WO2017094593A1

WO2017094593A1 - Non-heat-treated steel sheet having high yield strength in which hardness of a welding-heat-affected zone and degradation of low-temperature toughness of the welding-heat-affected zone are suppressed

Info

Publication number: WO2017094593A1
Application number: PCT/JP2016/084857
Authority: WO
Inventors: 皓至倉田; 晴弥川野; 喜一郎田代; 元樹柿崎
Original assignee: 株式会社神戸製鋼所
Priority date: 2015-12-04
Filing date: 2016-11-24
Publication date: 2017-06-08
Also published as: JP2017106107A; EP3385399A1; EP3385399A4; CN108350540A; KR20180085791A

Abstract

Provided is a non-heat-treated steel sheet having high yield strength in which hardness of a welding-heat-affected zone and degradation of low-temperature toughness of the welding-heat-affected zone are suppressed. A non-heat-treated steel sheet having high yield strength in which hardness of a welding-heat-affected zone and degradation of low-temperature toughness of the welding-heat-affected zone are suppressed, the non-heat-treated steel sheet characterized by including predetermined components in the steel, the Ceq specified by expression (1) is less than 0.44, the A value specified by expression (2) is 2.5 or greater, the B value specified by expression (3) is 2.37 or greater, the area ratios of the metallographic structure below at a position 1/4 of the thickness of the steel sheet satisfy the values of 80% or more of bainite and 0% to 0.26% of island martensite, and the maximum hardness of the bainite is 270 HV or greater. (1): Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5. (2): A value = 1.15 × Mn + 2.20 × Mo + 6.50 × Nb. (3): B value = 1.20 × Mn + 0.50 × Ni + 4.25 × Nb.

Description

Non-tempered steel sheet with high yield strength that suppresses low temperature toughness degradation of weld heat affected zone and hardness of weld heat affected zone

The present invention relates to a non-tempered steel sheet having high yield strength in which the low temperature toughness deterioration of the weld heat affected zone and the hardness of the weld heat affected zone are suppressed. More specifically, the present invention relates to a non-tempered steel sheet having high yield strength of API standard X80 class used for transportation line pipes such as oil and natural gas.

In the case of line pipes that transport natural gas and crude oil over long distances, there is a growing need to limit the increase in wall thickness by increasing the strength of the pipe material itself in order to reduce laying costs and transportation costs. Currently, in the American Petroleum Institute (API), X80 grade steel is standardized and put into practical use as high yield strength steel.

In addition to high yield strength, steel sheets used as line pipes as described above are desired to have high toughness, a short work period, and low cost. Controlling rolling is an example of a manufacturing method for satisfying these requirements. Controlled rolling is a technique in which crystal grains are refined by appropriately controlling the temperature and rolling reduction during rolling, and accelerated cooling is performed after hot rolling. In controlled rolling, tempering such as heating after accelerated cooling is unnecessary. A steel sheet obtained by such a method is generally called a non-tempered steel sheet.

Various technologies have been developed for non-tempered high yield strength steel sheets. For example, Patent Documents 1 to 4 disclose a method for producing a steel sheet having a high yield strength of API standard X80 class with no tempering.

JP 2006-328523 A International Publication No. 2010/052927 Pamphlet JP 2006-169591 A JP 2008-261012 A

Incidentally, since the line pipe is often laid in a cold region, it is essential that the low temperature toughness of the heat affected zone (Heat Affected Zone, HAZ) is excellent. Further, in recent years, it has been strongly desired to suppress the hardness of the weld heat affected zone from the viewpoint of welding workability.

However, since the steel sheets described in Patent Document 1 and Patent Document 2 do not control Ceq, which is an index for evaluating the toughness and hardness of the weld heat affected zone, toughness degradation in the weld heat affected zone and the weld heat impact. There is a risk of curing the part.

Further, in the methods described in Patent Literature 3 and Patent Literature 4, since a large amount of B, which is a component that degrades the low temperature toughness of the weld heat affected zone, is added, there is a risk of the low temperature toughness of the weld heat affected zone being deteriorated. .

The present invention has been made in view of such circumstances, and its purpose is to provide a non-tempered steel sheet having a high yield strength that suppresses the low temperature toughness deterioration of the weld heat affected zone and the hardness of the weld heat affected zone. It is to provide.

The non-heat treated steel sheet having a high yield strength with suppressed low temperature toughness deterioration of the weld heat affected zone and hardness of the weld heat affected zone according to the present invention, which has solved the above problems, is C% by mass. % Over 0.10% or less, Si: 0.15 to 0.50%, Mn: 1.20 to 2.50%, P: over 0% to 0.020% or less, S: over 0% to 0.0050% Hereinafter, Nb: 0.020 to 0.100%, Ti: 0.003 to 0.020%, N: 0.0010 to 0.0075%, Zr: 0.0001 to 0.0100%, Ca: 0.00. 0005 to 0.0030%, REM: 0.0001 to 0.0050%, Al: 0.010 to 0.050%, and B: 0.0003% or less (including 0%), and Mo : More than 0% and 0.30% or less, Cu: more than 0% and 0.30% or less, Ni: more than 0% and 0.30% or less, Cr 1 or more selected from the group consisting of more than 0% and 0.30% or less, and V: more than 0% and 0.050% or less, and the balance consists of iron and inevitable impurities, and is defined by the following formula (1) Ceq is less than 0.44, the A value defined by the following formula (2) is 2.50 or more, and the B value defined by the following formula (3) is 2.37 or more. At the 1/4 position of the thickness, the area ratio of the following metal structure satisfies bainite: 80 area% or more and island martensite: 0 area% or more and 0.26 area% or less, and the maximum hardness of the bainite is 270 HV. It has a gist where it meets the above.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
A value = 1.15 × Mn + 2.20 × Mo + 6.50 × Nb (2)
B value = 1.20 × Mn + 0.50 × Ni + 4.25 × Nb (3)
However, C, Mn, Cu, Ni, Cr, Mo, V, and Nb are mass%, respectively, and indicate the contents of C, Mn, Cu, Ni, Cr, Mo, V, and Nb.

In a preferred embodiment of the present invention, the non-tempered steel sheet is for a line pipe.

According to the present invention, by adopting the above configuration, a non-tempered steel sheet having a high yield strength with suppressed low temperature toughness deterioration of the weld heat affected zone and hardness of the weld heat affected zone can be obtained.

FIG. 1 is a graph showing the relationship between the bainite area ratio and the yield strength. FIG. 2 is a graph showing the relationship between the maximum hardness of bainite and the yield strength. FIG. 3 is a graph showing the relationship between the area ratio of the island martensite (hereinafter, the island martensite may be referred to as MA) and the yield strength.

First, the present inventors examined factors that govern the yield strength of non-tempered steel sheets. As a result, the yield strength of the non-tempered steel sheet has a close correlation with the area ratio of bainite and island martensite in the metal structure, and the maximum hardness of bainite, and when these are controlled within a predetermined range It was found that high yield strength of API standard X80 grade can be obtained.

Furthermore, the present inventors have studied from various angles in order to realize a non-tempered steel sheet having high yield strength and low-temperature toughness deterioration of the weld heat affected zone and suppressing the hardness of the weld heat affected zone. As a result, if the chemical composition is controlled so as to satisfy the relationships of the above formulas (1) to (3), the low temperature toughness deterioration of the weld heat affected zone is suppressed, and the hardness of the weld heat affected zone is further reduced. The present invention has been completed.

Further, such a non-tempered steel sheet is preferably heated from a steel material satisfying a predetermined component composition by hot rolling, and then from a cooling start temperature of 730 ° C. or higher to a cooling stop temperature of 370 to 550 ° C., Average cooling rate: It can be produced by cooling at 10 to 50 ° C./second.

In this specification, “high yield strength of API standard X80 class” means that the yield strength in the sheet width direction of the steel sheet is 555 MPa or more and 705 MPa or less.

First, the structure of the non-heat treated steel sheet of the present invention will be described.

In the non-tempered steel sheet according to the present invention, each structure with respect to the entire metal structure is bainite: 80 area% or more, island-like martensite: 0 area% or more, 0.26 area at a quarter position of the sheet thickness t of the steel sheet. % Or less, and the maximum hardness of bainite: 270 HV or more is satisfied.

Bainite: 80 area% or more Bainite is a structure that contributes to improving the yield strength, and is an important structure for securing high yield strength of API standard X80. When bainite is less than 80 area%, the yield strength decreases. Therefore, the lower limit of the area ratio of bainite when the area of the whole metal structure is 100% is 80 area% or more. The lower limit of the area ratio of bainite is preferably 82 area% or more, more preferably 84 area% or more.

FIG. 1 shows No. 1 in Table 2 using steel types A to X in Table 1 of Examples described later. 2 is a graph showing the relationship between the area ratio of bainite and the yield strength in a non-tempered steel sheet produced under the production conditions 1 to 24. As shown in FIG. 1, all examples satisfying the desired high yield strength of 555 MPa or more had a bainite area ratio of 80% or more in the metal structure, and in order to satisfy the high yield strength, the bainite area It can be seen that it is effective to increase the rate to 80% or more. In FIG. 1, there is an example in which the yield strength does not satisfy 555 MPa or more even though the bainite area ratio is 80% or more. However, these have a bainite hardness of less than 270 HV described later or an MA area ratio of 0. This is an example of over 26%.

Maximum hardness of bainite: 270 HV or higher The maximum hardness of bainite is important for obtaining high yield strength stably by suppressing variation in yield strength, and it is necessary to control it to 270 HV or higher. Thereby, high yield strength of API standard X80 class can be secured stably. The lower limit of the maximum hardness of bainite is preferably 275 HV or higher. However, considering the formability to the steel pipe, the upper limit of the maximum hardness of the bainite is preferably 310 HV or less, more preferably 300 HV or less.

FIG. 2 shows No. 1 in Table 2 using steel types A to X in Table 1 of Examples described later. 2 is a graph showing the relationship between the maximum hardness of bainite and the yield strength in a non-heat treated steel sheet produced under the production conditions 1 to 24. As shown in FIG. 2, all examples satisfying the desired 555 MPa or more have a maximum bainite hardness of 270 HV or more in the metal structure, and in order to satisfy high yield strength, the bainite maximum hardness is 270 HV or more. It can be seen that it is effective to increase it. Here, although the bainite maximum hardness is 270 HV or more, there is an example where the yield strength does not satisfy 555 MPa or more, but these have a bainite area ratio of less than 80% or an MA area ratio of 0.26%. It is an example of exceeding.

Here, “maximum hardness of bainite” means the average value of the upper three points when the Vickers hardness of bainite is measured by the method described in the examples described later. The present inventors have found that high yield strength can be stably obtained by controlling the maximum hardness of bainite.

Island-like martensite: 0 area% or more and 0.26 area% or less Island-like martensite is a structure that lowers the yield strength, so it is necessary to reduce the MA area ratio in order to ensure the desired high yield strength. is there. Therefore, the upper limit of the MA area ratio when the area of the entire metal structure is 100% is set to 0.26 area% or less. The upper limit of the MA area ratio is preferably 0.25 area% or less.

FIG. 3 shows No. in Table 2 using A to X in Table 1 of Examples described later. 2 is a graph showing the relationship between the area ratio of island martensite and the yield strength in a non-tempered steel sheet produced under the production conditions of 1 to 24. As shown in FIG. 3, in all examples satisfying 555 MPa or more, the MA area ratio in the metal structure is 0.26% or less, and in order to satisfy the high yield strength, the MA area ratio is 0.26%. It can be seen that the following control is effective. Here, although there is an example where the yield strength does not satisfy 555 MPa or more even though the MA area ratio is 0.26% or less, these may have a bainite area ratio of less than 80% or a bainite maximum hardness. This is an example of less than 270 HV.

The structure of the non-tempered steel sheet according to the present invention is as described above. The remaining structure other than the above is ferrite, martensite, or pearlite.

Next, the components in steel will be described.

First, the relationship between the Ceq, A value, and B value represented by the above formulas (1) to (3), the yield strength, the low temperature toughness of HAZ, and the hardness of HAZ will be described.

Ceq: Less than 0.44 Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
However, C, Mn, Cu, Ni, Cr, Mo, and V are mass%, respectively, and show content of C, Mn, Cu, Ni, Cr, Mo, and V.
Ceq defined by the above formula (1) is an important index for determining the low temperature toughness of HAZ and the hardness of HAZ. When Ceq is 0.44 or more, the low temperature toughness of HAZ and the hardness properties of HAZ deteriorate rapidly, and if Ceq is less than 0.44, good low temperature toughness of HAZ and hardness of HAZ part can be ensured. it can. Therefore, the upper limit of Ceq is set to less than 0.44. The upper limit of Ceq is preferably 0.43 or less, more preferably 0.42 or less. On the other hand, considering the lower limit of the content of each element, the lower limit of Ceq is preferably 0.37 or more, more preferably 0.38 or more.

A value: 2.50 or more A value = 1.15 × Mn + 2.20 × Mo + 6.50 × Nb (2)
However, Mn, Mo, and Nb are the mass%, respectively, and show content of Mn, Mo, and Nb.
The A value was found for the first time by the present inventors, and the contents of Mn and Mo effective for suppressing ferrite transformation among the elements constituting Ceq described above, and further the content of Nb Is a parameter controlled to satisfy the above equation (2). By controlling the A value to 2.50 or more, an increase in Ceq can be suppressed and a bainite area ratio important for realizing high yield strength can be secured. In order to increase the bainite area ratio, the higher the A value, the better. In order to ensure high yield strength of API standard X80 class, the lower limit of the A value is 2.50 or more. The lower limit of the A value is preferably 2.52 or more, more preferably 2.54 or more. On the other hand, considering the upper limit of the content of each element, the upper limit of the A value is preferably 3.00 or less, more preferably 2.95 or less.

B value: 2.37 or more B value = 1.20 × Mn + 0.50 × Ni + 4.25 × Nb (3)
However, Mn, Ni, and Nb are mass%, respectively, and show the content of Mn, Ni, and Nb.
The B value was found for the first time by the present inventors, and by reducing the transformation temperature of bainite, each content of Mn, Ni, and Nb effective for introducing high-density dislocations is expressed by the above formula ( 3) Parameters controlled to satisfy. By setting the B value to 2.37 or more, the maximum hardness of bainite can be ensured while suppressing an increase in Ceq. In order to increase the maximum hardness of bainite, the higher the B value, the better. In order to ensure high yield strength of API standard X80 class, the lower limit of the B value is 2.37 or more. The lower limit of the B value is preferably 2.39 or more. On the other hand, considering the upper limit of the content of each element, the upper limit of the B value is preferably 2.70 or less, more preferably 2.68 or less.

C: more than 0.04% and 0.10% or less C is an element indispensable for securing the high yield strength of the base material (steel plate), and therefore the lower limit of the C content is more than 0.04%. There is a need. The lower limit of the C amount is preferably 0.05% or more, more preferably 0.06% or more. However, when the amount of C is excessive, island martensite is likely to be generated, yield strength is lowered, and welding workability is lowered. Therefore, the upper limit of the amount of C needs to be 0.10% or less. is there. The upper limit of the C amount is preferably 0.09% or less, more preferably 0.08% or less.

Si: 0.15-0.50%
Si is an element that has a deoxidizing action and is effective for improving the yield strength of the base material. Therefore, the lower limit of the Si amount is set to 0.15% or more. The lower limit of the Si amount is preferably 0.18% or more, more preferably 0.20% or more. However, since the weldability and low temperature toughness of the HAZ deteriorate when the Si amount is excessive, the upper limit of the Si amount needs to be 0.50% or less. The upper limit of the Si amount is preferably 0.45% or less, more preferably 0.40% or less.

Mn: 1.20-2.50%
Mn is an element effective for improving the yield strength of the base material. For this reason, the lower limit of the amount of Mn needs to be 1.20% or more. The lower limit of the amount of Mn is preferably 1.50% or more, more preferably 1.70% or more. However, since the weldability deteriorates when the amount of Mn becomes excessive, the upper limit of the amount of Mn is made 2.50% or less. The upper limit of the amount of Mn is preferably 2.20% or less, and more preferably 2.00% or less.

P: more than 0% and 0.020% or less P is an element inevitably contained in the steel material, and when the P content exceeds 0.020%, the low temperature toughness of the HAZ is remarkably deteriorated. Therefore, the upper limit of the P amount is 0.020% or less. The upper limit of the amount of P is preferably 0.015% or less, more preferably 0.010% or less. Note that P is an impurity inevitably contained in the steel, and it is impossible to make the amount 0% in industrial production.

S: more than 0% to 0.0050% or less S is an element that affects the low temperature toughness of HAZ as in the case of P. When the amount of S exceeds 0.0050%, coarse sulfides are generated and HAZ Deteriorates low temperature toughness. Therefore, the upper limit of the amount of S is made 0.0050% or less. The upper limit of the amount of S is preferably 0.0030% or less, more preferably 0.0020% or less. In addition, S is an impurity inevitably contained in the steel, and it is impossible for industrial production to make the amount 0%.

Nb: 0.020 to 0.100%
Nb is an element effective for increasing the yield strength and the low temperature toughness of the base material without deteriorating the weldability, and therefore the lower limit of the Nb amount needs to be 0.020% or more. The lower limit of the Nb amount is preferably 0.030% or more, more preferably 0.040% or more. However, if the Nb amount becomes excessive and exceeds 0.100%, the low temperature toughness of the HAZ deteriorates, so the upper limit of the Nb amount is made 0.100% or less. The upper limit of the Nb amount is preferably 0.070% or less, more preferably 0.060% or less.

Ti: 0.003-0.020%
Ti is an element effective for improving the yield strength of the base metal, and further, it precipitates as TiN in the steel, and is necessary for improving the low temperature toughness of HAZ by suppressing coarsening of austenite grains in the HAZ during welding. It is an element. In order to exert such an effect, the lower limit of the Ti amount needs to be 0.003% or more. The lower limit of the Ti amount is preferably 0.005% or more, more preferably 0.007% or more. However, if the amount of Ti becomes excessive, solid solution Ti and TiC precipitates increase and the low temperature toughness of the HAZ deteriorates, so the upper limit of the amount of Ti is made 0.020% or less. The upper limit of the Ti amount is preferably 0.018% or less, more preferably 0.016% or less.

N: 0.0010 to 0.0075%
N is an element necessary for improving the low temperature toughness of HAZ by suppressing the coarsening of austenite grains in the HAZ during welding by precipitating as TiN in the steel. In order to exert such an effect, the lower limit of the N amount needs to be 0.0010% or more. The lower limit of the N amount is preferably 0.0020% or more, and more preferably 0.0030% or more. However, if the amount of N becomes excessive, the low temperature toughness of the HAZ deteriorates due to the presence of solute N, so the upper limit of the amount of N is made 0.0075% or less. The upper limit of the N amount is preferably 0.0070% or less, and more preferably 0.0065% or less.

Zr: 0.0001 to 0.0100%
Zr is an element that contributes to the improvement of low temperature toughness in HAZ by forming and dispersing an oxide, and for that purpose, the lower limit of the amount of Zr needs to be 0.0001% or more. The lower limit of the amount of Zr is preferably 0.0003% or more, more preferably 0.0005% or more. However, if the amount of Zr is excessive, coarse inclusions are formed and the low temperature toughness of the HAZ is deteriorated, so the upper limit of the amount of Zr needs to be 0.0100% or less. The upper limit of the amount of Zr is preferably 0.0050% or less, more preferably 0.0030% or less.

Ca: 0.0005 to 0.0030%
Ca has an action of controlling the form of sulfide, and is an element that suppresses the formation of MnS by forming CaS and improves the low temperature toughness of HAZ. For this reason, the lower limit of the Ca amount is 0.0005% or more. It is necessary to. The lower limit of the Ca content is preferably 0.0006% or more. However, if the Ca content exceeds 0.0030% and becomes excessive, the low temperature toughness of the HAZ deteriorates, so the upper limit of the Ca content is 0.0030% or less. The upper limit of the Ca content is preferably 0.0028% or less, and more preferably 0.0026% or less.

REM: 0.0001 to 0.0050%
REM, which is a rare earth element, is an element effective for controlling the morphology of sulfides, and suppresses the generation of MnS that is harmful to the low temperature toughness of HAZ. In order to exert such an effect, the lower limit of the REM amount is set to 0.0001% or more. The amount of REM is preferably 0.0003% or more, more preferably 0.0005% or more. However, even if a large amount of REM is contained, the effect is saturated, so the upper limit of the REM amount is 0.0050% or less. The upper limit of the REM amount is preferably 0.0040% or less, more preferably 0.0030% or less. In the present invention, REM means lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of Ce, La, and Nd, and it is more preferable to contain at least one of Ce and La.

Al: 0.010 to 0.050%
Al is a strong deoxidizing element, and in order to obtain a deoxidizing effect, the lower limit of the Al amount needs to be 0.010% or more. The lower limit of the Al content is preferably 0.015% or more, more preferably 0.018% or more. However, when the amount of Al becomes excessive, a large amount of AlN is generated, and the amount of TiN precipitation decreases, so that the low temperature toughness of HAZ is impaired. Therefore, the upper limit of the amount of Al needs to be 0.050% or less. The upper limit of the amount of Al is preferably 0.045% or less, and more preferably 0.042% or less.

B: 0.0003% or less (including 0%)
The amount of B is an element that significantly deteriorates the low-temperature toughness of HAZ. For this reason, the upper limit of the amount of B is set to 0.0003% or less. The upper limit of the amount of B is preferably 0.0002% or less, more preferably 0.0001% or less. When B is added in excess of 0.0003%, the combined addition of Mo causes an excessive increase in the yield strength of the base material.

Mo: more than 0% to 0.30% or less, Cu: more than 0% to 0.30% or less, Ni: more than 0% to 0.30% or less, Cr: more than 0% to 0.30% or less, and V: more than 0% One or more selected from the group consisting of 0.050% or less Mo, Cu, Ni, Cr, and V are effective elements for improving the yield strength. These elements may be added alone or in combination of two or more. The reasons for setting the ranges of the contents of these elements are as follows.

Mo: more than 0% and 0.30% or less Mo is an element effective for improving the yield strength of the base material, and therefore the lower limit of the Mo amount is preferably 0.01% or more. The lower limit of the amount of Mo is more preferably 0.05% or more, and more preferably 0.10% or more. However, if the Mo amount exceeds 0.30%, the low temperature toughness and welding workability in HAZ deteriorate, so the upper limit of the Mo amount is set to 0.30% or less. The upper limit of the Mo amount is preferably 0.25% or less, more preferably 0.20% or less.

Cu: more than 0% and 0.30% or less Cu is an element effective for increasing the yield strength, and therefore the lower limit of the Cu content is preferably 0.01% or more. The lower limit of the amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the amount of Cu becomes excessive, MA is likely to be generated, so the upper limit of the amount of Cu is made 0.30% or less. The upper limit of the amount of Cu is preferably 0.27% or less, more preferably 0.25% or less.

Ni: more than 0% and 0.30% or less Ni is an element effective for improving the yield strength of the base metal, and therefore the lower limit of the Ni content is preferably 0.01% or more. The lower limit of the Ni amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the amount of Ni becomes excessive, MA is easily generated. Furthermore, since it becomes very expensive as a structural steel material, the upper limit of the Ni amount is set to 0.30% or less from an economical viewpoint. The upper limit of the Ni content is preferably 0.27% or less, more preferably 0.25% or less.

Cr: more than 0% and 0.30% or less Cr is an element effective for improving the yield strength, and therefore the lower limit of the Cr content is preferably 0.01% or more. The lower limit of the Cr amount is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Cr content exceeds 0.30%, it becomes easy to produce MA, so the upper limit of the Cr content is 0.30% or less. The upper limit of the Cr amount is preferably 0.27% or less, more preferably 0.25% or less.

V: more than 0% and 0.050% or less V is an element effective for improving the yield strength. For this reason, the lower limit of the V amount is preferably 0.001% or more. The lower limit of the V amount is more preferably 0.002% or more, and still more preferably 0.003% or more. However, if the amount of V exceeds 0.050%, it becomes easy to produce MA, so the upper limit of the amount of V is made 0.050% or less. The upper limit of the V amount is preferably 0.030% or less, more preferably 0.010% or less.

The elements in steel used in the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that steel contains inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like. Examples of the inevitable impurities include As, Sb, Sn, O, and H.

Next, a method for manufacturing the steel sheet will be described.

The steel sheet of the present invention can be produced, for example, by producing a slab such as a slab, heating the obtained slab, performing hot rolling, and then performing accelerated cooling.

Hereinafter, each process will be described in detail.

First, in the casting process, in order to control the form of sulfide with REM and Ca, Al and Zr are added, Al ₂ O ₃ and ZrO are formed, deoxidation is performed, and then REM and Ca are added. It is preferable to do. In particular, Ca is an element that easily forms an oxide. In addition, Ca is more likely to form oxide (CaO) than sulfide (CaS), and it is preferable to control the time until completion of casting in order to prevent resulfurization from CaS. Therefore, in the molten steel treatment process, when adding Al, Zr, REM, and Ca in this order, it is preferable to produce a slab such that solidification is completed within 200 minutes from the addition of Ca. However, it is preferable to secure 4 minutes or more after adding REM until adding Ca having a higher sulfide forming ability than REM. By such a process, Ca and REM easily exist as sulfides without forming oxides.

After casting as described above, the slab is heated and hot rolled.

The heating temperature for heating the slab is preferably 1000 to 1200 ° C. If the heating temperature is too low, Nb in the steel does not sufficiently dissolve, and high yield strength cannot be ensured. Therefore, the lower limit of the heating temperature is more preferably 1100 ° C. or more, and further preferably 1120 ° C. or more. However, if the heating temperature is too high, the austenite grains become coarse and the low temperature toughness of the base material deteriorates, so the upper limit of the heating temperature is more preferably 1180 ° C. or less.

Next, hot rolling is performed. The hot rolling start temperature is preferably 900 to 1100 ° C. If the hot rolling start temperature is too low, rolling in the austenite recrystallization region cannot be ensured, austenite grains become coarse, and the low temperature toughness of the base material may deteriorate. Therefore, the lower limit of the hot rolling start temperature is more preferably 930 ° C or higher, and still more preferably 950 ° C or higher. On the other hand, if the hot rolling start temperature is too high, the austenite grains after recrystallization become coarse and the low temperature toughness of the base material may be deteriorated. Therefore, the upper limit of the hot rolling start temperature is more preferably 1090 ° C. or less, and still more preferably 1080 ° C. or less.

The rolling reduction from 950 ° C. to the hot rolling end temperature is preferably 40 to 80%. If the rolling reduction from 950 ° C. to the hot rolling finish temperature is too low, the strain introduced into the austenite grains cannot be secured, and the grains after bainite transformation become coarse, which may deteriorate the low temperature toughness of the base material. Therefore, the lower limit of the rolling reduction is more preferably 50% or more, and still more preferably 60% or more. On the other hand, if the rolling reduction from 950 ° C. to the hot rolling end temperature is too high, strain is excessively introduced into the austenite grains, and the hardenability is lowered. Therefore, the upper limit of the rolling reduction is more preferably 77% or less, and further preferably 75% or less.

The hot rolling end temperature is preferably 770 to 880 ° C. If the hot rolling end temperature is too low, strain is excessively introduced into the austenite grains, and the hardenability is lowered. Therefore, the lower limit of the hot rolling end temperature is more preferably 790 ° C. or higher, further preferably 800 ° C. or higher. On the other hand, if the hot rolling finish temperature is too high, the strain introduced into the austenite grains cannot be ensured, and the grains after the bainite transformation become coarse, which may deteriorate the low temperature toughness of the base material. Therefore, the upper limit of the hot rolling end temperature is more preferably 860 ° C. or less, and further preferably 850 ° C. or less.

Accelerated cooling is preferably performed as follows after the hot rolling is completed. It is not necessarily limited to this condition.

The cooling start temperature after the hot rolling is preferably 730 ° C. or higher. If the temperature is lower than 730 ° C., ferrite transformation is promoted and ferrite precipitates, so that the metal structure does not become bainite, and it may be difficult to ensure high yield strength of the base material. Therefore, the lower limit of the cooling start temperature is more preferably 735 ° C. or higher, and further preferably 740 ° C. or higher. Although the upper limit of cooling start temperature is not specifically limited, More preferably, it is 860 degrees C or less, More preferably, it is 850 degrees C or less.

Immediately after completion of hot rolling, accelerated cooling is preferably performed at an average cooling rate of preferably 10 to 50 ° C./second. By making the average cooling rate of accelerated cooling preferably 10 ° C./second or more, it is possible to prevent precipitation of ferrite by transforming untransformed austenite into a bainite structure, and further increase the maximum hardness of bainite. It is easy to improve the yield strength. Therefore, the lower limit of the average cooling rate is more preferably 13 ° C./second or more, and further preferably 15 ° C./second or more. On the other hand, at an average cooling rate exceeding 50 ° C./sec, martensitic transformation occurs near the surface of the steel sheet and the yield strength of the steel sheet increases, but the hardness of the steel sheet surface increases remarkably and the formability to the steel pipe deteriorates. Since it becomes easy, the upper limit of the average cooling rate is preferably 50 ° C./second or less. The upper limit of the average cooling rate is more preferably 45 ° C./second or less in consideration of the formability to the steel pipe.

The cooling stop temperature is preferably 370 to 550 ° C. By setting the cooling stop temperature to 370 to 550 ° C., the MA area ratio is reduced, and a high yield strength of 555 MPa or more is easily obtained. Therefore, the lower limit of the cooling stop temperature is more preferably 390 ° C. or higher, and further preferably 400 ° C. or higher. The upper limit of the cooling stop temperature is more preferably 540 ° C. or less, and further preferably 530 ° C. or less.

After cooling to 370 to 550 ° C., normal cooling such as cooling is performed to cool to room temperature, and the non-tempered steel sheet of the present invention is obtained. Specifically, the average cooling rate at this time is preferably about 0.1 to 5 ° C./second.

The plate thickness of the steel plate according to the present invention is not particularly limited, but in order to obtain a line pipe, the lower limit of the plate thickness is preferably 6 mm or more, more preferably 10 mm or more. On the other hand, the upper limit of the plate thickness is preferably 32 mm or less, more preferably 30 mm or less, from the viewpoint of securing a necessary cooling rate and suppressing the precipitation of ferrite.

The non-tempered steel sheet obtained as described above is particularly useful for line pipes. Moreover, the line pipe obtained by using the non-heat treated steel sheet of the present invention reflects the characteristics of the non-heat treated steel sheet, and has excellent low temperature toughness and hardness characteristics and yield strength of HAZ.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-237839 filed on Dec. 4, 2015. The entire contents of the specification of Japanese Patent Application No. 2015-237839 filed on December 4, 2015 are incorporated herein by reference.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and can be implemented with modifications within a range that can meet the purpose described above and below. They are all included in the technical scope of the present invention.

Steel grades A to X having the composition shown in Table 1 below (the balance being iron and inevitable impurities) were melted to form a slab, and then heated and hot-rolled under the conditions shown in Table 2 below. The steel sheet of thickness 20mm was manufactured by cooling on the conditions shown in.

Specifically, in this example, a 35Fe-30REM-35Si alloy containing 50% Ce and 20% La was used as the REM. In the molten steel treatment step, REM and Ca were added after deoxidation with Al and Zr. Moreover, REM and Ca were added in order of REM and Ca, and the time from REM addition to Ca addition was made into 4 minutes or more. Moreover, the slab was produced so that solidification might be completed within 200 minutes after Ca addition.

In addition, after cooling to the cooling stop temperature shown in Table 2, it was allowed to cool and cooled to room temperature. The average cooling rate at this time was approximately 1 ° C./second.

Measurement of area ratio of bainite A test piece of 20 mm × 15 mm × 15 mm was cut out from the steel plate, a cross section parallel to the rolling direction was polished, and nital corrosion was performed. Thereafter, the structure at 1/4 position of the plate thickness t was observed at 100 times using an optical microscope, and the area ratio of bainite was measured by image analysis when the entire metal structure was 100%. The measurement was performed for a total of 3 fields of view, and the average value was obtained. In this example, the same observation as that of bainite was performed for the remaining bainite and the remaining structure other than MA described later.

Measurement of area ratio of MA A test piece of 20 mm × 15 mm × 15 mm was cut out from the steel sheet, the cross section parallel to the rolling direction was polished, and subjected to repeller corrosion. Thereafter, the structure at the 1/4 position of the plate thickness t was observed at 1000 times using an optical microscope, and the area ratio of MA when the entire metal structure was 100% was measured by image analysis. The measurement was performed for a total of 3 fields of view, and the average value was obtained.

Measurement of the maximum hardness of bainite A test piece of 20 mm × 15 mm × 15 mm was cut out from the steel sheet to expose a cross section parallel to the rolling direction. Then, 20 points of the structure at 1/4 position of the plate thickness t were measured at equal intervals in a 100 μm × 100 μm range with a Vickers tester with a load of 5 gf (0.049 N). Among them, the average value of the top three points was the maximum hardness of bainite.

Measurement of Yield Strength From the steel sheet, a test piece was cut out based on the API5L standard so that the rolling direction and the vertical direction of the steel sheet were the length of the test piece, and 0.5% yield strength was measured as the yield strength. As the yield strength, an API standard X80 grade of 555 MPa to 705 MPa was regarded as acceptable.

Evaluation of low temperature toughness of weld heat affected zone (HAZ) A test piece of 12 mm × 32 mm × 55 mm was cut out from the steel plates 1 to 24 so that the rolling direction and the vertical direction of the steel plate were the length of the test piece, and used as a reproducible thermal cycle test piece. A thermal cycle with a maximum heating temperature of 1350 ° C. simulating a coarse-grained heat-affected zone in the vicinity of the melting line was applied to the reproduced thermal cycle test piece. Specifically, after heating to 1350 ° C. and holding for 5 seconds, the temperature range of 800 to 500 ° C. was cooled over 30 seconds. Then, the Charpy impact test was implemented by the method prescribed | regulated to API5L specification, and the low temperature toughness of HAZ was evaluated. As for the low temperature toughness of HAZ, a Charpy impact test was conducted at −10 ° C., and the absorbed energy passed 27 J or more.

Evaluation of hardness characteristics of weld heat affected zone (HAZ) Similar to the evaluation of low temperature toughness of weld heat affected zone, No. 2 in Table 2 above. Reproducible thermal cycle test specimens were collected from 1 to 24 steel plates and subjected to thermal cycling. A Vickers hardness test was performed to evaluate the hardness properties of the HAZ. The hardness of HAZ indicates the highest value of Vickers hardness when three points are measured at a load of 98N. Regarding the hardness characteristics of the HAZ, the hardness of the HAZ was determined to be less than 225 HV.

These results are shown in Table 2. Moreover, all the remaining structures other than bainite and MA were ferrite.

From these results, it can be considered as follows.

No. in Table 2. Nos. 17 to 24 are Nos. 1 and 2 in Table 2 that satisfy the preferable requirements defined in the present invention by using the steel types Q to X in Table 1 that satisfy the component composition defined in the present invention. This is an example manufactured under the manufacturing conditions of 17-24. These show that the low temperature toughness and hardness characteristics of HAZ are good, and a steel sheet having a high yield strength of 555 MPa or more is obtained.

In contrast, the following No. 1 to 16 do not satisfy any of the requirements defined in the present invention.

No. in Table 2. 1 is an example using the steel type A in Table 1 having a large Ceq, although the component composition of each element satisfies the requirements specified in the present invention, and the maximum hardness of the HAZ is high because of the large Ceq. As a result, the low temperature toughness of the HAZ was reduced.

No. in Table 2. 2 is an example using the steel type B of Table 1 with a large amount of B and a small A value and B value. Since the area ratio of bainite is low and the maximum hardness of bainite is low, the yield strength is low. Since the amount was large, the low temperature toughness of HAZ was lowered.

No. in Table 2. No. 3 is an example using the steel type C in Table 1 having a large B amount and Ti amount, and a small A value and B value, and the low temperature toughness of HAZ was lowered because the B amount and Ti amount were large. In addition, although A value and B value were small, since B exceeded 0.0003% and Mo was also compounded, the area ratio of bainite, the maximum hardness of bainite, and the yield strength increased.

No. in Table 2. No. 4 is an example using the steel type D of Table 1 having a small A value and B value, and the yield strength was low because the area ratio of bainite was low and the maximum hardness of bainite was low.

No. in Table 2. No. 5 is an example using the steel type E of Table 1 having a small A value. Since the area ratio of bainite was low, the yield strength was low.

No. in Table 2. 6 is an example using the steel type F of Table 1 with a small B value. Although the bainite has 80% by area or more, the yield strength is low because the maximum hardness of the bainite is low.

No. in Table 2. 7 is an example using the steel type G of Table 1 that does not include Mo, Cu, Ni, Cr, and V, and has a small A value and B value, and does not include Mo, Cu, Ni, Cr, and V. The yield strength was low because the area ratio of bainite was low and the maximum hardness of bainite was low.

No. in Table 2. No. 8 is an example using the steel type H of Table 1 with a small amount of C and a small B value. Since the amount of C was small and the maximum hardness of bainite was low, the yield strength was low.

No. in Table 2. No. 9 is an example using the steel type I of Table 1 with a small amount of Si and a small A value. Since the amount of Si was small and the area ratio of bainite was low, the yield strength was low.

No. in Table 2. 10 is an example using the steel type J of Table 1 with a small amount of Mn and a small A value and B value. Since the amount of Mn is small, the area ratio of bainite is low, and the maximum hardness of bainite is low, yield strength is low. Became lower.

No. in Table 2. 11 is an example using the steel type K of Table 1 with a small amount of Mn and Nb, and a small A value and B value. The amount of Mn and Nb is small, the area ratio of bainite is low, and the maximum hardness of bainite. , The yield strength was low.

No. in Table 2. No. 12 is an example using the steel type L in Table 1 having a small Nb amount, containing no Mo, Cu, Ni, Cr, and V, and having a small A value and B value. The Nb amount is small, and Mo, Cu, Since Ni, Cr, and V were not included, the area ratio of bainite was low, and the maximum hardness of bainite was low, so the yield strength was low.

No. in Table 2. No. 13 is an example using the steel type M of Table 1 with a large amount of Ni and a small A value and B value. MA was large, the area ratio of bainite was low, and the yield strength was low.

No. in Table 2. 14 is an example using the steel type N in Table 1 with a large amount of Cr and a small A value and B value. MA is large, the area ratio of bainite is low, the maximum hardness of bainite is low, and the yield strength is low. Became lower.

No. in Table 2. 15 is an example using the steel type O of Table 1 with a small amount of Mn, a large amount of Cr, and a small A value and B value. The amount of Mn is small, the amount of MA is large, the area ratio of bainite is low, The maximum hardness decreased and the yield strength decreased.

No. in Table 2. 16 is an example using the steel type P of Table 1 with a large amount of V and a small A value and B value. MA is large, the area ratio of bainite is low, the maximum hardness of bainite is low, and the yield strength is low. Became lower.

Claims

% By mass
C: more than 0.04% and 0.10% or less,
Si: 0.15 to 0.50%,
Mn: 1.20 to 2.50%,
P: more than 0% and 0.020% or less,
S: more than 0% and 0.0050% or less,
Nb: 0.020 to 0.100%,
Ti: 0.003 to 0.020%,
N: 0.0010 to 0.0075%,
Zr: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0030%,
REM: 0.0001 to 0.0050%,
Al: 0.010 to 0.050%, and B: 0.0003% or less (including 0%),
Furthermore, Mo: more than 0% but not more than 0.30%, Cu: more than 0% and not more than 0.30%, Ni: more than 0% and not more than 0.30%, Cr: more than 0% and not more than 0.30%, and V: 0 Including at least one selected from the group consisting of more than% and not more than 0.050%, the balance consisting of iron and inevitable impurities,
Ceq defined by the following formula (1) is less than 0.44,
The A value defined by the following formula (2) is 2.50 or more, and the B value defined by the following formula (3) is 2.37 or more,
The area ratio of the following metal structure at the 1/4 position of the plate thickness of the steel sheet,
Bainite: 80 area% or more, and island martensite: 0 area% or more and 0.26 area% or less,
A non-heat treated steel sheet having a high yield strength with suppressed low temperature toughness deterioration of the weld heat affected zone and hardness of the weld heat affected zone, wherein the bainite has a maximum hardness of 270 HV or higher.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
A value = 1.15 × Mn + 2.20 × Mo + 6.50 × Nb (2)
B value = 1.20 × Mn + 0.50 × Ni + 4.25 × Nb (3)
However, C, Mn, Cu, Ni, Cr, Mo, V, and Nb are mass%, respectively, and indicate the contents of C, Mn, Cu, Ni, Cr, Mo, V, and Nb.
The non-heat treated steel sheet according to claim 1, which is used for a line pipe.