CN1840726A - Steel material having excellent strength and toughness and manufacturing method thereof - Google Patents

Abstract

The invention provides a steel product with high strength and tenacity, and its preparing process, wherein the steel product comprises (by weight percent) C 0.02-0.15%, Si below 1%, Mn 0.3-2.5%, P below 0.05%, S below 0.004%, Al 0.001-0.1%, Ti below 0.02%, N below 0.009%, the metal structure being one of martensite or bainite or their tempering structure.

Description

Steel material having excellent strength and toughness and manufacturing method thereof

technical field

The present invention relates to a steel material having excellent strength and toughness and a method for producing the same.

Background technique

For steel, especially structural steel, it is often required to be excellent in both properties (strength and toughness). In order to meet the above requirements without adding expensive elements such as Ni, there have been many proposals and adoptions of methods for refining the structure through tempering and controlled rolling.

For example, Japanese Patent Publication No. 55-30050 discloses a method for producing tough steel. This method is to finely disperse AlN in the steel by specifying the chemical composition, the casting conditions of the iron ingot, and the heating conditions of the iron ingot during hot pressing, and use the AlN to suppress the growth of austenite grains and obtain a fine-grained structure.

This method can indeed obtain a fine-grained structure, but AlN is a precipitate that causes transverse cracks in the iron block during continuous casting, so it is difficult to apply to a high-efficiency production method such as continuous casting. JP-A-57-131320 discloses a method for producing a high-strength steel sheet having excellent low-temperature toughness. The method stipulates the temperature at the end of rolling and the cooling rate after that. However, since this method requires rolling at the temperature at which the austenite transforms from the non-recrystallized domain to the 2-phase domain, the rolling efficiency is very low. In addition, although the fracture transition critical temperature is improved, the absorption energy tends to decrease because precipitation tends to occur more easily. Therefore, this method cannot be said to be an effective method when the Charpy impact value requires the absorbed energy to be above a certain value.

In addition, JP-A-7-258730 and JP-A-7-258731 disclose a method for manufacturing a structural thick steel plate having excellent toughness and less acoustic anisotropy. In order to reduce acoustic anisotropy and ensure toughness, these methods make use of recrystallization of austenite to refine grains as much as possible, and perform rolling on non-recrystallized regions.

Although this method does not utilize the effect of controlled rolling, in order to obtain practical toughness, the temperature after rolling must be controlled at about 900°C or below 900°C, and the problem of low productivity caused by low-temperature rolling cannot be avoided. .

Although the above-mentioned methods ensure the required toughness by cleverly combining processing heat treatment with cooling and reheating to make the structure fine-grained, the reality is that the productivity of any of the above-mentioned methods is low in industrial scale production.

Contents of the invention

The present invention provides a steel material having excellent strength and toughness and a method for producing the steel material excellent in strength and toughness without the need to refine the structure and grains by controlled rolling, which causes a decrease in productivity.

The main contents of the steel material having excellent strength and toughness and its manufacturing method of the present invention are as follows.

1) A steel with excellent strength and toughness, characterized by:

Contains: % by weight, C: less than 0.02-0.15%, Si: less than 1%, Mn: 0.3-2.5%, P: less than 0.05%, S: less than 0.004%, sol.Al: 0.001-0.1%, Ti: 0.02 % or less, N: less than 0.009%, the metal structure is a structure containing one or both of martinite and bainite, or its tempered structure, and the average aspect ratio of old austenite grains is less than 1.5 , the average value of the short diameter of old austenite grains is 60-700 μm, and the content of Ti, N, S and the short diameter dr of old austenite grains satisfy the following formula (1) or (2):

When Ti/N<3.4,

$\mathrm{<span\; class="notranslate">\; Ti</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

When Ti/N≥3.4

$\mathrm{<span\; class="notranslate">\; 3.4</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; 315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

In the formula, the symbol of the element indicates the content of each element, and the unit is weight %, and the unit of dr is μm.

2) High heat input welding steel with excellent strength and toughness, characterized by: containing: weight%, C: less than 0.02-0.15%, Si: less than 1%, Mn: 0.3-2.5%, P: less than 0.05% , S: less than 0.004%, sol.Al: 0.001-0.1%, Ti: 0.004-0.02%, N: 0.001-0.009%, Ti/N is 0.4-4, and the metal structure is one of martinite and bainite One or both of the structures, or its tempered structure, the average aspect ratio of the old austenite grains is less than 1.5, the average value of the minor diameter of the old austenite grains is 60-700 μm, and Ti, N , S content and old austenite grain short diameter dr satisfy the following formulas (3) and (4):

$\mathrm{<span\; class="notranslate">\; Ti</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; 3.4</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; 315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

In the formula, the symbol of the element indicates the content of each element, and the unit is weight %, and the unit of dr is μm.

3) The method for producing steel having excellent strength and toughness as described in 1) or 2) above, wherein when the steel having the chemical composition as described in 1) or 2) above is subjected to hot working, the hot working is The minor diameter of the austenite grains at the end of hot working is 60 to 700 μm after finishing at 950°C or higher, and then directly quenched.

Here, the austenite grain (hereinafter referred to as γ grain) size refers to the old γ grain size in the metal structure of the steel obtained by cooling after hot working. The old γ grain boundary will easily appear due to corrosion in steel containing martinite and bainite, and the grain size can be identified and measured with an optical microscope. In addition, the steel material may be in any shape, and representative shapes include steel plate, steel pipe, section steel, and the like.

In order to develop a steel material with a tensile strength of 400 MPa or more and excellent toughness that can be obtained without microstructure in the production process and its manufacturing method, the present inventors used a small-scale rolling machine for trial testing in the laboratory field. Machines and heat treatment furnaces were tested under various conditions to investigate the strength and toughness of the obtained steel sheets.

At the beginning of the research, the aim was to improve production efficiency, and the premise was that the grain refinement method that relies on controlled rolling that causes low rolling efficiency, or the use of reheating quenching that requires a reheating process after rolling, is not used. Grain refinement method. In addition, the aim is to increase the finishing temperature of hot working as much as possible in order to improve productivity. However, if the material heating temperature and the calendering finishing temperature are increased without using controlled calendering, the gamma grains at the end of calendering must be coarse.

If controlled rolling is used, even if there are relatively coarse γ grains, the final structure can be refined and the toughness can be improved. However, since controlled rolling is not used, it is difficult to secure toughness unless the γ grains are sufficiently refined. In order to refine the γ grains, it is necessary to continue the rolling to a low temperature of 900° C. or lower, and the above-mentioned goal cannot be achieved.

In fact, the rolling test was carried out under uncontrolled rolling conditions. In order to ensure that the critical fracture transition temperature of the impact in the Charpy test is below -50°C, it is necessary to refine the γ grains to below 40 μm. For this reason, either end the rolling at 900°C or below, or reheat from the α region to reverse the transformation, and use AlN and NbC to lock the grain growth, then heating at about 950°C is easier to achieve a particle size of 40 μm or less. refinement. However, if the temperature exceeds 1000° C., the locked particles disappear by solid solution, coarsening occurs, and the toughness deteriorates remarkably.

The inventors of the present invention have made the following findings as a result of repeated studies on the development that steel materials having strength and toughness can be manufactured without performing γ-grain refining treatment in the production process.

(1) Steel materials in which the old γ grains become coarse grains deteriorate the toughness, but the reduction of S in the coarse grained state inhibits the precipitation of MnS, thereby significantly improving the transition temperature and absorption energy. However, in the case of finer γ grains, this effect cannot be expected.

(2) Ti N in steel also has adverse effects on toughness. By reducing N or Ti, the amount of Ti N precipitated is reduced, thereby improving the transition temperature. However, when the γ grains are fine grains, no improvement can be obtained.

(3) As described in (1) (2), the toughness improvement effect accompanying the purification by reducing the amount of MnS and Ti N precipitates does not contain bainite or martinite or tempering in the metal structure of the steel. When organized, it is almost impossible to get.

(4) For elements that form inclusions such as Ca and REM, it is also believed that coarse γ grains also have a negative effect on the transition temperature, so it is best to reduce them. However, this adverse effect is smaller than that of MnS and Ti N, and is not as important as the reduction of MnS and Ti N in terms of toughness.

(5) If the precipitation of MnS and Ti N is sufficiently reduced, the transition temperature will only increase slightly even if the γ particle size exceeds 60 μm. Furthermore, even if a site with a particle size exceeding 100 μm occurs, the increase in the transition temperature is slight.

(6) In the condition of limiting the precipitation of MnS and Ti N, because the hardenability can be increased by coarsening the γ grains, the hardenability can be increased, so if the γ grains are above 60 μm, high-strength steel can be manufactured at low cost. In addition, since there is no need to control rolling, the γ grains can be transformed from a completely recrystallized state to make the structure uniform, so high-quality products can be manufactured stably. As an approximate value of the recrystallized state, the average aspect ratio of the γ grains can be produced at a value of 1.5 or less under the condition that the average aspect ratio is appropriate.

(7) When coarsening the γ grains, it is necessary to appropriately reduce MnS and Ti N according to the short diameter dr of the γ grains. The γ grain size can be limited by the formula described later, and the height of the γ grain size can be guaranteed within a wide range. toughness. However, when the γ particle size exceeds 700 μm, the adverse effect of the coarsening on the toughness cannot be ignored.

The present invention was completed based on these findings, and since the present invention does not need to refine the structure by controlled rolling and tempering, productivity can be improved.

Description of drawings

Figure 1 shows the relationship between γ particle size and toughness.

Fig. 2 shows the collection position of the Charpy test piece.

Detailed ways

Specific embodiments of the present invention will be described below in conjunction with the accompanying drawings.

Figure 1 shows the relationship between γ particle size and toughness. Using steel (mouth symbol) containing Ti: 0.019%, S: 0.0041% and N: 0.0057% and steel containing Ti: 0.006%, S: 0.0009% and N: 0.0015% (■), changing various rolling temperatures Calendering, the result of impact fracture transition critical temperature was obtained through Charpy test.

It can be clearly seen from Figure 1 that no matter whether the content of Ti, S and N is large or small, the toughness will decrease as long as the grain size becomes larger, but as shown by the black square symbol in the figure, if the content of Ti, S and N is reduced, Especially when the content of S is reduced, the toughness is remarkably improved. In addition, the toughness improvement effect is remarkable when the γ particle size is large.

Next, the reason for limiting the chemical composition of the steel material in the present invention will be described (hereinafter, % represents weight %).

C is an element necessary for securing the strength, but if it is less than 0.02%, the necessary strength cannot be secured. On the other hand, if it exceeds 0.15%, the toughness of the weld heat-affected zone and the base metal will deteriorate during welding. Therefore, the content of C is 0.02 to 0.15%.

Si has a deoxidation effect and is also beneficial to improve the strength of the steel plate. However, if the content exceeds 1%, the toughness will deteriorate, so 1% is made the upper limit. In addition, as long as the deoxidation of steel is not affected, it does not matter how much Si can be reduced.

Mn has an effect of improving hardenability and is an effective component for securing strength. However, if the content is less than 0.3%, strength and toughness cannot be obtained due to insufficient hardenability. On the other hand, if the content exceeds 2.5%, the hardenability becomes too high as segregation increases, and the toughness of the weld heat-affected zone and the base metal during welding deteriorates. Therefore, the content of Mn is 0.3 to 2.5%.

P exists inevitably in steel as an impurity. If it exceeds 0.05%, the segregation to the grain boundary not only lowers the toughness, but also causes high-temperature cracks during welding, so it is 0.05%.

S combines with Mn, Ca or REM to form oxysulfides, which exist in steel as inclusions. In the case of low steel strength or very fine structure, these inclusions will not have a great adverse effect on toughness, but if it is a certain degree of coarse structure, its content must be limited to meet the formula described later. However, even if the formula is satisfied, adverse effects on toughness cannot be avoided when the content is 0.004% or more. More preferably, it is less than 0.003%.

Al is an essential element for deoxidation, and if 0.001% or more of sol.Al is not contained, the steel quality will deteriorate due to insufficient deoxidation. However, if the content exceeds 0.1%, the toughness of the base metal will deteriorate and the toughness of the weld will decrease, so 0.1% is made the upper limit.

Ti is generally contained in steel as an element capable of improving high-temperature ductility by fixing N in steel. However, since Ti N is the cause of the decrease in toughness, it is desirable not to add Ti as much as possible, and the allowable range in terms of toughness is limited by the formula described later. However, even if the formula is satisfied, if the content exceeds 0.02%, it is difficult to avoid deterioration of toughness.

In addition, excessive purification of steel materials subjected to high heat input welding may sometimes lead to excessive coarsening of γ grains and deterioration of toughness. Therefore, it is preferable to contain Ti at 0.004% or more, and to control the ratio of Ti/N to be in the range of 0.4 to 4.0.

N is an impurity that causes a decrease in high-temperature ductility, and usually, this adverse effect is avoided by adding Ti and fixing it as Ti N. However, in the present invention, since Ti N itself also becomes a cause of toughness deterioration, it is necessary to suppress the formation of Ti N. For this reason, either reduce N itself, or reduce the content of Ti.

In order to obtain excellent toughness, the range of N content needs to satisfy the formula described later. However, even if the formula is satisfied, if N exceeds 0.009%, or the toughness decreases due to Ti N, or the adverse effect of solid solution N that is not sufficiently fixed on toughness cannot be ignored. In addition, when N is 0.001% or less, the growth of γ grains becomes very easy under the condition that S decreases and MnS hardly exists. Therefore, when welding is performed with a large heat input of about 100 kJ/cm by submerged arc welding or the like, γ grains may be locally coarsened in the weld heat-affected zone.

Although the steel of the present invention has the property that the deterioration of toughness caused by the coarsening of γ grains is not easy to occur, it has a hardness distribution in the heat-affected zone of welding with large heat input, and the size of the crystal grains is uneven, so from the toughness The upper limit of the γ particle size allowed by the surface is about 300 μm. Therefore, on the premise of welding with large heat input, it is necessary to contain a certain degree of Ti N, which has the effect of inhibiting the growth of γ grains. It can contain more than 0.001% of N, and it can also contain some Ti.

On the other hand, for steels that do not need to be welded, and steels that can only be welded with a small heat input below 40kJ/cm, N can be reduced as much as possible within the economical allowable range.

The steel material of the present invention may contain the following elements for improving hardenability and strength as needed, in addition to the above-mentioned elements.

Cr is an element useful for improving hardenability. Although the minimum necessary hardenability can be ensured only by using the above-mentioned essential elements, it is used to further ensure hardenability when the steel material is a thick steel pipe or the like. If the content of Cr is 0.02% or more, in addition to hardenability, the effect of improving temper softening resistance can be obtained, so it is desirable to be 0.02% or more. However, if it exceeds 1.5%, the toughness of the weld cannot be avoided, so it should be 1.5% or less.

Mo is an element preferably contained in order to further improve hardenability and temper softening resistance when the steel material is a thick steel pipe or the like. However, if the content is less than 0.02%, these effects cannot be obtained, so it is desirable to contain Mo in an amount of 0.02% or more. However, if it exceeds 1%, the toughness of the welded portion will remarkably deteriorate, so the upper limit is preferably 1%.

B is particularly useful for increasing the hardenability and strength of the γ grain boundaries. The content is preferably 0.003% or less.

Nb is an essential additive element in steel materials produced by so-called controlled rolling, but in the present invention, controlled rolling is basically not used, so it is not an essential element. However, it is effective to further increase the strength, but if the content is increased, the toughness will be greatly impaired by strengthening precipitation when the rolling is completed at a high temperature of 1000°C or higher. Therefore, the content must be kept below 0.015%. More preferably below 0.01%.

V has the effect of increasing the strength by strengthening the precipitation, has relatively little influence on the toughness, and is effective for increasing the strength. If the content is 0.01% or more, in addition to the temper softening resistance, it also has the effect of improving hardenability, and it is preferably 0.01% or more. However, if it exceeds 0.15%, the toughness will deteriorate greatly, so the upper limit is preferably 0.15% or less.

Cu is effective in improving strength and corrosion resistance, so when further high yield strength and high corrosion resistance are required, Cu can be contained. If the content is 0.05% or more, the hardenability can be improved in direct quenching, so it is preferably 0.05% or more. However, even if it is added in excess of 1.5%, there is no performance improvement corresponding to an increase in cost, so the upper limit is preferably 1.5% or less.

Ni has the effect of increasing the toughness of the steel matrix (matrix) in a solid solution state, so Ni can be contained when it is necessary to obtain more excellent toughness stably. If the content is more than 0.05%, the effect of improving the hardenability can be achieved, so it is more than 0.05% ideally. However, if it exceeds 4%, the toughness improvement corresponding to the increase in alloy cost cannot be obtained, so the upper limit is preferably 4%.

Ca reacts with S in steel to form sulfate in molten steel. Unlike MnS and the like, this sulfate does not extend in the rolling direction by rolling, but is spherical even after rolling. Therefore, in order to suppress welding cracking or hydrogen induced cracking (HIC: Hydrogen Induced Cracking) starting from the front end of the extended inclusion, etc., it can be contained in the case of suppressing welding cracking or HIC. If the content is more than 0.0002%, it also has the effect of improving the toughness of the welded part, so it is more preferable to be more than 0.0002%. However, if it exceeds 0.004%, the toughness of the base material will decrease due to the decrease in cleanliness.

REM contributes to the refinement of the microstructure of the welded heat-affected zone and the fixation of S, but it will become inclusions and reduce the cleanliness. However, since the addition of inclusions formed by REM has relatively little effect on the deterioration of toughness, even if the content is less than 0.004%, the degree of toughness reduction of the base metal is within the allowable range.

Next, the metallic structure and prior austenite grains will be described.

1) Metal structure

In order to make the tensile strength above 450MPa, the metal structure of the steel needs to contain one or both of bainite and martinite transformed at low temperature, or their tempered structure, and the other contains pure granular iron, Nacre iron. Such a structure can be obtained by performing hot rolling, quenching from the γ region, and tempering if necessary.

2) Aspect ratio of old gamma grains

The reason why the average value of the aspect ratio of the old γ grains is set at 1.5 or less is to prevent a decrease in anisotropy and a decrease in strength. The γ grains that have undergone processing and have been dislocated inside also generate nuclei from the dislocated α phase in the grains, so the hardenability decreases and the strength decreases. In order to prevent this phenomenon, it is necessary to sufficiently recrystallize the γ grains (the recrystallized γ grains have an aspect ratio close to 1) before transforming. If the average value of the aspect ratio of the old gamma grains is 1.5 or less, the decrease in strength can be prevented.

In addition, the average value of the aspect ratio is to select the azimuth of the surface where the most elongated gamma grains can be observed, cut the sample for optical microscope, make the fibrous structure appear, measure the old gamma grains by image processing, and approximate it with each gamma grain The ratio of the major axis to the minor axis in the case of an ellipse is an average value.

3) Average short diameter of old gamma grains

In the present invention, in order to improve production efficiency, low-temperature processing for refining the structure is not performed, so the old γ grains are coarse grains. In addition, the reduction of Ti, N, and S due to coarse graining has a significant effect on toughness and strength. If the average minor diameter of the old γ grains is less than 60 μm, desired strength and toughness cannot be obtained. On the other hand, if it exceeds 700 μm, the toughness will deteriorate due to excessive roughening.

4) The relationship between Ti, N, S and the short diameter of old γ grains

When the content ratio of Ti and N is less than 3.4, Ti/N, if the following formula (1) is not satisfied, the content of Ti and S will be too much, and the amount of Ti N and MnS will increase to cause toughness to deteriorate. This formula is a formula summed up through many experiments, and the appropriate amount of Ti and S is specified according to the short diameter of the old γ grains.

When Ti/N<3.4,

$\mathrm{<span\; class="notranslate">\; Ti</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In addition, when the Ti/N content ratio Ti/N is 3.4 or more, that is, when the N content is less than Ti, if the following formula (2) is not satisfied, the N and S content will be too large, thereby increasing the amount of Ti N and MnS cause toughness to deteriorate.

When Ti/N≥3.4

$\mathrm{<span\; class="notranslate">\; 3.4</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; 315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

When the steel of the present invention is used for high heat input welding, the range of Ti/N is 0.4 to 4, and if the following formulas (3) and (4) are not satisfied, the γ grains in the heat-affected zone of the welded part are too coarse The toughness of this part deteriorates. That is, when welding with large heat input, the amount of Ti and N needs to be slightly larger, so that Ti N and MnS can be precipitated to a certain extent to suppress grain growth in the heat-affected zone.

$\mathrm{<span\; class="notranslate">\; Ti</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; 3.4</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; .</span>\mathrm{<span\; class="notranslate">\; 315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

In the formula, the symbol of the element indicates the content of each element, and the unit is weight %, and the unit of dr is μm.

Hereinafter, the manufacturing method of the steel material of this invention is demonstrated.

When hot-working the steel with the above chemical composition, by controlling the hot-working temperature so that the short diameter of the austenite grains is 60-700 μm at the end of the hot-working, the steel with better strength and toughness can be obtained by direct quenching after hot-working.

The hot-working temperature at which the short diameter of the austenite grains at the end of hot-working is 60-700 μm varies depending on the chemical composition and the degree of processing during hot-working, but generally, 950° C. or higher is guaranteed as a standard. In addition, as long as the old γ particle size does not exceed 700 μm, good performance can be obtained no matter how high the finishing temperature is, but it is difficult to ensure that the finishing temperature exceeds 1150°C in the actual production line. In addition, such a temperature will increase the loss of steel due to the occurrence of scale. From this point of view, the substantial upper limit of the finishing temperature is about 1150°C.

Although it is not necessary to perform hot rolling at a low temperature to ensure toughness, if the rolling is performed at a temperature below 900°C by 30% or more, the effect of refining the γ grains and controlling the rolling may occur, resulting in a significant decrease in strength. . Due to this property, it becomes a cause of unstable quality variation when steel materials are mass-produced, so processing at low temperatures must be avoided.

In order to avoid this adverse effect, the hot rolling end temperature must be controlled so that the old γ grain size does not fall below 60 μm, and the γ grains are water-cooled in a state where they are not work-hardened. Cooling for quenching after processing does not necessarily have to be water cooling, but at least it is desirable that the bainite or martinite of the transformed structure is fibrous and occupies more than 40% of the area, and a cooling rate corresponding to this is required. Its cooling conditions can be estimated from the CCT map.

[Example]

In the vacuum melting furnace, 150 kg of round steel ingots having 16 kinds of chemical compositions shown in Table 1 were melted. In addition, steels having 10 kinds of chemical compositions shown in Table 2 were smelted in an actual 250 t converter, and continuously cast into iron ingots with a thickness of 150 to 300 mm.

Table 1 (Remainder: Fe and unavoidable impurities) mark C Si mn P S Al Ti N other Ti/N Example 12345678 0.0960.1370.0360.0610.0950.0500.0490.052 0.300.190.160.190.180.160.320.10 1.351.262.211.430.910.781.071.34 0.0090.0090.0110.0100.0120.0090.0110.010 341216127558 0.0210.0230.0230.0260.0280.0290.0250.024 0.0050.0060.0170.0040.0060.0010.0020.008 14144061052220 B: 8, Ca: 2, Mg: 3Ca: 2, Mg: 4Ca: 2, Mg: 2Cr: 0.10, Mo: 0.29, Ca: 2, Mg: 3Ni: 0.48, Cr: 0.15, Mo: 0.61, Ca: 2, Mg: 2Ni: 0.19, Cr: 0.99, Mo: 0.14, Ca: 2, Mg: 3Cu: 1.21, Ni: 0.78, Ca: 2, Mg: 4, REM: 6Nb: 0.010, V: 0.029, Ca: 2, Mg: 4 6.404.114.257.025.571.823.173.87 comparative example 910111213 ^* 1415 ^* 16 ^* 0.1000.1380.0370.0610.0990.0510.0510.048 0.290.200.150.190.200.150.300.10 1.311.262.181.380.920.791.081.34 0.0100.0100.0100.0100.0110.0080.0100.010 51 ^* 58 ^* 1981922926 0.0260.0260.0280.0270.0260.0260.0270.025 0.0090.029 ^* 0.025 ^* 0.0020.0040.021 ^* 0.0110.010 33306998 ^* 11453219 B: 8, Ca: 2, Mg: 3Ca: 2, Mg: 3Ca: 2, Mg: 41Cr: 0.10, Mo: 0.31, Ca: 2, Mg: 3Ni: 0.50, Cr: 0.15, Mo: 0.63, Ca: 2, Mg: 2Ni: 0.19, Cr: 1.05, Mo: 0.15, Ca: 48, Mg: 3Cu: 1.23, Ni: 0.78, Ca: 2, Mg: 4, REM: 43Nb: 0.022, V: 0.049, Ca: 2, Mg: 3 2.769.563.680.224.0040.353.375.50

(S, N, B, Ca, Mg, REM are in ppm, others are by weight%,

*Indicates outside the scope of the present invention. Symbols 13, 15, and 16 are relational expressions that do not satisfy the present invention (see Table 3). )

Table 2 (Remainder: Fe and unavoidable impurities) mark C Si mn P S Al Ti N other Ti/N Example 1718192021 0.0720.1180.0820.0730.048 0.320.280.240.200.11 1.490.521.101.421.31 0.0110.0100.0090.0080.010 121251014 0.0270.0240.0240.0280.028 0.0110.0070.0060.0120.007 4019243323 Cu: 0.31, Ni: 0.35Cr: 0.10, Mo: 0.10, Nb: 0.06, Ca: 2, Mg: 3V: 0.110, Ca: 2, Mg: 3, REM: 10 2.753.623.013.693.07 comparative example 2223242526 0.0680.1240.0770.0710.050 0.320.280.250.190.11 1.450.501.181.381.26 0.0090.0080.0100.0110.010 298988 0.0240.0280.0270.0240.026 0.0060.0100.002**0.002**0.001** 9100*6549 Cu: 0.31, Ni: 0.34Cr: 0.10, Mo: 0.10, Nb: 0.004, Ca: 2, Mg: 3V: 0.106, Mg: 2, REM: 49 6.440.983.050.36 ^* 3.05

(S, N, B, Ca, Mg, REM are in ppm, others are by weight%,

*Indicates outside the scope of the present invention. **It is outside the range specified by the present invention when it is used as a large heat input steel material. )

As shown in Table 3, the steel ingot is forged into a thick plate with a thickness of 120-170 mm. After heating at 1180-1270 ° C, it is hot-rolled to obtain a hot-rolled steel plate with a thickness of 25-50 mm.

In addition, the continuously cast iron block was heated at 1200-1250° C. as shown in Table 4, and then hot-rolled to obtain a hot-rolled steel plate with a thickness of 25-40 mm. These hot-rolled steel sheets were water-quenched as shown in Table 3 and Table 4, and quenched-temper heat-treated partly.

table 3 mark old gamma grain short diameter aspect ratio Ti/N left side of formula 1 Right side of formula 1 Blank thickness mm Blank heating temperature ℃ Final product thickness mm Calender end temperature ℃ heat treatment Tempering temperature °C Yield strength MPa Tensile strengthMpa Transition temperature vTrs℃ Example 12345678 627269612106731057 1.11.21.21.01.11.21.11.5 6.404.114.257.025.571.823.173.87 0.0320.0150.0260.0120.0090.0050.0060.013 0.0450.0370.0390.0450.0120.0410.0080.049 300200150300150150120200 12501200127011801250121012701270 2525402035405035 9901080110010001110107011501070 Water quenching water cooling to 500°C stop water quenching water quenching water cooling to 500°C stop water cooling to 500°C stop water quenching water quenching 600-600600--600600 473437485530506501477518 563534570608586581560595 -73-97-59-107-91-82-54-92 comparative example 9 ^* 10 ^* 11 ^* 12 ^* 1314 ^* 1516 597358671887725057 ^* 1.21.31.11.21.01.31.11.4 2.769.563.680.224.0040.353.375.50 0.0500.0570.0390.0080.0190.0330.0180.015 0.047 ^** 0.037 ^** 0.0490.0400.014 ^** 0.0350.010 ^** 0.028 ^** 300200150300150150120200 12501200127011801250121012501270 2525402035405035 990109011109901110107011401060 Water quenching water cooling to 500°C stop water quenching water quenching water cooling to 500°C stop water cooling to 500°C stop water quenching water quenching 600-600600--600600 479426481504506499475544 569528574580584580566618 -45-26-16-20-18-2922-10

(* represents outside the specified scope of the present invention. * expression composition in the mark column is outside the specified scope of the present invention. ** expression does not satisfy the relational expression of the present invention.)

Table 4 mark old gamma grain short diameter aspect ratio Ti/N left side of formula 1 Right side of formula 1 Blank thickness mm Blank heating temperature ℃ Final product thickness mm Calender end temperature ℃ heat treatment Tempering temperature °C Yield strength MPa Tensile strengthMpa Transition temperature vTrs℃ Seam transfer temperature vTrs℃ Example 1718192021 80631447353 1.01.11.21.21.2 2.753.623.013.693.07 0.0210.0160.0100.0190.018 0.0340.0440.0180.0370.055 300300200200150 12501200125012301250 3025304040 1090106011501050990 Water quenching, water cooling to 500°C, stop water quenching, water quenching, water quenching 600-600600600 410398415427465 518511520527559 -89-87-77-73-70 -64-50-56-50-44 comparative example 2223 ^* 24 ^* 25 ^* 26 ^* 87397014451 1.21.11.11.21.2 6.44 ^* 0.983.050.36 ^* 3.05 0.0260.0160.0100.0090.007 0.0310.0950.0390.0180.057 300300200200150 12501200125012301250 3025304040 1090106011501050990 Water quenching, water cooling to 500°C, stop water quenching, water quenching, water quenching 600-600600600 428397415428458 526510518523553 -99-25-126-104-51 -43-39-36-34-22

(* indicates that it is outside the scope of the present invention. The * in the mark column indicates that the composition is outside the scope of the present invention.)

JIS4 No. Charpy test pieces and round bar tensile test pieces were prepared from each heat-extended steel plate for Charpy impact test and tensile test.

In addition, submerged arc welding was performed on the hot-rolled steel sheets of Nos. 17 to 26, and a Charpy impact test was performed. Welding is two-sided one-layer welding of V forging groove. The welding heat input is 70kj/cm for steel with a thickness of less than 30mm, and 100kj/cm for steel with a thickness of more than 30mm.

Fig. 2 shows the collection position of the Charpy test piece. As shown in the same figure, on the cross-section (notch portion) of the test piece 1 after the test, the weld metal portion 1 and the weld heat-affected zone 2 are almost half each.

Table 3 and Table 4 collectively show the results of each experiment. As can be clearly seen from Table 3, although the gamma grains are coarse as a result of finishing rolling at a high temperature of 990°C to 1100°C, in the examples of the present invention, it is shown that -50°C or below is sufficient for any application. toughness. Moreover, since the composition contains a small amount of Nb or hardly contains Nb, it is not good for ensuring strength. However, since the hardenability is enhanced by γ grain coarsening, a yield point strength exceeding 400 to 500 MPa can be obtained.

In Table 4, in addition to the mechanical properties of the base metal, the results of the simulated thermal cycle test are also shown.

Examples 17 to 21 also showed good weld heat-affected zone toughness. However, marks 22, 24, and 25 in the comparative example have good toughness after heat treatment, but the toughness of the heat-affected zone is greatly deteriorated due to excessive cleaning of the steel.

For the forged materials with the chemical compositions marked 1 to 8 in Table 1, hot rolling was carried out at a relatively low temperature below 900°C. Table 5 shows the rolling conditions and heat treatment conditions. The same tests as above were carried out from the heat-treated steel sheets. The results are also shown in Table 5.

table 5 mark old gamma grain short diameter aspect ratio Blank thickness mm Blank heating temperature ℃ Final product thickness mm Calender end temperature ℃ heat treatment Tempering temperature °C Yield strength MPa Tensile strengthMpa Transition temperature vTrs℃ comparative example 12345678 30 ^* 28 ^* 27 ^* 31 ^* 36 ^* 32 ^* 35 ^* 35 ^* 2.21.92.12.21.71.91.82.0 300200150300200150300200 12501200127011801200121012501270 2525402035405035 810830780800840830850880 Water quenching water cooling to 500°C stop water quenching water quenching water cooling to 500°C stop water cooling to 500°C stop water quenching water quenching 600-600600--600600 378337382435409399377445 492478498533517511493542 -92-100-73-125-96-96-71-98

(* means outside the scope of the present invention.)

Although the toughness is somewhat improved by low-temperature rolling, the strength is greatly reduced compared with Table 3. Considering that sufficient toughness has been secured at the stage shown in Table 3, such finishing of low-temperature rolling only impairs the strength and has no advantage. On the contrary, considering the fact that the steel reheated and quenched at high temperature in Table 4 now shows good strength and toughness, the steel shown in Table 5 whose temperature dropped excessively during the rolling process was heated before water cooling. Heating again in the furnace to above 1000°C, fully recrystallizing, coarsening the gamma grains and then water cooling, can obtain good performance.

[Invention effect]

By adopting the invention, high-strength and excellent toughness steel can be produced, the addition amount of alloy is small, and high productivity can be maintained continuously. This means that the production volume of high-performance steel can be increased without having to enhance new equipment, which is very beneficial in steel production.

Claims (3)

1. A steel material with excellent strength and toughness, characterized in that it contains: % by weight, C: less than 0.02-0.15%, Si: less than 1%, Mn: 0.3-2.5%, P: less than 0.05%, S: Less than 0.004%, sol.Al: 0.001-0.1%, Ti: 0.02% or less, N: 0.009% or less, the metal structure is a structure containing one or both of martinite and bainite, or its tempered structure, The average value of the aspect ratio of the old austenite grains is below 1.5, the average value of the short diameter of the old austenite grains is 60-700 μm, and the content of Ti, N, S and the short diameter dr of the old austenite grains satisfy the following Formula (1) or the following formula (2): When Ti/N<3.4, $\mathrm{<span\; class="notranslate">\; Ti</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$ When Ti/N≥3.4 $\mathrm{<span\; class="notranslate">\; 3.4</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$ In the formula, the symbol of the element indicates the content of each element, and the unit is weight %, and the unit of dr is μm. 2. A high heat input welding steel with excellent strength and toughness, characterized by: containing: weight %, C: less than 0.02-0.15%, Si: less than 1%, Mn: 0.3-2.5%, P: 0.05 % or less, S: less than 0.004%, sol.Al: 0.001 to 0.1%, Ti: 0.004 to 0.02%, N: 0.001 to 0.009%, Ti/N is 0.4 to 4, and the metal structure contains martinite and bainite One or both of the structures, or its tempered structure, the average aspect ratio of the old austenite grains is less than 1.5, the average value of the minor diameter of the old austenite grains is 60-700 μm, and Ti , N, S content and prior austenite grain short diameter dr satisfy the following formulas (3) and (4): $\mathrm{<span\; class="notranslate">\; Ti</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$ $\mathrm{<span\; class="notranslate">\; 3.4</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 8.1</span>}\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 0.315</span>}}{\sqrt{\mathrm{<span\; class="notranslate">\; dr</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 30</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 0.011</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$ In the formula, the symbol of the element indicates the content of each element, and the unit is weight %, and the unit of dr is μm. 3. A method for manufacturing the steel with excellent strength and toughness described in claim 1 or 2, characterized in that: when the steel with the chemical composition described in claim 1 or 2 is hot-worked, Hot working is completed at 950° C. or higher so that the minor diameter of the austenite grains at the end of hot working is 60 to 700 μm, and quenched directly.

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