JP3559073B2 - Ferrite thick steel plate with less material variation and method of manufacturing the same - Google Patents

Description

【００３７】
【表２】

【００３８】
【発明の効果】
この発明の厚鋼板は、工業的規模での生産における冷却工程で用いられる、いずれの冷却速度によっても、フェライト単相組織となる。従って、今後需要増が予想される、厚み方向の材質ばらつきの極めて少ない厚鋼板を、工業的に安定して供給できる。なお、この発明は形鋼の分野にも有利に適合する。
【図面の簡単な説明】
【図１】厚鋼材における冷却速度と強度との関係を示す図である。[0001]
[Industrial applications]
The present invention relates to a method for manufacturing a thick steel plate having a thickness of 30 mm or more, particularly a thick steel plate with less material variation in the thickness direction , which is used in the fields of architecture, marine structures, pipes, shipbuilding, storage tanks, civil engineering, construction machinery, and the like. .
[0002]
[Prior art]
Thick steel sheets are used in various fields as described above, and properties such as high strength and high toughness have been improved.However, in recent years, steel sheets are more uniform in the thickness direction, It is required that the variation between them is small.
[0003]
For example, on pages 11 to 21 of “Iron and Steel No. 74 (1988) No. 6,” as buildings become taller, they absorb vibration energy due to the deformation of the building in response to a huge earthquake and collapse. It has been reported that designs have been adopted to prevent this. Specifically, when an earthquake occurs, the framework of the building is collapsed in a predetermined shape, and the collapse of the building is prevented by plasticizing the framework. In other words, it is premised that the framework of the building behaves as intended by the designer when an earthquake occurs, and that the designer must fully understand the strength ratio of steel materials such as columns and beams of the building. It is. Therefore, it is indispensable that the steel materials used for the columns, beams, and the like are homogeneous, and the variation in the strength of the steel materials becomes a serious problem.
[0004]
Here, high tensile strength and high toughness are required for steel materials used for construction and shipbuilding, and this type of steel sheet is generally manufactured according to a controlled rolling control cooling method, a so-called TMCP method. However, when a thick steel material is manufactured by this TMCP method, the cooling rate changes in the thickness direction or between the steel materials, and the structure changes, so that the material varies in the thickness direction of the obtained steel material or between the steel materials. .
[0005]
On the other hand, in JP-A-63-179020, the difference in cross-sectional hardness in the plate thickness direction is reduced by controlling the components, the reduction amount, the cooling rate, and the cooling end temperature. In the production of thick steel plates exceeding this, a problem remains in that it is extremely difficult to suppress the difference in cross-sectional hardness because a cooling rate distribution in the thickness direction is inevitable.
[0006]
[Problems to be solved by the invention]
The present invention has solved the above problem, to propose a method of manufacturing the material with less variation thick steel plate between the thickness direction and steel, respectively of interest.
[0007]
[Means to solve the problem]
The variation in the quality of the steel plate is caused by a change in the structure due to a significant change in the cooling rate in the thickness direction from the surface of the steel material to the center in the cooling step or a change in the cooling rate due to a variation in manufacturing conditions. In order to avoid such a structure variation, it is important to obtain a homogeneous structure in a wide cooling rate range.
Therefore, the inventors returned to the origin and repeated the study on a method of obtaining a homogeneous structure even when manufacturing conditions changed, and by redesigning the component composition, regardless of the change in cooling rate, It has been found that a steel material having a uniform structure in the thickness direction and a small material variation can be obtained.
[0008]
That is, the present invention
C: less than 0.04 wt%,
Si: 0.60 wt% or less,
Mn: 0.2-0.5 wt% and
Al: contains 0.100 wt% or less, and
Cu: 0.7 to 2.0 wt%,
Ti: 0.01 ~ 0.20wt%,
Nb: 0.005 to 0.20 wt% and V: 0.005 to 0.20 wt%
Selected from among the one or comprise two or more, the balance Saishi the hot thickness subjected to rolling a steel slab comprising the composition of Fe and unavoidable impurities to produce a more steel plates 30 mm, Ac ₃ after heating to a temperature of to 1350 ° C., Ar ₃ or more finished rolling temperature, then precipitation treatment temperature range cooling rate of 500 ° C. or more predetermined temperature below Ar ₃ to 2 ° C. / s or higher 50 ° C. / s or less is in After accelerated cooling, subjected to precipitation treatment to cool 1 ° C. / s 30s or more of the following cooling rates in or the temperature range for 30s or more isothermal holding at the predetermined temperature, and performing the subsequent cooling, the tensile A method for producing a ferritic thick steel plate having a strength of at least 462 MPa and a small variation in the material in the thickness direction .
[0009]
[Action]
Next, the reasons for limiting the respective chemical components of the thick steel material of the present invention will be described.
C: less than 0.04 wt% In order to make the structure of the steel material mainly composed of ferrite or a ferrite single phase without depending on the cooling rate, it is necessary to suppress the amount of C to less than 0.04 wt%, preferably to 0.02 wt% or less. That is, the solid solubility limit of C in ferrite is approximately 0.02 wt%, and to obtain a ferrite single phase structure, it is necessary to suppress the content to preferably 0.02 wt% or less. Since it has the same characteristics and effects as the ferrite single phase structure, the C content is set to less than 0.04 wt%.
[0010]
Si: 0.60wt% or less
If the content of Si exceeds 0.6 wt%, the toughness of the welded portion deteriorates, so the content is set to 0.60 wt% or less. In addition, it is preferable to add 0.02 wt% or more in order to secure deoxidation and strength.
[0011]
Mn: 0.2 to 0.5 wt%
Mn is required to be 0.2 wt% or more to secure the strength. On the other hand, if it exceeds 0.5 wt%, it is difficult for the steel sheet having a thickness of 30 mm or more to form a ferrite-based structure over the entire thickness of the steel sheet, so the Mn content is limited to the range of 0.2 to 0.5 wt%.
[0012]
Al: 0.100 wt% or less
If the content of Al exceeds 0.100 wt%, the weldability is impaired. In addition, for deoxidation, it is preferable to add 0.010 wt% or more.
[0013]
The present invention is characterized in that by adjusting the components to the above-mentioned basic composition, a homogeneous structure, specifically, a ferrite structure of 90% or more can be obtained, which hardly depends on the production conditions, particularly the cooling rate. is there. This feature is evident from the experiment whose results are shown in FIG.
[0014]
That is, regarding the steel adjusted to the component according to the present invention (invention example) and the conventional steel used for building materials (conventional example), the tensile strength of the steel material obtained by variously changing the cooling rate in the manufacturing process. FIG. 1 shows the results of the investigation. It can be seen from the figure that by adjusting the components according to the present invention, a stable strength can be obtained independent of the cooling rate. Therefore, even if the cooling rate changes in the thickness direction of the thick steel sheet, the strength does not change depending on the cooling rate, and a thick steel sheet with less material variation in the thickness direction can be obtained.
[0015]
The invention example contains C: 0.019% by weight, Si: 0.049% by weight, Mn: 0.4% by weight, Cu: 1.01% by weight and Al: 0.024% by weight. On the other hand, the conventional example contains 0.14 wt% of C, 0.4 wt% of Si, 1.31 wt% of Mn, 0.024 wt% of Al, 0.015 wt% of Nb and 0.013 wt% of Ti, and the balance of iron and unavoidable The composition of the component became an impurity. Then, in the same manufacturing process, a large number of thick steel plates having a thickness of 15 mm were manufactured by changing the cooling rate, and the tensile strength was measured with a test piece taken from each thick steel plate.
[0016]
Further, in the present invention, the strength and the toughness level are controlled by adding a predetermined chemical component to the basic component. At this time, the already obtained homogeneous structure is not affected by the addition of a new component, so that a high-strength and / or high-toughness thick steel plate with little material variation can be easily obtained.
[0017]
First, in order to improve the strength, a precipitation strengthening component was selected from Cu: 0.7 to 2.0 wt%, Ti: 0.005 to 0.20 wt%, Nb: 0.005 to 0.20 wt%, and V: 0.005 to 0.20 wt%. One or more are added. Incidentally, after the addition of these precipitation strengthening components, it is necessary to perform the precipitation hardening process to be described later.
[0018]
Cu: 0.7 to 2.0 wt%
Cu is added to enhance precipitation strengthening, but if it exceeds 2.0 wt%, the toughness is rapidly deteriorated, whereas if it is less than 0.7 wt%, the effect of precipitation strengthening is small, so Cu is set to 0.7 to 2.0 wt%.
[0019]
Ti: 0.005 to 0.20wt%
Ti is added in an amount of 0.005 wt% or more to fix excess C in addition to precipitation strengthening and to improve the toughness of HAZ by existing as TiN. On the other hand, if it is added in excess of 0.20 wt%, it becomes difficult to form a ferrite single phase, so the upper limit is 0.20 wt%.
[0020]
Nb: 0.005 to 0.20wt%
Nb is added in an amount of 0.005 wt% or more in order to improve toughness and fix excess C in addition to precipitation strengthening. On the other hand, if it is added in excess of 0.20 wt%, it becomes difficult to form a ferrite single phase, so the upper limit is 0.20 wt%.
[0021]
V: 0.005 to 0.20 wt%
V is added in an amount of 0.005 wt% or more to fix excess C in addition to precipitation strengthening. However, if it is added in excess of 0.20 wt%, it becomes difficult to form a ferrite single phase. Is the upper limit.
[0022]
Further, in order to improve the strength, one or more selected from among Ni: 2.0 wt% or less, Cr: 0.5 wt% or less, Mo: 0.5 wt% or less, and W: 0.5 wt% or less are added. be able to. Since these components are effective even in a very small amount, the lower limit can be appropriately set.
[0023]
Ni: 2.0 wt% or less
Ni is effective in improving strength and toughness, and is effective in preventing Cu cracking during rolling when Cu is added, but is expensive and saturates its effect when added in excess, so 2.0 Add in the range of wt% or less. Note that if the addition is less than 0.05%, the above effect is insufficient, so the addition amount is preferably 0.05 wt% or more.
[0024]
Cr: 0.5 wt% or less
Cr has the effect of increasing the strength, but if added in excess of 0.5 wt%, it becomes difficult to form a ferrite single phase, and the weld toughness deteriorates, so Cr is added in a range of 0.5 wt% or less.
If the addition is less than 0.05 wt%, the effect of increasing the strength is insufficient, so the addition amount is preferably set to 0.05 wt% or more.
[0025]
Mo: 0.5 wt% or less
Mo has the effect of increasing the strength at ordinary and high temperatures, but if it exceeds 0.5 wt%, it becomes difficult to form a ferrite single phase, and the weld toughness deteriorates. Add in.
If the addition is less than 0.05 wt%, the effect of increasing the strength is insufficient, so the addition amount is preferably set to 0.05 wt% or more.
[0026]
W: 0.5 wt% or less W has the effect of increasing the high-temperature strength, but is expensive, and if it exceeds 0.5 wt%, the toughness deteriorates. Therefore, W is added in a range of 0.5 wt% or less.
If the addition is less than 0.05 wt%, the effect of increasing the strength is insufficient, so the addition amount is preferably set to 0.05 wt% or more.
[0027]
In order to improve the toughness of HAZ, at least one selected from REM and Ca can be added in an amount of 0.02 wt% or less.
REM contributes to the formation of ferrite precipitation nuclei and forms oxysulfide to suppress the growth of austenite grains to improve the toughness of HAZ. , 0.02 wt% or less. If the addition is less than 0.001 wt%, the above-mentioned effect of improving the HAZ toughness is insufficient, so that the addition amount is preferably 0.001 wt% or more.
[0028]
Ca is effective in improving the toughness of HAZ, and is also effective in improving the material quality in the sheet thickness direction by controlling the sulfide in steel, but adding more than 0.02 wt% increases the amount of nonmetallic inclusions. To 0.02 wt% or less because it causes internal defects. If the addition is less than 0.0005 wt%, the above effect is insufficient, so the addition amount is preferably 0.0005 wt% or more.
[0029]
Next , the steel slab adjusted to the above-described basic composition is heated to a temperature of Ac _{3 to} 1350 ° C., then the rolling is completed at a temperature of Ar ₃ or more, and then cooled.
[0030]
That is, the heating temperature is, Ac completely becomes insufficient homogenization can not be austenitizing is less than _3, whereas, since the surface oxidation becomes severe exceeds 1350 ° C., the temperature range of Ac ₃ to 1350 ° C. Heating is preferred. By setting the rolling finish temperature to Ar ₃ or more, the structure becomes finer, and the toughness of the base material is improved.
[0031]
Here, as the precipitation strengthening component, one or more selected from Cu: 0.7 to 2.0 wt%, Ti: 0.005 to 0.20 wt%, Nb: 0.005 to 0.20 wt%, and V: 0.005 to 0.20 wt%. on the addition of, after the completion of the rolling, precipitation treatment temperature range in which to a predetermined temperature below 500 ° C. or higher Ar ₃ 50 ℃ / s After following accelerated cooling at a cooling rate, 30s or more isothermal holding at the predetermined temperature I or rows precipitation treatment of cooling 1 ° C. / s or less in the cooling rate at 30s or more in the temperature range to, it is important to improve the strength.
[0032]
That is, the rate of cooling from the end of rolling to the precipitation temperature must be 50 ° C./s or less to suppress the precipitation of the bainite phase.
Next, after the accelerated cooling, Cu, Nb is kept by isothermal holding for at least 30 s in a temperature range of 500 ° C. or more and less than Ar ₃ or cooling for 30 s or more at a cooling rate of 1 ° C./s or less in the temperature range. One or more of (CN), Ti (CN) and V (CN) are precipitated to increase the strength. In addition, the precipitation treatment makes the structure uniform, and further reduces the material variation in the thickness direction.
[0033]
Here, the precipitates formed in the austenite region of Ar ₃ or more do not match the ferrite phase that transforms at low temperatures, so the amount of precipitation strengthening is small, and therefore it is necessary to perform the precipitation treatment at a temperature lower than Ar ₃ to obtain sufficient strengthening. There is. On the other hand, when the temperature is lower than 500 ° C., both the precipitation reaction and the structure homogenization are unlikely to occur, so the temperature range is set to 500 ° C. or more and less than Ar ₃ . The reason for setting the holding time to 30 s or longer is that sufficient precipitation strengthening cannot be performed if the holding time is shorter than 30 s. Further, precipitation strengthening can be obtained by holding at a cooling rate of 1 ° C./s or less at a cooling rate of 1 ° C./s or less within the temperature range. At a cooling rate exceeding 1 ° C./s, not only sufficient precipitation strengthening cannot be obtained, but also Thickness homogeneity is also impaired. In order to sufficiently strengthen the precipitation, a cooling rate of 0.1 ° C./s or less is desirable.
[0034]
Cooling after its does not have to be strictly controlled as in the prior art, it is possible either air or accelerated cooling, when the cooling at a cooling rate exceeding 50 ° C. / s, since the bainite phase is generated On the other hand, when the cooling rate is less than 0.5 ° C./s, the grain size becomes coarse and the toughness is deteriorated. Therefore, the cooling is preferably performed in the range of 0.5 to 50 ° C./s.
[0035]
【Example】
The steel slabs adjusted to the various component compositions shown in Table 1 were processed according to the conditions shown in Table 2 to produce thick steel plates having a thickness of 80 mm.
A tensile test and a Charpy test are performed on each of the obtained thick steel plates to investigate their mechanical properties, and to evaluate the variation in strength in the thickness direction, the hardness of the steel plate cross section is set to a pitch of 2 mm from the surface. And the hardness distribution in the thickness direction was investigated. Furthermore, in order to evaluate the toughness of HAZ , a heat cycle in which the steel sheet is heated to 1350 ° C and then cooled from 800 ° C to 500 ° C in 300 seconds ( corresponding to the heat history of HAZ when welding with a heat input of 500 kJ / cm ) , A Charpy test piece was collected, and the Charpy absorbed energy at 0 ° C. was measured. The volume fraction of the ferrite structure was measured by a point calculation method from an optical microscope photograph taken at 400 times. As shown in Table 2, the results of these investigations show that the thick steel plate according to the present invention has a tensile strength of 462 MPa or more and has a uniform structure. In comparison, it turns out that it is extremely small. It can also be seen that an increase in strength is realized by adding a precipitation strengthening element and performing a precipitation strengthening treatment.
[0036]
[Table 1]

[0037]
[Table 2]

[0038]
【The invention's effect】
The steel plate of the present invention has a ferrite single phase structure at any cooling rate used in a cooling step in production on an industrial scale. Therefore, it is possible to industrially stably supply a thick steel plate, which is expected to increase in demand in the future, and has very little material variation in the thickness direction. It should be noted that the present invention is also advantageously applied to the field of shaped steel.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between a cooling rate and a strength in a thick steel material.

Claims (1)

C: less than 0.04 wt%,
Si: 0.60 wt% or less,
Mn: 0.2-0.5 wt% and
Al: contains 0.100 wt% or less, and
Cu: 0.7 to 2.0 wt%,
Ti: 0.01 ~ 0.20wt%,
Nb: 0.005 to 0.20 wt% and V: 0.005 to 0.20 wt%
Selected from among the one or comprise two or more, the balance Saishi the hot thickness subjected to rolling a steel slab comprising the composition of Fe and unavoidable impurities to produce a more steel plates 30 mm, Ac ₃ after heating to a temperature of to 1350 ° C., Ar ₃ or more finished rolling temperature, then precipitation treatment temperature range cooling rate of 500 ° C. or more predetermined temperature below Ar ₃ to 2 ° C. / s or higher 50 ° C. / s or less is in After accelerated cooling, subjected to precipitation treatment to cool 1 ° C. / s 30s or more of the following cooling rates in or the temperature range for 30s or more isothermal holding at the predetermined temperature, and performing the subsequent cooling, the tensile A method for producing a ferritic thick steel plate having a strength of at least 462 MPa and a small variation in the material in the thickness direction .

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