JPS61136624A - Manufacture of high toughness steel material having thick section and yield point of 42-<90kgf/mm2 - Google Patents

Abstract

PURPOSE:To obtain thick steel material having superior strength, toughness and a desired yield point, by casting a steel contg. specified quantities of Mn, Nb, Al, etc., cooling, reheating, soaking and rolling said ingot under controlled temp.s, water cooling, then heating and tempering said plate. CONSTITUTION:The steel contg. 0.04-0.18wt% C, <=0.35% Si, 0.6-1.8% Mn, 0.005-0.04% Nb, 0.01-0.08% total Al, <=0.005% N, if necessary 0.0005-0.0015% B, 0.1-1.5% Cu, 0.2-1.5% Ni, 0.10-0.5% Cr, 0.05-0.6% Mo, 0.01-0.05% V, and the others of Ti, Ca, REM, etc., is cast. The steel slab is cooled so that up to 1/4 of the thickness becomes to 400-850 deg.C, next, heated and soaked at 900-1,050 deg.C, rolled at 800-920 deg.C by >=20% draft. Next, water cooling is started at the time point when surface temp. is attained to T>=Tr-20, and is stopped at the time point when attained <=350 deg.C recuperating temp. Next. said plate is heated to <=700 deg.C and tempered. Tr is obtd. by a specified formula.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention provides a thick section with a yield point of 42, which has excellent strength and toughness, and excellent weldability, and has a thickness of #f 2 or more and 90 klf/, and less than 2. Concerning the manufacturing method of high toughness steel materials.

(Conventional technology) In recent years, steel structures have become larger and have higher toughness, and the demands for steel materials used in them are thicker cross-sections and higher yield points, as well as low-temperature toughness and weldability without preheating. Excellent weld cracking resistance is required.

On the other hand, due to competition with concrete and other materials,
It is required to be as economical as possible.

In response to these demands, various new manufacturing methods have recently been developed to improve quality.

For example, as described in Japanese Patent Publication No. 55-30047, by heating the steel billet at a lower temperature than before, the austenite grain size before rolling is made finer, and the toughness improvement effect of subsequent rolling and water cooling is improved. Alternatively, as described in JP-A-57-131320, after continuous casting, the surface of the steel slab is heated to 1.100 to 750°C.
There are many methods, such as starting rough rolling at t-, finishing the rolling at ~650°C in Ar, then cooling with water to an appropriate temperature between 650 and 400°C, and then cooling at a cooling rate lower than air cooling. are seen here and there.

These methods are all methods for producing high-strength steel with high toughness, but they have disadvantages, especially when producing thick steel plates, such as significant strength differences in the thickness direction and easy occurrence of hydrogen embrittlement defects. In order to solve these problems, it was necessary to take completely other technical measures.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for removing the above-mentioned problems in manufacturing high-toughness steel materials with thick sections.

(Means for solving the problem) As a result of various studies, the present inventors have solved the problem by adopting various manufacturing conditions that can control the behavior of Nb in steel. I have come to the conclusion that this is possible.

In other words, Nb in steel mainly exists as carbon and nitrides of Nb at room temperature, but the steel billet after casting and the billet after blooming are in a high temperature state, and Nb is solidified as a single element in the steel base. Melt it.

This solid solution Nb has the effect of increasing the upper limit temperature of the non-recrystallized region during rolling and making it easier to roll to obtain a fine grain structure. In this case, Nb, which is properly cooled due to water cooling, is charcoal and Nb during tempering.
It is known that it precipitates as a nitride, and this precipitation hardening effect is used to improve strength.

Therefore, when a steel billet is once cooled to room temperature as in the past, Nb in the steel is precipitated as carbon and nitrides, so when it is heated and rolled again, this Nb carbon and nitrides are It is necessary to dissolve it into the base steel again, so in reality it is 110
Heating is performed at a high temperature of 0°C to 1300°C. Therefore, the present inventors attempted to omit the high-temperature heating process of the steel billet by directly using the solid solution Nbi. In other words, when cooling a steel billet, heat is removed from the surface of the billet, so naturally a temperature gradient occurs from the surface side to the inside of the billet. When the temperature reaches between 400 and 850°C, cooling is stopped, the heating furnace is charged, and the temperature is heated again from 900°C to 1050°C.
It has been found that it is possible to eliminate the difference in strength in the thickness direction of steel materials that have been subsequently rolled, water-cooled, and tempered by soaking the steel materials after heating them to a temperature of .degree.

That is, as mentioned above, Nb after water cooling is K during tempering.
Nb precipitates as carbon/nitride and provides precipitation hardening action. - When water-cooling a thick cross-section steel material, heat is removed from the surface of the steel material, so the water-cooling rate in the cross-sectional direction is different, with the cooling rate being high near the surface and low near the center. Therefore, the cross-sectional hardness after water cooling differs greatly, resulting in a large difference in strength between the surface and center of the steel material. After tempering, this difference disappears to some extent, but it still remains, resulting in a steel material with a difference in strength in the cross-sectional direction.

On the other hand, as mentioned above, reheating the steel billet after once cooling it is the next means to eliminate this difference in strength in the cross-sectional direction. In other words, the surface side of the steel piece/fi40
When cooled to a temperature of 0 to 850°C, the metal in the steel precipitates as Nb carbon/nitride, but the central part is still in a solid solution state due to the high temperature.

In this state, stop cooling once and return to 900-1050℃.
When heated to a temperature of The structure becomes fine-grained bainite, fine-grained martensite, etc., but it reaches room temperature with most of the Nb-Fi remaining in solid solution.

On the other hand, the amount of Nb carbon/nitride precipitated on the surface side of the steel piece is 900~
Even when reheated at 1050°C, almost no dissolution occurs and most of the solution remains. Therefore, during subsequent rolling, the effect on rolling in the non-recrystallized area is less υ than in the center of #4 material, and after water cooling, N
Since the carbon and nitrides remain in the precipitated state, the precipitation hardening phenomenon during subsequent tempering is also significantly smaller than in the central part of the steel material.

This quantitative difference in the precipitation hardening phenomenon in the cross-sectional direction of the steel material offsets the above-mentioned strength difference in the as-quenched state in the cross-sectional direction after water cooling, and as a result, steel materials with small strength differences in the cross-sectional direction of the steel material can be manufactured. becomes possible.

(Structure and operation of the invention) The present invention has been made based on the above knowledge, and its gist is that
81, 810.351i or less, Mn 0.6-1.8
%.

Nb0. OO5~0.04, Total At0
．． 01 to 0.081, N 0.0054 or less, and further contains B 0.0005 to 0.01 if necessary.
5, Cu0.1-1.5, N1 0.2-1
．． 5 4, Cr 0.1 0-0.5 96
, MO0.0 5-0.6%, V0.
0 1~0.0 54, 7i 0.005~0
．． 024, Ca 0.0054 or less, RIM 0.0
After casting or blooming a steel containing one or more of the following 24 or less and the remainder consisting of F's and unavoidable impurities, the obtained steel slab is cast from the surface to 1/4 of the thickness of the steel slab. is 4
Cool to a temperature of 00 to 850°C, then 900 to 850°C.
Heating to 1050℃, soaking, and then rolling at 800 to 920℃ until the reduction amount is 204 or more, and then water cooling when the surface temperature T (6) of the obtained steel material becomes T≧Tr-20. The water cooling is started, and the water cooling is stopped when the reheating temperature after water cooling on the steel surface becomes 350°C or less, and after the entire steel reaches room temperature, it is heated to a temperature of 700°C or less and tempered. The present invention relates to a method for producing a high toughness steel material with a thick cross section having a yield point of 42 kpfAm or more and less than 90 kgf/m2.

However, Tr=910-310XC(@-80XMn(19
-20XCu(d-15XCr(d-55xNl(99
-80xMo(←35(t-80)+30t: Thickness of steel material.

The present invention will be explained in detail below.

First of all, in the present invention, the yield point is 42yufA*2Ae
*t of less than 90 kgt/1 paa, but the lower limit of yield point t 42 kllfAal is the target steel type in the present invention, which is generally produced by tempering after water cooling. This is because it includes tension steel. That is, as mentioned above, in order to have versatility as a structural steel, it is necessary to have both toughness and weldability.
A so-called quenching and tempering process is generally used, in which tempering is performed at the following temperatures: In this case, steel with a low yield point generally has a low chemical composition, so even in steel with a thick cross section, hardened structures do not form much after water cooling, so the difference in strength in the cross-sectional direction is generally small.

Further, it is possible to easily obtain strength without quenching and tempering. This is why the yield point f was set at 421C17f/g or higher.

On the other hand, regarding the upper limit of the yield point, if the yield point is high, steel with a thick cross section requires the addition of a large amount of alloying elements to ensure its strength, and the formation of a hardened structure after quenching depends on the water cooling rate. Since a component system is selected such that . However, it is not necessary to specifically adopt the lining method according to the present invention. yield point t
-90kl? This is why it is set to less than f/m.

In addition, the present invention is directed to steel materials with thick cross sections, and as mentioned above, this is particularly applicable to high toughness and high tensile strength tI4t-
When manufacturing steel materials with thick sections, there is a significant difference in strength in the cross-sectional direction, and internal defects due to hydrogen are likely to occur. The purpose is to improve steel. Although the thickness is not particularly specified, the steel materials having a yield point that are the object of the present invention are usually those with a thickness of about 25 fi or more, which tends to appear in the strength difference in the cross-sectional direction.

Furthermore, in the manufacturing method of the present invention, after melting steel with a specific composition range as described below, casting or blooming is performed, and then steel material is produced by a manufacturing method under specific conditions as described below. However, casting or blooming in this case refers to continuous casting, individual casting using a slab-shaped or plume-shaped mold made according to the shape of the target steel material,
Or it means a means of manufacturing a steel ingot obtained by normal ingot making into a steel billet by blooming rolling, and the as-cast steel billet obtained by these continuous castings or the steel billet after being rolled to reduce the thickness as necessary, The as-cast slabs obtained by the above-mentioned single casting, or the slabs after rolling to reduce the thickness as required, or the slabs after blooming of the steel ingots obtained by normal ingot making. Rolling is carried out in the middle of cooling by the manufacturing means described later without cooling at a high temperature.

Next, the reason for limiting the range of composition of steel targeted by the present invention will be described.

First, C is an element necessary to ensure hardenability and strength. If the content is less than 0.041, the content is too low, making it impossible to secure the desired strength even with a relatively small cross-sectional thickness. do not have. On the other hand, the C content is 0. Exceeding is*'e results in deterioration of weld cracking resistance, significant reduction in hardening of the weld heat affected zone, and deterioration of properties necessary for use such as toughness and stress corrosion cracking resistance. As a precaution, the C content is 0.04.
~0.181.

Next, Si is effective in increasing the strength, but if the Si content is large, high tensile strength steel becomes susceptible to temper embrittlement, and notch toughness is impaired. Therefore, in order to ensure sufficient strength and not reduce notch toughness, the upper limit is set at 0.35 mm. Mn is also necessary to ensure hardenability and prevent cracking during hot working. However, if it exceeds 1.8, the transformation point will be lowered and cold cracking will easily occur during welding.

Moreover, if it is less than 0.64, it is difficult to ensure hardenability, and the effect in preventing hot cracking is reduced due to the relationship with the S content. Therefore, the ratio is set at 0.6 to 1.8.

Furthermore, as mentioned above, Nb is a main element for grain refining effect during rolling and precipitation hardening during tempering, but 0.0
If the amount is less than 0!15'j, the desired effect will not be obtained and the gist of the present invention will be violated. On the other hand, if it exceeds 0.04%, the above effect becomes saturated, but on the other hand, it becomes a factor that reduces the weld heat affected zone and the notch toughness of the welded steel. Therefore, appropriate Nb
The amount is between 0.005 and 0.04.

Next, kt combines with oxygen in the steel and becomes At20 during refining and deoxidation.
3, in addition to removing oxygen, it combines with N in the steel to form AtN and contributes to the refinement of the structure, so the total is 0.01
It is necessary to exceed the width, but the total amount added is 0.084
Exceeding this results in coarsening of grains and an increase in the amount of inclusions such as At203, which impairs toughness, especially in high-strength steels. Therefore, kA containing 11 f Total At is 0
,01 to 0.08 mm.

In addition, in the case of the steel making method according to the present invention, B and N, which are individually dissolved in solid solution in the steel billet at high temperature, undergo the processes of cooling the billet, stopping mid-cooling, reheating, and rolling. The coefficient is set to 0.005 or less because it becomes RN during passing and impairs the hardenability of B, and generally deteriorates the toughness of the weld heat affected zone.

The above are the basic elements, but the steel that is the object of the present invention further contains B r Cu INi as an element other than the above.
l Cr l lil[o l V I Tl #C
By adding REMt-, it is possible to further ensure complete hardenability and improve strength and toughness depending on the cross-sectional thickness of the steel material.

First of all, B has the effect of improving hardenability, but if it is less than 0.0005'1, the effect is small (if it exceeds 0.000015%, B compounds will form in the weld heat affected zone, etc., and the toughness will deteriorate significantly. However, the amount of B If it is within the range of 0.0005 to 0.0015, it exhibits a synergistic effect with Nb and has the effect of improving hardenability without deteriorating toughness.

Cu increases strength without deteriorating toughness, and addition of 0.1 or more, especially 1 or more, also adds precipitation hardening of Cu during tempering [remarkably effective in increasing strength]. But 1
．． If the amount added exceeds 51 t-, hot cracking will easily occur in the welded part during welding.

Nl makes it easier to obtain lower bainite and martensitic structures during post-rolling quenching treatment, and is effective in improving toughness, and is particularly important for ensuring full strength after quenching in the center of the cross-section in the case of steel materials with large cross-sectional thicknesses. Therefore, stones containing 0.21 or more are effective. However, even if a large amount of Nl is added, the effect will be saturated, so the upper limit of k was kept at 1.5%.

It is effective to contain 0.11 or more Cr in order to ensure hardenability and strength. On the other hand, when it exceeds 0.54, carbides increase and toughness deteriorates. Therefore 0.1~
It is set to 0.51.

It is effective to add Mo1l-i to ensure the full yield point and to prevent temper embrittlement, but if it is less than 0.051, the effect is small (if it exceeds 0.6'i, the heat affected zone deteriorates the low-temperature toughness of

■ is effective in suppressing temper softening and increasing strength due to its precipitation hardening effect, but if it is less than 0.014, the effect is small (0,
Exceeding 054'i deteriorates toughness and further promotes the occurrence of weld cracks.

Since T1 has the property of fixing Nt and activating Bft, it is set to 0. It is effective to add ooss or more.

However, if TI exceeds 0.01%, it will form a solid solution in the steel base, resulting in a significant deterioration of toughness.

It is effective to add C& or REV to control the morphology of sulfides and improve notch toughness in the direction perpendicular to the rolling direction.
When it exceeds 021, surface and internal defects occur frequently. In the present invention, REM refers to rare earth elements having an atomic number of 57 to 71 and Yt.

Next, the reasons for limiting the manufacturing conditions in the present invention will be fully explained.

First, the invention Meiwa developed without cooling the steel billet to room temperature.
The area from the surface to the thickness of the steel slab, 174, is 400 to 85
The cooling part is to be cooled until the temperature reaches 0℃, but the cooled area is from the surface of the steel piece to 1/4 of the thickness of the steel piece. This is because the intensity distribution is abnormally large. In this case, cooling may be done naturally or by water cooling evenly from the periphery of the steel billet, but if the billet is thick, forced cooling by water cooling may be used to create a temperature difference between the surface side and the center of the billet. is preferred.

The reason why the temperature range is between 1400 and 850°C is that in this temperature range, the Nb dissolved in the steel becomes Nb charcoal.
Because it precipitates as nitrides, the degree of precipitation is small at temperatures above 850°C, and the temperature distribution of Nb carbon and nitrides in the cross-sectional direction before water cooling does not result in the desired temperature distribution, resulting in a steel material with uniform strength in the cross-sectional direction. It cannot be manufactured. Furthermore, if the steel piece is cooled to less than 400°C, the temperature of the whole piece will drop and Nb charcoal will be formed in the center of the piece.
This is because the desired high strength cannot be obtained due to excessive precipitation of nitrides.

In addition, whether or not the above-mentioned part has reached the above temperature range is determined by temperature measurement using a thermocouple on the surface of the steel piece and the steel piece drilled up to position 174 of the thickness of the steel piece, or by calculation results based on the heat transfer coefficient. .

Next, the reheating and soaking temperature of the steel slab was set to 900 to 1050°C.If the temperature is lower than 900°C, precipitation of unprecipitated Nb will continue to occur, reducing precipitation hardening during subsequent tempering and reducing the strength improvement effect. This is because, if the temperature exceeds 1050°C, the once precipitated Nb carbon/nitride will melt again, making it impossible to obtain the desired steel material.

Next, in the present invention, rolling is carried out at a temperature between 800 and 920°C until the reduction amount is 20 or more, which means that the reduction in the non-recrystallized region of the Nb-containing steel is sufficient. This is because if the temperature exceeds 920° C. or the reduction amount is less than 201°C, the effect of controlled rolling will be small and a steel material with good toughness will not be obtained.

On the other hand, if the temperature is lower than 800° C., some of the Ar points may be lowered depending on the components, and there is a possibility that the lower limit of the water cooling starting temperature for subsequent water cooling may be exceeded.

There is no particular upper limit to the amount of reduction, but in order to obtain the desired toughness of the steel material, which is the object of the present invention, an upper limit of approximately 70 degrees is appropriate.

Furthermore, the water cooling start temperature is changed to the steel surface temperature T (G is T≧Tr
-211 However, Tr =910-310 XC('#-80XMn
('1-20XCu(la-15XCr(41-55
XNI(@80XMo(4)+35(t-8)+30t
: Steel material thickness (m) The point at which the illusion occurs is determined because water cooling from a temperature below this temperature may result in a temperature lower than the austenite → ferrite transformation start temperature to the inside of the steel, that is, the true A r 3 point. This is because it becomes impossible to effectively obtain a hardened structure by water cooling.

Note that Tr here represents a temperature slightly above the austenite → 7-elite transformation start temperature, and takes a different value depending on the chemical composition of the steel material, the cooling rate that changes depending on the plate thickness, etc. . That is, the above Trt
-As seen in the formula, the Tr value is Ce Mn +
The terms related to the weight content of Cu t Cr + Nl b and M (I and the steel thickness t (am)) were calculated by Mr. Ouchi and others first.

67 (1981) A1, page 143, by adding 30°C to the calculation formula for Ars. This 3
0'C means an increase in the A r s point due to the influence of B during controlled rolling, which is not included in the calculation formula, and allowances during production at the factory, and the water cooling start temperature is the temperature of the steel material. This is to ensure a sufficiently true Ar at the center and a temperature above the point.

In addition, water cooling is stopped when the surface temperature of the steel material after water cooling and reheating becomes 350°C or less, as in the present invention, when reheating is performed without cooling to room temperature after casting, and water cooling is performed after rolling. This is because water cooling must be stopped at a relatively high temperature because internal defects may occur. In this case, if the water cooling stop temperature exceeds 350℃, it will affect the quenching transformation.
r and leads to a decrease in strength. For such reasons, 113
The temperature was set at 50°C or lower. In addition, when determining whether the recuperation temperature is below 350℃, the water cooling time can be set based on the thickness of the steel material, the water cooling start temperature, water volume density, etc., and the temperature can be measured after water cooling. In the present invention, it is necessary to take the changes over time shown in Table (2).In the present invention, tempering is performed to a temperature of 700°C or less after cooling as described above, but this is because the hardened structure is softened. The purpose is to increase the toughness associated with this, and to strengthen the steel by the precipitation of Nb carbon and nitrides in the central part of the steel. However, we set the temperature below 700 degrees Celsius to ensure that it did not exceed that temperature.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

Example Steel having the chemical composition shown in Table 1 is melted and subjected to continuous casting, single billet casting, or normal ingot-blowing rolling under the casting conditions shown in Table 1. In the case of continuous casting, casting 11 is performed. Or, in the case of the steel ingot-blooming method, the steel is cooled from the state heated to at least 1,100°C during thinning rolling, or from the state after blooming, and manufactured according to the manufacturing conditions shown in Table 2. A tensile test piece and a 20V and 20V impact test piece were taken from the center of the cross section and the surface layer of each thick cross-section steel material and tested. The results of these tests are shown in the lK2 table. Also, Table 3 shows 74 lameter Trt for water cooling start temperature setting for reference.

According to these results, it can be seen that the steel materials Al to 24 according to the present invention all have small differences in strength in the cross-sectional direction, and have satisfactory strength and toughness.

On the other hand, steel type 3 with different chemical composition or casting conditions
0 to 43 has a large difference in strength in the cross-sectional direction, and steel types 1625 to 2 are those that do not meet the chemical composition specified in the present invention.
Steel grades 7 and 29 lack strength, steel grade A28 has poor toughness, and among those that deviate from the manufacturing conditions stipulated in the present invention, steel grade 42 and 44 lack strength, and steel grade 45 has poor toughness. There is a clear difference from the present invention, which utilizes the effect of temporarily stopping cooling during the cooling of the steel slab and the precipitation hardening of Nb carbon and nitride to achieve the desired effect.

Furthermore, if conditions that partially deviate from the limiting conditions of the present invention are adopted, the resulting steel material will not achieve the intended purpose compared to when steel pieces with the same composition are cast within the limiting conditions. . Further, the same applies when steel having a chemical composition that deviates from the chemical composition specified in the present invention is manufactured under limited conditions.

That is, it is clear that the steel targeted by the present invention can be cast only if it meets the conditions specified by the present invention.

(Effects of the Invention) As is clear from the above examples, according to the present invention, the steel material has a thick cross section, which has superior toughness and weldability, and has a smaller difference in strength including the cross-sectional direction, compared to steel obtained by conventional methods. can be obtained by a process omitted from conventional methods, and the industrial effects are extremely significant.

Claims (1)

[Claims] C0.04 to 0.18%, Si 0.35% or less, Mn 0.6 to 1.8%, Nb 0.005 to 0.04% by weight
, Total Al0.01-0.08%, N0.00
Contains 5% or less, and if necessary, B0.000
5-0.0015%, Cu0.1-1.5%, Ni0.
2-1.5%, Cr0.10-0.5%, Mo0.05
~0.6%, V0.01~0.05%, Ti0.005
~0.02%, Ca0.005% or less, REM0.02
After casting or blooming steel containing one or more of the following % or less, with the remainder consisting of Fe and unavoidable impurities, the obtained steel slab is treated so that the area from the surface to 1/4 of the thickness of the slab is 400
Cool to a temperature of ~850°C, then cool to a temperature of 900-10°C.
After heating to 50°C and soaking, rolling at 800 to 920°C until the reduction amount is 20% or more, and then the point at which the surface temperature T (°C) of the obtained steel material becomes T≧Tr-20. A yield point of 42 kgf/m characterized by starting water cooling at , stopping water cooling when the recuperation temperature of the steel surface after water cooling becomes 350°C or less, and then heating and tempering to a temperature of 700°C or less.
A method for manufacturing a high toughness steel material with a thick cross section of m^2 or more and less than 90 kgf/mm^2. However, Tr=910-310×C(%)-80×Mn(%
)-20xCu(%)-15xCr(%)-55xNi
(%)-80×Mo(%)+35(t-8)+30t;
Steel thickness (mm)