JP5953952B2 - Steel material excellent in impact resistance and method for producing the same - Google Patents

Description

  The present invention relates to a steel material used for a large structure such as a ship and a method for producing the same, and particularly to a steel material having a high uniform elongation that is effective in suppressing damage during a ship collision and the like, and an excellent impact resistance. It relates to a manufacturing method.

  In recent years, environmental pollution due to oil spills caused by landing of large tankers and collisions has become a problem. In order to prevent oil spills due to these accidents, efforts have been made from the hull structure side such as double hull structure, but sufficient countermeasures have not been studied for steel for hulls. Among them, as an approach from the steel material side of the hull, it has been proposed that the steel material itself absorbs a lot of energy at the time of collision, but has not yet reached a sufficient practical stage.

  As a method for improving the energy absorption capacity at the time of collision, Patent Document 1 proposes a technique in which the structure of a steel sheet is mainly composed of ferrite and the ferrite phase is strengthened. This technique is characterized in that the ferrite fraction F is 80% or more, and the ferrite hardness H defines a lower limit value (H ≧ 400−2.6 × F).

Further, Patent Document 2 proposes a technique for including a residual γ phase in the front and back layers of a steel plate. This technique contains C, Si, Mn, and Al, and further contains a strengthening element as necessary. The front and back layers of at least 1/8 or more of the plate thickness of the steel plate have an area ratio of 1.0 to 20%. It contains residual γ.
In these technologies, energy absorption at the time of collision is evaluated as a product of the strength (average of yield stress and rupture stress) of steel material and total elongation. Therefore, the absorption energy is increased by improving both strength and total elongation.

  In addition to these, Patent Document 3 discloses that the phase fraction of the ferrite phase in the steel sheet metallographic structure is 70% or more at the plate thickness center part and 50% or more at the plate thickness surface layer part to increase the uniform elongation. A technique for improving collision resistance is disclosed.

  Further, in Patent Document 4, the area fraction of ferrite in the total metal structure of the steel sheet is 90% or more, the average ferrite grain size is 3 to 12 μm, the maximum ferrite grain size is 40 μm or less, the average equivalent circle diameter of the second phase Has been proposed to improve the impact absorption by increasing the product of the uniform elongation and the breaking stress to 0.8 .mu.m or less.

Japanese Patent No. 3434431 Japanese Patent No. 3499126 Japanese Patent No. 3578126 JP 2007-162101 A

  The evaluation of the absorbed energy based on the total elongation used in Patent Document 1 and Patent Document 2 described above does not necessarily lead to the evaluation of the safety of the hull structure, and is not appropriate when discussing collision resistance. In other words, in order to evaluate the elongation deformation of a hull outer plate supported by a stiffener with a long span that cannot be compared with the gauge distance in a tensile test, the total elongation including the local elongation affected by the shape of the specimen is used. Evaluation of elongation is not suitable. Therefore, when considering the absorbed energy at the time of collision, it is necessary to evaluate with uniform elongation which is judged to be highly correlated with the elongation characteristics of the hull skin.

  For example, in the technique of Patent Document 1, the ferrite particle size is 5 μm or less, and the hardness of the ferrite is Hv 160 to 190 in the example (the same document, Table 2), which is high. Therefore, the total elongation (EL in the same table) is 23 to 32%, and the uniform elongation cannot be higher than this, so it is estimated that the total elongation is only about half of the total elongation.

  Further, in the technique of Patent Document 2, alloy elements are added in many cases so that the structure contains residual γ, and the steel of the examples has a high carbon equivalent (Ceq) or a steel type with high Si. It has become.

  For example, when looking at Table 1 of the same literature, Ceq is calculated to be about 0.38 for steel type A, and Si is 0.55 to 1.94% for steel types B to F, both of which are high. . Therefore, the ductility is generally low, and even if the uniform elongation is increased only by the residual γ on the surface layer, the uniform elongation is regulated by the portion with low ductility, so it is difficult to improve the uniform elongation. Guessed.

For these steel types, no test results relating to toughness or weldability are disclosed. In this document, the impact absorption energy is EL × (YP + TS / 2) in Table 2 and is the product of total elongation and strength. Therefore, considering the materials of these steel types from the normal thick steel plate materials, it is presumed that the steel types with high Si have low toughness and the steel types with high Ceq have problems in weldability.
In general, the necessary yield stress is determined by design requirements for steel for hulls, and the strength grade of the steel is changed according to the part to be used. In order to improve this, the cost increases due to the addition of alloy elements and the like, and the weldability deteriorates. Therefore, it is not preferable to improve the absorbed energy by increasing the strength.

  On the other hand, in the technology of Patent Document 3, uniform elongation is improved by suppressing the addition amount of the alloy element to a low level and increasing the structure fraction of the ferrite phase having low hardness and high ductility. Development of a manufacturing method for increasing the ferrite phase fraction of the thick surface layer portion to the same extent as that of the central portion of the plate thickness has not been achieved. In the examples, only a comparatively small plate thickness of 25 mm or less is disclosed. However, since the plate thickness increases and the amount and time of controlled cooling at the time of manufacture increase, It is extremely difficult to secure the fraction.

  Patent Document 4 discloses information on chemical components and metal structures of steel materials, but there are many practically uncertain points in the manufacturing method. In other words, the manufacturing method described in the detailed description recommends reheating after hot rolling and cooling. However, in steel sheets for shipbuilding that are indispensable for low-priced and mass production, processes such as reheating are the production costs. And there is concern about practical application from the viewpoint of manufacturing construction period. Further, although it is suggested in Patent Document 3 that a characteristic difference in the sheet thickness direction is likely to occur in cooling after rolling, this technique is not considered, and the characteristic evaluation of the example is also performed in the sheet thickness 1 / There are only four parts, and the characteristics of the plate thickness surface layer part where the characteristics are most concerned are not disclosed.

  In view of the above, it is considered that a steel material excellent in energy absorption performance at the time of collision of a ship still needs to be improved, and there is room for expansion of the manufacturable plate thickness. In particular, it is necessary to establish an ideal metal microstructure in consideration of the entire plate thickness including the plate thickness surface layer portion and to break through the manufacturing method thereof.

  The present invention increases the energy absorption capacity at the time of collision compared to the steel materials currently proposed without increasing the cost by adding alloying elements to the steel materials currently used and without changing the hull structure design. It is an object of the present invention to provide a steel material excellent in impact resistance that can be produced and a method for producing the same.

  The features of the present invention for solving such problems are as follows.

  In order to improve the uniform elongation without reducing the strength, the steel material of the present invention is a steel having a structure of two or more phases such as ferrite, which is a soft phase, and pearlite, bainite, martensite, which are hard phases. . The structure of this steel material was obtained while studying the basic policy of optimizing the mechanical properties of each phase and optimizing the combination, and based on the following knowledge Yes.

  In general, in a steel having a structure of two or more phases, the soft phase mainly plays a role of improving ductility, and the hard phase mainly plays a role of improving strength. Therefore, in order to improve the uniform elongation, the properties of the ferrite phase, which is a soft phase, were examined. It is clear that the uniform elongation is better for soft materials, but when there is another hard phase, the larger the difference between the two phases, the greater the concentration of strain on the soft phase, and the uniform elongation. The contribution of the soft phase to increases. When a bainite phase having a relatively low strength is considered as the hard phase, the hardness of the ferrite phase must be Hv 160 or less in order to increase the strain concentration on the ferrite phase. In order to obtain a tensile strength of 490 MPa or higher, Hv 140 or higher is required.

  In addition, since the uniform elongation decreases as the crystal grain size decreases, the influence of the ferrite crystal grain size of the duplex steel is investigated, and when the average crystal grain size is less than 2 μm, the uniform elongation decreases rapidly. confirmed. Here, since the local elongation is relatively unaffected by the crystal grain size, it was also confirmed that the decrease in the total elongation due to the decrease in the crystal grain size was relatively small compared to the decrease in the uniform elongation. Therefore, also from this, when evaluating ductility, it is necessary to distinguish between uniform elongation and total elongation.

  Further, when the relationship between the ratio of the soft phase and the hard phase and the uniform elongation was examined, the higher the fraction of the ferrite phase, the greater the uniform elongation was improved. In particular, the ferrite phase fraction was 75% or more of the entire plate thickness. And found that it is excellent in uniform elongation. When the hardness of the ferrite phase is Hv 140 or more and 160 or less, it was found that the influence of the surface layer portion of the plate thickness is particularly large, and it became clear that an increase in the ferrite phase fraction as a whole plate thickness is important.

  Thus, in order to ensure a predetermined ratio of the ferrite phase fraction, the cooling conditions must be adjusted appropriately. That is, the cooling process is roughly divided into two stages, a first stage focusing on transformation from the austenite phase structure to the ferrite phase at the end of rolling and a second stage causing transformation to the hard phase.

In the first stage cooling, the average temperature of the steel plate is relatively difficult to undergo the ferrite transformation (Ar ₃ -50) ° C. or higher, and the ferrite transformation is fractional from the viewpoint of phase equilibrium and temporally from the viewpoint of kinetics. It is ideal to quickly cool the steel sheet to an average temperature of (Ar ₃ -150) ° C. or more and (Ar ₃ -50) ° C. However, as the cooling rate increases, the difference in the cooling rate in the thickness direction of the steel plate increases, so that the transformation to the hard phase such as bainite or martensite occurs in place of the ferrite transformation at the plate thickness surface layer where the cooling rate is fast. It will happen. Therefore, it is necessary to suppress the transformation to the hard phase. When the cooling rate of the steel sheet surface is set to 100 ° C./second or more, if the temperature of the steel sheet surface is controlled to be less than 400 ° C., the generation of the hard phase Can be suppressed.

In addition, the ferrite phase is generated in the process where the surface temperature of the steel sheet is reheated by the heat at the center of the plate thickness after cooling. In this case, it is conceivable that the average cooling temperature of the steel sheet cannot be set to (Ar ₃ -150) ° C. or more and (Ar ₃ −50) ° C. or less by one cooling because the plate thickness is thick. It can be repeated once.

  On the other hand, a method of slowing the cooling rate to suppress the formation of the hard phase of the steel sheet surface layer part is also conceivable, but at the same time it takes a long time for cooling and lowers the production efficiency. The relationship between the cooling rate and the upper limit temperature for hard phase formation also changes in a complicated manner and is difficult to control. If the cooling rate is 100 ° C./second or higher, the transformation to the hard phase can be suppressed unless the cooling rate is 400 ° C. or lower, and control is easy.

  After cooling to a predetermined temperature by the above cooling method, the ferrite transformation at the center of the plate thickness can be rapidly advanced. A time of 10 seconds or more is required to make the phase fraction 75% or more.

  Next, the latter stage cooling for generating the hard phase was examined from the viewpoint of the influence of the structure on the strength. The strength is greatly influenced by the strength and fraction of the hard phase, but if the composition of steel is constant, it can be controlled to obtain an arbitrary strength by selecting the manufacturing conditions even if the structure changes. confirmed.

  That is, when the volume fraction of the hard phase is relatively large, a predetermined strength can be obtained by increasing the cooling stop temperature after rolling, or lowering the cooling rate to lower the strength of the hard phase. It is possible to obtain.

  On the other hand, when the volume fraction of the hard phase is relatively small, conversely, by lowering the cooling stop temperature after rolling or by increasing the cooling rate to increase the strength of the hard phase, the predetermined strength It is possible to obtain

  Such strength control is based on the principle that when the fraction of the hard phase is small, the concentration of carbon that is discharged from the ferrite phase during the transformation and becomes concentrated in the hard phase increases, and the hard phase becomes easier to harden. Therefore, it can be achieved relatively easily.

  The cooling rate control method may be allowed to cool as long as a predetermined condition is satisfied. However, it is conceivable that a heat insulating cover is provided on the steel material when keeping the temperature, or water cooling is performed when the cooling rate is increased.

  Finally, in steel materials used in ships and the like, toughness is one of the important mechanical properties, but in steel materials of a ferrite-based structure that is the subject of the present invention, toughness is mainly composed of ferrite crystal grains. Since it is affected by the diameter, it is desirable that the crystal grain size should be 40 μm or less. The grain size can be controlled by making the rolling reduction a certain value or more in the rolling process.

  The features of the present invention based on the above findings are as follows.

  In the first invention, the steel composition satisfies Ceq ≦ 0.36%, the structure is composed of a ferrite phase and a hard phase, the volume fraction of the ferrite phase is 75% or more in the whole plate thickness, and the hardness is Hv 140 or more and 160. Hereinafter, the steel material is excellent in collision resistance, characterized in that the average crystal grain size is 2 μm or more.

  However, Ceq is represented by the following formula (1).

  The second invention is characterized in that the ratio of the volume fraction of the ferrite phase in the plate thickness surface layer portion to the volume fraction of the ferrite phase in the plate thickness central portion is 0.925 or more and 1.000 or less. It is a steel material excellent in impact resistance described in the invention.

  3rd invention is mass% as steel composition, C: 0.05-0.16%, Si: 0.1-0.5%, Mn: 0.8-1.6%, Sol. The steel material having excellent collision resistance according to the first or second invention, wherein Al: 0.002 to 0.07% is included, and the balance is made of iron and inevitable impurities.

  4th invention is steel material excellent in the collision resistance as described in 3rd invention characterized by containing Ti: 0.003-0.03% by mass% further as steel composition. .

  According to a fifth aspect of the present invention, the steel composition further includes Nb: 0.005 to 0.05% by mass%, and is excellent in collision resistance according to the third or fourth aspect. It is a steel material.

  According to a sixth aspect of the present invention, the steel composition further includes, in mass%, Cr: 0.1 to 0.5%, Mo: 0.02 to 0.3%, V: 0.01 to 0.08%, Cu : It is a steel material excellent in impact resistance according to any one of the third to fifth inventions, characterized by containing one or more selected from 0.1 to 0.6% .

  The seventh aspect of the invention is the collision resistance according to any one of the third to sixth aspects of the invention, further comprising, as a steel composition, Ni: 0.1 to 0.5% by mass%. It is an excellent steel material.

The eighth invention is the cumulative reduction ratio in the temperature range of Ar _{3 to} 850 ° C. after heating the steel material having the steel composition according to any of the first invention or the third to seventh inventions. 50% or more rolling is performed, and then the pre-stage cooling is started at an average steel temperature of (Ar ₃ -50) ° C. or higher, the cooling speed of the steel surface is 100 ° C./second or higher, and the steel surface temperature is 400 ° C. or higher ( Ar ₃ -50) ° C. to below the temperature range, once or twice or more cooling is performed until the steel average temperature is _(Ar 3 -150) ℃ than _(Ar 3 -50) ℃ or less, then 10 perform cooling over seconds, to carry out subsequent cooling of steel average temperature _{(Ar 3 -150) ℃ 10 ℃} / sec or more from the above steel average cooling rate until the steel material average temperature is 300 ° C. or higher 600 ° C. or less This is a method for producing a steel material with excellent impact resistance. .

However, Ar ₃ point is represented by the following formula (2).

  According to the present invention, steel having two or more phases of ferrite and hard phase, which are substantially the same components as ordinary ship hull steel materials, is optimized, and the mechanical properties of each phase are optimized. It is possible to obtain a steel material having a high uniform elongation and excellent impact resistance. In addition, the production method can be efficiently and stably produced because there is no decrease in efficiency and no particular difficulty in controllability as compared with the usual method for producing steel for hulls.

  As a result, it is possible to provide steel materials with excellent energy absorption performance in the event of a ship collision without increasing costs due to the addition of alloying elements to the steel materials currently in use and without adding special production equipment. Industrially, the effect is extremely large. In addition, from the viewpoint of preventing oil spills caused by large tankers aground and collision, the environmental protection effect is also extremely large.

  The reasons for limiting the respective constituent requirements of the present invention will be described below.

1. Regarding the metal structure The steel material of the present invention is a steel material having substantially the same components as those of a normal hull steel material and excellent in impact resistance, that is, excellent in uniform elongation. That is, in order to improve the uniform elongation without reducing the strength, steel composed of two or more phases such as ferrite, which is a soft phase, and pearlite, bainite, martensite, etc., which are hard phases, are used. In addition to optimizing the mechanical properties, the combination is optimized.

  The structure of the steel material of the present invention consists of a ferrite phase and a hard phase. The hard phase is composed of a structure having a higher hardness than ferrite phases such as pearlite, bainite, and martensite.

Ferrite phase volume fraction: Uniform elongation improves as the volume fraction (phase fraction) of the ferrite phase increases by 75% or more in the entire plate thickness. Although the metal structure slightly changes in the plate thickness direction, it is necessary that the volume fraction of the ferrite phase is 75% or more over the entire plate thickness in order to obtain sufficient uniform elongation.
In the present invention, the plate thickness surface layer portion is a region from the surface of the plate to a depth of about 1/10 of the plate thickness. The plate thickness surface layer portion is a region where the cooling rate is relatively faster than that of the plate thickness center portion during cooling, a hard phase is easily generated, and uniform elongation is likely to be reduced. In consideration of the entire plate thickness, the fraction is not so large and its influence can be allowed to some extent in terms of characteristics, but if the characteristic difference from the central portion of the plate thickness increases, the influence cannot be ignored. Therefore, it is necessary to secure the ferrite phase fraction in this way also for the plate thickness surface layer portion.

  As described above, since the main factor affecting the ferrite phase volume fraction is the cooling rate, it is confirmed whether the ferrite phase volume fraction is within the range of the present invention over the entire plate thickness. For this, the volume fraction of the ferrite phase may be measured and confirmed for the central portion of the plate thickness having the smallest cooling rate in the plate thickness direction and the plate thickness surface layer portion having the largest cooling rate in the plate thickness direction.

Ratio of the volume fraction of the ferrite phase in the surface portion of the plate thickness to the volume fraction of the ferrite phase in the center portion of the plate thickness: 0.925 or more and 1.000 or less The ratio of the volume fraction of the ferrite phase in the plate thickness surface layer portion to the volume fraction of the ferrite phase in the center portion of the plate thickness (hereinafter also simply referred to as volume fraction ratio) is 0.925 or more and 1.000 or less. Is preferred. If the volume fraction ratio is 0.925 or more, the material difference between the plate thickness surface layer portion and the plate thickness center portion, particularly the difference in uniform elongation, is sufficiently small, and the structure that is substantially homogeneous in the plate thickness direction. This is preferable from the viewpoint of collision resistance. Furthermore, the volume fraction ratio is preferably 0.935 or more. In addition, as described above, the plate thickness surface layer portion has a cooling rate relatively faster than that of the plate thickness central portion during cooling, and a hard phase is easily generated. The ferrite fraction is higher than the surface layer. For this reason, the upper limit of the volume fraction ratio is 1.000.

Hardness of ferrite phase: 140 to 160 in Hv The lower the hardness of the ferrite phase, the more uniform elongation improves. Since the hardness of the ferrite phase is 160 or less at Hv and uniform elongation is excellent, it is 160 or less at Hv. On the other hand, in order to obtain a strength of TS490 MPa or more, it is set to Hv140 or more.

Average crystal grain size of ferrite phase: 2 μm or more The smaller the average crystal grain size of the ferrite phase, the lower the uniform elongation. In particular, when the average crystal grain size is less than 2 μm, the uniform elongation deteriorates abruptly. By setting the average crystal grain size of the ferrite phase to 2 μm or more, high uniform elongation can be stably obtained. The average crystal grain size of the ferrite phase is preferably 4 μm or more. If the ferrite structure is excessively large, the steel may be softened. Therefore, in order to stably obtain a tensile strength of 490 MPa or more, the average crystal grain size of the ferrite phase should be 40 μm or less. Is preferred.

2. Component Composition The reason for defining the component composition of the steel material will be described. In addition, all component% means the mass%.

Ceq: 0.36 or less The higher the Ceq, the higher the strength and the higher the strength of the ferrite, so that the uniform elongation decreases. When the Ceq exceeds 0.36, the uniform elongation decreases remarkably. Ceq is an index of the toughness of the heat affected zone, and if it exceeds 0.36, the heat affected zone toughness of the high heat input welding decreases. For this reason, Ceq is set to 0.36 or less. Here, Ceq is obtained by the following equation (1).

C: 0.05 to 0.16%
C is contained to ensure strength. However, if it is less than 0.05%, the effect is not sufficient, and if it exceeds 0.16%, a structure mainly composed of ferrite cannot be obtained and the uniform elongation is reduced. The range is 0.05 to 0.16%.

Si: 0.1 to 0.5%
Si is contained as a deoxidizing material and a strength improving element in the steelmaking stage, but if less than 0.1%, the effect is insufficient, and if it exceeds 0.5%, the ductility is lowered. The range is 0.5%.

Mn: 0.8 to 1.6%
Mn is contained to ensure strength, but if less than 0.8%, the effect is insufficient, and if it exceeds 1.6%, a ferrite-based structure cannot be obtained. 1.6%
The range.

Sol. Al: 0.002 to 0.07%
Al is contained for deoxidation. Sol. When the amount of Al is less than 0.002%, the effect is not sufficient, and when it exceeds 0.07%, surface flaws of the steel material are likely to occur. The Al content is in the range of 0.002 to 0.07%. Preferably, it is 0.01 to 0.05% of range.

  The above is the basic chemical component of the present invention, which consists of the balance Fe and unavoidable impurities, but can further contain Ti and Nb as selective elements in order to improve strength and toughness.

Ti: 0.003 to 0.03%
In order to further improve toughness, Ti can be contained. Ti generates TiN during rolling heating or welding, refines the austenite grain size, and improves the base metal toughness and the toughness of the weld heat affected zone. If the content is less than 0.003%, the effect is not sufficient. If the content exceeds 0.03%, the toughness of the weld heat affected zone is lowered. A range of 003 to 0.03% is preferable. More preferably, it is 0.005 to 0.02% of range.

Nb: 0.005 to 0.05%
In order to improve the strength, Nb can be contained. If the content is less than 0.005%, the effect is not sufficient, and if it exceeds 0.05%, the toughness of the weld heat affected zone is lowered. Therefore, when Nb is contained, the amount is 0.005 to 0. A range of 0.05% is preferable. More preferably, it is 0.005 to 0.03% of range.

  Furthermore, in order to improve an intensity | strength, 1 type (s) or 2 or more types of Cr, Mo, V, and Cu can be contained.

Cr: 0.1 to 0.5%
If Cr is less than 0.1%, the effect is insufficient, and if it exceeds 0.5%, the weldability and the toughness of the weld-affected zone decrease. Therefore, when Cr is contained, 0.1 to 0.5% It is preferable to set it as the range.

Mo: 0.02-0.3%
If the content of Mo is less than 0.02%, the effect is insufficient. If the content exceeds 0.3%, the weldability and the toughness of the heat affected zone are significantly reduced. A range of 3% is preferable.

V: 0.01 to 0.08%
When V is less than 0.01%, the effect is insufficient, and when it exceeds 0.08%, the toughness is remarkably lowered. Therefore, when V is contained, the content is preferably in the range of 0.01 to 0.08%. .

Cu: 0.1 to 0.6%
The effect of Cu is not sufficient if it is less than 0.1%, and if it is added in excess of 0.6%, the concern about Cu cracking increases. Therefore, when Cu is contained, the range is from 0.1 to 0.6%. It is preferable to do. More preferably, it is 0.1 to 0.3% of range.

  Furthermore, in order to improve toughness, Ni can also be contained.

Ni: 0.1 to 0.5%
If the Ni content is less than 0.1%, the effect is not sufficient. If the Ni content exceeds 0.5%, the cost of the steel material is remarkably increased. It is preferable to do.

3. About manufacturing conditions The steel material excellent in the collision resistance which concerns on this invention can be manufactured on the manufacturing conditions shown below.

  First, molten steel having the above composition is melted in a converter or the like, and is made into a steel material (slab) by continuous casting or the like, and then the steel material is heated to a temperature of 900 to 1150 ° C. and then hot-rolled. It is preferable.

  In order to obtain good toughness, it is effective to lower the heating temperature and reduce the crystal grain size before rolling, but if the heating temperature is less than 900 ° C, the rolling load becomes excessive, and if it exceeds 1150 ° C, austenite In addition to coarsening of the grains and a reduction in toughness, there is a possibility that the oxidation loss becomes remarkable and the yield decreases. A heating temperature of 900 to 1150 ° C. is preferable because stable rolling is possible and good toughness is obtained. The range of more preferable heating temperature from a viewpoint of toughness is 1000-1100 degreeC.

Rolling conditions: Ar ₃ points or more and 850 ° C. or less, cumulative reduction ratio of 50% or more A steel sheet having a desired thickness is produced by hot rolling a steel material. The start temperature of hot rolling is not particularly limited. Moreover, it is not necessary to provide a restriction in particular as rolling conditions other than the rolling conditions in the non-recrystallization temperature range of austenite described later. Prior to rolling in the non-recrystallization temperature range of austenite, which will be described later, the austenite recrystallization structure is refined and sized so that rolling with a cumulative reduction rate of 30% or more is performed in the austenite recrystallization temperature range. It is preferable.

In rolling, in order to improve toughness, work strain is introduced in the temperature range of Ar ₃ point or higher and 850 ° C. or lower, which is the non-recrystallization temperature range of austenite. The cumulative rolling reduction is 50% or more, and the ferrite crystal grain size after transformation is sufficiently refined to improve toughness. Therefore, the cumulative rolling reduction during rolling is set to 50% or more in a temperature range of Ar ₃ points or more and 850 ° C. or less. Preferably it is 55% or more. Although the upper limit of the cumulative rolling reduction need not be specified, it is preferably set to 80% or less industrially. Incidentally, Ar ₃ point is calculated by the following formula (2).

The rolling end temperature is preferably Ar ₃ points or more. If the rolling end temperature is lower than the Ar ₃ point, the processed ferrite structure remains, which may reduce the elongation of the steel finally obtained. Therefore, the rolling end temperature is preferably Ar ₃ point or higher.

  In the present invention, the first stage cooling, which is the first stage cooling, is performed on the steel sheet after the hot rolling, and then the second stage cooling, which is the second stage cooling, is performed.

The first stage cooling, which is the first stage cooling before cooling, focuses on the transformation from the austenite phase structure to the ferrite phase at the end of rolling, and the subsequent cooling allows the volume fraction of the ferrite phase, Now, it is performed in order to make the crystal grain size predetermined. Therefore, pre-cool the steel material average temperature starting from (Ar 3 _-50) ℃ temperatures above in terms in terms of ferrite transformation phase equilibrium fraction manner kinetically also temporally cool The cooling is performed to a temperature range of (Ar ₃ −150) ° C. or higher and (Ar ₃ −50) ° C. or lower.

In front of the cooling, the steel sheet average temperature from _(Ar 3 -50) ℃ temperatures _above, (Ar 3 -150) ℃ than _(Ar 3 -50) ℃ in the ideal by rapidly cooling to below the steel sheet average temperature Therefore, the cooling rate is a steel surface cooling rate of 100 ° C./second or more. However, as the cooling rate increases, the difference in the cooling rate in the thickness direction of the steel plate increases, so that the transformation to the hard phase such as bainite or martensite occurs in place of the ferrite transformation at the plate thickness surface layer where the cooling rate is fast. It will happen. Therefore, it is necessary to suppress the transformation to the hard phase, and when the cooling rate of the steel sheet surface is set to 100 ° C./second or more, as described later, the temperature of the steel sheet surface at the end of the previous stage cooling is less than 400 ° C. If it controls so that it may not become, the production | generation of the hard phase in a pre-stage cooling process can be suppressed. When the cooling rate is less than 100 ° C./second in terms of the steel material surface cooling rate, the ferrite transformation and the transformation of the hard phase progress in a complicated manner, making it difficult to control the transformation during cooling. By securing a cooling rate of 100 ° C./second or more at the steel surface cooling rate and cooling to a predetermined temperature range at once, the driving force of the ferrite transformation in the cooling process after the previous stage cooling can be increased. The volume fraction, hardness, and crystal grain size of the ferrite phase generated in the cooling step can be specified in the present invention.

The cooling method of the first stage cooling is performed once or twice to a temperature range of 400 ° C. or more (Ar ₃ -50) ° C. or less at the steel sheet surface temperature.

This transformation will proceed rapidly to hard phase when the steel sheet surface temperature is less than 400 ° C., the predetermined ferrite phase volume fraction can not be obtained, whereas (Ar 3 _-50) ℃ The exceed the overall thickness This is because the cooling effect is almost lost. Therefore, as the condition of the steel sheet surface temperature of the pre-cool, if cooled to 400 ° C. or higher at the steel sheet surface temperature (Ar 3 _-50) ℃ below the temperature range, while securing the cooling effect on the overall thickness plate, and, A ferrite phase having a predetermined volume fraction can also be obtained in the steel sheet surface layer portion. Moreover, when the steel plate average temperature does not reach a predetermined temperature by one cooling, the steel plate surface can be reheated with the heat at the center of the plate thickness and then repeatedly cooled under the same conditions. Here, after reheating the steel plate surface, the second and subsequent cooling is performed to prevent only the steel plate surface layer portion from being excessively cooled, and by doing so, including the central portion of the plate thickness. It is possible to balance the cooling behavior of the whole steel plate and the cooling behavior of the surface layer portion of the steel plate.

Cooling after the former stage cooling is performed for 10 seconds or more in a temperature range of (Ar ₃ -150) to (Ar ₃ -50) ° C. at an average steel temperature.

Cooling after the pre-stage cooling is performed in order to make the ferrite phase volume fraction, hardness, and crystal grain size predetermined. Regarding the cooling temperature range, when the average steel temperature is less than (Ar ₃ −150) ° C., it takes a long time to advance the ferrite transformation, and at a temperature exceeding (Ar ₃ −50) ° C., the ferrite transformation rate is predetermined. The fraction of is not reached. Therefore, the cooling temperature range is set to (Ar ₃ -150) ° C. or more and (Ar ₃ −50) ° C. or less in terms of the average steel material temperature. When the cooling time is less than 10 seconds, since ferrite transformation does not proceed sufficiently, desired dispersion control of ferrite phase (ferrite fraction: 75% or more, average crystal grain size: 2 μm or more) cannot be achieved. Diffusion from the ferrite phase of C to the austenite phase does not proceed sufficiently, and the hardness of the ferrite phase does not become Hv160 or less. Therefore, the cooling time is set to 10 seconds or more. In this way, by allowing to cool for 10 seconds or more in the temperature range of (Ar ₃ -150) to (Ar ₃ -50) ° C. at the steel material average temperature, the volume fraction of the ferrite phase, hardness, crystal grain size Can be predetermined.

  In addition, the average temperature of steel materials can use what was calculated | required by simulation calculation etc., when the shape of steel materials, surface temperature, cooling conditions, etc. were given.

In the latter stage cooling, which is the second stage cooling, the steel material is cooled from a temperature of (Ar ₃ -150) ° C. or higher to 300 ° C. to 600 ° C. at a cooling rate of 10 ° C./second or higher.

The latter stage cooling, which is the second stage cooling, controls the cooling start temperature, the cooling rate, and the cooling end temperature in order to secure a predetermined strength by causing the transformation from the austenite phase structure to the hard phase. Cooling start temperature is lower as the intensity is lowered, the steel material average temperature (Ar ₃ -150) for the less than ℃ not predetermined strength can be obtained, the cooling start temperature in order to secure a predetermined strength (Ar ₃ −150) C or higher.

The higher the steel average cooling rate, the higher the strength. However, when the steel average cooling rate is less than 10 ° C./second, the predetermined strength cannot be obtained. Therefore, in order to ensure the predetermined strength, the steel average cooling rate is 10 ° C. / More than a second.
The lower the cooling end temperature, the higher the strength, but the ductility deteriorates when cooled to below 300 ° C. Conversely, if the cooling is stopped at a temperature exceeding 600 ° C., a predetermined strength cannot be obtained. Therefore, the cooling end temperature is set to 300 ° C. or more and 600 ° C. or less in terms of the steel material average temperature from the viewpoint of optimization of strength and ductility.

  Examples will be described below. Table 1 shows the components of the test steel used in the examples. The remainder not shown consists of iron and inevitable impurities. Steel types A to H in Table 1 are steels having a component composition that satisfies the present invention, and steel type I has Ceq outside the scope of the invention (upper limit of more than 0.36%).

  The slab having these steel compositions was heated and then rolled into a steel plate having a thickness of 12 to 50 mm and cooled with various cooling patterns. Table 2 shows the manufacturing conditions. Steel numbers 1 to 10 are invention examples satisfying the component composition and production conditions of the present invention, and steel numbers 11 to 16 are comparative examples where production conditions or component compositions are outside the scope of the present invention.

  The microstructures of these steel plates were observed with an optical microscope, and the ferrite phase fraction and the ferrite crystal grain size (average crystal grain size) in the plate thickness center part and plate thickness surface layer part were measured. The hardness of the ferrite phase was measured with a micro Vickers hardness meter (load: 25 gf) for the plate thickness center portion and the plate thickness surface layer portion, and the average value thereof was taken.

  Further, as mechanical properties, strength, uniform elongation, and toughness were obtained. In the tensile test, a full-thick JIS1B test piece was sampled in the direction perpendicular to the rolling direction of the steel sheet and tested. Uniform elongation was evaluated as elongation at maximum stress. In the impact test, a JIS No. 4 standard test piece was taken in parallel with the rolling direction and brought close to the surface layer (the distance between the surface of the steel material and the end face of the test piece was 2 mm or less) and tested. Toughness was evaluated by vTrs (brittle ductile transition temperature).

  Table 3 shows the test results of the microstructure and mechanical properties of the steel sheet.

As shown in Table 3, the steel numbers 1 to 10, which are invention examples, all have excellent characteristics such that TS (tensile strength) is 520 MPa or more and uniform elongation is 22% or more. Moreover, YS (yield strength) of steel Nos. 1 to 10 is 390 MPa or more, vTrs is lower than −40 ° C., and all target properties are YS ≧ 355 MPa, TS ≧ 490 MPa, uniform elongation ≧ 20%, vTrs ≦ 0 ° C. Is satisfied.

  On the other hand, Steel Nos. 11 to 16 are comparative examples, and Steel No. 11 has a high Ceq, and even if the manufacturing conditions are devised, a predetermined characteristic cannot be obtained. The volume fraction of is small and the uniform elongation is inferior. In Steel No. 12, since the cooling start temperature in the previous stage is too low, the volume fraction of the ferrite phase in the center portion of the plate thickness and the surface layer portion of the plate thickness is small, and the uniform elongation is inferior. Steel No. 13 has a slow cooling rate with respect to the prescribed cooling rate (100 ° C./s or more) of the preceding stage cooling, so the volume fraction of the ferrite phase is small and the uniform elongation is inferior.

  In Steel No. 14, the stop temperature of the pre-cooling is too low, so the ferrite phase volume fraction is small and the uniform elongation is inferior. In Steel No. 15, the end temperature of the pre-cooling is too low, so the ferrite phase volume fraction is small and the uniform elongation is inferior. Steel No. 16 had a short cooling time between the former stage cooling and the latter stage cooling, so the ferrite phase volume fraction was low and the uniform elongation was inferior.

Claims (6)

As a steel composition, C: 0.05-0.16%, Si: 0.1-0.5%, Mn: 0.8-1.6%, Sol. Al: 0.002 to 0.07% is included, the balance is made of iron and inevitable impurities, and Ceq ≦ 0.36% is satisfied,
Structure becomes ferrite phase and a hard phase, the volume fraction of ferrite phase thickness total 75% or more, hardness is Hv140 or more 160 or less, the average crystal grain size Ri der least 40μm or less 2 [mu] m,
Der ratio 0.925 or 1.000 volume fraction of the ferrite phase in the sheet thickness surface layer portion to the volume fraction of the ferrite phase in the sheet thickness center portion is,
A steel material excellent in collision resistance, characterized in that the plate thickness is 12 to 50 mm .
However, Ceq is represented by the following formula (1).

The steel material having excellent collision resistance according to claim 1 , further comprising Ti: 0.003 to 0.03% by mass% as a steel composition. The steel composition according to claim 1 or 2 , further comprising Nb: 0.005 to 0.05% by mass% as a steel composition. Further, as a steel composition, in mass%, Cr: 0.1-0.5%, Mo: 0.02-0.3%, V: 0.01-0.08%, Cu: 0.1-0 The steel material excellent in impact resistance according to any one of claims 1 to 3 , comprising one or more selected from 0.6%. The steel material having excellent impact resistance according to any one of claims 1 to 4 , wherein the steel composition further contains Ni: 0.1 to 0.5% by mass. . A method for producing a steel material having excellent impact resistance according to any one of claims 1 to 5,
After heating the steel material, rolling is performed at a cumulative reduction ratio of 50% or more in a temperature range of Ar ₃ points or more and 850 ° C. or less, and then the former stage cooling is started from the steel material average temperature (Ar ₃ −50) ° C. or more. at a cooling rate of the surface 100 ° C. / sec or more, the steel material surface temperature 400 ° C. or higher _(Ar 3 -50) ℃ to below the temperature range, once or twice or more cooling, the steel material average temperature (Ar ₃ - performed until 0.99) ° C. or higher _(Ar 3 -50) ℃ or less, Thereafter, the cooling of more than 10 seconds, the steel average temperature _(Ar 3 -150) steel average cooling rate of 10 ° C. / sec or more from the above ° C. In the method for producing a steel material having excellent collision resistance, the latter stage cooling is performed until the average temperature of the steel material becomes 300 ° C. or more and 600 ° C. or less.
However, Ar ₃ point is represented by the following formula (2).