KR20180085791A

KR20180085791A - A non-tempered steel sheet having a low-temperature toughness deterioration of the weld heat-affected zone and a high yield strength which suppresses the hardness of the weld heat affected zone

Info

Publication number: KR20180085791A
Application number: KR1020187018576A
Authority: KR
Inventors: 고지 구라타; 하루야 가와노; 기이치로 다시로; 모토키 가키자키
Original assignee: 가부시키가이샤 고베 세이코쇼
Priority date: 2015-12-04
Filing date: 2016-11-24
Publication date: 2018-07-27
Also published as: EP3385399A4; EP3385399A1; WO2017094593A1; JP2017106107A; CN108350540A

Abstract

There is provided a non-tempered steel sheet having a low temperature toughness deterioration of a weld heat affected zone and a high yield strength by suppressing the hardness of a weld heat affected zone. Ceq of less than 0.44, an A value defined by the following formula (2) is not less than 2.50, and a B value defined by the following formula (3) is not less than 2.37, , The area ratio of the following metal structure at a 1/4 position of the plate thickness of the steel sheet satisfies bainite: 80% or more area, and isotactic martensite: 0% or more and 0.26% or less area% Characterized in that the maximum hardness of the knead is not less than 270 HV, and the low heat resistance of the weld heat affected zone and the high hardness of the weld heat affected zone are suppressed.
Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V /
A value = 1.15 x Mn + 2.20 x Mo + 6.50 x Nb (2)
B value = 1.20 x Mn + 0.50 x Ni + 4.25 x Nb (3)

Description

A non-tempered steel sheet having a low-temperature toughness deterioration of the weld heat-affected zone and a high yield strength which suppresses the hardness of the weld heat affected zone

The present invention relates to a non-corrugated steel sheet having a low heat resistance at a low temperature and a high hardness of a weld heat affected zone. More particularly, the present invention relates to a non-corrugated steel sheet having a high yield strength of API X80 grade used in a line pipe for transportation of petroleum, natural gas and the like.

BACKGROUND ART [0002] In the case of a line pipe for transporting natural gas or crude oil over a long distance, there is a growing demand for restricting the increase in thickness by increasing the strength of the pipe material itself with the aim of reducing the cost of laying down and transportation. Currently, the American Petroleum Institute (API) has standardized X80 grade steels as high yield strength steels and has been put to practical use.

Steel plates used as the line pipe as described above are required to have high toughness, short construction time, and low cost in addition to high yield strength. Control rolling is a manufacturing method for satisfying these requirements. Controlled rolling is a technique of finely grinding the crystal grains by appropriately controlling the temperature or rolling reduction rate at the time of rolling, and performing accelerated cooling after hot rolling. In the controlled rolling, tempering such as heating after accelerated cooling is unnecessary. A steel sheet obtained by such a method is generally called a non-corrugated steel sheet.

Various techniques have been developed for the non-durable high yield strength steel sheet. For example, Patent Documents 1 to 4 disclose a method of producing a steel sheet having a high yield strength of API standard X80 grade by non-tempering.

Japanese Patent Application Laid-Open No. 2006-328523 International Publication No. 2010/052927 pamphlet Japanese Patent Application Laid-Open No. 2006-169591 Japanese Patent Application Laid-Open No. 2008-261012

However, since the line pipe is often attached to cold regions, it is essential that the low temperature toughness of the heat affected zone (HAZ) is excellent. In addition, from the viewpoint of welding workability, it has been strongly desired to suppress the hardness of the weld heat affected zone in recent years.

However, since the steel sheets described in Patent Documents 1 and 2 do not control Ceq, which is an index for evaluating the toughness and hardness of the weld heat affected zone, to be low, there is a fear of toughness deterioration of the weld heat affected zone and curing of the weld heat affected zone have.

In the methods described in Patent Documents 3 and 4, since a large amount of B, which is a component that deteriorates the low temperature toughness of the weld heat affected zone, is added, there is a risk of deteriorating the low temperature toughness of the weld heat affected zone.

An object of the present invention is to provide a non-tempered steel sheet having a low heat resistance at the weld heat affected zone and a high hardness of the weld heat affected zone.

A non-tempering steel sheet having a low heat resistance at a low temperature and a high heat resistance at a welded heat affected zone of a weld heat affected zone according to the present invention which can solve the above problems is characterized by containing C: 0.04% to 0.10% 0.10 to 0.50% Mn, 1.20 to 2.50%, P: more than 0 to 0.020%, S: more than 0 to 0.0050%, Nb: 0.020 to 0.100%, Ti: 0.003 to 0.020% %, 0.0001 to 0.0100% of Zr, 0.0005 to 0.0030% of Ca, 0.0001 to 0.0050% of REM, 0.010 to 0.050% of Al, and 0.0003% or less of B (inclusive of 0% Mo: more than 0% to not more than 0.30%, Cu: not more than 0% to not more than 0.30%, Ni: more than 0% to not more than 0.30%, Cr: more than 0% to not more than 0.30%, and V: And Ceq of less than 0.44, an A value defined by the following formula (2) is not less than 2.50, and at least one of the following formulas (1) and (2), and the balance thereof is made of iron and inevitable impurities, 3) and the B value is 2.37 or more The area ratio of the following metal structure at a 1/4 position of the plate thickness of the steel sheet satisfies a bainite content of 80% or more and an isotactic martensite content of 0% or more and 0.26% or less by area, The maximum hardness of 270 HV or more is satisfied.

Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V /

A value = 1.15 x Mn + 2.20 x Mo + 6.50 x Nb (2)

B value = 1.20 x Mn + 0.50 x Ni + 4.25 x Nb (3)

The content of C, Mn, Cu, Ni, Cr, Mo, V, and Nb in terms of mass% is represented by C, Mn, Cu, Ni, Cr, Mo, V and Nb.

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

According to the present invention, it is possible to obtain a non-tempered steel sheet having a high yield strength by suppressing the low temperature toughness deterioration of the weld heat affected zone and the hardness of the weld heat affected zone.

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

First, the inventors of the present invention studied factors that govern the yield strength of the non-coarse steel sheet. As a result, the yield strength of the non-coarse steel sheet is closely correlated with the area ratio of bainite and stalactite martensite in the metal structure and the maximum hardness of bainite. By controlling them to a predetermined range, the API standard X80 High yield strength is obtained.

The inventors of the present invention have also studied various angles in order to realize a non-tempered steel sheet having a high yield strength and suppressing low temperature toughness deterioration of the weld heat affected zone and hardness of the weld heat affected zone. As a result, it has been found that the deterioration of the low temperature toughness of the weld heat affected zone can be suppressed and the hardness of the weld heat affected zone can be reduced by controlling the chemical composition so as to satisfy the relations of the above formulas (1) to (3) Thereby completing the invention.

Preferably, such a non-tempering steel sheet is obtained by heating and hot-rolling a steel material satisfying a predetermined component composition and then cooling it from a cooling start temperature of 730 DEG C or more to a cooling stop temperature of 370 to 550 DEG C, To < RTI ID = 0.0 > 50 C / sec. &Lt; / RTI >

In the present specification, " high yield strength of API standard X80 grade " means that the yield strength of the steel sheet in the sheet width direction is 555 MPa or more and 705 MPa or less.

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

The non-corrugated steel sheet according to the present invention is characterized in that, at a position 1/4 of the plate thickness t of the steel sheet, each structure of the metal structure is composed of at least 80% by area of bainite, %, And maximum hardness of bainite: 270 HV or more.

Bainite: over 80% area

Bainite is an organization contributing to yield strength and is an important organization for ensuring high yield strength of API X80 class. When the bainite is less than 80% by area, the yield strength is lowered. Therefore, the lower limit of the area ratio of bainite when the area of the entire metal structure is taken as 100% is set to 80% or more. The lower limit of the area ratio of bainite is preferably at least 82% by area, more preferably at least 84% by area.

Fig. 1 is a graph showing the results obtained by using the steel types A to X shown in Table 1 of the later embodiment. 1 to 24 are graphs showing the relationship between the area ratio of bainite and the yield strength in the non-tempering steel sheet produced by the production process. As shown in Fig. 1, all of the examples satisfying the desired high yield strength of 555 MPa or more have a bainite area ratio of at least 80% in the metal structure. To satisfy the high yield strength, the bainite area ratio is increased to 80% Is valid. 1 shows an example in which the yield strength does not satisfy the yield strength of 555 MPa or more even though the bainite area ratio is 80% or more. However, these are examples in which the later bainite hardness is less than 270 HV or the MA area ratio exceeds 0.26%.

Maximum hardness of bainite: 270HV or more

The maximum hardness of the bainite is important in order to stably suppress the gap of the yield strength to obtain a high yield strength, and it is necessary to control the bainite to 270 HV or more. As a result, the high yield strength of the API standard X80 class can be stably secured. The lower limit of the maximum hardness of bainite is preferably 275 HV or more. However, considering the formability into a steel pipe, the upper limit of the maximum hardness of bainite is preferably 310 HV or less, and more preferably 300 HV or less.

Fig. 2 is a graph showing the results of evaluation of the steel No. 1 to Table No. 2 shown in Table 2, using the steel types A to X of Table 1 of the later embodiment. 12 is a graph showing the relationship between the maximum hardness and the yield strength of bainite in the non-corrugated steel sheet produced by the manufacturing conditions 1 to 24; As shown in Fig. 2, all the examples satisfying the desired 555 MPa or more are all 270 or higher in bainite hardness in the metal structure, and it is effective to increase the maximum bainite hardness to 270 HV or more in order to satisfy the high yield strength . Here, even though the maximum hardness of bainite is 270 HV or more, the yield strength does not satisfy 555 MPa or more. However, these are examples where the bainite area ratio is less than 80% or the MA area ratio is more than 0.26%.

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

Scarlet martensite: 0 Area% or more 0.26 Area% or less

Since the island martensite is a structure which lowers the yield strength, it is necessary to reduce the area ratio of the MA in order to secure a desired high yield strength. Therefore, the upper limit of the MA area ratio when the area of the entire metal structure is taken as 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 is a graph showing the results obtained by using the values A to X in Table 1 of the later-described embodiments. 1 to 24 are graphs showing the relationship between the area ratio of the island martensite and the yield strength in the non-corrugated steel sheet. As shown in Fig. 3, in all the examples satisfying 555 MPa or more, the MA area ratio in the metal structure is 0.26% or less, and it is effective to control the MA area ratio to 0.26% or less in order to satisfy the high yield strength . Here, even though the MA area ratio is 0.26% or less, the yield strength does not satisfy 555 MPa or more. However, these are examples where the bainite area ratio is less than 80% or the maximum bainite hardness is less than 270 HV.

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

Next, the components in the steel will be described.

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

Ceq: less than 0.44

Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V /

Note that C, Mn, Cu, Ni, Cr, Mo and V represent the contents of C, Mn, Cu, Ni, Cr, Mo and V in mass%, respectively.

Ceq as 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 the HAZ and the hardness characteristics of the HAZ sharply deteriorate. When Ceq is less than 0.44, the low temperature toughness of a good HAZ and the hardness of the HAZ can be secured. Therefore, the upper limit of Ceq is set to be 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, and more preferably 0.38 or more.

A value: 2.50 or higher

A value = 1.15 x Mn + 2.20 x Mo + 6.50 x Nb (2)

Note that Mn, Mo, and Nb represent the contents of Mn, Mo, and Nb in mass%, respectively.

A value was first discovered by the inventors of the present invention. It was confirmed that the content of Mn and Mo, which is effective for suppressing ferrite transformation, among the elements constituting Ceq, and the content of Nb satisfy the above formula (2) Controlled parameter. By setting the value of A to 2.50 or more, it is possible to secure the bainite area ratio which is important for realizing high yield strength while suppressing the rise of Ceq. In order to increase the bainite area ratio, the higher the A value is, the better, and in order to secure the high yield strength of the API standard X80, the lower limit of the A value is set to 2.50 or more. The lower limit of the A value is preferably 2.52 or more, and more preferably 2.54 or more. On the other hand, in consideration of 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 higher

B value = 1.20 x Mn + 0.50 x Ni + 4.25 x Nb (3)

Note that Mn, Ni, and Nb represent the contents of Mn, Ni, and Nb in mass%, respectively.

The B value was first discovered by the inventors of the present invention. By lowering the transformation temperature of bainite, the respective contents of Mn, Ni, and Nb, which are effective for introducing a high-density dislocation, One parameter. By setting the B value to 2.37 or more, the maximum hardness of bainite can be secured while suppressing the rise of Ceq. In order to increase the maximum hardness of the bainite, the higher the B value, the better, and in order to secure the high yield strength of the API standard X80, the lower limit of the B value is set to 2.37 or more. The lower limit of the B value is preferably 2.39 or more. On the other hand, in consideration of 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: not more than 0.04% and not more than 0.10%

C is an indispensable element for securing the high yield strength of the base material (steel plate), and therefore, the lower limit of the C content needs to be more than 0.04%. The lower limit of the amount of C is preferably 0.05% or more, and more preferably 0.06% or more. However, if the amount of C is excessive, it becomes easy to form stalactite martensite, and the yield strength is lowered and the workability of the weld is lowered. Therefore, the upper limit of the C content is required to be 0.10% or less. The upper limit of the amount of C is preferably 0.09% or less, more preferably 0.08% or less.

Si: 0.15 to 0.50%

Si has a deoxidizing action and is an effective element for improving the yield strength of the base material, and therefore the lower limit of the amount of Si is 0.15% or more. The lower limit of the Si content is preferably 0.18% or more, and more preferably 0.20% or more. However, when the amount of Si is excessive, the weldability and the low temperature toughness of the HAZ are deteriorated, so the upper limit of the Si content is required to be 0.50% or less. The upper limit of the Si content is preferably 0.45% or less, and more preferably 0.40% or less.

Mn: 1.20 to 2.50%

Mn is an element effective for improving the yield strength of the base material, and therefore it is necessary to set the lower limit of the Mn amount to 1.20% or more. The lower limit of the Mn content is preferably 1.50% or more, and more preferably 1.70% or more. However, when the amount of Mn is excessive, the workability of the weld is deteriorated, so the upper limit of the amount of Mn is set to 2.50% or less. The upper limit of the Mn content is preferably 2.20% or less, and more preferably 2.00% or less.

P: more than 0% and less than 0.020%

P is an element inevitably included in the steel, and when the amount of P exceeds 0.020%, the low temperature toughness of the HAZ is remarkably deteriorated. Therefore, the upper limit of P amount is set to 0.020% or less. The upper limit of the P content is preferably 0.015% or less, and more preferably 0.010% or less. On the other hand, P is an impurity inevitably contained in the steel, and the amount of P is 0%, which is not possible in industrial production.

S: more than 0% and less than 0.0050%

S, like P, is an element that affects the low temperature toughness of HAZ. If S is more than 0.0050%, coarse sulfide is formed and low temperature toughness of HAZ is deteriorated. Therefore, the upper limit of the amount of S is set to 0.0050% or less. The upper limit of the amount of S is preferably 0.0030% or less, and more preferably 0.0020% or less. On the other hand, S is an impurity inevitably contained in the steel, and the amount of S is 0%, which is not possible in industrial production.

Nb: 0.020 to 0.100%

Nb is an effective element 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 content is required to be 0.020% or more. The lower limit of the amount of Nb is preferably 0.030% or more, and more preferably 0.040% or more. However, if the amount of Nb becomes excessive and exceeds 0.100%, the low-temperature toughness of the HAZ will deteriorate, so the upper limit of the amount of Nb is 0.100% or less. The upper limit of the amount of Nb is preferably 0.070% or less, and more preferably 0.060% or less.

Ti: 0.003 to 0.020%

Ti is an element effective for improving the yield strength of the base material and is an element necessary for improving the low temperature toughness of HAZ by suppressing the coarsening of the austenitic grains in the HAZ by precipitation as TiN in the steel. In order to exhibit such an effect, it is necessary to set the lower limit of Ti amount to 0.003% or more. The lower limit of the amount of Ti is preferably 0.005% or more, and more preferably 0.007% or more. However, when the amount of Ti becomes excessive, the amount of dissolved Ti or TiC precipitate increases and the low temperature toughness of the HAZ deteriorates. Therefore, the upper limit of the amount of Ti is set to 0.020% or less. The upper limit of the amount of Ti is preferably 0.018% or less, and more preferably 0.016% or less.

N: 0.0010 to 0.0075%

N is precipitated as TiN in the steel and is an element necessary for improvement of the low temperature toughness of HAZ by suppressing coarsening of austenite grains in HAZ at the time of welding. In order to exhibit such an effect, it is necessary to set the lower limit of the amount of N to 0.0010% or more. The lower limit of the amount of N 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 is deteriorated by the presence of solid solution N. Therefore, the upper limit of the amount of N is set to 0.0075% or less. The upper limit of the amount of N is preferably 0.0070% or less, and more preferably 0.0065% or less.

Zr: 0.0001 to 0.0100%

Zr is an element contributing to the improvement of low-temperature toughness in HAZ by forming and dispersing an oxide, and therefore the lower limit of the amount of Zr should be 0.0001% or more. The lower limit of the amount of Zr is preferably 0.0003% or more, and more preferably 0.0005% or more. However, when the amount of Zr is excessive, coarse inclusions are formed to deteriorate the low-temperature toughness of the HAZ. Therefore, the upper limit of the amount of Zr must 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 effect of controlling the form of sulfide and is an element which inhibits the formation of MnS by forming CaS and improves the low-temperature toughness of HAZ. Therefore, it is necessary to set the lower limit of Ca amount to 0.0005% or more . The lower limit of the amount of Ca is preferably 0.0006% or more. However, if the amount of Ca exceeds 0.0030% and becomes excessive, the low-temperature toughness of the HAZ deteriorates, so the upper limit of the amount of Ca is 0.0030% or less. The upper limit of the amount of Ca is preferably 0.0028% or less, and more preferably 0.0026% or less.

REM: 0.0001-0.0050%

REM, which is a rare earth element, is an effective element for controlling the shape of sulfides and inhibits the formation of MnS which is harmful to the low temperature toughness of HAZ. To achieve this effect, the lower limit of the amount of REM is set to 0.0001% or more. The amount of REM is preferably 0.0003% or more, and more preferably 0.0005% or more. However, even when a large amount of REM is contained, the effect becomes saturated, so the upper limit of the amount of REM is 0.0050% or less. The upper limit of the amount of REM is preferably 0.0040% or less, more preferably 0.0030% or less. In the present invention, REM means a lanthanoid element (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 more preferably contain at least one of Ce and La.

Al: 0.010 to 0.050%

Al is a strong acid element, and the lower limit of the amount of Al needs to be 0.010% or more in order to obtain a deoxidizing effect. The lower limit of the amount of Al is preferably 0.015% or more, and 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 the 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 B is an element that significantly deteriorates the low-temperature toughness of the HAZ. Therefore, the upper limit of the amount of B is 0.0003% or less. The upper limit of the amount of B is preferably 0.0002% or less, more preferably 0.0001% or less. On the other hand, in the case where B is added in excess of 0.0003%, the addition of Mo in combination causes an excessive increase in the yield strength of the base metal.

Mo: more than 0% to not more than 0.30%, Cu: not more than 0% to not more than 0.30%, Ni: more than 0% to not more than 0.30%, Cr: more than 0% to not more than 0.30%, and V: More than one kind

Mo, Cu, Ni, Cr, and V are effective elements for improving the yield strength. These elements may be added alone, or two or more of them may be used in combination. The reason for setting the range of the contents of these elements is as follows.

Mo: more than 0% and not more than 0.30%

Mo is an element effective for improving the yield strength of the base material, and therefore the lower limit of the amount of Mo 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, when the amount of Mo exceeds 0.30%, the low temperature toughness and weldability in the HAZ deteriorate, so the upper limit of the amount of Mo is set to 0.30% or less. The upper limit of the amount of Mo is preferably 0.25% or less, more preferably 0.20% or less.

Cu: more than 0% and not more than 0.30%

Cu is an effective element for increasing the yield strength, and therefore the lower limit of the amount of Cu 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, when the amount of Cu becomes excessive, MA tends to be generated, so the upper limit of the amount of Cu is set to 0.30% or less. The upper limit of the amount of Cu is preferably 0.27% or less, and more preferably 0.25% or less.

Ni: more than 0% and not more than 0.30%

Ni is an effective element for improving the yield strength of the base material, and therefore the lower limit of the amount of Ni is preferably 0.01% or more. The lower limit of the amount of Ni is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the amount of Ni becomes excessive, MA tends to be generated. In addition, since it becomes extremely expensive as a structural steel, the upper limit of the amount of Ni is 0.30% or less from the viewpoint of economy. The upper limit of the amount of Ni is preferably 0.27% or less, more preferably 0.25% or less.

Cr: more than 0% and not more than 0.30%

Cr is an effective element for improving the yield strength, and therefore the lower limit of the amount of Cr is preferably 0.01% or more. The lower limit of the amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the amount of Cr exceeds 0.30%, MA tends to be generated, so the upper limit of the amount of Cr is set to 0.30% or less. The upper limit of the amount of Cr is preferably 0.27% or less, more preferably 0.25% or less.

V: more than 0% and not more than 0.050%

V is an element effective for improving the yield strength, and therefore the lower limit of the V content is preferably 0.001% or more. The lower limit of the amount of V is more preferably 0.002% or more, and still more preferably 0.003% or more. However, when the amount of V exceeds 0.050%, MA tends to be generated, so the upper limit of the amount of V is set to 0.050% or less. The upper limit of the amount of V is preferably 0.030% or less, more preferably 0.010% or less.

The elements in the steel used in the present invention are as described above, and the remainder is substantially iron. However, it is a matter of course that inevitable impurities contained in the steel are included according to the conditions of raw materials, materials, manufacturing facilities, and the like. Examples of the inevitable impurities include As, Sb, Sn, O, H, and the like.

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

The steel sheet of the present invention can be produced, for example, by preparing a cast slab or the like, heating the obtained cast steel, and performing hot rolling followed by accelerated cooling.

Each step will be described in detail below.

First, in order to control the shape of the sulfide with REM and Ca in the casting step, it is preferable to add REM and Ca after adding Al and Zr to form Al ₂ O ₃ and ZrO and performing deoxidation. Particularly, Ca is an element that tends to form oxides. Further, Ca is more likely to form oxides (CaO) than sulfides (CaS), and it is preferable to control the time until completion of casting in order to prevent sulfuration from CaS. Therefore, in the molten steel treatment step, it is preferable to prepare a cast steel such that Al, Zr, REM and Ca are added in this order, so that solidification is completed within 200 minutes after Ca addition. However, it is preferable that the time from the addition of REM to the addition of Ca, which has higher sulfide forming ability than that of REM, is ensured for 4 minutes or more. By such a process, Ca and REM are likely to exist as a sulfide without forming an oxide.

After the casting is performed as described above, the cast steel is heated and hot-rolled.

The heating temperature at the time of heating the cast steel is preferably 1000 to 1200 占 폚. If the heating temperature is too low, the Nb in the steel is not sufficiently dissolved and the high yield strength can not be ensured. Therefore, the lower limit of the heating temperature is more preferably 1100 占 폚 or higher, and more preferably 1120 占 폚 or higher. However, if the heating temperature is excessively high, the austenite grains become coarse and the low-temperature toughness of the base material deteriorates. Therefore, the upper limit of the heating temperature is more preferably 1180 占 폚 or lower.

Next, hot rolling is performed. The hot rolling starting temperature is preferably 900 to 1100 캜. If the hot-rolling start temperature is too low, rolling in the austenite recrystallization zone can not be ensured, the austenite grains become large 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 DEG C or higher, and more preferably 950 DEG C or higher. On the other hand, if the hot rolling starting temperature is too high, the austenite grains after recrystallization become too large, and the low temperature toughness of the base material may be deteriorated. Therefore, the upper limit of the hot rolling starting temperature is more preferably 1090 占 폚 or lower, and more preferably 1080 占 폚 or lower.

The rolling reduction from 950 ° C to the hot rolling end temperature is preferably 40 to 80%. If the reduction rate from 950 ° C to the hot rolling end temperature is too low, the deformation to be introduced into the austenite lips can not be ensured, so that the lips after bainite transformation become coarse and the low temperature toughness of the base material may deteriorate. Therefore, the lower limit of the reduction ratio is more preferably 50% or more, and more preferably 60% or more. On the other hand, if the reduction rate from 950 ° C to the hot rolling end temperature is excessively high, the deformation introduction into the austenite lips becomes excessive, and the hardenability is deteriorated. Therefore, the upper limit of the reduction rate is more preferably 77% or less, and still more preferably 75% or less.

The hot rolling end temperature is preferably 770 to 880 캜. If the hot rolling end temperature is too low, the deformation introduction into the austenite lips becomes excessive and the hardenability is deteriorated. Therefore, the lower limit of the hot rolling end temperature is more preferably 790 占 폚 or higher, and more preferably 800 占 폚 or higher. On the other hand, if the hot rolling end temperature is too high, the deformation to be introduced into the austenite lips can not be ensured, so that the lips after the bainite transformation become coarse and the low temperature toughness of the base material may deteriorate. Therefore, the upper limit of the hot rolling finishing temperature is more preferably 860 占 폚 or lower, and more preferably 850 占 폚 or lower.

After completion of hot rolling, it is preferable to perform accelerated cooling in the following manner. On the other hand, it is not necessarily limited to this condition.

The cooling start temperature after completion of the hot rolling is preferably 730 캜 or higher. If the temperature is lower than 730 占 폚, ferrite transformation is promoted and ferrite is precipitated, so that the metal structure does not become bainite, so that it may become difficult to secure the high yield strength of the base metal. Therefore, the lower limit of the cooling start temperature is more preferably 735 占 폚 or higher, and more preferably 740 占 폚 or higher. The upper limit of the cooling start temperature is not particularly limited, but is more preferably 860 占 폚 or lower, and still more preferably 850 占 폚 or lower.

After the completion of hot rolling, accelerated cooling is immediately carried out at an average cooling rate of preferably 10 to 50 占 폚 / sec. By setting the average cooling rate of the accelerated cooling to not less than 10 占 폚 / sec preferably, the untransformed austenite is transformed into the bainite structure to prevent the precipitation of the ferrite, and the maximum hardness of the bainite is increased, . Therefore, the lower limit of the average cooling rate is more preferably not less than 13 占 폚 / second, and more preferably not less than 15 占 폚 / second. On the other hand, at an average cooling rate exceeding 50 deg. C / second, 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 remarkably increases, Therefore, the upper limit of the average cooling rate is preferably 50 DEG C / sec or less. The upper limit of the average cooling rate is more preferably 45 占 폚 / second or less in consideration of the formability into the steel pipe.

The cooling stop temperature is preferably 370 to 550 占 폚. By setting the cooling stop temperature at 370 to 550 캜, 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 DEG C or higher, and more preferably 400 DEG C or higher. The upper limit of the cooling stop temperature is more preferably 540 占 폚 or lower, and still more preferably 530 占 폚 or lower.

After cooling to 370 to 550 캜, ordinary cooling such as cold cooling is carried out and cooling to room temperature results in the non-cored steel sheet of the present invention. Concretely, it is preferable that the average cooling rate at this time is approximately 0.1 to 5 占 폚 / second.

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

The non-corrugated steel sheet obtained as described above is particularly useful for a line pipe. Further, the line pipe obtained by using the non-corrugated steel sheet of the present invention reflects the characteristics of the non-corrugated steel sheet, so that the low temperature toughness, hardness characteristics, and yield strength of the HAZ are excellent.

This application claims the benefit of priority based on Japanese Patent Application No. 2015-237839 filed on December 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.

Example

The present invention will be described in more detail with reference to the following examples, but it should be understood that the present invention is not limited to the following examples, And are included in the technical scope of the present invention.

The steel materials A to X having the composition shown in the following Table 1 (the remainder being iron and inevitable impurities) were made into a slab by melting and then heated and hot-rolled under the conditions shown in Table 2, To prepare a steel sheet having a thickness of 20 mm.

Specifically, in this embodiment, a 35 Fe-30REM-35Si alloy containing 50% of Ce and 20% of La was used as the REM. Further, in the molten steel treatment process, REM and Ca were added after deoxidation with Al and Zr. REM and Ca were added in the order of REM and Ca, and the time from addition of REM to addition of Ca was set to 4 minutes or more. In addition, a casting was made to complete solidification within 200 minutes after Ca addition.

After the cooling to the cooling stop temperature shown in Table 2 was performed, the cooling was carried out and the temperature was cooled to room temperature. The average cooling rate at this time was approximately 1 占 폚 / second.

Measurement of area ratio of bainite

A specimen of 20 mm x 15 mm x 15 mm was cut out of the steel sheet, and a cross section parallel to the rolling direction was polished to carry out releasing corrosion. Thereafter, the texture at 1/4 of the plate thickness t was observed at 100 times using an optical microscope, and the area ratio of bainite when the entire metal structure was 100% was determined by image analysis . Measurements were made for a total of three visual fields, and the average value was obtained. In this embodiment, the same observation as that of bainite was also performed on the remaining bainite and the residual structure other than the MA.

Measurement of area ratio of MA

A specimen of 20 mm x 15 mm x 15 mm was cut out of the steel sheet, and a cross section parallel to the rolling direction was polished to perform a repeller erosion. Thereafter, the texture at 1/4 of the plate thickness t was observed at 1,000 magnifications using an optical microscope, and the area ratio of MA when the entire metal structure was taken as 100% by image analysis was measured. Measurements were made for a total of three visual fields, and the average value was obtained.

Measurement of maximum hardness of bainite

A specimen of 20 mm x 15 mm x 15 mm was cut out from the steel sheet, and a cross section parallel to the rolling direction was exposed. Thereafter, the structure at 1/4 of the plate thickness t was measured with a Vickers tester having a load of 5 gf (0.049 N) at 20 points at regular intervals within a range of 100 m x 100 m. Among them, the average value of the upper three points was determined as the maximum hardness of the bainite.

Measurement of yield strength

A test piece was cut from the steel sheet on the basis of the API5L standard so that the rolling direction and the perpendicular direction of the steel sheet were the long side of the test piece, and the yield strength was measured as 0.5%. The yield strength was 555 MPa or more and 705 MPa or less, which is API standard X80 grade.

Evaluation of low temperature toughness of weld heat affected zone (HAZ)

No. of Table 2 above. Test specimens of 12 mm x 32 mm x 55 mm were cut out from the steel sheets of 1 to 24 so that the rolling direction and the vertical direction of the steel sheets were the longer sides of the test specimens. The reproduced heat cycle test piece was subjected to a heat cycle at a maximum heating temperature of 1350 DEG C simulating the assembled (coarse) heat affected zone in the vicinity of the molten wire. Specifically, after heating at 1350 캜 for 5 seconds, the temperature range of 800 - 500 캜 was cooled for 30 seconds. Thereafter, the Charpy impact test was carried out by the method specified in the API5L standard to evaluate the low temperature toughness of the HAZ. The low-temperature toughness of the HAZ was determined by carrying out the Charpy impact test at -10 ° C to pass the absorption energy of 27 J or more.

Evaluation of hardness characteristics of weld heat affected zone (HAZ)

Similarly to the evaluation of the low temperature toughness of the weld heat affected zone, From the 1 to 24 steel sheets, the reproducible heat cycle test specimens were collected and given a heat cycle. Vickers hardness test was conducted to evaluate the hardness characteristics of the HAZ. The hardness of the HAZ represents the highest value of Vickers hardness when measured at 3 points at a load of 98N. The hardness characteristics of the HAZ were determined so that the hardness of the HAZ was less than 225 HV.

These results are shown in Table 2. In addition, the rest of the structure except bainite and MA was ferrite.

From these results, it can be considered as follows.

No. of Table 2 17 to 24 are graphs showing the results obtained by using the steel grades Q to X in Table 1 satisfying the component composition specified in the present invention. 17-24. &Lt; / RTI > It can be seen from these that a steel sheet having good low temperature toughness and hardness characteristics of HAZ and high yield strength of 555 MPa or more is obtained.

On the other hand, 1 to 16 do not satisfy any of the requirements specified in the present invention.

No. of Table 2 1 shows an example in which the composition of each element satisfies the requirements specified in the present invention. However, since Ceq is large, the maximum hardness of the HAZ is increased, and accordingly, the HAZ The low-temperature toughness deteriorated.

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

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

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

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

No. of Table 2 6 is an example using the steel type F of Table 1 having a small B value and has a bendite content of 80% or more by area but has a low maximum hardness of bainite, so that the yield strength is low.

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

No. of Table 2 8 is an example using the steel type H in Table 1 having a small amount of C and a small value of B and a low C amount and a low maximum hardness of bainite.

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

No. of Table 2 10 is an example using the steel grade J in Table 1 having a small amount of Mn and a small A value and a low B value and has a low yield strength because of low Mn amount, low bainite area ratio and low maximum hardness of bainite.

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

No. of Table 2 12 is an example using the steel type L in Table 1 having a small amount of Nb and no Mo, Cu, Ni, Cr, and V and having a small A value and a small B value. , And V, the area ratio of bainite was low, and the maximum hardness of bainite was low, so that the yield strength was low.

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

No. of Table 2 14 is an example using the steel N of Table 1 having a large amount of Cr and a small A value and a small B value. It has a high MA, a low area ratio of bainite, a low maximum hardness of bainite, and a low yield strength.

No. of Table 2 15 is an example using the steel grade O of Table 1 in which the amount of Mn is small, the amount of Cr is large, the value of A is small and the value of B is small, and the Mn amount is small, MA is large, the area ratio of bainite is low, Lowered yield strength.

No. of Table 2 16 is an example using the grade P of Table 1 having a large amount of V and a small value of A and B and a high yield strength, a low area ratio of bainite, a low maximum hardness of bainite and a low yield strength.

Claims

In terms of% by mass,
C: more than 0.04% and not more than 0.10%
Si: 0.15 to 0.50%
Mn: 1.20 to 2.50%
P: more than 0% and not more than 0.020%
S: more than 0% and not more than 0.0050%
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: not more than 0.0003% (including 0%),
, And more preferably more than 0% but not more than 0.30% of Mo, more than 0% and not more than 0.30% of Mo, more than 0% and not more than 0.30% of Ni, more than 0% And the remainder is composed of iron and inevitable impurities,
Ceq defined by the following formula (1) is less than 0.44,
An A value defined by the following formula (2) is at least 2.50, and
The B value defined by the following formula (3) is not less than 2.37,
The area ratio of the following metal structure at the 1/4 position of the plate thickness of the steel sheet,
Bainite: at least 80% by area, and
0 > to not less than 0.26 area percent,
Characterized in that the maximum hardness of the bainite is 270 HV or more. The non-welded steel sheet has a low temperature toughness deterioration of the weld heat affected zone and a high hardness of the weld heat affected zone.
Ceq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V /
A value = 1.15 x Mn + 2.20 x Mo + 6.50 x Nb (2)
B value = 1.20 x Mn + 0.50 x Ni + 4.25 x Nb (3)
The content of C, Mn, Cu, Ni, Cr, Mo, V, and Nb in terms of mass% is represented by C, Mn, Cu, Ni, Cr, Mo, V and Nb.

The method according to claim 1,
Non-tempered steel plate for line pipe.