JPH07126797A - Method for manufacturing thick steel plate with excellent low temperature toughness - Google Patents

Abstract

(57) [Summary] [Objective] The present invention can achieve an average α grain size of 3 µm or less over the entire thickness without using expensive alloying elements or reducing productivity due to complicated thermal history. A method for producing a thick steel sheet having good low temperature toughness is provided. [Configuration] in terms of defining the hot rolling prior to the microstructure of a given component range of the steel strip, the steel strip Ac ₁ transformation point or more Ac ₃
After heating below the transformation point, rolling with a cumulative rolling reduction of 50% or more is performed within a temperature range from the Ac ₁ transformation point to less than the Ac ₃ transformation point, and after the completion of rolling, a cooling rate of 5 ° C / sec or more, if necessary. After accelerating cooling to a temperature of 650 ° C. or less, or after completion of rolling, accelerating cooling to a temperature of 650 ° C. or less at a cooling rate of 5 ° C./sec or more, and then tempering to an Ac ₁ transformation point or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thick steel sheet having excellent low temperature toughness.

[0002]

2. Description of the Related Art Since thick steel plates are used as structures,
In many cases, low temperature toughness is required from the viewpoint of ensuring the safety of structures. As a method for obtaining low temperature toughness in thick steel plates, refining the ferrite (α) crystal grain size is a typical method that does not cause deterioration of other properties without using expensive alloy elements. Various methods have been conventionally proposed and put into practical use.

As a typical method, for example, Japanese Patent Publication Sho 49
As disclosed in Japanese Patent Publication No. 7291, Japanese Patent Publication No. 57-21007, Japanese Patent Publication No. 59-14535, etc., by performing controlled rolling in the unrecrystallized temperature range of austenite (γ) and subsequently performing accelerated cooling. A method of refining α during the transformation from γ to α has been proposed. In the method utilizing the transformation from γ to α, it is possible to increase the γ / α conversion ratio by effectively utilizing the non-recrystallization rolling when γ is coarse, but when γ becomes fine, the γ / α conversion ratio can be increased. Is close to 1, the degree of α miniaturization is saturated, and extreme α miniaturization cannot be expected. Further, the fact that the productivity is extremely lowered when the reduction in the unrecrystallized region is increased remains as a very difficult problem to solve as an essential drawback of the method based on controlled rolling.

Further, a method of heat treatment has been proposed instead of hot working such as rolling. For example, [Iron and Steel, No. 77
, No. 1, 1991, pp. 171-178], by adding a large amount of V and N,
Has been developed, and a method of increasing the γ / α conversion ratio during transformation to obtain a fine α structure by normalizing treatment has been developed. However, in order to fully exert the grain-refining effect by normalizing, V is 0.01% or more and N is 0.01%.
It is necessary to add more than this, but the attainable α particle size is 5 μm
It is a degree.

Furthermore, [Materials and Processes, 3rd year, 6th
No. 1990, p. 1796], there is disclosed a method for obtaining ultrafine grain steel having a grain size of 3 μm or less by a complicated thermomechanical treatment including repeated γ-α transformation. This method performs accelerated cooling after controlled rolling, stops accelerated cooling at about 500 ° C, and then 900 ° C without cooling to room temperature.
Ultra-fine grained steel is obtained by reheating to 0 ° C and hot rolling at a predetermined temperature. However, α grain size is strongly influenced by the cooling stop temperature, and the cooling stop temperature is in the vicinity of 500 ° C except near. An α particle size of 3 μm or less has not been obtained, and it is considered that it is difficult to manufacture it industrially stably.

Therefore, all of the above-mentioned conventional methods are accompanied by high costs such as deterioration of productivity, increase of heat treatment steps, increase of alloying elements, etc., and stable α particle size can be obtained by some experiments. Except for the above-mentioned method, the limit is about 10 μm, and about 5 μm is a limit even with a complicated process that is strictly controlled, and it is not possible to expect a significant improvement in toughness by further refinement of α.

[0007]

DISCLOSURE OF THE INVENTION The present invention has good productivity and an average α particle size of about 3 μm or less without adding a large amount of expensive alloying elements, inferior productivity, or complicated repeating steps. The present invention provides a method for producing a thick steel sheet having excellent low temperature toughness, which can obtain the ultrafine grain α.

[0008]

As a result of studies to solve the above problems, the present invention has an industrial limit in the conventional refinement of α using the transformation from γ to α. It was discovered that ultra-fine α-structure can be obtained by direct processing, and the gist is that it is wt%.
C: 0.01 to 0.20%, Si: 0.03 to 1.
0%, Mn: 0.30 to 2.0%, Al: 0.005
0.1%, N: 0.001-0.01%, and if necessary, Cr: 0.01-0.50%, Ni:
0.01 to 3.0%, Mo: 0.01 to 0.50%, C
u: 0.01 to 1.5%, Ti: 0.003 to 0.10.
%, V: 0.005 to 0.20%, Nb: 0.003 to
0.05%, B: 0.0003 to 0.0020% of 1 type or 2 types or more, a balance of Fe and unavoidable impurities, and a steel slab having a microstructure corresponding to the following ₁ to Ac ₁ transformation. After heating the temperature above the Ac ₃ transformation point to below the Ac ₃ transformation point, rolling with a cumulative reduction of 50% or more is performed at the Ac ₁ transformation point or above A
c ₃ carried out in the temperature range of less than transformation point, if necessary after rolling completion, 5 ° C. / sec or more cooling rate or accelerated cooling to 650 ° C. temperature below, 5 ° C. / sec or more following the end of rolling After accelerated cooling to a temperature of 650 ° C or lower at the cooling rate of
It is a method for producing a thick steel sheet excellent in low temperature toughness, which is characterized by tempering to a temperature not higher than the c ₁ transformation point. Martensite single-phase structure, bainite single-phase structure, or mixed structure of both Ferrite and martensite, or ferrite and bainite, or mixed structure of ferrite, bainite, and martensite Ferrite single-phase structure with an average grain size of 20 μm or less or a mixed structure consisting of one or more of ferrite and martensite, bainite, and pearlite.

[0009]

First of all, the inventors of the present invention put a steel piece into a steel having an Ac ₁ transformation point or more A
It was found that, after heating below the c ₃ transformation point, rolling with a cumulative reduction of 50% or more in the temperature range between the Ac ₁ transformation point and less than the Ac ₃ transformation point, and by directly processing α, ultrafine graining can be performed. It was As a method of manufacturing a thick steel sheet for processing α, there is a so-called two-phase rolling method, which has been conventionally performed as an extension of controlled rolling, and in which rolling is performed in the α and γ coexisting regions of the cooling process following γ region processing. . In this method, it is possible to increase the strength by introducing a processed structure into α, but it is difficult to effectively reduce the grain size of α, and it is difficult to improve the toughness. However, the toughness often deteriorates.

On the other hand, according to the present invention, the ratio of α is higher at the same temperature in the heating process than in the cooling process, that is, the same ratio of α can be secured at a high temperature in the processing stage, so that the recovery of α can be achieved.
Recrystallization is facilitated, α is significantly fine-grained by recrystallization without leaving processing strain harmful to toughness, and γ coexisting at this time is also processed at extremely low temperature, so during cooling After being processed in the single phase region of the conventional γ, α which transforms into α becomes finer than that in the case where it is transformed, and α becomes significantly finer as a whole, so that excellent toughness can be obtained.

However, α is not necessarily uniformly made into ultrafine grains simply by heating and processing in the two-phase region. This is because if the structure before processing, especially the α particle size is coarse, recrystallization is difficult to occur even if the rolling reduction is increased. As a result of intensive studies on the conditions for uniform miniaturization in the processing of α, the inventors of the present invention changed the easiness of recrystallization depending on the microstructure before processing, and as a result, α was uniformly refined. I found that it was decided whether or not. Below, based on the experimental results, the reasons for limiting the structure before processing in the manufacturing method of the present invention will be described.

% By weight, C: 0.12%, Si: 0.3
3%, Mn: 1.45%, Ti: 0.011%, Al:
Regarding a steel slab containing 0.03% and N: 0.0028%, a martensite single-phase microstructure, a bainite + martensite microstructure, α Of four types of α + bainite mixed structure having a ratio of about 18% and α + bainite mixed structure having a ratio of α of about 55% and an average grain size of about 35 μm (hereinafter, the structure at this stage is referred to as a front structure). )) And then kept at various temperatures for 2 hours, and then rolled in one pass immediately after extraction from the heating furnace to reduce the temperature drop as much as possible, to a plate thickness of 4 mm and allowed to cool to room temperature. did. Heating temperature at that time (about processing temperature) and the obtained average α
The relationship between particle sizes is shown in FIG.

Regardless of the previous structure, if the processing temperature is below the Ac ₁ transformation point, α-stretched unrecrystallized grains result in insufficient recovery. On the other hand, if the temperature is at the Ac ₃ transformation point or higher, the conditions are the same as in ordinary hot rolling in which the alloy is processed and cooled in the γ single phase region, so the α grain size is 5 μm or more, and extreme grain refining cannot be achieved. When processed in the temperature range from the Ac ₁ transformation point to below the Ac ₃ transformation point, α is the finest grain size, but the degree of grain size varies greatly depending on the anterior structure, and the anterior structure is a martensite single-phase structure or bainite. + Martensite mixed structure, α ratio of about 18% α + bainite mixed structure gives a very fine α structure of 3 μm or less, while α ratio is about 55% and the average grain size is about 55%. Grain refinement is not sufficient with a mixed structure of α + bainite of 35 μm.

This is because when α is heated, the original grain size becomes coarser as the heating temperature rises, whereas when the martensite or bainite structure is heated, the original grain size of martensite or bainite is increased. Since the lath and block play the role of substantial grain boundaries, the grain size before processing is fine, and, if present before heating, or if fine cementite that precipitates during heating is present in the grain, At the temperature of the two-phase region, it becomes a fine γ and helps the uniform processing into α.
Since recrystallization is facilitated, if the pre-structure is martensite or bainite, it becomes easy to make α into ultrafine grains after processing. Even if α is present in the previous structure, if the ratio is small, it does not have a great adverse effect on the miniaturization of the whole. From the results of FIG. 1 and the results of other experiments by the present inventors, if the ratio of α in the pre-tissue is less than 20%, there is no problem in ultrafine graining.

It is difficult to make the α-based structure into ultrafine grains when the grain size is coarse. In Fig. 2, the same steel piece as in Fig. 1 was used, and when the pre-structure was a mixed structure of α and pearlite, the α-grain size of the pre-structure was changed by heat treatment conditions to investigate the relationship between the processing temperature and the average α-grain size. It is a figure. In this case, the ratio of α in the anterior tissue is about 80% or more, but the α particle size is about 20 μm.
If it is below, even if it has an α-based structure, it can be made into ultrafine grains when processed in the two-phase region. Since the other second phase separates into α and γ during heating, the α particle size before processing becomes finer particles than when originally α. Therefore, even in a structure in which α exceeds 20%, if the α particle size is set to 20 μm or less, ultrafine graining is possible regardless of the type of the remaining structure.

As described above, from the experimental results shown in FIGS.
As manufacturing conditions for superfine α to improve low temperature toughness, heating in a temperature range of Ac ₁ transformation point or more and less than Ac ₃ transformation point, processing is required, and the structure before processing is as follows. It is clear that the organization needs to satisfy either of them. Martensite structure, bainite structure, or a mixed structure of both: Ferrite and martensite, ferrite and bainite, or a mixed structure of ferrite, bainite, and martensite with a ferrite content of less than 20%. Ferrite single-phase structure of 20 μm or less or mixed structure consisting of one or more of ferrite and martensite, bainite, and pearlite

Next, the reasons for limitation regarding other manufacturing conditions will be described. When heating and processing in a temperature range of Ac ₁ transformation point or more and less than Ac ₃ transformation point, it is necessary to add a certain amount of processing or more in order to recrystallize α, and based on the experimental results of the present inventors, A cumulative rolling reduction of 50% or more is required. Since the temperature is lower in the temperature range than in the γ range, the strain of the applied processing is accumulated, so that the reduction rate of each pass does not matter. However, in order to effectively exert the rolling reduction effect of each pass on the recrystallization of α, it is preferable that the interval between the passes is short.

The rolling temperature range in the present invention is at most Ac.
Since it is less than ₃ transformation points, there is little concern about α growth, cementite, and coarsening of precipitates during cooling after rolling. Therefore, as cooling after rolling, air cooling or, depending on the desired steel plate strength level, subsequent to rolling, accelerated cooling to 650 ° C. or less at a cooling rate of 5 ° C./sec or more, or less than Ac ₁ transformation point after accelerated cooling It is possible to apply a tempering process at. The above are the reasons for limiting the present invention regarding the manufacturing method, but not only the manufacturing method for ensuring low temperature toughness,
The chemical composition also needs to be within the proper range. The reasons for limiting the chemical components in the present invention will be described below.

First, C is added as an effective component for improving the strength of steel. If it is less than 0.01%, it is difficult to secure the strength required for structural steel, and if it exceeds 0.20%, it is excessive. Since the addition of Al significantly reduces toughness, weld crack resistance, etc., the range was made 0.01 to 0.20%. Then S
i is an element effective as a deoxidizing element and for ensuring the strength of the base material. Addition of less than 0.03% results in insufficient deoxidation and is disadvantageous in securing strength. On the contrary, excessive addition of more than 1.0% forms a coarse oxide and causes ductility and toughness deterioration. Therefore, the range of Si is set to 0.03 to 1.0%.

Further, Mn is an element necessary for securing the strength and toughness of the base metal, and it is necessary to add at least 0.30% or more, but in a range that is acceptable in terms of material such as toughness and cracking of the welded portion. The upper limit was 2.0%. Al is an element effective for deoxidizing, refining the γ grain size, etc.
It is necessary to contain at least 05%, but if excessively added in excess of 0.1%, a coarse oxide is formed and ductility is extremely deteriorated. Therefore, it is limited to a range of 0.005 to 1.0%. There is a need to. N combines with Al and Ti to effectively work for γ grain refinement, but in order to clarify the effect, 0.0
On the other hand, it is necessary to contain it in an amount of 01% or more, but when it is added in excess, the amount of solute N increases and the toughness is adversely affected. The upper limit of the allowable range is 0.01%.

The above are the basic components of the steel of the present invention, but for the purpose of increasing the strength of the base material in accordance with the desired strength level, Cr, Ni, Mo, Cu, Ti, V, Nb, B are added if necessary. 1 type or 2 types or more of these can be contained. First, C
Both r and Mo are effective elements for improving the strength of the base material, but 0.01% or more is necessary for producing a clear effect, while if added over 0.50%, toughness is Since it has a tendency to deteriorate, the range is 0.01 to 0.50%.

Ni is a very effective element because it can improve the strength and toughness of the base material at the same time, but it is necessary to contain Ni in an amount of 0.01% or more in order to exert the effect. When the content is high, the strength and toughness are improved, but the effect is saturated even if added in excess of 3.0%. Therefore, considering economic efficiency, the upper limit is made 3.0%. Next, Cu has almost the same effect as Ni, but addition of more than 1.5% causes a problem in hot workability, so the range is limited to 0.01 to 1.5%.

Ti contributes to the improvement of the strength of the base material by its precipitation strength, and is an element effective for the refinement of γ grains by the formation of TiN, but 0.003 is necessary to exert the effect.
% Or more must be added. On the other hand, if it exceeds 0.10%, similarly to Al, a coarse oxide is formed to deteriorate toughness and ductility, so the upper limit is made 0.10%. V and Nb
All of these contribute to the improvement of the strength of the base material mainly by precipitation strengthening, but excessive addition deteriorates the toughness. Therefore, V is 0.005 to 0.20% and Nb is 0.003 to 0.05 as a range in which the effect can be exhibited without causing deterioration of toughness.
%. B is very effective in increasing the hardenability by increasing the hardenability of steel by adding a very small amount of 0.0003% or more, but if added in excess, it forms BN, which adversely affects the hardenability and reduces toughness. , The upper limit is 0.0020.
%.

[0024]

[Examples] Table 1 shows the chemical composition of the test steels used in the examples. Steel types 1 to 7 are within the chemical composition range of the present invention, and steel types 8 and 9 deviate from the chemical composition of the present invention by C and Ti, respectively. Table 2 shows the state of the pre-structure obtained by the pre-heat treatment, the manufacturing conditions, and the obtained strength and toughness of the thick steel plate obtained by using the test steel of Table 1.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

Steel No. A1 to A13 are thick steel plates manufactured by the method of the present invention, and the finally obtained α particle size is 3 μm.
Since the grain size is significantly smaller than m, the toughness value evaluated by the fracture surface transition temperature (vTrs) in the Charpy impact test is excellent at −100 ° C. in vTrs.

Steel No. B1 to B7 are comparative examples, all of which do not satisfy the conditions of the present invention, so that the toughness is significantly deteriorated as compared with the thick steel plate manufactured by the present invention. That is, since the chemical composition of Comparative Steels B1 and B2 is outside the range of the present invention, the finally obtained α grain size is fine, but the toughness is poor.
Further, since Comparative Steel B3 has a coarse α-grain size of the front structure,
In B4, the α grain size is coarse and the rolling reduction is too small, and in B5, the preceding structure satisfies the present invention, but the cumulative rolling reduction is insufficient. Becomes coarse and good toughness cannot be obtained.

On the other hand, since the comparative steels B6 and B7 have heating and rolling temperatures outside the range of the present invention, the toughness is deteriorated as compared with the thick steel plate manufactured according to the present invention. That is, since the heating and rolling temperatures of steel B6 are too low, α does not recrystallize, the grains do not become finer, and the processing strain remains and the toughness is not improved. On the contrary, since the heating temperature of Comparative Steel B7 is too high and exceeds the Ac ₃ transformation point,
Since there is no difference from normal hot rolling, α does not become fine grain and the toughness is inferior. According to the above examples, it is clear that the manufacturing method of the present invention can significantly improve the toughness as compared with the conventional manufacturing method.

[0032]

INDUSTRIAL APPLICABILITY The present invention uses expensive alloy elements,
It is an epoch-making method that can produce thick steel plate with good low temperature toughness without reducing productivity due to complicated heat history.
Industrial effects such as reduction of manufacturing cost and improvement of safety as a structure are extremely large.

[Brief description of drawings]

FIG. 1 is a chart showing the influence of the type of pre-structure on the relationship between processing temperature and ferrite grain size.

FIG. 2 is a chart showing the influence of the initial ferrite grain size on the relationship between the processing temperature and the α grain size when the preceding structure is a mixed structure of ferrite and pearlite.

Claims (4)

[Claims] 1. By weight%, C: 0.01 to 0.20%, Si: 0.03 to
1.0%, Mn: 0.30 to 2.0%, Al: 0.005
0.1%, N: 0.001-0.01%, a balance of Fe and unavoidable impurities, and a steel slab having a microstructure corresponding to the following ( ₁₎ to Ac ₁ transformation point or higher.
A method for producing a thick steel sheet excellent in low-temperature toughness, comprising rolling at a cumulative rolling reduction of 50% or more in a temperature range of Ac ₁ transformation point or more and less than Ac ₃ transformation point after heating to less than c ₃ transformation point. Martensite single-phase structure, bainite single-phase structure, or a mixed structure of both Ferrite and martensite or ferrite and bainite, or a mixed structure of ferrite, bainite and martensite Ferrite single-phase structure with an average grain size of 20 μm or less or a mixed structure consisting of one or more of ferrite and martensite, bainite, and pearlite. 2. By weight%, Cr: 0.01-0.50%, Ni: 0.01-
3.0%, Mo: 0.01 to 0.50%, Cu: 0.01 to
1.5%, Ti: 0.003 to 0.10%, V: 0.005
0.20%, Nb: 0.003 to 0.05% B: 0.0003
The method for producing a thick steel sheet having excellent low temperature toughness according to claim 1, wherein the steel sheet contains at least one of 0.002% to 0.0020%. 3. The method for producing a thick steel sheet excellent in low temperature toughness according to claim 1 or 2, further comprising accelerating cooling to a temperature of 650 ° C. or lower at a cooling rate of 5 ° C./sec or more subsequent to completion of rolling. . 4. After completion of rolling, after accelerated cooling to a temperature of 650 ° C. or lower at a cooling rate of 5 ° C./second or higher, Ac ₁
The method for producing a thick steel sheet excellent in low temperature toughness according to claim 1 or 2, characterized by tempering to a temperature not higher than a transformation point.