WO2024090614A1 - Steel plate having excellent heat-affected zone toughness and method for manufacturing same - Google Patents

Abstract

The present invention pertains to: a steel plate which is used in ships etc. and in which a heat-affected zone (HAZ) formed by welding has excellent toughness; and a method for manufacturing same.

Description

Steel material with excellent weld heat-affected zone toughness and method of manufacturing the same

The present invention relates to steel materials used in ships, etc., and relates to steel materials with excellent toughness of a welded heat-affected zone (HAZ) formed by welding and a method of manufacturing the same.

Recently, as the Arctic sea ice area is rapidly decreasing due to temperature rise due to global warming, interest in opening the Northern Sea Route is increasing. It is necessary for ships to pioneer the Northern Sea Route or to operate the Northern Sea Route as icebreakers that can crush glaciers in case of emergency.

The icebreaker refers to a ship that sails by breaking ice on the surface of the water to open a sea route. Most icebreakers to date have been military or exploration ships, but as interest in the Northern Sea Route has recently increased, their scope of use is expanding to include commercial ships and cruise ships. For example, Russia is the most active country in the construction of icebreakers due to its regional characteristics. As of 2020, about 40 icebreakers, including the Yermak, Arktika, and Sivir, are in operation worldwide, and the construction of icebreakers is expected to increase further in the future. It is expected that it will be done.

The steel used in the icebreaker hull must have excellent impact toughness even at extremely low temperatures to withstand the low temperatures of the Northern Sea Route, and at the same time requires high strength to protect the hull. In other words, it is necessary to secure both high strength and high toughness, and for this purpose, a large amount of alloy elements are added.

In order to improve productivity when building an icebreaker, shipbuilders are advantageous in increasing the heat input during welding of steel materials, and steel materials that do not deteriorate the toughness of the heat-affected zone when welding are required even when the heat input during welding increases as described above. However, as mentioned above, a large amount of alloying elements are added to the steel used in icebreakers to ensure strength, and as a result, the welded heat-affected zone suffers from a problem in which the toughness is significantly reduced.

In general, in order to secure the toughness of the weld heat-affected zone manufactured with high heat input, a method of increasing the nitrogen content and generating fine TiN precipitates to refine the grain size of the weld heat-affected zone (Patent Document 1) is used, but in this case, the high It is easy for base material impact toughness to decrease due to free nitrogen (Free N) depending on the nitrogen content. To prevent this, a decrease in toughness can be prevented by adding a large amount of boron (B) to form BN. However, if the amount added is not carefully controlled, further decrease in toughness may occur due to the formation of free boron (Free B). . In addition, when adding a large amount of nitrogen, there is a problem that micro cracks may be caused on the surface of the slab during the casting process for manufacturing slabs, so it is difficult to consider the method of utilizing high nitrogen as an effective method.

On the other hand, there are attempts to secure toughness by using fine oxides to refine the grain size of the weld heat-affected zone, but it is very difficult to actually finely disperse the oxides generated in advance at high temperatures evenly within the steel, and only the necessary oxides are selected. Therefore, there is uncertainty as to whether it is actually possible to make the entire steel material finer and whether it will have an effect in improving toughness.

Accordingly, there is a demand for steel manufacturing technology that can secure the strength and toughness of the base material and at the same time ensure excellent toughness in the weld heat-affected zone.

(Patent Document 1) Japanese Patent Publication No. 2005-200716

One aspect of the present invention is to provide a steel material capable of securing excellent toughness in the weld heat-affected zone even when a steel material having high strength and high toughness is welded with a certain amount of heat input, and a method of manufacturing the same.

The object of the present invention is not limited to the above-mentioned matters. The additional problems of the present invention are described throughout the specification, and those skilled in the art will have no difficulty in understanding the additional problems of the present invention from the content described in the specification of the present invention.

One aspect of the present invention is expressed in weight percent, C: 0.03-0.06%, Mn: 1.5-1.7%, Si: 0.05-0.2%, Al: 0.01-0.04%, Ni: 0.6-0.9%, Mo: 0.1-0.2 %, Cr: 0.1~0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less, the remainder includes Fe and inevitable impurities do,

In a welded heat-affected zone (HAZ) welded with a heat input of 150 to 200 KJ/cm, the MA fraction in the area from the fusion line (FL) to FL+1 mm is less than 4% by area, and the toughness of the welded heat-affected zone is excellent. Provides steel materials.

Another embodiment of the present invention is by weight percentage, C: 0.03-0.06%, Mn: 1.5-1.7%, Si: 0.05-0.2%, Al: 0.01-0.04%, Ni: 0.6-0.9%, Mo: 0.1- 0.2%, Cr: 0.1~0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less, the remainder is Fe and inevitable impurities. Heating the steel slab to 1050-1150°C;

Rough rolling the heated steel slab at a temperature of 900°C or higher;

Manufacturing a hot rolled steel sheet by performing final hot rolling at a temperature of 800° C. or higher after the rough rolling; and

A method for manufacturing a steel material having excellent weld heat-affected zone toughness is provided, including cooling the hot-rolled steel sheet at a cooling rate of 15°C/s until the temperature at point t/4 of the thickness (t) of the hot-rolled steel sheet is 700°C or lower.

According to the present invention, it is possible to provide a steel material that not only has excellent strength and toughness of the base material, but also secures excellent toughness in the weld heat-affected zone, and a method of manufacturing the same. The above steel materials can be applied in various fields such as icebreakers and structures in cryogenic environments.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

The terms used in this specification are for describing the present invention and are not intended to limit the present invention. Additionally, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary.

The meaning of “including” used in the specification specifies a configuration and does not exclude the presence or addition of another configuration.

Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in the dictionary are interpreted to have meanings consistent with related technical literature and current disclosure.

The inventor of the present invention has developed a technique for securing the toughness of the formed weld heat-affected zone, especially low-temperature toughness, by performing medium heat input welding, for example, welding with a heat input of about 150 to 200 KJ/cm, on steel materials with high strength and toughness. studied in depth. As a result, by minimizing the island-like martensite (MA) phase among the microstructures of the base material and optimizing the nitrogen (N) content to finely precipitate TiN precipitates, the melt line ( We came to the present invention after recognizing that the technical purpose of preventing cracks and reducing toughness could be achieved by controlling the MA phase near the Fusion Line (FL).

First, one aspect of the steel material of the present invention will be described in detail.

The steel is expressed in weight percent: C: 0.03-0.06%, Mn: 1.5-1.7%, Si: 0.05-0.2%, Al: 0.01-0.04%, Ni: 0.6-0.9%, Mo: 0.1-0.2%, Cr : 0.1~0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less, the remainder includes Fe and inevitable impurities. Hereinafter, each alloy composition will be described.

Carbon (C): 0.03~0.06% (hereinafter, the content for each alloy composition refers to weight%)

Since C is the most important element in securing basic strength, it needs to be contained in steel within an appropriate range. If the content of C exceeds 0.06%, the hardenability is improved and a large amount of island-like martensite (MA) is generated, which reduces the toughness of the weld heat-affected zone. If the content of C is less than 0.03%, the strength decreases, so the C The content is preferably 0.03 to 0.06%.

Silicon (Si): 0.05~0.2% and Aluminum (Al): 0.01~0.04%

The Si and Al are essential alloy elements for deoxidation by precipitating dissolved oxygen in the molten steel in the form of slag during the steelmaking and casting process. When manufacturing steel using a converter, it is preferable to contain more than 0.05% of Si and 0.01% of Al. . However, if contained in large amounts, it is an alloy element that can generate coarse Si and Al composite oxides or generate large amounts of island-like martensite within the microstructure of the weld heat-affected zone. Therefore, Si and Al should be kept to 0.2% or less and Al 0.04% or less. It is desirable to include it.

Manganese (Mn): 1.5~1.7%

Since Mn is a useful element that improves strength through solid solution strengthening and improves hardenability to generate a low-temperature transformation phase, it is preferable that Mn is included in an amount of 1.5% or more to secure a yield strength of 500 MPa or more. However, if it exceeds 1.7%, the Mn content may greatly reduce toughness by promoting the formation of upper bainite and martensite in the weld heat-affected zone and the base metal structure due to excessive increase in hardenability. It is preferable that it is 1.5 to 1.7%.

Nickel (Ni): 0.6~0.9%

Ni is an important element in improving impact toughness by facilitating cross slip of dislocations at low temperatures and increasing strength by improving hardenability. In order to improve the impact toughness of high-strength steel with a yield strength of 500 MPa or more and the impact toughness of the bainitic structure in the weld heat-affected zone, it is desirable to contain 0.6% or more, but if it exceeds 0.9%, the hardenability increases excessively. There is a problem of generating a low-temperature transformation phase, which reduces toughness, and also increases manufacturing costs, so it is preferable not to exceed 0.9%.

Niobium (Nb): 0.005~0.02%

The Nb precipitates in the form of NbC or NbCN to improve the strength of the base material. In addition, Nb dissolved in solid solution when reheated to a high temperature precipitates very finely in the form of NbC during rolling, which has the effect of suppressing recrystallization of austenite and refining the structure. Accordingly, it is preferable to contain 0.005% or more of Nb, but if it is added excessively, there is a possibility of causing brittle cracks at the edges of the steel material, and the toughness is reduced due to the formation of a large amount of island-like martensite (MA) in the weld heat-affected zone. Since problems may occur, it is desirable not to exceed 0.02%.

Titanium (Ti): 0.01~0.02%

The Ti precipitates as TiN upon reheating and significantly improves low-temperature toughness by inhibiting the growth of grains in the base metal and weld heat-affected zone. It is preferable to contain 0.01% or more for effective TiN precipitation. However, if it exceeds 0.02%, low-temperature toughness is reduced due to clogging of the casting nozzle or crystallization at the center, and the toughness of the weld heat-affected zone is reduced as the TiN precipitates become coarse by lowering the Ti/N ratio. It is desirable not to exceed 0.02%.

Nitrogen (N): 0.0035~0.0070% (35~70ppm)

The N combines with Ti to precipitate TiN, preventing the growth of old austenite crystal grains, and has the effect of refining the particle size. It is preferable to contain 35 ppm or more to form fine TiN precipitates. However, if it is included excessively, it may cause a decrease in toughness due to the generation of free nitrogen and slab cracks due to AlN precipitation, so it is preferable that it is 70 ppm or less. It is more preferable that the N is 45 to 60 ppm.

Molybdenum (Mo): 0.1~0.2%

The Mo is an element that improves strength by increasing hardenability. In the present invention, in order to secure the required strength, it is preferable that Mo is included in an amount of 0.1% or more. However, if it is included excessively, toughness may decrease due to excessive strength increase, so it is preferable not to exceed 0.2%.

Chromium (Cr): 0.1~0.3%

Cr is an element that improves strength through solid solution strengthening, and is preferably included in an amount of 0.1% or more to secure the strength required in the present invention. However, if added excessively, the strength may increase too much or the toughness may decrease due to carbide precipitation, so the content is preferably 0.3% or less.

Phosphorus (P): 0.008% (80ppm) or less and Sulfur (S): 0.002% (20ppm) or less.

The P and S are elements that cause embrittlement by causing grain boundary embrittlement or forming coarse inclusions. In order to improve brittle crack propagation resistance, it is desirable to manage P: 80 ppm or less and S: 20 ppm or less.

The remainder includes iron (Fe), and unintended impurities may inevitably be introduced from raw materials or the surrounding environment during normal manufacturing processes, so this cannot be excluded. Since these impurities can be known to anyone skilled in the art during the manufacturing process, all of them are not specifically mentioned in this specification.

The steel material of the present invention preferably has a base material yield strength of 500 MPa or more and an impact transition temperature of -40°C or less.

Meanwhile, in the heat-affected zone where the steel of the present invention is welded with medium heat input (about 150 to 200 KJ/cm), the MA fraction in the area of fusion line (FL) ~ FL + 1 mm is less than 4% in area fraction. , it is preferable that the impact toughness at -20°C measured in the range FL~FL+1mm is 33J or more. By minimizing the MA fraction in the FL ~ FL + 1 mm region of the weld heat-affected zone, low-temperature toughness in the medium heat input weld zone can be secured. There is no particular limitation on the microstructure of the weld heat-affected zone during the medium heat input welding, but in the steel material of the present invention, a large amount of alloy components are added to ensure strength, thereby forming a microstructure unfavorable to toughness, and thus the MA phase is minimized to improve toughness. It is important to secure. For example, the microstructure of the weld heat-affected zone may include a mixed phase of granular bainite and upper bainite.

Meanwhile, the base material microstructure of the steel material of the present invention is not particularly limited, but for example, the base material microstructure includes a mixed phase of acicular ferrite, granular bainite, and upper bainite. can do.

Next, one aspect of the steel manufacturing method of the present invention will be described in detail.

The steel material of the present invention can be manufactured through a process of reheating a steel slab that satisfies the above-described composition, performing rough rolling and finishing rolling, and then cooling. Below, each process is described in detail.

Slab reheating: 1050~1150℃

It is desirable to reheat the steel slabs meeting the above-described composition to a temperature range of 1050 to 1150°C. When the reheating temperature is set to 1050°C or higher, carbonitrides of Ti and/or Nb formed during casting may be dissolved in solid solution. In addition, in order to sufficiently dissolve the carbonitride of Ti and/or Nb, it is more preferable to heat to 1080°C or higher. However, when reheating to an excessively high temperature, there is a risk that austenite may coarsen, so the reheating temperature is preferably 1150°C or lower.

Rough rolling: above 900℃

The reheated steel slab is subjected to rough rolling to adjust its shape. The rough rolling temperature is preferably higher than the temperature (Tnr) at which recrystallization of austenite stops, and therefore, the rough rolling is preferably performed at a temperature of 900°C or higher. The effect of reducing the grain size can also be achieved through recrystallization of coarse austenite along with destruction of the cast structure such as dendrites formed during casting by rolling. In order to cause sufficient recrystallization and refine the structure, it is preferable that the total cumulative reduction rate of rough rolling is 40% or more.

Finishing rolling: above 800℃

Finishing rolling is performed to introduce the austenite structure of the rough-rolled steel sheet into a non-uniform microstructure. In order to provide maximum deformation within the structure, it is desirable to carry out the finishing rolling at a temperature of 800°C or higher. In order to create the finest structure possible, it is desirable that the cumulative reduction ratio of the finishing rolling is 50% or more. If the finishing rolling temperature is less than 800°C, ferrite precipitates during air cooling after completion of rolling and before water cooling, thereby lowering the strength, so it is preferable to carry out the finishing rolling at 800°C or higher.

Cooling after rolling: Cool at a cooling rate of more than 15℃/s until the temperature at point t/4 is below 700℃ (t: thickness of steel sheet)

If the cooling rate is less than 15°C/s or the cooling end temperature exceeds 700°C, the microstructure is not properly formed, making it difficult to secure a yield strength of 500MPa or more. The upper limit of the cooling rate is not particularly limited in the present invention, but in the technical field to which the present invention pertains, the cooling rate is possible at 100°C/s or more. Therefore, as a preferred example, the cooling rate is preferably 200°C/s or less.

Hereinafter, embodiments of the present invention will be described. Of course, various modifications to the following examples can be made by those skilled in the art without departing from the scope of the present invention. The following examples are for understanding of the present invention, and the scope of the present invention should not be limited to the following examples, but should be determined by the claims described below as well as their equivalents.

(Example)

A 300 mm thick steel slab having the composition shown in Table 1 below (the remainder being Fe and inevitable impurities) was reheated to a temperature of 1110°C, followed by rough rolling at 980°C, and finishing rolling at 860°C. . Afterwards, steel was manufactured by cooling to 620-560°C at a cooling rate of 25-37°C/s. However, in Table 2 below, in Comparative Example 5, reheating and rough rolling were performed under the same conditions as above using a steel slab having the composition of Invention Steel 2, but finishing rolling was performed at 730°C and then at 7°C/s. Steel was manufactured by cooling to 610°C at a cooling rate.

For the steel manufactured as above, the yield strength and impact transition temperature were measured and the results are shown in Table 2. In addition, the manufactured steel was welded with a heat input of between 150 and 200 KJ/cm, where the impact toughness and microstructure of the fusion line (FL) ~ FL + 1 mm of the weld heat-affected zone (HAZ) were measured. was analyzed and the results are shown in Table 2. The MA fraction is measured optically by the LePera etching method.

Steel grade	Steel composition (% by weight)
	C	Mn	Si	Al	Ni	Mo	Cr	Nb	Ti	N (ppm)	P (ppm)	S (ppm)
Invention Lecture 1	0.048	1.67	0.13	0.02	0.87	0.12	0.16	0.012	0.013	59	79	9
Invention Lecture 2	0.037	1.54	0.17	0.03	0.73	0.11	0.25	0.013	0.016	48	68	9
Invention Lecture 3	0.052	1.63	0.12	0.02	0.66	0.16	0.13	0.009	0.012	39	58	10
Invention Lecture 4	0.044	1.59	0.09	0.03	0.81	0.13	0.19	0.008	0.015	67	64	11
Comparison lecture 1	0.091	1.66	0.13	0.03	0.72	0.13	0.22	0.016	0.019	37	68	12
Comparison lecture 2	0.054	1.68	0.19	0.03	0.37	0.17	0.12	0.018	0.015	46	61	9
Comparison lecture 3	0.059	1.65	0.26	0.02	0.88	0.12	0.26	0.039	0.014	49	52	8
Comparison lecture 4	0.055	1.57	0.11	0.02	0.79	0.13	0.21	0.018	0.022	21	53	11

division	Steel grade	base material yield strength (MPa)	Base material impact transition temperature (℃)	heat input (kJ/cm)	Fusion Line(FL)~FL + 1mm area MA fraction (%)	Fusion Line Average CVN Energy @ -20℃ (J)	Fusion Line+1mm average CVN Energy @ -20℃ (J)
Invention Example 1	Invention Lecture 1	575	-68	178	2.1	73	59
Invention Example 2	Invention Lecture 2	537	-78	167	3.2	56	43
Invention Example 3	Invention Lecture 3	556	-63	189	1.7	42	68
Invention Example 4	Invention Lecture 4	549	-71	157	1.3	64	79
Comparative Example 1	Comparison lecture 1	586	-51	163	5.2	28	24
Comparative example 2	Comparison lecture 2	544	-57	175	2.3	26	35
Comparative Example 3	Comparison lecture 3	561	-53	166	6.7	19	11
Comparative example 4	Comparison lecture 4	559	-56	171	3.1	12	23
Comparative Example 5	Invention Lecture 2	472	-75	173	3.5	49	51

Inventive examples that satisfy the conditions of the present invention all have a yield strength of the base material of 500 MPa or more, an impact transition temperature of -40°C or less, and a heat-affected zone welded with a heat input of 150 to 200 KJ/cm, where the Fusion Line (FL) ~ It can be seen that the MA fraction of FL + 1mm is less than 4%, and the impact toughness at -20℃ measured in the area from Fusion Line to FL + 1mm satisfies 33J or more.

In contrast, Comparative Example 1 contains more C than the present invention, and as a large amount of island-like martensite (MA) phase is generated in the weld heat-affected zone, the impact toughness measured in the region of FL ~ FL + 1 mm is It can be seen that it is less than 33J at -20℃.

Comparative Example 2 contained less Ni content than suggested in the present invention, and the lack of Ni addition caused a decrease in toughness, so that even though the MA fraction was less than 4%, the impact toughness measured at FL was less than 33J at -20°C. You can see that it happens.

Comparative Example 3 contains a large amount of Si and Nb as presented in the present invention, and as a large amount of MA phase is generated in the weld heat-affected zone, the impact toughness measured in the range FL ~ FL + 1 mm is less than 33J at -20 ° C. You can see that it happens.

In Comparative Example 4, although the MA fraction was less than 4% because the Ti content was added more than that suggested by the present invention and the N content was low, the particle size increased as TiN was coarsely precipitated in the weld zone, FL ~ FL It can be seen that the impact toughness measured in the +1mm region is less than 33J at -20℃.

On the other hand, Comparative Example 5 satisfies the components suggested by the present invention, but does not meet the manufacturing process, and the MA fraction of the weld heat-affected zone after welding is 4% or less, and the impact measured in the range FL ~ FL + 1 mm Although the toughness was more than 33J at -20℃, some ferrite was formed during finishing rolling and air cooling, and the low-temperature transformation phase was not properly created due to the slow cooling rate, so it was confirmed that the base material was manufactured with a yield strength of 500 MPa or less.

Claims (8)

1. In weight percent, C: 0.03~0.06%, Mn: 1.5~1.7%, Si: 0.05~0.2%, Al: 0.01~0.04%, Ni: 0.6~0.9%, Mo: 0.1~0.2%, Cr: 0.1~ 0.3%, Ti: 0.01-0.02%, Nb: 0.005-0.02%, N: 0.0035-0.0070%, P: 0.008% or less, S: 0.002% or less, the remainder includes Fe and inevitable impurities,

   In a welded heat-affected zone (HAZ) welded with a heat input of 150 to 200 KJ/cm, the MA fraction in the area from the fusion line (FL) to FL+1 mm is less than 4% by area, and the toughness of the welded heat-affected zone is excellent. Steel.
2. In claim 1,

   A steel material with excellent weld heat-affected zone toughness having an impact toughness of 33J or more at -20°C in the FL to FL+1 mm range.
3. In claim 1,

   A steel with excellent weld heat-affected zone toughness, wherein the microstructure of the steel includes a mixed phase of acicular ferrite, granular bainite, and upper bainite.
4. In claim 1,

   A steel material with excellent weld heat-affected zone toughness, wherein the microstructure of the weld heat-affected zone includes a mixed phase of granular bainite and upper bainite.
5. In claim 1,

   A steel material with excellent weld heat-affected zone toughness having a base material yield strength of 500 MPa or more and an impact transition temperature of -40°C or less.
6. In weight percent, C: 0.03~0.06%, Mn: 1.5~1.7%, Si: 0.05~0.2%, Al: 0.01~0.04%, Ni: 0.6~0.9%, Mo: 0.1~0.2%, Cr: 0.1~ 0.3%, Ti: 0.01~0.02%, Nb: 0.005~0.02%, N: 0.0035~0.0070%, P: 0.008% or less, S: 0.002% or less, the remainder containing Fe and inevitable impurities. Heating to 1150°C;

   Rough rolling the heated steel slab at a temperature of 900°C or higher;

   Manufacturing a hot rolled steel sheet by performing final hot rolling at a temperature of 800° C. or higher after the rough rolling; and

   A method of manufacturing a steel material with excellent weld heat-affected zone toughness, comprising cooling the hot rolled steel sheet at a cooling rate of 15°C/s until the temperature at point t/4 of the thickness (t) of the hot rolled steel sheet is 700°C or lower.
7. In claim 6,

   A method of manufacturing a steel material with excellent weld heat-affected zone toughness, wherein the total reduction rate of the rough rolling is performed at 40% or more.
8. In claim 6,

   A method of manufacturing a steel material with excellent welding heat-affected zone toughness, wherein the cumulative reduction ratio of the finishing rolling is performed at 50% or more.