JP2019131852A - MANUFACTURING METHOD OF HIGH Ni THICK STEEL SHEET - Google Patents

Abstract

To provide a high Ni thick steel sheet less in surface stock removal amount and less in burn out after a heat treatment, and a manufacturing method therefor.SOLUTION: A slab having a chemical component constituted by, by mass%, C:0.03 to 0.10%, Si:0.02 to 1.5%, Mn:0.1 to 2.0%, Ni:2.0 to 10%, P:0.005% or less, S:0.002% or less and the balance Fe with inevitable impurities is heated, then no homogenization treatment is conducted, or holding temperature is set at 1200°C or lower and heating time with 10 hr. or less when the homogenization treatment is conducted, rough rolling with draft ratio by 2 is conducted, an intermediate heating at a temperature of 1180°C to 1250°C for 1.5 hr to 5 hr. is conducted and then finish rolling is conducted.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for producing a high Ni thick steel plate.

  High Ni thick steel plates are used in applications such as shipbuilding, bridges, architecture, offshore structures, pressure vessels, tanks, line pipes, liquefied natural gas storage tanks, and the like. In particular, it is effective to use the high Ni thick steel plate for applications requiring toughness at a cryogenic temperature of −160 ° C. or lower, such as in a storage tank for liquefied natural gas. As a method for producing such a high Ni thick steel plate, a method for producing a steel plate as disclosed in Patent Documents 1 to 9 is disclosed.

  When manufacturing thick steel plates, including high Ni thick steel plates, they are often subjected to processes called heat treatments such as annealing (annealing), tempering, and quenching after rolling, but this includes residual stress removal and structure control. The purpose is to adjust the material.

  In the heat treatment process, the mechanical properties such as desired strength and formability are obtained by controlling the heating temperature and cooling rate, but depending on the heat treatment method, the shape and dimensions may be reduced due to the generation of unwanted residual stress. It can change and be disqualified.

  For such a problem, for example, Patent Document 10 shows a method of performing a heat treatment while correcting the shape with a pressing roll, and corresponds to a shape change such as a warp of a plate generated by the heat treatment. Yes. Patent Document 11 discloses a method of performing press correction before heat treatment.

  However, in these methods, although it is possible to cope with the shape change due to deformation such as warpage of the plate due to heat treatment, deflection, torsion, etc., it is only correction, so plate width, plate thickness, due to phase transformation during heating and cooling, It was not possible to cope with dimensional changes in which the plate length itself changed greatly, and often resulted in the addition of mechanical care processes for steel plates such as cutting and polishing, and disqualification (defective products).

In Patent Document 12, a function of cutting to a predetermined plate size is added to the heat treatment step, so that it is possible to cope with changes in the plate size.
However, even with the technique of Patent Document 12, when the plate contracts from the planned size due to heat treatment, the insufficient length cannot be compensated for, and the yield is also deteriorated.

JP 09-143557 A JP, 2006-183387, A JP 2014-34708 A JP 2011-241419 A JP 2011-219849 A JP 7-316654 A International Publication No. 2010/038470 International Publication No. 2012/005330 International Publication No. 2013/046357 JP 2005-230914 A JP-A-2005-226106 JP 2001-25810 A

  The phenomenon that the plate dimensions change due to heat treatment accompanied by phase transformation has been known for a long time. For example, the martensite phase generated by quenching has a lower density than the ferrite phase and the pearlite phase, and thus the volume expands as a whole. On the other hand, when austenite is formed by heating, the volume shrinks.

  In the heat treatment process in which heating and cooling are performed, this expansion and contraction almost occur. At that time, if the amount of expansion and contraction generated during the heat treatment is finally offset and becomes substantially zero after the heat treatment, the dimensions before the heat treatment and the dimensions after the heat treatment step will hardly change. In fact, this is rare.

  Furthermore, in the heat treatment process of thick steel plates, when heat-treating high Ni thick steel plates that contain a high amount of Ni as an alloying element, there is a phenomenon that the plate thickness called “burn-out” is significantly reduced, resulting in poor yield. It was connected. Therefore, there has been a demand for a method for producing a high Ni thick steel plate with little dimensional change even after the heat treatment step.

  Patent Documents 7 to 9 each describe a high Ni thick steel plate with little Ni segregation and a method for producing the same. According to the manufacturing methods described in Patent Documents 7 to 9, it was possible to manufacture a high Ni thick steel plate with little burn-out. However, in the manufacturing methods described in Patent Documents 7 to 9, it is necessary to homogenize (soak) the cast slab (slab) at a high temperature of 1250 ° C. or higher for a long time before hot rough rolling. There is.

  When the homogenization treatment (rough heat treatment, soaking) before rough rolling is performed at a high temperature of 1250 ° C. or higher for a long time, a large amount of scale (oxide) is often generated on the surface of the high Ni thick steel plate. This scale of high Ni steel is not limited to the thick steel plate, and is firmly adhered to the metal, and therefore cannot be completely removed even by descaling in the rolling line. If the scale remains, wrinkles often remain on the surface of the steel sheet after quenching. At this time, since it is necessary to grind the surface by pickling or mechanical grinding, it is usually devised so that it is manufactured to be thicker than the required product thickness and the surface thickness is not less than the lower limit plate thickness. For this reason, there is the actual situation that the yield has deteriorated.

  In view of the above circumstances, an object of the present invention is to provide a high Ni thick steel plate and a method for manufacturing the same, which does not generate a large amount of scale on the surface of the high Ni thick steel plate and is less burned out after heat treatment.

The gist of the present invention for solving the above problems is as follows.
(1) In mass%,
C: 0.03-0.10%,
Si: 0.02 to 1.5%,
Mn: 0.1 to 2.0%,
Ni: 2.0 to 10%
P: 0.005% or less,
S: heating a slab having a chemical composition composed of 0.002% or less and the balance being Fe and inevitable impurities;
Thereafter, when the homogenization process is not performed or when the homogenization process is performed, the holding temperature is 1200 ° C. or less and the heating time is 10 hours or less.
Rough rolling to a reduction ratio of at least 2;
Intermediate heating is performed at a temperature of 1180 ° C. to 1250 ° C. for 1.5 hours to 5 hours,
A method for producing a high-Ni thick steel plate, which is followed by finish rolling.
(2) The slab is replaced by a part of the Fe in mass%,
Ti: 0.1% or less,
Nb: 0.1% or less,
B: 0.01% or less,
Cr: 1.5% or less,
Cu: 1.0% or less,
1 or 2 types or more, The manufacturing method of the high Ni thick steel plate as described in (1) characterized by the above-mentioned.
(3) The slab is replaced by a part of the Fe in mass%,
Mo: 1.0% or less,
W: 0.5% or less,
V: 0.5% or less,
The method for producing a high Ni thick steel plate according to (1) or (2), comprising one or more of the above.
(4) The slab is replaced by a part of the Fe in mass%,
Al: 0.05% or less,
The manufacturing method of the high Ni thick steel plate as described in any one of (1)-(3) characterized by containing.

  According to the present invention, the homogenization treatment is not performed or minimized, so that the scale is small, so that the amount of grinding is small because the scale wrinkles on the surface derived from the scale is small, and after the heat treatment, it is burned out. It is possible to provide a method for producing a high-Ni thick steel plate with a small amount. As a result, in the present invention, the homogenization process before rough rolling (soaking process) and an extra sheet thickness as a measure against scale wrinkles are eliminated, and the possibility that the scale wrinkles themselves will be degraded is also reduced. Cost reduction and high productivity can be achieved.

It is a figure explaining the deformation | transformation process of each layer in a heating process in the steel plate which has a layered density distribution of an alloy element in the sheet thickness direction. It is a figure explaining the difference ((A) prior art, (B) this invention) of the process of this invention, and the conventional temperature process.

  As a result of intensive studies on the wear-out of the high-Ni thick steel sheet, the present inventors have clarified that the wear-out is derived from a band structure caused by microsegregation during solidification.

The process of this elucidation will be explained.
Particularly, in a high Ni thick steel sheet containing a large amount of Ni as an alloy element, for example, when heat treatment is performed to heat to Ac1 or higher, which is a temperature at which austenite starts to be generated, the dimensional change before and after the heat treatment is significant. Examining the direction in which the dimensions change, among the plate width direction, plate thickness direction, plate length direction, especially the dimensional change in the plate thickness direction causing burnout is significant, in the plate width direction, plate length direction, In both cases, the dimensional change was about half of the change in the plate thickness direction.

Therefore, in order to elucidate the reason why the dimensional change significantly occurs only in the plate thickness direction after heat treatment, while the dimensional change in the plate width direction and the plate length direction is relatively small and the same level, Detailed elemental analysis was performed on each part of the high Ni thick steel plate.
As a result, the conventional high Ni thick steel sheet has a band structure in which a concentrated layer having a high Ni concentration and a deficient layer having a low concentration are repeatedly laminated in the thickness direction. It became clear that This band structure is more easily formed particularly in a thick steel plate as a target of the present invention. In addition, this band structure remained as it was even after the heat treatment in which burnout occurred.

  The band structure has a structure in which a plurality of thin layers having a uniform concentration in the plate width direction and the plate length direction are stacked in the plate thickness direction to some extent. That is, the change in the Ni concentration is poor in the plate width direction and the plate length direction, while the change in the Ni concentration depending on the location is significant in the plate thickness direction.

  In the plate width direction and the plate length direction where the Ni concentration change is poor, the dimensional change before and after the heat treatment is relatively small. Therefore, the dimensional change in the plate thickness direction is large because the Ni concentration change in the plate thickness direction ( Due to the presence of a concentrated layer and a depleted layer), and during transformation or reverse transformation at the time of heat treatment, the phase transformation timing is shifted between one Ni concentrated layer and the adjacent layer having a different Ni concentration. Or it was thought that the plastic deformation which arises in order to compensate the distortion by shrinkage was caused.

This point will be further described.
L ₁ shown in FIG. 1 is a layer that undergoes phase transformation at the lowest temperature. As the temperature is raised by heating, the composition of each concentrated layer and each non-concentrated layer (deficient layer) that are stacked is different, so that there is a difference in the temperature at which each layer actually undergoes phase transformation. That is, when the temperature increase, a layer L ₁ to phase transformation to shrink earlier at low temperatures, result in a layer L ₂ ~L _N to contract in phase transformation later at high temperatures, and phase transformation of each layer, There is a time lag in dimensional changes due to phase transformation. Therefore, the arrow length in FIG. 1 (A), as represented by _{L 1} contraction of 1, _{L 2} shrinkage of 2, difference occurs in _{L 1} and _{L 2} in the contraction degree of the dimension. Since the volume change due to the phase transformation is large, the phase transformation temperature is the lowest, and when the phase transformation occurs in the layer L ₁ that undergoes the _first phase transformation in the heating and heating process, the L ₁ contraction force 3 causes the plate length direction and the plate width direction. The length tends to shrink in the plate thickness direction (see FIG. 1B). However, since no cause layer L ₂ Hamada phase transformation of the adjacent L _1, layer L ₁ may also less plate length direction, can not be contracted in length in the sheet width direction. This is because even if the layer L ₁ is contracted, the length in the plate length direction and the plate width direction is changed by being restrained by the restraining force 4 from the adjacent layer L ₂ to L ₁ whose length has not changed. This is because it becomes difficult. For this reason, the actual L ₁ contraction force 5 of the layer L ₁ that has undergone phase transformation works in the plate thickness direction, which is the remaining direction other than the plate length direction and the plate width direction that are constrained. deflated (see FIG. 1 (C)), the amount from Itacho direction to be originally shrinkage phase transformation of L ₁ remains in the plate width direction is longer length is completed. Thereafter, the temperature rises, L ₂ undergoes phase transformation, and attempts to shrink in the plate length direction and the plate width direction, this time, the plate length direction and the plate width direction remain longer than the amount that should be shrunk. Constrained by L ₁ , it is not possible to sufficiently contract in this direction, and L ₂ is considered to contract excessively in the plate thickness direction like L ₁ . Due to such a dimensional change during the heating process, the shrinkage of each layer is likely to occur in the thickness direction, and as a whole, it is considered that the shrinkage is greatly reduced in the thickness direction. On the contrary, the expansion force works at the time of cooling and cooling, but it is thought that the change in the plate thickness direction becomes large by the binding force acting on each layer.
For convenience of explanation in FIG. 1, although a layer of phase transformation to L ₁ in the outermost layer at the lowest temperatures, necessarily, the outermost layer is not a layer which phase transformation at the lowest temperatures. In addition, the order of transformation of each layer differs depending on the production conditions and components, and therefore does not have special regularity.

  Therefore, in order to grasp the concentration change of the alloy element in the plate thickness direction, the Ni concentration change was analyzed as the Ni microsegregation degree evaluated by the ratio between the maximum concentration and the minimum concentration.

As a result, it has been clarified that when a high Ni thick steel plate having a large Ni micro-segregation degree is heat-treated, a large dimensional change, that is, burn-out occurs in the thickness direction before and after the heat treatment.
For this reason, it is considered that if the Ni microsegregation degree is reduced or the band structure is eliminated by eliminating the Ni microsegregation, the burnout is also eliminated.
In fact, Patent Documents 7 to 9 describe a high Ni thick steel plate with little Ni segregation, but as a result of heat-treating such a high Ni thick steel plate with little segregation, the burn-out is reduced. Was confirmed. The term “micro segregation” in the present invention means that it is not so-called macro segregation of casting, and does not necessarily mean that the order of the structure is micron order.

  The present invention will be described in detail below. The high Ni thick steel plate of the present invention is a steel plate having a thickness of 4 mm or more. More preferably, it is a steel plate having a thickness of 6 mm or more.

The reason for limitation of each manufacturing condition prescribed | regulated to this invention is demonstrated.
Initially, the conditions of the manufacturing process of the manufacturing method of the high Ni thick steel plate of this invention are demonstrated. The manufacturing process conditions of the present invention are for reducing microsegregation of Ni (reducing the band structure) and thereby preventing burning.

A high Ni thick steel plate is manufactured by hot rolling a slab having a chemical component limited to the present invention described later. The slab to be subjected to hot rolling may be obtained by continuous casting, casting, or ingot rolling, but may be obtained by adding hot working or cold working to them.
In the method for producing a high Ni thick steel plate according to the present invention, a characteristic production condition is the following hot rolling step.

(Homogenization treatment: not performed, holding temperature 1200 ° C. or less, and heating time 10 hours or less)
The band structure that causes burnout can be achieved by maintaining slabs and steel materials at a predetermined high temperature or higher for a predetermined long period of time as in the prior art, as in the slab soaking (homogenization) condition shown in FIG. It can be lost.
If rough rolling and finish rolling are performed under the conditions of the present invention, the Ni concentrated layer (band structure) can be eliminated without homogenization. However, homogenization is effective when it is necessary to reduce voids called dehydrogenation or porosity. The amount of scale generated before rough rolling increases with increasing temperature and time. For this reason, if the homogenization treatment before rough rolling is performed at a holding temperature of over 1200 ° C. or a heating time of over 10 hours, the scale increases. In order to suppress this, when homogenization is performed, the temperature x time is set by setting the holding temperature to 1200 ° C. or less and the heating time to 10 hours or less as in the slab soaking conditions shown in FIG. To reduce the occurrence of scale. In addition, 900 degreeC or more is preferable normally, and the holding temperature in the case of performing a homogenization process has more preferable 1000 degreeC or more.
In this way, if the homogenization process at the slab stage is reduced in temperature and time is reduced, the scale generated on the surface of the steel sheet becomes thinner. It becomes possible to prevent. When the homogenization treatment (soaking) before rough rolling is a normal condition that is not a low temperature and short time condition as in the present invention, the scale is not completely scaled by descaling in the rolling line, often in the case of high Ni steel. It cannot be removed and remains as a surface defect after quenching.

  On the other hand, if the homogenization process is not performed or the homogenization process is performed at a low temperature for a short time, the Ni microsegregation is not sufficiently reduced as it is. Therefore, in the present invention, Ni microsegregation is diffused and reduced by rough rolling and further intermediate heating between rough rolling and finish rolling.

(Rough rolling: reduction ratio is at least 2)
In the present invention, the reduction ratio here refers to the ratio between the slab thickness and the plate thickness after rough rolling. In the present invention, after the rough rolling and before the finish rolling, the heat treatment described later (intermediate heating) is performed, so the reduction ratio here is, in other words, the ratio between the slab thickness and the plate thickness before intermediate heating. it can.
The temperature and time required for the diffusion (segregation diffusion) of the Ni concentrated layer forming the band structure depend on the concentration of segregation and the band interval (diffusion distance). The segregation concentration is determined at the time of casting, but the band interval can be shortened by rolling, and segregation and diffusion can be efficiently performed by performing intermediate heating described later after rough rolling.
That is, in the rough rolling, before the intermediate heating is performed, the rolling is performed until the reduction ratio is at least 2. Thereby, the space | interval of the layer (Ni concentrated layer) with high Ni density | concentration in a band structure | tissue can be narrowed. As a result, since the diffusion distance of Ni for homogenization is shortened, the intermediate heating conditions described later are reduced in temperature and shortened (however, as long as the band structure can be reduced, the temperature is increased as long as possible). it can.
When the rolling ratio is less than 2, Ni is not sufficiently promoted in rough rolling, and Ni is segregated to form a band structure. As a result, burn-out cannot be reduced. Therefore, the reduction ratio is set to 2 or more. The upper limit of the reduction ratio is not particularly limited, but is preferably 20 or less for the operation of the manufacturing facility.

  Heating before rough rolling is performed up to a temperature at which rough rolling is possible. As long as rough rolling can be performed, as described above, the previous homogenization treatment (soaking) may not be performed or at least at a low temperature for a short time. As the homogenization process becomes milder, costs can be reduced and quality can be improved. As usual, the slab temperature immediately before rough rolling is preferably 950 ° C. or more and 1230 ° C. or less. This is because when the heating temperature is too high, scale generation increases. What is necessary is just to determine a heating time suitably according to the magnitude | size of a slab.

(Intermediate heating between rough rolling and finish rolling: 1.5 hours to 1250 ° C. for 1.5 hours to 5 hours)
Intermediate heating is performed after rough rolling with a reduction ratio of 2 or more, and the band structure is reduced by the synergistic effect of rough rolling and intermediate heating. Between rough rolling and finish rolling, heating at a temperature of 1180 ° C. or more and 1250 ° C. or less for 1.5 hours or more and 5 hours or less, in order to reduce the band structure and prevent burnout It is an important process. Usually, the intermediate heating of this component steel (high Ni steel) is not for diffusing segregation, and therefore is not usually performed at a high temperature for a long time as in the present invention.
In this intermediate heating, when the heating temperature is less than 1180 ° C. or the heating time is less than 1.5 hours, the diffusion of Ni is not sufficiently promoted, and Ni segregates to form a band structure. Cannot be reduced. Therefore, it is assumed that heating is performed at a temperature of 1180 ° C. or higher for 1.5 hours or longer.
On the other hand, if the intermediate heating condition is more than 1250 ° C. or more than 5 hours, it becomes difficult to deteriorate the surface properties of the steel sheet and to reduce the cost. is there.
The intermediate heating conditions of the present invention are high temperature and long time compared with normal intermediate heating conditions. However, in the present invention, the normal homogenization process before rough rolling as shown in FIG. 2 (A), which generates a scale above this intermediate heating condition, is not performed, or even when the homogenization process is performed, FIG. As in B), the conditions are low temperature and short time. Therefore, as can be understood by comparing FIGS. 2 (A) and 2 (B), in the present invention, since the time that the temperature is maintained above 1200 ° C. is short, the scale is generated. Maintained temperature x time is small. As a result, after intermediate heating, there is no or little scale generation, so scale removal is easy and incomplete removal of scale does not occur.

  Finish rolling is performed on the high Ni thick steel plate subjected to intermediate heating. The conditions for finish rolling are not particularly limited, but may be performed at a temperature of 900 to 1100 ° C. and a reduction ratio of about 1 to 30, as usual. After finish rolling, heat treatment is performed. Examples of the heat treatment include quenching and tempering under normal conditions.

It is as follows when the characteristic manufacturing conditions in the manufacturing method of the high Ni thick steel plate of this invention are put together.
1. Do not perform homogenization before rough rolling, or if low temperature and short time, prevent generation of scale flaws.
2. Increasing the rolling ratio of rough rolling (shortening the band interval of the band structure).
3. Reduce the band structure by increasing the intermediate heating time to a high temperature and long enough not to increase the cost (to the extent that the surface properties of the steel material are not deteriorated).
Combining these three, a method for producing a high Ni thick steel plate that does not burn out during heat treatment (no or little band structure) and does not require extra thickness as a measure against scale flaws is provided. It is.

  Next, the reasons for limiting the chemical components of the high Ni thick steel plate to be manufactured by the present invention and the slab to be the material will be described. In addition, all “%” for chemical components means mass%.

(C: 0.03-0.10%)
C improves the strength, hardenability, and toughness, so 0.03% or more is added. On the other hand, when the content exceeds 0.10%, the toughness and brittle crack propagation stopping performance are lowered, and the weldability is deteriorated. Therefore, C is contained in an amount of 0.03 to 0.10%. The amount of C is more preferably 0.05% or more and 0.10% or less.

(Si: 0.02-1.5%)
Si is an inexpensive deoxidizer, and further, strength is improved by solid solution strengthening, so 0.02% or more is added. On the other hand, if the content exceeds 1.5%, the surface properties are lowered, so the upper limit is made 1.5%. The Si content is more preferably 0.3% or more and 1.2% or less.

(Mn: 0.1 to 2.0%)
Mn is a deoxidizer and is further added in an amount of 0.1% or more in order to improve strength, hardenability, and toughness. On the other hand, if the content exceeds 2.0%, the wear-out increases and the toughness decreases, so the upper limit is made 2.0%. The amount of Mn is more preferably 0.3% or more and 1.8% or less.

(Ni: 2.0 to 10%)
Ni is the most important element among the alloy elements added in the present invention. Ni improves the corrosion resistance, so 2.0% or more is added. On the other hand, in the present invention, it is necessary to reduce the segregation of Ni. However, if the content exceeds 10.0%, the segregation of Ni is not sufficiently reduced. Further, if it exceeds 10.0%, the cost increases. The amount of Ni is more preferably 2.0% or more and 9.5% or less.

(P: 0.005% or less)
P is present in the steel as an impurity, and if it exceeds 0.005%, it segregates at the grain boundary and causes toughness to decrease, so its content is made 0.005% or less. The P content should be as small as possible, and the preferred P content is 0.003% or less. The lower limit is not particularly limited, but is preferably 0.001% or more from the viewpoint of manufacturing cost.

(S: 0.002% or less)
S is present in the steel as an impurity, and if it exceeds 0.002%, a large amount of stretched MnS that causes brittle fracture is generated and the toughness is lowered. 005% or less. The S content should be as small as possible, and the preferred S content is 0.001% or less. The lower limit is not particularly limited, but is preferably 0.0005% or more from the viewpoint of manufacturing cost.

The above is the limitation on the essential additive elements or elements contained as impurities. The balance is Fe and inevitable impurities. As an unavoidable impurity not mentioned above, for example, there is N. N is present in the steel as an impurity, and if it exceeds 0.05%, it causes a decrease in toughness due to an increase in the amount of N dissolved in the steel and the formation of precipitates. It is preferable to set it to 05% or less. The N content should be as small as possible, and the preferred N content is 0.01% or less. The lower limit is not particularly limited, but is preferably 0.0001% or more from the viewpoint of manufacturing cost.
Furthermore, as a selective addition component, one or more of the following elements may be added instead of Fe.

(Ti: 0.1% or less)
Since Ti improves the strength, Ti can be added in a range of 0.1% or less. In order to sufficiently obtain this effect, it is preferable to add 0.01% or more. On the other hand, if the content exceeds 0.1%, the toughness decreases due to the formation of inclusions, so the content is made 0.1% or less.

(Nb: 0.1% or less)
Nb can be added in the range of 0.1% or less because the strength is improved. In order to sufficiently obtain this effect, it is preferable to add 0.01% or more. On the other hand, if the content exceeds 0.1%, the toughness decreases due to the formation of inclusions, so the content is made 0.1% or less.

(B: 0.01% or less)
B can be added in a range of 0.01% or less because the hardenability is improved. In order to sufficiently obtain this effect, 0.0001% or more is preferably added. On the other hand, if it exceeds 0.01%, the toughness decreases due to the formation of inclusions, so the content is made 0.01% or less.

(Cr: 1.5% or less)
Since Cr improves strength, hardness, and hardenability, it can be added in a range of 1.5% or less. In order to sufficiently obtain this effect, 0.1% or more is preferably added. On the other hand, if it exceeds 1.5%, the toughness decreases, so the content is made 1.5% or less.

(Cu: 1.0% or less)
Since Cu improves strength and hardenability, it can be added within a range of 1.0% or less. In order to sufficiently obtain this effect, 0.1% or more is preferably added. On the other hand, if it exceeds 1.0%, the surface properties deteriorate, so the content is made 1.0% or less.

(Mo: 1.0% or less)
Since Mo improves strength, it can be added in a range of 1.0% or less. In order to sufficiently obtain this effect, it is preferable to add 0.01% or more. On the other hand, if the content exceeds 1.0%, the toughness decreases, so the content is made 1.0% or less.
(W: 0.5% or less)
Since W improves in strength, W can be added in a range of 0.5% or less. In order to sufficiently obtain this effect, it is preferable to add 0.01% or more. On the other hand, if the content exceeds 0.5%, the toughness decreases, so the content is made 0.5% or less.
(V: 0.5% or less)
Since V improves strength, V can be added in a range of 0.5% or less. In order to sufficiently obtain this effect, it is preferable to add 0.01% or more. On the other hand, if the content exceeds 0.5%, the toughness decreases, so the content is made 0.5% or less.

(Al: 0.05% or less)
Al may be added as a deoxidizer, but if it exceeds 0.05%, the toughness is reduced due to inclusions, so the content is made 0.05% or less. More preferably, it is 0.04% or less.
In addition, in order to acquire the effect as a deoxidizer, it is preferable that the content shall be 0.002% or more. More preferably, it is 0.005% or more.

  The slab shown in Table 1 was prepared with the chemical component in mass%.

  This slab was subjected to rough rolling, intermediate heating after rough rolling, and finish rolling under the conditions shown in Table 2. Here, the rolling ratio in rough rolling is the ratio between the slab thickness and the sheet thickness after rough rolling, and the rolling ratio in finish rolling is the ratio between the sheet thickness after rough rolling and the sheet thickness after finish rolling. It is. Intermediate heating is performed between rough rolling and finish rolling, and conditions are set in which the heating temperature and the time held at the heating temperature are changed. Condition number 8 is a condition in which finish rolling is performed directly from rough rolling and intermediate heating is not performed.

As these combinations, rolled materials were obtained by combinations of steel numbers and condition numbers as shown in Table 3. After these plates were cooled to room temperature, they were heated to 810 ° C. at 1 ° C./s, held for 10 minutes, and subjected to quenching heat treatment to cool to room temperature at 40 ° C./s. The plate thickness change rate before and after the heat treatment was measured. Moreover, the surface grinding amount was measured about the steel plate after heat processing, and weldability was examined. The results are shown in Table 3.
However, the plate thickness change rate was defined as (plate thickness before heat treatment−plate thickness after heat treatment) / plate thickness before heat treatment. In addition, the Ni concentration was measured using EPMA with a 1 mm × 1 mm field of view in a 1/4 thickness part of the steel plate at a pitch of 1 μm, and the ratio between the maximum Ni concentration and the minimum concentration obtained by line analysis in the plate thickness direction. A certain degree of microsegregation M _Ni was determined. The preferable degree of micro segregation M _Ni is 1.2 or less, and the preferable thickness change rate is 0.2% or less.
On the other hand, the surface grinding amount was evaluated by the surface thickness (surface grinding amount) (mm) of the steel plate after the heat treatment, which was ground to remove the generated scale wrinkles.
A preferable surface grinding amount was 1 mm or less. If there is a large amount of surface grinding, it may be less than the lower limit thickness of the product, so it is conceivable to set a rolling thickness that anticipates the surface grinding amount in advance. The amount is preferably 1 mm or less.
The weldability was evaluated by preparing a welded joint from the obtained base material and measuring the Vickers hardness of the welded portion.

In Table 3, the test levels 2, 4, 6, 8, 11, 13, 14, 15, 16, 18, 19, 20 which are the present invention are components, rough rolling, and intermediate heating conditions are limited to the present invention. As a result, the micro segregation degree M _Ni could be set to a preferable value of 1.2 or less, so that the plate thickness change rate could be suppressed within a preferable 0.2%. Moreover, since it was a suitable component range, weldability was also favorable.

  In Table 3, test levels 22 to 25 according to the present invention are for the purpose of investigating the effects of adding each selectively added component. In any of the test levels 22 to 25, the influence on the segregation of Ni due to the addition of the selective additive component was small, and thus the plate thickness change rate during the heat treatment became a small value.

  In Table 3, in particular, test level 28 according to the present invention was not homogenized at all, but microsegregation was sufficiently homogenized by rough rolling and intermediate heating, and the microsegregation degree was lowered to 1.18. The plate thickness change rate was as low as 0.2%.

  Moreover, these inventions had few scales produced | generated on the surface of the steel plate. For this reason, since no wrinkles due to heat treatment occurred, surface grinding could be omitted or even if it was done, the amount of surface grinding could be suppressed to a preferred 1 mm or less.

On the other hand, in test level 1 as a comparative example, the reduction ratio in rough rolling was smaller than 2, so that the diffusion of Ni was not sufficiently promoted, the degree of micro segregation M _Ni was large, and as a result, the plate thickness change rate was large. became.
Comparative Example a is test level 3, for intermediate heating temperature is low, the diffusion of Ni is insufficient, a large micro segregation ratio M _Ni, thickness change rate in the heat treatment is increased.
Test levels 5 and 10 as comparative examples were not subjected to intermediate heating, so Ni was not diffused, the degree of microsegregation M _Ni was large, and the plate thickness change rate during heat treatment was large.
In test level 7 as a comparative example, since the intermediate heating time was short, Ni was not diffused, the degree of microsegregation M _Ni was large, and the plate thickness change rate during heat treatment was large.

Test level 9, which is a comparative example, has a small degree of microsegregation M _{Ni and a} small plate thickness change rate in heat treatment, but because the amount of addition of C exceeds the range limited to the present invention, weldability is poor. became.
Comparative Test Level 12 is an example, often the content of Mn, a large micro segregation ratio M _Ni, thickness change rate is increased.
Test level 17, which is a comparative example, had a problem in corrosion resistance because the Ni content of steel code i was lower than the range limited to the present invention.
The test level 21, which is a comparative example, has a large amount of Ni exceeding the range limited to the present invention. Therefore, even when intermediate heating is sufficiently performed, the degree of microsegregation M _Ni becomes large. It became bigger. In this case, it is necessary to increase the heating temperature or to increase the heating time in exchange for generating a large amount of scale and increasing the surface grinding amount.

In test level 26, which is a comparative example, the heating temperature of the homogenization treatment (soaking treatment) exceeded 1200 ° C., and thus the production cost increased and a thick scale was generated on the surface during the homogenization treatment. As a result, since wrinkles were observed on the surface after quenching, the surface grinding amount was increased.
Test level 27, which is a comparative example, exceeded 10 hours of heating time for homogenization treatment, so that the amount of surface grinding after quenching was increased as in test level 26 (test levels 26 and 27 were current proper conditions). .
In test level 29, which is a comparative example, the intermediate heating temperature exceeded 1250 ° C., so the surface grinding amount after quenching was large.
Test level 30, which is a comparative example, had a large amount of surface grinding after quenching because the intermediate heating time exceeded 5 hours.

  According to the present invention, the burn-out of the high Ni thick steel plate can be reduced, so that the additional labor and cost of the mechanical care process can be reduced, and the deterioration (defective product) can be reduced. In addition, since the grinding after the heat treatment can be reduced, the yield is improved, and the manufacturing cost can be reduced by omitting the homogenization process and the grinding process. In addition, it is possible to provide high-quality products with less wrinkles and scales. As a result, it has excellent industrial applicability such that a high quality high Ni thick steel plate can be provided at a low price.

1 ... L ₁ contraction amount, 2 ... L ₂ contraction amount, 3 ... L ₁ contraction force, 4 ... restraining force from L ₂ to L ₁ , 5 ... actual L ₁ contraction force

Claims (4)

% By mass
C: 0.03-0.10%,
Si: 0.02 to 1.5%,
Mn: 0.1 to 2.0%,
Ni: 2.0 to 10%
P: 0.005% or less,
S: heating a slab having a chemical composition composed of 0.002% or less and the balance being Fe and inevitable impurities;
Thereafter, when the homogenization process is not performed or when the homogenization process is performed, the holding temperature is 1200 ° C. or less and the heating time is 10 hours or less.
Rough rolling to a reduction ratio of at least 2;
Intermediate heating is performed at a temperature of 1180 ° C. to 1250 ° C. for 1.5 hours to 5 hours,
A method for producing a high-Ni thick steel plate, which is followed by finish rolling. The slab is replaced by a part of the Fe in mass%,
Ti: 0.1% or less,
Nb: 0.1% or less,
B: 0.01% or less,
Cr: 1.5% or less,
Cu: 1.0% or less,
The manufacturing method of the high Ni thick steel plate of Claim 1 characterized by including 1 type, or 2 or more types of these. The slab is replaced by a part of the Fe in mass%,
Mo: 1.0% or less,
W: 0.5% or less,
V: 0.5% or less,
1 or 2 types or more of these are contained, The manufacturing method of the high Ni thick steel plate of Claim 1 or Claim 2 characterized by the above-mentioned. The slab is replaced by a part of the Fe in mass%,
Al: 0.05% or less,
The manufacturing method of the high Ni thick steel plate of any one of Claims 1-3 characterized by the above-mentioned.

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