JP2019501281A - High-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of welds and method for producing the same - Google Patents

Abstract

The present invention provides a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of welds, and a method for producing the same.
According to one aspect of the present invention, by weight, C: 0.05 to 0.09%, Mn: 1.5 to 2.0%, Ni: 0.3 to 0.8%, Nb: 0.005 -0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5%, Si: 0.05-0.3%, Al: 0.005-0.05%, P: 100 ppm or less, S: 40 ppm or less, including the remaining Fe and other inevitable impurities, the fine structure of the central portion includes 70% or more acicular ferrite and 10% or less pearlite in area%, The equivalent microstructure has a circle equivalent diameter of 15 μm (micrometer) or less, and the microstructure of the area of 2 mm or less immediately below the surface is composed of at least 30% ferrite and the remaining bainite, martensite, and pearlite in area%. And welding formed during welding Affected zone, in area%, a high strength steel and its manufacturing method having excellent brittle crack initiation resistance of brittle crack propagation resistance and weld containing 5% or less of the island martensite is provided.
According to the present invention, it is possible to obtain a high-strength steel material having high yield strength and excellent in brittle crack propagation resistance and brittle crack initiation resistance in welds.
[Selection] Figure 1

Description

  The present invention relates to a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of welds, and a method for producing the same.

In recent years, in designing structures used in the domestic, foreign, marine, architectural, and civil engineering fields, development of extra heavy steel having high strength properties has been demanded.
When using high-strength steel when designing a structure, it is possible to reduce the form of the structure, which not only provides economic benefits, but also reduces the thickness of the steel sheet. Ease of welding work can be ensured at the same time.
In general, in the case of high-strength steel, the total rolling reduction is reduced during the manufacture of extra-thick materials, and sufficient deformation is not performed as compared with thin materials, resulting in a coarser microstructure of extra-heavy materials. The low temperature physical properties that the crystal grain size has the greatest influence are lowered.
In particular, in the case of brittle crack propagation resistance, which indicates the stability of structures, there is an increasing number of cases that require assurance when applied to main structures such as ships, but when the microstructure becomes coarse, brittle crack propagation resistance Therefore, it is very difficult to improve the resistance to brittle crack propagation of extremely thick high-strength steel materials.

On the other hand, in high-strength steel with a yield strength of 390 MPa or more, in order to improve brittle crack propagation resistance, application of surface cooling at the time of finish rolling to refine the grain size of the surface layer, and application of bending stress at the time of rolling Various technologies, such as particle size adjustment by, were introduced.
However, in the case of the above technique, although it is advantageous for refining the structure of the surface layer part, since the reduction in impact toughness due to the remaining coarse structure excluding the surface layer part cannot be solved, the fundamental to brittle crack propagation resistance It is hard to say that it is a countermeasure.
In addition to steel materials applied to recent large container ships, etc., as design concepts aimed at improving ship safety by controlling the initiation of brittle cracks themselves, An increasing number of cases guarantee the brittle crack initiation resistance of the weld heat affected zone, which is considered to be the most fragile part related to the initiation of brittle cracks.
Generally, in the case of high-strength steel, the microstructure of the weld heat-affected zone (HAZ) is composed of a high-temperature, low-temperature transformation phase such as bainite, so the toughness of the heat-affected zone (HAZ) of the weld zone is very high. It has the disadvantage of becoming weak.

In particular, in order to evaluate the stability of the structure, in the case of brittle crack initiation resistance in CTOD evaluation (Cracking Opening Displacement) that is generally performed, it is generated from untransformed austenite during the generation of the low-temperature transformation phase. In fact, it is very difficult to improve the resistance to brittle cracking of high-strength steel materials because island martensite becomes a nucleation site for brittle cracking.
In the case of a conventional high strength steel having a yield strength of 400 MPa or more, in order to improve the brittle crack initiation resistance of the weld zone, the microstructure of the weld heat affected zone is refined using TiN or oxide (oxide metallurgy). ) Was used to form ferrite in the weld heat affected zone. However, this is partly useful for improving impact toughness by refining the structure, but has no significant effect on reducing the fraction of island martensite that has a large effect on the reduction in brittle crack initiation resistance.
In addition, the brittle crack initiation resistance of the base metal can ensure physical properties by transforming island-like martensite into another phase through tempering or the like, but it can be tempered by thermal history. In the case of a welding heat-affected zone where the effect of) disappears, it is impossible to apply this.
On the other hand, in order to minimize the generation of island martensite, it is necessary to reduce elements such as C and Nb. However, if this is reduced, it is difficult to ensure the strength level. In order to ensure, it is necessary to add a large amount of expensive elements such as Mo and Ni.

An object of one aspect of the present invention is to provide a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a weld.
Another object of the present invention is to provide a method for producing a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of welds.

  The high-strength steel material excellent in the brittle crack propagation resistance and the brittle crack initiation resistance of the welded portion of the present invention is, by weight, C: 0.05 to 0.09%, Mn: 1.5 to 2.0%. Ni: 0.3-0.8%, Nb: 0.005-0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5%, Si: 0.05 -0.3%, Al: 0.005-0.05%, P: 100 ppm or less, S: 40 ppm or less, the remaining Fe and other inevitable impurities are included, and the microstructure in the center is 70% in area%. It consists of one or more selected from the group consisting of the above-mentioned acicular ferrite, 10% or less pearlite, and the remaining ferrite, bainite, and island martensite (MA), and is equivalent to a pearlite circle. 15 μm in diameter (ma 1 or more selected from the group consisting of 30% or more of ferrite, and the remaining bainite, martensite, and pearlite in the area%. The weld heat affected zone formed during welding includes 5% or less of island martensite in area%.

The weight ratio of Cu and Ni (Cu / Ni weight ratio) can be set to 0.8 or less, preferably 0.6 or less.
The steel material preferably has a yield strength of 390 MPa or more.
The steel material may have a Charpy fracture surface transition temperature of −40 ° C. or less at the position of the steel material thickness ½t (t: the thickness of the steel plate) in the thickness direction of the steel material.

The manufacturing method of the high strength steel material excellent in the brittle crack propagation resistance and the brittle crack initiation resistance of the welded portion of the present invention is in% by weight, C: 0.05 to 0.09%, Mn: 1.5 to 2 0.0%, Ni: 0.3-0.8%, Nb: 0.005-0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5%, Si: After reheating a slab containing 0.1 to 0.3%, Al: 0.005 to 0.05%, P: 100 ppm or less, S: 40 ppm or less, remaining Fe and other inevitable impurities at 1000 to 1100 ° C. , Rough rolling at a temperature of 1100 to 900 ° C., and finishing the roughly rolled bar in a temperature range of Ar ₃ (ferrite transformation start temperature) + 60 ° C. to Ar ₃ ° C. based on the center temperature. Rolling to obtain a steel plate; cooling the steel plate to a temperature of 700 ° C. or lower; , Including.

For the last three passes during rough rolling, the rolling reduction per pass is preferably 5% or more, and the total cumulative rolling reduction is preferably 40% or more.
For the last three passes during rough rolling, the strain rate can be set to 2 / sec or less.
The grain size at the center of the thickness of the bar before rough rolling after rough rolling can be 150 μm or less, preferably 100 μm or less, more preferably 80 μm or less.
The reduction ratio at the time of finish rolling may be set so that the ratio of the thickness of the slab (mm) / the thickness of the steel sheet after the finish rolling (mm) is 3.5 or more, preferably 4 or more.

The cumulative rolling reduction during finish rolling is preferably maintained at 40% or higher, and the rolling reduction per pass excluding temper rolling is preferably maintained at 4% or higher.
Here, temper rolling refers to rolling performed to ensure the shape of the plate (so that the plate comes out flat), and the final 1-2 passes of normal finish rolling (less than 5%). It is good to carry out at a low rolling reduction of.
The steel sheet can be cooled at a central part cooling rate of 1.5 ° C./s or more.
The steel sheet can be cooled at an average cooling rate of 2 to 300 ° C./s.
Furthermore, the means for solving the problems described above do not enumerate all the features of the present invention. Various features of the present invention and the attendant advantages and advantages can be more fully understood with reference to the following specific embodiments.

  According to the present invention, it is possible to obtain a high-strength steel material having high yield strength and excellent in brittle crack propagation resistance and brittle crack initiation resistance in welds.

It is the photograph which observed the thickness center part of invention steel 3 with the optical microscope.

The inventors of the present invention propose the present invention as a result of research and experiment to improve the yield strength of thick steel materials, brittle crack propagation resistance and brittle crack initiation resistance of welds. It came to.
The present invention controls the steel composition, structure, and manufacturing conditions of the steel material to further improve the yield strength of the thick steel material, the brittle crack propagation resistance, and the brittle crack initiation resistance of the weld. .

The main concept of the present invention is as follows.
1) The steel composition is appropriately controlled in order to improve the strength through solid solution strengthening. In particular, the contents of Mn, Ni, Cu and Si are optimized for solid solution strengthening.
2) The steel composition is appropriately controlled in order to improve the strength through improving the hardenability. In particular, not only the C content but also the contents of Mn, Ni and Cu are optimized to improve the curability.
By improving the hardenability in this way, a fine structure is ensured up to the center of the thick steel material even at a low cooling rate.
3) It is preferable to control the weight ratio of Cu / Ni. Thus, when controlling the weight ratio of Cu / Ni, the surface quality can be further improved.
4) The composition is appropriately controlled in order to control the fraction of island martensite in the weld heat affected zone formed during welding. In particular, the contents of C, Si, and Nb that affect the generation of island martensite are optimized.
By optimizing the steel composition in this way, excellent brittle crack initiation resistance is ensured also in the weld heat affected zone.

5) It is preferable to control the structure of the steel material in order to improve strength and brittle crack propagation resistance. In particular, the structure of the central region and the surface layer region in the thickness direction of the steel material is controlled.
By controlling the microstructure in this way, the strength necessary for the steel material is ensured, and the microstructure that promotes the generation of cracks is excluded, thereby improving the brittle crack propagation resistance.
6) The finish rolling conditions are controlled in order to further refine the structure of the steel material. In particular, by controlling the temperature and reduction conditions during finish rolling, a fine structure is formed up to the center of the steel material by generating many deformation bands inside the austenite during finish rolling and securing many ferrite nucleation sites. Is ensured. This also promotes the generation of acicular ferrite.
7) It is preferable to control the rough rolling conditions in order to further refine the structure of the steel material.
In particular, a fine structure is ensured at the center by controlling the rolling conditions during rough rolling. This also promotes the generation of acicular ferrite.

Hereinafter, a high-strength steel material excellent in brittle crack propagation resistance according to one aspect of the present invention will be described in detail.
The high-strength steel material excellent in the brittle crack propagation resistance and the brittle crack initiation resistance of the weld zone, which is one aspect of the present invention, is C: 0.05 to 0.09%, Mn: 1.5 to 2.0%, Ni: 0.3-0.8%, Nb: 0.005-0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5%, Si : 0.05 to 0.3%, Al: 0.005 to 0.05%, P: 100 ppm or less, S: 40 ppm or less, containing the remaining Fe and other inevitable impurities, the fine structure in the center is area% And comprising at least one selected from the group consisting of 70% or more acicular ferrite, 10% or less pearlite, and the remaining ferrite, bainite, and island martensite (MA). Perlite circle equivalent diameter The microstructure of the surface portion in the region of 15 μm (micrometer) or less and 2 mm or less immediately below the surface is selected from the group consisting of 30% or more ferrite and the remaining bainite, martensite, pearlite in area% 1 The weld heat-affected zone that is composed of more than seeds and is formed at the time of welding includes 5% or less of island martensite in area%.

Hereinafter, the steel components and component ranges of the present invention will be described.
C (carbon): 0.05 to 0.09 wt% (hereinafter referred to as “%”)
Since C is the most important element for securing basic strength, it must be contained in the steel within an appropriate range. In order to obtain such an effect of addition, it is preferable to add 0.05% or more of C.
However, if the C content exceeds 0.09%, a large amount of island-like martensite is generated in the weld heat-affected zone, reducing the brittle crack initiation resistance, and the high strength and low temperature transformation of the base ferrite itself. The C content is preferably limited to 0.05 to 0.09% in order to reduce low temperature toughness due to a large amount of phase formation. Among them, the C content is more preferably limited to 0.061 to 0.085%, and further preferably limited to 0.065 to 0.075%.

Mn (manganese): 1.5 to 2.0%
Mn is a useful element that improves the strength by solid solution strengthening and improves the curability so that a low-temperature transformation phase is generated. Moreover, since the low temperature transformation phase can be generated even at a slow cooling rate due to the improvement of the curing ability, it is a main element for ensuring the strength of the central portion of the extra-thick material.
Therefore, in order to acquire such an effect, it is preferable to add 1.5% or more.
However, if the content of Mn exceeds 2.0%, excessive hardening ability increases, promotes the formation of upper bainite and martensite, and reduces impact toughness and brittle crack propagation resistance, It also reduces the toughness of the weld heat affected zone.
Therefore, the Mn content is preferably limited to 1.5 to 2.0%.
Among these, the Mn content is more preferably limited to 1.61 to 1.92%, and further preferably limited to 1.7 to 1.9%.

Ni (nickel): 0.3-0.8%
Ni is an important element that improves strength by facilitating cross slip of dislocations at low temperatures and improving impact toughness and hardenability. In order to obtain this effect, 0.3% or more is preferably added. However, when Ni is added in an amount of 0.8% or more, the low temperature transformation phase is generated due to the excessive increase in the hardenability, thereby lowering the toughness, and the expensive cost of Ni compared with other hardenability elements. Since the production cost may increase due to the cause, the upper limit of the Ni content is preferably limited to 0.8%.
Among these, the Ni content is more preferably limited to 0.37 to 0.71%, and further preferably limited to 0.4 to 0.6%.
Nb (niobium): 0.005 to 0.04%
Nb precipitates in the form of NbC or NbCN and improves the strength of the base material.
In addition, Nb dissolved at the time of reheating at high temperature has the effect of refining the structure by precipitating very finely in the form of NbC during rolling and suppressing recrystallization of austenite.
Therefore, Nb is preferably added in an amount of 0.005% or more. However, when excessively added, the formation of island martensite in the weld heat affected zone is promoted to reduce brittle crack initiation resistance, and there is a possibility that brittle cracks are induced at the end of the steel material. The upper limit of the amount is preferably limited to 0.04%.
Among these, the Nb content is more preferably limited to 0.012 to 0.031%, and further preferably limited to 0.017 to 0.03%.

Ti (titanium): 0.005 to 0.04%
Ti is a component that precipitates as TiN during reheating, suppresses crystal grain growth in the base metal and the weld heat affected zone, and greatly improves low temperature toughness. In order to obtain such an additive effect, 0.005% or more is preferably added.
However, if Ti is added excessively, the low temperature toughness may decrease due to clogging of the continuous casting nozzle or crystallization of the central portion, so the Ti content is limited to 0.005 to 0.04%. It is preferable.
Among these, the Ti content is more preferably limited to 0.012 to 0.023%, and further preferably limited to 0.014 to 0.018%.
Si: 0.05-0.3%
Si is an indispensable element for the production of clean steel because it improves the strength of the steel material by solid solution strengthening as a substitutional element and has a strong deoxidizing effect. Therefore, it is preferable to add 0.05% or more of Si. However, when added in a large amount, a coarse island-like martensite (MA) phase is generated, and the brittle crack propagation resistance and the brittle crack initiation resistance of the weld zone may be lowered. Therefore, the upper limit of the Si content is 0. It is preferable to limit it to 3%.
Among these, the Si content is more preferably limited to 0.1 to 0.27%, and further preferably limited to 0.19 to 0.25%.

Cu: 0.1 to 0.5%
Cu is a main element that improves the hardenability and causes solid solution strengthening to improve the strength of the steel material, and is an important element that increases the yield strength through the formation of epsilon Cu precipitates when applying tempering. . Therefore, it is preferable to add Cu 0.1% or more. However, if added in a large amount, slab cracks due to hot shortness may occur in the steelmaking process, so the upper limit of the Cu content is preferably limited to 0.5%.
Among these, the Cu content is more preferably limited to 0.15 to 0.31%, and further preferably limited to 0.2 to 0.3%.
The contents of Cu and Ni are preferably set so that the weight ratio of Cu / Ni is 0.8 or less, preferably 0.6 or less.
By setting the weight ratio of Cu / Ni as described above, the surface quality can be further improved.

Al: 0.005 to 0.05%
Al is a component that serves as a deoxidizer. However, when added excessively, inclusions may be formed to reduce toughness, so it is preferable to limit the Al content to 0.005 to 0.05%.
P: 100 ppm or less, S: 40 ppm or less P and S are elements that induce brittleness at the grain boundaries or form coarse inclusions to induce brittleness, and thus improve brittle crack propagation resistance. Therefore, it is preferable to limit to P: 100 ppm or less and S: 40 ppm or less.
The remaining component of the present invention is iron (Fe).
However, in a normal manufacturing process, unintended impurities may be inevitably mixed from the raw materials and the surrounding environment, and thus cannot be excluded.
Since such an impurity can be understood by any ordinary engineer, its contents are not specifically mentioned.

In the steel of the present invention, the microstructure of the central portion is 70% or more of acicular ferrite (area ferritic), 10% or less of pearlite, and the remaining ferrite, pearlite, and island martensite ( MA), and the microstructure of the surface portion in the region of pearlite having a circle equivalent diameter of 15 μm (micrometers) or less and 2 mm or less immediately below the surface is 30% in area%. Island-shaped martens composed of one or more selected from the group consisting of the above ferrite and the remaining bainite, martensite, and pearlite, and the weld heat affected zone formed during welding is 5% or less in area%. Includes site.
Ferrite means polygonal ferrite, and bainite means granular bainite and upper bainite.

If the fraction of acicular ferrite in the central microstructure is less than 70%, the toughness may be reduced due to the formation of coarse bainite.
Further, the fraction of the acicular ferrite is more preferably limited to 75% or more, and further preferably limited to 80% or more.
When the fraction of pearlite in the center exceeds 10%, the fraction of the upper bainite in the center is 10% or less because a microcrack is induced at the crack tip during brittle crack propagation to reduce brittle crack propagation resistance. It is preferable that
In particular, the pearlite fraction is more preferably limited to 8% or less, and further preferably limited to 5% or less.
If the equivalent circle diameter of the pearlite in the center exceeds 15 μm (micrometers), there is a problem that cracks are easily induced despite the low pearlite fraction. It is preferable that it is 15 micrometers (micrometer) or less.

When the microstructure of the surface portion in the region of 2 mm or less immediately below the surface contains ferrite of 30% or more, the brittle crack propagation resistance can be improved by effectively preventing the crack propagation on the surface during the brittle crack propagation. .
The ferrite fraction is more preferably limited to 40% or more, and further preferably limited to 50% or more.
If the island-like martensite in the weld heat-affected zone formed during welding of steel exceeds 5%, it acts as a crack start starting point and lowers brittle crack initiation resistance, so the island-like martensite in the weld heat-affected zone The fraction is preferably 5% or less.
The amount of heat input during welding can be 0.5 to 10 kJ / mm.

The welding method at the time of welding is not particularly limited, and examples thereof include FCAW (Flux Corded Arc Welding) and SAW (Submerged Arc Welding).
The steel material preferably has a yield strength of 390 MPa or more.
The steel material preferably has a Charpy fracture surface transition temperature of −40 ° C. or less at the position of the steel material thickness ½t (t: thickness of the steel plate) in the thickness direction of the steel material.
The steel material can have a thickness of 50 mm or more, preferably has a thickness of 60 to 100 mm, and more preferably has a thickness of 80 to 100 mm.

Hereinafter, the manufacturing method of the high strength steel material excellent in the brittle crack propagation resistance by the other side surface of this invention is demonstrated in detail.
According to another aspect of the present invention, a method for producing a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a weld zone is represented by weight%, C: 0.05 to 0.09%, Mn: 1 0.5-2.0%, Ni: 0.3-0.8%, Nb: 0.005-0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5 %, Si: 0.1 to 0.3%, Al: 0.005 to 0.05%, P: 100 ppm or less, S: 40 ppm or less, remaining slab containing Fe and other inevitable impurities at 1000 to 1100 ° C. After reheating, the stage of rough rolling at a temperature of 1100 to 900 ° C. and the roughly rolled bar (bar) with Ar ₃ (ferrite transformation start temperature) + 60 ° C. to Ar ₃ ° C. based on the center temperature The stage of finish rolling in the temperature range to obtain a steel plate, and the steel plate to a temperature of 700 ° C. or less Cooling.

Reheating the slab Reheating the slab prior to rough rolling.
The slab reheating temperature is preferably 1000 ° C. or higher. This is to dissolve Ti and / or Nb carbonitride formed during casting.
However, when reheating is performed at a temperature that is too high, austenite may be coarsened, so the upper limit of the reheating temperature is preferably 1100 ° C.

Rough rolling Roughly rolling the reheated slab.
The rough rolling temperature is preferably equal to or higher than the temperature (Tnr) at which recrystallization of austenite stops. A cast structure such as dendrite formed during casting is destroyed by rolling, and the effect of reducing the size of austenite can also be achieved. In order to obtain such an effect, the rough rolling temperature is preferably limited to 1100 to 900 ° C.
Furthermore, the rough rolling temperature is more preferably 1050 to 950 ° C.
In the present invention, in order to refine the structure of the central part during rough rolling, for the last three passes during rough rolling, the rolling reduction per pass is 5% or more, and the total cumulative rolling reduction is 40% or more. It is preferable to do.

In the structure recrystallized by the initial rolling during the rough rolling, crystal growth occurs at a high temperature. However, when performing the last three passes, the bar is air-cooled while waiting for rolling, so that the crystal grains grow. As a result, the growth rate becomes slow, and as a result, the rolling reduction ratio of the last three passes during rough rolling has the greatest influence on the grain size of the final microstructure.
Moreover, when the rolling reduction per pass of rough rolling becomes low, sufficient deformation may not be transmitted to the central portion, and the toughness may be reduced due to the coarsening of the central portion. Therefore, it is preferable to limit the rolling reduction per pass of the last three passes to 5% or more.
In particular, the rolling reduction per pass is more preferably 7 to 20%.
On the other hand, in order to refine the structure of the central part, the total cumulative rolling reduction during rough rolling is preferably set to 40% or more.
Among these, the total cumulative rolling reduction is more preferably 45% or more.

For the last three passes during rough rolling, the strain rate is preferably set to 2 / sec or less.
In general, due to the thickness of a thick bar during rough rolling, it is difficult to roll at a high pressure reduction rate, making it difficult to transmit the amount of reduction to the center of an extremely thick material, and the austenite grain size in the center There is a problem that becomes coarse. On the other hand, the lower the deformation speed, there is an advantage that the deformation can be transmitted to the central portion even with a small amount of reduction, and the particle size can be reduced.
Therefore, for the last three passes that have the greatest influence on the final grain size during rough rolling, by limiting the deformation speed to 2 / sec or less, the grain size at the center is made finer. The generation of acicular ferrite can be promoted.

Finish rolling The rough-rolled bar is finish-rolled at Ar ₃ (ferrite transformation start temperature) + 60 ° C. to Ar ₃ ° C. to obtain a steel plate.
This is for obtaining a finer microstructure. When rolling directly above the Ar ₃ temperature, the effect of ensuring that a fine structure is ensured up to the center of the steel material by generating a large number of deformation bands inside the austenite and securing a large number of ferrite nucleation sites. Can be obtained.
Also, in order to effectively generate many deformation bands inside the austenite, the cumulative reduction ratio during finish rolling is maintained at 40% or more, and the reduction ratio per pass excluding temper rolling is maintained at 4% or more. It is preferable.
A more preferable cumulative rolling reduction is 40 to 80%.
A more preferable rolling reduction per pass is 4.5% or more.
When the finish rolling temperature is lowered to Ar ₃ or lower, coarse ferrite is generated before rolling and is elongated for a long time during rolling, so that impact toughness is lowered. Also, as the finish rolling at Ar 3 + ₆₀ ° C. or higher, because it is not effective in granularity finer finish rolling temperature during finish rolling is preferably set to _{Ar 3 + 60 ℃ ~Ar 3 ℃} .

In the present invention, it is preferable to limit the rolling reduction in the non-recrystallized region during finish rolling to 40 to 80%.
As described above, by controlling the reduction rate in the non-recrystallized region, the number of nucleation sites of acicular ferrite increases, and thus the formation of these structures can be further promoted.
If the rolling reduction in the non-recrystallized region is too low, sufficient acicular ferrite cannot be secured. On the other hand, when too high, there exists a possibility that intensity | strength may fall by the production | generation of the cornerstone ferrite resulting from a high pressure reduction rate.

The grain size at the center of the thickness of the bar before rough rolling after rough rolling is 150 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less.
The grain size at the center of the thickness of the bar after rough rolling and before finish rolling can be controlled by rough rolling conditions and the like.
As described above, when controlling the grain size of the bar before rough rolling after rough rolling, it may further have the advantage of improving low temperature impact toughness because the final microstructure is refined by refining austenite crystal grains. it can.
The reduction ratio at the time of finish rolling can be set so that the ratio of slab thickness (mm) / steel plate thickness after finish rolling (mm) is 3.5 or more, preferably 4 or more.

When controlling the reduction ratio as described above, increasing the amount of reduction during rough rolling and finish rolling increases yield strength / tensile strength through refinement of the final microstructure, improves low-temperature toughness, and thickness It can further have the advantage of improving the toughness of the central part through a reduction in the grain size of the central part.
After finish rolling, the steel sheet can have a thickness of 50 mm or more, preferably 60 to 100 mm, and more preferably 80 to 100 mm.

Cooling The steel plate after finish rolling is cooled to 700 ° C. or lower.
When the cooling end temperature exceeds 700 ° C., a fine structure is not appropriately formed, and it may be difficult to ensure a sufficient yield strength, for example, a yield strength of 390 MPa or more.
A preferable cooling end temperature is 600 to 300 ° C.
When the cooling end temperature is less than 300 ° C., the amount of bainite produced may increase and the toughness may decrease.
The steel sheet can be cooled at a central part cooling rate of 1.5 ° C./s or more. When the center part cooling rate of the steel sheet is less than 1.5 ° C./s, a fine structure is not properly formed, and it may be difficult to ensure a sufficient yield strength, for example, a yield strength of 390 MPa or more.
Moreover, the steel sheet can be cooled at an average cooling rate of 2 to 300 ° C.

Hereinafter, the present invention will be described in more detail through examples.
However, the description of the embodiment is for illustrating the implementation of the present invention, and the present invention is not limited by the description of the embodiment. This is because the scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

(Example)
A steel slab having a thickness of 400 mm having the composition shown in Table 1 below was reheated at a temperature of 1060 ° C. and then subjected to rough rolling at a temperature of 1025 ° C. to produce a bar. The cumulative rolling reduction during rough rolling was equally applied to 50%.
The thickness of the coarsely rolled bar was 200 mm, and as shown in Table 2 below, the crystal grain size in the center after rough rolling and before finish rolling was 75 to 89 μm. The rolling reduction of the last 3 passes at the time of rough rolling was 7.2 to 14.3%, and the deformation rate at the time of rolling was in the range of 1.29 to 1.66 / s.
After rough rolling, as shown in Table 2 below, finish rolling is performed at a [finish rolling temperature-Ar ₃ (ferrite transformation start temperature)] temperature to obtain a steel sheet having a thickness shown in Table 3 below, and then a cooling rate of 4 Cooled to a temperature of 496 to 412 ° C. or lower at 5 ° C./sec.
The microstructure, yield strength, Kca value (brittle crack propagation resistance coefficient) and CTOD value (brittle crack initiation resistance coefficient) of the steel sheet produced as described above were investigated, and the results are shown in Tables 3 and 4 below. Indicated.

The Kca value in the following Table 4 is a value evaluated by conducting an ESSO test (test) on the steel sheet.
In addition, FCAW (0.7 kJ / mm) was welded, and the structure analysis and CTOD evaluation were performed on the weld heat affected zone. The results are shown in Tables 3 and 4 below.
The surface characteristics shown in Table 3 below were measured to determine whether or not star cracks on the surface due to hot shortness caused by the Cu / Ni addition ratio occurred.

As shown in Tables 1 to 4, in the case of Comparative Example 1, although the steel composition is consistent with the present invention, the temperature difference of [Finishing Rolling Temperature at Finish Rolling—Ar ₃ ] presented in the present invention is 60. The temperature was controlled at a temperature higher than or equal to 0 ° C., and sufficient reduction was not applied to the central portion, and the fraction of acicular ferrite (AF) in the central portion was less than 50%. % Ferrite is not generated, it can be seen that the Kca value measured at −10 ° C. does not exceed 6000 required for general steel for shipbuilding.
In the case of Comparative Example 2, the C content has a value higher than the upper limit of the C content of the present invention, and a large amount of bainite is generated at the center during rough rolling. Since the AF fraction of the microstructure is less than 50%, it has a value of 6000 or less at −10 ° C., and many island-like martensite (MA) structures are generated in the weld heat affected zone. It can be seen that the value has a value of 0.25 mm or less.

In the case of Comparative Example 3, the Si content has a value higher than the upper limit of the Si content of the present invention, and when a large amount of Si is added, a large amount of MA texture is generated in a large amount. Despite the fact that the microstructure of the part contains a large amount of AF, the Kca value has a low value of about 6000 at -10 ° C., and many island martensite (MA) structures are also formed in the weld heat affected zone. Thus, it can be seen that the CTOD value has a value of 0.25 mm or less.
In the case of Comparative Example 4, the Mn content has a value higher than the upper limit of the Mn content of the present invention, and the microstructure of the base material is provided as the upper bainite due to the high curability. The fraction of acicular ferrite (AF) is less than 50%. As a result, it can be seen that the Kca value also has a value of 6000 or less at -10 ° C.

In the case of Comparative Example 5, the Ni content has a value higher than the upper limit of the Ni content of the present invention, and the fine structure of the base material is granular bainite due to the high curing ability. And provided as upper bainite, the fraction of acicular ferrite (AF) is less than 50%. As a result, it can be seen that the Kca value also has a value of 6000 or less at -10 ° C.
In the case of Comparative Example 6, the content of Nb and Ti has a value higher than the upper limit of the content of Nb and Ti of the present invention, and all other conditions satisfy the conditions presented in the present invention. Nevertheless, it can be seen that a large amount of island martensite (MA) structure is generated in the weld heat affected zone due to high Ti and Nb, and the CTOD value is 0.25 mm or less.

In the case of Invention Example 7, it has a component exceeding the ratio of Cu / Ni presented in one preferred aspect of the present invention, and although other physical properties are very excellent, star cracks are present on the surface. It can be seen that the surface quality is abnormal.
In the case of Comparative Example 7, the content of C and Mn has a value lower than the lower limit of the content of C and Mn of the present invention. ) Fraction is less than 50%, most of the structure has ferrite and pearlite structure of 10% or more, and the average particle size of pearlite has a size of 15 μm or more. Therefore, the Kca value is −10 ° C. It can be seen that it has a value of 6000 or less.

On the other hand, in the case of Invention Examples 1 to 6 that satisfy the component range and the production range of the present invention and the ratio of Cu / Ni, the fraction of acicular ferrite (AF) in the fine structure at the center is 70% or more. The pearlite fraction in the center is 10% or less, the equivalent circle diameter of the pearlite in the center is 15 μm or less, and the MA phase fraction in the weld heat affected zone is less than 5%. I understand.
Inventive Examples 1 to 6 show that the yield strength is 390 MPa or more, the Kca value satisfies the value of 6000 or more at −10 ° C., the CTOD value is also excellent value of 0.25 mm or more, and the surface quality is also excellent.
FIG. 1 shows a photograph of the center of thickness of invention steel 3 observed with an optical microscope. As shown in FIG. 1, it can be confirmed that the microstructure in the central portion includes a large amount of the structure of acicular ferrite (AF), and the pearlite is finely dispersed.

  Although the present invention has been described with reference to the embodiments, those skilled in the art can make various modifications to the present invention without departing from the spirit and scope of the present invention described in the claims below. And understand that it can be changed.

Claims (17)

  By weight, C: 0.05 to 0.09%, Mn: 1.5 to 2.0%, Ni: 0.3 to 0.8%, Nb: 0.005 to 0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5%, Si: 0.05-0.3%, Al: 0.005-0.05%, P: 100 ppm or less, S: 40 ppm Hereinafter, the remaining microstructure contains Fe and other inevitable impurities, and the central microstructure has an area percentage of 70% or more of acicular ferrite, 10% or less of pearlite, and the remaining ferrite and bainite. And a fine group of surface portions in a region where the circle-equivalent diameter of the pearlite is 15 μm (micrometers) or less and 2 mm or less immediately below the surface. The weaving is composed of one or more selected from the group consisting of 30% or more ferrite and the remaining bainite, martensite and pearlite in area%, and the weld heat affected zone formed during welding is area%. A high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of welds, characterized by containing 5% or less of island martensite.   The high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a welded portion according to claim 1, wherein the steel material has a thickness of 50 mm or more.   The weight ratio of Cu to Ni (Cu / Ni weight ratio) is 0.8 or less, and is excellent in brittle crack propagation resistance and brittle crack initiation resistance of welds according to claim 1. High strength steel.   The high heat-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a welded portion according to claim 1, wherein a welding heat input amount during welding is 0.5 to 10 kJ / mm.   5. The brittle crack propagation resistance and the brittle crack initiation resistance of the welded portion according to claim 4, wherein the welding method at the time of welding is FCAW (Flux Cored Arc Welding) or SAW (Submerged Arc Welding). Excellent high strength steel.   The high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a welded portion according to claim 1, wherein the yield strength of the steel material is 390 MPa or more.   7. The brittle crack propagation resistance according to claim 1, and the brittle crack initiation resistance of a weld zone, wherein the steel material has a Kca value measured at −10 ° C. of 6000 or more. High strength steel with excellent properties.   2. The steel material has a Charpy fracture surface transition temperature of −40 ° C. or less at a position where the thickness of the steel material is 1/2 t (t: the thickness of the steel plate) in the thickness direction of the steel material. High strength steel with excellent brittle crack propagation resistance and brittle crack initiation resistance of welds. By weight, C: 0.05 to 0.09%, Mn: 1.5 to 2.0%, Ni: 0.3 to 0.8%, Nb: 0.005 to 0.04%, Ti: 0.005-0.04%, Cu: 0.1-0.5%, Si: 0.1-0.3%, Al: 0.005-0.05%, P: 100 ppm or less, S: 40 ppm Hereinafter, after reheating the slab containing the remaining Fe and other inevitable impurities at 1000 to 1100 ° C., rough rolling at a temperature of 1100 to 900 ° C., and the roughly rolled bar (bar) Based on the above, a stage of finish rolling in a temperature range of Ar ₃ (ferrite transformation start temperature) + 60 ° C. to Ar ₃ ° C. to obtain a steel sheet,
And a step of cooling the steel sheet to a temperature of 700 ° C. or lower. A method for producing a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a welded portion.   The method for producing a high-strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a welded portion according to claim 9, wherein the thickness of the finish-rolled steel sheet is 50 mm or more.   The last 3 passes (pass) at the time of the rough rolling have a rolling reduction per pass of 5% or more and a total cumulative rolling reduction of 40% or more. A method for producing a high-strength steel material having excellent brittle crack propagation resistance and weld brittle crack initiation resistance.   The brittle crack propagation resistance and weld zone of claim 9, wherein the last three passes during the rough rolling are performed at a strain rate of 2 / sec or less. A method for producing a high-strength steel material excellent in brittle crack initiation resistance.   The brittle crack propagation resistance according to claim 9 and the brittle crack initiation resistance of the welded portion according to claim 9, wherein the crystal grain size in the central portion of the thickness of the bar before rough rolling after the rough rolling is 150 µm or less. A method for producing high strength steel.   The reduction ratio at the time of the finish rolling is set so that the ratio of the thickness of the slab (mm) / the thickness of the steel sheet after the finish rolling (mm) is 3.5 or more. A method for producing a high-strength steel material having excellent brittle crack propagation resistance and brittle crack initiation resistance of welds as described in 1.   10. The brittle crack propagation resistance according to claim 9, wherein the cumulative rolling reduction during the finish rolling is maintained at 40% or more, and the rolling reduction per pass excluding the temper rolling is maintained at 4% or more. And the manufacturing method of the high strength steel materials excellent in the brittle crack start resistance of a welding part.   The high strength steel material excellent in brittle crack propagation resistance and brittle crack initiation resistance of a welded portion according to claim 9, wherein the steel plate is cooled at a central portion cooling rate of 1.5 ° C./s or more. Manufacturing method.   The steel sheet is cooled at an average cooling rate of 2 to 300 ° C / s. The high-strength steel material having excellent brittle crack propagation resistance and brittle crack initiation resistance of welds according to claim 9. Production method.

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