-balanced manner by appropriately controlling the types and ratios of the component composition of the steel sheet and appropriately controlling the rolling conditions and cooling conditions after rolling as described above. Steel plate can be made with good yield.

  Specifically, as shown in FIGS. 1 and 2, a high-strength steel sheet having an excellent balance between strength and low-temperature toughness is obtained by rolling and cooling a steel slab whose component composition is appropriately adjusted under appropriate conditions. The yield of the product is better than when the cooling stop temperature is not controlled.

  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.

  After the molten steel having the component composition shown in Table 1 (the balance is iron and inevitable impurities, the unit in the table is% by mass) is made into a slab by the continuous casting method, heating, hot rolling, Test steel (plate thickness 25 mm) was manufactured by accelerated cooling. The time until the start of accelerated cooling after hot rolling was within 150 seconds.

  About each obtained test steel, metal structure (bainite fraction), yield strength (YS: MPa), tensile strength (TS: MPa), and low temperature toughness (vTrs: degreeC) were measured on the following conditions, respectively.

Baynite fraction (%):
The metallographic structure of the test steel was obtained by cutting a cross section parallel to the rolling direction at 1/4 position of the plate thickness, mechanically polishing this cross section, and then corroding (3% by volume of nitric acid) with an optical microscope (400 times) The metal structure photograph taken at 5 fields of view (size per field of view: 150 μm × 200 μm) was subjected to image analysis to determine the average area ratio of bainite.

Yield strength (YS: MPa), tensile strength (TS: MPa):
The mechanical strength of the test steel was subjected to a tensile test (at room temperature) using a No. 4 test piece defined in JISZ2201, and the yield strength (YS: MPa) and tensile strength (TS: MPa) were measured. In addition, the said test piece was cut out from the plate | board thickness 1/4 position of test steel so that the direction perpendicular | vertical to a rolling direction might become a longitudinal direction. In addition, the upper yield point or 0.2% yield strength was measured as the yield strength.

Low temperature toughness (fracture surface transition temperature (vTrs): ° C):
Three Charpy test pieces (one side: 10 mm) specified by JISZ2242 are sampled so that the direction perpendicular to the rolling direction is the longitudinal direction at a thickness of 1/4 of the test steel, and the Charpy test is performed. And low temperature toughness was measured. Specifically, the low temperature toughness is determined based on JISZ2242, in which a test temperature is set between −160 to 20 ° C. at a pitch of 20 ° C., a transition curve is drawn, and a temperature with a brittle fracture surface ratio of 50% is set to the brittle wavefront transition temperature (vTrs ).

  As shown in FIG. 3, the example of the present invention is such that vTrs ≦ 0.506 × YS−362.5. This equation is derived based on the results of Tables 2 and 3, and is “excellent in balance between low temperature toughness and strength” in the present invention.

Bainite transformation end temperature (Bf point: (° C.)):
The Bf point was determined by preparing a CCT curve by a processing for master test. Specifically, a processed for master test piece (φ8 mm × 12 mm) was collected, heated to 1100 ° C. and held for 10 seconds, and then cooled to 1000 ° C. at an average cooling rate of 1.0 ° C./second. A reduction at a cumulative reduction rate of 25% is applied at a strain rate of 1.0 / second at 0 ° C. Then, it is cooled to 900 ° C. at an average cooling rate of 1.0 ° C./second, and is processed at a strain rate of 1.0 / second at 900 ° C. and a cumulative reduction rate of 50%. Then, hold at 900 ° C. for 2 seconds, cool to 50 ° C. at an average cooling rate (specifically 3 to 50 ° C./second) applied during accelerated cooling, and measure the temperature at which volume change during cooling occurs. The transformation temperature was determined. Furthermore, the final structure was identified by observing the cooled structure and measuring the Vickers hardness (based on JIS B 7735). From these results, a CCT curve was created and the Bf point was determined. As a measuring device, AVK manufactured by Akashi Seisakusho (test load 98 N) was used.

  In Tables 2 and 3, “FRT (° C.)” is the hot rolling end temperature, “SCT (° C.)” is the accelerated cooling start temperature, “FCT (° C.)” is the accelerated cooling stop temperature, and “cooling rate (° C./s) ) "Represents the average cooling rate during accelerated cooling, and" Bainite fraction (%) "represents the bainite area ratio with respect to the entire structure at the ¼ thickness position.

  Experiment No. 1-33 is an example which manufactured on the rolling and cooling conditions prescribed | regulated by this invention using the steel type which satisfy | fills the component composition of this invention. All of these satisfied the metal structure defined in the present invention (bainite: 80 area% or more), and exhibited excellent properties in the balance between the low temperature toughness and strength of the base material. In addition, when the metal structure of each experimental example was examined by a known method, the remainder other than bainite was ferrite and the like, and all of the MA structure was 6 area% or less and was hardly contained.

  Experiment No. which does not satisfy the requirements of the present invention. 34-72 were inferior in the balance between strength and low temperature toughness.

  Experiment No. whose heating temperature is lower than the specified temperature (1000 to 1250 ° C.) of the present invention. At 40 (950 ° C.), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

  Experiment No. in which the cumulative rolling reduction at 1000 ° C. or lower is lower than the specified rolling reduction (50% or more) of the present invention. At 41 (40%), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

Experiment No. in which the rolling end temperature (FRT) is higher than the specified temperature (Ar ₃ (719 ° C.) to 900 ° C.) of the present invention. At 42 (950 ° C.), the strength could be improved, but sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

Experiment No. 2 in which the accelerated cooling start temperature (SCT) after rolling is lower than the prescribed temperature of the present invention (Ar ₃ −30 ° C. (689 ° C.) or higher). At 35 (650 ° C.), the bainite fraction was low, and sufficient strength and low temperature toughness could not be secured.

  Experiment No. in which the accelerated cooling rate is slower than the average rate (3 ° C./second) defined in the present invention. In 47, sufficient strength and low temperature toughness could not be secured.

  No. in which the accelerated cooling rate and the cooling stop temperature do not satisfy the provisions of the present invention. 46 and no. No. 48 could not secure sufficient low temperature toughness, and the balance between strength and low temperature toughness was poor.

  Experiment No. in which the cooling stop temperature (FCT) is higher than the specified temperature (Bf point + 50 ° C. or less) of the present invention. In 34, 37, 38, 43, 44, 51-53, 55, 57, the balance between strength and low temperature toughness was poor.

  Experiment No. in which the cooling stop temperature (FCT) is lower than the specified temperature of the present invention (Bf point −50 ° C. or lower). In 36, 39, 45, 49, 50, 54, and 56, sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

  No. using a steel piece that does not satisfy the composition of the present invention. In 58-72, sufficient strength and low temperature toughness could not be obtained.

  In detail, in Experiment No. in which the C content falls below the regulation of the present invention (0.05%). In 58-60 (0.03%), sufficient strength could not be secured, and the balance between strength and low temperature toughness was poor. In particular, no. In 58 and 60, the cooling stop temperature (FCT) was outside the range of the temperature specified in the present invention (Bf point ± 50 ° C. or less), and sufficient low temperature toughness could not be secured.

  Experiment No. C content exceeding the regulation (0.15%) of the present invention. With 61 (0.18%), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

  Experiment No. in which the Si content falls below the regulation of the present invention (0.05%). At 62 to 64 (0.03%), sufficient strength could not be secured, and the balance between strength and low temperature toughness was poor. In particular, no. In 62 and 64, the cooling stop temperature (FCT) was out of the range of the temperature specified in the present invention (Bf point ± 50 ° C. or less), and sufficient low temperature toughness could not be secured.

  Experiment No. in which the Si content exceeds the definition of the present invention (0.50%). At 65 (0.55%), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

  Experiment No. in which the Mn content falls below the definition (1.0%) of the present invention. In 66-68 (0.75%), sufficient strength could not be secured, and the balance between strength and low temperature toughness was poor. In particular, no. No. 68 has a cooling stop temperature (FCT) outside the range of the temperature specified in the present invention (Bf point ± 50 ° C. or less), and sufficient low temperature toughness could not be secured.

  Experiment No. in which the Mn content exceeds the specified value (2.0%) of the present invention and the cooling stop temperature (FCT) is outside the specified temperature range (Bf point ± 50 ° C. or less) of the present invention. In 69 (2.20%), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor. Similarly, the steel No. Q in which the Mn content exceeds the provisions of the present invention was used. No. 70 could not secure low temperature toughness, and the balance between strength and low temperature toughness was poor.

  Experiment No. in which the Nb content falls below the provision of the present invention (0.005%). With 71 (0.003%), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

  Experiment No. in which the Al content exceeds the definition of the present invention (0.045%). At 72 (0.050%), sufficient low temperature toughness could not be secured, and the balance between strength and low temperature toughness was poor.

  In FIG. The graph which plotted the value of low temperature toughness of 1-72 and intensity | strength (YS) is shown. As can be seen from this graph, the inventive examples (No. 1 to 33) have a better balance between low temperature toughness and strength than the comparative examples (No. 34 to 72), and a constant line (vTrs = 0.506YS-362. It is possible to distinguish between 5).

Claims (5)

C: 0.05 to 0.15% (meaning mass%, hereinafter the same for the components),
Si: 0.05 to 0.5%,
Mn: 1.0-2.0%,
P: 0.02% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.005 to 0.045%,
Nb: 0.005 to 0.08%,
The steel slab composed of the remaining iron and inevitable impurities is heated to a temperature of 1000 to 1250 ° C., and then the cumulative rolling reduction at a temperature of 1000 ° C. or lower is 50% or more, and the rolling end temperature is Ar ₃ or higher and 900 or higher. After performing hot rolling at a temperature not higher than ° C., a temperature range from Ar ₃ −30 ° C. or higher to a bainite transformation end temperature + 50 ° C. to a bainite transformation end temperature −50 ° C. at an average cooling rate of 3 to 50 ° C./sec. The method for producing a high-strength steel sheet excellent in the balance between low-temperature toughness and strength, wherein the bainite area ratio with respect to the entire structure at the 1/4 position of the sheet thickness is 80% or more. Furthermore, as other elements,
Cu: 1.5% or less (excluding 0%),
Ni: 3.0% or less (excluding 0%),
Mo: 0.8% or less (excluding 0%),
2. At least one selected from the group consisting of Cr: 1.5% or less (not including 0%) and V: 0.08% or less (not including 0%) is provided. Manufacturing method of high strength steel sheet. Furthermore, as other elements,
B: 0.005% or less (excluding 0%),
Ca: 0.050% or less (excluding 0%),
3. At least one selected from the group consisting of Ti: 0.030% or less (not including 0%) and N: 0.010% or less (not including 0%). A method for producing a high-strength steel sheet according to 1. A method for controlling a cooling process after hot rolling to obtain a high-strength bainitic steel sheet having an excellent balance between low-temperature toughness and strength,
C: 0.05 to 0.15%,
Si: 0.05 to 0.5%,
Mn: 1.0-2.0%,
P: 0.02% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.005 to 0.045%,
Nb: 0.005 to 0.08%,
The steel slab composed of the remaining iron and inevitable impurities is heated to a temperature of 1000 to 1250 ° C., and then the cumulative rolling reduction at a temperature of 1000 ° C. or lower is 50% or more, and the rolling end temperature is Ar ₃ or higher and 900 or higher. Low temperature toughness and strength of bainite steel sheet, characterized in that the cooling stop temperature after accelerated cooling is within the range of the bainite transformation end temperature ± 50 ° C. in the cooling step after performing the hot rolling step to below ℃ Balance improvement method. The accelerated cooling start temperature in the cooling step is Ar ₃ -30 ° C or higher, and the average cooling rate is 3 to 50 ° C / second. The method for improving the balance between low temperature toughness and strength of a bainite steel sheet according to claim 4.