KR100256357B1 - The manufacturing method for high strength steel sheet with cu precipitation hardening type - Google Patents

Abstract

PURPOSE: Provided is a method for manufacturing Cu-precipitation reinforced high tensile strength steel sheets having superior tensile strength at low temperature for the application to marine structures. CONSTITUTION: The Cu-precipitation reinforced high tensile strength steel sheet is manufactured by hot rolling a steel comprising C 0.01-0.10wt.%, Si 0.01-0.50%, Mn 1.0-2.0wt.%, Al 0.01-0.10wt.%, Cu 1.0-2.0wt.%, Ni 0.5-2.0wt.%, Nb 0.01-0.10wt.%, Ca 0.0005-0.005wt.%, a balance of Fe and inevitable impurities under a conventional condition; water cooling; coiling the steel sheets in the temperature range of 400+120x (Cu%)±20°C according to the Cu content.

Description

Manufacturing method of copper precipitation strengthening type high tensile strength steel with excellent low temperature toughness

1 is a graph showing the change of the micro hardness according to the change in the winding temperature and the holding time of the present invention steel.

The present invention relates to a method for manufacturing high tensile steel sheets used for construction, pressure vessels, line pipes and offshore structures, and more particularly, to manufacturing high tensile steel sheets having excellent strength and low temperature toughness by precipitation of copper. It is about a method.

In general, steel, which uses copper (Cu) precipitation strengthening, has advantages such as excellent weldability, low temperature toughness, and compensation of precipitation loss due to precipitation strengthening. It is widely used for the purpose.

A number of conventional research results and patents are known for steel sheets using such precipitation strengthening of copper, among which US Patent No. 3,692,514, Japanese Patent Laid-Open No. 62-149845 and Japanese Patent Laid-Open No. 3-162523 are representative.

U.S. Patent No. 3,692,514 discloses excellent weldability in the case of Cu-added steel, and Japanese Patent Laid-Open No. 62-149845 proposes to improve the toughness of the welded portion by appropriate heat treatment. The present invention relates to a rolling control and heat treatment method for producing Cu precipitated hardened steel having excellent toughness. However, since the above methods precipitate Cu by heat treatment after hot rolling, the manufacturing cost is higher and the production process is complicated than when Cu is precipitated only by hot rolling.

Many studies have been conducted to solve such problems and to precipitate Cu by only hot rolling. As a result, proposals such as Japanese Patent Application Laid-Open No. 1-156420 and Japanese Patent Application Laid-open No. 2-197547 have been made.

Japanese Patent Application Laid-Open No. 1-156420 discloses that after the hot rolling is finished in the temperature range of 700-850 ° C to cool Cu, it is cooled to a temperature range of 600-650 ° C at a cooling rate of 3 ° C / s or more, It is a method of cooling below 0.2 ℃ / s with cooling rate up to 500 ℃ after finishing -10% finishing hot rolling. However, this method is applicable to the thick plate manufacturing process mainly for reversible rolling because the finish rolling temperature during the hot rolling process is extremely low, such as 600-650 ° C., but it is not possible for the thin plate which performs continuous hot rolling.

Japanese Laid-Open Patent Publication No. 2-197547 discloses partial precipitation of Cu by a simplified method of controlling the winding temperature in the range of 350-550 ° C. without using a complicated hot rolling method such as Japanese Patent Laid-Open No. 1-156420. Way. However, this method considers the increase in strength due to partial precipitation of Cu, compared to the addition of a large amount of Cu, and thus does not maximize the effect due to the addition of alloying elements.

Thus, the present inventors have solved the above-mentioned disadvantages of the prior art and conducted research and experiments to improve the strength and low temperature toughness only by changing the rolling conditions. As a result, the present inventors confirmed the following facts and proposed the present invention. .

In general, precipitation strengthening in steel is excellent in increasing strength, but relatively lower in toughness (especially low temperature toughness) of steel. In the case of lowering the toughness of steel, in particular, low temperature toughness causes Cu precipitation to increase strength, Cu precipitates. This is due to the fact that the distribution and size of and the coherence between matrix structures are greatly changed.

The present invention is to properly control the alloy component system and the rolling process of the steel, so that the Cu precipitates are not too coarse and at the same time the Cu precipitates and the matrix structure are not coherent, thereby increasing the strength due to the Cu precipitation and excellent low temperature toughness copper precipitation strengthening type An object of the present invention is to provide a method of manufacturing high tensile steel.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is a method for producing a Cu precipitation-reinforced high tensile steel sheet, in weight%, C: 0.01-0.10%, Si: 0.01-0.50%, Mn: 1.0-2.0%, Al: 0.01-0.10%, Cu: 1.0 -2.0%, Ni: 0.5-2.0%, Nb: 0.01-0.10%, Ca: 0.0005-0.005%, remainder: After hot-rolling and water-cooling the steel formed of Fe and other unavoidable impurity elements under normal conditions And a method for producing a Cu precipitation-reinforced high tensile steel sheet having excellent low temperature toughness, which is wound in a temperature range of 400 + 120 × (Cu%) ± 20 ° C. according to the Cu content.

Hereinafter, the present invention will be described in more detail.

In order to achieve the above object, in the present invention, it is preferable to first make an alloy component system of steel as described above, for the following reason.

C is the most economical and effective element for reinforcing steel, but when added in a large amount, the weldability, formability and toughness are reduced, so the C content is limited to 0.01-0.10% in the present invention. When the amount of C is less than 0.01%, it is not economical because other alloying elements must be added in order to exhibit the same strength, and when the addition amount of 0.10% or more is added, the weldability, formability and toughness decrease, as well as the rolling conditions of the present invention. It is not preferable because the matrix structure is formed of a ferrite-pearlite structure from the needle-like ferrite structure, so that the strength is greatly reduced or the upper bainite structure is formed and the toughness is reduced.

Si is also needed to deoxidize molten steel and is effective as a solid solution strengthening element, so an addition in the range of 0.01-0.50% is required. If the added amount is 0.01% or less, it is difficult to obtain clean steel because it does not sufficiently deoxidize the molten steel.When 0.5% or more is added, the red scale is formed by Si during hot rolling, and the surface shape of the steel sheet becomes very bad and the ductility is also lowered. Not desirable

Mn is an effective element to strengthen the solid solution of steel and should be added more than 1.0% to exhibit high strength with an increase in quenchability. However, the addition of 2.0% or more is not preferable because the segregation part is greatly developed at the center of thickness when casting to slab in the steelmaking process, and it damages the weldability of the final product.

Al is a deoxidation element, which serves to reduce impurities in steel and is effective in suppressing coarsening of austenite grains. Therefore, it is necessary to add more than 0.01%, but excessive addition of more than 0.1% may form a large amount of inclusions and deteriorate toughness. Therefore, the upper limit was limited to 0.1% to improve the cleanliness of the steel.

Cu is an element which is a feature of the present invention, and is precipitated by hot rolling and cooling control to greatly increase the strength of the steel, so that the amount of carbon can be reduced in the case of high strength, and thus the weldability and toughness of the steel can be improved. The solid solution of Cu in the ferrite is 2.1% at 850 ° C, and the addition of more than this results in segregation and precipitation at high temperatures, resulting in hot brittleness (quotation; "Copper in Iron and Steel", I. Le May, 1982, Publ. John Willey & Sons, p. 6) In the present invention, the upper limit is 2.0%. On the other hand, when the amount of Cu added is less than 1.0%, it is difficult to precipitate Cu during the hot rolling process without a separate heat treatment because the driving force necessary for causing precipitation is large and the deposition rate is very slow. Therefore, Cu was set at 1.0-2.0% in the present invention.

Since Ni acts to improve the toughness of steel and prevents cracking during hot rolling by Cu addition, it is generally known to add about 1/2 of Cu addition amount. (Citation Verification; "Copper in Iron and Steel", I.Le May, 1982, Publ.John Willey & Sons, p.101) If the amount of Ni is less than 0.5%, the hot brittleness is insufficient and if more than 2.0% is added Since the strength is high and the economy due to the addition of expensive elements is lost, the amount of Ni added is set at 0.5 to 2.0%.

Nb is a very useful element to refine the grains and at the same time plays a significant role in improving the strength of the steel, so at least 0.01% or more should be added. If it exceeds 0.10%, the low temperature toughness may be deteriorated due to excessive precipitation of Nb carbonitride. Therefore, the addition range was set at 0.01-0.10%.

Ca is an inclusion shape control element and improves low temperature toughness and ductility. To achieve this effect, at least 0.0005% or more of addition is required, but addition of 0.005% or more does not improve the effect.

In the present invention, after the steel is formed as described above, hot rolling, cooling, and winding are performed, which will be described in more detail below.

After the steel is formed of the alloy system as described above, hot rolling is performed by a conventional method, in which the finishing hot rolling is finished hot rolling in a temperature range of 800 ° C. or higher (usually 850-900 ° C.). In general, in order to improve the toughness of the steel, it is preferable to finish hot rolling in the range of 800-850 ° C., which is a low temperature range of the hot rolling finish temperature range, but in this case, the load applied to the rolling mill is greatly increased as the rolling temperature decreases. In order to increase the temperature, the present invention finishes hot rolling in a temperature range of 900 ° C. or higher, more preferably 900-910 ° C., which is a relatively high temperature range of the finishing hot rolling temperature range. The present invention exhibits excellent low temperature toughness even at such a high finish hot rolling temperature. However, in order to further improve low temperature toughness, it is desirable to lower the hot rolling finish temperature.

After the hot rolling is completed, water cooling is performed on the run-out table, thereby suppressing the precipitation of Cu before winding and at the same time preventing the coarsening of the ferrite and reducing the toughness. At this time, the cooling rate is different depending on the thickness of the hot rolled sheet, but is usually controlled in the range of 3-30 ℃ / s.

After water-cooling as described above, the winding temperature (CT) is a pattern that ensures strength and low temperature toughness at the same time by preventing the Cu precipitates from being excessively coarsened while at the same time achieving a degree of coherence between the Cu precipitates and the matrix structure. As a main requirement of the invention, it is preferable to limit the range to 400 + 120 × (Cu%) ± 20 ° C. for the following reason.

If the coiling temperature is higher than 400 + 120 × (Cu%) ± 20 ℃, the microstructure does not form as a needle-like ferrite, but changes to a ferrite-pearlite structure, and the Cu precipitation rate is fast, so that the Cu precipitate is too coarse and the strength of the material is increased. When it is lower than 400 + 120 × (Cu%)-20 ℃, the bainite transformation occurs and the strength increases, but fine Cu precipitates are formed to maintain the consistency with the matrix structure. This is also because it is insufficient.

In the present invention, the determination of the coiling temperature is very important as described above, because the coiling temperature for optimizing low temperature toughness and strength depends on the Cu content contained in the steel. That is, in the present invention, the winding temperature is controlled in the range of 400 + 120 × (Cu%) ± 20 ° C. according to the copper content present in the steel, thereby uniformly depositing Cu precipitate particles and increasing the size of the Cu precipitates. It is to optimize the strength and low temperature toughness of the steel by forming a needle-like ferrite structure that is not overly coarse and in which Cu precipitates and matrix structures do not match.

As described above, the change in the mechanical properties of the material according to the change in the winding temperature can be attributed to the change in the microstructure and the precipitate. First, as the coiling temperature increases, the microstructure changes in the order of bainite, needle ferrite, and ferrite-pearlite, and the change of the microstructure causes a decrease in strength and an improvement in impact toughness. Therefore, the change of the (tensile strength × impact toughness) value only by the change of the microstructure appears to be improved as the winding temperature is increased. However, in the present invention, the maximum value (tensile strength x impact toughness) within the range having acicular ferrite structure is shown, which is due to the change in impact toughness due to the difference in the degree of Cu precipitation. That is, when the Cu precipitate is finely precipitated to maintain consistency with the matrix structure, the strength increase effect due to the Cu precipitation is greatest, while the impact toughness is most degraded. Therefore, in order to maximize the strength and impact toughness of the material, when the Cu precipitates are grown to such an extent that the base structure and the Cu precipitates lose coherence, the loss of strength increase effect due to precipitation is small and the impact toughness decreases. It seems to be able to minimize it.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

After the invention steel and comparative steel were prepared as shown in Table 1 below, they were dissolved and manufactured in an ingot, and then hot rolled under the conditions as shown in Table 2 to produce a 6.5t thick plate, and a run-out table. water cooled on the table). At this time, the hot rolling finish temperature was changed in the range of 850-905 ℃, the water cooling rate on the run-out table after hot rolling was set to about 5 ℃ or more the same as the cooling rate in the actual field production process.

The coiling temperature was applied differently according to the steel grade and controlled in the range of 350-660 ℃.

In Table 1, the steel grade A is similar to the steel of the present invention, but the other components are not added to Cu and Ni, so the steel grade B added Cu and Ni but still small addition amount (Cu = 0.75%, Ni = 0.60%), The desired properties do not appear in the present invention was selected as a comparative steel. That is, in the present invention, the addition of Cu is an essential element for forming an appropriate amount of precipitate, because if the addition of Cu is out of the scope of the present invention, the mechanical properties aimed at in the present invention are not satisfied.

Table 2 shows the hot rolling and cooling conditions of the inventive steels (A, B) and the comparative steels (C, D, E), and Comparative Examples (1, 4, 5) see the comparative steel (A, B) Hot rolling was carried out under the conditions satisfying the scope of the invention, and Comparative Examples (2, 3) were comparative rolling of the comparative steel (B) under conditions outside the scope of the present invention. And Comparative Example (6) is the case that the coiling temperature is lower than the present invention conditions (CT = 500-540 ℃) in hot rolling (C) steel grade (Cu = 0.15%) of the steel sheet of the present invention, Comparative Example (8) Is a case where the coiling temperature is increased; Comparative Example (9) is a case where the coiling temperature is lower than the conditions of the present invention (560-600 ° C) in hot rolling of the steel (D) steel grade (Cu = 1.46%) of the present invention, and Comparative Example (11) shows the coiling temperature. Heightened case; Comparative Example (12) is a case where the coiling temperature is lower than the present invention conditions (CT = 590-630 ° C.) in hot rolling of the steel (E) grade (Cu = 1.73%) of the present invention, and Comparative Example (14) is wound When the temperature is increased; Invention Examples (7, 10, 13) show the case where (C, D, E) steel grades of the present invention were rolled under the conditions of the present invention.

Yield strength, tensile strength, total elongation, impact toughness, transition temperature and (tensile strength x impact toughness) values were measured to confirm the mechanical properties of the hot rolled sheet manufactured as described above, and the results are shown in Table 3 below. In this case, the yield strength, tensile strength and total elongation are measured using a tensile tester, and the impact toughness is measured by a Charpy V-notch test at a temperature of -60 ° C.

Table 3 shows changes in mechanical properties, that is, tensile properties and impact toughness, of the invention examples satisfying the scope of the present invention and the comparative examples not satisfying the same.

As can be seen from Table 3, first, the steel components do not satisfy the scope of the present invention. In Comparative Example (1), other chemical compositions of the steel components are similar to those of the present invention, but Cu and Ni are not added. (Steel grade (A)), even if the rolling and cooling conditions are within the scope of the present invention, the tensile strength is smaller than that of the present invention steel (the tensile strength of Comparative Example (1): 58.2 kg / mm2, the tensile strength of the invention steel) It is understood that the precipitation strengthening effect by adding Cu at 70 kgf / mm 2 or more significantly improves the strength of the material.

(B) Comparative examples (2,3,4,5) in which the coiling temperature was converted to the 350-650 ° C range by rolling the steel grades (Cu = 0.75%, Ni = 0.60%) with the addition of Cu and Ni but still small additions In the case of, the tensile strength of the material varies greatly from 53.9kgf / mm2 to 69.9kgf / mm2 as the winding temperature decreases, but the low temperature impact toughness is very small and the impact transition temperature is also high. From this, if the added Cu does not precipitate and is dissolved in the steel to increase the hardenability, the strength of the material depends on the transformation strengthening, so in the case of low temperature winding (CT = 350-450 ° C.), even if the strength is similar to that of the inventive steel, It is shown that the impact toughness is very bad and the strength is lowered at a winding temperature of 550-650 ° C. in the range of the present invention.

Looking at the case of changing the manufacturing conditions using the composition steel of the present invention are as follows. In Comparative Example (6), where the coiling temperature was as low as 400 ° C in the (C) steel grade with Cu added amount of 1.05%, the bainite was formed due to the low coiling temperature, so the strength was high but the impact toughness was lowered (tensile strength × The impact toughness value is very low (1819). In addition, in the comparative example (8) in which the coiling temperature was higher than the present invention, the tensile strength was decreased and the impact toughness was improved, and the tensile strength x impact toughness was 5,197. However, in the invention (7), which is manufactured under the manufacturing conditions of the present invention, the tensile strength is high and the impact toughness is also excellent, and the tensile strength x impact toughness is 5,671, which is the best. This tendency is the same in the composition steel of the present invention in both Comparative Examples (9, 11, 12, 14) and Inventive Examples (10, 13).

Example 2

When the coil is wound in the hot rolling process, the cooling rate is very slow due to the sensible heat of the coil itself, and the cooling rate of the coil after the winding is about 20-40 ° C./hr. Therefore, it can be said that the coiling temperature holding time of the coil corresponds to about 100 minutes.

In order to confirm the change in the microhardness of the steel due to the change in the precipitation degree of Cu according to the coiling temperature and the holding time, the coiling temperature of the steel (D), the inventive steel of Example 1, was changed to 530, 600 and 650 ° C. The change of the microhardness according to the change of the holding time was measured and the result is shown in FIG. The microhardness is a value measured under a load of 10 kg.

Looking at Figure 1 it can be seen that the change in the degree of Cu precipitation is very large depending on the coiling temperature. That is, in the case of 650 ° C., the precipitation rate is very fast, indicating the maximum precipitation strengthening in the holding time of about 3 minutes, and by maintaining more than that, the precipitate is coarsened to lose the precipitation strengthening effect of Cu. Accordingly, it can be seen that when the coil is wound at this temperature, the impact toughness is good but the strength is lowered. In addition, in the case of 530 ° C., the precipitate is finely maintained even after 90 minutes, but the strength is very high, but the impact toughness is lowered. However, in case of 600 ℃, the precipitation rate is appropriately matched with the cooling process after winding, so that the toughening effect of the material is maintained and the impact toughness is also improved. As a result of calculating the optimum coiling temperature range according to the Cu content, it can be seen that the optimum coiling temperature satisfying the present invention is in the range of CT = 400 + 120 × (Cu%) ± 20 ° C.

As described above, the present invention is to control the chemical composition of the steel and to control the hot rolling and winding conditions, Cu precipitates can form the structure of the needle-like ferrite finely and uniformly distributed in the matrix, by controlling the coiling temperature High strength due to precipitation can be easily obtained, and at the same time exhibits extremely excellent low temperature toughness. In addition, since the hot rolling can be finished at a relatively high temperature in the production of the present invention, the rolling productivity can be improved, and the welding rate is excellent even in the use of the steel, thereby improving the error rate and productivity in manufacturing parts. According to the present invention, the production cost can be reduced, and the effect is very large.

Claims (1)

In the method for producing a Cu precipitation-reinforced high tensile steel sheet, in weight%, C: 0.01-0.10%, Si: 0.01-0.50%, Mn: 1.0-2.0%, Al: 0.01-0.10%, Cu: 1.0-2.0% , Ni: 0.5-2.0%, Nb: 0.01-0.10%, Ca: 0.0005-0.005%, remainder: hot-rolled finish and water-cooled steel made of Fe and other unavoidable impurity elements under normal conditions, and then the Cu A method for producing a Cu precipitation-reinforced high tensile steel sheet having excellent low temperature toughness, which includes winding in a temperature range of 400 + 120 × (Cu%) ± 20 ° C. according to the content.